Blockade of LH and FSH secretion by LH-releasing hormone, by the LH-releasing hormone analogue, buserelin, and by combined treatment with LH-releasing hormone and oestradiol benzoate

The LH and FSH release-stimulating (experiment 1) and -blocking (experiment 2) effects of LH-releasing hormone (LHRH) and of the LHRH analogue D-Ser(But)6-des-Gly10-LHRH-ethylamide (buserelin), as well as the effect of combined treatment with LHRH and oestradiol benzoate (OB; experiment 3) on the 'supra-maximally' LHRH-stimulated release of LH and FSH were studied in rats ovariectomized for 2 weeks. Pretreatment with LHRH (250 or 500 ng/h) or buserelin (250 ng/h) for 6 days was effected by means of subcutaneously implanted Alzet osmotic minipumps; control rats received a 'sham pump', i.e. a piece of silicone elastomer with the dimensions of a minipump. Oestradiol benzoate (3 micrograms/injection) or solvent was injected subcutaneously 75 and 27 h before the induction of LH/FSH responses. Experiment 1 revealed that after infusion of LHRH and buserelin, both at the rate of 1 microgram/h, plasma LHRH concentrations were established which were about twice as low as the plasma buserelin concentrations. This might suggest that buserelin has a longer half-life than LHRH. As an LH and FSH release-stimulating substance, however, it appeared that buserelin was about as effective as LHRH. Experiment 2, however, suggested that as an LH/FSH release-blocking agent buserelin was much more effective than LHRH. In addition, after buserelin pretreatment the pituitary glands contained much less LH and FSH than after LHRH pretreatment at both dose levels used.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of delta-9-tetrahydrocannabinol on the hypothalamic-pituitary control of luteinizing hormone and follicle-stimulating hormone secretion in adult male rats

The main psychoactive component of marihuana, delta-9-tetrahydrocannabinol (THC) was injected into the 3rd cerebral ventricle. A single dose of THC (2 microliter of 10(-6) M) decreased serum LH temporarily but did not alter serum follicle-stimulating hormone (FSH) levels. The mediobasal hypothalamic (MBH) luteinizing hormone-releasing hormone (LHRH) content was elevated by 30 min after the injection. The elevation persisted for 1 h. Then, the LHRH content returned towards the preinjection level. In contrast, the LHRH in the organum vasculosum of the lamina terminalis did not change after a single dose of THC. The results indicate that THC alters pituitary LH release by inhibiting the release of LHRH which then increases in the MBH by continued synthesis or transport from rostral areas. In addition, the data support the existence of an FSH releasing factor, the release of which is not suppressed by this dose of THC. THC did not alter the release, storage or responsiveness to LHRH of cultured anterior pituitary cells, which further supports the view that its principal site of action is on the hypothalamus.     

Synthetic human seminal alpha-inhibin-92 selectively suppresses follicle-stimulating hormone release in vivo.

A 92-amino acid polypeptide, alpha-inhibin-92 (alpha-IB-92), has been isolated and characterized from human seminal plasma and found to be active in suppressing follicle-stimulating hormone (FSH) release in vitro. In the present in vivo study, intravenous injection of synthetic alpha-IB-92 (4 and 20 micrograms) significantly suppressed FSH release (P less than 0.001), whereas this peptide had no effect on luteinizing hormone (LH) release in 1-day orchidectomized male rats. In contrast, third ventricular injection of alpha-IB-92 (0.02, 0.4, 4, or 20 micrograms) had no effect on FSH and LH release in 1- or 2-day orchidectomized rats. These results indicate that alpha-IB-92 exerts a FSH suppressing activity by direct action on the pituitary gland.     

