Effect of testosterone on hypothalamic LH-RH content

Non-genomic actions of androgens

Previous work in the endocrine and neuroendocrine fields has viewed the androgen receptor (AR) as a transcription factor activated by testosterone or one of its many metabolites. The bound AR acts as transcription regulatory element by binding to specific DNA response elements in target gene promoters, causing activation or repression of transcription and subsequently protein synthesis. Over the past two decades evidence at the cellular and organismal level has accumulated to implicate rapid responses to androgens, dependent or independent of the AR. Androgen's rapid time course of action; its effects in the absence or inhibition of the cellular machinery necessary for transcription/translation; and in the absence of translocation to the nucleus suggest a method of androgen action not initially dependent on genomic mechanisms (i.e. non-genomic in nature). In the present paper, the non-genomic effects of androgens are reviewed, along with a discussion of the possible role non-genomic androgen actions have on animal physiology and behavior.

Reduction of GnRH and infertility in the R6/2 mouse model of Huntington's disease

Reductions in testosterone and luteinizing hormone levels and reduced sexual functions have been reported in Huntington's disease (HD) patients. Atrophy of the reproductive organs and loss of fertility have also been observed in the R6/2 mouse, which is currently the most studied transgenic model of HD. In an effort to define the cause of infertility we studied the expression of gonadotropin-releasing hormone (GnRH) in the medial septum, diagonal band of Broca and hypothalamus of R6/2 male mice during sexual maturation. We found a progressive reduction in the numbers of GnRH-immunoreactive neurons in the analysed brain areas of R6/2 mice starting at 5 weeks of age and becoming statistically significant with only 10% of the neurons remaining by 9 weeks of age. Atrophy of testes and seminal vesicles combined with a significant reduction in serum and testicular testosterone levels were detected in 12-week-old R6/2mice. These results suggest that infertility in the R6/2 males is due either to death of GnRH neurons or to a reduction in GnRH expression leading to a downstream impairment of the gonadotropic hormones. Gonadotropic hormone replacement did not mitigate weight loss or restore motor function in R6/2 males.

Gonadal steroid replacement reverses gonadectomy-induced changes in the corticosterone pulse profile and stress-induced hypothalamic-pituitary-adrenal axis activity of male and female rats

We investigated the effects of gonadal hormone replacement on the pulsatile parameters underlying basal circadian corticosterone secretion in castrated male and ovariectomized female rats using an automated sampling system. Blood was collected from freely moving, unanaesthetized rats every 10 min over a 24-h period and sampling was continued during a noise stress and after lipopolysaccharide (LPS) administration. Castrated male rats had markedly higher corticosterone levels than intact controls. This was reflected by increased number and frequency of pulses in addition to an increase in the pulse height and amplitude under both basal circadian and stress conditions. Hormone replacement with either testosterone or dihydrotestosterone returned these corticosterone levels and circadian profile to those found in intact males, confirming an androgen-mediated effect. Ovariectomized females had significantly lower basal and stress-induced corticosterone levels with lower frequency and amplitude of corticosterone pulses than intact females. 17beta-oestradiol replacement returned basal levels, pulsatile measurements and stress-induced corticosterone levels to those found in intact females. Three hours post-LPS administration, castrated males demonstrated significantly higher values of parvocellular paraventricular nucleus (PVN) arginine vasopressin and corticotrophin-releasing factor and anterior pituitary pro-opiomelanocortin mRNA while ovariectomized females showed significantly lower levels of all three transcripts compared to intact controls. PVN glucocorticoid receptor mRNA levels 3 h post-LPS administration were significantly decreased in castrated males and significantly increased in ovariectomized female rats. Replacement of gonadal steroids resulted in a return to the levels found in intact controls after LPS. Gonadal steroid replacement is sufficient to reverse changes in the pulsatile characteristics of corticosterone release after gonadectomy. In addition, gonadal steroid replacement reverses stress-induced alterations in hypothalamic-pituitary-adrenal (HPA) activity. These data demonstrate a major contribution of gonadal steroids to the regulation of HPA axis activity and to the pulsatile characteristics of corticosterone release.

