CONTROL OF GONADOTROPHIN SECRETION BY STEROID HORMONES IN CASTRATED MALE TRANSSEXUALS. II. EFFECTS OF ANDROGENS ALONE AND IN COMBINATION WITH OESTRADIOL ON THE SECRETIONS OF FSH AND LH

Twenty-nine infusions in twenty castrated male transsexual volunteers were carried out over a period of 7 h with subjects lying in the supine position. The effects of different doses of testosterone and its Sα-reduced metabolites as well as the effect of testosterone in combination with oestradiol on gonadotrophin secretion were evaluated. Different and varying degrees of suppression of plasma levels of FSH and LH were observed. The infusions of 2·4 mg testosterone, 5α-androstan-3α-17β-diol (3α-diol), 5α-androstan-3β17β-diol (3β-diol) but not dihydrotestosterone (DHT) caused significant suppression of LH. FSH, on the the other hand was not significantly inhibited by the androgens at this rate. At higher doses all four androgens suppressed LH secretion significantly. FSH was similarly suppressed by the androgens except by DHT. A differential effect on FSH and LH secretions was noted with the combined regime of testosterone and oestradiol. The combined regime did not cause a significantly higher degree of FSH suppression compared with either 200 μg of oestradiol or 12 mg of testosterone infused alone. The level of LH, however, was suppressed to a greater extent than either of the hormones when given alone. The inhibitory effect of testosterone demonstrated in this study could be due to the parent hormone or its 5α-reduced metabolites. Pharmacological doses of testosterone could exert a greater degree of LH suppression through its conversion to oestradiol. It is likely that oestradiol and testosterone act on gonadotrophin secretion through different mechanisms and that they have an additive suppressive effect on the secretion of LH but not FSH. The potencies for the androgens to suppress gonadotrophin secretion can be ranked as: 3α-diol = 3β-diol > testosterone > dihydrotestosterone.     

Effects of oestradiol on gonadotrophin levels in normal and castrated men

Context  Testosterone inhibits gonadotrophin release in men either directly or after aromatization to oestradiol. We hypothesized that in males the androgen receptor-mediated effect of testosterone on LH release is negligible relative to that of oestradiol.
Objective  To compare the effect of experimentally induced variations of plasma oestradiol levels on LH levels in normal (physiological testosterone levels) and castrated men (very low testosterone levels).
Design  Prospective, open label, intervention.
Subjects and interventions  We suppressed endogenous oestradiol in 10 young men with letrozole 2·5 mg once daily. In these men and in 10 young healthy castrated men, we restored plasma oestradiol levels with oestradiol patches (first week 100 μg/day, second week 50 μg/day, third week 25 μg/day and fourth week no oestradiol patch).
Measurements  The effect of the intervention on plasma levels of LH were monitored and compared between the groups.
Results  With the intervention, the mean plasma oestradiol level in the two groups varied from supraphysiological to below the lower reference range. Levels of LH mirrored plasma oestradiol levels in both the groups, as did testosterone in the intact group. Despite similar oestradiol levels, mean levels of LH were significantly higher in the castrated group compared to the intact group for all doses of oestradiol, and supraphysiological levels of oestradiol were unable to suppress LH into the physiological range in the castrated group.
Conclusions  Physiological plasma oestradiol levels have a substantial suppressive effect on LH in men. However, low-normal testosterone levels are a prerequisite for suppression of LH into the normal range.     

Androgenic and oestrogenic steroid participation in feedback control of luteinizing hormone secretion in male sheep

The acute castrate ram (wether) was used as an experimental model to investigate the site(s) of feedback on luteinizing hormone (LH) by testosterone, dihydrotestosterone and oestradiol. At the time of castration, wethers were implanted subdermally with Silastic capsules containing either crystalline testosterone (three 30 cm capsules), dihydrotestosterone (five 30 cm capsules) or oestradiol (one 6.5 cm capsule). Blood samples were taken at 10 min intervals for 6 h 2 weeks after implantation to determine serum steroid concentrations and to characterize the patterns of LH secretion. Pituitary LH response to exogenous LRH (5 ng/kg body weight) were also determined at the same time. The steroid implants produced serum concentrations of the respective hormones which were either one-third (testosterone) or two-to-four times (dihydrotestosterone, oestradiol) the levels measured in rams at the time of castration. Non-implanted wethers showed rhythmic pulses of LH (pulse interval 40-60 min) and had elevated LH levels (16.1 +/- 1.6 ng/ml; mean +/- SE) 2 weeks after castration. All three steroids suppressed pulsatile LH release and reduced mean LH levels (to below 3 ng/ml) and pituitary LH responses to LRH. Inhibition of pulsatile LH secretion by all three steroids indicated that testosterone as well as its androgenic and oestrogenic metabolites can inhibit the LRH pulse generator in the hypothalamus. Additional feedback on the pituitary was indicated by the dampened LH responses to exogenous LRH.     

