Suppression of tonic luteinizing hormone secretion and norepinephrine release near the GnRH neurons by estradiol in ovariectomized rats

Hindbrain Adrenergic/Noradrenergic Control of Integrated Endocrine and Autonomic Stress Responses

Hindbrain adrenergic/noradrenergic nuclei facilitate endocrine and autonomic responses to physical and psychological challenges. Neurons that synthesize adrenaline and noradrenaline target hypothalamic structures to modulate endocrine responses while descending spinal projections regulate sympathetic function. Furthermore, these neurons respond to diverse stress-related metabolic, autonomic, and psychosocial challenges. Accordingly, adrenergic and noradrenergic nuclei are integrative hubs that promote physiological adaptation to maintain homeostasis. However, the precise mechanisms through which adrenaline- and noradrenaline-synthesizing neurons sense interoceptive and exteroceptive cues to coordinate physiological responses have yet to be fully elucidated. Additionally, the regulatory role of these cells in the context of chronic stress has received limited attention. This mini-review consolidates reports from preclinical rodent studies on the organization and function of brainstem adrenaline and noradrenaline cells to provide a framework for how these nuclei coordinate endocrine and autonomic physiology. This includes identification of hindbrain adrenaline- and noradrenaline-producing cell groups and their role in stress responding through neurosecretory and autonomic engagement. Although temporally and mechanistically distinct, the endocrine and autonomic stress axes are complementary and interconnected. Therefore, the interplay between brainstem adrenergic/noradrenergic nuclei and peripheral physiological systems is necessary for integrated stress responses and organismal survival.

Kisspeptin/Neurokinin B/Dynorphin (KNDy) cells as integrators of diverse internal and external cues: evidence from viral-based monosynaptic tract-tracing in mice

Neurons in the hypothalamic arcuate nucleus (ARC) that co-express kisspeptin, neurokinin B and dynorphin (KNDy cells) are essential for mammalian reproduction as key regulators of gonadotropin-releasing hormone (GnRH) secretion. Although multiple endogenous and exogenous signals act indirectly via KNDy neurons to regulate GnRH, the identity of upstream neurons that provide synaptic input to this subpopulation is unclear. We used rabies-mediated tract-tracing in transgenic Kiss1-Cre mice combined with whole-brain optical clearing and multiple-label immunofluorescence to create a comprehensive and quantitative brain-wide map of neurons providing monosynaptic input to KNDy cells, as well as identify the estrogen receptor content and peptidergic phenotype of afferents. Over 90% of monosynaptic input to KNDy neurons originated from hypothalamic nuclei in both male and female mice. The greatest input arose from non-KNDy ARC neurons, including proopiomelanocortin-expressing cells. Significant female-dominant sex differences in afferent input were detected from estrogen-sensitive hypothalamic nuclei critical for reproductive endocrine function and sexual behavior in mice, indicating KNDy cells may provide a unique site for the coordination of sex-specific behavior and gonadotropin release. These data provide key insight into the structural framework underlying the ability of KNDy neurons to integrate endogenous and environmental signals important for the regulation of reproductive function.

Gonadotropin-inhibitory hormone reduces sexual motivation but not lordosis behavior in female Syrian hamsters (Mesocricetus auratus)

