Inhibitory effects of a luteinizing-hormone-releasing hormone agonist implant on ovine fetal gonadotrophin secretion and pituitary sensitivity to luteinizing-hormone-releasing hormone.

Sheep fetuses at day 70 of gestation (term = 145 days) were implanted subcutaneously with a biodegradable implant containing a luteinizing-hormone-releasing hormone (LHRH) agonist (buserelin) to investigate whether treatment with LHRH agonist would induce a state of desensitization of the fetal gonadotrophs and thus influence fetal gonadal development. Treatment with the LHRH agonist for 35-40 days caused a significant reduction in mean fetal plasma concentrations of LH and follicle-stimulating hormone (FSH) compared with control fetuses. LH pulses were evident in control fetuses but were completely abolished by buserelin treatment. Furthermore, the pituitary content of LH and FSH was significantly depleted in fetuses implanted with LHRH agonist. A bolus intravenous injection of 500 ng LHRH given to control fetuses caused a rapid and significant increase in plasma LH and FSH concentrations which was sustained for at least 60 min after injection. Pretreatment with buserelin completely abolished the LH and FSH responses to a bolus injection of LHRH. There were no differences between the sexes in fetal gonadotrophin concentrations or pituitary sensitivity to LHRH in control or agonist-treated fetuses. Furthermore, buserelin treatment for 35-40 days had no effect on the morphological appearance of the fetal gonads when compared with control fetuses, at least to day 110 of pregnancy. These results provide evidence for the induction of a state of desensitization of the LHRH receptors of the fetal pituitary gonadotrophs following long-term treatment with an LHRH agonist, but provide no evidence for a role for gonadotrophin secretion in gonadal development at this stage in fetal life.     

Effects of luteinizing hormone releasing hormone on gonadotrophins in the hamster

The changes in serum gonadotrophins in male hamsters following one injection of 15 μg luteinizing hormone releasing hormone (LHRH) (Group A) were compared with those following the last injection of LHRH in animals receiving an injection approximately every 12 hr for 4 days (Group B) or 12 days (Group C). Peak follicle stimulating hormone (FSH) levels (ng/ml) were 1776±218 (Group A), 2904±346 (Group B), and 4336±449 (Group C). Peak luteinizing hormone (LH) values (ng/ml) were 1352±80 (Group A), 410±12 (Group B), and 498±53 (Group C). Serum FSH:LH ratios, calculated from the concentrations measured 16 hr after the last LHRH injections, were higher in Groups B and C than in Group A. Similar injections of LHRH (100 ng or 15 μg/injection) for 6 days elevated the serum FSH:LH ratio in intact males. Five such LHRH injections (100 ng/injection) blunted the rise in serum LH in orchidectomized hamsters. Direct effects of LHRH on gonadotrophin secretory dynamics or altered brain-pituitary-testicular interactions may alter the ratio of FSH to LH in the hamster.     

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)     

Effects of factors from ovarian follicular fluid and Sertoli cell culture medium on in-vivo and in-vitro release of pituitary gonadotrophins in the rat: an evaluation of systems for the assay of inhibin.

Inhibin-like activity is present both in testicular and ovarian fluids. Various methods can be used for the detection of this activity. Indirect methods, using organ weights as an endpoint, lack the specificity required for reliable estimation of inhibin-like activity. With in-vivo bioassay systems, using estimation of circulating concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in intact or gonadectomized, immature or adult, male or female rats, a suppression of FSH concentrations only is usually observed after injection of inhibin-like material. The largest suppression of FSH concentrations can be obtained in short-term gonadectomized adult female or 35-day-old male rats. Addition of inhibin-like activity to cultured pituitary cells specifically suppresses the spontaneous release of FSH from the cells. After stimulation of cultured pituitary cells with LH-releasing hormone (LH-RH), the release of both FSH and LH are suppressed when inhibin-like activity is present. From dialysis experiments it appears that the molecular weight of the inhibin-like material in follicular fluid is greater than 10 000. However, acid ethanol extracts of this fluid contain a factor with a molecular weight smaller than 10 000, which does not suppress the spontaneous release of FSH from cultured pituitary cells, but diminishes the LH-RH-stimulated release of both LH and FSH. Furthermore, both follicular fluid and Sertoli cell culture medium can stimulate the release of FSH and LH from pituitary cells when these are cultured without addition of fetal calf serum. These results suggest that gonadal fluids contain several non-steroidal factors which can influence the release of gonadotrophins from pituitary cells.     

