Hypothalamic regulation of pituitary secretion of luteinizing hormone

Luteinizing hormone (LH) is secreted continuously from the anterior pituitary gland. The concentration in the blood of this gonadotropic hormone plays a regulatory role in the development of puberty in both sexes, in the induction of ovulation in females, and in the production of testosterone in males. The secretion of LH is in turn controlled by luteinizing hormone releasing hormone (LHRH) secreted by the hypothalamus. LH and LHRH are removed from the blood by degradation and excretion. This hormonal system is modelled by a system of ordinary differential equations based upon specific physiological and biochemical assumptions current among experimentalists in this field. The one exception is the assumption that LHRH may bind reversibly to a serum protein; an analysis of the data shows that this or a similar mechanism is a crucial specification. Data on the serum levels of LH and LHRH in two human subjects were fitted using the model. The data consist of the transients and subsequent decays created by a bolus intravenous injection of LHRH. Primary appointment: Chemistry Dept., Dalhousie University. Primary appointment: Mathematics Dept., Dalhousie University.     

Testosterone control of brain and anterior pituitary beta-adrenergic receptors

Orchidectomy of adult albino rats resulted in a quick, (approximately 70%) increase in the density of beta-adrenergic receptors in the anterior pituitary gland within the first day. There was a concurrent rapid increase in plasma levels of pituitary gonadotropins. The beta-receptor density continued to increase slowly for at least 16 days after castration, but it could be lowered significantly to the levels of sham-operated animals by treatment with testosterone (3 mg/kg/day) beginning on the fourth day after castration and continuing for 4 days. This treatment also completely reversed the elevation in plasma levels of luteinizing hormone (LH), and significantly reduced the circulating follicle-stimulating hormone (FSH) levels. Prolactin levels were not significantly altered by the treatments used in these studies. Most of the beta-adrenergic receptors induced by orchidectomy in the anterior pituitary were shown, using a beta 1-selective antagonist, practolol, or a beta 2-selective antagonist, IPS-339, to be of the beta 2-subtype. The density of the beta-adrenergic receptors in the cerebral cortex also increased significantly (10-24%) after castration, and returned to the levels of sham-operated animals following treatment with testosterone. No significant change in the density of the beta-adrenergic receptors in the hypothalamus resulted from either castration or testosterone replacement.     

Nutritional influences on sexual maturation in the rat

The effect of altered nutrition on sexual maturation may depend in part on the nature and timing of the dietary change. The data are conflicting as to whether rats undernourished before weaning but normally fed after weaning have delayed puberty, but such undernourished rats clearly weigh less at vaginal opening than do normally fed animals. Altered nutrition after weaning can change the timing of puberty, and in such cases the body weight at puberty of the animals given the modified diet is frequently abnormal. The factors regulating the age and weight at puberty of rats fed altered diets seem to include the degree of underfeeding, as reflected in the growth rate, and the composition of the diet. Undernourished immature male rats have low serum testosterone secondary to gonadotropin deficiency. Basal luteinizing hormone (LH) in these animals is either low or "inappropriately normal" relative to their hypoandrogenic state (low serum testosterone and sexual accessory gland weights), and serum LH increases after luteinizing hormone-releasing hormone (LHRH) or castration are normal or minimally reduced. Serum follicle-stimulating hormone (FSH) in undernourished rats is subnormal basally and after administration of LHRH, but not after castration, which suggests that the low basal serum FSH is due to inhibition of FSH output by a testicular factor. Spermatogenesis may be unaltered by dietary changes severe enough to cause hypoandrogenism, although very severe under-nutrition will impair sperm production.     

A model for the control of testosterone secretion

We produce here a model to explain the control of testosterone secretion. In this model the hypothalamic secretion of the hormone LHRH (luteinizing hormone releasing hormone) is controlled by a combination of local testosterone concentration and of the local concentration of the pituitary hormone LH (luteinizing hormone). Since LHRH stimulates the release of LH, and LH in turn stimulates the release of testosterone, the three hormones constitute a three-component "feedback" network. We show how this model is able to account for the pulsatility of the release of these three hormones. Furthermore, the model is consistent with results obtained from a wide range of experimental manipulations of the system. For example, it accounts for the changes observed in hormone release patterns after castration. In particular, it follows that no "neural clock", or "neural pulse-generator", is required to force the system into pulsatile behaviour.     