Reproductive success is maximized when female sexual motivation and behavior coincide with the time of optimal fertility. Both processes depend upon coordinated hormonal events, beginning with signaling by the gonadotropin-releasing hormone (GnRH) neuronal system. Two neuropeptidergic systems that lie upstream of GnRH, gonadotropin-inhibitory hormone (GnIH; also known as RFamide related peptide-3) and kisspeptin, are potent inhibitory and excitatory modulators of GnRH, respectively, that participate in the timing of the preovulatory luteinizing hormone (LH) surge and ovulation. Whether these neuropeptides serve as neuromodulators to coordinate female sexual behavior with the limited window of fertility has not been thoroughly explored. In the present study, either intact or ovariectomized, hormone-treated female hamsters were implanted for fifteen days with chronic release osmotic pumps filled with GnIH or saline. The effect of GnIH on sexual motivation, vaginal scent marking, and lordosis was examined. Following mating, FOS activation was quantified in brain regions implicated in the regulation of female sexual behavior. Intracerebroventricular administration of GnIH reduced sexual motivation and vaginal scent marking, but not lordosis behavior. GnIH administration altered FOS expression in key neural loci implicated in female reproductive behavior, including the medial preoptic area, medial amygdala and bed nucleus of the stria terminalis, independent of changes in circulating gonadal steroids and kisspeptin cell activation. Together, these data point to GnIH as an important modulator of female proceptive sexual behavior and motivation, independent of downstream alterations in sex steroid production.

Chronic estradiol-17β exposure suppresses hypothalamic norepinephrine release and the steroid-induced luteinizing hormone surge: role of nitration of tyrosine hydroxylase

Chronic exposure to estrogens is known to produce a variety of deleterious effects in women including breast and ovarian cancer and anovulation. In female rats, exposure to low levels of estradiol-17β (E2) decreases hypothalamic norepinephrine (NE) to suppress luteinizing hormone (LH) secretion and cause failure of ovulation. We hypothesized that E2 exposure most likely decreases NE release in the medial preoptic area (MPA) of the hypothalamus to produce this effect and that this may be due to E2-induced inflammatory changes in noradrenergic nuclei leading to nitration of an enzyme involved in NE synthesis. To test this, female Sprague Dawley rats were sham implanted or implanted with slow release E2 pellets (20ng/day) for 30, 60 or 90 days (E30, E60 and E90 respectively). At the end of the treatment period, the rats were implanted with a push-pull cannula in the MPA, ovariectomized and steroid primied to induce a LH surge and subjected to push-pull perfusion. Perfusates were analyzed for NE levels using HPLC-EC. Blood samples collected simultaneously were analyzed for LH levels. We measured interleukin-1β (IL-1β) and nitrate levels in brainstem noradrenergic nuclei that innervate the MPA. In control animals, there was a marked increase in NE levels in response to steroid priming at 1600h that was reduced in the E30 group, and completely abolished after 60 and 90 days of E2 exposure. LH profiles were similar to NE release profiles in control and E2-treated animals. We found that IL-1β levels increased in all three (A1, A2 and A6) noradrenergic nuclei with chronic E2 exposure, while nitrate levels increased only in the A6 region. There was an increase in the nitration of the NE synthesizing enzyme in the MPA in this group as well probably contributing to reduced NE synthesis. This could be a possible mechanism by which chronic E2 exposure decreases NE levels in the MPA to suppress the LH surge.

Differential and interactive effects of ligand-bound progesterone receptor A and B isoforms on tyrosine hydroxylase promoter activity

The classical progesterone receptors (PRs) are expressed in some hypothalamic dopaminergic and brainstem noradrenergic neurones. Progesterone influences prolactin and luteinising hormone release from the anterior pituitary gland, in part by regulating the activity of these catecholaminergic neurones. The present study aimed to determine the effects of PRs on tyrosine hydroxylase (TH) promoter activity. When CAD, SK-N-SH and CV-1 cells were transfected with TH promoter constructs and PR-A or PR-B expression vectors, progesterone treatment caused three- to six-fold increases in TH-9.0 kb promoter activity in PR-B expressing cells, although only a modest increase or no change in PR-A expressing cells. Using CAD cells, deletional analysis mapped the site of PR action to the -1403 to -1304 bp region of the TH promoter. Mutational analysis of putative regulatory sequences in this region indicated that multiple DNA elements are required for complete PR-B transactivation. Electrophoretic mobility shift assays were unable to demonstrate direct PR-B binding to TH promoter DNA sequences. However, chromatin immunoprecipitation analysis indicated PR-B was recruited to the TH promoter. Two different PR-B DNA binding domain mutants had opposing effects on PR-B-mediated TH promoter activation. A GS to AA mutation located in the p-box of the first zinc finger of PR-B inhibited progesterone transactivation of the TH promoter, whereas a C to A mutation in the zinc finger increased transactivation. PR-A was able to inhibit PR-B transactivation in a dose-dependent manner, although the degree of PR-A inhibition was dependent on the TH promoter deletion construct. These data indicate that ligand-bound PR-B is recruited to DNA elements in the TH promoter and acts as a transcriptional activator of the TH gene, and also that changes in the ratio of PR-A to PR-B may affect the ability of progesterone to increase TH expression.

