Pituitary and gonadal function in hypogonadotrophic hypogonadal (hpg) mice bearing hypothalamic implants

GnRH receptor values are 30-50% of normal in pituitaries of hpg male mice, and testicular LH receptors only 8% of normal (160.4 +/- 17.6 and 2013 +/- 208.1 fmol/testis respectively). In male hpg mice bearing fetal preoptic area (POA) hypothalamic implants for 10 days there was no change in pituitary GnRH receptors, pituitary gonadotrophin content, or seminal vesicle weight. However, testicular weights and LH receptors were doubled in 4/10 mice and 2 had increased serum FSH levels. Between 26 and 40 days after implantation pituitary GnRH receptors and pituitary LH increased to normal male levels, although at 40 days serum and pituitary FSH concentrations had reached only 50% of normal values. Testicular and seminal vesicle weights increased more than 10-fold by 40 days after implantation and LH receptors to 70% of normal. In hpg female mice bearing hypothalamic implants for 30-256 days pituitary gonadotrophin concentrations were normal, even though GnRH receptors reached only 60% of normal values (6.18 +/- 0.4 and 9.8 +/- 0.4 fmol/pituitary respectively). Serum FSH was substantially increased from values of less than 30 ng/ml in hpg mice to within the normal female range in hypothalamic implant recipients. Ovarian and uterine weights increased after hypothalamic grafting from only 4-5% to over 74% of normal values. LH receptors increased from 6.5 +/- 1.3 fmol/ovary for hpg mice to 566.9 +/- 39.2 fmol/ovary for implant recipients. Vaginal opening occurred about 23 days after implantation and these animals displayed prolonged periods of oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

An activated human follicle-stimulating hormone (FSH) receptor stimulates FSH-like activity in gonadotropin-deficient transgenic mice

FSH mediates its testicular actions via a specific Sertoli cell G protein-coupled receptor. We created a novel transgenic model to investigate a mutant human FSH receptor (FSHR(+)) containing a single amino acid substitution (Asp567Gly) equivalent to activating mutations in related glycoprotein hormone receptors. To examine the ligand-independent gonadal actions of FSHR(+), the rat androgen-binding protein gene promoter was used to direct FSHR(+) transgene expression to Sertoli cells of gonadotropin-deficient hypogonadal (hpg) mice. Both normal and hpg mouse testes expressed FSHR(+) mRNA. Testis weights of transgenic FSHR(+) hpg mice were increased approximately 2-fold relative to hpg controls (P < 0.02) and contained mature Sertoli cells and postmeiotic germ cells absent in controls, revealing FSHR(+)-initiated autonomous FSH-like testicular activity. Isolated transgenic Sertoli cells had significantly higher basal ( approximately 2-fold) and FSH-stimulated ( approximately 50%) cAMP levels compared with controls, demonstrating constitutive signaling and cell-surface expression of FSHR(+), respectively. Transgenic FSHR(+) also elevated testosterone production in hpg testes, in the absence of circulating LH (or FSH), and it was not expressed functionally on steroidogenic cells, suggesting a paracrine effect mediated by Sertoli cells. The FSHR(+) response was additive with a maximal testosterone dose on hpg testicular development, demonstrating FSHR(+) activity independent of androgen-specific actions. The FSHR(+) response was male specific as ovarian expression of FSHR(+) had no effect on hpg ovary size. These findings reveal transgenic FSHR(+) stimulated a constitutive FSH-like Sertoli cell response in gonadotropin-deficient testes, and pathways that induced LH-independent testicular steroidogenesis. This novel transgenic paradigm provides a unique approach to investigate the in vivo actions of mutated activating gonadotropin receptors.     

