Direct action of estradiol on gonadotropin-releasing hormone-1 neuronal activity via a transcription-dependent mechanism

Classical estrogen receptor alpha signaling mediates negative and positive feedback on gonadotropin-releasing hormone neuron firing

During the female reproductive cycle, the neuroendocrine action of estradiol switches from negative feedback to positive feedback to initiate the preovulatory GnRH and subsequent LH surges. Estrogen receptor-alpha (ERalpha) is required for both estradiol negative and positive feedback regulation of LH. ERalpha may signal through estrogen response elements (EREs) in DNA and/or via ERE-independent pathways. Previously, a knock-in mutant allele (ERalpha-/AA) that selectively restores ERE-independent signaling onto the ERalpha-/- background was shown to confer partial negative but not positive estradiol feedback on serum LH. The current study investigated the roles of the ERE-dependent and ERE-independent ERalpha pathways for estradiol feedback at the level of GnRH neuron firing activity. The above ERalpha genetic models were crossed with GnRH-green fluorescent protein mice to enable identification of GnRH neurons in brain slices. Targeted extracellular recordings were used to monitor GnRH neuron firing activity using an ovariectomized, estradiol-treated mouse model that exhibits diurnal switches between negative and positive feedback. In wild-type mice, GnRH neuron firing decreased in response to estradiol during negative feedback and increased during positive feedback. In contrast, both positive and negative responses to estradiol were absent in GnRH neurons from ERalpha-/- and ERalpha-/AA mice. ERE-dependent signaling is thus required to increase GnRH neuron firing to generate a GnRH/LH surge. Furthermore, ERE-dependent and -independent ERalpha signaling pathways both appear necessary to mediate estradiol negative feedback on serum LH levels, suggesting central and pituitary estradiol feedback may use different combinations of ERalpha signaling pathways.

Nonclassical estrogen modulation of presynaptic GABA terminals modulates calcium dynamics in gonadotropin-releasing hormone neurons

There is increasing recognition that estrogen exerts multifaceted regulatory effects on GnRH neurons. The acute effects of estrogen on calcium dynamics in these cells were examined using a transgenic mouse line that allows real-time measurement of intracellular calcium concentration ([Ca2+]i) in GnRH neurons in the acute brain slice preparation. 17-beta-Estradiol (E2) at 100 pm-100 nm was found to activate [Ca2+]i transients in approximately 40% of GnRH neurons with an approximate 15-min latency. This effect was not replicated by E2-BSA, which limits E2 action to the membrane, 17-alpha-estradiol, the inactive isomer at classical estrogen receptors (ERs), or G-1 the GPR30 agonist. E2 continued to activate [Ca2+]i transients when transcription was blocked. An ER alpha-selective agonist was equally potent in activating [Ca2+]i transients, and E2 remained effective in ERbeta knockout x GnRH-Pericam mice. E2's activation of [Ca2+]i transients continued in the presence of tetrodotoxin, which blocks action potential-dependent transmission, but was abolished completely by the further addition of a gamma-aminobutyric acid (GABA)A receptor antagonist. Exogenous GABA was found to initiate [Ca2+]i transients in GnRH neurons. Whole cell, voltage-clamp recordings of GnRH-green fluorescence protein neurons revealed that E2 generated discrete bursts of miniature inhibitory postsynaptic currents with a latency of approximately 15 min. These observations provide evidence for a new mechanism of nonclassical estrogen action within the brain. Estrogen interacts with the classical ERalpha at the level of the GABAergic nerve terminal to regulate action potential-independent GABA release that, in turn, controls postsynaptic calcium dynamics.

Membrane-initiated estrogen signaling in hypothalamic neurons

It is well known that many of the actions of 17beta-estradiol (E2) in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. But it is not well understood how estrogen signals via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by E2 in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

Gonadotropin-releasing hormone-1 neuronal activity is independent of hyperpolarization-activated cyclic nucleotide-modulated channels but is sensitive to protein kinase a-dependent phosphorylation

