Gonadal steroid modulation of LHRH-stimulated LH secretion by pituitary cell cultures

Enzymatically dispersed rat pituitary cells were grown in primary culture, and LHRH-stimulated LH secretion was measured. Testosterone (T) decreased and 17 beta-estradiol (E) increased pituitary responsiveness to LHRH. The effect of E on LH secretion was partly due to an increase in LH content. There was a latent period of 12 h for E and 18 h for T between the onset of steroid treatment and the manifestation of steroid action. Neither steroid was required to be continuously present in order to exert its effects. After steroid withdrawal, the effect of T persisted for 72 h and that of E for more than 96 h. The actions of both steroids were blocked by protein-synthesis inhibitors. These results are consistent with the hypothesis that steroid effects rely on a mechanism involving alterations in protein synthesis; the affected proteins may be involved in the process of LHRH action.     

Effect of level of food intake of ewes on the secretion of LH and FSH and on the pituitary response to gonadotrophin-releasing hormone in ovariectomized ewes

The effect of level of food intake on LH and FSH profiles and pituitary sensitivity to gonadotrophin-releasing hormone (GnRH) was investigated in two groups of 12 ovariectomized ewes. Ewes with a high intake (group H) had a mean daily intake (+/- S.E.M.) of 1.99 +/- 0.075 kg dry matter (DM)/head per day while ewes with a moderate intake (group M) consumed a mean of 1.02 +/- 0.021 kg DM/head per day. Ovaries were surgically removed from six ewes of each group on day 11 of the luteal phase and from the remainder 30 h after an injection of 100 micrograms prostaglandin analogue given on day 11 to induce luteolysis. During both the luteal phase and the follicular phase, mean LH and FSH concentrations and LH pulse frequencies and amplitudes were unaffected by the level of intake but mean plasma prolactin concentrations were higher (P less than 0.05) in group H than in group M ewes in the follicular phase. Mean LH and FSH concentrations at day 2 after ovariectomy were unaffected by treatment while mean prolactin concentrations were higher (P less than 0.05) in group H than in group M ewes. At day 7 after ovariectomy, mean LH and FSH concentrations were lower (P less than 0.05) in group H than in group M ewes although mean LH pulse frequencies and pulse amplitudes were not significantly affected by the level of intake at either time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Episodic ovarian inhibin secretion is not due to LH pulses in anoestrous ewes

In sheep, secretion of oestradiol by the ovary is stimulated by pulses of LH but the factors controlling ovarian inhibin secretion are not well understood. We have investigated the effect of a single injection of LH on the ovarian secretion of inhibin. Six anoestrous Finn-Merino ewes which had one ovary autotransplanted to a site in the neck had jugular and timed ovarian venous blood samples collected at 10-min intervals for a total of 5 h. The secretion rates of both inhibin (1-3 ng/min) and oestradiol (0.5-8 ng/min) were similar to those observed during the breeding season indicating significant follicular development in these animals. After injection of 2.5 micrograms NIH-LH-S25 intravenously the concentration of LH in plasma rose from a baseline of 1.8 +/- 0.1 (S.E.M.) microgram/l to a peak of 3.9 +/- 0.3 micrograms/l (P less than 0.01). This LH pulse stimulated a corresponding increase (P less than 0.01) in oestradiol secretion from a basal level of 0.9 +/- 0.2 ng/min to a peak of 4.6 +/- 0.6 ng/min that occurred within 30 min of injection. Although inhibin secretion was episodic in nature, increases were not related to either exogenous or endogenous LH pulses. We conclude that, in contrast to oestradiol, the secretion of inhibin by the ovary is not controlled acutely by changes in plasma levels of LH during anoestrus.     

