Mechanisms That Underlie Expression of Estradiol-Induced Excitatory Synaptic Potentiation in the Hippocampus Differ between Males and Females.

17ß-estradiol (E2) is synthesized in the hippocampus of both sexes and acutely potentiates excitatory synapses in each sex. Previously, we found that the mechanisms for initiation of E2-induced synaptic potentiation differ between males and females, including in the molecular signaling involved. Here, we used electrical stimulation and two-photon glutamate uncaging in hippocampal slices from adult male and female rats to investigate whether the downstream consequences of distinct molecular signaling remain different between the sexes or converge to the same mechanism(s) of expression of potentiation. This showed that synaptic activity is necessary for expression of E2-induced potentiation in females but not males, which paralleled a sex-specific requirement in females for calcium-permeable AMPARs (cpAMPARs) to stabilize potentiation. Nonstationary fluctuation analysis of two-photon evoked unitary synaptic currents showed that the postsynaptic component of E2-induced potentiation occurs either through an increase in AMPAR conductance or in nonconductive properties of AMPARs (number of channels × open probability) and never both at the same synapse. In females, most synapses (76%) were potentiated via increased AMPAR conductance, whereas in males, more synapses (60%) were potentiated via an increase in nonconductive AMPAR properties. Inhibition of cpAMPARs eliminated E2-induced synaptic potentiation in females, whereas some synapses in males were unaffected by cpAMPAR inhibition; these synapses in males potentiated exclusively via increased AMPAR nonconductive properties. This sex bias in expression mechanisms of E2-induced synaptic potentiation underscores the concept of latent sex differences in mechanisms of synaptic plasticity in which the same outcome in each sex is achieved through distinct underlying mechanisms.SIGNIFICANCE STATEMENT Estrogens are synthesized in the brains of both sexes and potentiate excitatory synapses to the same degree in each sex. Despite this apparent similarity, the molecular signaling that initiates estrogen-induced synaptic potentiation differs between the sexes. Here we show that these differences extend to the mechanisms of expression of synaptic potentiation and result in distinct patterns of postsynaptic neurotransmitter receptor modulation in each sex. Such latent sex differences, in which the same outcome is achieved through distinct underlying mechanisms in males versus females, indicate that molecular mechanisms targeted for drug development may differ between the sexes even in the absence of an overt sex difference in behavior or disease.

Latent Sex Differences in Molecular Signaling That Underlies Excitatory Synaptic Potentiation in the Hippocampus.

Excitatory synapses can be potentiated by chemical neuromodulators, including 17ß-estradiol (E2), or patterns of synaptic activation, as in long-term potentiation (LTP). Here, we investigated kinases and calcium sources required for acute E2-induced synaptic potentiation in the hippocampus of each sex and tested whether sex differences in kinase signaling extend to LTP. We recorded EPSCs from CA1 pyramidal cells in hippocampal slices from adult rats and used specific inhibitors of kinases and calcium sources. This revealed that, although E2 potentiates synapses to the same degree in each sex, cAMP-activated protein kinase (PKA) is required to initiate potentiation only in females. In contrast, mitogen-activated protein kinase, Src tyrosine kinase, and rho-associated kinase are required for initiation in both sexes; similarly, Ca2+/calmodulin-activated kinase II is required for expression/maintenance of E2-induced potentiation in both sexes. Calcium source experiments showed that L-type calcium channels and calcium release from internal stores are both required for E2-induced potentiation in females, whereas in males, either L-type calcium channel activation or calcium release from stores is sufficient to permit potentiation. To investigate the generalizability of a sex difference in the requirement for PKA in synaptic potentiation, we tested how PKA inhibition affects LTP. This showed that, although the magnitude of both high-frequency stimulation-induced and pairing-induced LTP is the same between sexes, PKA is required for LTP in females but not males. These results demonstrate latent sex differences in mechanisms of synaptic potentiation in which distinct molecular signaling converges to common functional endpoints in males and females.SIGNIFICANCE STATEMENT Chemical- and activity-dependent neuromodulation alters synaptic strength in both male and female brains, yet few studies have compared mechanisms of neuromodulation between the sexes. Here, we studied molecular signaling that underlies estrogen-induced and activity-dependent potentiation of excitatory synapses in the hippocampus. We found that, despite similar magnitude increases in synaptic strength in males and females, the roles of cAMP-regulated protein kinase, internal calcium stores, and L-type calcium channels differ between the sexes. Therefore, latent sex differences in which the same outcome is achieved through distinct underlying mechanisms in males and females include kinase and calcium signaling involved in synaptic potentiation, demonstrating that sex is an important factor in identification of molecular targets for therapeutic development based on mechanisms of neuromodulation.

