Correlative surges of LHRH, LH and FSH in pituitary stalk plasma and systemic plasma of rat during proestrus. Effect of anesthetics

60 YEARS OF NEUROENDOCRINOLOGY: MEMOIR: Harris' neuroendocrine revolution: of portal vessels and self-priming

Geoffrey Harris, while still a medical student at Cambridge, was the first researcher (1937) to provide experimental proof for the then tentative view that the anterior pituitary gland was controlled by the CNS. The elegant studies carried out by Harris in the 1940s and early 1950s, alone and in collaboration with John Green and Dora Jacobsohn, established that this control was mediated by a neurohumoral mechanism that involved the transport by hypophysial portal vessel blood of chemical substances from the hypothalamus to the anterior pituitary gland. The neurohumoral control of anterior pituitary secretion was proved by the isolation and characterisation of the 'chemical substances' (mainly neuropeptides) and the finding that these substances were released into hypophysial portal blood in a manner consistent with their physiological functions. The new discipline of neuroendocrinology - the way that the brain controls endocrine glands and vice versa - revolutionised the treatment of endocrine disorders such as growth and pubertal abnormalities, infertility and hormone-dependent tumours, and it underpins our understanding of the sexual differentiation of the brain and key aspects of behaviour and mental disorder. Neuroendocrine principles are illustrated in this Thematic Review by way of Harris' major interest: hypothalamic-pituitary-gonadal control. Attention is focussed on the measurement of GnRH in hypophysial portal blood and the role played by the self-priming effect of GnRH in promoting the onset of puberty and enabling the oestrogen-induced surge or pulses of GnRH to trigger the ovulatory gonadotrophin surge in humans and other spontaneously ovulating mammals.

Regulation of the cell proliferation and migration as extra-pituitary functions of GnRH

GnRH was originally identified as a hypothalamic factor which promotes gonadotropin release from the pituitary and was named gonadotropin-releasing hormone (GnRH). However, broad tissue distributions of GnRH and the GnRH receptor in various extrapituitary tissues and organs have been revealed and it has been suggested that GnRH has extrapituitary effects such as neuromodulation, immunomodulation, and regulation of follicular atresia and ovulation. Although a number of studies have been performed on these effects, little is known about the molecular mechanisms and physiological settings in which GnRH exerts its activities in extrapituitary organs or tissues. Our recent studies had demonstrated that GnRH is able to regulate both cell proliferation and cell migration at much lower concentration than that in the peripheral circulation by using human carcinoma cell lines. Moreover, stimulating activity of GnRH on the developing chick embryonic GnRH neurons was also demonstrated and strongly suggests possible involvement of GnRH in some of extrapituitary functions. This mini-review intends to provide solid evidence of GnRH activity in the regulation of cell proliferation and migration and its physiological relevance in extra-pituitary functions. Recent other research, including that in various invertebrates, provides new insight into the evolutionary scenarios of GnRH signaling systems, and GnRH functions. Both proliferating and migrating activities are important fundamental cellular activities and could provide an important clue into understanding what the driving force behind the evolution of the GnRH signaling system was.

17 β-estradiol rapidly increases ATP-sensitive potassium channel activity in gonadotropin-releasing hormone neurons [corrected] via a protein kinase signaling pathway

17Beta-estradiol (E2) both inhibits and excites GnRH neurons via presynaptic as well as postsynaptic mechanisms. Although it has been demonstrated that E2 can alter the excitability of GnRH neurons via direct actions, the intracellular signaling cascades mediating these actions are not well understood. Previously we have shown that the activity of one of the critical ion channels needed for maintaining GnRH neurons in a hyperpolarized state, the ATP-sensitive potassium channel (K(ATP)) channel, is augmented by E2 in ovariectomized females. However, the mRNA expression of the K(ATP) channel subunits Kir6.2 and SUR1 are unchanged with in vivo E2 treatment. Therefore, to elucidate the cellular signaling mechanism(s) modulating the channel activity, we did whole-cell patch-clamp recording of enhanced green fluorescent protein-GnRH neurons from ovariectomized female mice to study the acute effects of E2. E2 dose-dependently (EC(50) = 0.6 nM) enhanced the diazoxide (channel opener)-activated K(ATP) channel currents by 1.2- to 2.0-fold, which was antagonized by ICI 182,780. E2-BSA was equally as effective as E2, whereas 17 alpha-estradiol [corrected] had no effect. The protein kinase A (PKA) activator forskolin mimicked the effects of E2, whereas the PKA inhibitor H89 and the protein kinase C (PKC) inhibitor bisindolylmaleimide I blocked the effects of E2. Similar to E2, STX, a membrane estrogen receptor (ER) agonist that does not bind to ERalpha or ERbeta, also potentiated the diazoxide-induced K(ATP) channel current by 1.5-fold. Therefore, E2 can potentiate K(ATP) channel activity in GnRH neurons through a membrane ER-activated PKC-PKA signaling pathway.

Oestrogen, kisspeptin, GPR54 and the pre-ovulatory luteinising hormone surge

Ovulation is central to mammalian fertility, yet the precise mechanism through which oestrogen triggers the gonadotrophin-releasing hormone (GnRH) surge that generates the pre-ovulatory luteinising hormone (LH) surge has remained elusive. The recent discovery that kisspeptin-GPR54 signalling is an essential regulator of the neuroendocrine axis at puberty has led investigators to evaluate the role of kisspeptin in the pre-ovulatory GnRH surge mechanism. Kisspeptin neurones are known to express oestrogen and progesterone receptors and have their cell bodies located in brain regions implicated in the positive-feedback mechanism in several mammalian species. In rodents, kisspeptin neurones located in the rostral periventricular area of the third ventricle (RP3V) are positively regulated by oestrogen and most likely are activated by oestrogen at the time of positive feedback. A similar scenario appears to exist for a sub-population of kisspeptin neurones located in the mediobasal hypothalamus of sheep and primates. The majority of GnRH neurones express GPR54, and kisspeptin causes an intense electrical activation of these cells. In concordance with this, kisspeptin administration in vivo results in an abrupt and prolonged release of LH in all mammalian species examined to date. Functional evidence from immunoneutralisation and knockout studies suggests that RP3V kisspeptin neurones projecting to GnRH neurones are an essential component of the surge mechanism in rodents. Taken together, the studies undertaken to date provide substantial evidence in support of a key role of kisspeptin-GPR54 signalling in the generation of the oestrogen-induced pre-ovulatory surge mechanism in mammals.