Estrogen inhibits luteinizing hormone (LH), but not follicle-stimulating hormone secretion in hypophysectomized pituitary-grafted rats receiving pulsatile LH-releasing hormone infusions

The differential feedback actions of estrogen (E2) on gonadotropin secretion were studied by means of an in vivo isolated pituitary paradigm. Adult female rats were hypophysectomized (hypox) and the next day received single anterior pituitary transplants (graft) under the kidney capsule. At the same time rats underwent bilateral ovariectomy. On the third day each animal was fitted with a catheter system which allowed for intermittent infusions of LHRH (250 ng/5 min.h) and chronic blood sampling. Rats received LHRH infusions for 7 days. On the sixth day of LHRH infusions blood samples were collected for 4 h 5, 15, 25, 35, 45 min after each hourly LHRH pulse. After 1 h of sampling, animals received sc injections of 2 micrograms estradiol benzoate (EB; n = 5) or oil vehicle (n = 5). Plasma LH, FSH, E2, and PRL levels in samples from all groups were determined by RIA. In hypox/graft rats LH release, but not FSH release, was pulsatile in response to the hourly LHRH infusions. Injection of EB in the hypox/graft rats significantly (P less than 0.05) suppressed LH release within 3 h by 57%, while FSH was unaffected. PRL levels were elevated by approximately 10-fold in the hypox/graft animals compared to those in pituitary-intact rats. These levels, however, were not changed as a function of steroid treatment and, therefore, could not account for the effects of EB on LH secretion. On the basis of these observations we conclude that 1) a major inhibitory effect of an acute injection of EB on LH secretion is exerted by a direct action on pituitary gonadotropes, and 2) E2 can differentially affect the release of LH and FSH by an intrapituitary mechanism. It is hoped that development of this model will allow for further investigation of the cellular mechanisms that mediate feedback actions of E2 on pituitary gonadotropes exposed to intermittent LHRH stimulation.     

Evidence that pulsatile follicle-stimulating hormone secretion is independent of endogenous luteinizing hormone-releasing hormone

Although LHRH can stimulate the release of both LH and FSH from the pituitary, there are a number of instances in which the secretion of LH and FSH are divergent. Previous studies from our laboratory have indicated that pulsatile LH and FSH secretion are independently regulated by gonadal factors. We have, therefore, reexamined the role of LHRH in regulating pulsatile gonadotropin secretion by evaluating the effect of passive LHRH immunoneutralization on LH and FSH secretion in castrate adult male rats. Injection of 500 microliters ovine anti-LHRH serum no. 772 (LHRH-AS) into 2-week-castrate rats caused an 85% suppression of mean plasma LH levels by 2 h, which lasted through 48 h. Mean plasma FSH, however, was reduced by only 19% after 2 h and by only 59% after 48 h. When cannulated 2-week-castrate rats were bled every 10 min, both LH and FSH were secreted in a pulsatile manner. Injection of 500 microliters LHRH-AS caused an immediate abolishment of LH pulses and a rapid reduction in mean plasma LH through 24 h. Pulsatile FSH secretion, as characterized by the parameters of pulse frequency and amplitude, was unaffected by LHRH-AS, although mean plasma FSH levels were significantly reduced. Collectively, the results suggest that pulsatile FSH secretion is regulated by a separate factor(s) distinct from LHRH, but that LHRH is required for the maintenance of elevated FSH levels.     

Hypothalamic-pituitary interactions during the periovulatory secretion of follicle-stimulating hormone in the rat

We investigated the importance of anterior afferents to the medial basal hypothalamus (MBH) on the increases in plasma FSH during the periovulatory period in the 4-day cyclic rat. We served the anterior connections to the MBH either at 1200 h on proestrus (before the time of onset of the normal spontaneous LH surge in plasma and the associated first phase of FSH release) or near the end of the LH surge and first phase of FSH release at 2000 h on proestrus (before the onset of the second or selective phase of FSH release). Analyses of FSH and LH in blood collected through indwelling atrial catheters or from the trunk after decapitation showed that anterior deafferentation of the MBH at 1200 h on proestrus blocked the proestrous LH surge, the elevations in plasma FSH during proestrus and estrus, and ovulation. In contrast, when brain surgery was delayed until 2000 h on proestrus, the second phase of FSH release and ovulation occurred. In rats with retrochiasmatic transections made at 1200 h, a constant rate iv infusion of LHRH from 1500-1800 h on proestrus restored the LH surge, both phases of increased plasma FSH, and ovulation. The results suggest that 1) the prevolutory LH surge and the first phase of FSH release are dependent on rostral afferents to the MBH which result in hypothalamic LHRH release and 2) the role of rostral afferents to the MBH in the second phase of FSH release is solely to result in hypothalamic LHRH release during proestrus.     

