Kamila Vagnerova, MD Ines P Koerner MD PhD Patricia D Hum PhD

From the Department of Anesthesiology and Peri-Operative Medicine, Oregon Health and Science University, Portland, Oregon.

D. Hurn, PhD, Department of Anesthesiology and Peri-Operative Medicine, OHSU, 3181 SW Sam Jackson Park Road UHS-2, 97239, Portland, OR; E-mail: [email protected].

Abstract

Anesthesiologists are frequently confronted with patients who are at risk for neurological complications due to perioperative stroke or prior traumatic brain injury. In this review, we address the growing and fascinating body of data that suggests gender and sex steroids influence the pathophysiology of injury and outcome for these patients. Cerebral ischemia, traumatic brain injury, and epilepsy are reviewed in the context of potential sex differences in mechanisms and outcomes of brain injury and the role of estrogen, progesterone, and androgens in shaping these processes. Lastly, implications for current and future perioperative and intensive care are identified.

Introduction

An esthesiologists are frequently confronted with patients who are at risk for neurological complications due to perioperative stroke or prior traumatic brain injury. In this review, we address the growing and fascinating body of data suggesting that gender and sex steroids influence the outcome and optimal treatment plan for these patients. Clinical evaluations of neuroinjury and recovery mechanisms have resulted in many of our current concepts of neuroprotection. These concepts, however, must reflect new evidence for complex sex-linked patterns in the epidemiology, risk, and response to stroke, traumatic brain injury, and epilepsy in women versus men. In fact, tantalizing laboratory findings suggest that male and female cells simply do not respond identically to death or survival signals after injury. Furthermore, the presence or loss of hormonal steroids, i.e., the estrogens, progestins, and androgens, suppress or amplify innate gender- based differences in physiology and pathobiology. The purpose of this article is to: 1) review gender differences in mechanisms and outcomes of brain injury, 2) present evidence for the influence of sex steroids in these sex-specific responses, and 3) delineate implications for current and future perioperative and intensive care.

ISCHEMIC BRAIN INJURY

Sex Matters in Ischemic Stroke

Male sex is an acknowledged risk factor for stroke; and, in most international studies, ischemic stroke occurs more often in men than women. This sexually dimorphic epidemiology appears to be present until late in life, well beyond the menopausal years. When female and male animals are evaluated side-by-side, a male phenotype of “ischemic sensitivity" can be uncovered. In a remarkable study of more than 2000 genetically hypertensive and stroke-prone rodents, life expectancy was longer in the female than in the male. In addition, evidence of cerebral hemorrhage and vascular lesions was absent in females until an advanced age.[1] These early observations mimic human epidemiology. Furthermore, outcome from ischemic brain injury (IBI) is clearly sex-linked in genetically nonspecific animal models. Female rats and mice of many different inbred and outbred strains sustain smaller tissue damage and enjoy improved functional outcome compared with their male counterparts after an equivalent insult from focal or global cerebral ischemia.

Similar sex-specificity can be modeled in cell cultures grown without background sex steroids. Male neurons, for example, are more susceptible than female cells to challenges from pharmacological insults used to simulate brain injury, e.g., glutamate or per- oxynitrite.[2] This differential sensitivity may be related to a relative inability of male cells to maintain intracellular glutathione levels after nitrosative stress. In contrast, response to oxidants such as hydrogen peroxide is gender neutral.[2] These observations do not appear to be limited to neurons. Cell death after oxygen-glucose deprivation is less extensive in female astrocytes[3] and in hippocampal slices from females.[4] These findings suggest that sex-specific sensitivity to cerebral ischemia is partly a function of the sex of cells. However, hormonal influences should not be discounted in our understanding of post-IBI cell death and recovery.

Estrogen and IBI: What We Know From the Bench

Today, there is a large body of evidence suggesting a protective effect of estrogen in a variety of experimental models of stroke. Studies of focal as well as global brain ischemia in various rodent models have consistently shown that female animals sustain less tissue damage than males after similar insults.[5-8] This beneficial effect of female gender is lost in reproductively senescent animals[7] or after ovariectomy but can be restored by estrogen supplementation.[7,9-11] Estrogen treatment proved similarly beneficial in male animals.[12-14] Although most of these animal studies emphasized infarct size and cell loss early after the insult, chronic estrogen supplementation also improved functional outcome.[15,16] The effects of chronic estrogen exposure in these models may explain some of the female advantage in IBI, and they are the focus of recent studies in primary stroke prevention. However, since long-term treatment before a perioperative brain insult is obviously not an option for neuroprotection, the efficacy of acute treatment with estrogen in the perioperative setting at or after onset of ischemia has also been tested in experimental ischemia and found to reduce brain damage.[17,18] This benefit also extends to male animals.[12]

Researchers have invested considerable time and effort to determine the mechanisms of estrogen pro-tection. A profound understanding of these mechanisms is required to develop drugs that mirror estrogen's neuroprotection without the undesirable hormonal effects, particularly for male patients. Furthermore, understanding how a patient's hormonal profile may affect brain injury in the perioperative phase will help the clinical anesthesiologist design a more individualized anesthetic regimen. Estrogen elicits a cascade of cellular and subcellular actions that involve both genomic and non-genomic mechanisms after an ischemic insult. These actions can 1) stabilize the blood- brain barrier[19] and subsequently reduce brain edema,[20] 2) dilate vasculature21 to increase cerebral blood flow,[6,9,22 3]) suppress inflammation,[19,23,24] and 4) upregulate cell-survival mediators[.25-27] In addition, estrogen is an antioxidant that can prevent lipid peroxidation.[28-30] N-methyl-D-aspartate (NMDA) receptor activation may contribute to estrogen-mediated neuroprotection,[31] but, at least in higher doses, estrogen can also directly inhibit NMDA receptors,[32] ameliorating excitotoxicity. Recently, investigators also recognized benefits provided by estrogen that extend beyond acute injury and positively influence regeneration and plasticity of new neurons after ischemia.[33] This may contribute to the improved memory function outcome after ischemia that is seen in estrogen- supplemented animals.[16]

In classical estrogen signaling, 17β-estradiol (E₂), the predominant human estrogen, binds to an estrogen receptor (ER), usually ER-αor ER-β, which translocates to the nucleus and binds to an estrogen-response element (ERE) on the target gene to activate transcription. Both ER-αand ER-β are widely expressed under physiologic conditions in all cell types throughout the brain, i.e., neurons, glia, and endothelial cells, including in ischemia-sensitive areas such as neocortex and hippocampus.[34-37] Not surprisingly, concentrations of ER-αand ER-β are higher in adult females compared with males.[38] Transcriptional regulation of genes that do not carry an ERE has also been described via activation of a variety of non-ER transcription factors. In addition to classical transcriptional gene activation, E₂ elicits non-transcriptional rapid signaling action, possibly through interaction with membrane-bound G-proteins. Rapid actions include modification of protein phosphorylation and levels of intracellular second messengers such as cyclic adenosine monophosphate or calcium. Figure 1 and Table 1 provide information on studies that describe the main signaling pathways by which estrogen affects physiological changes to reduce IBI.

Table 1. Summary of Potential Estrogenic Mechanisms of Neuroprotection in Ischemic Brain Injury.

Figure 1. Relevant estrogen signaling pathways in ischemic brain injury.

Despite solid experimental evidence that E₂ is neuroprotective in stroke, some investigators have found no effect or even detrimental effects of E₂ in experimental ischemia. Such findings may be related to the dose of E₂ used, since neuroprotection may be lost or detrimental effects may occur at higher doses.[39,40] E₂ may also be less beneficial with increased severity of injury, e.g., prolonged or permanent vessel occlusion as opposed to transient occlusion.[41,42] In some models of comorbidity, such as diabetes, E₂ increases infarct size[43] and postischemic inflammation.[44,45] Some of these effects may be model-related. A recent study found that E₂ replacement reduced infarct size as well as systemic and brain inflammation only if it was initiated immediately after ovariectomy, but not after a prolonged period of hypoestrogenic- ity.[46] The mechanisms behind this dichotomy are unclear, but may be related to an inability to upregulate the expression of ERs in response to ischemia that was seen in animals exposed to prolonged hypoestro- genicity.[46] Other effects of ovariectomy besides the removal of endogenous sex hormones, such as reactive upregulation of pituitary hormones, may also account for some of the findings in animal studies. Unfortunately, this question has not been addressed. Age may also be a confounding factor, as ERalpha expression increases in reproductively senescent females.[47] Overall, while most experimental data support a beneficial effect of E₂ in IBI, the dissenting findings emphasize that E₂-mediated neuroprotection may depend on the specifics of the experimental and clinical situation. More work is clearly needed to define the circumstances under which E₂ can be expected to show its full neuroprotective potential.

Progesterone and IBI: What We Know From the Bench

Fewer studies have focused on the effects of pro-gesterone, the “other" female hormone, on IBI. Most reports identify beneficial effects of progesterone and its metabolite, allopregnanolone, in a variety of experi-mental injury models, including focal and global ce-rebral ischemia. Both cell survival, assessed as lesion volume,[48-52] or neuronal density,[53-55] as well as functional neurologic outcome[49,50,56] are improved by acute or chronic progesterone treatment in male[56] and female[7] animals. However, Murphy et al. reported that chronic progesterone treatment is associated with exacerbated striatal injury after focal ischemia in ovariectomized rats.[57]

After brain injury and ischemia, progesterone stimulates protective pathways that suppress inflam-matory response,[58-60] i.e., reduce expression of proin-flammatory cytokines and decrease post-stroke edema[58]; but the exact molecular mechanisms involved remain unclear. We know that progesterone, but not allopreg- nanolone, binds to the progesterone receptor, sigma receptors, and the putative membrane progesterone binding site,[61'62] which may be important steps in the neuroprotective pathways. Finally, both allopreg- nanolone and progesterone activate γ-aminobutyric acid-A (GABA_a) receptors, protecting neurons from in vitro ischemia.[63]

Female Sex Steroids in IBI: Clinical Trials and Implications for the Anesthesiologist and Intensivist

Despite the abundant epidemiologic and experimental data that support the beneficial effects of female sex hormones in brain injury, and specifically cerebral ischemia, no clinical studies have substantiated any benefit of estrogen or progesterone treatment in the context of stroke. Neither have estrogen nor progesterone been studied clinically as an acute treat-ment for perioperative brain injury, although experi-mental data suggest that they may reduce injury and improve outcome. Since side effects of an acute, likely single-dose, treatment regimen are expected to be limited in both men and women, clinical trials appear to be warranted.