Pulsatile release of GnRH-1 stimulates the anterior pituitary and induces secretion of gonadotropin hormones. GnRH-1 release is modulated by many neurotransmitters that act via G protein-coupled membrane receptors. cAMP is the most ubiquitous effector for these receptors. GnRH-1 neurons express hyperpolarization-activated cyclic nucleotide-modulated (HCN) channel protein in vivo. HCN channels are involved in neuronal pacemaking and can integrate cAMP signals. cAMP-dependent protein kinase (PKA) is also activated by cAMP signals, and PKA-dependent phosphorylation modulates voltage-activated channels. In this report, these two pathways were examined in GnRH-1 neurons as integrators of forskolin (FSK)-induced stimulation. The HCN3 isoform was detected in GnRH-1 neurons obtained from mouse nasal explants. ZD7288, a HCN channel blocker, significantly reduced the efficiency of FSK to stimulate GnRH-1 neurons, whereas blockade of PKA with Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium did not attenuate the FSK-induced stimulation. To ensure that disruption of HCN channels on GnRH-1 neurons was responsible for reduction of FSK stimulation, experiments were performed removing gamma-aminobutyric acid (GABA), the major excitatory input to GnRH-1 neurons in nasal explants. Under these conditions, Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium, but not ZD7288, altered the FSK-induced response of GnRH-1 neurons. These studies indicate that PKA-dependent phosphorylation is involved in the FSK-induced stimulation of GnRH-1 neurons rather than HCN channels, and HCN channels integrate the FSK-induced stimulation on GABAergic neurons. In addition, blockade of HCN channels did not modify basal GnRH-1 neuronal activity when GABAergic input was intact or removed, negating a role for these channels in basal GABAergic or GnRH-1 neuronal activity.

Neuroendocrine consequences of androgen excess in female rodents

Androgens exert significant organizational and activational effects on the nervous system and behavior. Despite the fact that female mammals generally produce low levels of androgens, relative to the male of the same species, increasing evidence suggests that androgens can exert profound effects on the normal physiology and behavior of females during fetal, neonatal, and adult stages of life. This review examines the effects of exposure to androgens at three stages of development--as an adult, during early postnatal life and as a fetus, on reproductive hormone secretions in female rats. We examine the effects of androgen exposure both as a model of neuroendocrine sexual differentiation and with respect to the role androgens play in the normal female. We then discuss the hypothesis that androgens may cause epigenetic modification of estrogen target genes in the brain. Finally we consider the clinical consequences of excess androgen exposure in women.

Rapid action of estrogens on intracellular calcium oscillations in primate luteinizing hormone-releasing hormone-1 neurons

Feedback controls of estrogen in LHRH-1 neurons play a pivotal role in reproductive function. However, the mechanism of estrogen action in LHRH-1 neurons is still unclear. In the present study, the effect of estrogens on intracellular calcium ([Ca(2+)](i)) oscillations in primate LHRH-1 neurons was examined. Application of 17beta-estradiol (E(2), 1 nm) for 10 min increased the frequency of [Ca(2+)](i) oscillations within a few minutes. E(2) also increased the frequency of [Ca(2+)](i) synchronization among LHRH-1 neurons. Similar E(2) effects on the frequency of [Ca(2+)](i) oscillations were observed under the presence of tetrodotoxin, indicating that estrogen appears to cause direct action on LHRH-1 neurons. Moreover, application of a nuclear membrane-impermeable estrogen dendrimer conjugate, not control dendrimer, resulted in a robust increase in the frequencies of [Ca(2+)](i) oscillations and synchronizations, indicating that effects estrogens on [Ca(2+)](i) oscillations and their synchronizations do not require their entry into the cell nucleus. Exposure of cells to E(2) in the presence of the estrogen receptor antagonist ICI 182,780 did not change the E(2)-induced increase in the frequency of [Ca(2+)](i) oscillations or the E(2)-induced increase in the synchronization frequency. Collectively, estrogens induce rapid, direct stimulatory actions through receptors located in the cell membrane/cytoplasm of primate LHRH-1 neurons, and this action of estrogens is mediated by an ICI 182,780-insensitive mechanism yet to be identified.