Interleukin 1 alpha inhibits prostaglandin E2 release to suppress pulsatile release of luteinizing hormone but not follicle-stimulating hormone.

Interleukin 1 alpha (IL-1 alpha), a powerful endogenous pyrogen released from monocytes and macrophages by bacterial endotoxin, stimulates corticotropin, prolactin, and somatotropin release and inhibits thyrotropin release by hypothalamic action. We injected recombinant human IL-1 alpha into the third cerebral ventricle, to study its effect on the pulsatile release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in conscious, freely moving, ovariectomized rats. Intraventricular injection of 0.25 pmol of IL-1 alpha caused an almost immediate reduction of plasma LH concentration; this decrease was statistically significant 20 min after injection and occurred through a highly significant reduction in the number of LH pulses, with no effect on pulse amplitude. In contrast, there was no change in pulse frequency but a small significant elevation in amplitude of FSH pulses. Intraventricular injection of the diluent had no effect on gonadotropin release. The results provide further evidence for separate hypothalamic control mechanisms for FSH and LH release. To determine the mechanism of the suppression of LH release, mediobasal hypothalamic fragments were incubated in vitro with IL-1 alpha (10 pM) and the release of LH-releasing hormone (LHRH) and prostaglandin E2 into the medium was measured by RIA in the presence or absence of norepinephrine (50 microM). IL-1 alpha reduced basal LHRH release and blocked LHRH release induced by norepinephrine. It had no effect on the basal release of prostaglandin E2; however, it completely inhibited the release of PGE2 evoked by norepinephrine. To evaluate the possibility that IL-1 alpha might also interfere with the epoxygenase pathway of arachidonic acid metabolism, epoxyeicosatrienoic acids were also measured. IL-1 alpha had no effect on the content of epoxyeicosatrienoic acids in the hypothalamic fragments as measured by gas chromatography and mass spectrometry. In conclusion, IL-1 alpha suppresses LH but not FSH release by an almost complete cessation of pulsatile release of LH in the castrated rat. The mechanism of this effect appears to be by inhibition of prostaglandin E2-mediated release of LHRH.     

Differential control of luteinizing hormone and follicle-stimulating hormone secretion by luteinizing hormone-releasing hormone pulse frequency in man

To test the hypothesis that the frequency of pulsatile LHRH stimulation can differentially control LH and FSH secretion in man, we administered low doses of LHRH in pulsatile fashion in several different regimens to men with idiopathic hypogonadotropic hypogonadism (IHH) and presumed endogenous LHRH deficiency. In study 1, four men with IHH received a constant amount of LHRH per day in three different frequencies. After an initial 7-day period of LHRH (5.0 micrograms every 2 h), the men received 2.5 micrograms every 1 h and 7.5 micrograms every 3 h, each for 4 days, in varying order. Frequent blood samples were obtained before LHRH administration and at the end of each regimen. Before LHRH administration, mean serum FSH and LH levels were low [28 +/- 3 (+/- SEM) and 6 +/- 2 ng/mL, respectively], and they increased into the normal adult male range during LHRH treatment. As the frequency of LHRH administration decreased from every 1 to 2 to 3 h, serum FSH levels progressively increased from 99 +/- 33 to 133 +/- 34 to 181 +/- 58 ng/mL (P less than 0.05). Serum LH levels (34 +/- 6, 33 +/- 6, and 34 +/- 5 ng/mL) were significantly higher than those before LHRH administration and did not differ significantly among the three regimens. Total serum testosterone (T), estradiol, and free T levels were increased by LHRH, but were not significantly different during the three regions of LHRH administration. In study 2, three men with IHH received the same amount of LHRH per dose, given in two different pulse frequencies; 2.5 micrograms LHRH were administered in frequencies of every 0.5 h and every 1.5 h, each for 4 days, in varying order. During the 0.5 h frequency, the mean serum FSH level was 42 +/- 13 ng/mL, and it rose to 80 +/- 19 ng/mL during the 1.5 h frequency (P less than 0.05). Corresponding mean serum LH levels were 25 +/- 5 and 27 +/- 4 ng/mL. Serum T and estradiol levels were not significantly different during the two LHRH regimens. We conclude that the frequency of LHRH stimulation can differentially control FSH and LH secretion by the human pituitary gland, and the pattern of hormonal stimulation may be a determinant of target organ response.     

