Testosterone stimulates LH-RH-like mRNA level in the rat hypothalamus

The effects of supraphysiological levels of testosterone on neural networks upstream of gonadotropin-releasing hormone neurons

OBJECTIVES

Several pathological conditions are associated with hyper-production of testosterone; however, its impacts are not well understood. Hence, we evaluated the effects of supraphysiological levels of testosterone on gonadotropin-releasing hormone (GnRH) system in the hypothalamus of male rats. Also, we assessed the expression of two excitatory (kisspeptin and neurokinin-B) and two inhibitory (dynorphin and RFamide-related-peptide) neuropeptides upstream of GnRH neurons as possible routes to relay androgen information.

MATERIALS AND METHODS

Gonadectomized (GDX) male rats received single injection of 100, 250 or 500 mg/kg testosterone undecanoate and three weeks later, posterior (PH) and anterior (AH) hypothalamus was dissected for evaluation of target genes using quantitative RT-PCR.

RESULTS

We found that GnRH mRNA in the PH was high in GDX rats and 500 mg/kg testosterone reduced GnRH level expression. Finding revealed extremely high level of Kiss1 mRNA in the PH of GDX rats. However, in GDX rats treated with different levels of testosterone, Kiss1 expression was not significantly different than control. We also found that testosterone replacement increased the Kiss1 mRNA level in the AH. Moreover, neurokinin-B mRNA level in PH of GDX rats was similar to control. However, excess testosterone levels were effective in significantly inducing the down-regulation of neurokinin-B expression. The basal level of dynorphin mRNA was increased following testosterone treatments in the AH, where we found no significant difference in the level of RFamide-related-peptide mRNA between the experimental groups.

CONCLUSION

Excess levels of testosterone could act differently from its physiological concentration to regulate hypothalamic androgen sensitive neurons to control GnRH cell.

Effects of active immunization against GnRH versus surgical castration on hypothalamic-pituitary function in boars

The objective was to compare effects of anti-GnRH immunization (immunocastration) versus surgical castration on hypothalamic-pituitary function in boars. Thirty-six boars were randomly divided into three groups (n = 12/group): control, surgically castrated, or immunized against GnRH at 10 wk of age (boostered 8 wk later). Compared to intact boars, immunocastration reduced (P < 0.05) serum concentrations of LH, FSH, testosterone and inhibin B and caused severe testicular atrophy, whereas surgical castration increased (P < 0.05) serum concentrations of LH and FSH. Both immunocastration and surgical castration consistently reduced hypothalamic GnRH synthesis, with decreased (P < 0.05) mRNA expressions of GnRH, GnRH up-stream gatekeeper genes kiss1 and its receptor (GPR54), and androgen receptor in the hypothalamic arcuate nucleus (ARC) and anteroventral periventricular nucleus (AVPV), as well as GnRH content in the median eminence. Inconsistently, mRNA expressions of gonadotropin-inhibitory hormone (GnIH) in ARC and AVPV as well as its receptor (GPR147) in pituitary were selectively reduced (P < 0.05), but mRNA expressions of estrogen receptor alpha and aromatase (CPY17A1) in pituitary were selectively increased (P < 0.05) in surgical castrates. In response to selectively attenuated suppressive signaling from GnIH and testosterone, mRNA expressions of GnRH receptor (GnRHR), LH-β and FSH-β in pituitary were increased (P < 0.05) in surgical castrates, whereas these pituitary gene expressions were decreased (P < 0.05) in immunocastrates, due to loss of hypothalamic GnRH signaling. We concluded that immunocastration and surgical castration consistently reduced hypothalamic GnRH synthesis due to a testosterone deficiency disrupting testosterone-Kisspeptin-GPR54-GnRH signaling pathways. Furthermore, selectively attenuated GnIH and testosterone signaling in the pituitary increased gonadotropin production in surgical castrates.

Gonadotropin-releasing hormone plasticity: a comparative perspective

Gonadotropin-releasing hormone 1 (GnRH1) is a key regulator of the reproductive neuroendocrine system in vertebrates. Recent developments have suggested that GnRH1 neurons exhibit far greater plasticity at the cellular and molecular levels than previously thought. Furthermore, there is growing evidence that sub-populations of GnRH1 neurons in the preoptic area are highly responsive to specific environmental and hormonal conditions. In this paper we discuss findings that reveal large variation in GnRH1 mRNA and protein expression that are regulated by social cues, photoperiod, and hormonal feedback. We draw upon studies using histochemistry and immediate early genes (e.g., c-FOS/ZENK) to illustrate that specific groups of GnRH1 neurons are topographically organized. Based on data from diverse vertebrate species, we suggest that GnRH1 expression within individuals is temporally dynamic and this plasticity may be evolutionarily conserved. We suggest that the plasticity observed in other neuropeptide systems (i.e. kisspeptin) may have evolved in a similar manner.