Gonadotropin-releasing hormone-1 neuronal activity is independent of cyclic nucleotide-gated channels

Pulsatile release of GnRH-1 is essential for secretion of gonadotropin hormones. The frequency of GnRH-1 pulses is regulated during the reproductive cycle by numerous neurotransmitters. Cyclic nucleotide-gated (CNG) channels have been proposed as a mechanism to integrate the cAMP signal evoked by many neurotransmitters. This study reports the expression of the CNGA2 subunit in GnRH-1 neurons obtained from mouse nasal explants and shows the ability of GnRH-1 neurons to increase their activity in response to forskolin (activator of adenylyl cyclases), or 3-isobutyl-1-methylxanthine (inhibitor of phosphodiesterases) even after removal of gamma-aminobutyric acid (A)-ergic input. Next, the endogenous activity of adenylyl cyclases was evaluated as a component of the oscillatory mechanism of GnRH-1 neurons. Inhibition of endogenous activity of adenylyl cyclases did not alter GnRH-1 activity. The potential involvement of CNGA2 subunit in basal or induced activity was tested on GnRH-1 neurons obtained from CNGA2-deficient mice. Without up-regulation of CNGA1 or CNGA3, the absence of functional CNGA2 did not alter either the endogenous GnRH-1 neuronal activity or the response to forskolin, negating CNG channels from cAMP-sensitive mechanisms leading to changes in GnRH-1 neuronal activity. In addition, the potential role of CNGA2 subunit in the synchronization of calcium oscillations previously described was evaluated in GnRH-1 neurons from CNGA2-deficient explants. Synchronized calcium oscillations persisted in CNGA2-deficient GnRH-1 neurons. Taken together, these results indicate that CNGA2 channels are not necessary for either the response of GnRH-1 neurons to cAMP increases or the basal rhythmic activity of GnRH-1 neurons.

Increased Fos immunoreactivity in suprachiasmatic nucleus before luteinizing hormone surge in estrogen-treated ovariectomized female rats

BACKGROUND/AIMS

The suprachiasmatic nucleus (SCN) is thought to control the timing of luteinizing hormone (LH) surges. The present study was designed to examine temporal patterns of Fos expression in the dorsomedial and ventrolateral parts of the SCN (SCNdm and SCNvl) of female rats during an LH surge. It also included examination of temporal changes in plasma LH levels and temporal changes in Fos levels in the anteroventral periventricular nucleus (AVPV) and gonadotropin-releasing hormone (GnRH) neurons.

METHODS

Ovariectomized rats injected with 20 microg estradiol benzoate (EB) or vehicle were sacrificed at various times from Zeitgeber time (ZT) 8:00 to 16:30 h (ZT8-16.5; ZT0 = lights on; ZT12 = lights off) on the 2nd day after the injection. Immunohistochemical analyses for Fos and GnRH and enzyme-linked immunosorbent assays for LH were then performed.

RESULTS

In both the SCNdm and SCNvl of EB rats, the number of Fos-immunoreactive cells significantly increased between ZT9.5-10.5 and ZT11-12. On the other hand, in EB rats there were significant peaks of LH levels and Fos levels in GnRH neurons and the AVPV between ZT11-12 and ZT13-14. There was no significant difference in the number of Fos-immunoreactive cells between EB and control rats in either the SCNdm or SCNvl at ZT9.5-10.5, or in the SCNdm at ZT11-12, whereas the SCNvl of EB rats contained more Fos-immunoreactive cells than that of control rats at ZT11-12.

CONCLUSION

These results suggest that in female rats during an LH surge, a peak in the Fos level in the SCN precedes peaks in Fos levels in the AVPV and GnRH neurons.

Estrogen receptor-alpha mediates estradiol attenuation of ATP-induced Ca2+ signaling in mouse dorsal root ganglion neurons