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)     

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.     

Changes in circulating levels of immunoreactive follicle stimulating hormone, luteinizing hormone, and testosterone during sexual development in the rhesus monkey, Macaca mulatta

Basal serum levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) and the responsiveness of these hormones to a challenge dose of luteinizing hormone releasing hormone (LHRH), were determined in juvenile, pubertal, and adult rhesus monkeys. The monkey gonadotrophins were analyzed using RIA reagents supplied by the World Health Organization (WHO) Special Programme of Human Reproduction. The FSH levels which were near the assay sensitivity in immature monkeys (2.4 +/- 0.8 ng/ml) showed a discernible increase in pubertal animals (6.4 +/- 1.8 ng/ml). Compared to other two age groups, the serum FSH concentration was markedly higher (16.1 +/- 1.8 ng/ml) in adults. Serum LH levels were below the detectable limits of the assay in juvenile monkeys but rose to 16.2 +/- 3.1 ng/ml in pubertal animals. When compared to pubertal animals, a two-fold increase in LH levels paralleled changes in serum LH during the three developmental stages. Response of serum gonadotrophins and T levels to a challenge dose of LHRH (2.5 micrograms; i.v.) was variable in the different age groups. The present data suggest: an asynchronous rise of FSH and LH during the pubertal period and a temporal correlation between the testicular size and FSH concentrations; the challenge dose of LHRH, which induces a significant rise in serum LH and T levels, fails to elicit an FSH response in all the three age groups; and the pubertal as compared to adult monkeys release significantly larger quantities of LH in response to exogenous LHRH.     

Effect of mouse epidermal growth factor on plasma concentrations of LH, FSH and testosterone in rams

Two experiments were conducted to examine the effects of mouse epidermal growth factor (EGF) on the concentrations of testosterone, LH and FSH in jugular blood plasma and on the pituitary responsiveness to LHRH. In 20 rams treated with subcutaneous doses of EGF at rates of 85, 98 or 113 micrograms/kg fleece-free body weight, mean plasma LH and testosterone concentrations were significantly reduced (P less than 0.05) at 6 h after treatment but not at 24 h. EGF treatment at 130 micrograms/kg fleece-free body weight suppressed the plasma content of these hormones for up to 48 h. Mean plasma FSH concentrations decreased significantly (P less than 0.05) for up to 48 h after EGF treatment, the effect being most pronounced in rams with mean pretreatment FSH values greater than or equal to 0.5 ng/ml. Intravenous injections of 1.0 micrograms LHRH given to each of 5 rams before and at 6 h, 24 h and 72 h after EGF treatment produced LH and testosterone release patterns which paralleled those obtained in 5 control rams similarly treated with LHRH. These results suggest that, in rams, depilatory doses of mouse EGF temporarily impair gonadotrophin and androgen secretion by inhibiting LHRH release from the hypothalamus. Such treatment appears to have no effect on the responsiveness of the pituitary to LHRH.     

A possible role for cyclic GMP in mediating the effect of luteinizing hormone releasing hormone on gonadotropin release in dispersed pituitary cells of the female rat.