Effects of central administration of gonadotropin-releasing hormone agonists and antagonist on elevated plus-maze and social interaction behavior in rats

The correlation between neuronal mechanism of anxiety and neuroanatomic expression/neuromodulatory role of gonadotropin-releasing hormone (GnRH), points to a role of GnRH in the modulation of anxiety. Therefore, we investigated the influence of GnRH agonists and antagonist on the anxiety-like behavior of rats in the elevated plus-maze and social interaction tests. GnRH agonists, leuprolide [100 or 200 ng/rat, intracerebroventricularly (i.c.v.)] or 6-D-tryptophan luteinizing hormone-releasing hormone (400 ng/rat, i.c.v.), significantly increased percentage of open arms entries, time spent in open arms, and time spent in social interaction. The observed anxiolytic effect of these agents was comparable with diazepam (0.5-1.0 mg/kg, intraperitoneally). Treatment with a GnRH antagonist [pGlu-D-Phe-Trp-Ser-Tyr-D-Ala-Leu-Arg-Pro-Gly-NH2, (100 ng/rat, i.c.v.)], significantly reduced percentage of open arm indices and decreased time spent in social interaction, indicating an anxiogenic-like effect. Further, castrated rats exhibited anxiogenic-like behavior in these tests, which was significantly attenuated by leuprolide (200 ng/rat, i.c.v.) or 6-D-tryptophan luteinizing hormone-releasing hormone (400 ng/rat, i.c.v.), indicating the noninvolvement of peripheral sex hormone in their anxiolytic-like effect, at least in castrated rats. In conclusion, this study indicated a putative role of GnRH in the control of anxiety, and further adds to the importance of investigating the possible role of the hypothalamus-pituitary-gonadal axis in regulating the anxiety-related disorders arising out of hypothalamus-pituitary-adrenal axis dysregulation.

Gonadotropin-releasing hormone agonist blocks anxiogenic-like and depressant-like effect of corticotrophin-releasing hormone in mice

Corticotrophin-releasing factor (CRF) is reported to inhibit the release of gonadotropin-releasing hormone (GnRH). In addition to the endocrine effects, GnRH is reported to influence the behavior via its neuronal interactions. We therefore, hypothesized that anxiety and depression produced by CRF could be also subsequent to the decrease in GnRH. To support such possibility, we investigated the influence of GnRH agonists on CRF or CRF antagonist induced changes in social interaction time in social interaction test, and immobility time in forced swim test in mice, as the indices for anxiety and depression, respectively. Results indicated that GnRH agonists [leuprolide (20-80 ng/mouse, i.c.v.), or d-Trp-6-LHRH (40-160 ng/mouse, i.c.v.)] dose dependently increased social interaction time and decreased immobility time indicating anxiolytic- and antidepressant-like effect, respectively. Such effects of GnRH agonists were even evident in castrated mice, which suggest that these effects were unrelated to their endocrine influence. Administration of CRF (0.1 and 0.3 nmol/mouse, i.c.v.) produced just opposite effects as that of GnRH agonist on these parameters. Further, it was seen that pretreatment with leuprolide (10 or 20 ng/mouse, i.c.v.) or d-Trp-6-LHRH (20 or 40 ng/mouse, i.c.v.) dose dependently antagonized the effects of CRF (0.3 nmol/mouse, i.c.v.) in social interaction and forced swim test. CRF antagonist [alpha-Helical CRF (9-41), (1 or 10 nmol/mouse, i.c.v.)] was found to exhibit anxiolytic- and antidepressant-like effect, and its sub-effective dose (0.1 nmol/mouse, i.c.v.) when administered along with sub-threshold dose of leuprolide (10 ng/mouse, i.c.v.), or d-Trp-6-LHRH (20 ng/mouse, i.c.v.) also produced significant anxiolytic- and antidepressant-like effect. These observations suggest reciprocating role of GnRH in modulating the CRF induced anxiogenic- and depressant-like effects.

Growth factors and steroid hormones: a complex interplay in the hypothalamic control of reproductive functions

The mechanisms through which LHRH-secreting neurons are controlled still represent a crucial and debated field of research in the neuroendocrine control of reproduction. In the present review, we have specifically considered two potential signals reaching these hypothalamic neurons: steroid hormones and growth factors. Examples of the relevant physiological role of the interactions between these two families of biologically acting molecules have been provided. In many cases, these interactions occur at the level of hypothalamic astrocytes, which are presently accepted as functional partners of the LHRH-secreting neurons. On the basis of the observations here summarized, we have formulated the hypothesis that a functional co-operation of steroid hormones and growth factors occurring in the hypothalamic astrocytic compartment represents a key factor in the neuroendocrine control of reproductive functions.

Redefining gonadotropin-releasing hormone (GnRH) cell groups in the male Syrian hamster: testosterone regulates GnRH mRNA in the tenia tecta