17ß-Estradiol Acutely Potentiates Glutamatergic Synaptic Transmission in the Hippocampus through Distinct Mechanisms in Males and Females.

Estradiol (E2) acutely potentiates glutamatergic synaptic transmission in the hippocampus of both male and female rats. Here, we investigated whether E2-induced synaptic potentiation occurs via presynaptic and/or postsynaptic mechanisms and which estrogen receptors (ERs) mediate E2's effects in each sex. Whole-cell voltage-clamp recordings of mEPSCs in CA1 pyramidal neurons showed that E2 increases both mEPSC frequency and amplitude within minutes, but often in different cells. This indicated that both presynaptic and postsynaptic mechanisms are involved, but that they occur largely at different synapses. Two-photon (2p) glutamate uncaging at individual dendritic spines showed that E2 increases the amplitude of uncaging-evoked EPSCs (2pEPSCs) and calcium transients (2pCaTs) at a subset of spines on a dendrite, demonstrating synapse specificity of E2's postsynaptic effects. All of these results were essentially the same in males and females. However, additional experiments using ER-selective agonists indicated sex differences in the mechanisms underlying E2-induced potentiation. In males, an ERß agonist mimicked the postsynaptic effects of E2 to increase mEPSC, 2pEPSC, and 2pCaT amplitude, whereas in females, these effects were mimicked by an agonist of G protein-coupled ER-1. The presynaptic effect of E2, increased mEPSC frequency, was mimicked by an ERα agonist in males, whereas in females, an ERß agonist increased mEPSC frequency. Thus, E2 acutely potentiates glutamatergic synapses similarly in both sexes, but distinct ER subtypes mediate the presynaptic and postsynaptic aspects of potentiation in each sex. This indicates a latent sex difference in which different molecular mechanisms converge to the same functional endpoint in males versus females. SIGNIFICANCE STATEMENT: Some sex differences in the brain may be latent differences, in which the same functional endpoint is achieved through distinct underlying mechanisms in males versus females. Here we report a latent sex difference in molecular regulation of excitatory synapses in the hippocampus. The steroid 17ß-estradiol is known to acutely potentiate glutamatergic synaptic transmission in both sexes. We find that this occurs through a combination of increased presynaptic glutamate release probability and increased postsynaptic sensitivity to glutamate in both sexes, but that distinct estrogen receptor subtypes underlie each aspect of potentiation in each sex. These results indicate that therapeutics targeting a specific estrogen receptor subtype or its downstream signaling would likely affect synaptic transmission differently in the hippocampus of each sex.

Considering Sex as a Biological Variable Will Be Valuable for Neuroscience Research.

The recently implemented National Institutes of Health policy requiring that grant applicants consider sex as a biological variable in the design of basic and preclinical animal research studies has prompted considerable discussion within the neuroscience community. Here, we present reasons to be optimistic that this new policy will be valuable for neuroscience, and we suggest some ways for neuroscientists to think about incorporating sex as a variable in their research.

Sex Differences in Molecular Signaling at Inhibitory Synapses in the Hippocampus.

The possibility that mechanisms of synaptic modulation differ between males and females has far-reaching implications for understanding brain disorders that vary between the sexes. We found recently that 17ß-estradiol (E2) acutely suppresses GABAergic inhibition in the hippocampus of female rats through a sex-specific estrogen receptor α (ERα), mGluR, and endocannabinoid-dependent mechanism. Here, we define the intracellular signaling that links ERα, mGluRs, and endocannabinoids in females and identify where in this pathway males and females differ. Using a combination of whole-cell patch-clamp recording and biochemical analyses in hippocampal slices from young adult rats, we show that E2 acutely suppresses inhibition in females through mGluR1 stimulation of phospholipase C, leading to inositol triphosphate (IP3) generation, activation of the IP3 receptor (IP3R), and postsynaptic endocannabinoid release, likely of anandamide. Analysis of sex differences in this pathway showed that E2 stimulates a much greater increase in IP3 levels in females than males, whereas the group I mGluR agonist DHPG increases IP3 levels equivalently in each sex. Coimmunoprecipitation showed that ERα-mGluR1 and mGluR1-IP3R complexes exist in both sexes but are regulated by E2 only in females. Independently of E2, a fatty acid amide hydrolase inhibitor, which blocks breakdown of anandamide, suppressed >50% of inhibitory synapses in females with no effect in males, indicating tonic endocannabinoid release in females that is absent in males. Together, these studies demonstrate sex differences in both E2-dependent and E2-independent regulation of the endocannabinoid system and suggest that manipulation of endocannabinoids in vivo could affect physiological and behavioral responses differently in each sex. SIGNIFICANCE STATEMENT: Many brain disorders vary between the sexes, yet the degree to which this variation arises from differential experience versus intrinsic biological sex differences is unclear. In this study, we demonstrate intrinsic sex differences in molecular regulation of a key neuromodulatory system, the endocannabinoid system, in the hippocampus. Endocannabinoids are involved in diverse aspects of physiology and behavior that involve the hippocampus, including cognitive and motivational state, responses to stress, and neurological disorders such as epilepsy. Our finding that molecular regulation of the endocannabinoid system differs between the sexes suggests mechanisms through which experiences or therapeutics that engage endocannabinoids could affect males and females differently.

