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.     

The effects of porcine follicular fluid in the postcastration rise of gonadotropins in the rabbit

The effects of repeated administration of porcine follicular fluid (PFF) on the follicle stimulating hormone (FSH) and luteinizing hormone (LH) levels after castration were studied in rabbits. Steroid-free dextran-charcoal extracted PFF was administered to male and female adult rabbits at 0800 and 1600 h for four days immediately following castration. Serum levels of FSH and LH were measured before, during and after the PFF administration and compared to controls. A significant increase in FSH and LH was observed within 24 h following castration in the controls. In the PFF-treated group, a clear suppression of serum FSH levels was observed during PFF administration in both males and females. FSH concentrations returned to the control levels within 24 h after PFF withdrawal. Even through LH levels showed no differences during PFF injection, compared to controls, a significant increase was observed following discontinuation of PFF administration.     

Follicle-stimulating hormone secretion in monosodium glutamate-lesioned rats: response to unilateral gonadectomy or porcine follicular fluid (inhibin)

The purpose of this study is to determine the acute response of pituitary FSH and LH release to unilateral gonadectomy in the MSG-treated rat, and to determine whether pFF (inhibin) can act effectively on pituitary FSH secretion in the MSG-lesioned rat. MSG (4 mg/kg B.W.) or saline was injected subcutaneously on postnatal days 2, 4, 6, 8, and 10 to male and female littermates which were used in the experiments after postnatal day 60. In the first experiment male and female littermates were bilaterally gonadectomized and bled serially for the next 72 h. At 0 h plasma FSH concentrations in MSG-treated rats were lower (p less than 0.05) than those in saline-treated controls, and for the 72 h immediately following bilateral gonadectomy FSH levels increased parallel to those of the controls, but after a significant delay. In the second experiment, MSG-treated male and female littermates were injected with 0.5 ml of pFF at several intervals following bilateral gonadectomy and decapitated 6 hours later. Injection of pFF significantly suppressed circulating FSH titers in all groups without affecting LH levels. In a third experiment, rats were unilaterally gonadectomized and blood samples were obtained at various intervals for 48 h. Following unilateral gonadectomy there was a significant transient increase in FSH levels in male or female MSG-treated rats as compared to their 0 h values; however, the absolute levels attained were barely equal to the basal concentrations observed in the saline-treated control rats. The conclusions from these data are: insufficient FSH secretion in response to unilateral gonadectomy may be responsible for the lack of compensatory gonadal hypertrophy in MSG-lesioned rats, pituitary response to inhibin is apparently unaltered by MSG toxicity, and the MSG-lesioned rat is a useful model to study the differential control mechanisms of FSH and LH secretion.     

Testosterone modulates the differential release of luteinizing hormone and follicle-stimulating hormone that occurs in response to changing gonadotropin-releasing hormone pulse frequency in the male monkey, Macaca fascicularis

Selective elevations of plasma follicle-stimulating hormone (FSH) levels are characteristic of some physiological conditions, such as the early stages of human puberty, and in some disorders of testicular function, such as idiopathic oligospermia. We tested the hypotheses that a slow gonadotropin-releasing hormone (GnRH) pulse frequency favors a selective elevation of plasma FSH and that this is influenced by the circulating steroidal milieu. We administered exogenous GnRH at frequencies of once every 90 min (q 90 min) and once every 240 min (q 240 min) to castrated prepubertal male monkeys who had received either empty (sham) or testosterone (T)-filled Silastic capsules at the time of castration. At the end of each experimental frequency period, mean plasma levels of luteinizing hormone (LH) and FSH were measured. Plasma T levels were also measured. Animals with T implants had plasma levels of this hormone that were in the adult range (approximately equal to 8 ng/ml), whereas those with sham implants had plasma T levels in the prepubertal range (less than or equal to 4 ng/ml). In animals with sham implants, mean plasma FSH levels were markedly elevated at the slower GnRH pulse frequency (39.5 +/- 3.6 ng/ml following GnRH q 240 min compared with 23.7 +/- 2.8 ng/ml following GnRH q 90 min). This selective FSH elevation was not apparent in animals with T implants. Mean plasma LH levels were similar (approximately equal to 8 micrograms/ml) at the two GnRH pulse frequencies, in both T-treated and sham-implanted animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of vagotomy on postcastration gonadotropin secretion in male rats

The postcastration increase in gonadotropins was studied in intact and vagotomized male rats. Rats underwent vagotomy or sham surgery immediately prior to castration. In the first experiment, rats were bled before castration and at 1, 2, 4, and 7 days after castration. Serum LH and FSH were significantly lower in vagotomized rats 1 day after castration. On days 2, 4, and 7 postcastration, serum gonadotropin levels were generally not different among experimental groups. In a second experiment, rats were decapitated at 12 or 24 hr after surgery and castration. Trunk blood was collected for assay of LH. Vagotomy had no effect on LH levels at 12 hr postcastration, but, at 24 hr postcastration, vagotomized rats had significantly lower serum LH than did sham-operated rats. These experiments indicate that vagotomy has a transient suppressive effect on gonadotropin release following castration. Such observations support the hypothesis that the vagus nerve may play a modulatory role in gonadotropin secretion.     