Much of the recent interest in clinical trials has focused on long-term, chronic female hormone treat-ment, i.e., hormone replacement therapy (HRT). Large studies, including the National Institute of Health- sponsored Women's Health Initiative, have linked HRT to increased risk for incidental stroke. The Women's Health Initiative studied two parallel groups: women after hysterectomy were randomized to receive either conjugated equine estrogen or placebo, and women with an intact uterus were randomized to receive combined estrogen and progestin or placebo.[64] Quite unexpectedly, in light of the epidemiologic data supporting a reduced incidence of stroke in premenopausal women, initial results from the Women's Health Initiative showed an increased risk for firsttime stroke in both groups of otherwise healthy postmenopausal women[65]; and the study was terminated prematurely. E₂'s hormonally inactive optical isomer, 17α-estradiol, was tested in a phase I clinical study and was determined to be safe for human use.66 However, data supporting an actual neuroprotective effect in humans are currently lacking.

Are there any clinical implications, then, for the anesthesiologist and intensivist? Despite the over-whelming experimental evidence for the neuroprotective effects of female sex steroids in IBI, there is an alarming paucity of clinical data. Consequently, there are currently no recommendations for the use of acute estrogen or progesterone treatment to afford protection in perioperative brain injury. Furthermore, no clinical studies have attempted to define gender differences in anesthetic neuroprotection. Although years of laboratory research have convinced most anesthesia providers that commonly used anesthetic drugs are more or less potent neuroprotectants,[67'68] all of the relevant research was performed exclusively in males. It is unclear at this time if females enjoy the same level of protection by anesthetics. Recent work on anesthetic preconditioning has shown, however, that isoflurane does not induce tolerance to experimental stroke in gonadally intact female rodents as it does in intact males; and, in fact, females sustain greater damage.[68] Neuroprotection by the anesthetic drugs isoflurane and xenon involves activation of Akt[69] or CREB,[70] neuroprotective mediators that can also be activated by E₂. Surprisingly, however, Kitano et al. also found that while isoflurane preconditioning induced Akt activation in brains of male mice, this was not the case in females.[68] In the context of anesthetic preconditioning, E₂ therefore may prevent rather than induce the activation of neuroprotective pathways. More definitive studies on this phenomenon are urgently needed to ensure optimal safety when developing an anesthesia plan and choosing the anesthetic drug, particularly, for female patients at risk for perioperative stroke.

In light of the adverse findings of the recent HRT trials and the complete lack of clinical studies on the effects of gender and sex hormones on perioperative IBI, many questions that are crucial to the routine and safe practice of anesthesiology and critical care have to remain unanswered at this time. Should anesthesia providers concerned about perioperative ischemia be more inquisitive about the hormone status of their female patients? Should we possibly try to manipulate this status to include recommendations at preanesthesia visits to continue or stop HRT and contraceptives?

Clinical evidence for the effects of endogenous hor-mones on perioperative IBI is abysmal; and there are no relevant clinical trials. Experimentally, infarct size after focal ischemia is inversely related to the levels of circulating estrogen in normal cycling female rats.[71] Although this suggests that elective high-risk surgery may be “safer" for the brain on days when endogenous estrogen levels are high, is this sufficient evidence to recommend scheduling such surgery according to the menstrual cycle of the premenopausal patient? Until we have more clinical data, most providers will likely answer this question in the negative, but more work is clearly needed to develop clinical evidence. Some have suggested discontinuing HRT and contraceptives prior to elective surgery to reduce the risk of perioperative thrombosis.[72] Will this practice, however, put the brain at higher risk in the case of perioperative ischemia? Should recommendations be based on the perceived risk of the planned surgery? If so, what data would we use to establish the relative risk of perioperative thrombosis versus stroke? Similarly, should HRT be continued in the intensive care unit (ICU) after stroke or traumatic brain injury (TBI)? Although outpatient cardiac medications will almost certainly be continued, hormones are more likely to be discontinued, since they tend to be viewed as unimportant or even dangerous due to their thrombogenic potential. What are the effects of acute hormone withdrawal on brain injury and outcome? Considering that E₂ suppresses inflammation after brain injury, should optimal levels be maintained in these critically injured patients, and what are optimal levels? All of these questions are admittedly pointed and provocative, and they are far from being answered definitively. Nevertheless, they may help to emphasize the gap between experimental data and clinical trials that needs to be narrowed before clinical practice can change. Hopefully, they will also raise awareness among anesthesia providers of the relevance that gender and sex hormones may have in the perioperative period.

TRAUMATIC BRAIN INJURY

Sex Differences and Neuroendocrine Abnormalities

TBI is a major cause of death and disability world-wide and is the leading cause of death between the ages of 15 and 44 yrs. Head injuries account for the majority of all trauma-related deaths; and at least 6.2 million people in Europe and 5.3 million in the United States live with disability, impairment, or handicap from TBI.[73]

Young adult males are at highest risk for TBI, but the male/female incidence ratio reaches 1:1 at age 65 yrs.[74] The strong relationship between age and TBI outcome has been demonstrated in numerous prognostic studies. Results from an IMPACT (International Mission on Prognosis and Analysis of Clinical Trials) study of TBI confirm the previously described direct association between age and volume of the lesion, particularly in acute subdural hematomas.[74] This ob-servation has direct consequences for health care planning due to increasing age of the population and increasing incidence of TBI in the elderly.[74-76]

Reports on gender-related differences in outcome after TBI have raised interest in hormonal influences and generated research into neuroprotective effects of estrogen, progesterone, and testosterone. Women were sometimes excluded from early studies due to concerns regarding effects on fecundity and the influence of hormonal fluctuation on drug pharmacokinetics. More recent studies indicate poorer TBI outcome in females,[77-81] whereas only a few investigators report a better outcome.[82] In a prospective study of severely and moderately brain-injured individuals, Kraus et al. found that females were 1.75 times more likely to die of their brain injury than males and were 1.57 times more likely to experience poor outcomes, i.e., severe disability or persistent vegetative state.[83] Mushkudiani et al. used data from an IMPACT study to describe and quantify the prognostic value of demographic characteristics, including gender, on six- month TBI outcomes assessed by the Glasgow Outcome Scale.[74] They extracted individual patient data based on age (n = 8719), gender (n = 8720), race (n = 5320), and education (n = 2201) from eight therapeutic phase III randomized clinical trials and three surveys involving moderate or severe TBI. Analysis demonstrated a reciprocal relationship between outcome and increasing age, but no correlation between gender and outcome was found in this study. The investigators concluded, therefore, that outcome after TBI is dependent on age, race, and, to a lesser extent, on education, but not on gender.

Despite a lack of consistent clinical data regarding gender differences in TBI, a growing body of evidence from laboratory and clinical research supports the influential role of sex hormones in injured brain. Several studies have also focused on gender-specific changes associated with TBI. Reproductive function, for example, is downregulated in episodes of severe illness, including TBI.[84,85] Hypopituitarism often occurs in the post-acute phase of TBI and may normalize later; however, it may also develop after the postacute phase. Schneider et al., described the prevalence of anterior pituitary insufficiency at 3 mo (56% of all patients) and 12 mo (36% of all patients) after TBI. At 3 mo, the extent of hypogonadism was directly proportional to the severity of disease.[86] At 12 mo, however, the clinical improvement noted was significantly less marked in male patients, due possibly to low testosterone levels or greater severity of disease. In fact, a decline in testosterone that is dependent on the severity of the injury and is also reversible has been reported in studies of the early phase after TBI.[87,88] Agha et al. evaluated the prevalence of anterior and posterior pituitary dysfunction in the early phase after TBI.[89] Eighty percent of patients had gonadotropin deficiency. In males, there was a direct correlation be-tween serum testosterone concentration and Glasgow Coma Scale assessment. Similarly, Dimopoulou et al. found that 53% of their TBI patients had an abnormal result in at least one hormonal axis tested during the early recovery period, and cortisol hyporesponsive- ness and gonadal dysfunction were equally common in males and females.[90] Interestingly, these endocrine abnormalities were associated with a higher brain computed tomography scan classification score.

In summary, we recommend that neuroendocrine abnormalities be assessed more carefully after TBI since they may have significant implications for recovery and rehabilitation. To confirm that sex steroids mediate gender differences in TBI outcome, more clinical studies are needed; however, there is already sufficient evidence to warrant considering restoration of gonadotropin levels in practice. For example, male TBI patients in the ICU or the operating room could benefit not only from restoring their testosterone levels, especially in the early phase, but also from monitoring these levels long-term for at least 3 mo or even 12 mo. Other studies suggest that progesterone treatment may improve outcomes for men and women.

Progesterone and TBI: What We Know From the Bench

The influence of sex steroids in trauma-induced brain damage was first considered with the observation that females develop less edema91 and sustain reduced cortical contusions compared with males.[92] Focus then centered around progesterone because edema is virtually absent in states of hyperprogester- onemia in females.[91,93] Progesterone is present in small but approximately equal concentrations in male and female brain, and progesterone receptors are widely distributed throughout the central nervous system.[94]

Research studies in animal models of TBI confirm the neuroprotective effects of progesterone (Table 2). Progesterone is beneficial in ischemic as well as trau-matic brain injury (Fig. 2). In fact, at comparable doses, progesterone yields effects that are reproducible across species and types of brain injury. In 2003, Goss et al. published a dose-response study demonstrating that 8-16 mg progesterone/kg body weight is optimal to promote cognitive recovery after TBI.[95] More recently, Sayeed et al. found that allopregnanolone is even more effective in facilitating central nervous system repair.[52]

Table 2. Neuro effect of progesterone in traumatic Brain injury.

Figure 2. Progesterone effects in ischemic and traumatic brain injury.