LH and PRL secretion in ovariectomized spontaneously hypertensive rats

The pulsatile patterns of LH and PRL secretion, and effects of stress on these patterns were examined in ovariectomized normotensive Wistar, spontaneously hypertensive, and genetically matched normotensive Wistar Kyoto rats. Judged by the overall mean of PRL concentrations, PRL levels in spontaneously hypertensive rats were not different from those in Wistar Kyoto rats, but the pulse amplitudes as well as the overall mean concentrations were significantly greater in both Wistar Kyoto and spontaneously hypertensive rats than in Wistar rats. Increases in PRL release during immobilization stress in spontaneously hypertensive rats occurred earlier than in Wistar rats, but the peak levels were not different between the two animal groups, though significantly lower in Wistar Kyoto rats than in Wistar rats. Overall mean of LH concentrations was higher, but pulse amplitude was smaller in spontaneously hypertensive rats than in Wistar Kyoto rats, and in both groups significantly smaller than in Wistar rats. The decrease in LH release owing to stress was similar in all animal groups. A significant association between LH and PRL peaks was evident in all animals, although the rate of association was lowest in Wistar Kyoto rats. In conclusion, the central nervous system mechanisms for both LH and PRL secretion differ markedly in both spontaneously hypertensive and Wistar Kyoto rats from those in Wistar rats, and no specific difference was found in spontaneously hypertensive rats compared with Wistar Kyoto and Wistar rats as controls.     

Hypoglycemia, but not insulin, acutely decreases LH and T secretion in men

Hypoandrogenemia is frequently associated with hyperinsulinemia in men with the metabolic syndrome. We questioned whether insulin or changes in blood glucose levels influence pituitary gonadotropin secretion or testicular steroidogenesis in healthy men. Also, the relationship between hypoglycemia-induced activation of the hypothalamus-pituitary-adrenal axis and altered steroidogenesis was examined. Euglycemic and hypoglycemic clamp experiments were performed in 30 healthy men over a period of 6 h. Half of the men were infused with insulin at a rate of 1.5 mU/min.kg; the other half were infused at a rate of 15.0 mU/min.kg. Plasma glucose was held constant during a euglycemic clamp session and was decreased stepwise in a hypoglycemic clamp session. LH and total/free T concentrations decreased under hypoglycemic conditions regardless of the rate of insulin infusion. With euglycemic conditions, LH and T levels remained unchanged. Dehydroepiandrosterone concentrations increased during hypoglycemia, but not during the euglycemic conditions. The FSH concentration was not affected by insulin or glycemic clamps. Hypoglycemia acutely suppresses T secretion, and this effect is apparently mediated by pituitary LH. Insulin is ineffective. As counterregulation to hypoglycemia begins at normoglycemic ranges in poorly controlled type 2 diabetes and probably also in patients with long-term perturbed glucose regulation in the metabolic syndrome, control of glucose-responsive neurons in the brain may contribute to hypoandrogenemia. Apart from down-regulation of hypothalamic release of GnRH, concurrent activation of the pituitary-adrenal axis (i.e. increased release of dehydroepiandrosterone) may add to the suppressive effect of hypoglycemia on gonadal steroidogenesis.     

Disruption of GnRH pulses by anti-GnRH serum and phentolamine obliterates pulsatile LH but not FSH secretion in ovariectomized rabbits

It has been hypothesized that the secretion of gonadotropins, i.e. luteinizing hormone (LH) and follicle-stimulating hormone (FSH), is driven by a synchronized neural network ('pulse generator'). This network, regulated in part by alpha-adrenergic activity, ultimately generates bursts of hypothalamic gonadotropin-releasing hormone (GnRH) release. In this study, we used the push-pull (PP) perfusion technique in ovariectomized rabbits to investigate three aspects of the ('GnRH/gonadotropin pulse generator') hypothesis. The objectives were to determine: (1) if plasma LH and FSH pulses occur concomitantly with mediobasal hypothalamic (MBH-) GnRH pulses, (2) changes in the patterns of pulsatile LH and FSH secretion when pulsatile MBH GnRH signals are interrupted by either local immunoneutralization of GnRH or intravenous infusion of the alpha-adrenergic antagonist phentolamine (PHEN, 4 mg/kg BW), and (3) whether third cerebroventricular (3VT-) GnRH patterns reflect neuronal GnRH release from the MBH. We found that while both plasma LH and FSH patterns were pulsatile, MBH GnRH pulses were significantly coupled only with LH pulses (94% coincidence). Both the local immunoneutralization of MBH GnRH pulses and the PHEN-induced suppression of MBH GnRH pulses obliterated the pulsatile secretion of LH, but not FSH. Neither MBH GnRH nor plasma LH or plasma FSH pulses were concurrent with 3VT GnRH pulses. However, the PP perfusion of the 3VT appeared to alter the pulsatile release of MBH GnRH and pituitary LH. The results support the hypothesis that in the absence of ovarian signals, the 'pulse generator' is maintained by tonic alpha-adrenergic input and that a 'cellular unity' of MBH GnRH release (GnRH pulses) drives the gonadotrophs to secrete LH in pulses. In contrast, the pulsatile release of FSH appears to involve additional nonovarian regulatory events to those controlling LH secretion.     

