Effects of opioid peptides on immunoreactive corticotropin-releasing factor release from the rat hypothalamus in vitro

Effects of opioid peptides on immunoreactive corticotropin-releasing factor (I-CRF) release from the rat hypothalamus were examined using a rat hypothalamic perifusion system and a rat CRF RIA in vitro. beta-Endorphin (0.3 - 30 nM), dynorphin (0.3 - 30 nM) and FK 33-824 (1 - 10 microM) suppressed basal I-CRF release in a dose-dependent fashion. At 2.2 nM concentrations of these peptides, mean percent inhibition was 56% for beta-endorphin; less than 5% for alpha-endorphin; 44% for dynorphin; 23% for leucine-enkephalin; 6% for methionine-enkephalin; less than 5% for FK 33-824; and less than 5% for D-ala2, D-leu5-enkephalin. The inhibitory effects of beta-endorphin and enkephalins were completely blocked by naloxone, but those of dynorphin were only partially blocked. These results suggest that opioid peptides act through opioid receptors and inhibit I-CRF release from the hypothalamus under our conditions. Therefore, endogenious opioid peptides may have a physiological role in the CRF-releasing mechanism of the hypothalamus.     

Localization of beta-endorphin in the rat heart and modulation by testosterone

Chromatographic analysis and radioimmunoassay were used to identify and quantitate beta-endorphin (BE) and beta-lipotropin (B-LPH) in the hearts (devoid of major blood vessels and atria) from intact male rats, castrated male rats, and castrated male rats treated with testosterone propionate (TP). BE and B-LPH in the plasma of these animals were also identified and measured. In comparison to intact animals, castration resulted in a significant elevation in the content of BE in the heart which was reversed by the administration of TP. The content of B-LPH in the heart was not affected by castration or castration in combination with TP. The ratio of BE to B-LPH in the heart of castrated animals was significantly elevated as compared with intact controls. Treatment of castrates with TP returned the ratio of BE to B-LPH to that observed in intact animals. The concentration of BE in the plasma was greater in castrated rats and castrated rats given TP than in intact males, whereas the concentration of B-LPH was diminished in castrated animals given TP. The ratio of BE to B-LPH was greater in castrated animals treated with TP than in castrated and intact animals. The content of BE and B-LPH, as well as the ratios of the two peptides, varied independently in the cardiac tissue and plasma. The present findings indicated that (i) BE and B-LPH are present in cardiac tissue, (ii) the amount of BE and B-LPH in the heart and the ratio of BE to B-LPH appear to be modulated by TP, and (iii) BE and B-LPH detected in the heart was not simply a reflection of the presence of these peptides in the plasma.     

Effect of castration on hypothalamic testosterone metabolism in the male rat

In vitro studies were performed of hypothalamic testosterone (T) metabolism 30 days after castration of adult male rats. No changes were seen in T conversion into dihydrotesterone and estrogens in the castrated rats. Plasma T levels were decreased while plasma estradiol concentrations did not differ from those of intact controls. It was suggested that the hypothalamic T metabolism probably is not androgen dependent.     

Influences of castration and testosterone on spring to summer changes in release of luteinizing-hormone-releasing hormone in rabbits.