Gonadotropin-releasing hormone (GnRH) regulates the production of testosterone via the hypothalamic-pituitary-gonadal axis and testosterone, in turn, regulates the GnRH system via negative feedback. We compared testosterone regulation of GnRH mRNA expression in four anatomically defined GnRH cell groups in juvenile and adult male Syrian hamsters, including a rostral population of GnRH cells in the tenia tecta. In situ hybridization histochemistry (ISHH) was used to measure GnRH mRNA in brains from castrated juveniles and adults treated with 0 mg or 2.5 mg testosterone pellets for one week. ISHH was performed on coronal sections using a 35S-cRNA probe generated from Syrian hamster GnRH cDNA. Testosterone treatment resulted in a significant reduction in mean area of GnRH neurones covered by silver grains within the tenia tecta, but only a trend toward decreased GnRH mRNA in the diagonal band of Broca/organum vasculosum of the lamina terminalis (DBB/OVLT), medial septum (MS), and caudal preoptic area (cPOA). The effects of testosterone were independent of age. Frequency distribution analyses unveiled a significant reduction in the number of heavily labelled cells following testosterone treatment within the tenia tecta and MS. Simple regression analyses revealed a significant positive correlation between plasma luteinizing hormone concentrations and GnRH mRNA only in the tenia tecta. These data indicate that, overall, GnRH mRNA is modestly reduced by testosterone, and the most robust attenuation of GnRH mRNA occurs within the tenia tecta. This is the first report to link mechanisms of steroid negative feedback with tenia tecta GnRH neurones, providing a new focus for investigating brain region-specific steroidal regulation of GnRH synthesis.

Effects of orchidectomy on levels of the mRNAs encoding gonadotropin-releasing hormone and other hypothalamic peptides in the adult male rhesus monkey (Macaca mulatta)

The testicular regulation of luteinizing hormone (LH) secretion in the adult rhesus monkey is mediated by an indirect action of testosterone to decelerate pulsatile gonadotrophin releasing hormone (GnRH) release. Whether this negative feedback action of testosterone involves regulation of GnRH gene expression is unknown. Therefore, the effect of bilateral orchidectomy on hypothalamic levels of the mRNA encoding this hypophysiotropic factor was examined. The feedback action of testosterone is generally considered to be mediated through non-GnRH cells, and the present experiment provided the opportunity to also examine testicular influences on mRNAs encoding putative hypothalamic factors implicated in the testicular regulation of LH secretion. Adult male rhesus monkeys were orchidectomized (n=5) or sham-orchidectomized (n=5) and killed 6 weeks later, after a castration-induced hypersecretion of LH was established. Separate preoptic and mediobasal hypothalamus containing areas were collected, and levels of GnRH mRNA, as well as those of mRNAs encoding pro-opiomelanocortin (POMC), the gamma-aminobutyric acid (GABA) synthesizing enzymes (glutamic acid decarboxylase 65 and 67; GAD65 and GAD67, respectively), neuropeptide Y, galanin and transforming growth factor (TGF)alpha, were quantified using RNase protection assay. Values were expressed in terms of optical density relative to that of cyclophilin mRNA levels. Bilateral orchidectomy produced a significant increase in GnRH mRNA levels that was restricted to the mediobasal hypothalamus and that was associated with a significant decrease in POMC, GAD65 and GAD67 mRNA levels in this region of the hypothalamus. In contrast, neuropeptide Y, galanin and TGFalpha mRNA levels were not affected by castration. These results indicate that, in the monkey, the deceleration of pulsatile GnRH release that is imposed by the testis, and presumably mediated by testosterone, is associated with a concomitant down regulation of GnRH gene expression in the mediobasal hypothalamus. They also support the notion that this hypothalamic feedback action may be mediated by POMC-and GABA-producing neurones in the mediobasal hypothalamus.

Effects of castration and chronic steroid treatments on hypothalamic gonadotropin-releasing hormone content and pituitary gonadotropins in male wild-type and estrogen receptor-alpha knockout mice

Testicular androgens are integral components of the hormonal feedback loops that regulate circulating levels of LHbeta and FSH. The sites of feedback include hypothalamic areas regulating GnRH neurons and pituitary gonadotropes. To better define the roles of androgen receptor (AR), estrogen receptor-alpha (ERalpha), and estrogen receptor-beta (ERbeta) in mediating feedback effects of sex steroids on reproductive neuroendocrine function, we have determined the effects of castration and steroid replacement therapy on hypothalamic GnRH content, pituitary LHbeta and FSHbeta messenger RNA (mRNA) levels, and serum gonadotropins in male wild-type (WT) and estrogen receptor-alpha knockout (ERKO) mice. Hypothalami from intact WT and ERKO males contained similar amounts of GnRH, whereas castration significantly reduced GnRH contents in both genotypes. Replacement therapy with estradiol (E2), testosterone (T), or dihydrotestosterone (DHT) restored hypothalamic GnRH content in castrated (CAST) WT mice; only the androgens were effective in CAST ERKOs. Analyses of pituitary function revealed that LHbeta mRNA and serum LHbeta levels in intact ERKOs were 2-fold higher than those in intact WT males. Castration increased levels of LHbeta mRNA (1.5- to 2-fold) and serum LHbeta (4- to 5-fold) in both genotypes. Both E2 and T treatments significantly suppressed LHbeta mRNA and serum LH levels in CAST WT males. However, E2 was completely ineffective, and T was only partially effective in suppressing these two indexes in the CAST ERKO males. DHT treatments stimulated a 50% increase in LHbeta mRNA and serum LH levels in WT males, whereas serum LH was significantly suppressed in DHT-treated ERKO males. Although the pituitaries from intact ERKO males contained similar amounts of FSHbeta mRNA, serum FSH levels were 20% higher than those in the intact WT males. Castration increased FSHbeta mRNA levels only in WT males, but significantly increased serum FSH levels in both genotypes. Both E2 and T treatments significantly suppressed serum FSH in CAST WT males, whereas only E2 suppressed FSHbeta mRNA. DHT treatments of CAST WT mice stimulated a small increase in serum FSH, but failed to alter FSHbeta mRNA levels. None of the steroid treatments exerted any significant effect on FSHbeta mRNA or serum FSH levels in CAST ERKOs. These data suggest that hypothalamic GnRH contents can be maintained solely through AR signaling pathways. However, normal regulation of gonadotrope function requires aromatization of T and activation of ERalpha signaling pathways in the gonadotrope. In addition, serum FSH levels in male ERKOs appear to be regulated largely by nonsteroidal testicular factors such as inhibin. Finally, these data suggest that hypothalamic ERbeta may not be involved in mediating the negative feedback effects of T on serum LH and FSH in male mice.