Castration reduces the effect of serotonin-1A receptor stimulation on prepulse inhibition in rats

This study examined the interaction between hormones and serotonin-1A (5-HT1A) receptor modulation of prepulse inhibition (PPI) of the acoustic startle response. Male and female rats were gonadectomized; some castrated rats received testosterone- or estrogen-filled implants. Rats were randomly injected with saline or 0.02 or 0.50 mg/kg 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A receptor agonist. All rats showed a dose-dependent disruption of PPI in response to 8-OH-DPAT. In untreated castrated rats, this disruption was significantly reduced (33% compared with 78% in sham-operated rats). Testosterone treatment reversed this reduction, but estrogen was less effective. Ovariectomized and sham-operated rats showed similar PPI in response to 8-OH-DPAT. These data suggest that the effect of 8-OH-DPAT on PPI in male rats depends on circulating hormone levels, particularly testosterone.

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.

Evidence for estrogen receptor-mediated regulation of human gonadotropin-releasing hormone promoter activity in human placental cells

Two fragments of the human gonadotropin-releasing hormone (GnRH) promoter, one containing 0.6 kb of the downstream promoter sequence (H-1) and another 1.8 kb fragment (H-2) containing the upstream promoter region with a deletion of the downstream promoter sequence, were fused to a promoterless luciferase (Luc) reporter construct and transfected into the human placental (JEG) cells. JEG cells transfected with both constructs showed insignificant changes in luciferase activity in response to estradiol. However, cotransfection of H-2-Luc construct with a vector expressing a human estrogen receptor (ER) cDNA results in dose-dependent decreases in luciferase activity in response to estradiol. This ER mediated down-regulation of promoter activity was retained in constructs with the GnRH promoter deleted to position 548 bp 5' to the upstream transcription start site. Further deletion of upstream promoter sequence to 169 bp reversed the estrogen responsiveness from inhibitory to stimulatory. Thus, this study demonstrated that the upstream GnRH promoter region can be regulated by estrogen in transfected JEG cells.

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.

Content and distribution pattern of luteinizing hormone-releasing hormone (LHRH) in the hypothalamus of neonatally estrogenized female rats

The rostro-caudal distribution of luteinizing hormone-releasing hormone (LHRH) in the hypothalamus of neonatally estrogenized (10 micrograms EB for 1-5 days), sterile female rats was compared with that of cycling rats at 50, 100 and 240 days of age. Serial slices of 400 microns in thickness in total were cut from a hypothalamic block, and the LHRH content of each slice was measured by radioimmunoassay. Two peaks of LHRH in the rostro-caudal direction were apparent in all groups. The rostral peak corresponded to the LHRH-producing cells in the septal-preoptic region and storage of LHRH in the organum vasculosum lamina terminalis (OVLT), while the caudal peak corresponded to that of LHRH in the median eminence. Thus, the total LHRH content of the regions of the hypothalamus was calculated separately either anterior or posterior to the suprachiasmatic nucleus, which were designated as the rostral hypothalamus (RH) and mediobasal hypothalamus (MBH), respectively. The total content of LHRH in the 50-day-old sterile rats was significantly higher than that in the estrous females in both RH and MBH. This was also true for the MBH of 240-day-old sterile females in comparison to that of estrous females but not so in 100-day-old females. The present results suggest that sterility in neonatally estrogenized rats is not due to a reduction in LHRH content of the hypothalamus but, rather, is due to changes in LHRH-releasing systems. Furthermore, in the control females, ovariectomy performed at 22 days of age induced a striking decrease in LHRH content in the MBH at 50 days of age, while it remained unchanged after ovariectomy in the neonatally estrogenized sterile rats. These findings suggest that the hypothalamic mechanism involved in the release of LHRH after ovariectomy was damaged by neonatal treatment with estrogen.