Effects of bovine follicular fluid on gonadotrophin secretion in intact and chronically ovariectomized ewes before and after desensitization of pituitary gonadotrophs to gonadotrophin-releasing hormone

Intact and chronically ovariectomized ewes were treated for 4 days with charcoal-treated bovine follicular fluid (FF) or charcoal-treated bovine serum during the late-anoestrous period, and the effects on basal and gonadotrophin-releasing hormone (GnRH)-induced secretion of LH and FSH observed. Subsequently, ewes received s.c. implants containing a sustained-release formulation of a potent GnRH agonist D-Ser(But)6-Azgly10-LHRH (ICI 118630) to desensitize pituitary gonadotrophs to hypothalamic stimulation, and the effects of bovine FF and bovine serum were re-assessed 2 weeks later. Chronic exposure (for 2-3 weeks) to ICI 118630 significantly reduced basal levels of LH and FSH in both intact and ovariectomized ewes and completely abolished both spontaneous LH pulses as well as exogenous GnRH-induced acute increases in plasma LH and FSH levels. Treatment with bovine FF significantly reduced plasma FSH levels, but not LH levels, in both intact and ovariectomized ewes before and after chronic exposure to ICI 118630. In intact ewes before exposure to ICI 118630, treatment with bovine FF actually enhanced pulsatile LH secretion and raised mean plasma LH levels by 240% (P less than 0.05). No such stimulatory effect of bovine FF on LH secretion was observed in intact ewes exposed to ICI 118630 or in ovariectomized ewes before or after exposure to ICI 118630, suggesting that the effect probably involved an alteration in ovarian steroid feedback affecting hypothalamic GnRH output. Treatment with bovine FF did not significantly affect the magnitude of GnRH-induced surges of LH or of FSH observed in either intact or ovariectomized ewes before exposure to ICI 118630.(ABSTRACT TRUNCATED AT 250 WORDS)     

Phenobarbital enhances pituitary LH release induced by LRH pulses in the ovariectomized rat

In the ovariectomized (OVX) rat, the plasma LH response was measured to a pulse of LRH (1.25 or 5 ng/100 g body weight, ia) given before and 1 h after ip administration of phenobarbital (80 mg/kg body weight). The LH response to the LRH pulses was increased 1 h after phenobarbital. In a second experiment, the pituitary LH content of OVX rats was measured 1 h after administration of phenobarbital or saline. No difference in pituitary LH content was found. It is concluded that in the OVX rat, phenobarbital increases the response to a pulse of LRH, presumably by suppressing endogenous pulsatile LRH. This, together with results of earlier experiments, further supports the hypothesis that under conditions where endogenous pulsatile LRH is present, there is always a certain degree of pituitary desensitization or refractoriness and that the removal of this endogenous LRH leads to recovery of pituitary sensitivity to LRH.     

Effects of modifying gonadotrophin-releasing hormone input before and after the oestrogen-induced LH surge in ovariectomized ewes with hypothalamo-pituitary disconnection

