Ovarian steroid modulation of gonadotropin secretion and pituitary responsiveness to luteinizing hormone-releasing hormone in the female hamster.

The significance of ovarian estradiol (E2) and progesterone secretion in the regulation of pituitary LH and FSH secretion and pituitary responses to LHRH was investigated in the hamster. Cycling females showed increased LH and FSH responses to LHRH on the morning of proestrus as compared to the responses observed on diestrus day 2. Pituitary responsiveness to LHRH declined on the evening of proestrus, after the preovulatory LH/FSH release. The secondary increase in serum FSH concentration on the morning of estrus was accompanied by a selective increase in the pituitary FHS response to exogenous LHRH. Hamsters ovariectomized (ovx) on diestrus day 2 exhibited daily afternoon LH surges but not FSH surges for at least 10 days after ovx. The magnitude of the LH surges in ovx hamsters was approximately 30-50% of that observed in proestrous females. The pituitary LH response to exogenous LHRH in ovx animals was about 25% as great as in proestrus hamsters. Serum FSH concentrations in ovx females increased by only 30% after LHRH injection, while similar treatment with LHRH resulted in 3- to 4-fold increments in serum FSH in proestrous hamsters. Implantation of E2 capsules in ovx hamsters resulted in increased gonadotropin responses to exogenous LHRH. Serum LH concentrations in the E2-implanted, LHRH-injected animals were as great as those observed after LHRH injection in proestrous females. Administration of LHRH, LH, or progesterone on the morning of proestrus failed to detectably alter the timing or magnitude of the proestrus afternoon FSH surge. The present results suggest that the increasing serum titers of estrogen on diestrus and early proestrus result in increased pituitary sensitivity to LHRH, and this increased sensitivity probably contributes to the magnitude of the preovulatory LH surge. The increases in LH and progesterone which occur during the afternoon do not seem to be responsible for triggering the proestrous FSH surge.     

Endogenous excitatory amino acid regulation of the progesterone-induced LH and FSH surge in estrogen-primed ovariectomized rats.

The role of endogenous excitatory amino acid neurotransmission in the regulation of progesterone and triamcinolone acetonide-induced LH and FSH release was examined. Estrogen-primed ovariectomized rats were utilized in this study. Progesterone or triamcinolone acetonide (1 mg/kg body weight) treatment led to a highly significant elevation of serum LH and FSH levels 5 h later. Treatment with the selective noncompetitive NMDA receptor antagonist, MK801, had no effect on serum LH and FSH levels when compared to estrogen controls. However, MK801 administered 1 h prior to progesterone or triamcinolone acetonide administration completely blocked their ability to induce LH and FSH surges. These studies demonstrate for the first time the involvement of endogenous excitatory amino acid neurotransmission in the mediation of progesterone and corticosteroid-induced LH and FSH surges.     

Lateral hypothalamic mediation of midbrain catecholaminergic influences on preovulatory surges of serum gonadotropin and prolactin in female rats

To investigate the involvement of the midbrain ascending pathways in the control of ovulation, bilateral coronal transections were produced at three rostro-caudal levels in the lateral hypothalamus as well as in the lateral forebrain on the day of proestrus. Although lateral forebrain transections failed to block spontaneous ovulation, transections in both the anterolateral hypothalamus (ALH) and the midlateral hypothalamus blocked ovulation and the proestrous surges of serum LH, FSH, and PRL. When electrochemical stimulation was applied to the diagonal band of Broca, ovulation could be induced in animals with ALH transections. Moreover, the effects of intraventricular saline, norepinephrine (NE; 40 microgram), or dopamine (40 microgram) on the induction of preovulatory surges of serum gonadotropin and PRL were examined in proestrous rats with ALH transections. Ovulation was restored by the intraventricular injection of NE, but not of dopamine. In ALH-transected animals which ovulated in response to intraventricular NE, serum concentrations of LH, FSH, and PRL were higher than those of ALH-transected animals injected with saline. These results suggest that the lateral hypothalamus plays an important role in the control of preovulatory surges of serum gonadotropin and PRL by mediating the midbrain catecholaminergic innervation of the preoptic-hypothalamic area.     