Estradiol-17beta-responsive A1 and A2 noradrenergic cells of the brain stem project to the bed nucleus of the stria terminalis in the ewe brain: a possible route for regulation of gonadotropin releasing hormone cells

We have studied brain stem cells in the ewe brain that project to the bed nucleus of the stria terminalis (BNST) and determined if these cells are activated by estradiol-17beta. This would predicate an indirect role in the estradiol-17beta regulation of gonadotropin releasing hormone (GnRH) cells, since these receive input from the BNST. Ovariectomized ewes received 50 mug estradiol-17beta benzoate (i.m.) 1 h prior to brain collection, so that activated cells could be identified by Fos immunohistochemistry. Retrograde tracer (FluoroGold; FG), was injected into the three divisions of the BNST and labeled cells were mapped to the A1 and A2 regions and the parabrachial nucleus (PBN) of the brain stem. With FG injection into the dorsal and lateral BNST, all FG-containing cells in the caudal A1 and 45% of those in A2 stained for dopamine-beta-hydroxylase (DBH), indicating noradrenergic type. No FG-labelled cells in the PBN were DBH-positive. In A1 and A2 respectively, 42% and 46% of FG-labelled cells were Fos-positive, with no double-labeling in cells of the PBN. In ewes receiving FG injections into the ventral BNST, estrogen receptor (ER)alpha-immunoreactive nuclei were found in 82% of A1-FG labeled and 38% of A2-FG labeled cells. No FG-labelled cells of the PBN were ERalpha-positive. Anterograde tracing from A1 with microruby injection identified projections to the PBN, BNST and preoptic area (POA). Thus, A1 and A2 noradrenergic neurons project to the BNST in the ewe brain, express ERalpha and are activated by estradiol-17beta. These noradrenergic, estrogen-responsive cells may provide indirect input to GnRH cells, via the BNST.

Noradrenaline involvement in the negative-feedback effects of ovarian steroids on luteinising hormone secretion

Noradrenaline has been shown to modulate the ovarian-steroid feedback on luteinising-hormone (LH) release. However, despite the high amount of evidence accumulated over many years, the role of noradrenaline in LH regulation is still not clearly understood. The present study aimed to further investigate the involvement of noradrenaline in the negative-feedback effect of oestradiol and progesterone on basal LH secretion. In experiment 1, ovariectomised (OVX) rats received a single injection of oil, oestradiol, or progesterone at 09.00-10.00 h and were decapitated 30 or 60 min later. Levels of noradrenaline and its metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), were determined in microdissections of the preoptic area (POA) and medial basal hypothalamus-median eminence (MBH-ME) and correlated with LH secretion. Basal LH levels were decreased 30 and 60 min after oestradiol or progesterone injection, and this hormonal response was significantly correlated with a reduction in POA MHPG levels, which reflect noradrenaline release. In addition, noradrenaline levels in the POA were increased, whereas noradrenaline turnover (MHPG/noradrenaline ratio) was decreased 60 min after the injection of both hormones. No effect was found in the MBH-ME. In experiment 2, i.c.v. administration of noradrenaline (60 nmol), performed 15 min before oestradiol or progesterone injection in jugular vein-cannulated OVX rats, completely prevented the ovarian steroid-induced inhibition of LH secretion. The data obtained provide direct evidence that LH secretion in OVX rats is positively regulated by basal noradrenergic activity in the POA, and its reduction appears to play a role in the negative-feedback effect of ovarian steroids on LH secretion in vivo.