Gonadotropin-releasing hormone neurons express K(ATP) channels that are regulated by estrogen and responsive to glucose and metabolic inhibition

Gonadotropin-releasing hormone (GnRH) is released in a pulsatile manner that is dependent on circulating 17beta-estradiol (E2) and glucose concentrations. However, the intrinsic conductances responsible for the episodic firing pattern underlying pulsatile release and the effects of E2 and glucose on these conductances are primarily unknown. Whole-cell recordings from mouse enhanced green fluorescent protein-GnRH neurons revealed that the K(ATP) channel opener diazoxide induced an outward current that was antagonized by the sulfonylurea receptor 1 (SUR1) channel blocker tolbutamide. Single-cell reverse transcription (RT)-PCR revealed that the majority of GnRH neurons expressed Kir6.2 and SUR1 subunits, which correlated with the diazoxide/tolbutamide sensitivity. Also, a subpopulation of GnRH neurons expressed glucokinase mRNA, a marker for glucose sensitivity. Indeed, GnRH neurons decreased their firing in response to low glucose concentrations and metabolic inhibition. The maximum diazoxide-induced current was approximately twofold greater in E2-treated compared with oil-treated ovariectomized females. In current clamp, estrogen enhanced the diazoxide-induced hyperpolarization to a similar degree. However, based on quantitative RT-PCR, estrogen did not increase the expression of Kir6.2 or SUR1 transcripts in GnRH neurons. In the presence of ionotropic glutamate and GABA(A) receptor antagonists, tolbutamide depolarized and significantly increased the firing rate of GnRH neurons to a greater extent in E2-treated females. Finally, tolbutamide significantly increased GnRH secretion from the preoptic-mediobasal hypothalamus. Therefore, it appears that K(ATP) channels and glucokinase are expressed in GnRH neurons, which renders them directly responsive to glucose. In addition, K(ATP) channels are involved in modulating the excitability of GnRH neurons in an estrogen-sensitive manner that ultimately regulates peptide release.

Estrogen positive feedback to gonadotropin-releasing hormone (GnRH) neurons in the rodent: the case for the rostral periventricular area of the third ventricle (RP3V)

Increasing levels of circulating estradiol during the follicular phase of the ovarian cycle act on the brain to trigger a sudden and massive release of gonadotropin-releasing hormone (GnRH) that evokes the pituitary luteinizing hormone surge responsible for ovulation in mammals. The mechanisms through which estrogen is able to exert this potent "positive feedback" influence upon the GnRH neurons are beginning to be unravelled. Recent studies utilizing mouse models with global and cell-specific deletions of the different estrogen receptors (ERs) have shown that estrogen positive feedback is likely to use an indirect pathway involving the modulation of ERalpha-expressing neurons that project to GnRH neurons. Conventional tract tracing studies in rats, and experiments involving conditional pseudorabies virus tract tracing from GnRH neurons in the transgenic mouse, indicate that the dominant populations of ERalpha-expressing neuronal afferents to GnRH neurons reside in the anteroventral periventricular, median preoptic and periventricular preoptic nuclei. Together these estrogen-sensitive afferents to GnRH neurons form a periventricular continuum that can be referred to as rostral periventricular area of the third ventricle (RP3V) neurons. The neurochemical identity of some RP3V neurons has been determined and there is mounting evidence for important roles of glutamate, GABA, kisspeptin and neurotensin-expressing RP3V neurons in estrogen positive feedback. The definition of the key cluster of estrogen-sensitive neurons responsible for activating the GnRH neurons to evoke the GnRH surge (and ovulation) should be of substantial value to on-going efforts to understand the molecular and cellular basis of the estrogen positive feedback mechanism.

Atrazine and reproductive function: mode and mechanism of action studies

Atrazine, a chlorotriazine herbicide, is used to control annual grasses and broadleaf weeds. In this review, we summarize our laboratory's work evaluating the neuroendocrine toxicity of atrazine (and related chlorotriazines) from an historic perspective. We provide the rationale for our work as we have endeavored to determine: 1) the underlying reproductive changes leading to the development of mammary gland tumors in the atrazine-exposed female rat; 2) the cascade of physiological events that are responsible for these changes (i.e., the mode of action for mammary tumors); 3) the potential cellular mechanisms involving adverse effects of atrazine; and 4) the range of reproductive alterations associated with this pesticide.

17beta-estradiol stimulates ascorbic acid and LHRH release from the medial basal hypothalamus in adult male rats