Push-pull cannulae were implanted toward the tuberal region of the hypothalamus in ten intact New Zealand male rabbits. In the first experiment, rabbits were perfused at different times after castration: 5-10 days (n = 10), 22-31 days (n = 9) and 50-64 days (n = 8). The release, mean amplitude and mean frequency of luteinizing-hormone-releasing hormone (LHRH) signals from 37 perfusions in ten animals were analysed in intact rabbits and at different times after castration. No significant changes in release of LHRH and in amplitude were observed, but the frequency was significantly higher 22-31 days after castration than in intact rabbits (intact: 0.86 +/- 0.12; castrated: 1.20 +/- 0.13 pulses h-1, P < 0.035; n = 9). In Expt 2, testosterone and placebo Silastic capsules were implanted in the castrated rabbits. Perfusions were performed in the following four periods, defined by season and time after testosterone and placebo implants: (i) spring; before implants, (ii) late spring; 0-2 weeks after implants, (iii) summer solstice; 2-4 weeks after implants and (iv) summer; 4-6 weeks after implants. Castrated rabbits were perfused during spring; castrated rabbits with testosterone capsule implants were perfused during late spring, around summer solstice and in summer and castrated rabbits with placebo implants were perfused during periods (iii) and (iv). Castrated animals with placebo implants showed no significant changes in mean LHRH release and amplitude, although the frequency was significantly higher around the summer solstice period than in castrated rabbits perfused in the spring. In castrated rabbits with testosterone implants LHRH release was significantly higher in late spring than around the summer solstice and in the summer. In addition, the concentrations of LHRH in late spring were significantly higher than those of intact and castrated animals. In contrast, mean LHRH amplitude and frequency did not change. Mean amount of LHRH released and amplitude in castrated rabbits with testosterone implants were significantly lower around the summer solstice than in late spring or summer and compared with intact animals around summer solstice and in castrated rabbits in early spring. These data demonstrate that there were no significant changes in the mean amplitude and release of LHRH after castration from 5 and up to 64 days in rabbits with hypothalamic push-pull cannulae, in contrast to the well established dramatic effect of castration on gonadotrophin concentrations. However, there was a small, but significant, increase in the mean frequency of LHRH pulses 22-31 days after castration compared with values from intact rabbits.(ABSTRACT TRUNCATED AT 400 WORDS)     

Multiple Factors Influencing the In Vitro Release of [Met5]-Enkephalin from Rat Hypothalamic Slices

This study examined several in vivo and in vitro factors which influence the release of [Met5]-enkephalin (Met-ENK) from male rat hypothalamic slices superfused in vitro. Met-ENK release was significantly stimulated by corticotropin-releasing hormone (CRH; 10(-12)-10(-8) M), an effect which was abolished in the presence of the CRH-receptor antagonist, alpha-helical CRF9-41 (10(-6) M). The amount of Met-ENK release diminished with time in experiments in which the slices were continuously exposed to CRH. The opioid receptor antagonist naloxone (10(-6) M) stimulated Met-ENK release, even in the presence of the Na+ -channel blocker tetrodotoxin (10(-6) M), a result indicating presynaptic opioid feedback inhibition of Met-ENK release. The role of gonadal steroids in the control of Met-ENK release in vitro was also examined. It was found that the basal and CRH-induced release of Met-ENK was not changed 1 week after castration. However, a significant increase in the basal release of this peptide was observed 4 weeks after gonadectomy, and the Met-ENK-releasing efficacy of CRH was found to be reduced. The Met-ENK content of hypothalami from 1-week castrates was not significantly changed from control levels, but was significantly reduced in those from 4-week castrates. These long-term effects of castration could be overcome by the subcutaneous implantation of testosterone-containing capsules at the time of castration.     

Mediation by gonadal steroids of plasma beta-endorphin and LH in castrated female and male rats

Immunoreactive beta-endorphin (IR-BE) was significantly decreased and luteinizing hormone (LH) significantly increased in female rats castrated for four weeks. Forty eight hours after a single injection of estradiol benzoate (EB), IR-BE levels increased, and LH levels were reduced. On the afternoon following the administration of a second injection of EB given six hours earlier, IR-BE levels were reduced below control values, whereas LH levels were significantly elevated. There was no change in IR-BE levels during the remainder of that afternoon whereas LH levels decreased over time. Similar to female rats, IR-BE was diminished and LH increased in castrated male rats. IR-BE was increased significantly above those values observed in intact animals 24 hr after a single injection of TP and returned to control levels by 48 hr after administration of TP. Injection of TP reduced LH to levels observed prior to castration. These findings suggest that gonadal steroids exert a feedback on the release of IR-BE from the pituitary of female and male rats opposite to their feedback effect on the release of pituitary gonadotropins.     