Differential regulation of pulsatile luteinizing hormone (LH) and follicle-stimulating hormone secretion in ovariectomized rats disclosed by treatment with a LH-releasing hormone antagonist and phenobarbital

Although pulsatile LH release in ovariectomized (OVX) rats appears to be controlled by pulsatile discharges of LHRH, the neuroendocrine regulation of episodic FSH release remains to be explored. The main objective of the present study is to compare and contrast the effects of a potent LHRH antagonist (ALHRH) and a central nervous system depressant, phenobarbital (PhB), on pulsatile LH and FSH release in OVX rats. Three to 4 weeks after ovariectomy, blood samples were obtained at 10-min intervals for 3 h, after which LHRH was injected and sampling continued for an additional hour. In control OVX rats, periodic increases in plasma LH and FSH levels occurred approximately every 30 to 60 min, respectively. Treatment of OVX rats with PhB several hours earlier resulted in a suppression of mean plasma levels and pulse frequencies of both LH and FSH. Interestingly, PhB suppressed the pulse amplitude of LH, but not of FSH. Phenobarbital increased pituitary LH responses to LHRH, but did not alter the FSH responses. When ALHRH was given to OVX rats 24 h before blood sampling, mean plasma LH levels as well as LH pulse frequency and amplitude were severely diminished. In striking contrast, ALHRH did not affect the frequency or amplitude of FSH pulses. However, mean plasma FSH levels were suppressed to 31% of levels measured in control OVX rats. These results demonstrate that in contrast to LH secretion, FSH secretion in OVX rats appears to be regulated by two distinct neuroendocrine mechanisms: an LHRH-dependent mechanism controlling the nonepisodic component of FSH secretion (baseline secretion) and a LHRH-independent mechanism controlling pulsatile FSH release.     

Interactions between 17 beta-estradiol and progesterone in the control of luteinizing hormone and follicle-stimulating hormone release in rat anterior pituitary cells in culture

After preincubation of rat anterior pituitary cells in culture for 48 h with 10(-9) M 17 beta-estradiol (E2), basal LH release is increased 1- to 2-fold, while the concentration of LHRH required to induce a half-maximal stimulation (ED50) of LH release is approximately 50% reduced. The presence of 10(-7) M progesterone (P) alone for 48 h has no effect on LH release, but it inhibits the sensitizing action of E2 on the LH response to LHRH. This inhibitory effect of P on the E2-induced increase of LH responsiveness to LHRH is similar to the effect of androgens. While not affecting the LHRH ED50 for FSH release, P stimulates both basal FSH release and the maximal FSH response to LHRH. This effect of P is potentiated by simultaneous incubation in the presence of E2. The stimulatory effect of E2 alone on LH release is exerted at an ED50 value of 1-2 x 10(-11) M. A similar ED50 value is found for the potentiating effect of E2 on the P-induced stimulation of FSH release. The stimulatory effect of E2 on LH responsiveness to LHRH appears to be due to changes in the sensitivity of the release mechanisms in gonadotrophs, since the total hormone content (release plus cell content) is not affected by E2 under the same experimental conditions. The stimulatory effect of P on FSH release is measured at an ED50 value of 1 x 10(-8) M and is maximal after approximately 8 h of incubation with the steroid. While not affecting total LH, P increases total FSH (release plus cell content), indicating that the stimulatory effect of P on FSH release could be secondary to changes in the hormone content of cells.     