Estradiol acutely potentiates hippocampal excitatory synaptic transmission through a presynaptic mechanism.

Although recent evidence suggests that the hippocampus is a source of 17ß-estradiol (E2), the physiological role of this neurosteroid E2, as distinct from ovarian E2, is unknown. One likely function of neurosteroid E2 is to acutely potentiate excitatory synaptic transmission, but the mechanism of this effect is not well understood. Using whole-cell voltage-clamp recording of synaptically evoked EPSCs in adult rat hippocampal slices, we show that, in contrast to the conclusions of previous studies, E2 potentiates excitatory transmission through a presynaptic mechanism. We find that E2 acutely potentiates EPSCs by increasing the probability of glutamate release specifically at inputs with low initial release probability. This effect is mediated by estrogen receptor ß (ERß) acting as a monomer, whereas ERα is not required. We further show that the E2-induced increase in glutamate release is attributable primarily to increased individual vesicle release probability and is associated with higher average cleft glutamate concentration. These two findings together argue strongly that E2 promotes multivesicular release, which has not been shown before in the adult hippocampus. The rapid time course of acute EPSC potentiation and its concentration dependence suggest that locally synthesized neurosteroid E2 may activate this effect in vivo.

Estradiol potentiation of NR2B-dependent EPSCs is not due to changes in NR2B protein expression or phosphorylation.

The hormone, 17ß-estradiol (E2), influences the structure and function of synapses in the CA1 region of the hippocampus. E2 increases the density of dendritic spines and excitatory synapses on CA1 pyramidal cells, increases CA1 cells' sensitivity to excitatory synaptic input mediated by the NMDA receptor (NMDAR), enhances NMDAR-dependent long-term potentiation, and improves hippocampus-dependent working memory. Smith and McMahon (2006 J Neurosci 26:8517-8522) reported that the larger NMDAR-mediated excitatory postsynaptic currents (EPSCs) recorded after E2 treatment are due primarily to an increased contribution of NR2B-containing NMDARs. We used a combination of electrophysiology, Western blot, and immunofluorescence to investigate two potential mechanisms by which E2 could enhance NR2B-dependent EPSCs: An increase in NMDAR subunit protein levels and/or a change(s) in NR2B phosphorylation. Our studies confirmed the E2-induced increase in NR2B-dependent EPSC amplitude, but we found no evidence that E2 affects protein levels for the NR1, NR2A, or NR2B subunit of the NMDAR, nor that E2 affects phosphorylation of NR2B. Our findings suggest that the effects of E2 on NMDAR-dependent synaptic physiology in the hippocampus likely result from recruitment of NR2B-containing NMDARs to synapses rather than from increased expression of NMDARs or changes in their phosphorylation state.

His and Hers: Sex Differences in the Brain.

While the 1990s bestseller Men Are from Mars, Women Are from Venus addressed behavior, the neurobiological sex differences in the male and female brain remain largely a mystery. Our author-an acclaimed neuroendocrinologist at Northwestern University-tells us what we know and why we don't know more.

Estradiol facilitates the release of neuropeptide Y to suppress hippocampus-dependent seizures.