Method of delivery also affects the efficacy of progesterone. The pharmacokinetics of progesterone indicates that the half-life of this neurosteroid in serum is approximately 15 min, and it is fully metabolized by 24 h.[96-98] This results in a spiking effect that is attenuated by subcutaneous delivery, as the bolus of drug seeps into tissues at a slower rate.[99-101] Cutler et al. demonstrated that, in a rat model of TBI, continuous progesterone release treatment is more beneficial than daily subcutaneous bolus injections over the same period of time.[102] Treatment is optimized by delivering a continuous infusion of progesterone over 5 days, and the adverse effects of acute progesterone withdrawal are reduced with gradual tapering by 1 week postinjury. In contrast to tapered withdrawal, acute progesterone withdrawal is characterized by increased apoptosis, inflammation, and anxiety behaviors during the acute recovery phase after TBI.

Acute progesterone withdrawal occurs when the linkage between allopregnanolone and receptors that activate GABA is suddenly terminated, causing an upregulation of NMDA and sigma receptor binding, which, in turn, leads to increased anxiety, depression, and seizure susceptibility.[103-108] Furthermore, continuous progesterone infusion provides a model better suited to inform clinical trials for progesterone use after TBI.

In summary, laboratory results indicate that, in males and females, progesterone treatment after TBI dramatically reduces edema109 and subsequent neuronal degeneration,[110,111] restores the integrity of the blood-brain barrier,[112] and improves spatial learning performance. Clinical findings substantiate benefits of progesterone treatment.

Progesterone and TBI: Clinical Trials and Implications for the Anesthesiologist and Intensivist

Progesterone is an attractive candidate for the treat-ment of TBI because it is lipid soluble and can, therefore, rapidly cross the blood-brain barrier, reaching equilibrium with plasma within an hour of admin-istration.[113] It also has a long history of safe use in men and women.[114] Although the neuroprotective potency of progesterone has been successfully studied in the laboratory, it is extremely difficult to translate its efficacy into clinical benefits.[73] Outcome after TBI depends not only on the nature and severity of the injury and the subsequent treatment, but also on the unique, constituent characteristics of each individual patient. For example, Farin et al. studied males and females with severe head injury and described greater susceptibility to brain swelling in females 50-yrs-of- age and younger, with a possible benefit from more aggressive monitoring and treatment of intracranial hypertension in this group.[115] They postulated that higher levels of estrogen relative to progesterone in these patients may be responsible for this sensitivity.

In 2005, Wright et al. reported that IV progesterone could be administered in effective doses via a peripheral line to adult victims of acute TBI.[116] Their formulation, which is now widely available in inexpensive, generic forms, was used in ProTECT, a clinical trial to assess the safety and potential benefit of administering progesterone to patients with acute TBI. In 2007, Wright et al. reported the results of this phase II, randomized, double-blind, placebo-controlled trial, conducted at an urban Level I trauma center.[94] One hundred adult trauma patients who arrived within 11 h of injury with a post-resuscitation Glasgow Coma Scale score of 4 to 12 were enrolled in this trial with proxy consent. Seventy-seven patients received progesterone, and 23 received placebo. The groups had similar demographic, clinical, and laboratory characteristics. Results showed that the 30-day mortality rate in the progesterone group was lower than in the control group. Furthermore, survivors of moderate TBI who received progesterone were more likely to have a moderate to good outcome than those randomized to placebo. Although no significant differences were observed between treatment and control patients in mean intracranial pressure or in the relationship of intracranial pressure to therapeutic intensity levels, this study lacked sufficient power to assess the effects of progesterone on intracranial pressure.[94]

The Wright et al. study concluded that progesterone caused no discernible harm and may be a beneficial treatment for TBI. The investigators speculated, however, that proxy consent delayed initiation of treatment by several hours. One study of TBI in an animal model suggests that progesterone may yield favorable effects as late as 24 h postinjury, but the benefit is greatest if treatment is administered within 2 h.[117] Early initiation of treatment, perhaps through exception to informed consent, would maximize potential therapeutic benefits and should be considered for future clinical trials of this agent.[94] A larger trial involving multiple clinical sites, randomization 1:1, and rapid initiation of treatment is still warranted.

In summary, noninvasive progesterone treatment is potentially beneficial for improving functional and cognitive recovery in TBI patients. It may also help to prevent and treat intracranial hypertension, although drug interactions with mannitol and other therapeutics must be addressed. Progesterone, unlike estrogen, can be administered to both genders without significant side effects. Ease of delivery and a relatively large window of opportunity also make progesterone very attractive. Although the clinical data may not be complete, anesthesiologists and intensivists can still consider using this neurosteroid in the ICU and the operating room, especially in the early phase of TBI. Special attention should be paid to women receiving contraceptives and HRT. If HRT is discontinued, the sudden decrease in progesterone and/or estrogen levels may negatively impact TBI outcome and can even cause acute withdrawal; therefore, continuation of hormonal treatment in the ICU and operating room may be beneficial and may also prevent withdrawal symptoms. Furthermore, TBI in pregnancy remains largely unexplored, but hyperprogesteronemia reduces brain edema.[91,93]

Epilepsy and Gender

Many epidemiologic studies suggest sex differences in the incidence of epilepsy. These differences appear to be closely linked to the type of seizure disorder. Overall epilepsy incidence may be higher in males,[118,119] but women are more likely to suffer from idiopathic generalized epilepsy[120,121] or absence seizures.[118] Because the menstrual cycle influences the occurrence of complex partial seizures involving the limbic system[122-127] and menopause largely eclipses gender differences in epilepsy incidence, sex steroids appear to play a significant role in these underlying disease processes.[128] In fact, increased seizure incidence is associated with low estrogen and low progesterone phases and with the follicular phase when plasma estradiol sharply increases; however, seizure activity decreases when progesterone is high relative to estrogen.[93] Experimental studies in animal models confirm these clinical findings.[129-135]

Such observations form the basis for hormone treat-ment of epilepsy. Progesterone is used to treat women with catamenial epilepsy.[122-124'126'131'136-138] Progesterone treatment also reduces limbic seizures in a variety of experimental models;[129'139'140] however, its beneficial effects are observed only at low physiological levels.[141] Progesterone's anti-epileptic mechanisms likely involve GABA_A receptor modulation.[142-145] In contrast' estrogen can increase seizure potential in animals' but it may also provide some protection against neuronal injury from seizure.[146'147] These neuroprotective effects are dependent on a number of variables' including 1) treatment duration' 2) latency before seizure testing' 3) mode of administration' 4) estrogen dose and hormonal status' 5) estrogenic species' 6) the region/neurotransmitter system involved' 7) seizure type/model used' and 8) sex.

Care should be taken' however' when prescribing hormone therapy for women with epilepsy; and the decision to continue or discontinue contraceptives' HRT' or other hormone preparations during the perioperative period should be made with special consideration in these patients. Conjugated equine estrogens' such as Premarin' for example' can be epileptogenic[148'149] and do not contain 17β-estradiol, well-documented for its neuroprotective properties (see Ischemic Brain Injury' this article). Further studies are necessary to gather the evidence required to direct prescription of hormonal preparations that may affect both seizure control and prevention of seizure-induced neuronal damage.

The Male Side of the Story: Androgens and Brain Injury

Less is known about male sex steroids and brain injury. This is due in part to poor agreement about “normal" levels of androgens in men over their lifespan. Testosterone cycles diurnally' declines progressively with age (andropause)' and decreases rapidly in response to stress and illness;[150'151] however' no widely accepted normal range for serum testosterone is established for aging men' and many studies have simply applied cutoff values that are defined for young adult men.[152] Furthermore' the significance of the andropause to men's health is unclear.

Emerging data from laboratory studies suggest that testosterone and its potent metabolite' dihydrotestos-terone' are important factors in the male response to cerebral ischemia and trauma. Male animals sustain larger ischemic damage compared with age-matched females for a comparable insult' suggesting a male “ischemia-sensitive" phenotype. In male rats' androgen replacement in castrates increases histological damage from stroke'[13'153'154] whereas' stressors' such as halothane anesthesia' administered before an episode of cerebral ischemia reduce testosterone levels' resulting in a 50% reduction in brain damage.[155] Furthermore' testosterone replacement after stroke accelerates functional recovery in castrated rats.[156] One interpretation of this interesting paradox is that testosterone has deleterious effects in the case of acute stroke but is beneficial during the recovery phase. Beneficial effects of androgens after peripheral nerve damage or brain trauma have also been reported in animals'[157-161] in part via interaction with glial elements.[156'160'162]

Several small-scale studies of the andropause suggest that loss of testicular and adrenal androgens has a negative impact on cognition and memory163-165 and contributes to the well-recognized loss of muscle function and bone density. More directly relevant to perioperative complications' decreased testosterone levels have been associated with poor outcome after acute ischemic events.[166] Androgen levels are inversely associated with stroke severity' infarct size' and 6-month mortality; and total and free testosterone levels tend to normalize within 6 mo after stroke. These data do not necessarily suggest a direct causal relationship because brain injury provokes an acute stress reaction that causes a reduction in plasma testosterone. However' stress-induced acute reduction of androgens could be relevant to progression of stroke damage' e.gv by decreasing fibrinolytic activity'[167'168] which would delay lysis of a preformed thrombus. Under normal physiological conditions' androgens inhibit arterial thrombosis;[169] however' their role in vascular disease has not been well studied. Potential mechanisms by which androgens could enhance poststroke recovery include normalization of reperfusion' promotion of axonal regeneration' synaptogenesis' and neurogenesis.[170]

Most of these observations come from anecdotal or small-scale clinical studies. Nevertheless' evaluation of the andropause with its gradual loss of male sex steroid production is gaining importance to the new area of men's health. The importance of androgens to anesthetic mechanisms or to perioperative complications remains an uncharted territory. However' new findings could have broad applications to men with brain injury from stroke or reperfusion injury after invasive neurosurgical procedures.

CONCLUSIONS

Biologic sex and sex steroids are important factors in clinical and experimental brain injury and in epilepsy. Estrogen and' to a lesser degree progesterone' have accumulated an impressive reputation as neuroprotectants in physiologically relevant doses in laboratory studies' but there are large gaps between experimental data and the application to women. Data surrounding TBI are more clear. Laboratory data strongly show that progesterone treatment after TBI reduces edema' improves outcomes and restores blood-brain barrier function. Clinical studies agree with these data, and there are continuing human trials for progesterone treatment after TBI. The question of why the male brain is more sensitive to some types of brain injury is an active area of research; however, androgenic effects remain largely evaluated at the bench rather than the bedside.