The Characteristics of Medial Septal Neurons Antidromically Identified as Projecting to the Median Eminence and their Response to Gonadal Steroids

Abstract We have obtained extracellular single unit recordings from 129 neurons in the medial septum of the male rat that projected to the median eminence. These cells were identified on the basis of their antidromic responses to electrical stimulation in the median eminence. The antidromically-evoked spike was always followed by a period of inhibition (-100 ms) which often (80%) persisted at stimulus intensities below threshold for antidromic activation. The identified neurons displayed one of three patterns of activity, the relative proportions of which were dependent upon the anesthetic. In rats anesthetized with urethane, 15% of the antidromically identified septal neurons displayed an oscillating, 25% a continuous, and 61% a low irregular pattern of spontaneous activity or no activity. In contrast, in barbiturate-anesthetized animals only 6% exhibited an oscillating and 3% a continuous pattern of activity. The remaining neurons (91%) displayed either a low firing rate or were silent. We then assessed the responses of these identified neurons to systemic administration of gonadal steroids. Systemic administration of 17beta-estradiol elicited a rapid decrease in the firing rate of 9 of 29 neurons tested. The inhibitory response was specific to those cells displaying an oscillating (6/9) or continuous (3/6) firing pattern of which 60% were inhibited. In contrast, systemic administration of testosterone failed to elicit such changes in the activity of the identified septal neurons (n = 12) including one neuron which was subsequently shown to be inhibited by estradiol. As such identified septal neurons may contain gonadotropin-releasing hormone, these results suggest a role for estradiol but not testosterone in the short-term regulation of the secretion of this releasing hormone in the male rat.

Antihormonal activities of 5 alpha-reductase and aromatase inhibitors

The problem of developing androgen antagonists has been tackled so far only by synthesizing steroids able to displace testosterone and other androgens from their specific receptor sites. The observation that testosterone has to be converted intracellularly either to 5 alpha-reduced metabolites (DHT, 3 alpha-diol, etc.) or to estrogens, in order to become fully active on androgen-dependent structures (both central and peripheral), has opened the possibility of creating molecules which prevent these conversions, and which could then block the actions of testosterone. The availability of these new compounds has allowed a better understanding of the selective physiological role of each of the metabolites of testosterone, and to provide the basis for the development of new hormone antagonists to be used in those clinical conditions for which an inhibition of the actions of testosterone is foreseen. The usefulness of these enzyme inhibitors is underlined by some examples described in this paper. The results obtained may permit the formulation of the following conclusions: (1) The conversion of testosterone to its 5 alpha-reduced metabolites occurring in the neuroendocrine structures may represent an essential step for the appearance of the inhibitory feedback effect testosterone exerts on LH secretion; (2) Testosterone exhibits its negative feedback effect on FSH secretion as such and not following the local aromatization to estrogens; (3) Testosterone exerts its effect on the intrahypothalamic stores of LHRH acting as such and not following its local conversion either to 5 alpha-reduced metabolites or to estrogenic molecules; (4) Some of the new enzyme inhibitors (e.g. 4-OH-A) may represent an interesting tool for the treatment and/or the prevention of BPH and possibly of other androgen-dependent diseases (prostate carcinoma, acne etc.), as shown by their ability to prevent the in vitro conversion of testosterone to its 5 alpha-reduced metabolites both in the normal prostate of the rat and in the human BPH tissue.