In the present investigation, 17beta-estradiol (E(2)) and tamoxifen, an antiestrogen, were evaluated for their effects on the release of ascorbic acid (AA) and luteinizing hormone-releasing hormone (LHRH). Medial basal hypothalami (MBH) from adult male rats were incubated with graded concentrations of E(2) (10 (-9) to 10(-6) M) or a combination of E(2) (10(-7) M) and tamoxifen (10(-7) and 10(-6) M ) in 0.5 ml of Krebs Ringer bicarbonate buffer for 1 hr. AA and LHRH in the incubation medium were measured by high-performance liquid chromatography and radioimmunoassay, respectively. E(2) significantly elevated both AA and LHRH release and the minimal effective dose was 10(-7) M. A combination of E(2) (10(-7) M) and tamoxifen (10(-6) M) totally blocked E(2)-induced AA and LHRH release. The stimulatory effect of E(2) was also suppressed in the presence of N(G)-monomethyl-L-arginine, a competitive inhibitor of nitric oxide synthase (NOS), illustrating that the release is mediated by nitric oxide (NO). To further characterize the role of NO, the tissues were incubated with E(2) or a combination of E(2) + (6 anilino-5, 8-quinolinedione) LY 83583 (10(-6) and 10(-5) M), an inhibitor of NOS. LY 83583 was effective in suppressing E(2)-induced AA and LHRH release, demonstrating that the effect was mediated by cyclic GMP. Incubation of the tissues with E(2) or a combination of E(2) + 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (O.D.Q.) (10(-5) and 10(-4) M), a specific inhibitor of soluble guanylyl cyclase failed to alter AA release but significantly suppressed LHRH release. The role of a prostaglandin synthesis blocker in E(2)-induced AA and LHRH release was tested by incubating the tissues with E(2) or a combination of E(2) + indomethacin (1.8 x 10 (-7) or 1.8 x 10(-6) M). Indomethacin produced a significant decrease in E(2)-induced AA and LHRH release, suggesting that the release process required prostaglandins as an intracellular mediator. In conclusion, E(2) stimulated both AA and LHRH release and the effect was mediated by NO and prostaglandins.

Galanin enhancement of gonadotropin-releasing hormone-stimulated luteinizing hormone secretion in female rats is estrogen dependent

The hypothalamic peptide GnRH is the primary neuroendocrine signal regulating pituitary LH in females. The neuropeptide galanin is cosecreted with GnRH from hypothalamic neurons, and in vitro studies have demonstrated that galanin can act at the level of the pituitary to directly stimulate LH secretion and also augment GnRH-stimulated LH secretion. Several lines of evidence have suggested that the hypophysiotropic effects of galanin are important for the generation of preovulatory LH surges. To determine whether the pituitary actions of galanin are enhanced by the preovulatory steroidal milieu, LH responses to galanin administration (with or without GnRH) were examined in: 1) ovariectomized (OVX); 2) OVX, estrogen (E)-primed; and 3) OVX, E- and progesterone-treated female rats. Results from the study indicate that galanin enhances GnRH-stimulated LH secretion only in the presence of E (in OVX, E-primed, or E- and progesterone-treated rats). Galanin alone does not directly stimulate LH secretion under any of the steroid conditions examined. In the absence of gonadal steroids (OVX rats), galanin inhibits GnRH-stimulated LH secretion. These findings suggest that the primary pituitary effect of galanin is to modulate GnRH-stimulated LH secretion, and that the potentiating effects of galanin occur only in the presence of E.

Testosterone-dependent effects of galanin on pituitary luteinizing hormone secretion in male rats

Galanin is a 29-amino-acid peptide that colocalizes with GnRH in hypothalamic neurons. High concentrations of galanin are present in portal vessel blood of both male and female rats, and galanin receptors are present on gonadotropes in both sexes. Results from studies of female rats indicate that galanin acts at the level of the pituitary to directly stimulate LH secretion and also to enhance GnRH-stimulated LH secretion. The effects of galanin on pituitary LH secretion in male rats are relatively uncharacterized; thus, the present in vivo study was conducted 1). to examine the ability of galanin to affect basal or GnRH-stimulated LH secretion in male rats and 2). to determine whether the effects of galanin on LH secretion in male rats are testosterone-dependent. All three doses of galanin used (1, 5, and 10 micro g/pulse) significantly enhanced GnRH-stimulated LH secretion in intact male rats. Only the highest dose of galanin directly stimulated LH secretion (without GnRH coadministration) in intact males. Galanin did not directly stimulate LH secretion or enhance GnRH-stimulated LH secretion in castrated male rats. In fact, the highest dose of galanin inhibited GnRH-stimulated LH secretion in castrated males. Upon testosterone replacement, the ability of galanin to directly stimulate LH secretion and to enhance GnRH-stimulated LH secretion was restored in castrated males. These results suggest a role for galanin in the regulation of LH release in male rats and demonstrate that testosterone upregulates the ability of the pituitary to respond to the stimulatory effects of galanin.

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.

Leptin effects on pulsatile gonadotropin releasing hormone secretion from the adult rat hypothalamus and interaction with cocaine and amphetamine regulated transcript peptide and neuropeptide Y

Leptin may act as a negative feedback signal to the hypothalamic control of appetite through suppression of neuropeptide Y (NPY) secretion and stimulation of cocaine and amphetamine regulated transcript (CART). We aimed at studying the effects of leptin, CART and NPY on the hypothalamic control of the pituitary-gonadal system. Pulsatile gonadotropin-releasing hormone (GnRH) secretion was studied in vitro using retrochiasmatic hypothalamic explants from adult rats. In the female, GnRH pulse amplitude was significantly increased by leptin (10(-7) M) and CART (10(-6) M) irrespective of the estrus cycle phase while no such effects were seen in the male. The GnRH interpulse interval was not affected in both sexes. Passive immunoneutralization against CART caused a reduction in GnRH pulse amplitude in the female. A slight but significant increase in GnRH pulse amplitude was caused by NPY (10(-7) M) in the female. However, GnRH pulse amplitude was not affected by a Y5-receptor antagonist (10(-6) M) while the interpulse interval was significantly increased as shown previously in the male. The increase in GnRH pulse amplitude caused by leptin was totally prevented by coincubation with an anti-CART antiserum whereas it was not affected by coincubation with the NPY Y5-receptor antagonist (10(-7) M). In conclusion, leptin and NPY show separate permissive effects on GnRH secretion in the adult rat hypothalamus. In both sexes, NPY is prominently involved in the control of the frequency of pulsatile GnRH secretion through the Y5 receptor subtype. Leptin causes a female-specific facilitatory effect on GnRH pulse amplitude which is mediated by CART and which occurs irrespective of the estrus cycle phase.