Inhibition of pituitary-gonadal function in male rats by a potent GnRH antagonist

The inhibitory effects of the potent GnRH antagonist, [Ac-D-pCl-Phe1,2,D-Trp3,D-Arg6,DAla10]GnRH (GnRHant) upon pituitary-gonadal function were investigated in normal and castrated male rats. The antagonist was given a single subcutaneous (s.c.) injections of 1-500 micrograms to 40-60 day old rats which were killed from 1 to 7 days later for assay of pituitary GnRH receptors, gonadal receptors for LH, FSH, and PRL, and plasma gonadotropins, PRL, and testosterone (T). In intact rats treated with low doses of the antagonist (1, 5 or 10 micrograms), available pituitary GnRH receptors were reduced to 40, 30 and 15% of the control values, respectively, with no change in serum gonadotropin, PRL, and T levels. Higher antagonist doses (50, 100 or 500 micrograms) caused more marked decreases in free GnRH receptors, to 8, 4 and 1% of the control values, which were accompanied by dose-related reductions in serum LH and T concentrations. After the highest dose of GnRHant (500 micrograms), serum LH and T levels were completely suppressed at 24 h, and serum levels of the GnRH antagonist were detectable for up to 3 days by radioimmunoassay. The 500 micrograms dose of GnRHant also reduced testicular LH and PRL receptors by 30 and 50% respectively, at 24 h; by 72 h, PRL receptors and LH receptors were still slightly below control values. In castrate rats, treatment with GnRHant reduced pituitary GnRH receptors by 90% and suppressed serum LH and FSH to hypophysectomized levels. Such responses in castrate animals were observed following injection of relatively low doses of GnRHant (100 micrograms), after which the antagonist was detectable in serum for up to 24 h. These data suggest that extensive or complete occupancy of the pituitary receptor population by a GnRH antagonist is necessary to reduce plasma gonadotropin and testosterone levels in intact rats. In castrate animals, partial occupancy of the available GnRH receptor sites appears to be sufficient to inhibit the elevated rate of gonadotropin secretion.     

Effects of short-term treatment with testosterone on the secretion of FSH and LH in castrated golden hamsters exposed to short days

Castrated hamsters which were transferred from long (14L:10D) to short (9L:15D) days and received testosterone-filled capsules for 1 week after transfer failed to show a significant suppression in the plasma levels of FSH and LH after capsule removal. In contrast, gonadotrophin concentrations were suppressed in hamsters in which the long-day castration response had been blocked with exogenous testosterone. After castration on long days and exposure to 10 weeks of short days pituitary gland weight and gonadotrophin content, as well as plasma FSH titres, were higher in control animals than in those that had received testosterone implants for 7 weeks of short days. The results suggest that failure of castrated hamsters to respond to the suppressive effects of short days reflects castration-induced changes in hypothalamo-pituitary physiology rather than a neuroendocrine mechanism by which photoperiod modulates gonadotrophin secretion.     

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.     

Effect of castration and steroid treatment on the release of gonadotropins by the rat pituitary-hypothalamus complex in vitro

Comparative in vitro studies on the release of LH and FSH by pituitary-hypothalamus complex (PHC) with intact portal plexus and whole pituitary (PI) from adult male rats showed that PHC released LH at a greater rate and in larger amounts than PI. PHC and PI released FSH in comparable amounts and rates. Attempts were made to correlate serum gonadotropin levels to that released by PHC and PI at 1, 3, 7, 14, 21 and 46 days of post-castration (PC). Sham operated animals served as controls. Castration increased serum LH and FSH levels but in different profiles. CPHC and CPI (PHC and PI from castrated rats) released less LH than NPHC and NPI (PHC and PI from sham operated controls) till day 14 PC after which CPHC and CPI released more LH than NPHC and NPI respectively. Castration abolished the intrinsic capacity of PHC to secrete more LH than PI. CPHC and CPI secreted significantly less FSH than NPHC and NPI at 1, 3 and 7 days PC. At days 14 and 21 of post-castration PCNCP or CPI and NPHC or NPI released similar amounts of FSH. Administration of 5 alpha-dihydrotestosterone (DHT, 1 mg/rat/day) or estradiol valerate (EV, 1 microgram/rat/day) immediately following castration prevented the rise in serum LH and FSH but increased the amounts of LH and FSH released by CPHC and CPI. The treatment caused a marked stimulation of FSH released by CPI.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential effects of follicle-stimulating and luteinizing hormones on testosterone production by mouse testes