Significance of testosterone in regulating hypothalamic content and in vitro release of beta-endorphin and dynorphin

The effects of castration and testosterone replacement on hypothalamic pools of beta-endorphin and dynorphin and on the basal and corticotropin-releasing factor (CRF)-stimulated release of these peptides from hypothalamic slices in vitro were studied. The experiments were done in adult male rats. The hypothalamic content of both peptides increased significantly within 1 week of castration, and levels remained elevated for up to 4 weeks. Testosterone treatment, begun at the time of castration, prevented these increases. In addition, testosterone replacement 6 weeks after castration reversed peptide levels to normal. Basal in vitro release rates of beta-endorphin and dynorphin were significantly lower from hypothalamic slices derived from 1-week castrated animals than from intact males, and when testosterone was administered in various doses in vivo, basal release rates in vitro increased in a dose-related manner. Hypothalami from rats that had been castrated for 4 weeks, however, showed basal release rates similar to those in tissues from intact controls, a finding indicating that castration initially alters both opioid peptide synthesis and release; later, release is normalized, whereas synthesis remains elevated. CRF was found to stimulate beta-endorphin and dynorphin release from hypothalami from intact and from 1- and 4-week-castrated rats, a result indicating that castration does not alter the response of beta-endorphin and dynorphin neurons to this stimulus.

LHRH messenger RNA in neurons in the intact and castrate male rat forebrain, studied by in situ hybridization

The purpose of the present study was to localize luteinizing hormone-releasing hormone (LHRH) mRNA within the male rat forebrain using an in situ hybridization approach. The expression of LHRH mRNA was compared in castrate and intact males to approach questions on the chronic influences of circulating testicular steroids on the gene expression of the peptide. Frozen 10 micron sections fixed in paraformaldehyde were obtained from the forebrain region of intact and 2 week post-castrate adult male rats. LHRH mRNA was autoradiographically detected using an oligomer (59mer) complementary to the mRNA coding for amino acids -5 to 15 of the human LHRH preprohormone. Individual brain sections were incubated in prehybridization buffer for 2 h to reduce nonspecific binding. Following this, 20 microliter of hybridization buffer containing 65,000-120,000 cpm of the 59mer were applied to sections and hybridized at 37 degrees C for 3 days. The sections were then rinsed over a 48 h period, dehydrated, dipped in Kodak NTB2 liquid emulsion and exposed for 22 days. Autoradiograms were developed and counterstained with fast green and cresyl violet. As reported in the female, LHRH message-containing cells were localized in ventral septal regions, the diagonal bands of Broca, preoptic area and anterior hypothalamus. On occasion, LHRH gene expressing cells were found to appear in loose clusters. Labeled cells were never found in control sections treated with hybridization buffer lacking the 59mer. The total number of LHRH mRNA-containing cells localized in intact rats did not differ significantly from the castrate group.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of orchidectomy and replacement therapy on the ultrastructure and gonadotropin-releasing hormone content of the median eminence of the rat

The effects of 2 weeks of orchidectomy and replacement therapy with testosterone upon the content and distribution of gonadotropin-releasing hormone (GnRH) in the median eminence were determined by means of radioimmunoassay and electron microscopy. Photographic montages were prepared from electron micrographs of the lateral median eminence at the point of deepest invagination of the tuberoinfundibular sulcus. Morphometric analysis of photographs of tissues immunohistochemically stained for GnRH was performed to determine changes in the volume density of GnRH-containing axon profiles following the experimental treatments. A decrease in GnRH content after orchidectomy was observed both by morphometric analysis of axon volume density and radioimmunoassay of total GnRH content. Testosterone treatment of orchidectomized animals prevented the postorchidectomy loss of GnRH. Morphometric analysis of conventional electron micrographs revealed an increase in the number of axons containing no dense-core vesicles following orchidectomy, but no decrease in volume density of the neuropil. The results indicate that the change in volume density of immunostained axons was related to the loss of immunostainable dense-core vesicles and not to a change in the size or number of axons. The area corresponding to the location of the highest concentration of GnRH-containing axons was observed to be largely avascular and separated from the vessels of the tuberoinfundibular sulcus by a "border zone" composed of glial foot processes. The unique morphology of the GnRH area has suggested the name "compact zone" to distinguish it from the palisade zone with which it is continuous medially. GnRH axons in this region are probably part of a tract extending farther caudally rather than a terminal field.