Definitive evidence for the existence of morphological plasticity in the external zone of the median eminence during the rat estrous cycle: implication of neuro-glio-endothelial interactions in gonadotropin-releasing hormone release

Despite intense investigation, the demonstration of morphological plasticity in the external zone of the median eminence concerning the gonadotropin-releasing hormone system has never been reported. In this study, we investigate whether dynamic transformations of the gonadotropin-releasing hormone nerve terminals and/or tanycytes in the external zone of the median eminence of the hypothalamus occurred during the rat estrous cycle, by following individual gonadotropin-releasing hormone-immunoreactive nerve terminals on serial ultrathin sections observed by electron microscopy. Female rats were killed at 16.00 diestrus II (n = 3), i.e. when estrogen levels are basal and gonadotropin-releasing hormone release is low, and at 16.00 proestrus (n = 4), i.e. when estrogen levels peak and the preovulatory gonadotropin-releasing hormone surge occurs. Our results show that, in the median eminence obtained from proestrus rats, 12+/-2% of the gonadotropin-releasing hormone nerve terminals were observed to make physical contact with the parenchymatous basal lamina, i.e. the pericapillary space. In the median eminence obtained from diestrus II rats, no contacts were observed. On proestrus, numerous physical contacts between gonadotropin-releasing hormone nerve terminals and the basal lamina occurred by evagination of the basal lamina and/or by emerging processes from gonadotropin-releasing hormone nerve terminals. The quantification of the evagination of the basal lamina revealed that the basal lamina was at least twofold more tortuous in appearance during proestrus. These results demonstrate for the first time the existence of dynamic plastic changes in the external zone of the median eminence, allowing gonadotropin-releasing hormone nerve terminals to contact the pericapillary space on the day of proestrus, thus facilitating the release of the neurohormone into the pituitary portal blood.

Serotonergic 5-HT2A receptors important for the oestradiol-induced surge of luteinising hormone-releasing hormone in the rat

Serotonin (5-HT) plays a role in mediating the oestradiol-induced surge of luteinising hormone (LH), but so far the 5-HT receptor subtype involved has not been identified. Our previous in-situ hybridization and pharmacological studies suggest that the action of 5-HT involves the 5-HT2A receptor. The aim of the present study was to investigate this possibility by the direct approach of determining whether 5-HT2A receptor antagonists block the oestradiol-induced surge of luteinising hormone releasing hormone (LHRH). Adult female Wistar rats, which had shown at least two consecutive 4-day oestrous cycles, were ovariectomised under halothane anaesthesia in the morning of dioestrus and injected with vehicle (arachis oil) alone or oestradiol benzoate (OB). At 12.00 h of the next day, presumptive pro-oestrus, the animals were injected intraperitoneally with one of three 5-HT2A antagonists, a selective 5-HT reuptake inhibitor (fluoxetine), or the appropriate vehicles; hypophysial portal blood was then collected under alphaxalone anaesthesia between 15.00 and 19.00 h. The amount of LHRH released into hypophysial portal blood during consecutive 30-min periods was determined by radioimmunoassay. As expected, oestradiol, but not oil, triggered a surge of LHRH in hypophysial portal blood with a peak at about 16.00 h of presumptive pro-oestrus. This oestradiol-induced surge of LHRH was blocked by ketanserin, ritanserin and the highly selective 5-HT2A receptor antagonist, RP62203, but not by fluoxetine. These results provide the first direct evidence that the 5-HT2A receptor plays an important role in the oestradiol-induced surge of LHRH.

Evaluation of LH secretory dynamics during the rat proestrous LH surge

The preovulatory luteinizing hormone (LH) surge results from the integration of complex interactions among gonadal steroids and hypothalamic and pituitary hormones. To evaluate changes in LH secretory dynamics that occur during the rat LH surge, we have 1) obtained frequently sampled serum LH concentration time series, 2) used both waveform-dependent and waveform-independent convolution analyses, and 3) independently assessed proestrous LH half-life and basal non-gonadotropin-releasing hormone (GnRH)-dependent LH secretion during the LH surge. Waveform-independent pulse analysis revealed a 24-fold increase in the maximal pulsatile LH secretory rate attained during late proestrus compared with early proestrus. A 15-fold increase was quantified for the mean LH secretory rate. In complementary analyses, we applied a measured LH half-life of 17 +/- 2.7 min and a median basal LH secretion rate of 0.0046 microgram. l-1. min-1 for convolution analysis, revealing a 16-fold increase in the mass of LH released/burst and more than sixfold rise in the amplitude of the secretory peaks. Evaluation of the approximate entropy of the LH surge profiles was performed, showing an increase in the orderliness of the LH release process during the surge. We conclude that both quantitative (mass/burst) and qualitative (approximate entropy) features of LH release are regulated during the proestrous LH surge.

Characterization of attenuated proestrous luteinizing hormone surges in middle-aged rats by deconvolution analysis

Reproductive aging in female rats is associated with attenuated preovulatory LH surges. In this study, detailed analyses of the episodic characteristics of the proestrous LH surge were conducted in young and middle-aged regularly cyclic rats. On proestrus, blood samples were withdrawn at 3-min intervals for 6 h and analyzed for LH concentrations by RIA in triplicate. Deconvolution analysis of immunoreactive LH concentrations revealed that there was no difference in the detectable LH secretory burst frequency between young and middle-aged rats. However, in middle-aged rats with an attenuated LH surge on proestrus, the mass of LH secreted per burst and the maximal rate of LH secretion per burst were only one fourth (p < 0.01) of those in young and middle-aged rats with normal LH surges. Furthermore, middle-aged rats with attenuated LH surges had a 4-fold decrease (p < 0.01) in the maximal rate of LH secretion per burst compared to young and middle-aged females with normal LH surges. The apparent half-life of endogenous LH was similar among the 3 groups. The attenuated LH surges of middle-aged rats were related specifically to a decrease in LH burst amplitude with no change in pulse frequency. The orderliness of moment-to-moment LH release as quantified by the regularity statistic, approximate entropy, was comparable in the 3 groups. Our findings of a markedly decreased amount of LH released per burst and preserved orderliness of the LH release process strongly suggest that a deficient GnRH drive and/or reduced responsivity to the GnRH signal, rather than altered timing of the signal, accounts for the age-related decline in reproductive function in female rats as presaged by an attenuated proestrous LH surge in middle age.