Differential gonadotropin secretion: blockade of periovulatory LH but not FSH secretion by a potent LHRH antagonist

The dependence of periovulatory gonadotropin secretion on LHRH was assessed with the use of a potent LHRH antagonist [ ALHRH ; (Nac-L- Ala1 ,p-Cl-D-Phe2,D-Trp3,6)LHRH]. Blood samples were collected hourly from 14.00 h proestrus (P) through 09.00 h estrus (E) from intact cycling female rats. ALHRH was administered at 09.00 or 13.00 h P before the proestrous increases in gonadotropins had commenced or at 23.00 h P after the LH and primary FSH surges had occurred but preceding the secondary FSH surge. Antagonist given at 09.00 or 13.00 h P completely blocked the LH release with levels remaining undetectable in most animals (less than 30 ng/ml) throughout the sampling period. However, administration of antagonist at these times failed to block completely the primary FSH surge although peak values were reduced when compared with controls, which displayed normal gonadotropin surges. In addition, ALHRH administered at 23.00 h failed to alter the magnitude or other characteristics of the secondary FSH surge when compared with controls. The present study demonstrates that the estrous surge of FSH in the rat is independent of acute hypothalamic release of LHRH. Furthermore, although the proestrous release of FSH is to a large extent LHRH dependent, our data suggest that some other mechanism may also contribute to this primary FSH surge.     

Effects of naloxone and morphine on the proestrous surge of prolactin and gonadotropins in the rat

The effects of naloxone hydrochloride and morphine sulfate on the proestrous surge of PRL and gonadotropins (LH and FSH) were investigated in normal cycling Sprague-Dawley rats. Blood samples (0.45-0.50 ml) were withdrawn without anesthesia every 20 min from 1400-2000 h through an atrial cannula implanted the same morning. RIA revealed that a single iv injection of naloxone (0.2 mg/kg) at 1400 h completely suppressed the surge of PRL, and this was reversed by a concomitant injection of morphine (10 mg/kg). Morphine itself did not alter the peak of the PRL surge. Morphine suppressed only the early phase of the LH surge, and this was reversed by naloxone. Naloxone alone did not change the peak of the LH surge but maintained higher levels than controls during the declining phase. The FSH surge was not altered by either morphine or naloxone. These results suggest that endogenous opioid peptides may have a role in regulating the PRL and LH surges during proestrus in the rat.     

Role of the progesterone receptor in restrained glutamic acid decarboxylase gene expression in the hypothalamus during the preovulatory luteinizing hormone surge

Previous work by our laboratory demonstrated that activation of the progesterone receptor through exogenous administration of progesterone suppressed glutamic acid decarboxylase-67 (GAD(67)) mRNA in the hypothalamus of the estrogen-primed ovariectomized rat. Since GAD(67) is the major synthetic enzyme for the inhibitory transmitter, gamma-aminobutyric acid, the finding raised the possibility that the endogenous activation of the progesterone receptor may act to restrain GAD(67) expression during the natural preovulatory gonadotropin surge during proestrus in the rat, thereby allowing GnRH secretion and the resultant LH surge. To test this hypothesis, the progesterone receptor antagonist, RU486, was administered to regularly cycling proestrous rats and the effect on GAD(67) and GAD(65) mRNA levels in the preoptic area (POA) and medial basal hypothalamus (MBH) was examined. Serum luteinizing hormone (LH) levels were also examined in order to identify correlations between changes in POA and MBH GAD levels and production of the LH surge. GAD(67) mRNA levels in the POA were increased in the cycling rat during proestrus at 18.00 h at the peak and just preceding the termination of the LH surge. There was no change in GAD(67) mRNA levels in the MBH, and GAD(65) expression was also unchanged during proestrus in the POA and MBH. Treatment with the antiprogestin RU486 resulted in an increase in GAD(67) mRNA levels at 12.00 and 14.00 h in the POA, and in the MBH at 14.00, 16.00, and 18.00 h during proestrus, effects which preceded and correlated with the attenuated LH surge in RU486-treated rats at 18.00 h. GAD(65) mRNA levels were also elevated by RU486 at 14.00 and 16.00 h in the POA, and at 14.00 h in the MBH during proestrus. These findings suggest that the progesterone receptor plays a role in restraining GAD expression in the hypothalamus during proestrus, and that this effect may be important for the production of the GnRH and LH surge.     