Autoregulation of testicular luteinizing hormone receptors in hypogonadal (hpg/hpg) mice

The autoregulation of testicular luteinizing hormone (LH) receptors was studied in hypogonadal (hpg/hpg) and normal mice. The basal concentration of LH receptors was more than three-fold higher in hpg/hpg than in normal mice. After injection of hCG, hpg/hpg mice showed a decrease in LH receptor levels which was not observed in normal mice. Plasma testosterone was undetectable in hpg/hpg mice, even after treatment with a single dose of hCG. Plasma prolactin levels were higher in hpg/hpg than in normal mice. The increase in basal LH receptor levels is thought to be due to a compensatory mechanism in which elevated prolactin could play a role. The differences between hpg/hpg and normal mice in the autoregulation of LH receptors observed could be due to the hypersensitivity of the physiologically immature testis in hpg/hpg mice to the action of hCG, to gonadotropin deficiency, particularly during the earlier stages of development, or to a direct effect of the hpg locus on the metabolism of LH receptors.These studies were supported by NIH Grants HD 12642 and HD 12671 (AB) and Grant CA-24145 (WGB).     

Pituitary Androgen Receptor Signalling Regulates Prolactin but Not Gonadotrophins in the Male Mouse

Production of the androgen testosterone is controlled by a negative feedback loop within the hypothalamic-pituitary-gonadal (HPG) axis. Stimulation of testicular Leydig cells by pituitary luteinising hormone (LH) is under the control of hypothalamic gonadotrophin releasing hormone (GnRH), while suppression of LH secretion by the pituitary is controlled by circulating testosterone. Exactly how androgens exert their feedback control of gonadotrophin secretion (and whether this is at the level of the pituitary), as well as the role of AR in other pituitary cell types remains unclear. To investigate these questions, we exploited a transgenic mouse line (Foxg1^Cre/+; AR^fl/y) which lacks androgen receptor in the pituitary gland. Both circulating testosterone and gonadotrophins are unchanged in adulthood, demonstrating that AR signalling is dispensable in the male mouse pituitary for testosterone-dependent regulation of LH secretion. In contrast, Foxg1^Cre/+; AR^fl/y males have a significant increase in circulating prolactin, suggesting that, rather than controlling gonadotrophins, AR-signalling in the pituitary acts to suppress aberrant prolactin production in males.     

Postnatal ovarian follicle development in hypogonadal (hpg) and normal mice and associated changes in the hypothalamic-pituitary ovarian axis

Significant uterine growth occurred in normal and hypogonadal (hpg) mice between Days 7 and 21 but thereafter no further growth was observed in hpg mice. The ovaries of hpg mice were significantly smaller than those of normals at all ages, but there was no significant difference between the number of non-growing follicles in the ovaries of mutants and their normal littermates at any age studied, and normal and hpg mice showed a marked reduction in the number of non-growing follicles during the first month of life. The size and composition of the growing follicle population in hpg mice, however, differed markedly from those in normal animals and by 21 days of age the number of growing follicles in mutants was significantly reduced. There was no significant difference in the number of Type 3b follicles before 60 days of age, but the number of all other follicle types was significantly less in hpg mice at all ages studied. Follicles in which the antrum is fully developed (Type 7 and 8) were never seen in the ovaries of mutants and corpora lutea were never observed. Interstitial tissue development was also very poor in hpg ovaries. The hypothalamic GnRH content in normal mice remained low until Day 20, before rising sharply to adult levels (approximately 800 pg) between Days 20 and 30. The pituitary FSH content increased over the first 10 days of life to reach a peak of about 5000 ng, before declining to the adult value of about 2000 ng by Day 30, whilst the plasma FSH concentration was high in the first 10 days, but fell to adult levels over the next 20 days. Pituitary LH content increased significantly between Days 5 and 10 to reach the adult level of about 600 ng. Hypothalamic GnRH was undetectable at all ages in hypogonadal mice, but the pituitary content of FSH and LH had risen to the attenuated mutant adult value by Day 15, and unlike normals, plasma FSH concentrations were not elevated during the neonatal period. These results suggest that minimal gonadotrophic stimulation of the ovary from birth has no effect on the total number of follicles but reduces the number of growing follicles and prevents follicle growth beyond the early antral stage. Gonadotrophins therefore appear to have a role in the initiation and continuance of follicle growth in the adult mouse.     