In adult mice, direct intratesticular injection of ovine follicle-stimulating hormone (o-FSH-13; AFP 2846-C, from NIAMDD, less than 1% LH contamination) at 10, 100 or 1000 ng significantly elevated concentrations of testosterone (T) within the testis. These effects were rapid, with peak values attained by 15 min, and transient, with return to values comparable to that in the contralateral, saline-injected testis within 90 min. Intratesticular injection of FSH (1 microgram) significantly increased testicular T levels in 15- and 60-day old mice. This contrasted with the effects of intratesticular administration of human chorionic gonadotropin (hCG), which stimulated T production significantly at 30 days of age through adulthood. In adult mice, the equivalent LH to the possible contamination in the FSH preparation (1 ng) had no effect. Intratesticular injection of 10 ng LH produced comparable stimulation to that by 100 ng FSH (approximately 7-fold). Systemic pre-treatment with a charcoal-treated porcine follicular fluid (PFF) extract for 2 days reduced plasma FSH levels [86 +/- 17 (5) vs 700 +/- 8 (6); P less than 0.05], but had no effect on plasma LH. Twenty-four hours after the last treatment, the response to intratesticular injection of hCG (2.5 mIU), FSH (100 ng) or LH (10 ng) was also significantly attenuated in these mice. Intratesticular injection of PFF had no direct effect on testicular T levels. In vitro T production in the presence of hCG, LH or FSH were differentially affected by the concentrations of calcium (Ca2+) or magnesium (Mg2+) in the incubation media. The stimulatory effects of FSH were apparent at significantly lower levels of Ca2+ or Mg2+, than were those of LH or hCG. The results of these studies indicate that FSH is capable of stimulating testicular T production. Furthermore, the responsiveness to FSH is qualitatively different than that to LH/hCG in terms of the age pattern, as well as the dependence on Ca2+ or Mg2+. In addition, plasma FSH levels appear to influence testicular responsiveness to direct exogenous administration of gonadotropins. These studies indicate that FSH stimulation of T production can be differentiated from those of LH, and that these effects of FSH can be observed under physiological conditions.     

Effects of acutely increased plasma testosterone concentrations on pulsatile luteinizing hormone secretion in the male rat

To assess the role of testosterone (T) in regulating the minute-to-minute release of pulsatile luteinizing hormone (LH) secretion in the adult male rat, we investigated the negative feedback of acute increases in plasma T concentrations on pulsatile LH secretion in acutely castrated male rats. At the time of castration, we implanted T-filled Silastic capsules, s.c., which maintained plasma T concentrations at approximately 1.8 ng/ml and suppressed LH pulses. On the next day, the capsules were removed; blood sampling (every 6 min) was started 8 h after implant removal, thereby allowing LH pulses to be reinitiated. Immediately following a control bleeding interval of 2 h, either T or vehicle alone was infused s.c., and blood sampling continued for another 4 h. In animals receiving vehicle alone, LH pulse frequency and mean LH levels increased over the 6 h bleeding period. The administration of 200 ng T/min caused a rapid rise in plasma T concentrations of about 4 ng/ml ("physiological") and prevented the increase in pulse frequency that occurred in the control group; it did not, however, reduce pulse frequency over the 4 h infusion period. When T was infused at the rate of 400 ng/ml, plasma T concentrations rose to approximately 18 ng/ml ("supraphysiological") and LH pulse frequency was significantly reduced, but not completely inhibited, during the last 2 h of the infusion. The pulse amplitude of luteinizing hormone did not change significantly in any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Testosterone selectively increases serum follicle-stimulating hormonal (FSH) but not luteinizing hormone (LH) in gonadotropin-releasing hormone antagonist-treated male rats: evidence for differential regulation of LH and FSH secretion