A decrease in opioid tone amplifies the luteinizing hormone surge in estrogen-treated ovariectomized rats: comparisons with progesterone effects

It is well known that the estrogen-induced LH surge in ovariectomized (ovx) rats is invariably far less in magnitude than the preovulatory LH surge or that induced by progesterone (P) in estrogen-primed ovx rats. Recent studies show that a decrease in hypothalamic inhibitory opioid tone by the neural clock (NC) is responsible for the induction of the preovulatory LH surge on proestrus. Therefore, we hypothesized that the diminished LH response in estrogen-treated ovx rats may be due to an inadequate reduction in opioid tone. To test this hypothesis the effects of transiently decreasing the opioid tone with an opiate receptor antagonist, naloxone (NAL), on LH secretion in estrogen-primed, short term (5 days) and long term (4 weeks) ovx rats were examined. NAL (2 mg/h) was infused iv from 1100-1400 h on day 2 in rats receiving either sc implants (two, 15 mm each) filled with 17 beta-estradiol (300 micrograms/ml in oil) or sc estradiol benzoate (EB; 10 micrograms/rat) injections at 1000 h on day 0. For comparison of NAL- and P-induced LH responses, EB-primed short and long term ovx rats received P injection (2 mg/rat, sc) instead of NAL infusion at 1100 h. Estrogen treatment alone induced a spontaneous rise in plasma LH on the afternoon of day 2, with peak LH levels ranging between 1.5-2.4 ng/ml. NAL infusion markedly enhanced the LH surge in both groups of ovx rats. In short term ovx rats NAL-induced peak LH levels (5-6 ng/ml) were less than those observed in rats receiving supplemental P treatment or that observed previously on proestrus (10-15 ng/ml). However, in long term EB-primed ovx rats, NAL infusion evoked LH surges equivalent to those observed after P injection. In addition, analysis of the episodic LH secretion pattern showed that NAL infusion accelerated the frequency and amplitude of LH discharge and significantly changed the contour of LH episodes. These results show that a transient decrease in inhibitory opioid tone before a spontaneous LH rise in estrogen-treated ovx rats can accelerate episodic LH secretion to culminate in LH surges that resemble those induced by P and the preovulatory LH surge. Therefore, these observations are in accord with the view that the NC-induced curtailment in the inhibitory opioid tone may be inadequate in estrogen-treated rats; NAL infusion and P treatment intensify decrements in inhibitory opioid tone to reinstate the preovulatory-type LH surge in these rats.     

Immunoneutralization of oxytocin attenuates preovulatory prolactin secretion during proestrus in the rat

Recent studies have demonstrated that oxytocin (OT) stimulates prolactin (PRL) release from the anterior pituitary gland and that the secretion of OT into pituitary portal blood changes during the rat estrous cycle. To better define the role of OT on PRL release during the reproductive cycle, the effect of administration of antiserum specific for OT on preovulatory PRL secretion in female rats was studied. Intravenous injection of OT antiserum into cyclic female rats between 13.30 and 14.00 h of proestrus neutralized the elevated levels of OT in pituitary portal blood and significantly reduced the subsequent PRL surge. The characteristic proestrous surge of luteinizing hormone (LH) was not affected by the OT antiserum treatment. These data support a physiological role for OT in the regulation of PRL release during the reproductive cycle.     

Effects of hyperprolactinemia on estrous cyclicity, serum luteinizing hormone, prolactin, estradiol, and progesterone concentrations, and catecholamine activity in microdissected brain areas