Evidence for a pituitary site of gonadal steroid stimulation of GnRH receptors in female mice

Treatment of GnRH-deficient (hpg) female mice with oestradiol-17 beta (E2) for 7 days increased GnRH receptors from 4.1 +/- 0.4 fmol/pituitary (control) to 7.2 +/- 0.7 fmol/pituitary (GnRH-treated), and consistently increased pituitary FSH content. Treatment of hpg female mice with E2 plus progesterone (P) for 14 days stimulated GnRH receptors more than did E2 alone, although values still remained lower than those of normal intact female mice. In contrast, GnRH treatment of intact hpg female mice alone, or combined with E2 + P, increased GnRH receptors to values similar to those of intact normal female mice. In contrast, the receptor rise after GnRH treatment alone of ovariectomized hpg mice was significantly less than in intact hpg mice similarly treated. However, the combination of GnRH + E2 + P treatment of ovariectomized hpg mice increased GnRH receptors to normal intact female values, indicating the synergistic actions of these hormones on GnRH receptor up-regulation at the pituitary. Oestradiol treatment of ovariectomized normal female mice prevented the receptor fall after ovariectomy, and when combined with exogenous GnRH further increased receptors to values identical to those of intact female mice receiving GnRH alone. Ovariectomy of hpg mice had no effect on GnRH receptor, serum or pituitary LH and FSH values. There was no change in serum LH concentration after GnRH treatment of hpg female mice, but serum FSH increased and this was accentuated by ovariectomy, indicating that in intact mice an ovarian factor(s) normally inhibits GnRH-stimulated FSH release. This factor did not appear to be an ovarian steroid since serum FSH was not suppressed in intact or ovariectomized GnRH-treated hpg mice concurrently receiving E2 + P treatment. These results suggest that: (1) gonadal steroids alone have a major direct stimulatory action on the pituitary to increase GnRH receptors; (2) the oestrogen-induced increase in GnRH receptors is enhanced in the presence of GnRH; (3) steroids exert inhibitory feedback on gonadotrophin secretion that is mediated at some cellular regulatory locus other than the GnRH-receptor complex.     

Neonatal androgenization of hypogonadal (hpg) male mice does not abolish estradiol-induced FSH production and spermatogenesis

Background  

Testicular development is arrested in the hypogonadal (hpg) mouse due to a congenital deficiency in hypothalamic gonadotropin-releasing hormone (GnRH) synthesis. Chronic treatment of male hpg mice with estradiol induces FSH synthesis and secretion, and causes testicular maturation and qualitatively normal spermatogenesis. As estradiol negative feedback normally inhibits FSH production in the male, this study tested whether this paradoxical response to estradiol in the male hpg mouse might be due to inadequate masculinisation or incomplete defeminization in the neonatal period. Previous studies have demonstrated that treatment of hpg mice with testosterone propionate in the immediate neonatal period is necessary to allow full reproductive behaviors to be expressed following suitable endocrine stimulation at adult ages.     

Targeted ablation of pituitary gonadotropes in transgenic mice.

LH, FSH, and TSH are heterodimeric glycoprotein hormones composed of a common alpha-subunit and unique beta-subunits. The alpha-subunit is produced in two distinct specialized cell types of the pituitary gland: gonadotropes, which synthesize LH and FSH, and thyrotropes, which synthesize TSH. We have demonstrated that 313 base pairs of the bovine-alpha subunit promoter direct expression of diphtheria toxin A chain specifically to the gonadotropes in transgenic mice. Animals carrying this transgene generally exhibit reproductive failure and lack of gonadal differentiation, consistent with gonadotrope ablation. Lack of gonadotrope activity was verified by RIA and immunohistochemical staining for LH. The phenotype of these transgenic mice is nearly identical to mice homozygous for the spontaneous mutation, hpg, which is due to a deletion in the gene encoding GnRH. Thyrotrope function was judged normal based on overall growth of the animals, appearance of their thyroids, T4 levels measured by RIA, and immunohistochemical staining for TSH. The ablation of gonadotropes but not thyrotropes suggests that separate cis-acting elements are necessary for expression of the alpha-subunit gene in these two cell types. Pituitary content of ACTH and GH was apparently normal, while PRL synthesis and storage were reduced. Thus, in a pituitary almost completely devoid of gonadotropes, most other pituitary functions were normal. This suggests that most pituitary cells are able to differentiate independently of terminal gonadotrope differentiation and can function in the absence of paracrine signaling provided by gonadotropes.     