A mechanism underlying gender-related differences in pain perception may be estrogen modulation of nociceptive signaling in the peripheral nervous system. In rat, dorsal root ganglion (DRG) neurons express estrogen receptors (ERs) and estrogen rapidly attenuates ATP-induced Ca2+ signaling. To determine which estrogen receptor mediates rapid actions of estrogen, we showed ERalpha and ERbeta expression in DRG neurons from wild-type (WT) female mice by RT-PCR. To study whether ERalpha or ERbeta mediates this response, we compared estradiol action mediating Ca2+ signaling in DRG neurons from WT, ERalpha knockout (ERalphaKO), and ERbetaKO mice in vitro. ATP, an algesic agent, induced [Ca2+]i transients in 48% of small DRG neurons from WT mice. 17beta-Estradiol (E2) inhibited ATP-induced intracellular Ca2+ concentration ([Ca2+]i) with an IC50 of 27 nM. The effect of E2 was rapid (5-min exposure) and stereo specific; 17alpha-estradiol had no effect. E2 action was blocked by the ER antagonist ICI 182,780 (1 microM) in WT mouse. Estradiol coupled to bovine serum albumin (E-6-BSA), which does not penetrate the plasma membrane, had the same effect as E2 did, suggesting that a membrane-associated ER mediated the response. In DRG neurons from ERbetaKO mice, E2 attenuated the ATP-induced [Ca2+]i flux as it did in WT mice, but in DRG neurons from ERalphaKO mice, E2 failed to inhibit the ATP-induced [Ca2+]i increase. These results show that mouse DRG neurons express ERs and the rapid attenuation of ATP-induced [Ca2+]i signaling is mediated by membrane-associated ERalpha.

Diurnal and estradiol-dependent changes in gonadotropin-releasing hormone neuron firing activity

A robust gonadotropin-releasing hormone (GnRH) surge is a prerequisite signal for the luteinizing hormone (LH) surge that triggers ovulation. In rodents, the GnRH surge is initiated by elevated estradiol and a diurnal switch in estrogen action from negative to positive feedback. The ability of constant estradiol treatment to induce daily LH surges was tested in adult mice that were ovariectomized (OVX) or OVX and treated with estradiol implants (OVX+E). LH in OVX mice showed no time-of-day difference. In contrast, OVX+E mice showed a large LH surge (8- to 124-fold relative to the a.m.) in p.m. samples on d 2-5 post-OVX+E. Targeted extracellular recordings were used to examine changes in firing activity of GnRH neurons in brain slices. There was no time-of-day difference in cells from OVX mice. In contrast, OVX+E cells recorded in the p.m. showed an increased mean firing rate and instantaneous firing frequency, which could increase GnRH release, and decreased duration of quiescence between bouts of firing, possibly reflecting increased pulse frequency, compared with cells recorded in the a.m. In the a.m., OVX+E cells showed changes in GnRH neuron firing reflecting negative feedback compared with OVX cells, whereas in the p.m., OVX+E cells exhibited changes suggesting positive feedback. These data indicate that differences in pattern and level of individual GnRH neuron firing may reflect the switch in estradiol action and underlie GnRH surge generation. The persistence of altered GnRH neuron activity in slices indicates that this approach can be used to study the neurobiological mechanisms of surge generation.

Firing pattern and rapid modulation of activity by estrogen in primate luteinizing hormone releasing hormone-1 neurons

We have shown previously that cultured LHRH-1 neurons, derived from monkey olfactory placode region, exhibit pulsatile LHRH-1 release at hourly intervals and spontaneous intracellular calcium oscillations, which synchronize at a frequency similar to LHRH-1 release. Brief application of estrogen induced a rapid increase in the frequency of intracellular calcium oscillations and the frequency of synchronizations. The estrogen-induced frequency of intracellular calcium oscillations was mediated by estrogen receptors (ER), whereas the frequency of synchronizations was not mediated by ER. In the present study, we further examined the rapid action of estrogen using patch-clamp recording in primate LHRH-1 neurons. Cell-attached patch-clamp recording showed that LHRH-1 neurons exhibited monophasic or biphasic action currents that were sensitive to an increase in extracellular K+ and the sodium channel blocker tetrodotoxin. The majority (90%) of LHRH-1 neurons showed irregular firing patterns composed of bursts and irregular beatings of action currents, which further formed a "cluster" firing pattern. Brief application of 17beta-estradiol (1 nM) increased the firing frequency and burst duration of LHRH-1 neurons with a latency of 60-120 sec for up to 25 min. ICI182,780, an ER antagonist, blocked the 17beta-estradiol-induced increase in the firing activity of LHRH-1 neurons. These results suggest that 1) primate LHRH-1 neurons exhibit complex firing patterns composed of activities with different time domains, 2) estrogen causes rapid stimulatory action of firing activity, and 3) this estrogen action is mediated by ER in primate LHRH-1 neurons.