A 'window of time' during which testosterone determines the opiatergic control of LH release in the adult male rat

Male rats castrated before puberty (when 26 days of age) showed a progressively decreasing susceptibility to the inhibitory effects of morphine (5 mg/kg) upon LH secretion for up to 28 days after gonadectomy (approximately 100%, 40% and 10% inhibition at 5, 12 and 28 days after castration), but thereafter morphine again caused approximately 50% reduction in serum LH values; the minimum inhibition found at 28 days after castration (age 54 days) occurred at the time at which male rats normally reach puberty. When rats were castrated at 59 days of age, morphine maximally suppressed serum LH concentrations (to less than 70%) 2 and 5 days after castration, but had no effect thereafter. In prepubertal castrates, testosterone replacement between Days 26 and 50 of life resulted in responses to morphine similar to those found in rats castrated after puberty, i.e. serum LH levels were not reduced. Morphine significantly reduced LH levels in prepubertal castrates given testosterone after 60 days of age. Treatment with morphine consistently elevated serum prolactin concentrations (greater than 100%) in castrated rats of all ages, regardless of the time elapsed after gonadectomy. These results indicate a transient fall in the inhibitory opioidergic tone upon LH secretion as the normal age of puberty approaches, that the ability of opiates to alter LH release in adulthood may depend upon testicular steroids secreted during the peripubertal period, and that the LH responses do not reflect general changes in the neuroendocrine response to opiates after castration since the prolactin response to morphine remains intact in rats castrated before and after puberty.     

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

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

Effect of prolonged incubation of male rat whole pituitary or pituitary-hypothalamus complex with testosterone on release of gonadotrophin and prolactin in vitro.

Investigations were undertaken to study the effect of in vitro addition of testosterone (0.3 mM) on the release of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) by pituitary-hypothalamus complex (PHC) or the whole pituitary (PI) incubated for 72 hr, with incubation media changed every 24 hr. PHC or PI were from adult intact or castrated (7 days post castration) rats. The tissues incubated with or without testosterone were further exposed to 0.1 nM luteinizing hormone-releasing hormone (LHRH) for 4 hr. Incubation media and the pituitary were analyzed for PRL and gonadotrophin content. While PHC from normal and castrated rats released increasing amounts of LH with diminishing amounts of FSH and PRL at different periods of incubation, PI showed a decrease in the amounts of gonadotrophin and PRL released. Co-incubation of PHC or PI of intact or castrated rats with testosterone stimulated the release of LH and FSH during the first or second-24 hr incubation but inhibited the release of PRL in all the three incubations of 24 hr each. The extent of PRL inhibition increased with increasing incubation period. Testosterone had no effect on LHRH induced release of PRL but inhibited LHRH induced release of LH and FSH by pituitaries from constructs of normal rats. Testosterone reduced intrapituitary contents of PRL and FSH of intact and castrated rats. The data are interpreted to suggest that hypothalamus is essential for the maintenance of functional pituitary in vitro and that intrinsic differences exist in mechanisms regulating the secretion of LH, FSH and PRL.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of neonatal castration on the content of hypothalamic LHRH, pituitary LH and plasma LH in developing male rats.

The content of hypothalamic LHRH and concentration of LH in pituitary and plasma were measured on day 5, 7, 10, 14, 17, 22, 25, 30, 45, 52 and 60 in male rats which were bilaterally castrated on day 2. The levels of plasma LH were significantly higher in all the groups of castrated rats than in normal male rats of corresponding ages. The concentration of plasma LH did not rise progressively but showed day to day fluctuation apparently due to alteration of sexual differentiation of the hypothalamus. The concentration of pituitary LH was significantly lower in neonatally castrated rats compared to normal male rats except on days 17, 25 and 30. The content of hypothalamic LHRH declined initially following castration, but from day 17 onwards significantly higher levels of hypothalamic LHRH were maintained in neonatally castrated rats than in intact control. Initial decline in the content of hypothalamic LHRH may be because of stimulation of release of LHRH which exceeds maximal rate of synthesis and subsequent increase in the content of hypothalamic LHRH may be due to enhanced LHRH synthesis as a result of castration.     