Semi-quantitative ultrastructural analysis of the localization and neuropeptide content of gonadotropin releasing hormone nerve terminals in the median eminence throughout the estrous cycle of the rat

The ultrastructural appearance of gonadotropin releasing hormone-immunoreactive elements was studied in the external zone of the median eminence of adult female Wistar rats. On the one hand, the purpose of the study was to determine the distribution of gonadotropin releasing hormone terminals towards the parenchymatous basal lamina at the level of hypothalamo-hypophyseal portal vessels, throughout the estrous cycle. On the other hand, we have semi-quantified the gonadotropin releasing hormone content in nerve terminals or preterminals during this physiological condition. A morphometric study was coupled to a colloidal 15 mn gold postembedding immunocytochemistry procedure. Animals were killed at 09.00 on diestrus II, 0.900, 10.00, 13.00, 17.00 and 18.00 on proestrus and 09.00 on estrus (n = 4-8 rats/group). A preliminary light microscopic study was carried out to identify an antero-posterior part of median eminence strongly immunostained by anti-gonadotropin releasing hormone antibodies but which was, in addition, easily spotted. This last condition was necessary to make a good comparison between each animal. Contacts between gonadotropin releasing hormone nerve terminals and the basal lamina were observed only the day of proestrus. Such contacts, however, were rare and in the great majority of cases, gonadotropin releasing hormone terminals are separated from basal lamina by tanycytic end feet. The morphometric analysis showed no significant variation in average distance between gonadotropin releasing hormone terminals and capillaries throughout the estrous cycle. Consequently, it did not appear that a large neuroglial plasticity exists during the estrous cycle. However, the observation of contacts only on proestrus together with some ultrastructural images evoke the possibility of a slight plasticity. The semi-quantitative results show that the content of gonadotropin releasing hormone in the nerve endings presented two peaks on proestrus: one at 09.00 (23 +/- 5 particles/micrograms2, P < 0.03) before the onset of luteinizing hormone surge, and the second at 18.00 (16 +/- 2 particles/micrograms2, P < 0.01) concomitantly with the luteinizing hormone surge, when compared to baseline values on proestrus 10.00 (8 +/- particles/micrograms2).

Central administration of antisense oligodeoxynucleotides to neuropeptide Y (NPY) mRNA reveals the critical role of newly synthesized NPY in regulation of LHRH release

Compelling evidence shows that the episodic and cyclic secretion of hypothalamic luteinizing hormone releasing hormone (LHRH), the primary stimulator of pituitary LH release, is subject to regulation by neuropeptide Y (NPY). We have reported earlier that sequential treatment of ovariectomized (ovx) rats with estrogen and progesterone to stimulate a preovulatory-type LH surge elevated the levels of both NPY and preproNPY mRNA levels in the hypothalamus concomitant with dynamic changes in LHRH activity. The present study was designed to determine whether these elevations in NPY content and gene expression represent new synthesis of NPY that is crucial to elicit LHRH discharge. Ovx, steroid-primed rats received intracerebroventricular injections of an unmodified 20-mer oligodeoxynucleotide (oligo) complementary to the NPY mRNA sequence. Control rats were injected similarly with either saline or the sense or missense oligos. Results showed that control rats displayed a characteristic surge-type elevation in plasma LH levels that was not affected by the administration of missense or sense oligos. However, in rats injected with the antisense oligo, the steroid-induced LH surge was completely blocked. In an additional experiment, NPY peptide levels were measured in microdissected hypothalamic sites following the injection of antisense or missense oligos. NPY antisense oligo administration blocked the significant increases in NPY levels in the median eminence-arcuate area, the medial preoptic area and lateral preoptic area seen in control rats. These results suggest that sequential ovarian steroid treatment augments NPY synthesis in the hypothalamus and this newly synthesized NPY is critical for induction of the LHRH and LH surge.

Evidence that hypothalamic neuropeptide Y gene expression increases before the onset of the preovulatory LH surge

Neuropeptide Y (NPY), a 36 amino acid residue peptide, is involved in stimulation of LHRH and LH surges on proestrus and those induced by ovarian steroids in ovariectomized (ovx) rats. Recently, we observed that NPY gene expression in the medial basal hypothalamus (MBH) was increased before the onset of the LH surge in the ovarian steroid-primed ovx rats. Since the ovarian steroidal milieu during the estrous cycle is markedly different from that prevailing after ovarian steroid injections in ovx rats, we evaluated in cycling rats the temporal relationship between MBH preproNPY mRNA levels and the preovulatory LH surge on the day of proestrus and compared that with diestrus II, concomitant with basal LH levels. PreproNPY mRNA levels in the MBH were measured by solution hybridization/RNAse protection assay, using a cRNA probe. On the day of diestrus II, preproNPY mRNA levels changed little between 1000 and 1800 h. Quite unexpectedly, preproNPY mRNA levels at 1000 h on proestrus were similar to diestrus II levels, despite additional exposure to ovarian steroids during this interval. However, from these low levels at 1000 h, the preproNPY mRNA profile displayed a biphasic rise. During the first phase, preproNPY mRNA rose significantly at 1200 h and remained elevated at 1300 and 1400 h concomitant with basal serum LH levels. Thereafter, a second rise in preproNPY mRNA began at 1500 h, peaked rapidly at 1600 h and declined significantly at 1800 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen-inducible neurotensin immunoreactivity in the preoptic area of the female rat