Despite considerable evidence emphasizing that sex differences, sex steroids and pharmacological hormones can alter outcome from brain injury, the implications for perioperative management are only beginning to be scrutinized. Although anesthetics are often thought to be neuroprotective in their own right, we know relatively little about interactions between anesthetics and sex steroids. In part, this is because preclinical studies are largely conducted in male animals. Further evidence can easily be gathered to direct the use and withdrawal of sex steroids in patients with neurovascular risk factors and seizures. Hopefully, this review will raise awareness among anesthesiologists and in- tensivists to the presence of clinically relevant gender differences in brain injury and to the relevance that sex hormones may have in the perioperative patient.

Acknowledgements

The authors thank Ms. Kathy Gage, Grants and Publications Writer for the Department of Anesthesiology and Perioperative Medicine, OHSU,for her outstanding editorial work in the preparation of this review. They also gratefully acknowledge Research Coordinator Robin Feidelsonfor expert manuscript preparation and creative figure design.

References

Yamori Y, Horie R, Handa H, Sato M, Fukase M. Pathogenetic similarity of strokes in stroke-prone spontaneously hypertensive rats and humans. Stroke 1976;7:46-53 

Du L, Bayir H, Lai Y, Zhang È, Kochanek PM, Watkins SC, Graham SH, Clark RS. Innate gender-based proclivity in response to cytotoxicity and programmed cell death pathway. J Biol Chem 2004;279:38563-70 

Liu M, Hurn PD, Roselli CE, Alkayed NJ. Role of P450 aromatase in sex-specific astrocytic cell death. J Cereb Blood Flow Metab 2007;27:135-41 

Li H, Pin S, Zeng Z, Wang MM, Andreasson KA, McCullough LD. Sex differences in cell death. Ann Neurol 2005;58:317-21 

Hall ED, Pazara KE, Linseman KL. Sex differences in postischemic neuronal necrosis in gerbils. J Cereb Blood Flow Metab 1991;11:292-8 

Alkayed NJ, Harukuni I, Kimes AS, London ED, Traystman RJ, Hurn PD. Gender-linked brain injury in experimental stroke. Stroke 1998;29:159-65 

Alkayed NJ, Murphy SJ, Traystman RJ, Hurn PD, Miller VM. Neuroprotective effects of female gonadal steroids in reproductively senescent female rats. Stroke 2000;31:161-8 

McCullough LD, Blizzard K, Simpson ER, Oz OK, Hurn PD. Aromatase cytochrome P450 and extragonadal estrogen play a role in ischemic neuroprotection. J Neurosci 2003;23:8701-5 

Pelligrino DA, Santizo R, Baughman VL, Wang Q. Cerebral vasodilating capacity during forebrain ischemia: effects of chronic estrogen depletion and repletion and the role of neuronal nitric oxide synthase. Neuroreport 1998;9:3285-91 

Wise PM, Dubal DB. Estradiol protects against ischemic brain injury in middle-aged rats. Biol Reprod 2000;63:982-5 

Horsburgh K, Macrae IM, Carswell H. Estrogen is neuroprotective via an apolipoprotein E-dependent mechanism in a mouse model of global ischemia. J Cereb Blood Flow Metab 2002;22:1189-95 

Toung TJ, Traystman RJ, Hurn PD. Estrogen-mediated neuroprotection after experimental stroke in male rats. Stroke 1998;29:1666-70 

Hawk T, Zhang YQ, Rajakumar G, Day AL, Simpkins JW. Testosterone increases and estradiol decreases middle cerebral artery occlusion lesion size in male rats. Brain Res 1998;796:296-8 

Jover T, Tanaka H, Calderone A, Oguro K, Bennett MV, Etgen AM, Zukin RS. Estrogen protects against global ischemia- induced neuronal death and prevents activation of apoptotic signaling cascades in the hippocampal CA1. J Neurosci 2002;22 and 2115-24 

Li È, Blizzard KK, Zeng Z, DeVries AC, Hurn PD, McCullough LD. Chronic behavioral testing after focal ischemia in the mouse: functional recovery and the effects of gender. Exp Neurol 2004;187:94-104 

Gulinello M, Lebesgue D, Jover-Mengual T, Zukin RS, Etgen AM. Acute and chronic estradiol treatments reduce memory deficits induced by transient global ischemia in female rats. Horm Behav 2006;49:246-60 

Zhang YQ, Shi J, Rajakumar G, Day AL, Simpkins JW. Effects of gender and estradiol treatment on focal brain ischemia. Brain Res 1998;784:321-4 

McCullough LD, Alkayed NJ, Traystman RJ, Williams MJ, Hurn PD. Postischemic estrogen reduces hypoperfusion and secondary ischemia after experimental stroke. Stroke 2001;32:796-802 

Liu R, Wen Y, Perez E, Wang È, Day AL, Simpkins JW, Yang SH. 17beta-Estradiol attenuates blood-brain barrier disruption induced by cerebral ischemia-reperfusion injury in female rats. Brain Res 2005;1060:55-61 

O'Donnell ME, Lam TI, Tran LQ, Foroutan S, Anderson SE. Estradiol reduces activity of the blood-brain barrier Na-K-Cl cotransporter and decreases edema formation in permanent middle cerebral artery occlusion. J Cereb Blood Flow Metab 2006;26:1234-49 

Mendelsohn ME, Karas RH. Estrogen and the blood vessel wall. Curr Opin Cardiol 1994;9:619-26 

Hurn PD, Littleton-Kearney MT, Kirsch JR, Dharmarajan AM, Traystman RJ. Postischemic cerebral blood flow recovery in the female: effect of 17 beta-estradiol. J Cereb Blood Flow Metab 1995;15æ666-72 

Mori M, Tsukahara F, Yoshioka T, Irie K, Ohta H. Suppression by 17beta-estradiol of monocyte adhesion to vascular endothelial cells is mediated by estrogen receptors. Life Sci 2004;75:599-609 

Wen Y, Yang S, Liu R, Perez E, Yi KD, Koulen P, Simpkins JW. Estrogen attenuates nuclear factor-kappa B activation induced by transient cerebral ischemia. Brain Res 2004;1008:147-54 

Alkayed NJ, Goto S, Sugo N, Joh HD, Klaus J, Crain BJ, Bernard O, Traystman RJ, Hurn PD. Estrogen and Bcl-2: gene induction and effect of transgene in experimental stroke. J Neurosci 2001;21:7543-50 

Fujita K, Kato T, Shibayama K, Imada H, Yamauchi M, Yoshimoto N, Miyachi E, Nagata Y. Protective effect against 17beta-estradiol on neuronal apoptosis in hippocampus tissue following transient ischemia/recirculation in mongolian gerbils via down-regulation of tissue transglutaminase activity. Neurochem Res 2006;31:1059-68 

Èu Y, Zhang W, Klaus J, Young J, Koerner I, Sheldahl LC, Hurn PD, Martinez-Murillo F, Alkayed NJ. Role of cocaine- and amphetamine-regulated transcript in estradiol-mediated neuroprotection. Proc Natl Acad Sci USA 2006;103:14489-94 

Keller JN, Germeyer A, Begley JG, Mattson MP. 17Beta-estradiol attenuates oxidative impairment of synaptic Na + /K+~ATPase activity, glucose transport, and glutamate transport induced by amyloid beta-peptide and iron. J Neurosci Res 1997;50:522-30 

Vedder H, Anthes N, Stumm G, Wurz C, Behl C, Krieg JC. Estrogen hormones reduce lipid peroxidation in cells and tissues of the central nervous system. J Neurochem 1999;72:2531-8 

Behl C, Manthey D. Neuroprotective activities of estrogen: an update. J Neurocytol 2000;29:351-8 

Connell BJ, Crosby KM, Richard MJ, Mayne MB, Saleh TM. Estrogen-mediated neuroprotection in the cortex may require NMDA receptor activation. Neuroscience 2007;146:160-9 

Weaver CE Jr, Park-Chung M, Gibbs TT, Farb DH. 17beta- estradiol protects against NMDA-induced excitotoxicity by direct inhibition of NMDA receptors. Brain Res 1997;761:338 -41 

Suzuki S, Gerhold LM, Bottner M, Rau SW, Dela CC, Yang E, Zhu H, Yu J, Cashion AB, Kindy MS, Merchenthaler I, Gage FH, Wise PM. Estradiol enhances neurogenesis following ischemic stroke through estrogen receptors alpha and beta. J Comp Neurol 2007;500:1064-75 

Mor G, Nilsen J, Horvath T, Bechmann I, Brown S, Garcia- Segura LM, Naftolin F. Estrogen and microglia: a regulatory system that affects the brain. J Neurobiol 1999;40:484-96 

Gudino-Cabrera G, Nieto-Sampedro M. Estrogen receptor im- munoreactivity in Schwann-like brain macroglia. J Neurobiol 1999;40:458-70 

Azcoitia I, Sierra A, Garcia-Segura LM. Localization of estrogen receptor beta-immunoreactivity in astrocytes of the adult rat brain. Glia 1999;26:260-7 

Shughrue PJ, Merchenthaler I. Estrogen is more than just a “sex hormone“: novel sites for estrogen action in the hippocampus and cerebral cortex. Front Neuroendocrinol 2000;21:95-101 

Shughrue PJ, Bushnell CD, Dorsa DM. Estrogen receptor messenger ribonucleic acid in female rat brain during the estrous cycle: a comparison with ovariectomized females and intact males. Endocrinology 1992;131:381-8 

Carswell HV, Bingham D, Wallace K, Nilsen M, Graham DI, Dominiczak AF, Macrae IM. Differential effects of 17beta- estradiol upon stroke damage in stroke prone and normoten- sive rats. J Cereb Blood Flow Metab 2004;24:298-304 

Bingham D, Macrae IM, Carswell HV. Detrimental effects of 17beta-oestradiol after permanent middle cerebral artery occlusion. J Cereb Blood Flow Metab 2005;25:414-20 