Role of the fetal pituitary in cryptorchidism induced by exogenous maternal oestrogen during pregnancy in mice

A single subcutaneous injection of 5 or 1 mg oestradiol given to pregnant female mice on Day 14 of pregnancy resulted in all male offspring being cryptorchid. Pituitary LH content, testicular weights and structure, seminal vesicle weights and the structure of the reproductive tract as a whole were monitored on the day of birth and at 2, 4, 8 and 14 weeks of age. Apart from an initial significant reduction in pituitary LH at the time of birth, no other marked differences were seen between control and treated animals except that all oestrogen-treated males lacked a gubernaculum and the testes were freely mobile within the abdomen. Hypogonadal (hpg) male mice lacking GnRH are cryptorchid but have a normal gubernaculum and their testes develop and descend normally if treated with gonadotrophins. When the mothers of hpg mice were treated with oestradiol the male offspring lacked a gubernaculum. These results indicate that perturbations of the fetal hypothalamic/pituitary axis play no significant part in oestrogen-induced cryptorchidism in mice.     

Reversal of reproductive deficiency in the hpg male mouse by neonatal androgenization.

Some aspects of reproductive function in the GnRH-deficient hypogonadal (hpg) mutant mouse can be restored by transplanting normal fetal brain tissue containing GnRH cells into the central nervous system of adult hpg mice. However, hpg males showing physiological response to the graft fail to display sexual behavior and are infertile. We hypothesized that the reproductive deficit of these males is due to insufficient perinatal exposure to testicular androgens as a consequence of the GnRH deficiency. To test this hypothesis we androgenized hpg males by giving them neonatal injections of testosterone propionate (TP). Controls consisted of hpg males not androgenized neonatally and of normal males. All three groups received a TP implant in adulthood, and their copulatory behavior and reproductive capability were recorded. In addition, other hpg males, not androgenized neonatally, received fetal brain transplants containing GnRH neurons and were also tested for copulatory behavior and reproductive capability before and after receiving a TP implant. Three of 8 neonatally androgenized hpg males expressed the full repertoire of male sexual behavior, including intromission and ejaculation, and sired several litters. Three of 7 control hpg males that were not androgenized neonatally but received TP implants in adulthood also displayed mounting and intromission, but there was no evidence of ejaculation, and these males failed to impregnate normal females. Of the 8 hpg males that responded to a fetal transplant with testicular growth, only 1 displayed mounting behavior. However, when given a TP implant, 4 of 8 hpg males with grafts displayed mounting and intromissions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Androgenic regulation of luteinizing hormone secretion: relationship to androgen binding in sheep pituitary

Castrated ram lambs (wethers) were investigated for sensitivity to androgen feedback and to determine whether this feedback inhibition of luteinizing hormone (LH) was associated with changes in pituitary androgen receptors. Administration of Silastic capsules containing either dihydrotestosterone or testosterone was found to produce dose-dependent inhibitory effects on serum LH levels in wethers. Physiological dosages of these androgens (i.e., those that produce serum levels of dihydrotestosterone [0.24 ng/ml] or testosterone [2.1 ng/ml] similar to those of intact rams) resulted in differential inhibition of serum LH and LH content of the anterior pituitary. Whereas the inhibitory effect of dihydrotestosterone on pituitary LH content was much more dramatic than that seen with testosterone, the high dosage of testosterone also produced a substantial decrease in pituitary LH content. Responses of the pituitary to changes in serum androgen were compared to responses of the seminal vesicle, which served as a control androgen target organ. Androgen levels were positively correlated with seminal vesicle weights, but pituitary weights were unaffected by castration and/or androgen replacement. Treatments with dihydrotestosterone were associated with decreased cytosol androgen binding activity (i.e., receptors) in pituitary and seminal vesicle, suggesting that both of these tissues were sites of androgen action. Although testosterone inhibited serum LH levels, pituitary cytosol androgen receptors were not affected by changes in serum testosterone. We conclude from these data that dihydrotestosterone is a physiological regulator of pituitary LH secretion in the ram and that further study is needed to investigate the complex actions of testosterone and its metabolites on pituitary function.     

