Effect of endogenous LH secretion on ovarian cyclic AMP and cyclic GMP levels in the rat

Injection of LH (2 and 10 μg) into proestrus rats increased ovarian cyclic AMP levels and concomitantly decreased the levels of cyclic GMP. When injected into diestrus rats, cyclic AMP increases were even greater, whereas cyclic GMP levels were not significantly different from controls receiving saline injections. Ovarian cyclic nucleotide levels were also examined on different days of the cycle. On the afternoon of proestrus (1700 h), the time when circulating levels of LH are at their maximum, the concentration of cyclic AMP showed a moderate but insignificant increase. At the same time, cyclic GMP levels were significantly decreased. An inverse relation between cyclic AMP and cyclic GMP levels was seen on each day of the cycle. When rats were injected with pentobarbital (35 mg/kg) on the afternoon of proestrus (1300 h) to block the LH surge, the expected increases in ovarian cyclic AMP and decreases in cyclic GMP were effectively blocked. These results indicate that ovarian cyclic AMP and cyclic GMP levels are regulated by circulating LH. The apparent differences in direction of nucleotide response to LH, suggest divergent roles for the nucleotides in ovarian function.     

Pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) responses to gonadotropin-releasing hormone during the rat estrous cycle: an increased ratio of FSH to LH is secreted during the secondary FSH surge

The hormonal interactions required for the generation of a secondary surge of FSH on the evening of proestrus have not been clearly defined. The role of GnRH in driving a surge of FSH has been questioned by findings in previous studies. In the current study, gonadotropin secretion was measured from pituitary fragments obtained from rats at 0900 and 2400 h on each day of the estrous cycle. Pituitary fragments were perifused in basal (unstimulated) conditions or in the presence of GnRH pulses to determine whether a selective increase in basal release of FSH and/or an increase in the responsiveness to GnRH occurs during the secondary FSH surge. Each anterior pituitary was cut into eighths and placed into a microchamber for perifusion. Seven pulses of GnRH (peak amplitude = 50 ng/ml; duration = approximately 2 min) were administered at a rate of one per hour starting at 30 min. Fractions of perfusate were collected every 5 min and frozen until RIA for LH and FSH. The mean total amount of LH or FSH secreted during the hour interval following each of the last six pulses of GnRH (or the corresponding basal hour) was calculated. Analysis of variance with repeated measures indicated that the evening secretion of LH on proestrus (2400 h) dropped significantly (p less than 0.05) from a maximum on the morning of proestrus (0900 h), whereas the FSH secretion remained elevated at this time. Therefore, the ratio of FSH to LH secreted in response to GnRH pulses was highest during the secondary FSH surge and lowest on the morning of proestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of intraventricular injection of delta receptor antagonist ICI 154, 129 on the secretion of luteinizing hormone and prolactin in the proestrous rat.

In order to ascertain the role of delta receptors in the control of gonadotropin secretion, a preferential delta receptor antagonist ICI 143,129 was microinjected into the third ventricle through chronically implanted cannulae and the effects on the serum concentration of luteinizing hormone (LH) and prolactin (PRL) were determined in female rats in proestrus. When the injection was given at 1030 h, ICI 154,129 (50 micrograms) exerted no significant effects on either LH or PRL. However, in the rat given a microinjection of ICI 154,129 at 1300 h, an afternoon rise in LH occurred in advance and was of greater magnitude, with the peak time more than 1 h earlier and the peak amplitude approximately 100% greater than that in the control rat, respectively. The injection also suppressed the PRL rise during the plateau phase. The results indicate that delta receptors are involved in the mediation of the inhibitory influence of endogenous opioids on the surge of LH in proestrus, and that delta receptors mediate the facilitatory influence of opioids on the PRL surge during the plateau phase.     