Brain region-specific regulation of luteinizing hormone-releasing hormone messenger ribonucleic acid in the male ferret: interactions between pubertal maturation and testosterone

This study examined the regulation of LHRH messenger RNA (mRNA) during pubertal maturation and by testosterone in male ferrets. Prepubertal and postpubertal ferrets were either intact or were castrated and treated with daily injections of oil or 5 mg/kg testosterone propionate for 14 days. In situ hybridization for LHRH mRNA was performed using an 35S-labeled 48-base oligonucleotide complementary to the human LHRH-coding region. Computerized image analysis was performed on cells in the preoptic area, retrochiasmatic area, arcuate nucleus (ARC), and median eminence; cells were classified as labeled if the number of pixels representing silver grains over the cell was 5 or more times the number of background silver grain pixels. Both pubertal maturation of intact males and castration of prepubertal males resulted in an increase in the number of labeled cells in the ARC. These effects were not observed in any of the other three brain regions, suggesting that ARC LHRH-producing neurons are of primary importance in the presumed increase in LHRH release that occurs as a consequence of either pubertal maturation or castration of prepubertal males. Castration of adults did not increase the number of labeled cells in any brain area, but resulted in an increase in silver grains per labeled cell only in the preoptic area. Thus, LHRH mRNA is regulated during puberty primarily in the ARC, and the particular cell group in which LHRH mRNA is most strongly regulated by testosterone changes with pubertal maturation.

Regulation of preoptic area gonadotrophin-releasing hormone (GnRH) mRNA expression by gonadal steroids in the long-term gonadectomized male rat

Testosterone exerts important feedback actions on the hypothalamus and pituitary of the male rat to control reproductive hormone secretion. Marked fluctuations occur in plasma-luteinizing hormone (LH) concentrations, hypothalamic gonadotrophin-releasing hormone (GnRH) content and GnRH mRNA expression following castration and it appears as though a stable post-castration equilibrium is not attained until 3-4 weeks after gonadectomy. In the present study, we have investigated the effects of long-term (7 week) gonadectomy on GnRH mRNA expression in the male rat and determined whether estrogen or androgen receptor-mediated mechanisms are involved in regulating its expression. Accordingly, in situ hybridization was undertaken using a 35S-labelled antisense oligonucleotide probe complementary to bases 102-149 of the rat GnRH cDNA to quantify cellular GnRH mRNA expression in the medial septum (MS), diagonal band of Broca (DBB) and rostral preoptic area (rPOA) of intact males, rats gonadectomized for 7 weeks and gonadectomized animals implanted with silastic capsules containing testosterone (T), estrogen (E) or dihydrotestosterone (DHT). We found no difference between any of the treatment groups in the number of cells expressing GnRH mRNA in the MS/DBB or rPOA. Similarly, the GnRH mRNA content of cells in the MS/DBB was not different between the treatment groups. In contrast, cellular GnRH mRNA expression in the rPOA was elevated 7 weeks following castration (intact: 0.95 +/- 0.07 silver grains/microm2/cell; gonadectomized: 1.26 +/- 0.03; mean +/- S.E.M., P < 0.05) and this was restored to intact levels by either T (1.02 +/- 0.07) or E (1.02 +/- 0.08) treatment. DHT replacement had no effect on cellular levels of GnRH mRNA in gonadectomized rats (1.26 +/- 0.03). Frequency analysis of relative GnRH mRNA expression/cell showed that the rostral preoptic GnRH population responded to the steroid treatment in an homogeneous manner. These results show that GnRH mRNA expression is elevated specifically within the rPOA of the long-term gonadectomized male rat when LH secretion has stabilized at a constant high level. Further, we show that the gonadal steroid regulation of cellular GnRH mRNA content at such time occurs only through an estrogen receptor-mediated pathway.

Regulation of gonadotropin-releasing hormone gene expression in vivo and in vitro

The pulsatile release of gonadotropin-releasing hormone (GnRH) into the portal vasculature is responsible for the maintenance of reproductive function. Levels of GnRH decapeptide available for this process can be regulated at transcriptional, posttranscriptional, and posttranslational levels. In the immortalized neuronal GT1 cell lines which synthesize and secrete GnRH, regulation of GnRH biosynthesis has been studied using activators of the protein kinase A (PKA), protein kinase C (PKC), and calcium second messenger systems. These substances, while stimulating GnRH release, cause a universal inhibition of all biosynthetic indices measured to date, including decreases in transcription of the proGnRH gene, GnRH mRNA levels, mRNA stability, and translational efficiency. In contrast, in the animal, the mechanism for the regulation of GnRH gene expression appears to be primarily posttranscriptional, since changes in GnRH mRNA levels often occur in the absence of changes in GnRH primary transcript levels an index of GnRH gene transcription. For example, GnRH mRNA levels increase in response to stimulation with glutamate analogs, while GnRH primary transcript levels are unchanged. However, parallel changes in GnRH mRNA and primary transcript have been observed on proestrus prior to the LH/GnRH surge, suggesting that the regulation of GnRH mRNA levels in vivo involves a complex interplay of transcriptional and posttranscriptional processes.