In vitro release of hypothalamic corticoliberine in the rat: incubation of slices from the whole hypothalamus

An experimental system allowing both the incubation and rapid transfert of rat hypothalamic slices has been developed in order to approach the regulation of CRF secretion. The release of CRF has been quantified by a specific radioimmunoassay. Under basal conditions, immunoreactive CRF release reached an optimum of 96.2 +/- 10.4 pg/3 hypothalami/20 min. A depolarizing concentration of KCl (56 mM) or veratridine (50 microM) applied for 20 min. induced a 222 and 257% increase, respectively, in CRF release. The in vitro CRF values released under basal and stimulated conditions are comparable to those of other hypothalamic neuropeptides. Furthermore, in vitro CRF release from the hypothalamus is in the same order of magnitude as in vivo CRF secretion estimated by hypophysial portal blood collection or median eminence push-pull cannulation.     

Modulation of luteinizing hormone(LH) release by clonidine and opioid peptides in castrated male rats

The effect of clonidine, a central alpha-adrenergic agonist, on the suppression of LH release induced by beta-endorphin or FK33-824, an endogenous opioid peptide or its synthetic analog, was investigated in castrated male rats, with or without pretreatment with reserpine. Pulsatile LH secretion was inhibited by intravenous injection of FK33-824 (400 micrograms/kg), or intraventricular injection of beta-endorphin (5 micrograms). Without pretreatment with reserpine, intraperitoneal administration of clonidine (1 mg/kg) failed to reverse the inhibition of LH release induced by these peptides. However, with pretreatment with reserpine (10 mg/kg), clonidine abolished the inhibitory effect on LH secretion induced by these peptides in castrated male rats. These data indicate that, unlike the results in ovariectomized, steroid-primed rats, pretreatment with reserpine allows the alpha-adrenergic system to act more peripherally than the opioid neuronal system in a neuronal network-regulating LH release in castrated male rats.     

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.     

Androgen metabolism by rat epididymis. 3. Effect of castration and anti-androgens.

The in vivo and in vitro metabolism of 3H-testosterone by rat epididymis and the changes in epididymal weight have been studied after castration and treatment with anti-androgens. The utilization of 3H-testosterone was greatly reduced after castration as was the formation of 5alpha-reduced 17 beta-hydroxy metabolites. The formation of the 17 -keto metabolites was unaffected. Castration had no effect on the ratio between water and ether soluble radioactivity. Administration of testosterone propionate, necessary for giving normal stimulated prostate weight (150 mug/day), restored the metabolism of testosterone to approximately normal values. Estradiol benzoate and progesterone inhibited metabolism of testosterone in vitro and greatly reduced the formation of DHT (17 beta-hydroxy-5alpha-androstan-3-one) and 3 alpha-diol(5 alpha-androstane-3 alpha-17 beta-diol) by experiments both in vivo and in vitro. No effect of cyproterone acetate could be demonstrated on either the in vitro or in vivo metabolism of testosterone. Castration for 14 days reduced the epididymal weight to about 30% of that found in intact animals. Administration of testosterone propionate restored the epididymal weight to about 80% of normal. Estradiol benzoate and cyproterone acetate given to intact rats led to a decrease in the epididymal weight. Progesterone had no such effect. In 14 days castrated rats receiving testosterone propionate all three anti-androgens reduced the weight of the epididymis. In conclusion, our results show that the metabolic conversion of testosterone in epididymis to DHT and 3 alpha-diol is dramatically dependent on the hormonal status of the animal; castration or treatment with anti-androgens causes a reduced formation of the "active" androgens whilst testosterone replacement treatment restores the metabolism of testosterone to normal.     

Immunoreactive beta-endorphin in the plasma, pituitary and hypothalamus of young female rats on the day of estrus and intact and chronically castrated old constant estrous female rats

Immunoreactive beta-endorphin (IR-beta-ENDO) was compared in the plasma, pituitary and hypothalamus of young female rats on the day of estrus and old constant estrous (CE) female rats, and in intact and chronically castrated old CE female rats. The concentration of IR-beta-ENDO in the plasma and the content and concentration of IR-beta-ENDO in the neurointermediate lobe of the pituitary were significantly greater in the old CE female rats than in the young female rats on the day of estrus. The content and concentration of IR-beta-ENDO in the anterior pituitary and hypothalamus were similar in the two age groups. To determine if estrogen contributed to the increase in plasma and pituitary levels of IR-beta-ENDO observed in the old animals, a group of old CE female rats were castrated and compared to sham operated control CE rats. Thirty days after castration, levels of plasma, pituitary and hypothalamic IR-beta-ENDO were comparable in the intact and the chronically castrated old female rats. These data indicate that in old CE female rats, plasma and pituitary IR-beta-ENDO are significantly increased in comparison to young female rats on the day of estrus, and that these increased levels of IR-beta-ENDO observed in old female rats do not appear to be influenced by gonadal estrogen.     