Ovarian-steroid modulation of locus coeruleus activity in female rats: involvement in luteinising hormone regulation

The noradrenergic nucleus locus coeruleus (LC) has been reported to regulate luteinising hormone (LH) secretion in female rats. Both oestrogen and progestin receptors have been demonstrated in LC neurones, suggesting that these cells are possibly responsive to variations in circulating levels of ovarian steroids. We therefore evaluated changes in the activity of LC neurones during the oestrous cycle and after ovarian-steroid treatment in ovariectomised (OVX) rats, as determined by immunoreactivity to Fos-related antigens (FRA), which comprises all of the known members of the Fos family. Effects of ovarian steroids on the firing rate of LC neurones were also determined in a slice preparation. The number of FRA/tyrosine hydroxylase (TH)-immunoreactive (ir) neurones in the LC increased from 14.00-16.00 h on pro-oestrus, coinciding with the onset of the LH surge and rise in plasma progesterone. FRA immunoreactivity was unaltered during dioestrus. Oestradiol-treated OVX rats (OVX+E) displayed marked reduction in FRA/TH-ir neurones in LC compared to oil-treated OVX rats. Accordingly, oestradiol superfusion significantly reduced the spontaneous firing rate of LC neurones in slices from OVX rats. Compared to OVX+E, oestradiol-treated rats injected with progesterone at 08.00 h (OVX+EP) exhibited higher number of FRA/TH-ir neurones in the LC at 10.00 h and 16.00 h, and great amplification of the LH surge. Bath application of progesterone significantly increased the spontaneous firing rate of OVX+E LC neurones. Our data suggest that ovarian steroids may physiologically modulate the activity of LC neurones in females, with possible implications for LH secretion. Moreover, oestradiol and progesterone appear to exert opposite and complementary effects (i.e. whereas oestradiol inhibits, progesterone, after oestradiol priming, stimulates LC activity).

Phenobarbital blockade of the preovulatory luteinizing hormone surge: association with phase-advanced circadian clock and altered suprachiasmatic nucleus Period1 gene expression

The suprachiasmatic nucleus (SCN) controls the timing of the preovulatory luteinizing hormone (LH) surge in laboratory rodents. Barbiturate administration during a critical period on proestrus delays the surge and prolongs the estrous cycle 1 day. Because a nonphotic timing signal (zeitgeber) during the critical period that phase advances activity rhythms can also induce the latter effect, we hypothesized that barbiturates delay the LH surge by phase-advancing its circadian timing signal beyond the critical period. In experiment 1, locomotor rhythms and estrous cycles were monitored in hamsters for 2-3 wk preinjection and postinjection of vehicle or phenobarbital and after transfer to darkness at zeitgeber time (ZT) 6 on proestrus. Phenobarbital delayed estrous cycles in five of seven hamsters, which exhibited phase shifts that averaged twofold greater than those exhibited by vehicle controls or phenobarbital-injected hamsters with normal cycles. Experiment 2 used a similar protocol, but injections were at ZT 5, and blood samples for LH determination were collected from 1200 to 1800 on proestrus and the next day via jugular cannulae inserted the day before proestrus. Phenobarbital delayed the LH surge 1 day in all six hamsters, but it occurred at an earlier circadian time, supporting the above hypothesis. Experiment 3 investigated whether phenobarbital, like other nonphotic zeitgebers, suppresses SCN Period1 and Period2 transcription. Two hours postinjection, phenobarbital decreased SCN expression of only Period1 mRNA, as determined by in situ hybridization. These results suggest that phenobarbital advances the SCN pacemaker, governing activity rhythms and hormone release in part by decreasing its Period1 gene expression.