Effect of gonadotropin-releasing hormone antagonists on serum follicle-stimulating hormone and luteinizing hormone under conditions of singular follicle-stimulating hormone secretion

Previous work has indicated that in long-term ovariectomized rats a potent antagonist to gonadotropin-releasing hormone (GnRH) suppressed serum luteinizing hormone (LH) more successfully than follicle-stimulating hormone (FSH). The present studies examined whether the rise in serum FSH which occurs acutely after ovariectomy, or during the proestrous secondary surge, depends on GnRH. In Experiment A, rats were ovariectomized at 0800 h of metestrus and injected with (Ac-dehydro-Pro1, pCl-D-Phe2, D-Trp3,6, NaMeLeu7)-GnRH (Antag-I) at 1200 h of the same day, or 2 or 5 days later. Antag-I blocked the LH response completely, but only partially suppressed serum FSH levels. Experiment B tested a higher dose of a more potent antagonist [( Ac-3-Pro1, pF-D-Phe2, D-Trp3,6]-GnRH; Antag-II) injected at the time of ovariectomy. The analog suppressed serum LH by 79% and FSH by 30%. Experiment C examined the effect of Antag-II on the day of proestrus on the spontaneous secondary surge of FSH, as well as on a secondary FSH surge which can be induced by exogenous LH. Antag-II, given at 1200 h proestrus, blocked ovulation and the LH surge expected at 1830 h, as well as increases in serum FSH which occur at 1830 h and at 0400 h. Exogenous LH triggered a rise in FSH in rats suppressed by Antag-II. In Experiment D proestrous rats were injected with Antag-II at 1200 h and ovariectomized at 1530 h. By 0400 h the antag had suppressed FSH in controls, but in the ovariectomized rats, a vigorous FSH response occurred.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism of the stimulatory action of antisteroid RU486 on follicle-stimulating hormone secretion in the cyclic Rat

These experiments explored the mechanism underlying FSH hypersecretion on estrous afternoon in rats injected with RU486 (RU) on proestrus. Four-day cyclic rats were injected with RU at 12:00 h on proestrus (1 or 4 mg/0.2 ml oil; s.c.), and its effects on LH and FSH secretion at 18:30 h on estrus were compared with those of antiprogestagens ZK299 (ZK) (1 or 4 mg/0.2 ml oil; s.c.) and Org31806 (OR) (2 or 8 mg/0.2 ml oil; s.c.). Additionally, rats treated with RU or nembutal (PB) (60 mg/kg; i.p. at 13:00 h on proestrus) were injected with an LHRH antagonist (LHRHa) at 10:00 h on estrus (1 mg/0.2 ml saline; s.c.) or progesterone (P) (7.7, 15.5 or 30.9 mg/0.2 ml oil; s.c.) on proestrus at 10:00 h in RU-injected rats and at 14:00 h in PB-injected rats. Animals were killed by decapitation at 18:30 h on estrus and serum LH and FSH concentrations were determined. Rats treated with 1 or 4 mg of RU or Org or 4 mg of ZK recorded increased serum FSH on estrous afternoon, while 1 mg ZK had no effect. PB increased mainly serum LH levels and, to a lesser extent, FSH levels. P decreased serum FSH concentrations in both RU- and PB-injected rats. LHRHa reversed the effects of PB on FSH secretions, but reduced FSH hypersecretion induced by RU only. These results are interpreted to mean that, in the absence of proestrous afternoon P-inhibitory action of the neural stimulus controlling LHRH release, FSH secretion on estrous afternoon involves two components: one is LHRH dependent while, in contrast to LH secretion, the other is LHRH independent, and only expressed in a low estrogen background.     

Effects of the antiandrogen hydroxyflutamide on progesterone secretion by preovulatory rat follicles in vivo and in vitro.