Regulation of thyroid hormones on the production of testosterone in rats

The effects of a thyroidectomy and thyroxine (T4) replacement on the spontaneous and human chorionic gonadotropin (hCG)-stimulated secretion of testosterone and the production of adenosine 3',5'-cyclic monophosphate (cAMP) in rat testes were studied. Thyroidectomy decreased the basal levels of plasma luteinizing hormone (LH) and testosterone, which delayed the maximal response of testosterone to gonadotropin-releasing hormone (GnRH) and hCG in male rats. T4 replacement in thyroparathyroidectomized (Tx) rats restored the concentrations of plasma LH and testosterone to euthyroid levels. Thyroidectomy decreased the basal release of hypothalamic GnRH, pituitary LH, and testicular testosterone as well as the LH response to GnRH and testosterone response to hCG in vitro. T4 replacement in Tx rats restored the in vitro release of GnRH, GnRH-stimulated LH release as well as hCG-stimulated testosterone release. Administration of T4 in vitro restored the release of testosterone by rat testicular interstitial cells (TICs). The increase of testosterone release in response to forskolin and androstenedione was less in TICs from Tx rats than in that from sham Tx rats. Administration of nifedipine in vitro resulted in a decrease of testosterone release by TICs from sham Tx but not from Tx rats. The basal level of cAMP in TICs was decreased by thyroidectomy. The increased accumulation of cAMP in TICs following administration of forskolin was eliminated in Tx rats. T4 replacement in Tx restored the testosterone response to forskolin. But the testosterone response to androstenedione and the cAMP response to forskolin in TICs was not restored by T4 in Tx rats. These results suggest that the inhibitory effect of a thyroidectomy on the production of testosterone in rat TICs is in part due to: 1) the decreased basal secretion of pituitary LH and its response to GnRH; 2) the decreased response of TICs to gonadotropin; and 3) the diminished production of cAMP, influx of calcium, and activity of 17beta-HSD. T4 may enhance testosterone production by acting directly at the testicular interstitial cells of Tx rats.     

Pituitary desensitization and the regulation of pituitary gonadotropin-releasing hormone (GnRH) receptors following chronic administration of a superactive GnRH analog and testosterone

We recently demonstrated that chronic daily administration of a superactive GnRH analog to intact rats resulted in an initial stimulation of serum LH levels with a subsequent return of LH levels to baseline at a time when testosterone levels were marked decreased. These data demonstrated pituatary desensitization following chronic GnRH analog treatment. Administration of GnRH analog with a dose of testosterone which did not markedly lower serum LH levels when administered alone prevented the stimulation of LH secretion by analog. The present studies were undertaken to determine the effects of GnRH analog and testosterone administration on the regulation of pituitary GnRH receptors. Pituitary GnRH receptor binding was increased by analog treatment alone at 20 days and returned to control levels at 40 and 60 days of treatment in parallel to the observed changes in serum LH, demonstrating that one mechanism by which chronic GnRH analog treatment leads to pituitary desensitization is down-regulation of pituitary GnRH receptors. Testosterone administration alone decreased pituitary GnRH receptor binding. Combined GnRH analog and testosterone administration prevented the increase in pituitary GnRH receptors observed with analog administration alone. These studies demonstrate that changes in pituitary GnRH receptor binding correlate with changes in serum LH and that the stimulatory effects of analog administration on LH are sensitive to inhibition by small doses of testosterone.     