The patterns of gonadotrophin-releasing hormone (GnRH) input to the pituitary gland that affect the expression of a positive-feedback event by oestrogen on LH secretion were investigated in ovariectomized ewes with hypothalamo-pituitary disconnection (HPD). In experiment 1, ovariectomized HPD ewes were given hourly i.v. pulses of 250 ng GnRH and an i.m. injection of 50 micrograms oestradiol benzoate (OB). The ewes were given a bolus pulse of 2.25 micrograms GnRH 16 h after injection of OB, followed by half-hourly pulses of 250 ng GnRH for 14 h (treatment A). The LH surge response was significantly (P less than 0.05) greater in these ewes compared with that in ewes given a continuous infusion of GnRH (250 ng/h) after the OB injection, followed by a continuous infusion of 500 ng GnRH/h after the bolus pulse of GnRH (treatment B). When no GnRH was administered after the OB injection, except for the bolus pulse of GnRH (treatment C), the surge response was significantly (P less than 0.05) reduced compared with that in treatment A, and was reduced compared with treatment B. These data suggest that GnRH pulses are important in the generation of the OB-induced LH surge, but that a baseline secretory component can prime the pituitary to some extent. In experiment 2, a doubling of the continuous infusion dose of GnRH used in treatment B to 500 ng/h before the bolus pulse of GnRH and to 1 micrograms/h afterwards (treatment D) gave a similar response compared with treatment A, suggesting that if the baseline input of GnRH is of sufficient magnitude, it can overcome the lack of pulsatile input. In experiment 3, halving the GnRH pulse amplitude used in treatment A from 250 to 125 ng (treatment E) did not reduce the LH surge response, implying that when the GnRH input is in a pulsatile mode, the amplitude of GnRH pulses is less important than the pulsatile nature per se. In experiment 4, removal of GnRH input after the bolus pulse of GnRH (treatment F) significantly (P less than 0.05) reduced the surge response compared with when pulses were maintained (treatment A), indicating that GnRH input is still required once the LH surge has been initiated. Collectively, these experiments show that several forms of GnRH delivery, both pulsatile and baseline, can result in the full expression of a positive-feedback response in ovariectomized ewes treated with oestrogen.     

The role of LH in the autofeedback inhibition of LH and FSH secretion in ovariectomized rats

This study examined the role of exogenous LH in the autofeedback regulation of LH and FSH release in ovariectomized rats. The rats were implanted with third ventricular cannulae three weeks after ovariectomy and fitted with silastic jugular cannulae one week later. Baseline hormone levels were established on the day of experimentation in conscious, unrestrained animals. Thereafter, experimental animals received intraventricularly (IVT) either a 9 ug or 20 ug dose of a purified preparation of human (h)LH that did not crossreact in our rLH RIA. In response to 20 ug, but not 9 ug of LH, plasma levels of both LH and FSH were significantly reduced during the following one hour period compared to values in controls receiving buffer IVT. Administration of ovine (o)LH (6 ug, IVT), a preparation which crossreacts in the rLH RIA, significantly elevated plasma levels of detectable LH during the experimental period indicating that LH reaches the blood stream from the third ventricle and, thus, may effect endogenous hormone release at either the pituitary or hypothalamic levels. However, in animals preinjected with 9 or 20 ug of hLH IVT one hour earlier the surge of both LH and FSH in response to LHRH (25 ng iv) was not different from that in the buffer-injected controls receiving LHRH which indicates that pituitary responsiveness was not suppressed by the effective dose of hLH. The results of this study suggest that the inhibitory shortloop feedback of LH on endogenous LH and FSH secretion in ovariectomized rats occurs at the level of the hypothalamus.     

Prolactin suppresses luteinizing hormone secretion and pituitary responsiveness to luteinizing hormone-releasing hormone by a direct action at the anterior pituitary

In pathological or experimental hyperprolactinemia, the elevated circulating levels of PRL are the usual cause of the impairment in gonadotropic function. The present study was undertaken to determine whether PRL could suppress basal LH secretion and LHRH-stimulated LH release by a direct action at the anterior pituitary. Anterior pituitaries from ovariectomized rats were incubated in medium 199 alone or in medium 199 containing ovine PRL, and basal and the LHRH-stimulated LH release were followed for 2 or 3 h in vitro. Ovine PRL at 40 and 80 micrograms/ml suppressed basal LH release by 41% and 72%, respectively, at 2 h of incubation. This suppressive effect of both concentrations of PRL continued to the third hour of incubation. LHRH at 5 ng/ml increased the release of LH from pituitaries incubated in medium alone by 57%, 61%, and 107% at 1, 2, and 3 h of incubation, respectively. However, in the pituitaries treated with 40 micrograms/ml ovine PRL, the stimulatory effects of LHRH were diminished at all time points measured. Pretreatment of anterior pituitaries with ovine PRL for 6 h significantly inhibited by 81% the LHRH (5 ng/ml) stimulation of LH release at 2 h of incubation. On the other hand, inhibition of endogenous PRL release by 10(-6) M bromocriptine enhanced the stimulatory effects of 5 ng/ml LHRH by 2.5-fold at 2 h of incubation. The inhibitory effects of PRL on basal and stimulated LH secretion appeared unique, since neither BSA nor vasopressin could elicit similar suppressive effects on LH. These results suggest that in anterior pituitaries exposed to elevated levels of PRL, LH secretion and pituitary responsiveness to LHRH could be impaired. This phenomenon may contribute in part to the antigonadotropic effects of PRL.     