Inhibition of release of a novel pituitary polypeptide, 7B2, follicle-stimulating hormone, and luteinizing hormone from rat anterior pituitary cells in vitro by human beta-inhibin

We have recently purified a novel pituitary polypeptide designated 7B2. By raising polyclonal antibodies to a synthetic 7B2 fragment in rabbits, we have developed a sensitive and specific radioimmunoassay for this novel polypeptide, and it has been used for the study of the release of immunoreactive 7B2 from rat anterior pituitary cells in vitro. In addition, immunocytochemical study shows that 7B2 is present in the gonadotropin cells of rat anterior pituitary. The aim of the present studies is to investigate the effect of human beta-inhibin, testosterone, and combined testosterone plus human beta-inhibin on the induced release of immunoreactive 7B2, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) in rat anterior pituitary cell culture in vitro. Our results show that both human beta-inhibin and testosterone effectively suppress the stimulatory effect of luteinizing hormone-releasing hormone (LHRH) on immunoreactive 7B2, FSH, and LH release. The present data indicate that the regulation of secretion of 7B2 and pituitary gonadotropins may be under a similar type of feedback mechanism.     

Effects of intermittent pulsatile infusion of luteinizing hormone-releasing hormone on dihydrotestosterone-suppressed gonadotropin secretion in castrate rams

The feedback effects of dihydrotestosterone (DHT) on gonadotropin secretion in rams were investigated using DHT-implanted castrate rams (wethers) infused with intermittent pulsatile luteinizing hormone-releasing hormone (LHRH) for 14 days. Castration, as anticipated, reduced both serum testosterone and DHT but elevated serum LH and follicle-stimulating hormone (FSH). Dihydrotestosterone implants raised serum DHT in wethers to intact ram levels and blocked the LH and FSH response to castration. The secretory profile of these individuals failed to show an endogenous LH pulse during any of the scheduled blood sampling periods, but a small LH pulse was observed following a 5-ng/kg LHRH challenge injection. Dihydrotestosterone-implanted wethers given repeated LHRH injections beginning at the time of castration increased serum FSH and yielded LH pulses that were temporally coupled to exogenous LHRH administration. While the frequency of these secretory episodes was comparable to that observed for castrates, amplitudes of the induced LH pulses were blunted relative to those observed for similarly infused, testosterone-implanted castrates. Dihydrotestosterone was also shown to inhibit LH and FSH secretion and serum testosterone concentrations in intact rams. In summary, it appears that DHT may normally participate in feedback regulation of LH and FSH secretion in rams. These data suggest androgen feedback is regulated by deceleration of the hypothalamic LHRH pulse generator and direct actions at the level of the adenohypophysis.     

Influence of testosterone and/or luteinizing hormone releasing hormone analogue on precocious sexual development in the juvenile rainbow trout

The influence of testosterone, luteinizing hormone releasing hormone (LHRH) agonist and combinations of these hormones on gonadotropic hormone (GtH) levels in the sexually immature trout was investigated. Both the steroid and releasing hormone preparations, testosterone in Silastic capsules and cholesterol-pelleted LHRH-A, were formulated for sustained release and long-term biological action following a single hormone implantation. Marked increases in pituitary GtH followed testosterone and/or testosterone and LHRH analogue treatment combined, but the low pituitary GtH level in controls remained unchanged after LHRH analogue administration alone. Plasma GtH titers increased with time after testosterone treatment, indicating a positive steroid feedback effect by androgen on GtH in the juvenile rainbow trout. When combined with testosterone treatment, LHRH analogue augmented plasma GtH levels compared to fish receiving testosterone treatment alone. In males the elevated plasma GtH levels were associated with testes stimulation and onset of spermatogenesis; in females, however, no significant stimulation of the ovaries was observed. It can be concluded from these studies that the testosterone stimulus is sufficient to induce onset of sexual development in immature males but not females. Whereas LHRH analogue releases GtH from the testosterone-primed trout pituitary, LHRH treatment alone under these conditions fails to stimulate the juvenile trout reproductive system.     