Neurotensin (NT) neurons in the rat preoptic area are implicated in female-specific regulation of reproduction. Estrogen markedly increases expression of mRNA encoding the neurotensin/neuromedin N (NT/N) precursor in several cell groups of the preoptic area, including the anteroventral periventricular nucleus, periventricular preoptic nucleus, and medial preoptic nucleus. In the present study, immunohistochemistry was performed on tissue from ovariectomized females with or without estradiol treatment to test the hypothesis that increased levels of NT accompany hormonal induction of NT/N mRNA in these cell group. Since colchicine treatment is required for visualization of NT-immunoreactive cell bodies, an additional objective of this study was to determine whether colchicine alters expression of NT/N mRNA in this area. Estradiol caused a pronounced increase in the number of NT-immunoreactive cell bodies in the anteroventral periventricular nucleus, as well as adjacent parts of the periventricular preoptic nucleus and medial preoptic nucleus. In the absence of colchicine, estradiol increased the number of NT-immunoreactive fibers in these same regions. Surprisingly, NT-immunoreactive cell bodies with intense staining were abundant in certain parts of the medial preoptic nucleus regardless of hormonal condition. NT-immunoreactive cell bodies were also numerous in certain regions where NT/N mRNA-expressing cells are scarce, and in two of these regions, the median preoptic nucleus and vascular organ of the lamina terminalis, estradiol substantially reduced the number of immunoreactive cell bodies. Treatment of ovariectomized females with colchicine induced expression of NT/N mRNA in the same regions where NT-immunoreactive cell bodies were unexpectedly numerous, thus providing a compelling explanation for the discordant distributions of the mRNA and peptide. Together with previous findings, the present results indicate that increased levels of NT accompany hormonal induction of NT/N mRNA in the anteroventral periventricular nucleus, as well as adjacent parts of the periventricular preoptic nucleus and medial preoptic nucleus. In other regions of the preoptic area, colchicine-inducible expression of NT/N mRNA confounds assessment of hormonal influences on NT synthesis. Multiple populations of neurons capable of NT synthesis can be distinguished in the rostral preoptic area on the basis of differential responsiveness to estrogen or colchicine, thereby providing additional evidence for functional heterogeneity among NT-synthesizing neurons in this region.

Ethanol acutely reduces LH and prolactin secretion: possible involvement by dopamine

Ethanol (ETOH) administered acutely to castrate male rats caused a decline in pituitary luteinizing hormone (LH) and prolactin (PRL) secretion. This was associated with an elevation in hypothalamic and median eminence stores of dopamine (DA) that was related to the dose of alcohol given. Pituitary stalk transection (PST) resulted in a significant rise in plasma PRL levels compared to sham control animals, which suggests that DA in the hypophysial portal blood exerted an inhibitory influence on pituitary PRL secretion. The DA agonist bromocriptine failed to alter mean plasma LH levels in stalk-transected rats. The ETOH-treated castrated rats showed a significant rise in circulating PRL after injection of the DA receptor antagonist haloperidol metabolite II (HAL), but the administration of the DA receptor agonist R(-)-apomorphine HCL (APO) caused plasma PRL to decline to near undetectable levels. Plasma LH levels remained unchanged in the HAL- and APO-treated rats and were similar to those of sham controls. These results suggest that lactotroph DA receptors were still functional. Thus our previous finding of ETOH-induced reduction on LH secretion may be attributable to an inhibitory effect by DA on the luteinizing hormone-releasing hormone (LHRH) peptidergic neurons rather than a direct inhibition by DA on the pituitary gonadotroph.

Sexually dimorphic distribution of neurotensin/neuromedin N mRNA in the rat preoptic area

Neurotensin release from estrogen-responsive neurons in the rostral preoptic area of the female rat may play an important role in triggering preovulatory secretion of gonadotropin-releasing hormone on proestrus. We investigated the possibility of sexually differentiated biosynthesis of neurotensin in the rostral preoptic area, using in situ hybridization histochemistry to detect neurotensin/neuromedin N (NT/N) mRNA in adult male rats and adult female rats at proestrus and the first day of diestrus. In sections through the anteroventral periventricular nucleus (AVPv), the number of labeled cells in proestrous females was four times that in males. Diestrus females exhibited half the number of labeled cells present at proestrus, and there was evidence for a significant correlation between circulating estradiol level and number of labeled cells in the AVPv. In the rostral portion of the medial preoptic nucleus (MPN), two contiguous groups of labeled cells were especially prominent. One group, in the medial half of the MPN, was located closer to the midline in females than in males and displayed greater labeling in males than in females. Furthermore, labeling in the rostral MPN was greater at proestrus than at diestrus. These results indicate that biosynthesis of neurotensin and neuromedin N in the rostral preoptic area may be sexually differentiated and, in the female, may vary across the estrous cycle in parallel with circulating estradiol levels, consistent with the view that neurotensin neurons in this area are involved in the regulation of preovulatory secretion of gonadotropin-releasing hormone. The sex- and region-specific expression of NT/N mRNA in the rostral preoptic area suggests functional heterogeneity of neurotensin neuronal populations in this area and implies complex regulation of NT/N gene expression in the rat brain.