Both testosterone (T) and gonadotropin-releasing hormone (GnRH)-antagonist (GnRH-A) when given alone lower serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in intact and castrated rats. However, when graded doses of testosterone enanthate (T.E.) were given to GnRH-A-treated intact male rats, a paradoxical dose-dependent increase in serum FSH occurred; whereas serum LH remained suppressed. This surprising finding led us to ask whether the paradoxical increase in serum FSH in GnRH-A-suppressed animals was a direct stimulatory effect of T on the hypothalamic-pituitary axis or the result of a T effect on a testicular regulator of FSH. To test these hypotheses, we treated adult male castrated rats with GnRH-A and graded doses of T.E. In both intact and castrated rats, serum LH remained undetectable in GnRH-A-treated rats with or without T.E. However, addition of T.E. to GnRH-A led to a dose-dependent increase in serum FSH in castrated animals as well, thus pointing against mediation by a selective testicular regulator of FSH. These data provide evidence that pituitary LH and FSH responses may be differentially regulated under certain conditions. When the action of GnRH is blocked (such as in GnRH-A-treated animals), T directly and selectively increases pituitary FSH secretion.     

Detection of folliculostatin: a sensitive and specific bioassay using the female rat

To establish a sensitive and specific bioassay for the FSH-suppressing activity present in porcine follicular fluid (pFF), we examined the latency of pFF action when injected IV in the acutely ovariectomized (ovax) metestrous rat. By 2h post injection (5.5h after ovax), FSH was suppressed significantly in pFF vs. porcine serum-injected controls. LH was unaltered. In an experiment establishing a dose-response curve for pFF 4.5h after injection, 1.77 mg of pFF protein significantly suppressed FSH. The index of precision (-0.2188) and precision of slope (1.088) were well within acceptable limits for bioassays. We conclude that the ovax metestrous rat, injected 3.5h after surgery and sacrificed at 4.5 or 5.5h, is a sensitive and specific bioassay for folliculostatin.     

Pituitary-gonadal axis before puberty: evaluation of testicular steroids in the male rat

Orchidectomy was performed on 26-day-old rats via a single midscrotal incision following which 1 of 6 steroids was administered subcutaneously twice daily for 7 days. Each hormone treatment set had its own controls both castrate and intact. Levels of serum LH and FSH were measured by radioimmunoassay. It was found that LH was suppressed to intact levels by testosterone or its active metabolites at doses within the "physiologic dosage range" (equivalent in biological activity to endogenously secreted androgens). FSH suppression with androgens occurred at considerably higher doses; only testosterone could maintain FSH at intact levels with a physiologic dosage. Both 5alpha-dihydrotestosterone, and 3alpha-androstanediol suppressed LH well and FSH partially; 3beta-androstanediol and fluoxymesterone were ineffective over the same dosage range. Estradiol suppressed both LH and FSH. It was concluded that LH is more easily suppressed than FSH by androgens, that there is poor correlation between biologic potency and their gonadotropin-suppression ability, and that testosterone is almost certainly not the final active intracellular androgenic hormone. It was suggested that while a small amount of testicular androgen can maintain the low levels of LH, complete control of FSH secretion may require conversion of testosterone to estrogens.     

Changes in gonadotropin secretion following complete or hemicastration in the adult rat.

The effect of sham castration, hemicastration or complete castration on gonadotropin and testosterone secretion was studied in adult male rats. Untreated control rats were autopsied 1, 10, 20, 30 and 40 days following assignment to treatment groups. Sham-castrated controls were autopsied 1, 2 and 3 days after surgery. Complete and hemicastrates were autopsied 1, 2, 3, 10, 20, 30 and 40 days after surgery. Serum levels of both FSH and LH were elevated by 24 h postcastration and the levels of both gonadotropins continued to rise throughout the course of the experiment. Serum levels of LH rose following hemicastration and remained above control values through day 30. Serum FSH levels were not significantly affected by hemicastration. Compensatory testicular hypertrophy was not observed in hemicastrated rats.     

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.     

Divergent regulation of gonadotropin subunit mRNA levels by androgens in the female rat.

To examine the effects of gonadal steroids on the pretranslational regulation of the gonadotropin subunits in the female, adult female rats, beginning 7 or 28 days after ovariectomy, received daily injections of testosterone propionate (T), dihydrotestosterone propionate (D), or estradiol benzoate (E) for 7 days. Intact cycling females and ovariectomized rats that received vehicle served as controls. Serum was obtained for LH and FSH levels to assess changes in gonadotropin secretion. Total RNA from individual rats was recovered and analyzed by blot hybridization with specific radiolabeled cDNA probes for the alpha, LH beta, and FSH beta subunits. Autoradiographic bands were quantitated and standardized to mRNA levels in the intact animals. Ovariectomy resulted in a rise in serum gonadotropin levels and all three gonadotropin subunit mRNA levels. Estrogen replacement resulted in suppression of alpha, LH beta, and FSH beta mRNAs whether given at 7 or 28 days after ovariectomy. In contrast, whereas androgen replacement decreased alpha and LH beta mRNAs, D or T did not consistently suppress FSH beta mRNAs. We conclude that chronic estrogen administration to the castrated female rat uniformly suppresses all three gonadotropin subunit mRNA levels. In female rats, as in male rats, chronic androgen administration fails to negatively regulate FSH beta mRNAs.     