The negative feedback effects of testicular steroids are predominantly at the hypothalamus in the ram

This study aimed to delineate the hypothalamic and/or pituitary actions of testosterone and its primary metabolites 5 alpha-dihydrotestosterone and estradiol (E) in adult castrated rams (wethers) during the breeding season. In Exp 1, wethers were treated for a week with twice daily injections (im) of peanut oil, 8, 16 or 32 mg/day testosterone propionate (TP) or dihydrotestosterone benzoate (DHTB) or an sc silastic implant containing 1 or 3 cm E. TP decreased plasma LH concentrations, increased (P less than 0.05) LH interpulse interval, did not have consistent effects on LH pulse amplitude, and had minimal effects on plasma FSH concentrations. DHTB decreased LH and FSH concentrations and increased (P less than 0.05) LH interpulse interval. E reduced (P less than 0.05) plasma LH and FSH concentrations and increased LH interpulse interval but had no effects on LH pulse amplitude. In Exp 2, hypothalamo-pituitary disconnected wethers given 125 ng GnRH every 2 h, were treated with either peanut oil, 32 mg/day TP or DHTB or 3 cm E. None of the treatments affected plasma LH or FSH concentrations or LH pulse amplitude. Exp 3 investigated the effects on GnRH of treatment of wethers either with peanut oil or TP. TP reduced GnRH concentrations (P less than 0.05) and pulse amplitude (P less than 0.01) and increased interpulse interval (P less than 0.05). These data provide evidence that, during the breeding season, the principal site of negative feedback of testicular steroids in the ram is the hypothalamus, resulting in decreased GnRH secretion; feedback effects at the pituitary are minimal.     

Direct evidence in men for a role of endogenous oestrogens on gonadotrophin release

In six healthy subjects serum oestradiol was selectively decreased by administering an aromatase activity inhibitor, hydrotestolactone (HT). After HT administration serum oestradiol (Oe2) decreased from 18.7 +/- 2.3 (SEM) to 6.7 +/- 0.6 pg/ml whereas testosterone (T) and dihydrotestosterone (DHT) blood levels were not modified. These oestradiol changes were associated with a significant increase in serum LH and FSH concentrations (P less than 0.001). The administration of tamoxifen, an oestrogen antagonist, to 5 subjects caused a sharp increase in LH and FSH levels (P less than 0.001). Oe2 was unchanged after the treatment with tamoxifen, whereas T levels were significantly higher. The sum of these data suggests that oestradiol under physiological conditions plays a specific role in the feedback mechanism of gonadotrophin release.     

Differential effects of testosterone, 5 alpha-dihydrotestosterone and oestradiol-17 beta on plasma concentrations of LH in castrated ferrets

The biological activity of testosterone often depends on the conversion of testosterone within the target cell to an androgenic or oestrogenic metabolite. The purpose of this study was to compare the relative ability of testosterone and two of its metabolites, dihydrotestosterone (DHT) and oestradiol, to suppress LH secretion in castrated male ferrets. Castrated ferrets were treated with five different doses of steroid by implanting various numbers of s.c. silicone elastomer capsules packed with either testosterone, DHT or oestradiol. The lowest dose of oestradiol (0.1 mm capsule length/100 g body weight, mean estimated total release rate of 25 ng/day) significantly suppressed plasma concentrations of LH in castrated ferrets. Higher amounts of DHT (2.5 mm capsule length/100 g body weight, mean estimated total release rate of 88 ng/day) were required for a significant reduction in plasma concentrations of LH. Concentrations of LH were also significantly lowered by testosterone when administered at a 2.5 mm capsule length/100 g body weight; however, estimated total release rate was 312 ng/day from these capsules. The fact that oestradiol was more effective than DHT, and that DHT was more effective than testosterone in inhibiting LH secretion in castrated ferrets, suggests that in gonadally intact ferrets, testosterone may be converted to DHT or oestradiol within target cells that mediate steroid negative feedback on LH secretion.     