Differences between the regulation of cholesterol side-chain cleavage in Leydig cells from mice and rats

A Leydig cell culture system has been used to study the in vitro modulation by luteinizing hormone (LH) of steroidogenesis in Leydig cells isolated from mice and immature rats. Mouse Leydig cells precultured for 24 h in the presence of increasing concentrations of LH (1 ng-1 microgram/ml) showed a dose-dependent decrease of the maximal LH-stimulated testosterone production. After pretreatment with 1 microgram LH/ml, maximal LH-stimulated testosterone production. After production in the presence of excess 20 alpha-hydroxycholesterol (a cholesterol side-chain cleavage substrate) were reduced to approx. 50% of control values. The possible site of action of LH is probably prior to pregnenolone, because testosterone production in the presence of excess pregnenolone was not affected by the LH pretreatment. Immature rat Leydig cells showed no decrease of maximal steroid production after 24 h culture in the presence of 1 microgram LH/ml. These results indicate that the regulation of the cholesterol side-chain cleavage activity during long-term LH action is different in mouse and rat Leydig cells. The properties of the cholesterol side-chain cleavage enzyme in mouse and rat Leydig cells were further investigated with different hydroxylated cholesterol derivatives as substrates. Steroid production by mouse Leydig cells in the presence of (22R)-22 hydroxycholesterol was similar as in the presence of LH. In contrast, steroidogenesis in rat Leydig cells in the presence of (22R)-22 hydroxycholesterol was at least 10-fold higher than in the presence of LH. It is concluded that the cholesterol side-chain cleaving enzyme in the mouse Leydig cell operates at its maximal capacity during short-term LH stimulation and can be inhibited after long-term LH action, whereas in the rat Leydig cell only a fraction of the potential activity is used during short-term LH stimulation, which is not affected during long-term LH action.     

Effects and interactions of LH and LHRH agonist on testicular morphology and function in hypophysectomized rats

The effects of single or combined daily treatment with an LHRH agonist and low or high doses of LH upon the testes of adult hypophysectomized rats were studied for up to 2 weeks in which changes in testicular histology, particularly the interstitial tissue, were examined by morphometry and related to functional assessment of the Leydig cells in vivo and in vitro. Compared to saline-treated controls, LHRH agonist treatment did not alter testis volume or the composition of the seminiferous epithelium or any of the interstitial tissue components although serum testosterone and in-vitro testosterone production by isolated Leydig cells were significantly reduced. With 2 micrograms LH for treatment, testis volume was increased, spermatogenesis was qualitatively normal, total Leydig cell volume was increased, serum testosterone values were initially elevated but subsequently declined and in-vitro testosterone production was enhanced. Testis volume with 20 micrograms LH treatment was unchanged compared to saline treatment, the seminiferous epithelium exhibited severe disruption but total Leydig cell volume was greatly increased due to interstitial cell hyperplasia. This group showed elevated serum testosterone concentrations and major increases in testosterone production in vitro. Treatment with LHRH agonist with either dose of LH resulted in reduced testis volume, moderate to very severe focal spermatogenic disruption and increased total Leydig cell volume although serum testosterone values and in-vitro testosterone production were markedly reduced compared to control rats. It is concluded that, in the absence of the pituitary, LHRH agonist fails to disrupt spermatogenesis and the previously described antitesticular action of LHRH agonists in intact rats is therefore dependent upon the presence of LH, which alone or in combination with LHRH agonist, may focally disrupt spermatogenesis in hypophysectomized rats whereas the Leydig cells undergo hyperplasia. The findings show that impairment of spermatogenesis is accompanied by alterations of the interstitial tissue and suggest that communication between these two compartments is involved in the regulation of testicular function.     

Testosterone inhibits 11-ketotestosterone-induced spermatogenesis in African catfish (Clarias gariepinus).