Serum and ovarian progesterone levels and in vitro production of progesterone by preovulatory follicles were measured on proestrus in pregnant mare's serum gonadotropin (PMSG) primed immature rats in which the luteinizing hormone (LH) surge and ovulation were blocked by administration of the antiandrogen hydroxyflutamide. Serum progesterone levels observed at 12:00 on proestrus were significantly elevated, twofold above those observed in vehicle-treated controls, by in vivo administration of 5 mg hydroxyflutamide 4 h earlier. In control rats, proestrous progesterone did not increase until 16:00, in parallel with rising LH levels of the LH surge. No LH surge occurred in the hydroxyflutamide-treated rats, ovulation was blocked, and serum progesterone declined throughout the afternoon of proestrus, from the elevated levels present at 12:00. Administration of human chorionic gonadotropin (hCG) at 11:00 advanced the elevation of serum progesterone by 2 h in vehicle-treated controls and prevented the decline in progesterone levels in hydroxyflutamide-treated rats. The patterns of change in ovarian tissue concentrations with time and treatment were essentially similar to those observed for serum progesterone. In in vitro experiments, progesterone secretion during 24 h culture of preovulatory follicles obtained on PMSG-induced proestrus was significantly increased, sixfold, by addition to the culture media of 370 microM but not of 37 microM hydroxyflutamide. Testosterone (50 nM) and hCG (20 mIU/mL) caused 26- and 14-fold increases, respectively, in progesterone secretion by cultured follicles. Hydroxyflutamide significantly reduced the stimulatory effect of testosterone but not of hCG on progesterone secretion in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)     

Preovulatory secretion of progesterone, luteinizing hormone, and prolactin in 4-day and 5-day cycling rats

Timing of ovulation and changes in plasma progesterone, luteinizing hormone (LH), and prolactin (PRL) during periovulatory stages were determined in Holtzman rats exhibiting regular 4- or 5-day cycles under a daily artificial illumination from 0500 to 1900 h. The 5-day cycling rats ovulated between 0130 and 0930 h on estrus, whereas some of the 4-day cycling animals ovulated as early as about 0130 h and others as late as 1130 h on estrus. Onset time of preovulatory LH and progesterone surges was about 1500 h on proestrus in both the 4- and the 5-day cycling rats. Peak levels of plasma LH and progesterone were measured at 1700 to 1900 h on proestrus, while the first rises and peak values of plasma PRL were evident a few hours earlier than those of plasma LH in the rats with two cycle lengths. Plasma LH levels at 1900 h on proestrus as well as plasma progesterone levels at 1600 and 2300 h on proestrus and at 0130 and 0330 h on estrus were significantly lower in the 5-day cycling rats than in the 4-day cycling animals (p less than 0.05). In contrast, PRL levels from 1500 through 2300 h on proestrus remained consistently higher in 5-day cycling rats than in 4-day cycling rats, and significant differences in PRL levels between these rats were apparent at 1500, 1600, and 2100 h (p less than 0.05-0.01). Thus, these results demonstrate that the 5-day cycling rats exhibit the attenuated magnitude of LH surge accompanied by the augmented preovulatory PRL release, and that plasma progesterone levels reflect the magnitude of LH surge. A tentative working hypothesis concerning the etiology of the 5-day cycle has been proposed.     

A secondary surge of prolactin on the estrus afternoon

It has been described that throughout the estrous cycle of the rat, plasma prolactin (PRL) is basal except on proestrus afternoon when a preovulatory surge occurs. However, there have been controversies about PRL levels on the estrus day. Thus, the aim of this study was to evaluate the existence of a secondary surge of PRL on estrus afternoon and correlate it with plasma estradiol levels. The jugular vein of cycling rats was cannulated at 14:00 h on proestrus and a blood sample was withdrawn at 17:00 h for plasma LH measurement and determination of the preovulatory LH surge occurrence. In order to exclude the regular cycling rats that do not present the gonadotropins preovulatory surge and do not ovulate, only rats showing the LH surge on proestrus were considered in this study. Blood samples were collected hourly during estrus from midnight to 9:00 h (group 1) and from 10:00 to 18:00 h (group 2). In group 1, PRL showed a descending profile from midnight to 9:00 h, whereas the estradiol concentrations were constant. In group 2, a secondary surge of PRL was observed in 20 of 25 (80%) rats and plasma estradiol remained constant, but was higher in animals with the PRL surge. Thus the present data suggest the occurrence of a secondary surge of PRL in the afternoon of estrus that seems to be related to plasma estradiol levels of estrus day, which might exert only a permissive role in this surge generation.     