Effect of 17β-estradiol on the preovulatory surge of LH in the bovine female

The hypothesis tested was that increasing concentration of 17beta-estradiol (E(2)) subsequent to luteolysis stimulates the preovulatory surge of LH and that a decline in E(2) after the initial rise is not necessary to trigger the preovulatory surge of LH in the bovine female. Beef cows were synchronized to Day 16 of the estrous cycle. At Hour 0, all cows were ovariectomized and received one of four E(2) treatments: 1) luteal phase E(2) (LE; n=5), 2) increasing then decreasing E(2) (DE; n=5), 3) increasing and subsequent maintenance of high E(2) (IE; n=4), and 4) no E(2) (NE; n=3). Cows in the LE group received one E(2) implant at Hour 0 which provided low concentrations of E(2). Cows in the DE group received one E(2) implant at 0, 8, 16, 24, 32 and 40 hours; implants were subsequently removed at 8-hour intervals, thus mimicking the preovulatory rise and fall of E(2). Cows in the IE group were treated with the same regimen of E(2) implants as cows of the DE group, except that no E(2) implants were removed. Blood samples were collected at Hour 0 and at hourly intervals from Hour 2 through 80, for serum LH and E(2) quantification. The number of cows responding with a surge of LH was 0/3, 0/5, 4/5 and 3/4 for the NE, LE, DE and IE treatments, respectively. The proportion of cows responding with an LH surge was different (P<0.01) when data for cows in the NE and LE groups were pooled and compared with the pooled data of cows in the DE and IE groups. The mean time of the LH surge was not different (P>0.80) for cows responding with an LH surge (DE and IE treatments). Thus, increased levels of E(2) greater than luteal phase concentrations are needed to initiate preovulatory surges of LH, and it appears that concentrations of E(2) need to reach a certain level but do not need to decrease after this initial rise to stimulate a surge release of LH.

Galanin: a hypothalamic-hypophysiotropic hormone modulating reproductive functions

Galanin (GAL) is widely distributed in the peripheral and the central nervous systems. In the brain, the highest GAL concentrations are observed within the hypothalamus and, particularly, in nerve terminals of the median eminence. This location, as well as GAL actions on prolactin, growth hormone, luteinizing hormone (LH), and LH-releasing hormone (LHRH) secretion, suggest the possibility that GAL may act as a putative hypothalamic-hypophysiotropic hormone. To establish this, GAL and LHRH levels were measured in hypophyseal portal plasma samples using specific radioimmunoassays. Rat galanin (rGAL) concentrations in portal blood were approximately 7-fold higher than those observed in peripheral plasma in male and female (estrus, diestrus) rats, indicating an active secretory process of rGAL into the portal vasculature. Frequent (10 min) sampling revealed that rGAL and LHRH were secreted into the portal circulation in a pulsatile manner with a pulse frequency of one pulse per hour. Interestingly, both hormone series depicted a high degree of coincident episodes. In fact, the probability of random coincidence, calculated by the algorithm HYPERGEO, was less than 0.01. Moreover, the retrograde tracer Fluoro-Gold, when given systemically, was taken up by GAL neurons in the hypothalamus, including a subset of neurons expressing rGAL and LHRH, strengthening the notion of the existence of a GAL neuronal system connected to the hypophyseal portal circulation. These observations reinforce the concept that GAL regulates pituitary hormone secretion. To analyze this in further detail, the effects of rGAL on LH secretion were evaluated under basal and stimulated conditions. rGAL induced a small but dose-dependent increase in LH secretion from cultured, dispersed pituitary cells. Interestingly, rGAL enhanced the ability of LHRH to stimulate LH release. The tight link between GAL and LHRH neuronal systems is strengthened by the observation that during the estrous cycle of the rat, rGAL and LHRH contents in the median eminence show an identical profile (r = 1.00). These data indicate that GAL should be considered as a hypothalamic-hypophysiotropic hormone and as an important neuromodulator of LHRH secretion and action. The colocalization and cosecretion of GAL and LHRH and the cooperative action at the level of the anterior pituitary afford important evidence for the functional significance of coexistence of neurotransmitters in neurons of the central nervous system.

Endopeptidase-24.15 in rat hypothalamic/pituitary/gonadal axis

Endopeptidase-24.15 (E.C. 3.4.24.15; EP-24.15) cleaves several substrates found in the hypothalamic/pituitary/gonadal axis, including gonadotropin-releasing hormone (GnRH) and the opioid peptides of the dynorphin family. We have examined the activity of EP-24.15 in these tissues as a function of maturation, of the estrous cycle, and in response to ovariectomy and estrogen replacement. A developmental regulation of EP-24.15-specific activity is apparent in anterior pituitary, in hypothalamus, and in the gonads. EP-24.15 is increased in the preoptic area and is decreased in the anterior pituitary in both male and female rats prior to puberty. The specific activity of EP-24.15 was increased following ovariectomy in the anterior pituitary and within medial and lateral preoptic nuclei. Testicular specific activity of EP-24.15 increased with age in a linear fashion, while ovarian EP-24.15 activity increased immediately prior to puberty, but returned to prepubertal levels by 65 days of age. The relevance of EP-24.15 to the metabolism of specific peptides is discussed.

The effect of prior surgical disconnection of the anterior mediobasal hypothalamus on LHRH release in vitro

The degenerative responses of luteinizing hormone-releasing hormone (LHRH)-containing neurons within the mediobasal hypothalamus (MBH) after knife cut lesions (FC) made in the frontal plane of the retrochiasmatic hypothalamus include a reduced number of LHRH-immunoreactive (ir) nerve terminals in the median eminence, reduction in LHRH content of the MBH and growth of novel irLHRH-containing neural processes into FC scar tissue. We have now investigated basal and secretogogue-evoked LHRH release in vitro from the preoptic area-MBH (POA-MBH) of adult male rats at 10 or 60 days after FC. Basal LHRH release rate (P less than 0.05) and total (P less than 0.01) amount released 60 days after FC were reduced when compared to control (CONT) hypothalami, but not shams. A 30 min pulse of naloxone (NAL, 1 mg/ml) stimulated greater than 2-fold relative increase in LHRH release for all groups; however, the total amount of LHRH released by FC hypothalami was less (P less than 0.05) than that of CONT, but not sham POA-MBHs. Although exposure to elevated KCl significantly increased (P less than 0.01) LHRH release for all 3 groups, the FC secretory response was less than that of both CONT (P less than 0.05) and sham (P less than 0.01) groups. In the second experiment single POA-MBH were perifused at 10 days (sham and FC) or 60 days (CONT, sham and FC) after surgery.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptology of luteinizing hormone-releasing hormone neurons in the preoptic area of the male rat: effects of gonadectomy