Pulsatile release of follicle-stimulating hormone in ovariectomized rats is inhibited by porcine follicular fluid (inhibin)

While the pulsatile release of LH in ovariectomized animals has been well established, little information exists concerning the pulsatile release of FSH and the factors involved in its regulation. The present studies analyzed the characteristics of episodic FSH secretion and examined the effects of ovarian feedback signals on that pattern. From conscious unrestrained ovariectomized rats, blood was collected at 10-min intervals for a period of 3 h. A pulsatile pattern of FSH was observed in the plasma of control rats, with an average frequency of 4 peaks/3, h, while LH oscillations occurred at an average of almost 6 peaks per 3 h. The injection of porcine follicular fluid (PFF; 0.5 ml), a source of inhibin, reduced FSH peak frequency to 2.5 peaks/3 h (P less than 0.005). In the third hour of blood sampling, PFF also significantly lowered the average FSH peak amplitude (P less than 0.05), the trough value, and the average mean value for that hour. In contrast, none of these parameters for LH was altered by PFF. Estradiol benzoate (EB; 20 micrograms, sc, 24 h before bleeding) significantly reduced LH peak frequency, mean trough value, and average mean value, whereas it only reduced the mean FSH average value and mean trough value. Combined treatment with EB and PFF synergized to decrease the mean value, trough value, peak frequency, and peak amplitude of episodic LH release. Pituitary responsiveness to LHRH was evaluated after the treatments described above. PFF inhibited the response of FSH to LHRH, but not that of LH. EB by itself potentiated the release of both hormones. The combined EB/PFF treatment resulted in a decreased pituitary response in terms of FSH and LH compared to that of the EB-treated control group. We conclude that the release of FSH occurs in a pulsatile fashion and, further, that this pattern of release is regulated by PFF (inhibin). It was also shown that an intriguing synergism between estradiol and inhibin exists to suppress LH release.     

Luteinizing hormone (LH)-releasing hormone: chronic effects on LH and follicle-stimulating hormone cells and secretion in adult male rats

We investigated whether chronic administration of LHRH to normal adult rats could increase the percentages of anterior pituitary gland (APG) cells that contain immunoreactive LH and/or FSH and gonadotropin secretion. Vehicle or 1 microgram LHRH was injected sc twice daily for 6 days, and rats were decapitated 16 h after the last injection. Treatment with LHRH caused nearly a doubling in the numerical density of LH and FSH cells and in the percentage of APG cells that contained LH or FSH. It also caused a shift in the gonadotroph population from LH and LH/FSH cells to LH/FSH cells. It did not change the mean size of gonadotrophs or APG weight. These changes at the light microscopic level were not accompanied by any apparent changes in LH cells at the ultrastructural level. However, they were accompanied by an approximate doubling of the basal serum LH and FSH concentrations, an increase in the APG FSH concentration, and an increase in the basal FSH release rate (measured in vitro). The results indicate that exogenous LHRH can be administered to increase numbers of gonadotrophs in the APG, synthesis of FSH in gonadotrophs, and basal serum LH and FSH concentrations.     

Pulsatile follicle-stimulating hormone secretion is independent of luteinizing hormone-releasing hormone (LHRH): pulsatile replacement of LHRH bioactivity in LHRH-immunoneutralized rats

We recently reported that passive immunoneutralization of endogenous LHRH in castrate male rats completely abolishes pulsatile LH secretion and, within 1 h, lowers mean plasma LH by 86%. While pulsatile FSH secretion, in terms of pulse amplitude and frequency, is not affected, mean plasma FSH is gradually lowered but only by 49% after 24 h. In the present study, we have examined the effect of replacing pulsatile LHRH biological activity on LH and FSH secretion in 4-week castrate male rats in which endogenous LHRH has been immunoneutralized by ovine anti-LHRH serum 772 (LHRH-AS) for 24 h. The LHRH-AS requires the 3-10 amino acid sequence of LHRH including the amidated C terminus for complete recognition. In order to circumvent the antiserum blockade, we utilized the LHRH agonist [Des Gly10]-LHRH ethyl amide (DG-LHRH) which is minimally recognized by the LHRH-AS but which possesses 2.6-fold the LH-releasing activity of LHRH. Twenty-four hours after injecting 500 microliter LHRH-AS into cannulated, castrate rats, sequential blood samples were taken every 10 min for 4 h. Bolus 3-ng injections of either DG-LHRH or saline were given iv either every 30 min during the 4-h collection period or every 30 or 60 min for 10 h before the initiation of and continuing through the 4-h collection period. Each DG-LHRH injection stimulated the release of a single pulse of LH, while pulsatile FSH secretion was unaffected. No synchrony was observed between the DG-LHRH pulses and the endogenous FSH pulses. Short term DG-LHRH treatment partially restored, and long term DG-LHRH treatment every 60 min completely restored, mean plasma FSH to the level observed in nonantiserum-treated castrate control rats. Long term DG-LHRH treatment every 30 min caused a rise in mean plasma FSH which exceeded the plasma FSH level of the nonantiserum-treated controls. The mean plasma level of LH was entirely dependent on the frequency of the DG-LHRH injection. The results of this study clearly demonstrate that pulsatile FSH secretion is independent of LHRH but that LHRH is required to elevate and/or maintain high mean plasma FSH levels. Trough levels of LH, however, are dependent on the frequency of LHRH-induced pulsatile LH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)     