In continuing studies on cyclic nucleotide involvement in the regulation of gonadotropin release, we have measured the cyclic nucleotide content and rate of LH and FSH release during stimulation by LHRH of dispersed overnight cultured cells from the pituitaries of adult female rats. The minimal effective concentration of LHRH was 0.1 nM and half maximal stimulation of gonadotropin release was observed in the presence of 1.0 nM LHRH. Significant release of both LH and FSH was detectable after only 10 min in the presence of 5 nM LHRH. The presence of fetal calf serum (FCS) in the overnight culture medium increased basal cGMP levels significantly, whereas horse serum (HS) had no effect, therefore all experiments were conducted on cells cultured in the presence of HS. Treatment of the cultured cells with the phosphodiesterase inhibitors theophylline (TH) or isobutyl-methyl-xanthine (MIX) revealed a preferential stimulatory effect of TH on basal cAMP levels and of MIX on cGMP levels. Throughout these experiments, LHRH had no effect on cAMP levels. In the presence of MIX, concentrations of the releasing hormone as low as 1 nM induced a significant rise in the level of cGMP whereas in its absence, cGMP levels appeared to be unchanged by LHRH. The increase was detectable after 10 min of incubation. MIX alone slightly increased LH and FSH release and significantly potentiated the response of the cells to increasing doses of LHRH up to, but not beyond, 10 nM. The data support the possibility that cGMP may be involved in the mechanism of action of LHRH.     

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.     

The induction of divergent gonadotropin secretion in vitro by variations in luteinizing hormone releasing hormone pulse regimen

The effects of luteinizing hormone releasing hormone (LHRH) pulse amplitude, duration, and frequency on divergent gonadotropin secretion were examined using superfused anterior pituitary cells from selected stages of the rat estrous cycle. Cells were stimulated with one of five LHRH regimens. With low-amplitude LHRH pulses (regimen 1) in the presence of potentially estrogenic phenol red, LH response in pituitary cells from proestrus 1900, estrus 0800, and diestrus 1,0800 were all significantly larger (P less than 0.05) than the other stages tested. In the absence of phenol red, responsiveness at proestrus 1900 was significantly larger than proestrus 0800, proestrus 1500, and estrus 0800 (P less than 0.01, 0.05, and 0.05, respectively); other cycle stages tested were smaller. No significant differences were observed between cycle stages for follicle-stimulating hormone (FSH) secretion in the presence or absence of phenol red. Because pituitary cells at proestrus 1900 were the most responsive to low-amplitude 4 ng LHRH pulses, they were also used to study the effects of LHRH pulses of increased amplitude or duration and decreased frequency. Increasing the amplitude (regimen 2) or the duration (regimens 3 to 5) increased FSH secretion; this effect was greatest with regimens 3 and 5. When regimens 3 and 5 were studied in pituitary cells obtained at proestrus 1500, FSH was significantly increased by both regimes, but most by regimen 5; furthermore, LH release was significantly reduced. When regimens 3 and 5 were studied in pituitary cells obtained at estrus 0800, FSH release was elevated most significantly by regimen 5. Thus, variations in LHRH pulse regimen were found to be capable of inducing significant divergence in FSH release from superfused anterior pituitary cells derived from specific stages of the estrous cycle.     

Gonadotroph-rich cell lines derived from pituitary clonal cells (2A8) grafted under the kidney capsule

Summary Gonadotroph-rich cell lines were established from multipotential pituitary clonal cells (2A8) which were implanted under kidney capsule of hypophysectomized female rats. These cell lines secrete gonadotrophins (FSH and LH) continuously over two months after establishment; LHRH stimulated the secretion of hormones into the culture medium. Many of the cells reacted immunohistochemically to antiserum to FSH or LH, while a small number reacted to antiserum to prolactin or TSH. They did not contain normal secretory granules such as those of gonadotrophs in vivo.Supported by USPHS Grant HD 11826 and NIH Grant P30 HD 10202. The authors wish to thank James Chambers (Immunocytochemistry), and Pat Koym and John Rhode (Radioimmunoassay) for their excellent technical assistance. We also express our thanks to NIAMDD for providing pituitary hormones     

Effects of thymulin and GnRH on the release of gonadotropins by in vitro pituitary cells obtained from rats in each day of estrous cycle