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

Influence of testosterone on LHRH release, LHRH mRNA and proopiomelanocortin mRNA in male sheep

The mechanism whereby testosterone (T) reduces pulsatile LHRH and LH release is unknown. We tested the hypothesis that hypothalamic levels of LHRH mRNA decrease and proopiomelanocortin (POMC) mRNA increase coincident with reduced LHRH release induced by either long-term or short-term T treatment in male sheep. Experiment 1 examined the effect of long-term T exposure on LHRH and LH release and LHRH and POMC mRNA levels. Yearling Suffolk rams were castrated and assigned to one of four treatments: 1) castrated (n = 4); 2) castrated, portal cannula (n = 5); 3) castrated+T (n = 4) and 4) castrated+T, portal cannula (n = 4). T-treated males received ten 10-cm silastic T-implants immediately after castration. Surgical placement of devices for collecting hypophyseal-portal blood occurred 2 to 3 months after castration. Seven to 10 days after surgery, blood samples were collected at 10-min intervals for 8 h from portal cannulated males or for 5 h from non-cannulated males to assess pulsatile LHRH and/or LH release. Immediately after blood sample collection, hypothalamic tissue was collected for in situ measurement of LHRH or POMC mRNA. T-treatment decreased (P < 0.01) mean LHRH and LH and decreased (P < 0.01) LHRH and LH pulse frequency. T did not significantly affect (P > 0.10) silver grain area per LHRH neuron, but decreased (P < 0.01) silver grain area per POMC neuron. Portal cannulation tended to decrease (P = 0.057) silver grain area per LHRH neuron without significantly affecting (P > 0.10) LHRH cell numbers while reducing (P < 0.01) silver grain area per POMC neuron and POMC cell numbers. A second experiment examined the effect of 72 h of T-infusion on LHRH and POMC mRNA levels. Castrated yearling males were assigned to receive either vehicle (n = 4) or T (768 ug/kg/day; n = 4). Blood samples were collected at 10 min intervals for 4 h prior to and during the final 4 h of infusion. Infusion of T decreased (P < 0.01) mean LH and LH pulse frequency. T did not significantly affect (P > 0.10) silver grain area per LHRH neuron or LHRH cell numbers. T reduced (P < 0.01) silver grain area per POMC neuron without affecting (P > 0.10) POMC cell number. We reject our hypothesis and conclude that reduced LHRH or heightened POMC gene expression are not mechanisms whereby T reduces pulsatile LHRH release in male sheep.

Hypertrophy of gonadotropin releasing hormone-containing neurons after castration in the teleost, Haplochromis burtoni

In the African cichlid fish, Haplochromis burtoni, males are either territorial or nonterritorial. Territorial males suppress reproductive function in the nonterritorial males, and have larger gonads and larger gonadotropin-releasing hormone- (GnRH) containing neurons in the preoptic area (POA). We describe an experiment designed to establish the causal relationship between large GnRH neurons and large testes in these males by determining the feedback effects of gonadal sex steroids on the GnRH neurons. Territorial males were either castrated or sham-operated, 4 weeks after which they were sacrificed. Circulating steroid levels were measured, and the GnRH-containing neurons were visualized by staining sagittal sections of the brains with an antibody to salmon GnRH. The soma areas of antibody-stained neurons were measured with a computer-aided imaging system. Completely castrated males had markedly reduced levels of circulating sex steroids [11-ketotestosterone (11KT) and testosterone (T)], as well as 17 beta-estradiol (E2). POA GnRH neurons in castrates showed a significant increase in mean soma size relative to the intact territorial males. Hence, in mature animals, gonadal steroids act as a brake on the growth of GnRH-containing neurons, and gonadal products are not responsible for the large GnRH neurons characteristic of territorial males.

Localization of oestrogen receptors in preoptic neurons containing neurotensin but not tyrosine hydroxylase, cholecystokinin or luteinizing hormone-releasing hormone in the male and female rat