The self-priming action of LHRH is under negative FSH control through a factor released by the ovary: observations in female rats in vivo

Female rats were treated with Metrodin (highly purified urinary FSH from menopausal women) or saline during the oestrous cycle. On the day of pro-oestrus they were anaesthesized with phenobarbital and received four repetitive LHRH injections 1 h apart. This treatment with FSH suppressed the unprimed LH response to the first LHRH injection. During the subsequent injections the maximal LHRH self-priming was delayed by 3 h till the fourth LHRH stimulation. At this time, LH release in response to LHRH was equally as high as shown in the saline controls after the second LHRH injection. Ovariectomized rats did not show the self-priming effect and FSH treatment was ineffective in suppressing LHRH-induced LH release. Administration of FSH followed by an additional 4- or 24-h period before LHRH stimulation were equally effective in suppressing the unprimed LH release and delaying (up to 3 h) the maximal priming of LH release by LHRH. Even 4-20-fold increased amounts of LHRH did not affect the suppressed unprimed release of LH after FSH treatment. Treatment with FSH did not change oestradiol and progesterone levels. It was concluded that FSH treatment suppresses the unprimed LHRH-induced LH release and delays maximal LHRH self-priming by enhancing the release of an ovarian factor.     

Effects of endogenous opioid peptides and opiates on luteinizing hormone and prolactin secretion in ovariectomized rats

Adult female rats were implanted with permanent cannulae in the third ventricle of the brain and ovariectomized. 3 weeks later, blood samples were withdrawn every 5 min from intraatrial cannulae placed the previous day. After a control sampling period of 30 min, the rats received an intraventricular bolus injection of saline (2 microliter) or beta-endorphin (beta E; 10 micrograms); sampling was continued for an additional 2 h. Saline injection caused no effect on luteinizing hormone (LH) and prolactin (PRL) secretion. beta E stimulated PRL secretion within 5-10 min, the values peaked in the next 10 min. Thereafter, as PRL levels fell, a suppression of LH secretion became apparent. Inhibition of LH release started 20-35 min after beta E injection and lasted for 35-65 min. The antecendent PRL secretion was apparently not responsible for the observed delayed LH response, since blockade of PRL response with bromocriptine failed to affect the beta E-induced LH suppression. Further, continuous intraventricular infusion of beta E (5 or 10 micrograms/h) for 3 h markedly suppressed the amplitude and frequency of LH episodes in long-term ovariectomized rats. Bolus intraventricular injection of other endogenous opioid peptides and opiate receptor agonists produced different PRL and LH responses. Dynorphin (10 micrograms) similarly suppressed LH release but was only moderately effective in stimulating PRL. Leucine enkephalin (50 micrograms) stimulated LH and inhibited PRL release, while methionine-enkephalin (50 micrograms) selectively stimulated PRL release. The methionine-enkephalin analogs, FK-33824 (50 ng) and DALAMID (50 micrograms), evoked sequential PRL and LH responses similar to those seen after beta E injection. Interestingly, morphiceptin (a specific mu receptor agonist; 10 micrograms) markedly suppressed LH release, but only sparingly stimulated PRL release. Delta receptor peptide (a specific delta receptor agonist; 10 micrograms) selectively suppressed LH release. Bremazocine (a specific kappa receptor agonist; 0.5 mg/kg) administered intravenously suppressed LH release selectively. These studies show that of the four endogenous opioid peptides tested beta E was most effective in evoking sequential PRL and LH responses, and these effects may be mediated by either epsilon receptors or multiple opiate receptor subtypes; stimulation of kappa receptors by bynorphin or bremazocine suppressed LH release, and further studies would be needed to understand the mode of action of the two enkephalins and the delta opiate receptors in eliciting disparate PRL and LH responses.     

