Centrally Applied Somatostatin Inhibits the Estrogen-Induced Luteinizing Hormone Surge via Hypothalamic Gonadotropin-Releasing Hormone Cell Activation in Female Rats1

Evidence that Nitric Oxide Is Critical for LH Surge Generation in Female Sheep

Elevated and sustained estradiol concentrations cause a gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) surge that is necessary for ovulation. In sheep, several different neural systems have been implicated in this stimulatory action of estradiol and this study focused on somatostatin (SST) neurons in the ventral lateral region of the ventral medial nucleus (vlVMN) which express c-Fos during the surge. First, we determined if increased activity of SST neurons could be related to elevated GnRH secretion by assessing SST synapses onto GnRH neurons and neurons coexpressing kisspeptin, neurokinin B, dynorphin (KNDy). We found that the percentage of preoptic area GnRH neurons that receive SST input increased during the surge compared with other phases of the cycle. However, since SST is generally inhibitory, and pharmacological manipulation of SST signaling did not alter the LH surge in sheep, we hypothesized that nitric oxide (NO) was also produced by these neurons to account for their activation during the surge. In support of this hypothesis we found that (1) the majority of SST cells in the vlVMN (>80%) contained neuronal nitric oxide synthase (nNOS); (2) the expression of c-Fos in dual-labeled SST-nNOS cells, but not in single-labeled cells, increased during the surge compared with other phases of the cycle; and (3) intracerebroventricular (ICV) infusion of the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester, completely blocked the estrogen-induced LH surge. These data support the hypothesis that the population of SST-nNOS cells in the vlVMN are a source of NO that is critical for the LH surge, and we propose that they are an important site of estradiol positive feedback in sheep.

Evidence That Endogenous Somatostatin Inhibits Episodic, but Not Surge, Secretion of LH in Female Sheep

Two modes of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion are necessary for female fertility: surge and episodic secretion. However, the neural systems that regulate these GnRH secretion patterns are still under investigation. The neuropeptide somatostatin (SST) inhibits episodic LH secretion in humans and sheep, and several lines of evidence suggest SST may regulate secretion during the LH surge. In this study, we examined whether SST alters the LH surge in ewes by administering a SST receptor (SSTR) 2 agonist (octreotide) or antagonist [CYN154806 (CYN)] into the third ventricle during an estrogen-induced LH surge and whether endogenous SST alters episodic LH secretion. Neither octreotide nor CYN altered the amplitude or timing of the LH surge. Administration of CYN to intact ewes during the breeding season or anestrus increased LH secretion and increased c-Fos in a subset GnRH and kisspeptin cells during anestrus. To determine if these stimulatory effects are steroid dependent or independent, we administered CYN to ovariectomized ewes. This SSTR2 antagonist increased LH pulse frequency in ovariectomized ewes during anestrus but not during the breeding season. This study provides evidence that endogenous SST contributes to the control of LH secretion. The results demonstrate that SST, acting through SSTR2, inhibits episodic LH secretion, likely acting in the mediobasal hypothalamus, but action at this receptor does not alter surge secretion. Additionally, these data provide evidence that SST contributes to the steroid-independent suppression of LH pulse frequency during anestrus.

Co-expression of c-Fos with oestradiol receptor α or somatostatin in the arcuate nucleus, ventromedial nucleus and medial preoptic area in the follicular phase of intact ewes: alteration after insulin-induced hypoglycaemia

The aim of this study was to investigate how acute insulin-induced hypoglycaemia (IIH) alters the activity of cells containing oestradiol receptor α (ERα) or somatostatin (SST) in the arcuate nucleus (ARC) and ventromedial nucleus (VMN), and ERα cells in the medial preoptic area (mPOA) of intact ewes. Follicular phases were synchronized with progesterone vaginal pessaries. Control animals were killed at 0 h or 31 h (n = 5 and 6, respectively) after progesterone withdrawal (PW; time zero). At 28 h, five other animals received insulin (INS; 4 iu/kg) and were subsequently killed at 31 h. Hypothalamic sections were immunostained for ERα or SST each with c-Fos, a marker of neuronal transcriptional activation. Insulin did not alter the percentage of activated ERα cells in the ARC; however, it appeared visually that two insulin-treated animals (INS responders, with no LH surge) had an increase in the VMN (from 32 to 78%) and a decrease in the mPOA (from 40 to 12%) compared to no increase in the two INS non-responders (with an LH surge). The percentage of activated SST cells in the ARC was greater in all four insulin-treated animals (from 10 to 60%), whereas it was visually estimated that activated SST cells in the VMN increased only in the two insulin responders (from 10 to 70%). From these results, we suggest that IIH stimulates SST activation in the ARC as part of the glucose-sensing mechanism but ERα activation is unaffected in this region. We present evidence to support a hypothesis that disruption of the GnRH/LH surge may occur in insulin responders via a mechanism that involves, at least in part, SST cell activation in the VMN along with decreased ERα cell activation in the mPOA.