The neurochemical identity of preoptic neurons containing oestrogen receptors was investigated in the male and female rat using a sequential double-staining immunocytochemistry procedure. Single-immunostaining revealed large populations of cells with nuclear immunoreactivity to the oestrogen receptor in the medial preoptic area of the male and female rat. Optimal double-staining of sections for the oestrogen receptor and one of several neuropeptides or tyrosine hydroxylase, was achieved with short-term (two- to four-day) gonadectomized rats treated with colchicine where necessary. Neurotensin-immunoreactive cells were distributed in a sexually dimorphic manner in the region of the anteroventral preoptic nucleus and exhibited oestrogen receptor immunoreactivity in both sexes. Double-labelled cells in this area of the female rat comprised 50% and 11% of the total neurotensin- and oestrogen receptor-containing cell populations, respectively, compared with 25% and 4% in the male (P less than 0.01). The numbers of neurotensin-immunoreactive cells in the region of the medial preoptic nucleus were similar in male and female rats with double-labelled cells making up 20-38% and 3-5% of the total numbers of cells containing neurotensin and oestrogen receptors, respectively, in both sexes. Neurons immunoreactive for tyrosine hydroxylase were distributed in a gender-specific manner within the anterior periventricular area but were not immunoreactive for the oestrogen receptor in either sex. Following colchicine treatment, cholecystokinin-immunoreactive cells were identified predominantly within periventricular regions of the preoptic area and similarly, did not possess immunoreactivity to the oestrogen receptor in either the male or the female rat. Neurons containing luteinizing hormone-releasing hormone were found immediately lateral to the cell populations containing oestrogen receptors and immunoreactivity to the oestrogen receptor was not identified within any neurons containing luteinizing hormone-releasing hormone in either the male or female rat. The absence of oestrogen receptor immunoreactivity in neurons containing tyrosine hydroxylase, cholecystokinin or luteinizing hormone-releasing hormone suggests that gonadal steroids acting through this receptor do not influence these cells directly in either sex. In particular, it appears that gender-specific patterns of luteinizing hormone secretion cannot be attributed to sex differences in oestrogen receptor localization within luteinizing hormone-releasing hormone neurons. These experiments also show that the sexually dimorphic neurotensin neurons in the preoptic area possess oestrogen receptors and that female rats have larger number of neurons co-localizing neurotensin and oestrogen receptors. As such, these neurons may be involved in mediating sex-specific actions of the gonadal steroids in the preoptic area.

Effects of castration on LH-RH patterns in intrahypophysial microdialysates

A microdialysis system was used to monitor LH-RH patterns in the extracellular fluid of the adenohypophysis of testes-intact and short-term castrate rats. Male rats received guide cannulae implants fitted with stylets that extended into the anterior pole of the anterior pituitary gland. At the same time, animals were either castrated or received sham surgeries. On day 4 following surgeries, microdialysis probes were inserted into the guide cannulae and artificial CSF was pumped through the system at a flow rate of 2.5 microliters/min. Continuous samples were obtained from each animal over 5- or 10-min intervals throughout 4-7 sessions. Placements of probe tips were verified by histological examination of stained tissue sections. In vitro tests of microdialysis probe performance revealed an exchange rate of 4% at the 2.5 microliters/min flow rate. In vivo patterns of LH-RH in microdialysates obtained from sham-operated and castrate rats were pulsatile, as determined by the computer algorithm ULTRA. Pulses of LH-RH occurred at a higher frequency (P less than 0.05) in the castrates (1.30 +/- 0.26 pulses/h, n = 6) versus the sham-castrates (0.87 +/- 0.06 pulses/h, n = 11). Mean LH-RH pulse amplitude (castrates delta 0.24 +/- 0.03 pg, testes-intact delta 0.42 +/- 0.06 pg) and mean LH-RH levels (castrate 0.37 +/- 0.04 pg/10 min, intact 0.48 +/- 0.06 pg/10 min), however, were not significantly changed by castration (delta = difference between trough and peak LH-RH value of an LH-RH pulse).(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo pulsatile LHRH release into the anterior pituitary of the male rat: effects of castration

The effects of castration on the concentration of luteinizing hormone releasing hormone (LHRH) in the anterior pituitary (PIT) was studied in freely-behaving male rats using a push-pull cannula for sampling. In over 70 perfusions of the PIT of rats sampled before and at multiple days after castration there was no consistent change detected in the overall amount, secretory pulse amplitude or frequency or the LHRH signal reaching the PIT.

Gonadotropin-releasing hormone mRNA in the rat: distribution and neuronal content over the estrous cycle and after castration of males

The decapeptide gonadotropin-releasing hormone (GnRH) stimulates release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary. In the present study we used a 51-base oligonucleotide probe and in situ hybridization to study the neuronal content of GnRH mRNA at several time points in the estrous cycle and 7 days after castration of male rats. GnRH mRNA containing cells were found in the medial septum (SEPT), the vertical and horizontal limbs of the diagonal band of Broca (DBB), and throughout the preoptic area (POA) from the organum vasculosum of the lamina terminalis (OVLT) to its caudal merger with the anterior hypothalamus. The number of neurons producing detectable quantities of GnRH mRNA was not different either among females killed at 0700 h proestrus, 1000 h estrus, or 1900 h of diestrus 1 or between intact male rats and male rats killed 1 week after castration. We did, however, detect a significant difference in the number of GnRH mRNA producing neurons between males and females (P less than 0.05), where females had 20% more labeled cells. We detected no significant difference in the relative copy number of GnRH mRNA molecules (grains per labeled cell) either over the estrous cycle or between intact and castrate males. However, females overall had 24% more grains per labeled cell than males (P less than 0.05). These results suggest that gonadal steroid regulation of GnRH both over the estrous cycle and after short-term castration of males is mediated primarily by cellular processes subsequent to GnRH gene regulation. Furthermore, these results suggest that biosynthetic activity of GnRH is higher in females than in males.