Is the postovulatory release of follicle-stimulating hormone in the rabbit mediated by luteinizing hormone-releasing hormone?

Studies were performed to determine whether the postovulatory secretion of FSH in the rabbit is an LHRH-mediated event. Does were mated and then injected at 12 and 18 h postcoitum with pentobarbital (30 mg/kg BW), an agent known to block endogenous LHRH release. The injection of this barbiturate had no measurable effect on the postovulatory FSH secretion pattern. Administration of the LHRH antagonist [Ac-D-p-C1-Phe1,2, Phe3, D-Arg6, D-Ala10]LHRH (0.5 mg/doe) prevented all gonadotropin release in response to LHRH injection (10 micrograms/kg BW). When this same dose of the antagonist was injected at 18 h postcoitum, the postovulatory FSH secretion pattern was unaffected. Finally, to prove that the pituitary was sensitive to LHRH at 18-h postcoitum, LHRH (10 micrograms/kg BW) was injected into rabbits mated 18 h earlier; this treatment led to a marked increase in FSH secretion showing that the pituitary is responsive to LHRH at this time. The results of this study show that two drugs which block LHRH-mediated gonadotropin release have no effect on the postovulatory secretion of FSH and support the concept that this episode of FSH secretion occurs via a pathway which does not include the hypothalamic secretion of LHRH.     

Effect of destruction of the dorsal anterior hypothalamus on follicle-stimulating hormone secretion in the rat

The role of the paraventricular nucleus-dorsal anterior hypothalamus (PVN-DAHA) in the control of anterior pituitary gland secretion of FSH and LH in castrated male and female rats was examined. Bilateral radiofrequency lesions of the PVN-DAHA in chronically ovariectomized (OVX) rats lowered plasma FSH levels by 33% (P less than 0.005) compared to values in unoperated and sham-operated control rats; plasma LH concentrations were unaltered. RIA of median eminence (ME) LHRH concentrations in these animals revealed no differences among the three experimental groups. Other categories of diencephalic destruction did not result in this pattern of selectively reduced FSH release. Bilateral radiofrequency destruction of the PVN-DAHA also attenuated by 50% (P less than 0.025 to P less than 0.005) the progesterone-induced surge of FSH in estrogen-primed OVX rats. Progesterone-induced LH release was unaffected by PVN-DAHA lesions. Other lesion categories failed to show the same result. Bilateral ablation of the PVN-DAHA in male rats resulted in a selective diminution of the postcastration rise of plasma FSH beginning 48 h postcastration (P less than 0.05 to P less than 0.005) and persisting for 14 days (P less than 0.005) after orchidectomy, thus revealing the time course and permanence of this procedure on plasma FSH levels. The postcastration rise of plasma LH levels was not affected by PVN-DAHA lesions. The concentration of ME LHRH was the same among orchidectomized male rats whether they bore PVN-DAHA lesions, sham lesions, or no lesions. In summary, destruction of the PVN-DAHA was found to reduce significantly the elevation of plasma FSH, but not LH, in the OVX rat and the estrogen-progesterone-stimulated OVX rat. PVN-DAHA lesions also attenuated the postcastration rise of FSH, but not that of LH, in the male. The failure of lesions of the PVN-DAHA to alter ME LHRH concentrations in the face of decreased FSH release does not prove that LHRH release is totally unaffected by this procedure. This finding is, however, consistent with the concept that diminished FSH secretion could be the result of a deficiency of a hypothalamic releasing factor (FSH-releasing factor?) other than that of the LHRH decapeptide.     