Benzodiazepine suppression of corticotropin-releasing factor (CRF)-induced beta-endorphin release from rat neurointermediate pituitary.

Dopamine and gamma-aminobutyric acid (GABA) inhibit POMC peptide release from the pituitary intermediate lobe, via interaction with D2 or GABA-A/benzodiazepine receptors. Here, we examined the effects of an antianxiety triazolobenzodiazepine, adinazolam, on corticotropin-releasing factor (CRF)-stimulated POMC peptide secretion from the rat neurointermediate pituitary. Neurointermediate lobes (NILS) were incubated with CRF (10(-7) M), then adinazolam (10(-8) or (10(-9) M) was added, with CRF remaining in the medium. Aliquots were removed at 15-min intervals and frozen for radioimmunoassay of beta-endorphin. Adinazolam alone did not significantly affect secretion as compared to controls or CRF alone. Adinazolam incubated with CRF led to significant inhibition of beta-endorphin secretion, as compared to CRF alone. In addition, adinazolam was as effective as dopamine or the CRF antagonist, alpha-helical CRF, in preventing CRF-induced beta-endorphin release. Adinazolam appears to act directly on the pituitary to suppress hormone release induced by a stress-related hypothalamic peptide.     

Effects of hyperprolactinemia on the control of luteinizing hormone and follicle-stimulating hormone secretion in the male rat

Experiments were conducted to determine the effects of acute hyperprolactinemia (hyperPRL) on the control of luteinizing hormone and follicle-stimulating hormone secretion in male rats. Exposure to elevated levels of prolactin from the time of castration (1 mg ovine prolactin 2 X daily) greatly attenuated the post-castration rise in LH observed 3 days after castration. By 7 days after castration, LH concentrations in the prolactin-treated animals approached the levels observed in control animals. HyperPRL had no effect on the postcastration rise in FSH. Pituitary responsiveness to gonadotropin hormone-releasing hormone (GnRH), as assessed by LH responses to an i.v. bolus of 25 ng GnRH, was only minimally effected by hperPRL at 3 and 7 days postcastration. LH responses were similar at all time points after GnRH in control and prolactin-treated animals, except for the peak LH responses, which were significantly smaller in the prolactin-treated animals. The effects of hyperPRL were examined further by exposing hemipituitaries in vitro from male rats to 6-min pulses of GnRH (5 ng/ml) every 30 min for 4 h. HyperPRL had no effect on basal LH release in vitro, on GnRH-stimulated LH release, or on pituitary LH concentrations in hemipituitaries from animals that were intact, 3 days postcastration, or 7 days postcastration. However, net GnRH-stimulated release of FSH was significantly higher by pituitaries from hyperprolactinemic, castrated males. To assess indirectly the effects of hyperPRL on GnRH release, males were subjected to electrical stimulation of the arcuate nucleus/median eminence (ARC/ME) 3 days postcastration. The presence of elevated levels of prolactin not only suppressed basal LH secretion but reduced the LH responses to electrical stimulation by 50% when compared to the LH responses in control castrated males. These results suggest that acute hyperPRL suppresses LH secretion but not FSH secretion. Although pituitary responsiveness is somewhat attenuated in hyperprolactinemic males, as assessed in vivo, it is normal when pituitaries are exposed to adequate amounts of GnRH in vitro. Thus, the effects of hyperPRL on pituitary responsiveness appear to be minimal, especially if the pituitary is exposed to an adequate GnRH stimulus. The suppression of basal LH secretion in vivo most likely reflects inadequate endogenous GnRH secretion. The greatly reduced LH responses after electrical stimulation in hyperprolactinemic males exposed to prolactin suggest further that hyperPRL suppresses GnRH secretion.     