Stimulation of hypothalamic LHRH levels and release by gonadal steroids

The inhibitory feedback effects of steroids on pituitary LH release are believed to be mediated via steroidal effects on the hypothalamic LHRH activity. We have examined the direct effects of individual steroids (T, DHT and E2) on hypothalamic LHRH levels and on LHRH release in vitro. In castrated male rats, replacement of either steroid in physiological doses, resulted in augmentation of the MBH LHRH levels by steroidal action within the MBH. LHRH analyses of microdissected diencephalic nuclei revealed that this accumulation occurred exclusively in LHRH terminals in the ME. Careful examination of the time course of steroid action showed that whereas LH release was suppressed within hours of steroid treatment, the LHRH response occurred after 3-4 days of steroid exposure in 2-week castrated rats and 7-14 days in 8-week castrated rats. This temporal dichotomy in the LH and LHRH responses to steroid action was further substantiated by the differential effects of low, sub-physiological levels of steroids on these two responses. Very low levels of T or E2 evoked maximal accumulation of the MBH LHRH, but LH release in vivo and the rate of LHRH release in vitro were not affected. Surprisingly, physiological levels of T which suppressed LH release concomitant with elevations in LHRH levels, augmented the in vitro rate of LHRH release. In fact, the LHRH release rate was found to be correlated with LHRH concentrations in hypothalami of intact, castrated and castrated rats treated with T. Thus it appears that in the hypothalamo-pituitary axis there are different thresholds of responsiveness to steroids. Apparently, the LHRH neurons, particularly the processes involved in LHRH accumulation are most sensitive to low levels of steroids; however, higher physiological levels of steroids are required to suppress pituitary LH release as well as to promote LHRH release. On the basis of our cumulative data, it is reasonable to speculate that steroid-induced accumulation of LHRH in the ME may not be a consequence of decrease in LHRH release, but may involve synthesis of the neurohormone.

Pituitary gonadotropin-releasing hormone receptors. Effects of castration, steroid replacement, and the role of gonadotropin-releasing hormone in modulating receptors in the rat

To study the role of gonadotropin-releasing hormone (GnRH) receptors in the regulation of gonadotropin secretion, we used D-125I-alanine6 des glycyl10 GnRH ethylamide (D-125I-Ala analog), a nondegradable, superagonist GnRH analog to assess GnRH receptors on rat pituitary membranes. Receptor affinity in intact adult rats was 5.0 X 10(9) M-1 and was unchanged after castration in both sexes. Castration of adult male and female rats produced a twofold increase in GnRH binding capacity by 7 d and binding capacity remained elevated for the subsequent 14 d. GnRH receptor number rose more rapidly after castration in males than females, and the time-course of receptor rise was similar to the increase in serum gonadotropin levels. The increase in GnRH binding capacity was prevented by gonadal steroid replacement at the time of castration in both sexes. Injections of the GnRH analog, D-Ser6 (TBu) des Gly10 GnRH ethylamide for 4 d produced a 70% increase in GnRH receptor number in intact male rats and testosterone-replaced castrates. The same regimen, however, failed to increase the elevated receptor numbers present after castration. Administration of rabbit anti-GnRH serum concomitant with castration inhibited the rise in both GnRH receptor number and luteinizing hormone. The changes in pituitary GnRH receptors parallel previously demonstrated changes in hypothalamic secretion of GnRH. Thus, GnRH probably regulates its own receptor in vivo and gonadal steroids may influence pituitary GnRH receptors by changing hypothalamic GnRH secretion.