Vergouwen MD, Anderson RE, Meyer FB. Gender differences and the effects of synthetic exogenous and non-synthetic estrogens in focal cerebral ischemia. Brain Res 2000;878:88-97 

Gordon KB, Macrae IM, Carswell HV. Effects of 17beta- oestradiol on cerebral ischaemic damage and lipid peroxidation. Brain Res 2005;1036:155-62 

Yong Y, Xie HJ, Zhang YF, Yang QD, Liao DF, Yang HL, Yan PK, Liu ZJ. 17beta-estradiol potentiates ischemia-reperfusion injury in diabetic ovariectomized female rats. Brain Res 2005;1054:192-9 

Xu HL, Baughman VL, Pelligrino DA. Estrogen replacement treatment in diabetic ovariectomized female rats potentiates postischemic leukocyte adhesion in cerebral venules. Stroke 2004;35:1974-8 

Santizo RA, Xu HL, Galea E, Muyskens S, Baughman VL, Pelligrino DA. Combined endothelial nitric oxide synthase upregulation and caveolin-1 downregulation decrease leukocyte adhesion in pial venules of ovariectomized female rats. Stroke 2002;33:613-6 

Suzuki S, Brown CM, la Cruz CD, Yang E, Bridwell DA, Wise PM. Timing of estrogen therapy after ovariectomy dictates the efficacy of its neuroprotective and antiinflammatory actions. Proc Natl Acad Sci USA 2007;104:6013-8 

Sohrabji F, Bake S. Age-related changes in neuroprotection: is estrogen pro-inflammatory for the reproductive senescent brain? Endocrine 2006;29:191-7 

Kumon Y, Kim SC, Tompkins P, Stevens A, Sakaki S, Loftus CM. Neuroprotective effect of postischemic administration of progesterone in spontaneously hypertensive rats with focal cerebral ischemia. J Neurosurg 2000;92:848-52 

Jiang N, Chopp M, Stein D, Feit H. Progesterone is neuroprotective after transient middle cerebral artery occlusion in male rats. Brain Res 1996;735:101-7 

Chen J, Chopp M, Li Y. Neuroprotective effects of progesterone after transient middle cerebral artery occlusion in rat. J Neurol Sci 1999;171:24-30 

Betz AL, Coester HC. Effect of steroids on edema and sodium uptake of the brain during focal ischemia in rats. Stroke 1990;21:1199-204 

Sayeed I, Guo Q, Hoffman SW, Stein DG. Allopregnanolone, a progesterone metabolite, is more effective than progesterone in reducing cortical infarct volume after transient middle cerebral artery occlusion. Ann Emerg Med 2006;47:381-9 

Morali G, Letechipia-Vallejo G, Lopez-Loeza E, Montes P, Hernandez-Morales L, Cervantes M. Post-ischemic administration of progesterone in rats exerts neuroprotective effects on the hippocampus. Neurosci Lett 2005;382:286-90 

Gonzalez-Vidal MD, Cervera-Gaviria M, Ruelas R, Escobar A, Morali G, Cervantes M. Progesterone: protective effects on the cat hippocampal neuronal damage due to acute global cerebral ischemia. Arch Med Res 1998;29:117-24 

Cervantes M, Gonzalez-Vidal MD, Ruelas R, Escobar A, Morali G. Neuroprotective effects of progesterone on damage elicited by acute global cerebral ischemia in neurons of the caudate nucleus. Arch Med Res 2002;33:6-14 

Gibson CL, Murphy SP. Progesterone enhances functional recovery after middle cerebral artery occlusion in male mice. J Cereb Blood Flow Metab 2004;24:805-13 

Murphy SJ, Traystman RJ, Hurn PD, Duckles SP. Progesterone exacerbates striatal stroke injury in progesterone-deficient female animals. Stroke 2000;31:1173-8 

Gibson CL, Constantin D, Prior MJ, Bath PM, Murphy SP. Progesterone suppresses the inflammatory response and nitric oxide synthase-2 expression following cerebral ischemia. Exp Neurol 2005;193:522-30 

He J, Hoffman SW, Stein DG. Allopregnanolone, a progesterone metabolite, enhances behavioral recovery and decreases neuronal loss after traumatic brain injury. Restor Neurol Neurosci 2004;22:19-31 

Grossman KJ, Goss CW, Stein DG. Effects of progesterone on the inflammatory response to brain injury in the rat. Brain Res 2004;1008:29-39 

Rupprecht R, Reul JM, Trapp T, van Steensel B, Wetzel C, Damm K, Zieglgansberger W, Holsboer F. Progesterone receptor-mediated effects of neuroactive steroids. Neuron 1993;11:523-30 

Monnet FP, Mahe V, Robel P, Baulieu E-E. Neurosteroids, via sigma receptors, modulate the [3H]norepinephrine release evoked by N-methyl-D-aspartate in the rat hippocampus. Proc Natl Acad Sci USA 1995;92:3774-8 

Ardeshiri A, Kelley MH, Korner IP, Hurn PD, Herson PS. Mechanism of progesterone neuroprotection of rat cerebellar Purkinje cells following oxygen-glucose deprivation. Eur J Neurosci 2006;24:2567-74 

Rossouw JE, Anderson GL, Prentice RL, LaCroix AZ, Kooperberg C, Stefanick ML, Jackson RD, Beresford SA, Howard BV, Johnson KC, Kotchen JM, Ockene J. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the women's health initiative randomized controlled trial. JAMA 2002;288:321-33 

Hendrix SL, Wassertheil-Smoller S, Johnson KC, Howard BV, Kooperberg C, Rossouw JE, Trevisan M, Aragaki A, Baird AE, Bray PF, Buring JE, Criqui MH, Herrington D, Lynch JK, Rapp SR, Torner J. Effects of conjugated equine estrogen on stroke in the women's health initiative. Circulation 2006;113:2425-34 

Dykens JA, Moos WH, Howell N. Development of 17alpha- estradiol as a neuroprotective therapeutic agent: rationale and results from a phase I clinical study. Ann NY Acad Sci 2005;1052:116-35 

Koerner IP, Brambrink AM. Brain protection by anesthetic agents. Curr Opin Anaesthesiol 2006;19:481-6 

Kitano H, Young JM, Cheng J, Wang L, Hurn PD, Murphy SJ. Gender-specific response to isoflurane preconditioning in focal cerebral ischemia. J Cereb Blood Flow Metab 2007;27:1377-86 

Gray JJ, Bickler PE, Fahlman CS, Zhan X, Schuyler JA. Isoflurane neuroprotection in hypoxic hippocampal slice cultures involves increases in intracellular Ca^+ and mitogen-activated protein kinases. Anesthesiology 2005;102:606-15 

Ma D, Hossain M, Pettet GK, Luo Y, Lim T, Akimov S, Sanders RD, Franks NP, Maze M. Xenon preconditioning reduces brain damage from neonatal asphyxia in rats. J Cereb Blood Flow Metab 2006;26:199-208 

Liao SL, Chen WY, Kuo JS, Chen CJ. Association of serum estrogen level and ischemic neuroprotection in female rats. Neurosci Lett 2001;297:159-62 

Shiffman MA. Estrogen and thromboembolic disorders: should patients stop hormones prior to cosmetic surgery? J Wom Health (Larchmt) 2003;12:853-5 

Schouten JW. Neuroprotection in traumatic brain injury: a complex struggle against the biology of nature. Curr Opin Crit Care 2007;13:134-42 

Mushkudiani NA, Englel DC, Steyerberg EW, Butcher I, Lu J, Marmarou A, Slieker F, McHugh GS, Murray GD, Maas AI. Prognostic value of demographic characteristics in traumatic brain injury: results from the IMPACT study. J Neurotrauma 2007;24:259-69 

Kannus P, Palvanen M, Niemi S. Time trends in severe head injuries among elderly Finns. JAMA 2001;286:673-4 

Luukinen H, Viramo P, Koski K, Laippala P, Kivela SL. Head injuries and cognitive decline among older adults: a population- based study. Neurology 1999;52:557-62 

Farace E, Alves WM. Do women fare worse: a metaanalysis of gender differences in traumatic brain injury outcome. J Neurosurg 2000;93:539-45  

Wagner AK, Sasser HC, Hammond FM, Wiercisiewski D, Alexander J. Intentional traumatic brain injury: epidemiology, risk factors, and associations with injury severity and mortality. J Trauma 2000;49:404-10 

Kirkness CJ, Burr RL, Mitchell PH, Newell DW. Is there a sex difference in the course following traumatic brain injury? Biol Res Nurs 2004;5:299-310 

Bayir H, Marion DW, Puccio AM, Wisniewski SR, Janesko KL, Clark RS, Kochanek PM. Marked gender effect on lipid peroxidation after severe traumatic brain injury in adult patients. J Neurotrauma 2004;21:1-8 

Gan BK, Lim JH, Ng IH. Outcome of moderate and severe traumatic brain injury amongst the elderly in Singapore. Ann Acad Med Singapore 2004;33:63-7 

Groswasser Z, Cohen M, Keren O. Female TBI patients recover better than males. Brain Inj 1998;12:805-8 

Kraus JF, Peek-Asa C, McArthur D. The independent effect of gender on outcomes following traumatic brain injury: a preliminary investigation. Neurosurg Focus 2000;8:e5 

Van den BG. Novel insights into the neuroendocrinology of critical illness. Eur J Endocrinol 2000;143:1-13 

Feldman HA, Longcope C, Derby CA, Johannes CB, Araujo AB, Coviello AD, Bremner WJ, McKinlay JB. Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts male aging study. J Clin Endocrinol Metab 2002;87: 589-98 

Schneider HJ, Schneider M, Saller B, Petersenn S, Uhr M, Husemann B, von RF, Stalla GK. Prevalence of anterior pituitary insufficiency 3 and 12 months after traumatic brain injury. Eur J Endocrinol 2006;154:259-65 

Woolf PD, Hamill RW, McDonald JV, Lee LA, Kelly M. Transient hypogonadotrophic hypogonadism after head trauma: effects on steroid precursors and correlation with sympathetic nervous system activity. Clin Endocrinol 1986;25: 265-74 

Cernak I, Savic VJ, Lazarov A, Joksimovic M, Markovic S. Neuroendocrine responses following graded traumatic brain injury in male adults. Brain Inj 1999;13:1005-15 