Activation of cells containing estrogen receptor alpha or somatostatin in the medial preoptic area, arcuate nucleus, and ventromedial nucleus of intact ewes during the follicular phase, and alteration after lipopolysaccharide

Cells in the medial preoptic area (mPOA), arcuate nucleus (ARC), and ventromedial nucleus (VMN) that possess estrogen receptor alpha (ER alpha) mediate estradiol feedback to regulate endocrine and behavioral events during the estrous cycle. A percentage of ER alpha cells located in the ARC and VMN express somatostatin (SST) and are activated in response to estradiol. The aims of the present study were to investigate the location of c-Fos, a marker for activation, in cells containing ER alpha or SST at various times during the follicular phase and to determine whether lipopolysaccharide (LPS) administration, which leads to disruption of the luteinizing hormone (LH) surge, is accompanied by altered ER alpha and/or SST activation patterns. Follicular phases were synchronized with progesterone vaginal pessaries, and control animals were killed at 0, 16, 31, and 40 h (n = 4-6/group) after progesterone withdrawal (PW [time 0]). At 28 h, other animals received LPS (100 ng/kg) and were subsequently killed at 31 h or 40 h (n = 5/group). Hypothalamic sections were immunostained for c-Fos and ER alpha or SST. LH surges occurred only in control ewes with onset at 36.7 ± 1.3 h after PW; these animals had a marked increase in the percentage of ER alpha cells that colocalized c-Fos (%ER alpha/c-Fos) in the ARC and mPOA from 31 h after PW and throughout the LH surge. In the VMN, %ER alpha/c-Fos was higher in animals that expressed sexual behavior than in those that did not. SST cell activation in the ARC and VMN was greater during the LH surge than in other stages in the follicular phase. At 31 or 40 h after PW (i.e., 3 or 12 h after treatment, respectively), LPS decreased %ER alpha/c-Fos in the ARC and the mPOA, but there was no change in the VMN compared to that in controls. The %SST/c-Fos increased in the VMN at 31 h after PW (i.e., 3 h after LPS) with no change in the ARC compared to controls. These results indicate that there is a distinct temporal pattern of ER alpha cell activation in the hypothalamus during the follicular phase, which begins in the ARC and mPOA at least 6-7 h before the LH surge onset and extends to the VMN after the onset of sexual behavior and LH surge. Furthermore, during the surge, some of these ER alpha-activated cells may be SST-secreting cells. This pattern is markedly altered by LPS administered during the late follicular phase, indicating that the disruptive effects of this stressor are mediated by suppressing ER alpha cell activation at the level of the mPOA and ARC and enhancing SST cell activation in the VMN, leading to the attenuation of the LH surge.

Somatostatin inhibition of GnRH neuronal activity and the morphological relationship between GnRH and somatostatin neurons in rats

In rodents, GnRH neurons are diffusely distributed from the medial septum through to the medial preoptic area and control gonadal functions through the pituitary. The activity of GnRH neurons is regulated by a variety of bioactive substances, including the inhibitory peptide somatostatin. In the present study, we focused on somatostatin because intracerebroventricular injection of somatostatin inhibits the LH surge in rats and reduces LH secretion in ewes. Somatostatin also decreases GnRH release from rat hypothalamic slices. In mice, somatostatin is also thought to suppress GnRH neuronal activity through contact on the soma of GnRH neurons. However, similar data are missing in rats. Moreover, rat GnRH neurons receive only a few synaptic inputs. In this study, we assessed the morphological relationship between GnRH and somatostatin neurons. Confocal microscopy on the sections from the medial septum through medial preoptic area revealed about 35 close contacts per rat between the GnRH and somatostatin neuronal fibers in the organum vasculosum of the lamina terminalis region. No contact of somatostatin fibers on the GnRH neuronal somata was observed. Multicell RT-PCR for somatostatin receptor mRNA in rat GnRH neurons was also performed, which revealed moderate expression of somatostatin receptor subtypes 1-5. In addition, patch clamp experiments were carried out in acute slice preparations. Somatostatin suppressed neuronal firing in cells recorded in a cell-attached configuration and also induced whole-cell outward currents in GnRH neurons. These findings suggest that somatostatin directly inhibits the activity of rat GnRH neurons through volume transmission in the organum vasculosum of the lamina terminalis region.