Suppression of the proestrus prolactin surge in the rat estrous cycle by urethane anesthesia

Urethane-anesthetized male rats have been used for the analysis of prolactin (PRL)-releasing substances on PRL secretion. However, there are only a few reports investigating the effect of urethane anesthesia on PRL secretion in female rats. In this study, we intended to examine the effects of urethane anesthesia on PRL secretion during proestrus in the rat. Proestrus PRL surge was completely blocked when urethane was administered to rats prior to the critical period of proestrus both at doses of 1.0 g/kg and 1.5 g/kg. Additionally, urethane, at a dose of 1.5 g/kg, was also effective in blocking spontaneous ovulation. An experiment examining pituitary PRL concentration at 1800 h confirmed that urethane (1.0 g/kg) anesthesia prevents the PRL surge from the pituitary. Similarly, urethane anesthesia blocked the LH surge from the pituitary, but LH levels in the urethane-treated group were higher than those in the pentobarbital-treated group.     

Rapid recovery of estrogen-induced pituitary gonadotropin surges after luteectomy in rhesus monkeys

In the presence of a functional corpus luteum, positive estrogen feedback on the surge modes of gonadotropin secretion is blocked in rhesus monkeys. We investigated the effects of luteectomy (Lx) on the time required for recovery of pituitary responsiveness (LH/FSH surges) to positive estrogen feedback. Estradiol-17 beta-3- benzoate (EB, 50 microgram/kg sc) was given: 1) 24th prior to, 2) the day of, or 3) 24 h after luteal ablation. Daily measurements of serum follicle stimulating hormone (FSH), luteinizing hormone (LH), estradiol-17 beta (e2) and progesterone (P) were made on each monkey for 5 days. Serum P fell to undetectable levels within 24 h after Lx, whereas E2 levels in circulation peaked within 24h after injection of EB. Among early follicular phase monkeys, this EB treatment results in typical midcycle type LH/FSH surges within 48h. Lx alone was not soon followed by significant changes in pituitary gonadotropin secretion. When circulating P levels were undetectable the pituitary responded fully to EB; that is, typical midcycle type FSH/LH surges occurred. When serum P was in the midst of declining after Lx, gonadotropin surges were present, but attenuated. However, when P levels remained elevated for more than 24 h after EB injection, the surge modes of FSH/LH secretion remained fully blocked. These results demonstrate that the suppressive influence of luteal secretions (principally progesterone) on positive estrogen feedback regulation of the surge modes of pituitary gonadotropin secretion is quite transient in these primates.     

Daily variations in the sensitivity of proestrous LH surge in the inhibitory effect of intraventricular injection of 5-HT or GABA in rats.

Intraventricular injection of 5-hydroxytryptamine (5-HT) into female rats at 11:00 h on the day of proestrus inhibited the preovulatory surge of luteinizing hormone (LH) and ovulation. A similar response was observed after the activation of the serotonergic system by stimulation of the median raphe nucleus. A diurnal rhythm of these responses was observed. In rats acclimated to a 14-h:10-h light:dark cycle the potency of 5-HT to inhibit the LH surge and ovulation was 2.06 and 2.3 times greater, respectively, when injected at 11:00 h than at 13:00 h. Also stimulation of the median raphe nucleus at 11:00 h was significantly more effective in inhibiting these parameters than stimulation at 13:00 h. Similarly, the ability of gamma-amino-butyric acid (GABA) to inhibit the preovulatory LH surge and ovulation was greater in rats injected in the morning than in the afternoon. The results of this study indicate that during proestrus the sensitivity of 5-HT and GABA to induce inhibition of preovulatory LH release and ovulation shows daily variations with maximal effect before the critical period.     