About one-third of women with epilepsy have a catamenial seizure pattern, in which seizures fluctuate with the menstrual cycle. Catamenial seizures occur more frequently when the ratio of circulating estradiol to progesterone is high, suggesting that estradiol is proconvulsant. We used adult female rats to test how estradiol-induced suppression of GABAergic inhibition in the hippocampus affects behavioral seizures induced by kainic acid. As expected, estradiol decreased the latency to initiate seizures, indicating increased seizure susceptibility. At the same time, however, estradiol also shortened the duration of late-stage seizures, indicating decreased seizure severity. Additional analyses showed that the decrease in seizure severity was attributable to greater release of the anticonvulsant neuropeptide, neuropeptide Y (NPY). First, blocking hippocampal NPY during seizures eliminated the estradiol-induced decrease in seizure duration. Second, light and electron microscopic studies indicated that estradiol increases the potentially releasable pool of NPY in inhibitory presynaptic boutons and facilitates the release of NPY from inhibitory boutons during seizures. Finally, the presence of estrogen receptor-alpha on large dense-core vesicles (LDCVs) in the hippocampus suggests that estradiol could facilitate neuropeptide release by acting directly on LDCVs themselves. Understanding how estradiol regulates NPY-containing LDCVs could point to molecular targets for novel anticonvulsant therapies.

Maintenance of high-frequency transmission at purkinje to cerebellar nuclear synapses by spillover from boutons with multiple release sites.

Cerebellar Purkinje neurons maintain high firing rates but their synaptic terminals depress only moderately, raising the question of how vesicle depletion is minimized. To identify mechanisms that limit synaptic depression, we evoked 100 Hz trains of GABAergic inhibitory postsynaptic currents (IPSCs) in cerebellar nuclear neurons by stimulating Purkinje axons in mouse brain slices. The paired-pulse ratio (IPSC(2)/IPSC(1)) of the total IPSC was approximately 1 and the steady-state ratio (IPSC(20)/IPSC(1)) was approximately 0.5, suggesting a high response probability of postsynaptic receptors, without an unusually high release probability. Three-dimensional electron microscopic reconstructions of Purkinje boutons revealed multiple active zones without intervening transporters, suggestive of "spillover"-mediated transmission. Simulations of boutons with 10-16 release sites, in which transmitter from any site can reach all receptors opposite the bouton, replicated multiple-pulse depression during normal, high, and low presynaptic Ca influx. These results suggest that release from multiple-site boutons limits depletion-based depression, permitting prolonged, high-frequency inhibition at corticonuclear synapses.

Estrogen mobilizes a subset of estrogen receptor-alpha-immunoreactive vesicles in inhibitory presynaptic boutons in hippocampal CA1.

Although the classical mechanism of estrogen action involves activation of nuclear transcription factor receptors, estrogen also has acute effects on neuronal signaling that occur too rapidly to involve gene expression. These rapid effects are likely to be mediated by extranuclear estrogen receptors associated with the plasma membrane and/or cytoplasmic organelles. Here we used a combination of serial-section electron microscopic immunocytochemistry, immunofluorescence, and Western blotting to show that estrogen receptor-alpha is associated with clusters of vesicles in perisomatic inhibitory boutons in hippocampal CA1 and that estrogen treatment mobilizes these vesicle clusters toward synapses. Estrogen receptor-alpha is present in approximately one-third of perisomatic inhibitory boutons, and specifically in those that express cholecystokinin, not parvalbumin. We also found a high degree of extranuclear estrogen receptor-alpha colocalization with neuropeptide Y. Our results suggest a novel mode of estrogen action in which a subset of vesicles within a specific population of inhibitory boutons responds directly to estrogen by moving toward synapses. The mobilization of these vesicles may influence acute effects of estrogen mediated by estrogen receptor-alpha signaling at inhibitory synapses.

Morphological sex differences and laterality in the prepubertal medial amygdala.

The medial amygdala (MeA) is crucial in the expression of sex-specific social behaviors. In adult rats the regional volume of the MeA posterodorsal subnucleus (MeApd) is approximately 50% larger in males than in females. The MeApd is also sexually dimorphic in prepubertal rats. We have recently shown that the left MeApd is significantly larger in prepubertal males than females. In contrast with volumetric sex differences elsewhere in the brain, however, we found no sex difference in the number of left MeApd neurons. In the present study we investigated the cellular bases of the sex difference in MeApd regional volume by quantifying the volume occupied by dendrites, axons, synapses, or glia, and by measuring MeApd dendritic morphology in 26-29-day-old male and female rats. We find that the volume occupied by dendritic shafts and glia completely accounts for the sex difference in left MeApd regional volume. Dendritic length measurements in the left hemisphere confirm that males have greater overall dendritic length, which is due to greater branching rather than to longer dendrite segments. In the right hemisphere the pattern of sex differences was different: Males have more MeApd neurons than females, whereas the dendritic morphology of individual neurons is not sexually dimorphic. These results highlight the importance of evaluating laterality in the MeA and suggest that the left and right MeA could play different roles in neuroendocrine regulation and sexually dimorphic social behaviors.