Effect of ethanol on androgen receptors in the anterior pituitary, hypothalamus and brain cortex in rats

The purpose of this study was to investigate ethanol-induced changes in androgen receptor sites in the anterior pituitary, hypothalamus, and brain cortex. Young adult male King-Holtzman rats were fed for 5 months a nutritionally complete liquid diet, with ethanol or isocaloric sucrose constituting 36% of the total calories. Androgen receptor sites were measured by sucrose density gradient and charcoal assay using tritiated dihydrotestosterone (DHT). Scatchard plot analysis of the data revealed that apparent dissociation constants of DHT-receptor complex for the anterior pituitary, hypothalamus, and brain cortex from alcohol-fed animals were estimated to be 0.7 +/- 0.13, 0.6 +/- 0.16 and 0.9 +/- 0.15 nM, respectively. These values are identical to those of their isocaloric controls. The concentrations of cytosol androgen receptors of the pituitary, hypothalamus, and brain cortex from alcohol-fed rats were 8.0 +/- 1.2, 6.2 +/- 1.0 and 4.9 +/- 0.7 fmol/mg protein, respectively. This represents about a 34, 24, and 22% reduction when compared to the values of the isocaloric control animals. In contrast to control rats, neither castration nor androgen or LHRH replacement to castrated alcohol-fed rats altered an alcohol-induced reduction of androgen receptor contents. Serum LH and testosterone levels were significantly decreased in alcohol-fed rats but these hormone levels were increased by administration of LHRH or norepinephrine. Such reduction of androgen receptors, serum LH and testosterone, but enhancement of these hormone levels by treatment with neurohormone and neurotransmitter in these animals suggests that ethanol exerts an adverse effect on the hypothalamic-pituitary unit and the neurotransmitter-hypothalamic hormone relationship, resulting in impairment of the androgen-induced sexual events and a suppression of the pituitary gonadotropin secretion.     

A new male hypogonadism mutant rat (hgn/hgn): concentrations of testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) in the serum and the responsiveness of accessory sex organs to exogenous T, FSH, human chorionic gonadotropin, and luteinizing hormone-releasing hormone

To determine the etiology of male hypogonadism in a newly found mutant rat (hgn/hgn, with a single autosomal recessive trait), concentrations of testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured, and the responsiveness of the urogenital organs, hypothalamus, and pituitary gland to testosterone (1 mg/kg s.c. for 7 days), FSH (0.3 AU/kg s.c. for 7 days), human chorionic gonadotropin (hCG) (40 IU/kg s.c. for 7 days), and luteinizing hormone-releasing hormone (LHRH) (0.5 or 5.0 micrograms/kg s.c. for 7 days) were tested. Treatment with testosterone only increased the weights of all of the accessory sex organs, whereas treatment with FSH, hCG, or LHRH did not. Levels of serum FSH and LH were extremely higher and testosterone was lower in hgn/hgn males than in normal males. Serum FSH and LH decreased to levels found in intact animals after treatment with testosterone, suggesting that hypothalamic responsiveness to exogenous testosterone is present in the hgn/hgn males. Thus, the status of the hgn/hgn males was indicated to be due to primary Leydig cell dysfunction.     

Analysis of androgen action on pituitary gonadotropin and prolactin secretion in ewes

To study the role of androgens in the control of gonadotropin and prolactin secretion in ther ewe, we have characterized androgen receptors in pituitary cytosol, and investigated the effect of androgens on pituitary hormone release in vivo and in vitro. High affinity, low capacity receptors, with an affinity for methyltrienolone (R1881) greater than 5 alpha-dihydrotestosterone (5 alpha-DHT) greater than testosterone (T) much greater than androstenedione (A4), estradiol-17 beta (E2) and progesterone (P), were identified in pituitary cytosol. Addition of 1 nM 5 alpha-DHT, but not A4, inhibited luteinizing hormone (LH) release from pituitary cells in vitro, induced by 10(10) to 10(-7) M luteinizing hormone releasing hormone (LHRH). The release of follicle-stimulating hormone (FSH) with 10(-9) M LHRH was inhibited when cells were incubated with 1 nM 5 alpha-DHT. 5 alpha-DHT had no effect when higher or lower doses of LHRH were used. In ovariectomized ewes, neither an i.v. injection of 1 mg, nor intracarotid injections of up to 1 mg, 5 alpha-DHT affected plasma LH, FSH or prolactin levels, despite dose-related increases in plasma 5 alpha-DHT levels. Daily or twice daily i.m. injections of 5 mg 5 alpha-DHT in oil did not affect LH or FSH levels, but daily injections of 20 mg significantly reduced plasma LH levels within 4 days and plasma FSH levels within 6 days. Thus, despite the presence of androgen receptors in the ewe pituitary, we conclude that androgens per se are of minimal importance in the regulation of pituitary LH, FSH and prolactin secretion in the ewe. The low binding affinity of A4 and the lack of its effect on hormone secretion in vitro suggests that A4 may act as an estrogen precursor rather than an androgenic hormone. The function of the pituitary androgen receptor remains to be established.     