Brain serotonin turnover and plasma prolactin and growth hormone concentrations during changes in osmotic balance in the domestic fowl

Summary Young cockerels injected 24 h earlier with 0.9% saline,para-chorophenylalanine (pCPA, brain serotonin depletor) or alpha-methylpara-tyrosine (AMPT, brain catecholamine depletor) were deprived of access to water for 24 h. Plasma prolactin concentrations were markedly elevated by water deprivation and returned to normal on rehydration. pCPA, but not AMPT, significantly reduced the increase in prolactin. Concentrations of growth hormone were not affected by water deprivation. Brain serotonin concentrations were reduced by treatment with pCPA. Groups of cockerels were maintained under normal conditions or without access to drinking water for 12 h or 24h. Some were injected with the monoamine oxidase inhibitor pargyline, which increased the prolactin and decreased the growth hormone concentration in the plasma of the hydrated birds. The inhibitory effect of pargyline on growth hormone was augmented following water deprivation. Serotonin levels were not significantly affected by water deprivation but turnover (defined as accumulation of serotonin after pargyline treatment) was increased in the hypothalamus but not in remaining tissue. Injecting 30% saline solution intravenously markedly increased plasma prolactin whilst growth hormone concentrations were decreased. Serotonin turnover was increased in the hypothalamus but not in other brain regions. The results show that secretion of prolactin and growth hormone by the pituitary gland during osmotic imbalance in the fowl may be mediated by changes in hypothalamic scrotonin turnover.     

Hypersecretion of follicle-stimulating hormone (FSH) on estrous afternoon in rats treated with RU486 in proestrus

Summary 1. Intact or ovariectomized (OVX) cyclic rats injected or not with RU486 (4 mg/0.2 ml oil) from proestrus onwards were bled at 0800 and 1800h on proestrus, estrus and metestrus. Additional RU486-treated rats were injected with: LHRH antagonist (LHRHa), estradiol benzoate (EB) or bovine follicular fluid (bFF) and sacrified at 1800 h in estrous afternoon. LH and FSH serum levels were determined by RIA.2. RU486-treated intact or OVX rats had decreased preovulatory surges of LH and FSH, abolished secondary secretion of FSH and hypersecretion of FSH in estrous afternoon. The latter was decreased by LHRHa and abolished by EB or bFF. In contrast, EB induced an hypersecretion of LH in RU486-treated rats at 1800h in estrus.3. It can be concluded that in the absence of the proestrous progesterone actions, the absence of the inhibitory effect of the ovary in estrus evoked a LHRH independent secretion of FSH.     

Effect of cycloheximide [correction of cyclohexamide] injected at proestrus on ovarian protein synthesis, peptide and steroid hormone levels, and ovulation in the hamster

Injecting 2 or 4 mg of cycloheximide (cyclo) at the onset of the proestrous release of gonadotropins prolongs the estrogen (E2) surge, diminishes progesterone (P4) secretion, and prevents ovulation by 0900 h of the next morning (Saidapur and Greenwald, 1981). The present study was designed to determine the effects of 0, 2, 4, or 8 mg cyclo injected at 1400 h proestrus (Day 4) on ovarian protein synthesis and other parameters. Ovulation was delayed until 1400 h estrus by 2 mg cyclo or prevented by 8 mg, and the latter treatment resulted in the death of all animals by 48 h. After 4 mg cyclo, ovulation was delayed in some animals, but the most characteristic feature was the development of large cystic follicles that ultimately transformed into corpora hemorrhagica. All animals lived after the injection of 4 mg cyclo. Ovaries collected 2, 8, 16, or 24 h after treatment were incubated with [3H]leucine for 1 h to assess the effects of cyclo on protein synthesis. Injection of phenobarbital at 1300 h proestrus, which blocks follicle-stimulating hormone (FSH) and luteinizing hormone (LH) surges, reduced ovarian protein synthesis at 1600 h to 61% of the control value. The incorporation of [3H]leucine was reduced to 75%, 37%, and 35% of the 1600-h control value by 2, 4, and 8 mg cyclo, respectively, but without affecting surge levels of FSH and LH. However, by 0600 h estrus, protein synthesis was increased significantly in all the cyclo-treated groups, which provides insight into the half-life of the compound (approximately equal to 8 h for 2-4 mg cyclo). At 1600 and 2200 h proestrus cyclo resulted in serum FSH and LH levels similar to controls, but increased serum prolactin and prolonged E2 levels at Day 4 of 2200 h and decreased serum P4 at both times. The second surge in FSH, which is in progress by 0600 h estrus, was abolished by 4 or 8 mg cyclo but not by the 2-mg dose. This is the first time for any species that ovarian protein synthesis has been measured in the proestrous normal or cyclo-treated animal. We conclude for the hamster that 4 mg cyclo is the optimal dose for blocking ovarian protein synthesis and ovulation and inducing formation of cystic follicles.     