Agha A, Rogers B, Mylotte D, Taleb F, Tormey W, Phillips J, Thompson CJ. Neuroendocrine dysfunction in the acute phase of traumatic brain injury. Clin Endocrinol (Oxf) 2004;60:584-91 

Dimopoulou I, Tsagarakis S, Theodorakopoulou M, Douka E, Zervou M, Kouyialis AT, Thalassinos N, Roussos C. Endocrine abnormalities in critical care patients with moderate-to-severe head trauma: incidence, pattern and predisposing factors. Intensive Care Med 2004;30:1051-7 

Roof RL, Duvdevani R, Stein DG. Gender influences outcome of brain injury: progesterone plays a protective role. Brain Res 1993;607:333-6 

Bramlett HM, Dietrich WD. Neuropathological protection after traumatic brain injury in intact female rats versus males or ovariectomized females. J Neurotrauma 2001;18:891-900 

Hoffman GE, Merchenthaler I, Zup SL. Neuroprotection by ovarian hormones in animal models of neurological disease. Endocrine 2006;29:217-31 

Wright DW, Kellermann AL, Hertzberg VS, Clark PL, Frankel M, Goldstein FC, Salomone JP, Dent LL, Harris OA, Ander DS, Lowery DW, Patel MM, Denson DD, Gordon AB, Wald MM, Gupta S, Hoffman SW, Stein DG. ProTECT: a randomized clinical trial of progesterone for acute traumatic brain injury. Ann Emerg Med 2007;49:391-402 

Goss CW, Hoffman SW, Stein DG. Behavioral effects and anatomic correlates after brain injury: a progesterone dose- response study. Pharmacol Biochem Behav 2003;76:231-42 

Thau RB, Lanman JT. Metabolic clearance rates (MCR) and production rates (PR) plasma progesterone in pregnant and pseudopregnant rabbits. Endocrinology 1975;97:454-7 

Robinson J, Merry BJ, Lightfoot ME, Hall AK. Dynamics of progesterone metabolism in the pseudopregnant rat. J Endocrinol 1981;90:359-66 

Gangrade NK, Boudinot FD, Price JC. Pharmacokinetics of progesterone in ovariectomized rats after single dose intravenous administration. Biopharm Drug Dispos 1992;13: 703-9 

Fang SM, Lin CS, Lyon V. Progesterone retention by rat uterus I. Pharmacokinetics after uterine intraluminal instillation. J Pharm Sci 1977;66:1744-8 

Lyles R, Elkind-Hirsch K, Goldzieher JW, Besch PK. Plasma gonadotropin-releasing hormone profiles after intravenous and subcutaneous bolus injection in thin and obese women. Obstet Gynecol 1988;71:44-8 

Oberye J, Mannaerts B, Huisman J, Timmer C. Local tolerance, pharmacokinetics, and dynamics of ganirelix (Orgalutran) administration by Medi-Jector compared to conventional needle injections. Hum Reprod 2000;15:245-9 

Cutler SM, Pettus EH, Hoffman SW, Stein DG. Tapered progesterone withdrawal enhances behavioral and molecular recovery after traumatic brain injury. Exp Neurol 2005;195:423-9 

Biggio G, Follesa P, Sanna E, Purdy RH, Concas A. GABA_A- receptor plasticity during long-term exposure to and withdrawal from progesterone. Int Rev Neurobiol 2001;46:207-41 

Lambert A, Rodgers M, Mitchell R, Wood AM, Wardle C, Hilton B, Robertson WR. In-vitro biopotency and glycoform distribution of recombinant human follicle stimulating hormone (Org 32489), Metrodin and Metrodin-HP. Hum Reprod 1995;10:1928-35 

Barbaccia ML, Roscetti G, Trabucchi M, Purdy RH, Mostallino MC, Concas A, Biggio G. The effects of inhibitors of GABAer- gic transmission and stress on brain and plasma allopreg- nanolone concentrations. Br J Pharmacol 1997;120:1582-8 

Concas A, Follesa P, Barbaccia ML, Purdy RH, Biggio G. Physiological modulation of GABA(A) receptor plasticity by progesterone metabolites. Eur J Pharmacol 1999;375:225-35 

Barbaccia ML, Colombo G, Affricano D, Carai MA, Vacca G, Melis S, Purdy RH, Gessa GL. GABA(B) receptor-mediated increase of neurosteroids by gamma-hydroxybutyric acid. Neuropharmacology 2002;42:782-91 

Gulinello M, Gong QH, Smith SS. Progesterone withdrawal increases the anxiolytic actions of gaboxadol: role of alpha4betadelta GABA(A) receptors. Neuroreport 2003;14:43-6 

Roof RL, Havens MD. Testosterone improves maze performance and induces development of a male hippocampus in females. Brain Res 1992;572:310-3 

Ghoumari AM, Ibanez C, El-Etr M, Leclerc P, Eychenne B, O'Malley 

BW,Baulieu EE, Schumacher M. Progesterone and its metabolites increase myelin basic protein expression in organotypic slice cultures of rat cerebellum. J Neurochem 2003;86:848-59 

Ibanez C, Shields SA, El-Etr M, Leonelli E, Magnaghi V, Li WW, Sim FJ, Baulieu EE, Melcangi RC, Schumacher M, Franklin RJ. Steroids and the reversal of age-associated changes in myeli- nation and remyelination. Prog Neurobiol 2003;71:49-56 

Schumacher M, Weill-Englerer S, Liere P, Robert F, Franklin RJ, Garcia-Segura LM, Lambert JJ, Mayo W, Melcangi RC, Parducz A, Suter U, Carelli C, Baulieu EE, Akwa Y. Steroid hormones and neurosteroids in normal and pathological aging of the nervous system. Prog Neurobiol 2003;71:3-29 

Lancel M, Faulhaber J, Holsboer F, Rupprecht R. Progesterone induces changes in sleep comparable to those of agonistic GABA_A receptor modulators. Am J Physiol 1996;271:E763- E772 

Allolio B, Oremus M, Reincke M, Schaeffer HJ, Winkelmann W, Heck G, Schulte HM. High-dose progesterone infusion in healthy males: evidence against antiglucocorticoid activity of progesterone1. Eur J Endocrinol 1995;133:696-700 

Farin A, Deutsch R, Biegon A, Marshall LF. Sex-related differences in patients with severe head injury: greater susceptibility to brain swelling in female patients 50 years of age and younger. J Neurosurg 2003;98:32-6 

Wright DW, Ritchie JC, Mullins RE, Kellermann AL, Denson DD. Steady-state serum concentrations of progesterone following continuous intravenous infusion in patients with acute moderate to severe traumatic brain injury. J Clin Pharmacol 2005;45:640-8 

Roof RL, Duvdevani R, Heyburn JW, Stein DG. Progesterone rapidly decreases brain edema: treatment delayed up to 24 hours is still effective. Exp Neurol 1996;138:246-51 

Hauser WA, Annegers JF, Kurland LT. Incidence of epilepsy and unprovoked seizures in Rochester, Minnesota: 1935-1984. Epilepsia 1993;34:453-68 

Kotsopoulos IA, van MT, Kessels FG, de Krom MC, Knottnerus JA. Systematic review and meta-analysis of incidence studies of epilepsy and unprovoked seizures. Epilepsia 2002;43:1402-9 

Tsuboi T, Christian W. Epilepsy, a clinical, electroencepha- lographic, and statistical study of 466 patients. Berlin: Springer-Verlag Neurology Series, 1976. Schriftenr Neurol 1976;17:1-168 

Kleveland G, Englelsen BA. Juvenile myoclonic epilepsy: clinical characteristics, treatment and prognosis in a Norwegian population of patients. Seizure 1998;7:31-8 

Herzog AG, Klein P, Ransil BJ. Three patterns of catamenial epilepsy. Epilepsia 1997;38:1082-8 

Morrell MJ. Epilepsy in women: the science of why it is special. Neurology 1999;53:S42-S48 

Murri L, Galli R. Catamenial epilepsy, progesterone and its metabolites. Cephalalgia 1997;17:46-7 

Herkes GK, Eadie MJ, Sharbrough F, Moyer T. Patterns of seizure occurrence in catamenial epilepsy. Epilepsy Res 1993;15:47-52 

Morrell MJ. Hormones and epilepsy through the lifetime. Epilepsia 1992;33 (suppl 4):S49-S61 

Schachter SC. Hormonal considerations in women with seizures. Arch Neurol 1988;45:1267-70 

Christensen J, Kjeldsen MJ, Andersen H, Friis ML, Sidenius P. Gender differences in epilepsy. Epilepsia 2005;46:956-60 

Edwards HE, Burnham WM, Mendonca A, Bowlby DA, MacLusky NJ. Steroid hormones affect limbic after discharge thresholds and kindling rates in adult female rats. Brain Res 1999;838:136-50 

Beyenburg S, Stoffel-Wagner B, Bauer J, Watzka M, Blumcke I, Bidlingmaier F, Elger CE. Neuroactive steroids and seizure susceptibility. Epilepsy Res 2001;44:141-53 

Bonuccelli U, Melis GB, Paoletti AM, Fioretti P, Murri L, Muratorio A. Unbalanced progesterone and estradiol secretion in catamenial epilepsy. Epilepsy Res 1989;3:100-6 

Buterbaugh GG. Estradiol replacement facilitates the acquisition of seizures kindled from the anterior neocortex in female rats. Epilepsy Res 1989;4:207-15 

Buterbaugh GG, Hudson GM. Estradiol replacement to female rats facilitates dorsal hippocampal but not ventral hippocampal kindled seizure acquisition. Exp Neurol 1991;111:55-64 

Hom AC, Buterbaugh GG. Estrogen alters the acquisition of seizures kindled by repeated amygdala stimulation or pentylenetetrazol administration in ovariectomized female rats. Epilepsia 1986;27:103-8 

Hom AC, Leppik IE, Rask CA. Effects of estradiol and progesterone on seizure sensitivity in oophorectomized DBA/2J mice and C57/EL hybrid mice. Neurology 1993;43:198-204 

Bauer J. Interactions between hormones and epilepsy in female patients. Epilepsia 2001;42 (suppl 3):20-22 