The purpose of the present study was to determine the duration of PRL treatment required to suppress estrous cyclicity and preovulatory LH surges and to ascertain whether such treatment affects cyclicity by altering catecholamine activity and/or ovarian steroid secretion. Rats exhibiting 4-day estrous cycles were treated with ovine PRL (oPRL) or vehicle beginning on diestrous day 1 at 0900 h, diestrous day 2 at 2400 h, proestrus at 0600 h, or proestrus at 1200 h. Jugular veins of rats were cannulated to the level of the right atrium on diestrous day 2. Unrestrained rats were bled on proestrus. Preovulatory LH surges and ovulation were completely blocked, and vaginal cytology remained leukocytic on the expected day of proestrus and estrus when oPRL treatment was begun on diestrous day 1. Such treatment elevated progesterone levels beginning on diestrous day 2 and suppressed the preovulatory rise in estradiol observed on proestrous morning and afternoon in control rats. To determine the effect of oPRL on catecholamine activity, alpha-methylparatyrosine was administered to groups of oPRL- or vehicle-treated rats at 0900 or 1500 h on diestrous day 1, diestrous day 2, or proestrus. Animals were killed 0, 45, or 90 min later. Norepinephrine and dopamine concentrations were measured in the median eminence, suprachiasmatic nucleus, medial preoptic nucleus, striatum, ventral aspect of the stria terminalis, and posterior pituitary gland by radioenzymatic assay. Controls exhibited increased norepinephrine turnover rates in the median eminence, suprachiasmatic nucleus, and medial preoptic nucleus on proestrous afternoon concomitant with preovulatory LH surges. In contrast, oPRL-treated rats showed no such increase. In addition, median eminence dopamine turnover rates were elevated beginning on the afternoon of diestrous day 1 in oPRL-treated rats compared to control values. No other differences in norepinephrine and dopamine turnover rates were observed in oPRL-treated rats compared with controls in any other brain area on any day examined. Thus, the data indicate that elevated PRL concentrations have profound effects on reproductive cyclicity by disrupting ovarian steroid secretion and essential preovulatory neurochemical events in selected brain areas involved in the regulation of LHRH.     

Anterior pituitary gland secretion after forebrain ablation: periovulatory gonadotropin release

We sought to determine whether the estrous phase of FSH release in cyclic female rats is dependent on the immediate presence of the diencephalon. A piece of forebrain, the diencephalon and part of the telencephalon, was surgically removed from female rats between 1130--1300 or 2000-2200 h on proestrus. Blood was withdrawn through indwelling venous cannulae during the afternoon and evening of proestrus and the early morning of estrus for RIA of plasma LH, FSH, and PRL concentrations. In rats sham operated at either time period, plasma LH, FSH, and PRL levels rose from 1345 to 1800 h on proestrus. Whereas the plasma LH and PRL concentrations fell from 1800 h on proestrus to 0300 h on estrus, the plasma FSH concentrations remained elevated during this period. The removal of the piece of forebrain around noon on proestrus blocked the rises in the plasma LH and FSH levels and caused high plasma PRL concentrations from 1345 h on proestrus to 0300 h on estrus. The removal of the piece of forebrain during the evening of proestrus did not interfere with the fall in plasma LH concentrations or the maintenance of elevated plasma FSH concentrations during either late proestrus or the early morning estrus, but did cause high plasma PRL levels during that time interval. Hypophysectomy combined with removal of the forebrain piece during the evening of proestrus resulted in a drop in plasma FSH and PRL concentrations. The results confirm that in the rat, 1) the prosencephalon plays an acute stimulatory role n causing the preovulatory LH surge and the proestrous phase of FSH release, 2) the prosencephalon exerts effects during the afternoon and/or early evening of proestrus that cause the estrous phase of FSH release, and 3) the estrous phase of FSH release occurs in the absence of acute diencephalic stimulation.     

Possible role of luteinizing hormone and follicle-stimulating hormone in modulating inhibin secretion and expression during the estrous cycle of the rat

In the female rat, plasma immunoreactive inhibin alpha (irl alpha) levels show marked changes during proestrus and estrus. We investigated the modulating effect of LH and FSH on these changes by injecting the GnRH antagonist DNal-DCpa-DPal-Dpr-(Ac)Dal-Leu-Arg-Pro-Asn-NH2, with or without exogenous LH replacement. Administration of the antagonist at noon on proestrus abolished the primary (proestrus) LH and FSH surge and markedly reduced the secondary (estrus) FSH surge. This treatment also reduced the release of irl alpha normally measured during proestrus afternoon, and partially prevented the decrease in irI alpha secretion on proestrus evening. Exogenous LH injected at 1545 h on proestrus had no measurable effect on irI alpha or FSH levels in control rats; however, in antagonist-treated animals, it restored the secondary FSH surge to control values while augmenting the late proestrus fall in irI alpha. This suggests that the decrease in inhibin secretion measured after exogenous LH treatment represents the mechanism through which LH induced the secondary FSH surge in antagonist-blocked rats. We also used in situ hybridization techniques to examine the changes in the expression of inhibin subunits in the ovary at 0200 h on estrus. The antagonist reduced expression of the alpha-, beta A-, and beta B-subunits in all follicle and tissue types, with the exception of the granulosa cells of large tertiary (possibly preovulatory) follicles where the signal appeared greatly enhanced. These changes were reversed by LH. The alteration in inhibin subunit messages caused by blockade of the primary gonadotropin surge suggests the presence of a cross-regulation between LH and inhibin/activin secretion, so that a decline in circulating LH levels might stimulate inhibin/activin secretion in the granulosa cells of preovulatory follicles, while reducing the production of these proteins in less mature follicles and in other ovarian cell types.     