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)     

Feedback regulation of gonadotropic hormone secretion in neonatal pigs

The effect of castration and of administration of charcoal-treated porcine follicular fluid (pFF) containing inhibin-like activity on plasma concentration of gonadotropic hormones was studied in neonatal pigs. Plasma follicle-stimulating hormone (FSH) concentration averaged 25.1 +/- 1.5 ng/ml (mean +/- SEM) in 1-wk-old females and gradually declined to 20.2 +/- 0.7 ng/ml 6 wk later. Ovariectomy did not significantly influence plasma FSH concentration. In males, concentration averaged 8.0 +/- 0.7 ng/ml before castration but rose significantly within 2 days after castration. Injection of luteinizing hormone-releasing hormone (LHRH) did not influence plasma FSH concentrations in intact males, but did in females and in 7-wk-old males castrated at 1 wk. Plasma luteinizing hormone (LH) concentrations in 1-wk-old females (2.2 +/- 0.4 ng/ml) gradually declined and were not influenced by castration. Concentrations of plasma LH in 1-wk-old male piglets (2.8 +/- 0.7 ng/ml) were not significantly influenced by castration within 2 days but were significantly higher 6 wk later. LHRH induced a significant rise in plasma LH concentrations in all animals. Injection of pFF resulted in a decline of plasma FSH concentrations in intact and castrated males and in intact females, but did not influence plasma LH concentrations. These data demonstrate a sex-specific difference in the control of plasma FSH, but not in plasma LH concentration in the neonatal pig. Plasma FSH concentrations, but not plasma LH concentrations, are suppressed by testicular hormones in 1-wk-old piglets. Plasma FSH concentrations can be suppressed in both neonatal male and female pigs by injections of pFF.     

Release of luteinizing hormone-releasing hormone (LHRH) and neuroactive substances in unanesthetized animals as estimated with push-pull cannulae (PPC)

In this report, we have reviewed recent information gathered by probing with a push-pull cannula (PPC) the in vivo activity of the suprachiasmatic nucleus (SCN), hypothalamus, and anterior pituitary gland of freely moving animals. In male and female rats, probing of the SCN with the PPC revealed distinct oscillatory patterns of 5-hydroxy indole-acetic acid (5-HIAA) output very much dependent on the position of the cannula. In males, it was also possible to demonstrate, for the first time, in vivo output of immunoreactive vasopressin (VP) most likely from the SCN. Interestingly, the output of VP was stimulated by local activation of probable 5-hydroxytryptamine (5-HT) terminals with 5-hydroxytryptophan (5-HTP), a precursor of 5-HT synthesis. Probing the hypothalamus of rats and rabbits revealed that the in vivo release of luteinizing hormone-releasing hormone (LHRH) (frequency and amplitude of the LHRH signal) can be altered by administration of estrogen to ovariectomized rats; in both species, progesterone stimulated the amplitude of the LHRH signal, but only when this steroid was infused in pulses--the physiological mode of circulating progesterone in the rat. Further, in male rabbits, pulses of progesterone did not stimulate LHRH release. Last, probing the anterior pituitary with the PPC revealed that a series of push-pull perfusions could be performed in the same animal under different experimental conditions for nearly 60 days of experimentation. It also resolved the apparent paradox that after castration, decreased instead of increased activity of the neural LHRH apparatus was noticed when the PPC was positioned in the hypothalamus. Moving the PPC to the anterior pituitary revealed that castration was accompanied by an increase in the amplitude and frequency of the LHRH signals arriving in the anterior pituitary of castrated male rats. This mode of operation of the LHRH pulse generator is clearly compatible with the mode of luteinizing hormone (LH) release in gonadectomized animals. Finally, based on these results, a hypothetical model of the operation of the LHRH pulse generator has been proposed.     

Effects of aging on pituitary and testicular luteinizing hormone-releasing hormone receptors in the rat