Developmental changes in GABA receptor subunit composition within the gonadotrophin-releasing hormone-1 neuronal system

It is becoming increasingly evident that GABA plays an important role in the regulation of gonadotrophin-releasing hormone (GnRH)-1 neurones via the GABAA receptor. The aim of the present study was to characterise expression of the GABAA receptor within the GnRH-1 system across development. The expression pattern of five GABAAalpha subunits and one GABAAbeta subunit was first examined within individual GnRH-1 neurones by the polymerase chain reaction. A significant increase in the expression of GABAAalpha2 and a significant decrease in the expression of GABAAalpha6 over time were found. Of the other subunits examined, two (alpha1 and alpha3) showed no differences in expression and two (alpha4 and beta3) showed variable low incidence of expression. Given the reciprocal relationship of alpha2 and alpha6 expression, we hypothesised that there is a developmental switch in the expression of these subunits in GnRH-1 neurones. To investigate this hypothesis, single- and double-label immunocytochemistry for GABAAalpha2 and alpha6 and GnRH-1 was performed in tissue from ages E12.5 to adulthood, as well as in nasal explants. We show that GABAAalpha2 and alpha6 are present in the GnRH-1 neuronal system both in vivo and in vitro and that the levels of expression are altered as a function of age.

Kainate/estrogen receptor involvement in rapid estradiol effects in vitro and intracellular signaling pathways

Although the interactions between sex steroids and GnRH have been extensively studied, little is known about the mechanism of estradiol (E2) effects on GnRH secretion. In the present study, we used retrochiasmatic hypothalamic explants of 50-d-old male rats, and we observed that E2 significantly increased the glutamate-evoked GnRH secretion in vitro within 15 min in a dose-dependent manner. E2 also significantly increased the L-arginine-evoked GnRH secretion. E2 effects were time dependent because the initially ineffective 10(-9) M concentration became effective after 5 h of incubation. The E2 effects involved the estrogen receptor (ER) alpha because they were similarly obtained with the specific ER alpha agonist 1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole. The use of glutamate receptor agonists and antagonists indicated that E2 effects on GnRH secretion evoked by both glutamate and L-arginine involved the 2-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid/kainate receptors. Similar E2 effects on the kainate-evoked secretion were observed throughout development in both sexes. The observation of similar E2 effects using explants containing the median eminence alone indicated that the median eminence was a direct target for E2 rapid effects on the glutamate-evoked GnRH secretion. The signaling pathways involved in E2 effects included an increase in intracellular calcium and the activation of protein kinase A, protein kinase C, and MAPK. It is concluded that E2 can stimulate the glutamate- and nitric oxide-evoked GnRH secretion in vitro through a rapid pathway involving the ER and kainate receptor as well as through a slower mechanism responding to lower E2 concentrations.

Clocks and the black box: circadian influences on gonadotropin-releasing hormone secretion

Although the mechanisms underlying hypothalamic surge secretion of gonadotropin-releasing hormone (GnRH) in rodent models have remained enduring mysteries in the field of neuroendocrinology, the identities of two fundamental constituents are clear. Elevated ovarian oestrogen, in conjunction with circadian signals, combine to elicit GnRH surges that are confined to the afternoon of the proestrus phase. The phenomenon of oestrogen positive feedback, although extensively investigated, is not completely understood, and may involve the actions of this steroid directly on GnRH perikarya, as well as on the activity of neuronal afferents. Additionally, whereas many studies have focused upon regulation of GnRH surge secretion by the neuroanatomical biological clock, the suprachiasmatic nucleus, it remains unclear why this daily signal is capable of stimulating surges only in the presence of oestrogen. This review re-examines multiple models of circadian control of reproductive neurosecretion, armed with the recent characterisation of the intracellular transcriptional feedback loops that comprise the circadian clock, and attempts to evaluate previous studies on this topic within the context of these new discoveries. Recent advances reveal the presence of oscillating circadian clocks throughout the central nervous system and periphery, including the anterior pituitary and hypothalamus, raising the possibility that synchrony between multiple cellular clocks may be involved in GnRH surge generation. Current studies are reviewed that demonstrate the necessity of functional clock oscillations in generating GnRH pulsatile secretion in vitro, suggesting that a GnRH-specific intracellular circadian clock may underlie GnRH surges as well. Multiple possible steroidal and neuronal contributions to GnRH surge generation are discussed, in addition to how these signals of disparate origin may be integrated at the cellular level to initiate this crucial reproductive event.