Holmes GL, Weber DA. The effect of progesterone on kindling: a developmental study. Brain Res 1984;318:45-53 

Herzog AG. Progesterone therapy in women with complex partial and secondary generalized seizures. Neurology 1995;45: 1660-2 

Frye CA, Murphy RE, Platek SM. Anti-sense oligonucleotides, for progestin receptors in the VMH and glutamic acid decarboxylase in the VTA, attenuate progesterone-induced lordosis in hamsters and rats. Behav Brain Res 2000;115:55-64 

Tauboll E, Lindstrom S. The effect of progesterone and its metabolite 5 alpha-pregnan-3 alpha-ol-20-one on focal epileptic seizures in the cat's visual cortex in vivo. Epilepsy Res 1993;14:17-30 

Hoffman GE, Moore N, Fiskum G, Murphy AZ. Ovarian steroid modulation of seizure severity and hippocampal cell death after kainic acid treatment. Exp Neurol 2003;182: 124-34 

Barbaccia ML, Roscetti G, Trabucchi M, Mostallino MC, Concas A, Purdy RH, Biggio G. Time-dependent changes in rat brain neuroactive steroid concentrations and GABA_A receptor function after acute stress. Neuroendocrinology 1996;63:166-72 

Baulieu EE, Schumacher M, Koenig H, Jung-Testas I, Akwa Y. Progesterone as a neurosteroid: actions within the nervous system. Cell Mol Neurobiol 1996;16:143-54 

Gulinello M, Gong QH, Li X, Smith SS. Short-term exposure to a neuroactive steroid increases alpha4 GABA(A) receptor subunit levels in association with increased anxiety in the female rat. Brain Res 2001;910:55-66 

Wohlfarth KM, Bianchi MT, Macdonald RL. Enhanced neurosteroid potentiation of ternary GABA(A) receptors containing the delta subunit. J Neurosci 2002;22:1541-9 

Velisek L, Veliskova J, Ravizza T, Giorgi FS, Moshe SF. Circling behavior and [14C]2-deoxyglucose mapping in rats: possible implications for autistic repetitive behaviors. Neurobiol Dis 2005;18:346-55 

14é. Azcoitia I, Sierra A, Garcia-Segura LM. Estradiol prevents kainic acid-induced neuronal loss in the rat dentate gyrus. Neuroreport 1998;9:3075-9 

Marcus EM, Watson CW, Goldman PL. Effects of steroids on cerebral electrical activity. Epileptogenic effects of conjugated estrogens and related compounds in the cat and rabbit. Arch Neurol 1966;15:521-32 

Logothetis J, Harner R, Morrell F, Torres F. The role of estrogens in catamenial exacerbation of epilepsy. Neurology 1959;9:352-60 

Andersson AM, Carlsen E, Petersen JH, Skakkebaek NE. Variation in levels of serum inhibin B, testosterone, estradiol, luteinizing hormone, follicle-stimulating hormone, and sex hormone-binding globulin in monthly samples from healthy men during a 17-month period: possible effects of seasons. J Clin Endocrinol Metab 2003;88:932-7 

Woolf PD, Hamill RW, McDonald JV, Lee LA, Kelly M. Transient hypogonadotropic hypogonadism caused by critical illness. J Clin Endocrinol Metab 1985;60:444-50 

Makinen J, Jarvisalo MJ, Pollanen P, Perheentupa A, Irjala K, Koskenvuo M, Makinen J, Huhtaniemi I, Raitakari OT. Increased carotid atherosclerosis in andropausal middle-aged men. J Am Coll Cardiol 2005;45:1603-8 

Yang SH, Perez E, Cutright J, Liu R, He Z, Day AL, Simpkins JW. Testosterone increases neurotoxicity of glutamate in vitro and ischemia-reperfusion injury in an animal model. J Appl Physiol 2002;92:195-201 

Cheng J, Alkayed NJ, Hurn PD. Deleterious effects of dihy-drotestosterone on cerebral ischemic injury. J Cereb Blood Flow Metab 2007;27:1553-62 

Yang SH, Liu R, Wen Y, Perez E, Cutright J, Brun-Zinkernagel AM, Singh M, Day AL, Simpkins JW. Neuroendocrine mechanism for tolerance to cerebral ischemia-reperfusion injury in male rats. J Neurobiol 2005;62:341-51 

Pan Y, Zhang H, Acharya AB, Patrick PH, Oliver D, Morley JE. Effect of testosterone on functional recovery in a castrate male rat stroke model. Brain Res 2005;1043:195-204 

Jones KJ. Gonadal steroids as promoting factors in axonal regeneration. Brain Res Bull 1993;30:491-8 

Kujawa KA, Jacob JM, Jones KJ. Testosterone regulation of the regenerative properties of injured rat sciatic motor neurons. J Neurosci Res 1993;35:268-73 

Tanzer L, Jones KJ. Gonadal steroid regulation of hamster facial nerve regeneration: effects of dihydrotestosterone and estradiol. Exp Neurol 1997;146:258-64 

Garcia-Estrada J, Del Rio JA, Luquin S, Soriano E, Garcia- Segura LM. Gonadal hormones down-regulate reactive gliosis and astrocyte proliferation after a penetrating brain injury. Brain Res 1993;628:271-8 

Jones KJ, Storer PD, Drengler SM, Oblinger MM. Differential regulation of cytoskeletal gene expression in hamster facial motoneurons: effects of axotomy and testosterone treatment. J Neurosci Res 1999;57:817-23 

Silver J, Miller JH. Regeneration beyond the glial scar. Nat Rev Neurosci 2004;5:146-56 

Barrett-Connor E, Goodman-Gruen D. Cognitive function and endogenous sex hormones in older women. J Am Geriatr Soc 1999;47:1289-93 

Tan RS, Pu SJ, Culberson JW. Role of androgens in mild cognitive impairment and possible interventions during an- dropause. Med Hypotheses 2003;60:448-52 

Janowsky JS, Chavez B, Orwoll E. Sex steroids modify working memory. J Cogn Neurosci 2000;12:407-14 

Jeppesen LL, Jorgensen HS, Nakayama H, Raaschou HO, Olsen TS, Winther K. Decreased serum testosterone in men with acute ischemic stroke. Arterioscler Thromb Vasc Biol 1996;16:749-54 

Caron P, Bennet A, Camare R, Louvet JP, Boneu B, Sie P. Plasminogen activator inhibitor in plasma is related to testosterone in men. Metabolism 1989;38:1010-5 

Marques-Vidal P, Sie P, Cambou JP, Chap H, Perret B. Relationships of plasminogen activator inhibitor activity and lipoprotein (a) with insulin, testosterone, 17 beta-estradiol, and testosterone binding globulin in myocardial infarction patients and healthy controls. J Clin Endocrinol Metab 1995;80:1794-8  

Li S, Li X, Li J, Deng X, Li Y, Cong Y. Experimental arterial thrombosis regulated by androgen and its receptor via modulation of platelet activation. Thromb Res 2007;121:127-34 

Chen R, Cohen LG, Hallett M. Nervous system reorganization following injury. Neuroscience 2002;111:761-73 

Liu X, Fan XL, Zhao Y, Luo GR, Li XP, Li R, Le WD. Estrogen provides neuroprotection against activated microglia-induced dopaminergic neuronal injury through both estrogen receptor- alpha and estrogen receptor-beta in microglia. J Neurosci Res 2005;81:653-65 

Regan RF, Guo Y. Estrogens attenuate neuronal injury due to hemoglobin, chemical hypoxia, and excitatory amino acids in murine cortical cultures. Brain Res 1997;764:133-40 

Singer CA, Figueroa-Masot XA, Batchelor RH, Dorsa DM. The mitogen-activated protein kinase pathway mediates estrogen neuroprotection after glutamate toxicity in primary cortical neurons. J Neurosci 1999;19:2455-63 

Dubal DB, Shughrue PJ, Wilson ME, Merchenthaler I, Wise PM. Estradiol modulates bcl-2 in cerebral ischemia: a potential role for estrogen receptors. J Neurosci 1999;19:6385-93 

Brinton RD, Tran J, Proffitt P, Montoya M. 17 beta-Estradiol enhances the outgrowth and survival of neocortical neurons in culture. Neurochem Res 1997;22:1339-51 

Murphy DD, Cole NB, Greenberger V, Segal M. Estradiol increases dendritic spine density by reducing GABA neurotransmission in hippocampal neurons. J Neurosci 1998;18: 2550-9 

Toran-Allerand CD. Mechanisms of estrogen action during neural development: mediation by interactions with the neu- rotrophins and their receptors? J Steroid Biochem Mol Biol 1996;56:169-78 

Santizo RA, Anderson S, Ye S, Koenig HM, Pelligrino DA. Effects of estrogen on leukocyte adhesion after transient forebrain ischemia. Stroke 2000;31:2231-5 

Bruce-Keller AJ, Keeling JL, Keller JN, Huang FF, Camondola S, Mattson MP. Antiinflammatory effects of estrogen on microglial activation. Endocrinology 2000;141:3646-56 

Garcia-Ovejero D, Veiga S, Garcia-Segura LM, DonCarlos LL. Glial expression of estrogen and androgen receptors after rat brain injury. J Comp Neurol 2002;450:256-71 

Tanapat P, Hastings NB, Reeves AJ, Gould E. Estrogen stimulates a transient increase in the number of new neurons in the dentate gyrus of the adult female rat. J Neurosci 1999;19: 5792-801 

Roof RL, Duvdevani R, Braswell L, Stein DG. Progesterone facilitates cognitive recovery and reduces secondary neuronal loss caused by cortical contusion injury in male rats. Exp Neurol 1994;129:64-9 

Wagner AK, Willard LA, Kline AE, Wenger MK, Bolinger BD, Ren D, Zafonte RD, Dixon CE. Evaluation of estrous cycle stage and gender on behavioral outcome after experimental traumatic brain injury. Brain Res 2004;998:113-21 

Roof RL, Hoffman SW, Stein DG. Progesterone protects against lipid peroxidation following traumatic brain injury in rats. Mol Chem Neuropathol 1997;31:1-11 