Use of a pulsed infusion of luteinizing hormone releasing hormone to mimic seasonally induced endocrine changes in the ram.

Adult Soay rams were housed indoors under natural lighting during the spring non-mating season when gonadotrophin secretion was low. Four animals received small doses (100 ng or 500 ng) of synthetic LH releasing hormone (LH-RH) infused into the jugular vein by a mechanical device for 60 s every 2 h for 33-57 days: two other rams acted as controls. The prolonged treatment with LH-RH resulted in growth of the testes and the development of the sexual skin flush; these effects were lost when treatment stopped. The plasma concentrations of LH, FSH and testosterone were low at the beginning; each short infusion of LH-RH resulted in a transitory increase in the level of LH and testosterone while the concentration of FSH was only marginally affected. After prolonged treatment with 500 ng pulses of LH-RH the plasma concentrations of all three hormones were permanently raised. The response to the individual injections of LH-RH was also modified, the peak in LH being reduced in amplitude but more prolonged while the FSH and testosterone responses were both enhanced. When the pulsed infusion was stopped the concentration of LH and testosterone declined rapidly while the decline in FSH levels took many days. These endocrine changes induced by the pulsed infusion are comparable to those that occur naturally in the ram during testicular redevelopment before the mating season.     

Luteinizing hormone regulation by sex steroids in men with germinal and Leydig cell tumours

OBJECTIVE We examined the gonadotrophin secretion in patients with increased plasma concentrations of testosterone and oestradiol due to hCG-producing tumours. DESIGN Comparison of plasma gonadotrophin concentrations before and after stimulation by GnRH, in eight men with hCG-producing tumours resulting in Increased testosterone and oestradiol plasma levels, and In 29 men with Leydig cell tumours resulting in increased oestradiol and normal to low testosterone plasma levels. PATIENTS Eight men with hCG-producing tumours (six with testicular tumours, two with extratesticular tumours), 29 men with Leydig cell tumours and 15 normal men. The six men with germinal cell tumours of the testis were studied before and after unilateral orchidectomy. MEASUREMENTS Plasma concentrations of hCG, testosterone and oestradiol were measured before and after intramuscular injection of hCG. LH and FSH were measured before and after intravenous Injection of 100 μg GnRH. RESULTS Plasma LH and FSH concentrations were low In patients with germ cell tumours, who exhibited increased plasma testosterone and oestradiol concentrations, and were normal in patients with Leydig cell tumours, in whom oestradiol only was increased. Plasma LH and FSH were normalized in the five patients with successful (e.g. normal hCG, testosterone and oestradiol) unilateral orchldectomy. Basal plasma testosterone concentrations correlated positively (P <001) with plasma oestradiol concentrations in patients with germ cell tumours and negatively (P <0 01) in patients with Leydig cell tumours. CONCLUSIONS In patients with hCG-secreting germ cell tumours complete suppression of plasma LH and FSH with increased plasma concentrations of both testosterone and oestradiol are often discovered. No such gonadotrophin suppression is found In patients with Leydig cell tumours, but the negative correlation observed between plasma testosterone and oestradiol in these patients suggests a weak negative feedback effect of oestradiol on LH secretion, which cannot be demonstrated by basal LH measurements in plasma.     

Effects of luteinizing hormone releasing hormone on plasma follicle-stimulating hormone and luteinizing hormone levels in the ferret.

Heterologous radioimmunoassays for FSH and LH were employed to examine the effect of synthetic LH-RH upon gonadotrophin secretion in the ferret. Intravenous injection of 4 microng LH-RH induced a surge of FSH and of LH secretion in male and in female animals. In intact and in castrated males, the rise of LH was much more marked than that of FSH. The gonadotrophin response to LH-RH was greater in anoestrous than in oestrous females; FSH secretion was not enhanced during oestrus. Ovariectomized females behaved as anoestrous females with respect to LH secretion, while FSH secretion remained unchanged. Treatment of ovariectomized females with progesterone did not alter the pattern of response to LH-RH, but oestradiol treatment depressed the reaction to match that seen in oestrous females. Repetitive injections of LH-RH induced repetitive surges of FSH and LH in anoestrous females, but only of LH during oestrus: slow i.v. infusion of LH-RH induced a sustained elevation of plasma LH levels both in oestrous and in anoestrous females; again FSH levels rose only in anoestrous females. Injection of synthetic TRH did not alter gonadotrophin secretion in corresponding groups of male or female ferrets.     