The effects of thymulin and GnRH on FSH and LH release were studied in suspension cultures of anterior pituitary cells from female adult rats sacrificed on each day of the estrous cycle. The spontaneous release of gonadotropins by pituitaries, as well as their response to GnRH or thymulin addition, fluctuated during the estrous cycle. Adding thymulin to pituitary cells from rats in diestrus 1 increased the concentration of FSH; while in cells from rats in estrus, FSH level decreased. Thymulin had a stimulatory effect on the basal concentration of LH during most days of the estrous cycle. Adding GnRH increased FSH release in cells from rats in diestrus 1, diestrus 2, or proestrus, and resulted in higher LH levels in cells obtained from rats in all days of the estrous cycle. Compared to the GnRH treatment, the simultaneous addition of thymulin and GnRH to cells from rats in diestrus 1, diestrus 2, or proestrus resulted in lower FSH concentrations. Similar results were observed in the LH release by cells from rats in diestrus 1, while in cells from rats in proestrus or estrus, LH concentrations increased. A directly proportional relation between progesterone serum levels and the effects of thymulin on FSH release was observed. These data suggest that thymulin plays a dual role in the release of gonadotropins, and that its effects depend on the hormonal status of the donor's pituitary.     

Effects of rat serum on gonadotrophin release in vitro

Gonadotrophin release from a monolayer of cultured pituitary cells from the rat was increased by the addition of fresh rat serum or rat serum albumin to the culture medium. Similar effects were obtained when horse, chicken, lamb or porcine serum was used. Fetal bovine serum or bovine serum albumin showed a stimulatory effect on FSH and LH release but, unlike the case with rat serum, a dose-response release could not be demonstrated. In addition, fresh rat serum caused the accumulation of secretory granules in the gonadotrophs. The addition of LHRH to the culture medium also stimulated gonadotrophin release in a dose-response dependent manner. The dependence of the gonadotrophs on LHRH was markedly increased by the addition of 50% fresh rat serum to the culture medium. These results indicate that fresh rat serum contains some unknown factor(s) which 1) stimulates gonadotrophin release, 2) causes an accumulation of secretory granules, and 3) is important for maintaining secretory capacity of gonadotrophs.     

Similar effects of inhibin and cycloheximide on gonadotropin release in superfused pituitary cell cultures

The actions of two inhibin preparations and cycloheximide on gonadotropin release were investigated in superfused pituitary cell cultures. Pituitary cells isolated from 18-day-old male rats were grown in Matrigel-coated superfusion chambers in chemically defined medium. After stationary culture for 4 days, the cell monolayers were superfused at a constant speed (0.25 ml/min) and were intermittently stimulated (6 min/h) with 10 nM gonadotropin-releasing hormone (GnRH). Groups of cultures were exposed to the test substances for varying time periods during stationary culture and/or during superfusion. Inhibitory effects of both inhibin preparations on the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in response to GnRH pulses were observed after 2 h of exposure and became maximal after about 6 h. Basal secretion of FSH between GnRH pulses was also suppressed, whereas the basal interpulse secretion of LH was not changed. When exposure to inhibin was discontinued, the secretion of both FSH and LH progressively increased and returned to control values by approximately 6 h. Cycloheximide (500 ng/ml) affected gonadotropin release with dynamics similar to those observed for the inhibin preparation. These data support the hypothesis that inhibition of gonadotropin synthesis may be an important step in the molecular mechanism of action by which inhibin regulates gonadotropin release.     

Studies on factors affecting release of gonadotrophins during the superfusion of isolated rat pituitaries.

Isolated pituitary glands from adult male rats were maintained in a continuous flow system. Gn-RH (1000 pmol/ml) caused a characteristic release of cyclic AMP, LH, and FSH. Cyclic AMP (1000 nmol/ml) liberated a similar amount of both gonadotrophins. Theophylline (1 mmol/ml) enhanced the effect of cyclic AMP by 21% for LH and 41% for FSH. The infusion of oestradiol (184 pmol/ml) alone or before Gn-RH infusion did not produce a significant effect on the secretion of either gonadotrophin or cyclic AMP. In contrast, there was a significant reduction in the amount of LH (P less than 0-025) and FSH (P less than 0-05) released by pituitaries infused with oestradiol and cyclic AMP.     