Male fish produce 11-ketotestosterone as a potent androgen in addition to testosterone. Previous experiments with juvenile African catfish (Clarias gariepinus) showed that 11-ketotestosterone, but not testosterone, stimulated spermatogenesis, whereas testosterone, but not 11-ketotestosterone, accelerated pituitary gonadotroph development. Here, we investigated the effects of combined treatment with these two types of androgens on pituitary gonadotroph and testis development. Immature fish were implanted for 2 wk with silastic pellets containing 11-ketotestosterone, testosterone, 5alpha-dihydrotestosterone, or estradiol-17beta; cotreatment groups received 11-ketotestosterone in combination with one of the other steroids. Testicular weight and pituitary LH content were higher (two- and fivefold, respectively) in the end control than in the start control group, reflecting the beginning of normal pubertal development. Treatment with testosterone or estradiol-17beta further increased the pituitary LH content four- to sixfold above the end control levels. This stimulatory effect on the pituitary LH content was not modulated by cotreatment with 11-ketotestosterone. However, the stimulatory effect of 11-ketotestosterone on testis growth and spermatogenesis was abolished by cotreatment with testosterone, but not by cotreatment with estradiol-17beta or 5alpha-dihydrotestosterone. Also, normal pubertal testis development was inhibited by prolonged (4 wk) treatment with testosterone. The inhibitory effect of testosterone may involve feedback effects on pituitary FSH and/or on FSH receptors in the testis. It appears that the balanced production of two types of androgens, and the control of their biological activities, are critical to the regulation of pubertal development in male African catfish.     

The different mechanisms for suppression of pituitary and testicular function

The differential mechanisms reducing androgen secretion by LHRH agonists are discussed with relevance to clinical therapy. LH secretion can be desensitised by exposure to agonists using high doses, frequent injections or sustained release/constant infusion. The desensitized pituitary is refractory to hypothalamic stimulation. Pituitary receptor suppression is associated with depletion of pituitary gonadotrophin content, and a decline of LH and FSH secretion to a basal rate. Recovery of LH responsiveness to endogenous LHRH stimulation requires restitution of gonadotrophin content (about 7 days in rats). After long-term infusions in normal men, testosterone secretion recovers within 7-10 days. The binding capacity of testicular LH/hCG receptors is reduced in rats after supraphysiological gonadotrophin stimulation, by agonists or directly by hCG, concomitantly the steroidogenic capacity of the testis in vitro is impaired. Qualitative changes in androgen biosynthesis are a marked fall in testosterone production and dose-dependent enhancement of progesterone production. After 12 months of buserelin injections, the changes in hCG-stimulated rat testes are an increased ratio of progesterone/17-OH-progesterone (inhibition of 17-hydroxylase), a reduced capacity for secretion of androstenedione and testosterone (block of 17,20-desmolase), and increased 5 alpha-pregnane-3,20-dione (this steroid inhibits the 17,20-desmolase, similarly to progesterone). After treatment, Leydig cell function recovers completely. Leydig cell hyperplasia is observed as a result of the steroidogenic changes. These findings in rats have not been observed in dogs, monkeys or in humans.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cortisol affects testicular development in male common carp, Cyprinus carpio L., but not via an effect on LH secretion

Previous work showed that prolonged elevated cortisol levels, implicated in the stress adaptation, inhibits testicular pubertal development in male common carp, as well as an impairment of the synthesis of the 11-oxygenated androgens. This may be a direct effect of cortisol on the testis or via the gonadotropin secretion by the pituitary. The aim of the present study was to investigate whether cortisol has an effect on pituitary LH secretion. Juvenile common carp were fed with cortisol containing food pellets. Elevated cortisol levels blocked the increase in testosterone levels and pituitary LH content, but induced higher plasma LH levels at the end of puberty. The in vitro LH release capacity was correlated to the pituitary LH content. At the final stage of pubertal development, when a significant difference in pituitary LH content was observed, sGnRHa-induced LH release was also decreased. Testosterone has been shown to induce development of pituitary gonadotrophs, leading to an increase in LH content and GnRH-inducible LH release, but a decrease in plasma LH levels. We observed decreased plasma testosterone levels as a consequence of prolonged cortisol treatment. It is hypothesised that cortisol inhibits the testicular testosterone secretion and thereby, prevents LH storage. In vitro, this leads to a reduced GnRH-inducible LH release, but in vivo to increased LH plasma levels. It is very unlikely that the impaired testicular development is due to an effect of cortisol on LH secretion.