Noradrenaline release in the medial preoptic area during the rat oestrous cycle: temporal relationship with plasma secretory surges of prolactin and luteinising hormone

During the rat oestrous cycle, the afternoon of pro-oestrous is characterised by preovulatory surges of luteinising hormone (LH) and prolactin. On the afternoon of oestrous, a secretory surge of prolactin has also been reported. Because the medial preoptic area (MPOA) is known to regulate prolactin and LH secretory surges and noradrenaline has been demonstrated to stimulate these hormones release, we evaluated whether noradrenaline release in the MPOA was temporally associated with plasma prolactin and LH surges in cycling rats. During the 4 days of oestrous cycle, noradrenaline concentrations were determined in microdialysates from the MPOA, collected at 30-min intervals from 10.30 h to 19.00 h. Plasma prolactin and LH levels were measured in blood samples withdrawn hourly from 14.00 h to 19.00 h on pro-oestrous and from 13.00 h to 18.00 h on the other days of the cycle. On the afternoons of both pro-oestrous and oestrous, noradrenaline levels increased at 14.00 h and remained elevated until 16.30 h. Conversely, they were low and constant throughout metoestrous and dioestrous. Correlating with noradrenaline release in the MPOA, plasma prolactin surges occurred during the afternoons of both pro-oestrous and oestrous. On pro-oestrous, the afternoon LH surge was also preceded by the increase in MPOA noradrenaline whereas, during oestrous, LH secretion was low and unaltered. A temporal association between noradrenaline release and prolactin secretion suggests that noradrenergic neurotransmission in the MPOA regulates prolactin surges in female rats. Moreover, our data also suggest that MPOA noradrenaline requires specific conditions to physiologically regulate LH secretion, which seems to occur during the afternoon of pro-oestrous.

Estrogen and progesterone do not activate Fos in AVPV or LHRH neurons in male rats

In rodents, females but not males, in response to escalating levels of estrogen, express a luteinizing hormone (LH) surge that is prompted by a surge in luteinizing hormone-releasing hormone (LHRH). It cannot take place if estrogen-sensitive afferents located in the anteroventral periventricular nucleus (AVPV) are either absent or disabled. Males appear to lack the ability to exhibit an LH surge, but it is unclear what level of the CNS contributes to this dimorphic response. This study was conducted to determine whether estrogen followed by progesterone treatment (E + P) of gonadectomized males evokes Fos activation in LHRH and AVPV neurons as it does in females. The results indicated that, consistent with the males' inability to express an LH surge in response to E + P treatment, LHRH and AVPV neurons in males failed to show increased Fos activation. Examination of neuron nuclear antigen (NeuN, a neuron-specific marker), estrogen receptor (ERalpha) and progesterone receptor (PR) neurons in AVPV neurons indicated that, while essentially all the neurons of the caudal AVPV in males and females are steroid responsive, the male possessed half the number of steroid responsive neurons within the caudal AVPV (where activation of Fos is maximal in females) compared to the female. Together, these data indicate that the male lacks a substantial population of steroid receptive AVPV neurons and is unable to respond to the presence of E and P and activate either AVPV or LHRH neurons.

Tamoxifen is an estrogen antagonist on gonadotropin secretion and responsiveness of the hypothalamic-pituitary- adrenal axis in female monkeys