Sexually dimorphic synaptic organization of the medial amygdala.

The medial amygdala is important in social behaviors, many of which differ between males and females. The posterodorsal subnucleus of the medial amygdala (MeApd) is particularly sensitive to gonadal steroid hormones and is a likely site for gonadal hormone regulation of sexually dimorphic social behavior. Here we show that the synaptic organization of the MeApd in the rat is sexually dimorphic and lateralized before puberty. With the use of whole-cell voltage-clamp recording and quantitative electron microscopy, we found that, specifically in the left hemisphere, prepubertal males have approximately 80% more excitatory synapses per MeApd neuron than females. In the left but not the right MeApd, miniature EPSC (mEPSC) frequency was significantly greater in males than in females; mEPSC amplitude was not sexually dimorphic. Paired-pulse facilitation of EPSCs, an index of release probability, also was not sexually dimorphic, suggesting that greater mEPSC frequency is caused by a difference in excitatory synapse number. Electron microscopy confirmed that the asymmetric synapse-to-neuron ratio and the total asymmetric synapse number were significantly greater in the left MeApd of males than of females. In contrast to results for excitatory synapses, we found no evidence of sexual dimorphism or laterality in inhibitory synapses. Neither the frequency nor the amplitude of mIPSCs was sexually dimorphic or lateralized. Likewise, the number of symmetric synapses measured with electron microscopy was not sexually dimorphic. These findings show that the excitatory synaptic organization of the left MeApd is sexually differentiated before puberty, which could provide a sexually dimorphic neural substrate for the effects of hormones on adult social behavior.

Evidence that disinhibition is associated with a decrease in number of vesicles available for release at inhibitory synapses.

We used three-dimensional reconstruction from serial electron micrographs to investigate two structural changes that could underlie estrogen-induced disinhibition of hippocampal CA1 pyramidal cells: a decrease in the number of inhibitory inputs per neuron and/or a change in inhibitory boutons that could limit GABA release. We analyzed 373 boutons forming 510 inhibitory synapses in estrogen-treated and control animals. Our results show that estrogen specifically decreases the number of synaptic vesicles adjacent to the presynaptic membrane of inhibitory synapses without affecting the overall number of vesicles. We detected no difference in the density of inhibitory inputs. These findings provide a novel mechanism for the functional effects of estrogen on synaptic inhibition and represent the first in vivo evidence that the number of presynaptic vesicles available for release is a regulated property of synapses that affects synaptic physiology.

Acute inhibition of neurosteroid estrogen synthesis suppresses status epilepticus in an animal model.

Status epilepticus (SE) is a common neurological emergency for which new treatments are needed. In vitro studies suggest a novel approach to controlling seizures in SE: acute inhibition of estrogen synthesis in the brain. Here, we show in rats that systemic administration of an aromatase (estrogen synthase) inhibitor after seizure onset strongly suppresses both electrographic and behavioral seizures induced by kainic acid (KA). We found that KA-induced SE stimulates synthesis of estradiol (E2) in the hippocampus, a brain region commonly involved in seizures and where E2 is known to acutely promote neural activity. Hippocampal E2 levels were higher in rats experiencing more severe seizures. Consistent with a seizure-promoting effect of hippocampal estrogen synthesis, intra-hippocampal aromatase inhibition also suppressed seizures. These results reveal neurosteroid estrogen synthesis as a previously unknown factor in the escalation of seizures and suggest that acute administration of aromatase inhibitors may be an effective treatment for SE.

Sex differences in cerebellar synaptic transmission and sex-specific responses to autism-linked Gabrb3 mutations in mice.