Postcastration rise in plasma gonadotropins is blocked by a luteinizing hormone-releasing hormone antagonist

The dependence of the acute increases in plasma gonadotropins following castration on luteinizing hormone-releasing hormone (LHRH) was assessed with the use of a potent LHRH antagonist [ALHRH; (Nac-L-Ala1,p-Cl-D-Phe2,D-Trp3,6) LHRH]. Blood samples were collected from male and female rats at the time of castration and 2, 4, 8, 12, 24 and 48 h following and plasma gonadotropin levels were determined. Immediately following castration (diestrus I for females) animals received one of the following treatments: females-vehicle, 100 micrograms ALHRH, 50 micrograms estrogen benzoate (EB), or 100 micrograms ALHRH + 50 micrograms EB; males-vehicle, 100 micrograms ALHRH, 500 micrograms testosterone propionate (TP), or 100 micrograms ALHRH + 500 micrograms TP. ALHRH blocked the selective increase in plasma follicle-stimulating hormone (FSH) observed in female rats as well as the parallel increases in both gonadotropins seen in male rats following castration. Administration of EB or ALHRH + EB to females significantly suppressed both gonadotropins compared with control levels. However, EB alone did not completely block the rise in plasma FSH in females. In males, all three treatments significantly suppressed the increases in both gonadotropins when compared with control levels. These data demonstrate that hypothalamic LHRH plays an essential role in the acute elevations of plasma gonadotropins following castration in rats. In addition, these data suggest that the selective rise of FSH in females is dependent on LHRH stimulation of pituitary gonadotropes.     

Altered sexual development in male rats after oestrogen administration during the neonatal period.

Male rats given 250 mug oestradiol benzoate by subcutaneous injection on Day 4 of postnatal life showed a marked delay in the onset of the pubertal increase in the weight of the testes and seminal vesicles and in spermatogenesis but not a complete failure of sexual development. The increase in plasma testosterone concentration at puberty was also delayed in oestrogen-treated males but the eventual increase in seminal vesicle weight was closely related in time to the delayed increase in plasma testosterone concentration. Both plasma LH and FSH concentrations were reduced for about 10 days after oestrogen administration as compared to control values. After 22 days of age, plasma LH concentration did not differ significantly from the control values. The plasma FSH concentration of the oestrogen-treated males showed a delayed rise to values equal to or higher than those of controls of the same age. The delayed rise in plasma FSH concentration in the oestrogen treated males preceded the delayed rise in plasma testosterone in these animals. The decrease in plasma FSH concentration from the high prepubertal values to the lower values in adults occurred at different ages in the control and in oestrogen-treated rats but in both groups the decrease occurred as plasma testosterone levels were increasing and the first wave of spermatogenesis was reaching completion. The increase in plasma FSH concentration after castration was reduced in oestrogen-treated males during the period throughout which FSH levels in the intact animals were subnormal but the levels in oestrogen-treated males castrated after the delayed rise in FSH had occurred did not differ from control values. It is suggested that the delayed sexual maturation of male rats treated with high doses of oestrogen in the neonatal period is related principally to abnormalities in the secretion of FSH.     

Feedback control of FSH secretion in the male rat

Site of feedback control of FSH secretion in the male rat was studied by measuring changes in serum LH, FSH and hypothalamic LH-RH by radioimmunoassay in rats after castration and after 500 rad X-irradiation to the testis. The rise in serum LH and FSH in castrated animals was associated with a significant fall in hypothalamic LH-RH 16 and 24 days after castration. Serum FSH rose significantly after X-irradiation without a significant change in serum LH or hypothalamic LH-RH content up to 30 days after irradiation. When pituitary halves from X-irradiated animals were incubated in vitro in the presence or absence of synthetic LH-RH, there was a significant rise in FSH (but not LH) released in the incubation medium in the absence of added LH-RH. The response of the pituitaries to LH-RH was, however, not different between control and irradiated rats. It is concluded that the testicular FSH-inhibitory substance acts predominantly at the pituitary gland on the LH-RH independent release of FSH.