Changes in the pulsatile secretion of LH after the removal of and subsequent resuckling by pups in ovariectomized lactating rats

Changes in the pulsatile secretion of LH after removal of pups and subsequent resuckling were examined in ovariectomized lactating rats, and the change after removal of pups was compared with that after the removal of ovaries in cyclic female rats. The day of parturition was designated day 0 of lactation. All lactating rats were ovariectomized on day 2 of lactation. They were deprived of their pups for 6, 12, 18, 24 or 45 h before blood sampling on day 8 of lactation, or were resuckled by their pups for 1, 4, 7 or 12 h before blood collection after separation from pups for 24 h. Cyclic female rats were ovariectomized on the day of dioestrus and blood samples were taken 12, 18, 24 or 48 h or 6 days after ovariectomy. Typical LH pulses appeared in some animals from 12 h after the removal of pups. The mean LH level and the frequency and amplitude of LH pulses gradually increased after removal of pups, until after 45 h of separation the frequency reached the high level observed 6 days after ovariectomy in cyclic rats. The subsequent resuckling by pups after a 24-h separation decreased these three parameters of LH pulses rapidly. In contrast, the frequency of LH pulses was unchanged after ovariectomy in cyclic rats, although the mean LH level and the amplitude of LH pulses increased. These results suggest that the suckling stimulus suppresses pulsatile LH secretion in a different manner from that of ovarian steroids.     

Inhibin suppresses luteinizing hormone (LH)-releasing hormone self-priming: direct action on follicle-stimulating hormone secretion and opposition of estradiol-enhanced LH secretion.

Previous studies have suggested that the ovary produces a factor that maintains the pituitary in a state of low LHRH responsiveness that must be overcome by the self-priming action of LHRH. To determine the role of inhibin in maintaining low LHRH responsiveness in pituitaries of diestrous female rats, endogenous inhibin was passively immunoneutralized in vivo, and the pituitaries were removed 18-20 h later and examined for LHRH responsiveness in vitro. Pituitaries from diestrous control rats produced the biphasic pattern of gonadotropin secretion that typifies LHRH self-priming: an initial low secretory response to LHRH (lag phase), followed by a protein synthesis-dependent transition to an enhanced rate of secretion with continued LHRH exposure (primed phase). Immunoneutralization of endogenous inhibin [antiserum (AS) treated] resulted in an increased rate of LH secretion during the lag phase, while no change was observed in the primed phase rate of LH secretion. FSH secretion from pituitaries of AS-treated rats was increased during the lag phase to a rate of secretion similar to that observed during the primed phase of FSH secretion from control pituitaries, and it was increased further during the primed phase of secretion. These results suggest that inhibin is at least partially responsible for the low secretion of LH observed during the lag phase response to LHRH exposure and is totally responsible for the lowered rate of FSH secretion during the lag phase. The observation that the enhanced rate of gonadotropin secretion observed with AS-treated pituitaries during the lag phase was resistant to inhibition of protein synthesis provides further evidence that a partial transition from the lag to the primed phase had already occurred. Pituitaries from ovariectomized rats were also examined in order to place the contribution of inhibin in perspective with the total ovarian influence on pituitary responsiveness to LHRH. Unexpectedly, LH secretion during the lag phase was similar to the low secretion rate of diestrous control pituitaries, and the higher primed rate of secretion failed to fully develop, suggesting that an additional ovarian factor was required to induce and maintain pituitary responsiveness to LHRH in terms of LH secretion. FSH secretion from the ovariectomized rats was similar to that observed from pituitaries of AS-treated rats, thus further supporting the concept that inhibin is fully responsible for the suppression of FSH secretion in response to LHRH. Plasma from the AS-treated rats revealed a 2-fold increase in estradiol levels compared with diestrous control rats.(ABSTRACT TRUNCATED AT 400 WORDS)     

Pulsatile administration of gonadotrophin-releasing hormone stimulates, in oestrogen-treated anaesthetized ovariectomized ewes, a surge release of LH qualitatively and quantitatively different from that induced by oestradiol in conscious ovariectomized ewes