Aging exerts profound influences on the function of the hypothalamic-pituitary-testicular-axis. This work has been performed in order to verify whether, in male rats, the decreased secretion of LH and testosterone (T) occurring in old animals is reflected by modifications of luteinizing hormone-releasing hormone (LHRH) receptors at the level of the anterior pituitary and of the testes. To this purpose, the affinity constant (Ka) and the maximal binding capacity (Bmax) for the LHRH analog [D-Ser(tBu)6]des-Gly10-LHRH-N-ethylamide were evaluated, by means of a receptor binding assay, in membrane preparations derived from the anterior pituitary and testicular Leydig cells of male rats of 3 and 19 months of age. Serum levels of LH and T were measured by specific RIAs. The results obtained show that, in aged male rats, the concentration of pituitary LHRH receptors is significantly lower than that found in young animals. On the other hand, the concentration of LHRH binding sites is significantly increased on the membranes of Leydig cells of old rats. In no instance the Ka for the LHRH analog is significantly affected. Serum levels of LH and T are significantly lower in old than in young male rats. In conclusion, these results suggest that the reduced secretion of LH in old male rats may be linked, at least partially, to a decrease of the number of pituitary LHRH receptors. The impaired production of testosterone occurring in aged rats is accompanied by a significant increase of the number of testicular LHRH receptors, indicating that also the intratesticular mechanisms controlling testosterone release undergo significant alterations with aging.     

Luteinizing hormone secretion from the quail pituitary in vitro

An enzymatically dispersed pituitary preparation from Japanese quail (Coturnix coturnix) was used to study the dynamics of gonadotropin release. After an 18-h incubation, the cells were challenged with different luteinizing hormone-releasing hormones (LHRH) for 90 min. Using pituitary cells from mature males, mammalian and chicken LHRH I (Gln8-LHRH) had approximately equal luteinizing hormone (LH)-releasing activity whereas chicken LHRH II (His5, Trp7, Tyr8-LHRH) was 8-9 times more potent. The LHRH agonist (Trp6, Pro9-NEt-LHRH) had 15 times greater potency than chicken LHRH I. Pre-incubation with an LHRH antagonist (D-Phe2, D-Trp6-LHRH) significantly suppressed LH release. Acid extracts of median eminence released LH from pituitary cells, extracts from short-day and long-day males had equal activity, while tissue extracts from castrated males had significantly greater LH-releasing activity. Pituitary cells from sexually immature males released LH in response to chicken LHRH I in a similar profile to cells from mature males. These data indicate that the quail LHRH receptor in the male recognizes several different molecular species of LHRH and the response to LHRH is comparable between short- and long-day males. Pituitary cells from ovulating females were variably sensitive to LHRH peptides, possibly due to changes in pituitary sensitivity during the ovulatory cycle. Pituitary cells from immature females did not release LH in response to chicken LHRH I. However, pituitary cells from immature females photostimulated for 1 wk displayed a response to chicken LHRH I and II similar to that of pituitary cells from males.(ABSTRACT TRUNCATED AT 250 WORDS)     

Induction of the self-priming effect of luteinizing hormone releasing hormone during the sexual maturation of the male rat

Pubertal and young adult male rats release more luteinizing hormone (LH) in response to luteinizing hormone releasing hormone (LHRH) if pretreated with LHRH than if pretreated with saline. Immature male rats do not show this self-priming effect. In order to examine the role of acute changes in testicular steroids in this process, immature (29-30 days old) or pubertal (50-51 days old) male rats were castrated or sham operated under ketamine HCl anesthesia. Beginning immediately after completion of the surgery, they were given three priming injections of 10 ng LHRH/100 g body wt or saline at 30-min intervals. Thirty minutes after the third priming injection, a blood sample was obtained by cardiac puncture followed immediately by a challenge injection of 50 ng LHRH/100 g body wt given to both saline and LHRH primed groups. Ten minutes after the challenge injection a final blood sample was obtained by heart puncture. Serum was assayed for LH concentration by radioimmunoassay. Sham-operated pubertal rats showed a typical self-priming effect. Animals pretreated with LHRH released significantly (P less than 0.01) more LH in response to the challenge injection than did rats pretreated with saline. Acute castration also resulted in a significant (P less than 0.001) self-priming effect in pubertal rats. As anticipated, sham castrated immature males did not show a self-priming effect. Acutely castrated immature rats however, showed a significant (P less than 0.05) self-priming effect. These data provide support for the hypothesis that, prior to puberty, increases in testosterone during the priming process inhibit the expression of the self-priming effect.     

Effect of bombesin on basal and stimulated secretion of some pituitary hormones in humans

The effect of bombesin (5 ng/kg/min X 2.5 h) on basal pituitary secretion as well as on the response to thyrotropin releasing hormone (TRH; 200 micrograms) plus luteinizing hormone releasing hormone (LHRH; 100 micrograms) was studied in healthy male volunteers. The peptide did not change the basal level of growth hormone (GH), prolactin, thyroid-stimulating hormone (TSH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH). On the contrary, the pituitary response to releasing hormones was modified by bombesin administration. When compared with control (saline) values, prolactin and TSH levels after TRH were lower during bombesin infusion, whereas LH and FSH levels after LHRH were higher. Thus bombesin affects in man, as in experimental animals, the secretion of some pituitary hormones.     