Antiprogestagen RU486 prevents the LH-dependent decrease in the serum concentrations of inhibin in the rat

Summary 1. In the rat, the LH-dependent ovarian progesterone rise mediates several actions of the primary surge of LH on the ovary. This experiment was aimed at elucidating the effects of the antiprogestagen RU486 on the LH-dependent decrease in both the serum concentrations and the ovarian content of inhibin.2. All rats in this experiment were treated with an antagonist of LHRH (1 mg/200 µl saline at 0800 h in proestrus) to supress the endogenous release of LH. One group of rats received 32 µg LH/250 µl saline at 1200 h in proestrus. Other group was given 4 mg RU486/200 µl oil at 0800 h in proestrus. The third group was injected with both RU486 and LH. Rats from the control group were injected with 250 µl saline and 200 µl oil. Animals were decapitated at 1700 h in proestrus and trunk blood and ovaries collected to determine the serum concentrations of LH, FSH, progesterone, 17ß-estradiol and inhibin as well as the ovarian content of inhibin.3. The ovulatory dose of LH in LHRHa-treated rats decreased both the serum concentrations and the ovarian content of inhibin and increased the serum concentrations of FSH. The administration of RU486 blocked the effect of LH on the serum concentrations of inhibin but not that on the ovarian content of inhibin.4. Since the antiprogestagen RU486 blocked the effect of LH on the serum concentrations of inhibin, we conclude that ovarian progesterone, besides mediating the effects of the primary LH surge on the ovulatory process and luteinization, participates in the LH-dependent drop in the serum concentrations of inhibin in proestrous afternoon.     

Vasopressin regulation of the proestrous luteinizing hormone surge in wild-type and Clock mutant mice

In the female mouse, ovulation and estrous cyclicity are under both hormonal and circadian control. We have shown that mice with a mutation in the core circadian gene Clock have abnormal estrous cycles and do not have a luteinizing hormone (LH) surge on the afternoon of proestrus due to a defect at the hypothalamic level. In the present study, we tested the hypotheses that vasopressin (AVP) can act as a circadian signal to regulate the proestrous release of LH, and that this signal is deficient in the Clock mutant. We found that Avp expression in the suprachiasmatic nucleus (SCN) and AVP 1a receptor (Avpr1a) expression in the hypothalamus is reduced in Clock mutant mice compared to wild-type mice. Intracerebroventricular (i.c.v.) injection of AVP on the afternoon of proestrus is sufficient to induce LH secretion, which reaches surge levels in 50% of Clock mutant mice. The effect of AVP on the Clock mutant LH surge is mediated by AVPR1A, as co-infusion of AVP and an AVPR1A-specific antagonist prevents AVP induction of LH release, although infusion of an AVPR1A antagonist into wild-type mice failed to prevent a proestrous LH surge. These results suggest that reduced hypothalamic AVP signaling plays a role in the absence of the proestrous LH surge in Clock mutant mice. The results also support the hypothesis that AVP produced by the SCN may be a circadian signal that regulates LH release.     