Neuroendocrine regulation of the luteinizing hormone-releasing hormone pulse generator in the rat

We have analyzed the mechanisms by which several known regulators of the LHRH release process may exert their effects. For each, we have attempted to determine how and where the regulatory input is manifest and, according to our working premise, we have attempted to identify factors which specifically regulate the LHRH pulse generator. Of the five regulatory factors examined, we have identified two inputs whose primary locus of action is on the pulse-generating mechanism--one endocrine (gonadal negative feedback), and one synaptic (alpha 1-adrenergic inputs) (see Fig. 29). Other factors which regulate LHRH and LH release appear to do so in different ways. The endogenous opioid peptides, for example, primarily regulate LHRH pulse amplitude (Karahalios and Levine, 1988), a finding that is consistent with the idea that these peptides exert direct postsynaptic or presynaptic inhibition (Drouva et al., 1981). Gonadal steroids exert positive feedback actions which also result in an increase in the amplitude of LHRH release, and this action may be exerted through a combination of cellular mechanisms which culminate in the production of a unique, punctuated set of synaptic signals. Gonadal hormones and neurohormones such as NPY also exert complementary actions at the level of the pituitary gland, by modifying the responsiveness of the pituitary to the stimulatory actions of LHRH. The LHRH neurosecretory system thus appears to be regulated at many levels, and by a variety of neural and endocrine factors. We have found examples of (1) neural regulation of the pulse generator, (2) hormonal regulation of the pulse generator, (3) hormonal regulation of a neural circuit which produces a unique, punctuated synaptic signal, (4) hormonal regulation of pituitary responsiveness to LHRH, and (5) neuropeptidergic regulation of pituitary responsiveness to LHRH. While an attempt has been made to place some of these regulatory inputs into a physiological context, it is certainly recognized that the physiological significance of these mechanisms remains to be clarified. We also stress that these represent only a small subset of the neural and endocrine factors which regulate the secretion or actions of LHRH. A more comprehensive list would also include CRF, GABA, serotonin, and a variety of other important regulators. Through a combination of design and chance, however, we have been able to identify at least one major example of each type of regulatory mechanism.

In vivo models for the study of gonadotropin and LHRH secretion

The objective of this paper is to describe recent data from my laboratory dealing with the in vivo control of LHRH secretion in male rabbits and rats using the technique of push-pull perfusion (PPP) that allows repetitive determinations of this neuropeptide in quasi-normal physiological conditions. In addition, we have applied this method to simultaneously measure LHRH and LH in freely behaving male rats bearing a push-pull cannula (PPC) in the anterior pituitary. A description of the validation of this technique and its potential use will be discussed as well as data indicating that castration in the male rat induces a significant increase in the LHRH and LH signals; however, following testosterone treatment, in spite of a clear return of LH output to intact levels, even higher levels of LHRH reaching the anterior pituitary were detected. Curiously, in the rabbit no changes in LHRH release were noticed with castration, but following testosterone treatment, a transient but robust 5-8-fold increase in LHRH release was noticed. In short, these studies have demonstrated the existence of apparently opposite rather than similar responses in the testicular control of the hypothalamic-hypophysial axis of the male rat as compared with those of the male rabbit.

Activation of intracellular pathways with forskolin and phorbol ester increases LHRH mRNA level in the rat hypothalamus superfused in vitro

Two intracellular signal transduction mechanisms such as cAMP-protein kinase a and phosphatidylinositol (PI) turnover-protein kinase c are known to be dually involved in the regulation of luteinizing hormone-releasing hormone (LHRH) release. However, it is not yet evident that the activation of two intracellular pathways affects the LHRH gene expression. The present study aims, therefore, to determine whether the activation of two intracellular pathways affects changes in LHRH mRNA. To this end, we took advantage of an in vitro superfusion system, where rat hypothalamic tissues were superfused with media containing forskolin (FKN) and/or phorbol-12-myristate-13-acetate (PMA). Superfusates were collected at 10-min intervals and LHRH release was determined by radioimmunoassay. After a 2-h superfusion period, the post-superfusion hypothalami were recovered and poly (A) RNA fractions were isolated. Alterations in LHRH mRNA in response to FKN and/or PMA were determined by an RNA-blot hybridization assay using a 32P-end-labeled LHRH oligonucleotide (29-mer) probe. In vitro perfusion of hypothalamic fragments with PMA and/or FKN stimulated LHRH release as well as LHRH mRNA. The combined infusion of FKN and PMA did not produce an additive effect on the LHRH mRNA levels, but it was effective in synergistically increasing LHRH secretion in vitro. These data clearly demonstrate that the biosynthetic machinery of LHRH is influenced by activation of two intracellular pathways, both cAMP-protein kinase a and phosphatidyl-inositol turnover-protein kinase c, indicating the transsynaptic regulation of hypothalamic LHRH gene expression.