Relation between circulating levels of testosterone lh and fsh in intact and castrated, adult, male rats after testosterone administration.

Serum levels of LH, FSH and testosterone were measured by radioimmunoassay in intact and castrated, adult, male rats after testosterone was administered subcutaneously for seven days in doses ranging from 25 to 200 mug per 100 g body weight per day. Such treatment increased circulating testosterone both in intact and castrated rats, but its effects on serum gonadotrophins were different in these animal groups. All doses of testosterone suppressed serum LH and FSH in the normal rat. In the castrates, treatment with the lowest dose of testosterone resulted in serum LH levels significantly above the high castrate levels, while serum FSH tended to drop. Administration of the highest doses of testosterone did not depress serum LH and FSH in the castrates to those of intact, normal animals, though serum testosterone in these castrates was much higher than in normal, male rats. It is concluded, that the sensitivity of the hypothalamic-pituitary system for daily, subcutaneous testosterone administration during seven days is not the same in the intact and castrated, adult, male rat and that testicular factors different from testosterone may play a role in regulating production and/or secretion of gonadotrophins by the hypophysis in male animals.     

Effects of luteolysis during late pregnancy on pituitary responsiveness to gonadotrophin-releasing hormone in the rat

We investigated whether the increase in the gonadotrophin response to gonadotrophin-releasing hormone (GnRH) during the last days of pregnancy and the occurrence of parturition on day 22 of pregnancy in rats are due to the increase in the plasma concentrations of oestradiol-17 beta after luteolysis, which occurs around day 20. In a first series of experiments we studied the effects of s.c. implantation of two capsules containing oestradiol on basal and GnRH-stimulated secretion of LH and FSH before and after luteolysis. Before luteolysis, ovariectomy increased basal LH and FSH; oestradiol treatment prevented this increase partly (FSH) or completely (LH). Ovariectomy also lowered the LH response to the infusion of GnRH (100 ng/h). Oestradiol treatment on the other hand, increased the LH and FSH responses of both intact and ovariectomized rats above the level in intact non-treated control rats. After luteolysis, ovariectomy increased basal FSH only. Treatment with oestradiol did not prevent the increase in basal FSH and ovariectomy diminished the LH response to GnRH infusion. Oestradiol treatment maintained the LH response in ovariectomized rats at the control level and increased the FSH responses of both intact and ovariectomized rats to a higher level than in control rats. Furthermore, the LH and FSH responses of the oestradiol-treated groups of intact and ovariectomized rats were higher after luteolysis than before. In a second series of experiments two capsules containing progesterone were s.c. implanted before or after luteolysis. Progesterone treatment suppressed the plasma concentration of oestradiol and the gonadotrophin responses to infusion of GnRH on the expected day of parturition in both groups of rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Gonadotropin secretion in cryptorchid and castrate rams and the acute effects of exogenous steroid treatment.

Gonadotropin secretion in cryptorchid and castrate rams and the acutve been determined. Rams made cryptorchid at 6 weeks of age had increased serum levels of luteinizing hormone (LH) and follicle stimulating hormone (FSH) when determined at 9 months of age. These levels approached those of the castrate animal; and yet serum levels of testosterone (T) were unchanged. Even though mean serum LH concentrations were elevated sixfold to eightfold over those of intact ram levels, a temporal relationship between this hormone and T was observed similar to that reported in the intact ram. Intramuscular injections of dihydrotestosterone had no effect on circulating levels of LH or FSH in either cryptorchid or castrate rams, whereas T effectively reduced these gonadotropins in castrate but not in cryptorchid rams. Only estradiol-17beta (E2) was effective in both cryptorchid and castrate rams. Estradiol was a potent inhibitor of LH secretion; however, its effect on FSH levels was less dramatic. This suggests that testicular products other than E2 may be important in the regulation of FSH production and/or release. Importantly, the inhibition of LH secretion lasted less than 12 h; whereas, the negative effects of E2 on FSH secretion lasted 72 to 144 h. In conclusion, results from this study show that T is not the single factor responsible for regulation of LH and FSH secretion in male sheep. Estradiol may be an important regulator of gonadotropin secretion, but 5alpha-reduction plays no apparent role in this process.     