Evidence for ovarian “inhibin”: Suppression of the secondary rise in serum follicle stimulating hormone levels in proestrous rats by injection of porcine follicular fluid

The present studies were carried out to see if porcine follicular fluid could inhibit increases in serum follicle stimulating hormone (FSH) levels when injected into the rat. For these studies the pentobarbital-treated proestrous rat was chosen as the major test animal model. If an artificial surge of luteinizing hormone (LH) is administered to these rats, it can induce a synchronized secondary rise in FSH secretion rate. Normal saline-treated rats were also used as test animals. They exhibit preovulatory endogenous "surges" of LH and FSH, and also a secondary FSH rise.Porcine follicular fluid was harvested from medium-sized and large (3- to 10-mm diameter) follicles and treated with charcoal to remove endogenous steroids. Charcoal-treated porcine serum served as a control solution. The fluid was injected intraperitoneally in two 0.5-ml doses into pentobarbital-treated proestrous rats immediately and 3 hr after LH injection. Follicular fluid, but not the serum, suppressed the secondary, LH-induced FSH rise (P < 0.01) in a dose-dependent manner, without altering the effects of LH upon serum ovarian steroid levels or follicular rupture. It was effective down to a total dose of 200 mul. Porcine follicular fluid also blocked the secondary FSH surge in normal proestrous rats exhibiting endogenous LH/FSH primary surges. Thus, it would appear that porcine follicular fluid contains a non-steroidal substance(s) that can block the secretion of FSH that is secondary to a natural or artificial LH surge.     

Fos expression in the female rat brain during the proestrous prolactin surge and following mating

A prolactin (PRL) surge occurs in the female rat during proestrus in response to elevated estradiol levels. The elevated release of ovarian steroids on the day of proestrus is also associated with sexual receptivity. Mating triggers twice-daily PRL surges that supplant the proestrous PRL surge and are responsible for maintaining luteal function during the first half of pregnancy. In order to understand the neuronal mechanisms controlling the proestrous- and mating-induced PRL surges, we examined patterns of Fos expression by immunocytochemistry in specific brain regions as a measure of neuronal activity. Intact female rats were sacrificed at 09.00, 15.00, and 18.00 h on the day of proestrus and the day of diestrus. Brain tissues were also collected at 21.00 h on the day of proestrus from rats receiving intromissions or mounts from males or taken directly from their homecage. On the day of proestrus, the number of neurons expressing Fos in the medial preoptic area (mPOA), medial amygdaloid nucleus (mAMYG), and ventromedial nucleus of the hypothalamus (VMH) was few and was associated with low plasma PRL levels at 09.00 h; however, the number of Fos-positive cells in these brain regions significantly increased at 15.00 and 18.00 h when the proestrous PRL surge occurred. Mating during the evening of proestrus resulted in a dramatic increase in the number of Fos-positive cells in the mPOA, mAMYG, and VMH as well as in the arcuate nucleus (ARC). Nonmated animals in diestrus showed low and unchanged PRL levels and Fos expression in all the brain areas throughout the day. These results suggest that the mPOA, mAMYG, VMH and ARC may be important brain sites for the integration of stimuli associated with proestrous- and mating-induced PRL surges.     