Somatostatin inhibition of gonadotropin-releasing hormone neurons in female and male mice

Previous studies indicate that somatostatin regulates gonadotropin secretion. We investigated here whether somatostatin has direct effects on GnRH neurons in the adult male and female mice. Dual-labeling immunofluorescence experiments revealed the presence of somatostatin-immunoreactive fibers adjacent to GnRH neurons, and three-dimensional confocal reconstructions demonstrated apparent somatostatin fiber appositions with 50-60% of GnRH neurons located throughout the brain in both male and female mice. Perforated patch-clamp recordings from GnRH-green fluorescent protein neurons revealed that approximately 70% of GnRH neurons responded in a dose-dependent manner to 10-300 nm somatostatin with an acute membrane hyperpolarization and cessation of firing. This effect persisted in the presence of tetrodotoxin and amino acid receptor antagonists, indicating a direct postsynaptic site of action on the GnRH neuron. The identity of the somatostatin receptors underlying this action was assessed using GnRH neuron single-cell RT-PCR. Of the somatostatin receptor subtypes, the sstr2 transcript was the most prevalent and detected in both males and females. The expression of sstr2 by GnRH neurons was confirmed in the sstr2 knockout/LacZ knock-in mouse line. Electrophysiological studies demonstrated that the sstr2-selective agonist seglitide exerted acute hyperpolarizing actions on GnRH neurons identical to those of somatostatin. Together, these studies reveal somatostatin, acting through sstr2, to be one of the most potent inhibitors of electrical excitability of male and female GnRH neurons identified thus far.

Prepubertal changes in the synthesis, storage and release of growth hormone and luteinising hormone and in the immunoreactivity of oestrogen receptor-alpha in lamb pituitary cells. A morphofunctional study

The present study was designed to determine the changes in the synthesis, storage and release of luteinising hormone (LH) and growth hormone (GH) in the hypophyseal cells by investigating the presence of oestrogen receptor-alpha (ERalpha) in developing prepubertal female lambs. The experiment was carried out on 14 prepubertal (17-week-old) and 14 peripubertal (32-week-old) ovary-intact lambs. Morphofunctional changes in the cells of the adenohypophyseal population were assayed with immunohistochemistry (IH), in situ hybridisation (ISH), Real-time PCR and radioimmunoassay (RIA). Blood samples (n=14) were taken every 2 weeks from 17 to 32 weeks of age for estimation of GH and LH by RIA. Computer image analysis was used to determine the percent of cells exhibiting IH and/or ISH reaction. The percentage of cells stained for LHbeta and GH increased for both LH- and GH-producing cells and were higher (P<0.001) in the peripubertal than prepubertal group. The percentage of mRNA LHbeta-expressing cells decreased and were lower for the peripubertal (P<0.001) than prepubertal group. The GH mRNA in pituitaries of prepubertal lambs was higher in comparison to peripubertal ones (P<0.001). The percentage of ERalpha positive cells increased significantly (P<0.001) in peripubertal compared to prepubertal lambs and this increase was significant (P<0.001) in both LH- and GH-producing cells. Plasma LH concentrations increased from 27 weeks of age, while GH concentrations gradually decreased from 17 weeks of age (P<0.05). The histomorphological changes in the LH- and GH-producing cells reflect the increasing pattern of the regulation of secretory processes of these hormones and an escalating regulatory role of oestrogen in the physiology of these cells during the prepubertal period. These results support the involvement of both hormones in the events leading up to puberty.

Prenatal programming of the female reproductive neuroendocrine system by androgens

It has been clear for several decades that the areas of the brain that control reproductive function are sexually dimorphic and that the 'programming actions' of the male gonadal steroids are responsible for sex-specific release of the gonadotrophins from the pituitary gland. The administration of exogenous steroids to fetal/neonatal animals has pinpointed windows of time in an animals' development when the reproductive neuroendocrine axis is responsive to the organisational influences of androgens. These 'critical' periods for sexual differentiation of the brain are trait- and species-specific. The neural network regulating the activity of the gonadotrophin releasing hormone (GnRH) neurones is vital to the control of reproductive function. It appears that early exposure to androgens does not influence the migratory pathway of the GnRH neurone from the olfactory placode or the size of the population of neurones that colonise the postnatal hypothalamus. However, androgens do influence the number and the nature of connections that these neurones make with other neural phenotypes. Gonadal steroid hormones play key roles in the regulation of GnRH release acting largely via steroid-sensitive intermediary neurones that impinge on the GnRH cells. Certain populations of hormonally responsive neurones have been identified that are sexually dimorphic and project from hypothalamic areas known to be involved in the regulation of GnRH release. These neurones are excellent candidates for the programming actions of male hormones in the reproductive neuroendocrine axis of the developing female.