Regulation of luteinizing hormone secretion in male sheep by endogenous estrogen

Both androgens and estrogens are implicated in the regulation of gonadotropin secretion in the male. Three experiments were conducted to determine the physiological importance of estradiol (E2) in the feedback regulation of LH secretion in male sheep. In the first experiment, LH secretion in castrate rams (wethers) was shown to be susceptible to the picomolar concentrations of E2 found in intact rams. In the second experiment, aminoglutethimide (AG) was administered to testosterone-implanted wethers to block aromatization of testosterone and ascertain the possible consequences of E2 deprivation on testosterone-mediated LH suppression. AG had no apparent effect on serum testosterone, but reduced serum E2 by half and increased serum LH approximately 3-fold. These data suggest that aromatization of testosterone to E2 is a physiologically important step in the regulation of LH secretion in the ram and that aromatization occurs, at least in part, in peripheral tissues. The relative contributions of peripherally and centrally derived E2 in the regulation of LH remain uncertain. In the third experiment, immunoneutralization of endogenous E2 in intact rams by active immunization against E2 was shown to stimulate pulsatile LH secretion and elevate basal and mean LH concentrations. Associated with this LH response was a significant increase in serum testosterone, such that immunized rams exposed to a nonstimulatory 16-h light, 8-h dark photoperiod had testosterone concentrations equivalent to those of control rams exposed to a stimulatory 8-h light, 16-h dark photoperiod. Together, these findings emphasize the importance of E2 in the control of male reproduction and suggest the possibility of improving year-round reproductive performance of the domestic ram through E2 immunoneutralization.     

Androgen feedback-dependent and -independent control of photoinduced LH secretion in male tree sparrows (Spizella arborea)

Photoperiodic control of gonadotrophin secretion in male tree sparrows was studied by examining changes in plasma LH in castrated birds retained on short daylengths and in castrated birds transferred to long daylengths. Plasma LH concentrations were markedly higher in photostimulated birds than in non-photostimulated birds throughout the 25-day experiment, and implantation of the antiandrogen cyproterone (free alcohol), which should have blocked the action of castration-resistant androgens, did not increase plasma LH in either group. Such results, obtained from birds in which testosterone feedback was inoperative, indicate that the gonadostimulatory effect of long daylengths in intact males must be mediated, at least in part, by an androgen feedback-independent mechanism. To determine whether changes in testosterone feedback facilitate gonadotrophin secretion during photostimulation, two feedback performance characteristics (i.e. set point (minimum concentration of testosterone that suppresses plasma LH) and sensitivity (change in plasma LH per unit change in testosterone)) were quantified by evaluating plasma LH responses of non-photostimulated castrated birds and of photostimulated castrated birds to replacement testosterone (0-4.16 mumol). The data indicate that, in addition to stimulating LH secretion by an androgen feedback-independent mechanism, long daylengths reduce feedback inhibition of LH secretion by increasing the putative set point and decreasing the sensitivity of the testosterone feedback mechanism. The feedback-independent effect is the predominant effect of photostimulation on LH secretion in male tree sparrows.     

Studies of the role of sex steroids in the feedback control of FSH concentrations in men

Studies on the role of sex steroids in the feedback control of follicle stimulating hormone (FSH) concentrations in 22-35 year old men are reported. 10 normal volunteers and 8 men undergoing evaluation for azoospermia or oligospermia received steroids by constant infusion for 96 hours at twice the estimated daily production rate of normal men to obtain stable levels in peripheral blood. 15 mg of testosterone increased plasma testosterone and estradiol levels 2-fold and suppressed FSH and luteinizing hormone (LH) approximately 40% during infusion Days 3 and 4. 90 mcg of estradiol/day caused similar suppression of FSH and LH and the addition of 4.5 mg of 17 alpha-hydroxyprogesterone had no additive effect. 7.5 mg of dihydrotestosterone/day produced no detectible changes in FSH or LH. No evidence for a selective effect of any of the steroids on FSH secretion was seen. These data support the concept that although there is a specific seminiferous tubular factor regulating FSH secretion, testicular steroids also modulate FSH secretion.     