Regulation of hypothalamic luteinizing hormone-releasing hormone levels by testosterone and estradiol in male rhesus monkeys

Castration of rhesus monkeys produces hypersecretion of pituitary luteinizing hormone (LH) and a marked reduction in hypothalamic luteinizing hormone-releasing hormone (LH-RH) content. We performed the present study to determine whether treatment with gonadal steroids would reverse the effect of castration by increasing LH-RH content. We found that, when administered in doses that suppressed serum LH, both testosterone (T) and estradiol (E) significantly increased LH-RH in the infundibular nucleus/median eminence. The LH-RH content of 8 other regions, some known to contain LH-RH neurons, was not significantly affected. Thus, gonadal steroids act within a discrete region of the basal hypothalamus to modify LH-RH content. The finding that both T and E were effective in male monkeys supports the hypothesis that aromatization is involved in the negative feedback mechanism.

Modulation of gonadotropin-releasing hormone neuronal activity as evidenced by uptake of fluorogold from the vasculature

Peripheral injections of the tracer fluorogold (FG) and immunocytochemistry were used to study the modulation of gonadotropin-releasing hormone (GnRH) cell secretory activity in adult mice. Intraperitoneal administration of FG would make it available to all GnRH terminals outside the blood-brain barrier. The degree of capture of the dye would be linked to exocytotic (e.g., secretory) events at the nerve terminal. Single injections of tracer were made into intact mice of both sexes, and this resulted in the retrograde labeling of two-thirds of the GnRH cell bodies. Administration of identical doses to 3 week castrate mice revealed a reduction in the percentage of GnRH cells, with detectable FG, to 40% of the total. Castration did not diminish the number of GnRH cells visualized. When castrate animals received two doses of FG, the number of GnRH cells with tracer was increased to slightly greater than intact levels. This suggests that the secretory rate of individual GnRH cells might be reduced under conditions of castration. In addition, when ovariectomized females treated with estrogen and progesterone to induce luteinizing hormone (LH) surge were injected with FG just prior to that surge, over 80% of the GnRH neurons were robustly labeled with FG. These latter data are interpreted as representing GnRH neurons at maximally synchronized activity. This study suggests that peripheral administration of FG can be used in this species to follow alterations in neurosecretory rates.

Estradiol stimulates preoptic area-anterior hypothalamic proGnRH-GAP gene expression in ovariectomized rats

The decapeptide gonadotropin-releasing hormone (GnRH) and the 56-amino acid GnRH-associated peptide (GAP) are derived from a common precursor translated from the proGnRH-GAP mRNA. Studies using solid-phase hybridization techniques (i.e., Northern blot analysis, dot blot analysis, or in situ hybridization autoradiography) have yielded a controversy as to whether estradiol stimulates, inhibits, or has any effect on proGnRH-GAP gene expression in the preoptic area-anterior hypothalamus (POA-AH) of the ovariectomized (OVX) rat. Using a sensitive and quantitative solution hybridization-nuclease protection assay, which ensures complete hybridization of target RNA to probe RNA, we examined the effects of OVX and estradiol replacement on the amount of proGnRH-GAP mRNA in individual POA-AH dissections. Rats sacrificed at different intervals after OVX showed a significant time-dependent decrease (34-60%) in the levels of POA-AH proGnRH-GAP mRNA relative to sham-operated animals; OVX rats treated with estradiol, however, had proGnRH-GAP mRNA levels comparable to those of sham-OVX animals. To verify these observations, levels of the proGnRH-GAP peptide, measured by radioimmunoassay with antibodies directed against the cleavage and amidation site between the GnRH and the GAP portions fo the precursor molecular, were also found to decrease (37%) after OVX and increase (63-85%) following estradiol replacement, relative to intact rats. These data support the view that estradiol stimulates the levels of both proGnRH-GAP mRNA and its primary translation product in the POA-AH region of the OVX rat.

Progesterone increases messenger ribonucleic acid (mRNA) encoding luteinizing hormone releasing hormone (LHRH) level in the hypothalamus of ovariectomized estradiol-primed prepubertal rats

In order to investigate the mechanism underlying ovarian steroid action on gene expression of hypothalamic luteinizing hormone releasing hormone (LHRH), changes in LHRH mRNA level were determined by RNA-blot hybridization assay. Twenty-eight-day-old female rats were ovariectomized (OVX) and implanted with Silastic capsule containing either 17 beta-estradiol (E) or vehicle (V). Two days later (day 30), OVX + E-primed rats were given s.c. progesterone (P, 1 mg) 6 h prior to decapitation. Four experimental groups were studied: (1) intact, (2) OVX + V, (3) OVX + E, and (4) OVX + E + P-treated rats. Poly(A) RNA fractions from hypothalami (40-50/group) were isolated, blotted onto nitrocellulose paper and hybridized with 32P-end-labeled LHRH oligonucleotides (29 mer) which are complementary to rat LHRH mRNA. The hypothalamic LHRH mRNA signal markedly attenuated 2 days following ovariectomy. E replacement to OVX rats slightly increased LHRH mRNA level, which is lower than that of the intact group. However, a single injection of P to OVX + E-treated rats notably augmented the LHRH mRNA level over that observed in the intact group. In addition, LHRH content and release in vitro were examined to correlate with changes in LHRH gene expression. Ovariectomy and the replacement of E and/or P resulted in a similar fashion of changes in LHRH release and content as compared to alteration of LHRH mRNA level. This study clearly demonstrates that P increases LHRH mRNA level in the hypothalamus of OVX + E-primed immature rats.(ABSTRACT TRUNCATED AT 250 WORDS)