The selective estrogen receptor modulator, tamoxifen, effectively slows the progression of estrogen-positive breast cancer and reduces the possibility of this cancer developing in women at high risk. Despite the widespread acceptance of tamoxifen as a therapeutic agent for this disease, its effects on other estrogen-dependent pathways, particularly on neural circuits regulating brain function and peripheral hormone secretion, are poorly understood. The present study, using previously ovariectomized rhesus monkeys, examined the effects of tamoxifen, in both the presence and absence of estradiol replacement, on the reproductive and hypo-thalamic-pituitary-adrenal (HPA) axes. In Experiment 1, monkeys randomly assigned to three groups (n = 8 each) were treated with placebo and either two doses of estradiol, two doses of tamoxifen alone, or two doses of tamoxifen plus high-dose estradiol to assess the effects on negative feedback suppression of luteinizing hormone (LH). Both doses of tamoxifen effectively antagonized the negative feedback efficacy of estradiol on LH secretion. In contrast, neither the low- or high-dose tamoxifen alone had any effect on LH secretion, as concentrations during tamoxifen treatments were indistinguishable from those during placebo. In Experiment 2, females were randomly assigned to one of four treatment groups (placebo, n = 6; estradiol, n = 5; tamoxifen only, n = 5; or tamoxifen plus estradiol, n = 6) to assess the effects on glucocorticoid negative feedback and pituitary and adrenal responsiveness to exogenous corticotropin- releasing hormone (CRH). Tamoxifen also antagonized the facilitating effects of estradiol on basal and CRH-induced ACTH and cortisol secretion. However, this antagonism produced basal and CRH-stimulated cortisol and ACTH concentrations that were lower than placebo-treated females. Interestingly, tamoxifen in the absence of estradiol produced a similar diminution in ACTH and cortisol response. These data suggest that, in the presence of estradiol, tamoxifen not only antagonized estrogenic facilitation of HPA responsivity but also actually attenuated the response compared with the placebo-treatment condition. Taken together, these data indicate that tamoxifen acts as an estrogen antagonist on the neural circuits controlling the neuroendocrine regulation of the hypothalamic-pituitary-ovarian and adrenal axes in ovariectomized macaque females.

The menopausal transition: characteristics and management

The menopausal transition is a complex period in a woman's life, reflecting ovarian ageing and concomitant hormonal changes, in addition to social and metabolic changes. These changes, in turn, influence the signs and symptoms common to this period. Symptoms which are influenced by the hormonal fluctuations occurring during the menopausal transition include vasomotor symptoms and vaginal dryness; others are breast tenderness, poor sleep and pre-menstrual dysphoria. Hormonal therapy has been shown to be first-line therapy for many of these symptoms. Other types of pharmacotherapies may be helpful, including selective serotonergic uptake inhibitors for vasomotor symptoms. Characteristic signs of the menopausal transition include abnormal uterine bleeding, best managed with hormonal pharmacotherapy; diminishing bone mineral density, which may warrant diagnostic intervention, and may benefit from dietary and lifestyle modifications; and increased body-mass index and worsening lipid profile, which also may benefit from dietary and lifestyle modifications.

Oestradiol-dependent and -independent modulation of tyrosine hydroxylase mRNA levels in subpopulations of A1 and A2 neurones with oestrogen receptor (ER)alpha and ER beta gene expression

Oestradiol (E2) induces luteinizing hormone-releasing hormone (LHRH) hypersecretion, thereby triggering LH surge release in ovariectomized (OVX) rats. Neural signals responsible for the surge are marked by a morning increase in LHRH gene expression and an afternoon increase in LHRH release. Evidence suggests that subpopulations of noradrenergic neurones may be responsible for one or both of these signals. To further investigate this issue, we examined effects of E2 on the activity of A1 and A2 noradrenergic neurones, as reflected in changes in tyrosine hydroxylase (TH) mRNA expression, on the day of LH surge release. We then used dual-label in situ hybridization to determine whether E2-induced changes occurred primarily in A1 and A2 subdivisions wherein most noradrenergic neurones expressed oestrogen receptor (ER)alpha and/or ER beta mRNA. We found that in all subdivisions, levels of TH mRNA were higher in E2- than oil-treated rats at 12.00 h. These differences resulted from a decline in TH mRNA expression in oil-treated rats, as well as a rise in levels in E2-treated rats between 10.00 h and 12.00 h. During the afternoon, TH mRNA expression in most A1 and A2 subdivisions peaked at 14.00 h when LH surge release began. However, in all but the middle and caudal A2 subdivisons, levels were similar in E2-treated and control rats at this time. This was attributable to a widespread increase in TH mRNA expression between 12.00 h and 14.00 h in OVX rats. There was no evidence that E2 induced changes in TH mRNA expression preferentially in regions wherein most neurones contained ER alpha or ER beta mRNA. Our findings suggest that E2 activation of middle and caudal A2 neurones, in conjunction with the widespread E2-independent activation of noradrenergic neurones in other subdivisions, may play a role in the induction of LH surge release.