Attenuation of preovulatory gonadotrophin surges by epostane: a new inhibitor of 3 beta-hydroxysteroid dehydrogenase

Epostane, an inhibitor of 3 beta-hydroxysteroid dehydrogenase, was administered orally to pro-oestrous rats to evaluate further a possible role for preovulatory progesterone secretion in eliciting surges of LH and FSH. Whereas a dose of 10 mg epostane/kg had essentially no effects on preovulatory gonadotrophin surges and ovulation, 200 mg epostane/kg markedly attenuated LH and FSH surges and blocked ovulation. A dose of 50 mg epostane/kg exerted effects on LH and FSH surges and ovulation intermediate between those of doses of 10 and 200 mg/kg. Plasma concentrations of progesterone were significantly lower in all anovulatory epostane-treated rats at 18.00 and 22.00 h on proestrus than those measured in vehicle-treated rats. Concurrent injection of 2 mg progesterone in rats given 200 mg epostane/kg restored gonadotrophin surges to normal, but consistently failed to reverse the inhibitory effects of epostane on ovulation. Peak plasma progesterone levels produced by the progesterone injections were eight- to tenfold higher than the highest levels measured in vehicle-treated rats during the afternoon of pro-oestrus. Insertion of progesterone capsules was less effective than injections of progesterone in restoring gonadotrophin surges to normal, even though peak plasma progesterone concentrations achieved after insertion of two 20 mm long progesterone capsules were double the peak progesterone concentrations measured in control rats. Nevertheless, taken together with recent reports showing attenuation of preovulatory gonadotrophin surges by the progesterone antagonist RU 486 (17 beta-hydroxy-11 beta-[4-dimethyl-aminophenyl]-17 alpha-[prop-1-ynl] estra-4,9-diene-3-one), the present results provide support for a role of preovulatory progesterone secretion in enhancing oestrogen-dependent LH/FSH surges on pro-oestrus.     

Plasma prolactin and luteinizing hormone profiles during the estrous cycle of the female rat: effects of surgically induced persistent estrus

Experiments were carried out to investigate the hypothalamic control mechanism for prolactin (PRL) and luteinizing hormone (LH) secretion in the female rat. Anterior medial preoptic nucleus (AMPO) or suprachiasmatic nucleus (SCN) lesions were produced by passing 5-10 microA of direct current (tip negative). Persistent estrus (PE) began as early as 6 days and as late as 30 days after electrolytic lesioning. Blood samples obtained during diestrus, proestrus and estrus revealed well-described profiles of plasma PRL and LH in sham-lesioned animals, indicating that our cannulation and blood sampling procedure had no adverse effects on the plasma hormone levels. Individual sham-operated animals sampled on successive or alternate proestrous afternoons showed precise timing of the PRL and LH surges. However, when a shift occurred in the PRL surge a comparable shift would also occur in the LH surge, indicating a coupling between the mechanisms regulating the PRL and the LH surge. The AMPO-, SCN- or combine-lesioned PE animals exhibited low basal levels of plasma PRL and LH. Small secretory bursts occurred one to three times during the 6-hour sampling periods. Animals with incomplete SCN lesions had plasma PRL titers significantly higher than the other 3 groups. Plasma progesterone levels were significantly lower in the PE animals (p less than 0.01), whereas plasma estrogen levels were not significantly different from proestrous controls. These experiments indicate that during the afternoon of proestrus, the surges of plasma PRL and LH are very precise in the time of onset. Moreover, the mechanisms controlling the surge of PRL and LH are temporally coupled.(ABSTRACT TRUNCATED AT 250 WORDS)     

A daily signal for the LH surge in the rat.

We have demonstrated that a brief step-like increment in circulating estradiol concentrations to approximately 100 pg/ml, achieved by SC insertion of Silastic capsules containing estradiol-17 beta and their withdrawal after 29 1/2 hr, elicited a daily LH surge on 4 consecutive afternoons in rats ovariectomized 2 weeks previously. Since the duration of this stimulus was similar to that of the preovulatory increment in serum estradiol concentrations, it was postulated that the endogenous estrogen signal in intact rats might also trigger repetitive LH surges if ovulation and formation of corpora lutea were prevented. To test this hypothesis, rats were ovariectomized at 10:00 AM on proestrus (day 1) and blood samples were obtained at 12:30 and 5:00 PM on days 1 to 4. Although an LH surge occurred on proestrus, no subsequent LH discharges were observed. The absence of an LH surge on consecutive days could not be attributed to a difference between the endogenous estradiol stimulus and the exogenous stimulus which elicited repetitive LH surges in long-term ovariectomized rats. Rather, it was determined that in recently ovariectomized rats, in contrast to long-term ovariectomized rats, a daily LH surge occurred only if elevated serum estradiol concentrations were maintained. Thus, by leaving implants in place, an LH discharge was elicited on 10 consecutive days. These results support the concept that a neural signal for the LH surge is emitted each day throughout the estrous cycle of the rat, and that prolonged maintenance of elevated circulating estradiol concentrations is essential for the expression of these signals.     