Hypothalamic expression of human growth hormone induces post-pubertal hypergonadotrophism in male transgenic growth retarded rats

Growth hormone (GH) is known to regulate peripheral components of the hypothalamo-pituitary gonadal (HPG) axis, but it remains unclear whether GH exerts a significant influence on the activity of the hypothalamo-pituitary components of the HPG axis. In this study, we investigated the development of HPG axis function in the male transgenic growth retarded (Tgr) rat, a model of moderate systemic GH deficiency caused by hypothalamic expression of human (h)GH. Impaired postnatal somatotroph expansion and moderate GH deficiency in male Tgr rats were accompanied by a two- to three-fold increase in pituitary gonadotrophin content, but without a significant change in the pituitary gonadotroph population. A three- to nine-fold elevation in basal circulating luteinising hormone concentration was seen in postpubertal Tgr rats, with a smaller increase in follicle-stimulating hormone. Despite this hypergonadotrophism, there was no corresponding increase in steroidogenic (circulating testosterone and seminal vesicle weights) or gametogenic (spermatozoa counts in seminiferous tubules) activity in the postpubertal Tgr testis. Following puberty, the plasma leptin concentration also became progressively elevated in Tgr males. Circulating gonadotrophin and leptin levels were normalised in Tgr rats by peripheral physiological replacement of rat GH, but plasma testosterone concentration was unaffected. These results confirm that hGH exerts a positive influence on the central control of gonadotrophin secretion in the Tgr rat, but the absence of a corresponding elevation in the steroidogenic or gametogenic function of the Tgr testis implies that the peripheral GH/insulin-like growth factor I axis may also exert a permissive influence on testicular function. The relative contribution of somatogenic and lactogenic mechanisms and the potential influence of elevated leptin and decreased sensitivity to androgen feedback to the development of postpubertal hypergonadotrophism in Tgr males remain to be determined.

Effects of central infusion and immunoneutralization of growth hormone on the timing of puberty and plasma leptin levels in the female rat

Growth hormone (GH) levels increase during puberty though its role in puberty onset is still unclear. An interaction is suggested between GH and leptin, as triggering factor of puberty. To evaluate the role of GH on the timing of puberty and its relation with leptin, we centrally administered recombinant human GH (rhGH; 1 microg/day) to normally fed or food-restricted (FR) prepubertal female rats, and monitored time of vaginal opening (VO). Median time of VO was equally postponed in FR animals and in normally fed rhGH-infused rats: median time of VO was respectively 35 and 34 vs. 27 d. Central infusion of rhGH in FR rats partially restored the delay in VO. Plasma leptin levels were increased in rhGH-infused animals, normally fed or FR. Centrally infused anti-rat GH (0.6 microg/day) did not affect plasma leptin levels, but advanced median time of VO (25 vs. 28 d) in pair-fed female rats but not in ad lib-fed animals. The effects of the centrally infused compounds appear to depend on the dietary regime imposed on the prepubertal animals. Furthermore, plasma leptin levels show no direct or predictive relation to the time of VO. The data indicate an involvement of GH in puberty onset, but do not explain the mechanism employed.

Profiling neurotransmitter receptor expression in mouse gonadotropin-releasing hormone neurons using green fluorescent protein-promoter transgenics and microarrays

The definition of neurotransmitter receptors expressed by individual neuronal phenotypes is essential for our understanding of integrated neural regulation. We report here a single-neuron strategy using green fluorescent protein (GFP)-promoter transgenic mice and oligonucleotide microarrays that has enabled us to provide a qualitative profile of the neurotransmitter receptors expressed by the gonadotropin- releasing hormone (GnRH) neurons, critical for the neural regulation of fertility. Acute brain slices were prepared from adult female GnRH-GFP transgenic mice and single GnRH neurons identified and patched. The contents of GnRH neurons underwent reverse transcription and cDNA amplification using the switch mechanism at the 5' end of RNA templates system, and hybridization to mouse gene oligonucleotide arrays. Fifty different neurotransmitter receptor subunit mRNAs were detected in GnRH neurons. Many of the classical amino acid and aminergic receptors were present in addition to 14 distinct, and in most cases novel, neuropeptidergic receptor signaling families. Four of the latter were selected for functional validation with gramicidin-perforated patch-clamp electrophysiology. Galanin, GnRH and neuromedin B were all found to exert direct depolarizing actions upon GnRH neurons whereas somatostatin induced a potent hyperpolarizing response. These studies demonstrate a relatively straightforward approach for transcriptome profiling of specific neuronal phenotypes. The stimulatory actions of GnRH and galanin upon GnRH neurons found here indicate that positive ultrashort feedback loops exist among the GnRH neuronal population.