Effects of alcohol feeding on androgen receptors in the rat pituitary gland

Specific binding of testosterone-1 beta, 2 beta-3H by cytosol from anterior pituitary gland of alcohol-fed, isocaloric control, and castrated control and alcohol-fed rats with or without testosterone treatment has been investigated by charcoal assay. The number of androgen binding sites was significantly reduced in alcohol-fed rats (8 +/- 1.0 fmoles/mg cytosol protein), when compared to the isocaloric control value (13.2 +/- 2.1 fmoles/mg protein), with no significant change in Kd (0.7 +/- 0.14 nM). Castration significantly increased the number of receptor sites in control rats and when castrated control animals were treated with testosterone the binding sites were decreased to the intact control level. In contrast, castration or testosterone given to castrated alcohol-fed rats did not alter alcohol-induced reduction of the receptor sites. The binding affinity (Kd) is identical in all groups. The concentration of serum luteinizing hormone (LH) was significantly lower in alcohol-fed rats when compared to that of normal controls. An increased serum LH level with a decreased testosterone level was noted in castrated control rats. However, castration of alcohol-fed rats had little or no effects on the concentrations of LH and testosterone. Administration of testosterone suppressed castration-induced high LH in control rats but alcohol-induced reduction of LH level was not altered by this treatment. These findings indicate that alcohol exerts a suppressive effect on the content of androgen receptors and secretory functions of gonadotropins in the pituitary gland.     

Variations in the number of pituitary LHRH receptors correlated with altered responsiveness to LHRH

Isolated pituitary cells from metestrous, ovariectomized (OVX), and ovariectomized-estradiol treated (OVX-EB) rats were employed to study the gonadotropin response to luteinizing hormone-releasing hormone (LHRH) challenge and to quantitate LHRH receptors, using a labeled LHRH analog. Ovariectomy (3–4 weeks post castration) resulted in a reduction of LHRH receptor concentration from 34.4 ± 2.1 in metestrous females to 14.3 ± 0.9 fmoles/10⁶ cells. Concomitantly, the luteinizing hormone (LH) response to a near-maximal dose of LHRH (5 ng/ml) decreased from a 3-fold stimulation in intact females to 1.13-fold stimulation in cells from OVX rats. Replacement therapy with EB (50 ug/rat for 2 days) to OVX rats restored LH response and LHRH binding sites (a 2.5-fold stimulation in LH secretion and 32.0 ± 2.1 fmoles/10⁶ cells, respectively). The LH response to LHRH stimulation was not altered after one day of EB treatment although the number of LHRH binding sites was increased. The changes in the number of LHRH binding sites were not accompanied by any alterations in the affinity of the LHRH analog ($\text{K}{}_{\mathrm{d}}\text{? 0.5 × 10}{}^{\mathrm{?9}}\text{M}$). It is concluded that variations in LHRH receptor number reflect the degree of pituitary sensitivity to LHRH and it may suggest that LHRH and estradiol modulation of gonadotropin release is mediated by these receptors.     

Effects of hypothalamic neurohormones on prolactin release from pituitary allografts in the hamster

Recent reports indicate that luteinizing hormone-releasing hormone (LHRH) releases prolactin (PRL) under some circumstances. We examined the chronic effects of LHRH, growth hormone-releasing hormone (GHRH), and corticotrophin-releasing hormone (CRH) on the release of PRL, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) by pituitary allografts in hypophysectomized, orchidectomized hamsters. Entire pituitary glands removed from 7-week-old-male Golden Syrian hamsters were placed under the renal capsule of hypophysectomized, orchidectomized 12-week-old hamsters. Beginning 6 days postgrafting, hamsters were injected subcutaneously twice daily with 1 microgram LHRH, 4 micrograms GHRH, or 4 micrograms CRH in 100 microliter of vehicle for 16 days. Six hosts from each of the four groups were decapitated on Day 17, 16 hr after the last injection. Prolactin, LH, and FSH were measured in serum collected from the trunk blood. Treatment with LHRH significantly elevated serum PRL levels above those measured in the other three groups, which were all similar to one another. Serum LH levels in hosts treated with vehicle were elevated above those measured in the other three groups. Serum FSH levels in hosts treated with LHRH were greater than FSH levels in any of the other three groups. These results indicate that chronic treatment with LHRH can stimulate PRL and FSH release by ectopic pituitary cells in the hamster.