Ultrastructural analysis of the synaptic input to luteinizing hormone-releasing hormone neurons has previously shown that in male rats these cells acquire an increased density of innervation with increasing age [Witkin J. W. (1987) Neuroscience 22, 1003-1013]. To determine if this aging phenomenon might be due to changes in the steroid environment, we examined luteinizing hormone-releasing hormone neurons, using methods identical to the earlier study, in sham operated and 1 day and 4 week castrated male rats. The density of synaptic input to luteinizing hormone-releasing hormone neurons did not vary among the three experimental groups. Furthermore, there were no differences in the relative numbers of various morphological categories of synaptic boutons (characterized by vesicle type: clear vs dense and round vs pleomorphic) among the groups. Long term castration did result in a slight decrease in immunocytochemically detectable luteinizing hormone-releasing hormone neuron numbers. These results suggest that gonadal steroid deprivation does not alter the total density or morphological characteristics of synaptic input to the luteinizing hormone-releasing hormone neuron in the male rat.

Acute ethanol treatment lowers hypophyseal portal plasma luteinizing hormone-releasing hormone (LH-RH) and systemic plasma LH levels in orchidectomized rats

Acute ethanol (EtOH) treatment resulted in a significant decline in hypophyseal portal plasma levels of luteinizing hormone-releasing hormone (LH-RH) in orchidectomized rats. This was accompanied by a similar decrement in circulating luteinizing hormone (LH) without, however, modifying plasma follicle-stimulating hormone (FSH) levels at time when peak blood alcohol concentrations occurred. Thus, acute alcohol intoxication lowers circulating LH levels primarily by impairing hypothalamic neuronal secretion of LH-RH into the portal blood.

The female rat as a model describing patterns of pulsatile LH secretion during puberty and their control by melatonin

A characteristic of the advent of final stages of sexual maturation in the rat, the equivalent of puberty in the human, is the development of a specific pattern of pulsatile GnRH secretion, reflected by an equivalent pattern of LH secretion. Many hypotheses have been put forward to explain these last changes, including a change at the hypothalamic level and a modification of the ovary. These models are developed to include melatonin, an indoleamine produced by the pineal gland, which appears to play a role in the establishment of the final pattern of LH secretion, thus affecting both vaginal opening and subsequent estrous cycles. Prior to first ovulation, pulsatile release of LH acquires at least two types of patterns, one with low-amplitude, high-frequency pulses, announcing a pattern observed in the follicular phase of primates, and one with high-amplitude, low-frequency pulses, mimicking what is seen in the luteal phase of primates. From the results presented, it is postulated that chronic melatonin administration increases the number of patterns typical of luteal phases, and reduces those of the follicular type, resulting in a decrease frequency of LH pulses and longer intervals between estrous cycles.

Neonatal testosterone modifies LH secretion in the adult female rat by altering the opioid-noradrenergic interaction in the medial preoptic area

Noradrenergic-opioid interaction in the medial preoptic area (MPOA) was examined in ovariectomised adult rats which were oestrogen-treated and had been injected neonatally with either testosterone propionate (TP) or vehicle (oil). The first experiment involved electrical stimulation of the ventral noradrenergic tract (VNAT) in anaesthetised rats. Blood samples were collected before and after the stimulation to determine plasma levels of luteinising hormone (LH). Approximately half of the animals received naloxone i.v. 15 min before the onset of stimulation. In all groups, stimulation of VNAT elicited a significant increase in plasma LH concentration. However, pretreatment with naloxone in androgenised rats, but not in oil-treated animals, almost doubled the LH increment due to stimulation. Naloxone had no effect on plasma LH concentrations in unstimulated control rats. In the second experiment hypothalamic slices containing the MPOA were preincubated with [3H]noradrenaline [( 3H]NA) and then subjected to electrical field stimulation under the conditions of (a) no drug added and (b, c) morphine superfusion without and with naloxone. The opioid agonist morphine significantly reduced the net release of [3H]NA in normal and TP-treated female rats. Addition of equimolar naloxone reversed this effect in normal females, whereas in the androgenised group it not only reversed this effect but elicited a significant increase in [3H]NA release. From these data we conclude that (1) neonatal testosterone treatment alters noradrenergic-opioid interaction regulating LH secretion in adult females and (2) the site of this change may be the presynaptic opioid input to the noradrenergic terminals in the MPOA.

Norepinephrine stimulates LH-RH secretion into the hypophysial portal blood of the rat

The present study was designed to investigate the effect of intracerebroventricular infusion of norepinephrine (NE) on the secretion of luteinizing hormone-releasing hormone (LH-RH) into the hypophysial portal blood of steroid-primed ovariectomized rats. Saline infusion into the third ventricle caused no significant change in LH-RH levels. NE infusion (20 micrograms) resulted in a significant release of LH-RH (p less than 0.05) into the portal blood 10-30 min later. This endogenous LH-RH was similar to synthetic LH-RH when characterized by thin-layer chromatography. LH secretion in similarly treated rats but with intact portal vessels, also was significantly elevated (p less than 0.05) at 20 and 40 min after the start of NE infusion. These results show that NE stimulated the secretion of LH-RH into the hypophysial portal blood and this correlated with an enhanced release of LH.

LH-RH in picomole concentrations evokes excitation and inhibition of rat arcuate neurones in vitro

A medial sagittal brain slice was developed, which enabled electrophysiological recording from spontaneously active neurones adjacent to the infundibular recess of the rat arcuate nucleus. Luteinizing hormone-releasing hormone (LH-RH) (10 nM-10 pM) significantly altered the frequency of 21 out of 31 units tested, exciting 14 and inhibiting 7 others. The excitatory responses were often not maintained during the exposure to LH-RH but either returned to pre-exposure frequency or displayed an inhibition of discharge. A neural model incorporating recurrent inhibition of LH-RH-excited neurones is proposed to explain these responses, thought to be endogenously evoked by collateral innervation of the arcuate nucleus by medial preoptic neurones projecting to the median eminence.