A change in basal FSH release accompanies the onset of the second or selective phase of increased serum FSH in the cyclic rat

Experiments were undertaken to investigate if changes occur at the level of the anterior pituitary gland to result in selective follicle-stimulating hormone (FSH) release during late proestrus in the cyclic rat. At 1200 h proestrus, prior to the preovulatory luteinizing hormone (LH) surge in serum and the accompanying first phase of FSH release, serum LH and FSH concentrations were low. At 2400 h proestrus, after the LH surge and shortly after the onset of the second or selective phase of FSH release, serum LH was low, serum FSH was elevated about 4-fold, pituitary LH concentration was decreased about one-half and pituitary FSH concentration was not significantly decreased. During a two hour $\text{in}$$\text{vitro}$ incubation, pituitaries collected at 2400 h released nearly two-thirds less LH and 2.5 times more FSH than did pituitaries collected at 1200 h. Addition of luteinizing hormone releasing hormone (LHRH) to the incubations caused increased pituitary LH and FSH release. However, the LH and FSH increments due to LHRH in the 2400 h pituitaries were not different from those in the 1200 h pituitaries. The results indicate that a change occurs in the rat anterior pituitary gland during the period of the LH surge and first phase of FSH release which results in a selective increase in the basal FSH secretory rate. It is suggested that this change is primarily responsible for the selective increase in serum FSH which occurs during the second phase of FSH release.     

Correlation of hypothalamic LHRH, pituitary LH and plasma LH in developing rats.

Hypothalamic LHRH, pituitary LH and plasma LH levels were measured in rats of both sexes from day 5-60 after birth. The content of hypothalamic LHRH was very high in one-week-old male and female rats. It declined gradually till day 17 in the female rat and sharply on day 10 in the male rat. Subsequently the content of hypothalamic LHRH increased and showed peak values on day 25 in the female rat and on day 45 in the male rat. It decreased markedly at respective times of puberty in both sexes (day 37 in the female rat and day 52-60 in the male rat). Results of the study suggest that maturation of hypothalamo-hypophyseal-axis proceeds in three distinct stages. Observations on days 17, 25 and 37 in the female rat and on days 5, 7, 10 and 22 in the male rat clearly show an inverse relationship between hypothalamic LHRH and plasma LH and a parallel relationship between pituitary and plasma LH. Marked decline in the content of hypothalamic LHRH at respective times of puberty in both sexes indicates that the release of threshold levels of LHRH from the hypothalamus may apparently be the event initiating the pubertal changes in rat.     

Relationship between release of LH and incorporation of tritiated thymidine in the anterior pituitary gland of the castrated female rat

Castration of female rats during diestrus increases the concentration of circulating LH from days 3 and the incorporation of 3H thymidine into pituitary DNA from day 5. Both effects are completely abolished by the administration of dihydrotestosterone. Although the i.v. injection of LHRH markedly enhances the concentration of LH in serum, it does not modify the incorporation of 3H thymidine into pituitary DNA. Castration might produce a maximal stimulation in 3H thymidine incorporation and a further stimulation of LH release with LHRH is unable to enhance the incorporation of the radioactive precursor. The results suggest a relationship between LH secretion and DNA synthesis in the pituitary gland of the rat.     

Seasonally and experimentally induced changes in testicular function of the Australian bush rat (Rattus fuscipes)

Serum concentrations of LH, FSH and testosterone were measured monthly throughout the year in male bush rats. Testicular size and ultrastructure, LH/hCG, FSH and oestradiol receptors and the response of the pituitary to LHRH were also recorded. LH and FSH rose in parallel with an increase in testicular size after the winter solstice with peak gonadotrophin levels in the spring (September). The subsequent fall in LH and FSH levels was associated with a rise in serum testosterone which reached peak levels during summer (December and January). In February serum testosterone levels and testicular size declined in parallel, while the pituitary response to an LHRH injection was maximal during late summer. The number of LH/hCG, FSH and oestradiol receptors per testis were all greatly reduced in the regressed testes when compared to active testes. In a controlled environment of decreased lighting (shortened photoperiod), temperature and food quality, the testes of sexually active adult males regressed at any time of the year, the resultant testicular morphology and endocrine status being identical to that of wild rats in the non-breeding season. Full testicular regression was achieved only when the photoperiod, temperature and food quality were changed: experiments in which only one or two of these factors were altered failed to produce complete sexual regression.