Bovine serum albumin-estrogen compounds differentially alter gonadotropin-releasing hormone-1 neuronal activity

Steroid hormones regulate a host of physiological processes and behaviors. These actions can occur by genomic mechanisms involving gene transcription or by nongenomic mechanisms proposed to involve receptors associated with the plasma membrane. BSA-conjugated steroid hormones have been extensively used to elucidate signal transduction pathways for these hormones. We have previously shown, using calcium imaging, that 17beta-estradiol (E2) significantly increases GnRH-1 neuronal activity. During the course of these experiments, it became apparent that three different BSA-estrogen compounds have been used in a variety of cell types: 17beta-estradiol 6-O-carboxymethyloxime-BSA (E2-6-BSA); 1,3,5(10)-estratrien-3,16alpha,17beta-triol-6-one 6-O-carboxymethyloxime-BSA (E-6-BSA); and 1,3,5(10)-estratrien-3,17beta-diol 17-hemisuccinate-BSA (E2-17-BSA). The effects of these compounds on GnRH-1 neuronal activity were compared using calcium imaging. E-6-BSA and E2-17-BSA, but not E2-6-BSA, significantly increased all parameters of GnRH-1 neuronal activity. In addition, the effects of these two BSA compounds were reversed by the estrogen receptor antagonist ICI 182,780 but not by inhibition of gene transcription. The effects of E2-17-BSA, but not E-6-BSA were reversed by treatment with pertussis toxin, which blocks G protein-coupled receptors. These data indicate that these compounds cannot be used interchangeably and clearly have different binding properties and/or different effects on target tissues.

Estrogen signaling in the hypothalamus

Estrogen has multifaceted effects on the hypothalamus that regulate a number of homeostatic functions including reproduction, temperature, energy balance, stress, and motivated behaviors. Estrogen targets all of the major hypothalamic neuroendocrine and autonomic cellular groups to activate multiple signaling pathways. Originally it was thought that all of these actions of estrogen could be ascribed to its binding to its "classical" intracellular receptor and to alterations in gene transcription. However, we now know that this steroid hormone activates multiple signaling pathways to affect neuronal excitability and gene transcription. Although the "classical" genomic signaling pathway has been recognized for almost half a century, until recently little attention has been paid to the rapid membrane-initiated signaling by estrogen in neurons. It has been known since the 1970s that estrogen can rapidly alter neuronal firing within seconds, indicating that some cellular effects of estrogen could occur via rapid, non-transcriptional mechanisms. Therefore, this chapter reviews the current status of estrogen signaling in the hypothalamus via membrane-initiated and nuclear-mediated events that affect the excitability of hypothalamic neurons and, ultimately, neuroendocrine and autonomic functions.

Oestrogen receptor beta-immunoreactive neurones in the ovine hypothalamus: distribution and colocalisation with gonadotropin-releasing hormone

Oestrogen powerfully affects the secretion of gonadotropin-releasing hormone (GnRH) from the brain in all species investigated, including sheep. Until recently, it was hypothesised that such regulation occurs indirectly because few or no GnRH neurones were found to express oestrogen receptor (ER) alpha. The discovery of a second oestrogen receptor, ERbeta, and its subsequent localisation in numerous GnRH neurones in the rat, led to a reconsideration of this hypothesis. However, colocalisation of immunoreactive ERbeta protein in GnRH neurones has only been demonstrated in the rat, raising the possibility that such putative direct regulation of GnRH neurones by oestrogen may be peculiar to this species. We have previously shown that steroid receptors in the sheep brain are acutely sensitive to fixation and the full complement of immunoreactive cells can only be visualised after antigen retrieval. The aims of this study were therefore to map immunocytochemically the distribution of ERbeta neurones in the ewe brain, and to determine which proportion of GnRH neurones express ERbeta. Brain sections (20 microm) from four ewes killed in anestrus were subjected to high temperature antigen retrieval and immunocytochemistry. Numerous ERbeta-immunoreactive cells were located throughout the hypothalamus and, following dual-label immunocytochemistry, over 50% of the GnRH neurones were found to express immunoreactive ERbeta. The functional significance of these ERbeta-expressing GnRH neurones in the ovine brain remains to be determined.