Factors altering the secretion of LHRH from superfused fragments of rat hypothalamus

This paper deals with the effect of temperature (t degrees), ions, prostaglandin (PGE2) and testosterone on the rate of release of LHRH from the superfused medial basal hypothalamus (MBH) of normal or castrated male rats in an effort to further validate the biological properties of this preparation. LHRH in the perfusate is nondetectable when the MBH is at 0 C, warming to 37 C produces a sharp rise in LHRH and then stabilization at a level at least threefold the one calculated for the 0 C experiment. Basal release rate of LHRH is t degrees and calcium dependent, as is the stimulatory action of potassium. Infusion of high K+ induces approximately a fivefold increase in the release rate of LHRH and stays at this high level for as long as the infusion lasts (50 min). LHRH stimulation by PGE2 (pulse) proceeds in the absence of external calcium, and the effect lasts longer than the potassium (pulse) action. Surprisingly, MBHs from long-ter castrated males (60-90 days) release less LHRH per minute than intact male MBHs during the steady state phase of LHRH secretion; testosterone propionate (TP, 200 microgram/100 g BW x 3 days) increases significantly the rate of release in castrate rats and is not related to the amount of LHRH assayed in the MBH. MBHs of untreated castrated rats or castrated rats treated with TP, show a reduced LHRH secretion in response to PGE2. It is concluded that this preparation can be used at advantage to study the rate of LHRH release in a variety of experimental paradigms.

Differential effects of castration and testosterone replacement on basal and LHRH-stimulated cAMP and cGMP accumulation and on gonadotropin release from the pituitary of the male rat

Castration of male rats decreased cAMP levels, and increased cGMP levels and gonadotropin release from anterior pituitaries incubated in vitro. Testosterone (T) replacement via silastic tubes filled with the steroid increased cAMP and decreased cGMP levels and gonadotropin release. Incubation of hemipituitaries from intact males with luteinizing hormone releasing hormone (LHRH, 5 nM for 2 h) resulted in increased cAMP and cGMP accumulation and gonadotropin release. Castration abolished LHRH-induced cAMP accumulation, but increased the effect of LHRH on cGMP accumulation and gonadotropin release. Testosterone replacement restored cAMP stimulation by LHRH but decreased LHRH-induced elevation of cGMP levels and gonadotropin release. These data illustrate parallel increases by castration of LHRH-induced cGMP accumulation and of gonadotropin release. Furthermore, these two parameters are influenced in the opposite direction by replacement therapy. These results support the concept of a role for cGMP in LHRH action as well as providing evidence of a link between the feedback action of T and cGMP in the pituitary gland.

Administration of gonadal steroids to the castrated male rat prevents a decrease in the release of gonadotropin-releasing hormone from the incubated hypothalamus

The influence of testosterone on gonadotropin-releasing hormone (GnRH) secretion was assessed indirectly by altering the serum testosterone concentration of male rats and measuring GnRH release from their incubated hypothalami 1 wk later.GnRH release from hypothalami of castrated rats was 13.4+/-1.2 (SE) pg/h, compared to 35.3+/-3.8 pg/h from hypothalami of intact rats (P < 0.001). GnRH release from the hypothalami of castrated rats treated with testosterone propionate, 100 or 500 mug daily, was 25.0+/-3.4 pg/h and 27.9+/-3.6 pg/h, which is significantly greater (P < 0.05 and P < 0.01, respectively) than that from hypothalami of castrated rats treated only with sesame oil.A similar decrease in GnRH release from hypothalami of hypophysectomized rats and prevention of this decrease by treating the hypophysectomized rats with testosterone propionate is evidence that the observed effects of testosterone are not mediated via luteinizing hormone and(or) follicle-stimulating hormone secretion. Treatment of castrated rats with either dihydrotestosterone propionate or estradiol benzoate also prevented the decrease in GnRH release from the hypothalami of castrated rats. We conclude that testosterone, dihydrotestosterone, and estradiol all prevent the decrease in GnRH release from hypothalami of castrated rats treated with these steroids. The possibility exists that these steroids may also maintain GnRH secretion in vivo.