The nature of the gonadotrophin-releasing hormone (GnRH) stimulus of the pituitary necessary for the oestrogen-induced plasma LH surge was studied in ovariectomized ewes. The sheep were treated with oestradiol benzoate (50 micrograms i.m.) at 0 h, and the hypothalamic contribution to the LH surge was blocked by pentobarbitone anaesthesia over the time during which the surge was expected (11-31 h). Pituitary responsiveness to exogenous GnRH (100 ng) administered i.v. in a pulsatile mode (once per hour or once per 20 min) over the period 15-30 h was assessed from plasma concentrations of LH. Neither of the GnRH treatments induced patterns of LH secretion similar to those seen in conscious ovariectomized ewes given oestrogen only. Plasma LH secretion in response to hourly GnRH pulses was less (P less than 0.01) than that associated with oestrogen-induced plasma LH surges in conscious control ewes. With pulses of GnRH administered every 20 min the amount of LH released was greater (P less than 0.05) than that in oestrogen-treated conscious control ewes. In contrast to the single surge induced by oestradiol in conscious ewes, GnRH pulses given every 20 min elicited phasic patterns of LH secretion consisting of two or three distinct surges. The failure of GnRH treatment to elicit an LH surge similar to an oestrogen-induced surge could reflect inappropriate GnRH treatment regimens, and/or inadequate priming of the pituitary with GnRH after induction of anaesthesia but before GnRH treatment.     

Effect of the body condition of ewes on the secretion of LH and FSH and the pituitary response to gonadotrophin-releasing hormone

The effects of body fat content (body condition) of ewes on hypothalamic activity and gonadotrophin-releasing hormone (GnRH) secretion and on pituitary sensitivity to GnRH were investigated using Scottish Blackface ewes. Two groups of 12 ewes were fed so that they achieved either a high body condition score (2.98, S.E.M. = 0.046; approximately 27% of empty body weight as fat) or a low body condition score (1.94, S.E.M. = 0.031; approximately 19% of empty body weight as fat) by 4 weeks before the period of study. Thereafter, they were differentially fed so that the difference in mean condition score was maintained. Oestrus was synchronized, and on day 11 of the subsequent cycle half of the ewes of each group were ovariectomized. On day 12, the remaining ewes were injected (i.m.) with 100 micrograms prostaglandin F2 alpha analogue and ovariectomized 30 h later. Numbers of large ovarian follicles and corpora lutea present at ovariectomy were recorded. Blood samples were collected at 15-min intervals for 12 h on day 10 of the cycle (luteal phase) and at 10-min intervals from 24 to 30 h after prostaglandin injection (follicular phase). At days 2 and 7 after ovariectomy, samples were collected at 15-min intervals for 8 h and ewes were then injected with 10 micrograms GnRH and samples were collected for a further 3 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intensive direct cavernous sinus sampling identifies high-frequency, nearly random patterns of FSH secretion in ovariectomized ewes: combined appraisal by RIA and bioassay.

Analyses of FSH secretion suggest pulsatile, nonpulsatile, or compositely pulsatile and nonpulsatile release modes. This may reflect the reduced signal-to-noise ratio inherent in FSH pulse estimation procedures and/or immunological-biological assay inconsistencies. To address these issues, we sampled cavernous sinus and jugular venous blood concomitantly from ovariectomized sheep at either 5-min or 1-min intervals. Samples from the former were assayed by RIA, and those from the latter by RIA and bioassay. Waveform-independent peak detection revealed FSH pulses occurring at high frequency. Pulsatile FSH secretion accounted for 28% of total secretion. Approximate entropy analysis showed that FSH secretion was nearly random. There was synchronous release of LH and FSH, but most FSH secretion was not associated with LH release; 13% of discrete FSH and LH pulses were concordant. We infer that FSH secretion exhibits pulsatile and basal/nonpulsatile features, with high-entropy features. Linear and nonlinear statistical measures revealed joint sample-by-sample synchrony of FSH and LH release, indicating pattern coordination despite sparse synchrony of pulses. We postulate that pattern synchrony of FSH and LH release is effected at the level of the gonadotrope. Concordant FSH and LH pulses probably result from pulsatile GnRH input, but other mechanisms could account for independent FSH pulses.