Central administration of corticotrophin releasing hormone but not arginine vasopressin stimulates the secretion of luteinizing hormone in rams in the presence and absence of testosterone.

This study tested the hypothesis that central administration of corticotrophin-releasing hormone (CRH) and/or arginine vasopressin (AVP) will affect the secretion of LH in rams and that testosterone is necessary for these actions to occur. Plasma LH levels were measured in castrated rams during 1 h infusion of either 100 microliter vehicle/mock cerebrospinal fluid (CSF) or mock CSF containing 25 microgram CRH, 25 microgram AVP or 25 microgram of each peptide through guide cannulae into the third cerebral ventricle. These intracerebroventricular (i.c.v.) infusions were given to the castrated rams following injections (i.m.) each 12 h of oil or 8 mg testosterone propionate for 7 days. Blood samples were collected every 10 min for 4 h before i.c.v. infusion, during infusion and for 4 h following the infusion. Infusion of vehicle did not affect any endocrine parameters. In contrast, the plasma concentrations of LH and the amplitude of LH pulses were increased significantly during and following infusion of CRH, and this effect was not influenced by whether the castrated rams were treated with testosterone propionate or whether the CRH was administered in combination with AVP. Infusion of AVP alone did not affect LH secretion. The frequency of LH pulses and the plasma concentrations of FSH did not change with any of the i.c.v. treatments. The plasma concentrations of cortisol were significantly increased by CRH and AVP infusions. The plasma concentrations of cortisol achieved during and following i.c.v. infusion of CRH and AVP combined were greater than the concentrations achieved as a result of treatment with AVP alone but were similar to those with CRH. There was no effect of testosterone propionate on cortisol levels. These results show that CRH, but not AVP, is capable of acting either centrally or at the pituitary level to increase the secretion of LH in rams and these actions are not affected by testosterone. The stimulatory effects of CRH on LH secretion are to increase the amplitude of GnRH pulses and/or the responsiveness of the pituitary to the actions of GnRH with no effect on the frequency of GnRH pulses. The secretion of FSH in rams is not influenced by either CRH or AVP. The effect of CRH to increase LH pulse amplitude occurs in the face of increased cortisol levels, further reinforcing our belief that this adrenal steroid does not affect the reproductive axis in this species.     

Effect of neonatal treatment with the sex-opposite steroid on gonadotropin responsiveness in rats

This study was performed to determine if two sex differences in gonadotropin responses to negative feedback, the acute postcastration rise and the effect of follicular fluid (FF) in the acute castrate, could be reversed by neonatal treatment with sex-opposite steroids. Female rats that received testosterone propionate (TP-females) and male rats that received estradiol benzoate (EB-males) neonatally were studied as adults. EB-males showed an LH response to gonadectomy which was much less than control males, and did not differ from control females, which could suggest the hypothesis that neonatal estrogen 'feminizes' the male response to gonadectomy. However, as the postgonadectomy response in both LH and FSH was depressed in both TP-females and EB-males in comparison to their respective matched sex controls, neonatal steroid treatment appears simply to impair hypothalamic-pituitary function. This is not a result of decreased pituitary responsiveness to GnRH in TP-females and EB-males. On the other hand, neonatal steroid treatment does not change the sex-specific response to imposition of peptide negative feedback (i.e., FF administration). In control and TP-females, FF significantly suppressed serum FSH levels, both in intact animals and 9 h after gonadectomy. In both control and EB-males, FF suppressed FSH in intact animals, but failed to do so in acute castrates. Thus, neonatal steroid treatment does not reverse the sex differences in gonadal-gonadotropin interrelations, but rather causes an impairment in the acute recognition of loss of negative feedback at the hypothalamic level.     