Effects of a potent antagonist to gonadotropin-releasing hormone on male rats: luteinizing hormone is suppressed more than follicle-stimulating hormone

Previous work with female rats showed that serum levels of follicle-stimulating hormone (FSH) are suppressed by gonadotropin-releasing hormone (GnRH) antagonists less than are levels of serum luteinizing hormone (LH), suggesting a lesser dependency of FSH on GnRH stimulation. The differential regulation of LH and FSH is known to have some aspects that are sexually asymmetrical, and it was of interest to see if males also show differential gonadotropin suppressibility after injection of an antagonist to GnRH. Male rats were prepared for serial sampling 4 wk after castration. After a blood sample was removed at Time Zero, [Ac-3-Pro1, pF-D-Phe2, -D-Trp3,6]-GnRH (Antag) was injected subcutaneously in oil; doses were 0, 4, 20, 100, 500, and 2500 micrograms. Blood was sampled at 2, 5, 12, 24 and 36 h postinjection. All doses above 4 micrograms had lowered LH levels by 2 h, and LH remained suppressed for 12 to 24 h at the three higher doses. By contrast, serum FSH was unaffected by any dose at 5 h, and was only marginally suppressed by the highest doses thereafter. As in females, therefore, FSH secretion in male rats appears not to be as dependent on GnRH as is LH secretion.     

Differential regulation of preovulatory luteinizing hormone and follicle-stimulating hormone release by opioids in the proestrous rat

We have investigated the role of mu- and kappa-opioid receptors in the central control of preovulatory LH and FSH release in the proestrous rat. Animals were anesthetized with chloral hydrate at 14:00 h on proestrus day. Following femoral artery cannulation, they were mounted in a stereotaxic apparatus. Morphine and U-50488H (benzene-acetamide methane sulphonate) were infused intracerebroventricularly either alone or in combination with naloxone and MR1452, respectively. Controls received sterile saline alone. Blood samples were obtained at hourly intervals between 15:00 h and 17:00 h. Plasma LH and FSH levels were measured by radioimmunoassay. Morphine did not significantly change plasma LH levels at 15:00 h and 16:00 h sampling intervals. A significant increase was observed at 17:00 h compared to the controls (p<0.05). U-50488H significantly increased LH levels at 16:00 h and 17:00 h (p<0.05). The co-administration of naloxone and MR1452 with mu- and kappa-agonist had no significant effect on LH levels at any sampling interval. In all groups, LH levels showed a linear rise over the sampling period between 15:00 h and 17:00 h. None of the treatments significantly altered plasma FSH levels which however, declined towards the end of the afternoon surge. In conclusion, we suggest that the secretion of LH and FSH is differentially regulated by mu- and kappa-opioid receptors. It is thought that in all groups chloral hydrate interfered with the LH surge secretory systems.     

Central, stereoselective receptors mediate the acute effects of opiate antagonists on luteinizing hormone secretion

Although a central site of acute opiate action in regulating luteinizing hormone (LH) secretion has been suggested by the ability of centrally implanted opiate antagonists to increase LH levels, opiate antagonists are lipophilic and could influence the pituitary in situ. Also, the physiological significance of opiate receptor blockade with antagonists rests on the assumed, but untested, stereoselectivity of these receptors. Therefore, a lipophobic quaternized derivative of naltrexone (MRZ 2663-Naltrexone methobromide) and dextro- (+) and levo- (-) stereoisomers of naloxone were used to study the site- and stereoselectivity of gonadotropin responses to opiate antagonists in vivo. Male rats were injected intracerebroventricularly (icv) or intravenously (iv) with the quaternary or tertiary congeners of naltrexone and subcutaneously (sc) with (-) or (+)-naloxone. Rats injected icv with 20 ug of quaternary naltrexone displayed significant increases in serum luteinizing hormone (LH). The onset of the response was rapid with serum LH levels being significantly elevated 15 minutes after the injection and returning to basal levels 30 minutes later. Rats injected iv with 10 mg/kg of quaternary naltrexone failed to show significant LH responses. Rats injected either centrally or periphally with equivalent doses of tertiary naltrexone showed LH responses that were similar to those found in animals injected icv with quaternary naltrexone. As little as 0.5 mg/kg of (-)-naloxone resulted in significant elevations in serum LH that were higher than those elicited by up to 10 mg/kg of (+)-naloxone, indicating that this effect of naloxone is stereoselective. These data support the argument that opioids can acutely modulate LH secretion through actions at stereoselective opioid receptors in the central nervous system.