Different neuroendocrine mechanisms regulate the acute pituitary follicle-stimulating hormone response to orchidectomy and ovariectomy

The following experiments were conducted to determine whether a sex difference exists in neuroendocrine mechanisms controlling acute pituitary follicle-stimulating hormone (FSH) responses to castration. Adult male rats and 4-day cycling female rats on diestrus 1 were injected intraperitoneally with either phenobarbital sodium (PhB, 80 mg/kg b.w.) or vehicle at 08:00 h. Following a blood collection at 10:00 h, rats given PhB or vehicle were either sham castrated or castrated under ether. Additional blood samples were obtained, and supplemental PhB or vehicle injections were given at 3, 8, 13, 18, and 24 h after castration. Administration of PhB to male rats completely prevented acute increases in plasma luteinizing hormone (LH) and FSH levels after orchidectomy (ORDX). In contrast, PhB treatment did not prevent initial rises in plasma FSH levels at 8 h after ovariectomy (OVX) and only partially suppressed OVX-induced increases in plasma FSH levels between 13 and 24 h. Plasma LH levels were not elevated by 24 h after OVX. In order to specifically evaluate the role of LH-releasing hormone (LHRH) in mediating the PhB-sensitive rises in gonadotropins after castration, groups of male rats and female rats on estrus were injected subcutaneously with 400 micrograms of a potent LH-RH antagonist (ALHRH) or oil at 12:00 h. At 10:00 h on the next morning, an initial blood sample was taken, and all rats were castrated under ether. Additional blood samples were taken at times indicated in the previous experiment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of recombinant inhibin on luteinizing hormone and follicle-stimulating hormone secretion in the rat

We investigated the effect of the iv injection of recombinant human (rh) inhibin on FSH and LH secretion in the female rat under various experimental circumstances. Rh inhibin caused dose-related decreases in mean plasma FSH, but not LH, levels in ovariectomized female rats 14 days old and older. The duration of this inhibition was proportional to the dose of rh inhibin, but no consistent changes in FSH secretion were observed until 4 h after treatment. Maximum suppression of FSH release was observed at about 15 micrograms rh inhibin/kg BW and lasted 8-10 h. Measurement of the area under the curve from 4-12 h after injection of inhibin indicated a dose-related decrease in total FSH secreted. When blood samples were withdrawn every 10 min to evaluate pulsatile gonadotropin release, analysis of FSH pulse parameters indicated that rh inhibin (25 micrograms/kg) interfered with pulse frequency, amplitude, and peak levels in both intact and ovariectomized rats. In contrast, pulsatile LH secretion was not measurably altered. These results demonstrate that rh inhibin acts primarily at the level of the pituitary to inhibit all parameters of FSH secretion and suggest that this effect is at least not entirely mediated by changes in GnRH receptors.     

Dose-response study with a new LH-RH analogue, D-Ser (TBU)6 LH-RH 1-9 (EA)10 during the follicular phase of the menstrual cycle.

D-Ser (TBU)6 LH-RH 1-9 (EA)10 (HOE 766) a highly active LH-RH analogue, was studied with regard to its effects on the release of follicle stimulating hormone (FSH), luteinizing hormone (LH) and oestradiol-17beta (Oe2) during the follicular phase of the menstrual cycle. Forty-two regularly menstruating women were allowed to five different treatment groups with different doses (1.25 microgram; 2.5 microgram; 5.0 microgram; 10.0 microgram; 20.0 microgram) of HOE 766 given as intravenous bolus injections and the plasma concentrations of FSH, LH and Oe2 were measured up to 24 h after injection using specific radioimmunoassays. In the majority of cases, peak values of both FSH and LH occurred 4 h after injection being significantly different from pre-injection levels (P less than 0.02 in the 1.25 microgram treatment group, P less than 0.005 for the other treatment groups). Statistical analysis of maximum values as well as the absolute and relative increase in the different treatment groups revealed a dose-dependent effect of HOE 766. Maximum values of Oe2 occurred 8 h after injection and were found to be significantly different from pre-injection levels (P less than 0.005). However, no dose dependent effect was observed. It was concluded that HOE 766 is a potent and long-acting stimulator of FSH, LH and OE2 release in women. The effect of HOE 766 is dose dependent for FSH and LH but not for Oe2.