Influence of different neural systems on the secretion of sex hormones in potassium deficient mice

The influence of different neural systems that modulate GnRH secretion by hypothalamic neurons was investigated in mice exposed to hypokalemic conditions, in which the pulsatile release of GnRH has been shown to be altered and associated with a significant decrease of plasma sex steroids. Our results demonstrate that the potentiation of the inhibitory pathways mediated by opiates and GABA may be implicated in the decrease of sex hormones secretion produced by hypokalemia since treatment with higher doses of naloxone or flumazenil are required to restore progesterone or testosterone levels in potassium deficient mice. The combination treatment of prazoxin and naloxone suggests that the inhibitory action of opiates take place through its action on noradrenergic neurons. It is also possible that the inhibition of GnRH release could be due to a decrease in the tonic stimulatory action of noradrenergic pathway implicated in the control of GnRH release. Our results also reveal that it is unlikely that the glutamatergic system may play any relevant direct role in the decrease of sex steroid secretion observed in potassium deficient mice. Finally, these results together with the normal pattern of estradiol levels found along the estrus cycle in potassium deficient mice indicate that factors different from estradiol and acting on neural systems implicated in the regulation of GnRH-secreting neurons participate in the generation of the preovulatory surge of GnRH.

Ovarian ageing and the menopausal transition

The underlying cause of the menopausal transition is a dwindling supply of FSH-responsive follicles available for ovulation. Additional factors may include dysregulation of existing follicles and concurrent follicle and oocyte deficits that may be strictly anatomic or consequences of the hormonal milieu. In the early transition, menstrual irregularity is infrequent but cycle length shortens by 1-4 days. Oestrogen production may be overall elevated, even in ovulatory cycles. As anovulatory cycles become more common, and amenorrhoea of greater duration, evidence of impaired hypothalamic-pituitary function is present. Oestradiol has been implicated as an agent responsible for the impaired positive feedback response. A model of the early menopausal transition suggests that the loss of FSH restraint by the inhibins, due to a critically diminished follicle pool, is the early event that precedes overt follicle failure and may initiate intervals of hyperoestrogenaemia. The hormonal fluctuations in the early and late menopausal transition may account for some of the signs and symptoms seen during these phases.

Loss of luteinizing hormone surges induced by chronic estradiol is associated with decreased activation of gonadotropin-releasing hormone neurons

Chronic exposure of young ovariectomized rats to elevated circulating estradiol causes loss of steroid-induced LH surges. Such LH surges are associated with cFos-induced activation of GnRH neurons; therefore, we hypothesized that chronic estradiol treatment abolishes LH surges by decreasing activation of GnRH neurons. Regularly cycling rats were ovariectomized and immediately received an estradiol implant or remained untreated. Three days or 2 or 4 wk later, the estradiol-treated rats received vehicle or progesterone at 1200 h, and 7 hourly blood samples were collected for RIA of LH. Thereafter, all rats were perfused, and the brains were examined for immunocytochemical localization of cFos and GnRH. The GnRH neurons from untreated ovariectomized rats rarely expressed cFos. As reported, LH surges induced by 3 days of estradiol treatment were associated with a 30% increase in cFos-containing GnRH neurons, and progesterone enhanced both the amplitude of LH surges and the proportion of cFos-immunopositive GnRH neurons. As hypothesized, the abolition of LH surges caused by 2 or more weeks of estradiol was paralleled by a reduction in the percentage of cFos-containing GnRH neurons, and this effect was delayed by progesterone. These results suggest that chronic estradiol abolishes steroid-induced LH surges in part by inactivating GnRH neurons.