The role of gonadal steroids in feedback regulation of gonadotropin secretion at different stages of primate development

The serum gonadotropin response to castration was assessed in 8 foetal, 2 neonatal, 30 juvenile, and 2 adult rhesus monkeys (M. mulatta). In the 30 castrated juvenile monkeys and 8 sham-operated controls, concentrations of oestrone, oestradiol, androstenedione, dihydrotestosterone, testosterone and 17OH-progesterone were measured in 10 ml serum pools before, one month after, and one year after the surgical procedure. Castration during foetal life (83-137 days gestation) was followed within 48-72 h by a significant rise in serum FSH levels in males, but had no effect on the already high levels in females. Similarly, castration of males during the first post-natal month raised serum FSH and LH into the adult castrate range; however, after 3 months of age serum gonadotropin levels again declined to the normal juvenile range in spite of the open feedback loop. Orchiectomy of prepubertal juvenile monkeys (age 3 month-2 8/12 years) had no immediate effect on serum gonadotropins, but was followed by a delayed rise in FSH (at age 2 3/12-4 3/12 years) and LH (at age 2 7/12-4 4/12 years) to adult castrate levels. Orchiectomy of older prepubertal (by serum testosterone) or adult males resulted within a few days in a progressive and sustained rise in serum FSH and a more gradual rise in LH. Prepubertal gonadotropin regulation appeared to be sexually dimorphic, since ovariectomy in juvenile females (age 3 months-1 5/12 years) was followed by generally elevated, if somewhat erratic, serum FSH values, with a secondary rise in both FSH and LH levels at 2-2 1/12 years. In both sexes, prepubertal castration caused a significant and sustained decline in serum concentrations of oestradiol; castrated males also showed a decline in serum testosterone levels. Although prepubertal castration also caused in both sexes a slight decline in serum oestrone, and ovariectomy a decline in serum androstenedione and dihydrotestosterone, these effects were not sustained one year later, and values were not significantly different from sham-operated controls. Taken together, these data lend support to a model of primate sexual maturation in which the primary regulator of gonadotropin secretion in both sexes during the prolonged juvenile phase is central inhibition of the hypothalamic GnRH regulator. However, during foetal and neonatal life, and again following the onset of puberty, the major modulator of gonadotropin secretion becomes sex steroid-mediated feedback inhibition.     

THE OESTROGEN PROVOCATION TEST

The oestrogen feedback and gonadotrophin release in ten amenorrhoeic women were investigated, using intramuscular injection of 1 mg oestradiol benzoate. Serial estimations of serum oestradiol and gonadotrophins (LH and FSH) were made over a period of 72 h following the injection. Five patients demonstrated positive feedback release of LH to the oestrogen stimulus with elevated levels of LH significantly above baseline (P less than 0.001), which occurred between 48 and 72 h after the injection. Two of the five patients also demonstrated elevated FSH levels accompanying these LH peaks, The hypothalamic-pituitary axis was postulated to be intact in these five patients, and all ovulated on clomiphene. None of the remaining five subjects showed any increase in serum gonadotrophin levels in response to oestradiol and none ovulated on clomiphene when given a dose of up to 200 mg daily x5 days. This 'oestrogen provocation test" seems to be a useful means of evaluating the functional capacity of the hypothalamic-pituitary axis for gonadotrophin release and has proved useful in predicting responsiveness to clomiphene in amenorrhoeic women.     

Effect of testosterone on gonadotrophin-releasing hormone receptors in the castrated rat: preliminary evidence for a stimulatory effect of testosterone on gonadotrophin function in the male rat

In the long-term castrated rat the negative feedback effect of testosterone is markedly reduced and the raised levels of plasma LH seen in the castrated animals are not suppressed by physiological concentrations of plasma testosterone. In this study we have measured pituitary gonadotrophin-releasing hormone (GnRH) receptor content as well as plasma and pituitary LH on days 1, 10 and 40 after castration and noted the effect of testosterone replacement on these parameters. We found that the negative feedback effect of physiological concentrations of testosterone on plasma and pituitary LH, pituitary GnRH receptor content and response to exogenous GnRH was attenuated 10 and 40 days after castration. It is suggested that the lack of effect of testosterone in the long-term castrated rat is due to its inability to reduce the pituitary GnRH receptor content. On increasing testosterone to supraphysiological levels, the negative feedback effect was reinstated. We also found that in rats 40 days after castration, physiological and subphysiological concentrations of testosterone significantly increased pituitary GnRH receptor content and this may explain the previous findings that low concentrations of testosterone can enhance the effect of GnRH and increase plasma LH levels.