Neurons of the cerebellar nuclei (CbN) transmit cerebellar signals to premotor areas. The cerebellum expresses several autism-linked genes, including GABRB3, which encodes GABAA receptor ß3 subunits and is among the maternal alleles deleted in Angelman syndrome. We tested how this Gabrb3 m-/p+ mutation affects CbN physiology in mice, separating responses of males and females. Wild-type mice showed sex differences in synaptic excitation, inhibition, and intrinsic properties. Relative to females, CbN cells of males had smaller synaptically evoked mGluR1/5-dependent currents, slower Purkinje-mediated IPSCs, and lower spontaneous firing rates, but rotarod performances were indistinguishable. In mutant CbN cells, IPSC kinetics were unchanged, but mutant males, unlike females, showed enlarged mGluR1/5 responses and accelerated spontaneous firing. These changes appear compensatory, since mutant males but not females performed indistinguishably from wild-type siblings on the rotarod task. Thus, sex differences in cerebellar physiology produce similar behavioral output, but provide distinct baselines for responses to mutations.

A role for the basal forebrain cholinergic system in estrogen-induced disinhibition of hippocampal pyramidal cells.

Estrogen transiently disinhibits hippocampal CA1 pyramidal cells in adult female rats and prolongs the decay time of IPSCs in these cells. Estrogen-induced changes in synaptic inhibition are likely to be causally related to subsequent enhancements in excitatory synaptic function in CA1 pyramidal cells. Currently, it is unknown how or on what cells estrogen acts to regulate synaptic inhibition in the hippocampus. We used whole-cell voltage-clamp recording of synaptically evoked IPSCs, spontaneous IPSCs, and miniature IPSCs in CA1 pyramidal cells to evaluate estrogen-induced changes in synaptic inhibition in ovariectomized rats that either were pretreated with the estrogen receptor (ER) antagonist tamoxifen or in which basal forebrain cholinergic neurons were eliminated by previous infusion of 192IgG-saporin toxin into the medial septum. We found that estrogen-induced disinhibition and prolongation of IPSCs are entirely dependent on a tamoxifen-sensitive ER. Estrogen-induced disinhibition is partially dependent on basal forebrain cholinergic neurons, but the prolongation of IPSCs is not at all dependent on these cells. Paired-pulse experiments and recordings of action potential-related spontaneous IPSCs suggest that estrogen-induced disinhibition is associated with a decrease in probability of release at GABAergic synapses, which decreases the amplitude of IPSCs produced by inhibitory neuron action potentials. Our findings lend novel insights into estrogen regulation of inhibitory synapses in the hippocampus and point to estrogen action on basal forebrain cholinergic neurons as critically involved in mediating the effects of estrogen in the hippocampus.

Selective estrogen receptor modulators regulate phasic activation of hippocampal CA1 pyramidal cells by estrogen.

Previous studies demonstrated that estrogen induces two sequential waves of CA1 pyramidal cell activation, evidenced by induction of c-Fos at 2 and 24 h after a single estrogen treatment. The second wave of activation is paralleled by suppression of immunoreactivity for glutamic acid decarboxylase-65kD (GAD65) in CA1 and decreased synaptic inhibition of CA1 pyramidal cells. Here, we report that pretreatment with either of the selective estrogen receptor (ER) modulators, tamoxifen (T) or CI628, has no effect on the first wave of c-Fos expression at 2 h but completely blocks the second wave of c-Fos and the suppression of GAD65 at 24 h. Interestingly, T, given 4 h after estrogen, failed to block c-Fos expression or suppression of GAD65 at 24 h. Electrophysiological experiments showed that the T metabolite, 4OH-T, or CI628 can inhibit the so-called rapid estrogen effect, to potentiate excitatory postsynaptic currents (EPSCs) in CA1 pyramidal cells. Thus, estrogen seems to act within 4 h via classical ERs and/or a rapid estrogen effect, such as EPSC potentiation, to produce activation/disinhibition of pyramidal cells 24 h later. In contrast, the initial activation of pyramidal cells, at 2 h after estrogen, seems to involve neither classical ERs nor rapid potentiation of EPSCs.

Cellular and molecular effects of steroid hormones on CNS excitability.

The steroid hormones 17beta-estradiol (estradiol) and progesterone not only regulate the reproductive system but have other central nervous system effects that can directly affect a variety of behaviors. Generally, estradiol has been shown to have activating effects, including the ability to increase seizure activity, while progesterone has been shown to have depressant effects, including anticonvulsant properties. Because levels of these hormones fluctuate across the menstrual cycle, it is important to understand how changes in these hormone levels may influence levels of excitability in the brain, especially in women who have seizure patterns that are related to their menstrual cycle, a phenomenon known as catamenial epilepsy. This paper reviews the effects of estradiol and progesterone on excitatory and inhibitory neurotransmitters, respectively, and the possible cellular and molecular mechanisms underlying the changes in brain excitability mediated by these hormones.