A paradoxical castration effect on LH-RH levels in male rat hypothalamus and serum

Serum and hypothalamic luteinizing hormone releasing hormone (LH-RH) was lowered in young mature male rats after castration. Testosterone injections raised the hypothalamic LH-RH content significantly. The mean value of serum LH level was elevated by testosterone, but not significantly. Hypothalamic LH-RH content was also lowered by hypophysectomy. In this circumstance, testosterone injections significantly increased LH-RH content. These results suggest that there may be a positive feedback of testosterone upon the hypothalamic LH-RH release and synthesis mechanisms.     

Sex-differences in adrenocortical responsiveness during development in rats

It is known that the stress hyporesponsive period (SHRP), which seems to be related to an immature hypothalamo-pituitary-adrenal (HPA) regulatory system, occurs during the first 2 weeks after birth in rats. In the present study, we investigated the effects of sex-steroid hormones on adrenocortical responsiveness to adrenocorticotropic hormone (ACTH) in neonatal rats. The levels of cyclic adenosine 3',5'-monophosphate (cAMP), corticosterone, and adenylate cyclase activity increased with the dose of ACTH in adrenal cells of males and females in vitro. The ACTH responsiveness in adrenal cells increased with age (7-35 days of age), that is, the loss in responsiveness to ACTH just after birth began to recover in 14-35-day-old rats, but the responsiveness in 14-day-old rats was attenuated in males compared with females. Although castration markedly augmented the responsiveness in male rats, testosterone-replacement in the castrated male rats inhibited the enhancement. Furthermore, the responsiveness in 14-day-intact female rats was suppressed by treatment with testosterone. Expression levels of ACTH receptor mRNA in adrenals increased with age in the female rat, but not in the male. Castration enhanced the level of ACTH receptor mRNA to three-fold of that in intact male rats at 14 days of age, but replacement treatment with testosterone in castrated male rats lowered the elevated levels. Testicular androgens are thought to evoke a gender-specific response in neonates, and the temporal decrease of adrenal ACTH-responsiveness might be due to the topically immature adrenal system as well as the central nervous system in mammals.     

Role of gonadal negative feedback on the gonadotrophin responses to gonadotrophin-releasing hormone (GnRH) in ram lambs from two lines of sheep selected for their luteinizing hormone response to GnRH.

Divergent selection has resulted in two lines of lambs (high and low) that have a 5-fold difference in their ability to release luteinizing hormone (LH) in response to 5 micrograms of gonadotrophin-releasing hormone (GnRH). Baseline gonadotrophin concentrations, the gonadotrophin responses to a GnRH challenge and the concentrations of testosterone and oestradiol were compared in lambs which were castrated at birth and intact lambs from both selection lines at 2, 6, 10 and 20 weeks of age. The pattern of LH and follicle-stimulating hormone (FSH) secretion was similar in the two lines, but differed between the intact and the castrated lambs. Basal LH and FSH secretion were significantly higher in the castrates than in the intact lambs from both selection lines. The high-line lambs had significantly higher basal FSH concentrations at all ages tested and significantly higher basal LH concentrations during the early postnatal period. The magnitude of the gonadotrophin responses to GnRH differed significantly between the intact and the castrated lambs within each line, the amount of gonadotrophins secreted by the castrated lambs being significantly greater. The removal of gonadal negative feedback by castration did not alter the between-line difference in either LH or the FSH response to the GnRH challenge. Throughout the experimental period, the concentration of testosterone in the intact lambs was significantly greater than in the castrated lambs in both selection lines, but no significant difference was seen in the concentrations of oestradiol. No significant between-line differences were found in the peripheral concentrations of testosterone or oestradiol in the intact lambs from the two selection lines. Therefore, despite similar amounts of gonadal negative feedback in the selection lines, there were significant between-line differences in basal gonadotrophin concentrations, at 2 and 6 weeks of age, and in the LH and FSH responses to an exogenous GnRH challenge, at all ages tested. Removal of gonadal negative feedback did not affect the magnitude of the between-line difference in the response of the lines to GnRH stimulation. The results indicate that the effects of selection on gonadotrophin secretion are primarily at the level of the hypothalamo-pituitary complex.