Goodman Y, Bruce AJ, Cheng B, Mattson MP. Estrogens attenuate and corticosterone exacerbates excitotoxicity, oxidative injury, and amyloid beta-peptide toxicity in hippocampal neurons. J Neurochem 1996;66:1836-44 

Subramanian M, Pusphendran CK, Tarachand U, Devasagayam TP. Gestation confers temporary resistance to peroxidation in the maternal rat brain. Neurosci Lett 1993;155:151-4 

Djebaili M, Hoffman SW, Stein DG. Allopregnanolone and progesterone decrease cell death and cognitive deficits after a contusion of the rat pre-frontal cortex. Neuroscience 2004;123:349-59 

Pettus EH, Wright DW, Stein DG, Hoffman SW. Progesterone treatment inhibits the inflammatory agents that accompany traumatic brain injury. Brain Res 2005;1049:112-9 

Suzuki T, Bramlett HM, Ruenes G, Dietrich WD. The effects of early post-traumatic hyperthermia in female and ovariecto- mized rats. J Neurotrauma 2004;21:842-53 

Jones NC, Constantin D, Prior MJ, Morris PG, Marsden CA, Murphy S. The neuroprotective effect of progesterone after traumatic brain injury in male mice is independent of both the inflammatory response and growth factor expression. Eur J Neurosci 2005;21:1547-54  

Kelly MJ, Qiu J, Ronnekleiv OK. Estrogen modulation of G-protein-coupled receptor activation of potassium channels in the central nervous system. Ann NY Acad Sci 2003;1007: 6-16 

Toran-Allerand CD, Guan X, MacLusky NJ, Horvath TL, Diano S, Singh M, Connolly ES Jr, Nethrapalli IS, Tinnikov AA. ER-X: a novel, plasma membrane-associated, putative estrogen receptor that is regulated during development and after ischemic brain injury. J Neurosci 2002;22:8391-401 

Toran-Allerand CD. Minireview: a plethora of estrogen receptors in the brain: where will it end? Endocrinology 2004;145:1069-74 

Mermelstein PG, Becker JB, Surmeier DJ. Estradiol reduces calcium currents in rat neostriatal neurons via a membrane receptor. J Neurosci 1996;16:595-604 

Minami T, Oomura Y, Nabekura J, Fukuda A. 17 beta-estradiol depolarization of hypothalamic neurons is mediated by cyclic AMP. Brain Res 1990;519:301-7 

Beyer C, Raab H. Nongenomic effects of oestrogen: embryonic mouse midbrain neurones respond with a rapid release of calcium from intracellular stores. Eur J Neurosci 1998;10: 255-62 

Wu TW, Wang JM, Chen S, Brinton RD. 17beta-estradiol induced Ca(2+) influx via L-type calcium channels activates the Src/ERK/cyclic-AMP response element binding protein signal pathway and BCL-2 expression in rat hippocampal neurons: a potential initiation mechanism for estrogen-induced neuroprotection. Neuroscience 2005;135:59-72 

Hayashi S, Ueyama T, Kajimoto T, Yagi K, Kohmura E, Saito N. Involvement of gamma protein kinase C in estrogen-induced neuroprotection against focal brain ischemia through G protein-coupled estrogen receptor. J Neurochem 2005;93: 883-91 

Cordey M, Pike CJ. Neuroprotective properties of selective estrogen receptor agonists in cultured neurons. Brain Res 2005;1045:217-23 

Ansonoff MA, Etgen AM. Estradiol elevates protein kinase C catalytic activity in the preoptic area of female rats. Endocrinology 1998;139:3050-6 

Singh M, Setalo G Jr, Guan X, Warren M, Toran-Allerand CD. Estrogen-induced activation of mitogen-activated protein kinase in cerebral cortical explants: convergence of estrogen and neurotrophin signaling pathways. J Neurosci 1999;19:1179-88 

Won CK, Ji HH, Koh PO. Estradiol prevents the focal cerebral ischemic injury-induced decrease of forkhead transcription factors phosphorylation. Neurosci Lett 2006;398:39-43 

Abraham IM, Herbison AE. Major sex differences in nongenomic estrogen actions on intracellular signaling in mouse brain in vivo. Neuroscience 2005;131:945-51 

Zhou Y, Watters JJ, Dorsa DM. Estrogen rapidly induces the phosphorylation of the cAMP response element binding protein in rat brain. Endocrinology 1996;137:2163-6 

Watters JJ, Dorsa DM. Transcriptional effects of estrogen on neuronal neurotensin gene expression involve cAMP/protein kinase A-dependent signaling mechanisms. J Neurosci 1998;18: 6672-80 

Lagrange AH, Wagner EJ, Ronnekleiv OK, Kelly MJ. Estrogen rapidly attenuates a GABAB response in hypothalamic neurons. Neuroendocrinology 1996;64:114-23 

Disshon KA, Boja JW, Dluzen DE. Inhibition of striatal dopamine transporter activity by 17beta-estradiol. Eur J Pharmacol 1998;345:207-11 

Sohrabji F, Miranda RC, Toran-Allerand CD. Identification of a putative estrogen response element in the gene encoding brain-derived neurotrophic factor. Proc Natl Acad Sci USA 1995;92:11110-4 

El Ashry D, Chrysogelos SA, Lippman ME, Kern FG. Estrogen induction of TGF-alpha is mediated by an estrogen response element composed of two imperfect palindromes. J Steroid Biochem Mol Biol 1996;59:261-9 

Xu Y, Berelowitz M, Bruno JF. Characterization of the promoter region of the human somatostatin receptor subtype 2 gene and localization of sequences required for estrogen- responsiveness. Mol Cell Endocrinol 1998;139:71-7 

Teixeira C, Reed JC, Pratt MA. Estrogen promotes chemotherapeutic drug resistance by a mechanism involving Bcl-2 protooncogene expression in human breast cancer cells. Cancer Res 1995;55:3902-7  

Lee K, Richardson CD, Razik MA, Kwatra MM, Schwinn DA. Multiple potential regulatory elements in the 5' flanking region of the human alpha la-adrenergic receptor. DNA Seq 1998;8:271-6 

Miller MM, Hyder SM, Assayag R, Panarella SR, Tousignant P, Franklin KB. Estrogen modulates spontaneous alternation and the cholinergic phenotype in the basal forebrain. Neuroscience 1999;91:1143-53 

Adan RA, Cox JJ, Beischlag TV, Burbach JP. A composite hormone response element mediates the transactivation of the rat oxytocin gene by different classes of nuclear hormone receptors. Mol Endocrinol 1993;7:47-57 

Bale TL, Dorsa DM. Cloning, novel promoter sequence, and estrogen regulation of a rat oxytocin receptor gene. Endocrinology 1997;138:1151-8 

Zhu YS, Pfaff DW. DNA binding of hypothalamic nuclear proteins on estrogen response element and preproenkephalin promoter: modification by estrogen. Neuroendocrinology 1995;62:454-66 

Kofler B, Evans HF, Liu ML, Falls V, Iismaa TP, Shine J, Herzog H. Characterization of the 5'-flanking region of the human preprogalanin gene. DNA Cell Biol 1995;14:321-9 

Webb P, Nguyen P, Valentine C, Lopez GN, Kwok GR, Mclnerney E, Katzenellenbogen BS, Enmark E, Gustafsson JA, Nilsson S, Kushner PJ. The estrogen receptor enhances AP-1 activity by two distinct mechanisms with different requirements for receptor transactivation functions. Mol Endocrinol 1999;13:1672-85 

Kennedy AM, Shogren KL, Zhang M, Turner RT, Spelsberg TC, Maran A. 17beta-estradiol-dependent activation of signal transducer and activator of transcription-1 in human fetal osteoblasts is dependent on Src kinase activity. Endocrinology 2005;146:201-7 

Paech K, Webb P, Kuiper GG, Nilsson S, Gustafsson J, Kushner PJ, Scanlan TS. Differential ligand activation of estrogen receptors ERalpha and ERbeta at AP1 sites. Science 1997;277: 1508-10 

McKay LI, Cidlowski JA. Molecular control of immune/ inflammatory responses: interactions between nuclear factorkappa B and steroid receptor-signaling pathways. Endocr Rev 

1999;20:435-59 

Kang JS, Yoon YD, Han MH, Han SB, Lee K, Kang MR, Moon EY, Jeon YJ, Park SK, Kim HM. Estrogen receptor-independent inhibition of tumor necrosis factor-alpha gene expression by phytoestrogen equol is mediated by blocking nuclear factor- kappaB activation in mouse macrophages. Biochem Pharmacol 2005;71:136-43 

Feldman I, Feldman GM, Mobarak C, Dunkelberg JC, Leslie KK. Identification of proteins within the nuclear factor-kappa B transcriptional complex including estrogen receptor-alpha. Am J Obstet Gynecol 2007;196:394-11 

Nathan L, Pervin S, Singh R, Rosenfeld M, Chaudhuri G. Estradiol inhibits leukocyte adhesion and transendothelial migration in rabbits in vivo: possible mechanisms for gender differences in atherosclerosis. Circ Res 1999;85:377-85 

Lei DL, Long JM, Hengemihle J, O'Neill J, Manaye KF, Ingram DK, Mouton PR. Effects of estrogen and raloxifene on neuroglia number and morphology in the hippocampus of aged female mice. Neuroscience 2003;121:659-66 

Shih HC, Lin CL, Lee TY, Lee WS, Hsu C. 17beta-Estradiol inhibits subarachnoid hemorrhage-induced inducible nitric oxide synthase gene expression by interfering with the nuclear factor kappa B transactivation. Stroke 2006;37:3025-31 

Baker AE, Brautigam VM, Watters JJ. Estrogen modulates microglial inflammatory mediator production via interactions with estrogen receptor {beta}. Endocrinology 2004;145:5021-32 

Park EM, Cho S, Frys KA, Glickstein SB, Zhou P, Anrather J, Ross ME, Iadecola C. Inducible nitric oxide synthase contributes to gender differences in ischemic brain injury. J Cereb Blood Flow Metab 2006;26:392-401 

White MM, Zamudio S, Stevens T, Tyler R, Lindenfeld J, Leslie K, Moore LG. Estrogen, progesterone, and vascular reactivity: potential cellular mechanisms. Endocr Rev 1995;16:739-51