New evidence for estrogen receptors in gonadotropin-releasing hormone neurons

Estrogen exerts a critical regulatory influence upon the biosynthetic and secretory activity of the gonadotropin-releasing hormone (GnRH) neurons. It seems likely that estrogen regulates the behavior of the GnRH neuron through multiple transsynaptic, neuronal-glial, and direct membrane modes of action. Advances in our understanding of these mechanisms over the last 3 years are highlighted. In addition, very recent studies have begun to provide evidence for the expression of estrogen receptors (ERs) in GnRH neurons in the rodent. Although not yet firmly established, the current consensus supports the hypothesis that GnRH neurons express ERbeta. Evidence exists for ERbeta mRNA expression by GnRH neurons throughout development and ERbeta immunoreactivity has now also been detected in these cells. Murine GnRH neurons have further been shown to express estrogen receptor-related receptor-alpha, an orphan receptor thought to constitutively activate estrogen response elements. Together, these findings provide a cornerstone for the reassessment of the role of ERs and related receptors in the direct genomic and potential nontranscriptional actions of estrogen upon the GnRH neuron.

Neurotensin gene expression increases during proestrus in the rostral medial preoptic nucleus: potential for direct communication with gonadotropin-releasing hormone neurons

Neurotensin (NT)-containing neurons in the rostral portion of the medial preoptic nucleus (rMPN) of the brain may play a key role in regulating the pattern of secretion of GnRH, thereby influencing the reproductive cycle in females. The major goals of this study were to determine whether NT messenger RNA (mRNA) levels in the rMPN exhibit a unique pattern of expression in temporal association with the preovulatory LH surge and to assess whether NT neurons may communicate directly with GnRH neurons. We analyzed NT gene expression in rats using in situ hybridization over the day of proestrus and compared this with diestrous day 1. We also determined whether the high-affinity NT receptor (NT1) is expressed in GnRH neurons using dual-label in situ hybridization and whether this expression varies over the estrous cycle. We found that NT mRNA levels in the rMPN increase significantly on the day of proestrus, rising before the LH surge. No such change was detected on diestrous day 1, when the LH surge does not occur. Furthermore, we observed that a significant number of GnRH neurons coexpress NT1 mRNA and that the number of GnRH neurons expressing NT1 mRNA peaks on proestrus. Together with previous findings, our results suggest that increased expression of NT in the rMPN may directly stimulate GnRH neurons on proestrus, contributing to the LH surge. In addition, our results suggest that responsiveness of GnRH neurons to NT stimulation is enhanced on proestrus due to increased expression of NT receptors within GnRH neurons.

Chronic elevation of estradiol in young ovariectomized rats causes aging-like loss of steroid-induced luteinizing hormone surges

This study was designed to test the hypothesis that the loss of LH surges in response to the stimulatory actions of estradiol and progesterone in middle-aged, persistent-estrous (PE) rats may be caused by chronic elevations in circulating estradiol. Five groups of regularly cycling young rats received an s.c. estradiol implant immediately after ovariectomy (Day 0). For determination of LH surges, blood samples were collected hourly between 1200-1900 h from each of the five groups at one of the following times: 3 days, or 1, 2, 4, or 8 wk later. On the next day, either progesterone (0.5 mg/100 g BW) or corn oil was injected s.c. at 1200 h, and samples were obtained as before. Incidence and amplitude of estradiol-induced LH surges decreased during the first 2 wk of estradiol treatment, after which no surges occurred. Progesterone enhanced the incidence and amplitude of estradiol-induced LH surges thus delaying their disappearance. These results support our hypothesis and demonstrate that the stimulatory actions of estradiol and progesterone on the LH surge sequentially diminish with time after exposure to estradiol in young rats. Thus, young rats chronically treated with estradiol may be a useful model for studying the mechanisms whereby LH surges are abolished in middle age during the hyperestrogenic state of PE.