Modulation of endogenous opioid influence on luteinizing hormone secretion by progesterone and estrogen

Studies were undertaken using the opiate receptor antagonist naloxone (NAL) to evaluate the relative influence of endogenous opioid peptides (EOP) on LH secretion in cycling and ovariectomized, steroid-treated adult rats. Intact animals received NAL (2 mg/kg, sc) or saline (control vehicle) at 0800 and 1400 h on estrus, 0800 h on diestrus day 1,2000 h on diestrus day 2, and before (at 0800, 1200, and 1400 h) and during the preovulatory LH surge (at 1600 and 1800 h) on proestrus. NAL stimulated LH release by 2- to 3-fold at all stages of the estrous cycle, including during the proestrous gonadotropin surge. Ovariectomized rats were treated with estradiol benzoate (EB; 7.5 micrograms/rat, sc) and 2 days later received NAL (2 mg/kg, sc) or saline at 1000, 1200, 1400, 1600, and 2000 h. NAL induced a relatively small (44-73%), but significant, increase in LH release before (1000, 1200, and 1400 h), during (1600 h), and after (2000 h) the afternoon LH rise. While progesterone (P) treatment (5 mg, sc, day 2 at 1000 h) of EB-primed ovariectomized rats augmented NAL-induced LH release before the LH surge (1200 h), it abolished the LH secretory response to NAL during the LH surge (1400, 1600, and 1800 h). The NAL-induced LH response returned after the LH surge at 2000 h. Likewise, administration of P on proestrus morning (0900 h) abolished the LH secretory response to NAL during the LH surge. These studies indicate that central opioid neurons participate in the tonic inhibition of LH secretion at all stages of the estrous cycle of the rat. The ability of exogenous P to advance and amplify the LH surge on proestrus and in EB-primed ovariectomized rats appears to result in part from a reduction in the EOP inhibitory influence on LH secretion and may indicate a role for EOP in mediating the stimulatory effects of endogenous steroids on LH secretion in the female rat.     

Pro-gonadotropin-releasing hormone (ProGnRH) and GnRH content in the preoptic area and the basal hypothalamus of anterior medial preoptic nucleus/suprachiasmatic nucleus-lesioned persistent estrous rats

The content of GnRH and its precursor peptide were quantified in female rats bearing lesions in the anterior medial preoptic nucleus (AMPO) and the suprachiasmatic nucleus (SCN), and the effects of the lesions on the synthetic activity of the GnRH neurons were evaluated. Electrolytic lesions which induced persistent estrous (PE), or irregular estrous cycles, were produced by passing 5-10 microA of direct current into the AMPO or the SCN of female rats which exhibited regular 4 days estrous cycles before the lesions. Approximately 5 weeks after lesion placement, blood samples were withdrawn from catheterized, freely moving animals and plasma LH, PRL, estrogen, and progesterone were determined by RIA. The preovulatory surges of LH and PRL were eliminated in AMPO- or SCN-lesioned PE rats. Moreover, the LH surge was eliminated and the PRL surge significantly attenuated after estrogen and progesterone treatment of rats bearing complete lesions, irrespective of the presence of ovaries. Irregular cycling animals with incomplete AMPO or SCN lesions, exhibited attenuated LH surge and PRL surge similar to proestrous controls. In one incidence this occurred spontaneously, and could also be induced by sequential estrogen and progesterone injections. After ovariectomy, plasma LH levels were significantly lower in the lesioned animals as compared to sham operated rats (P less than 0.05). Similar secretory patterns of LH and PRL were obtained from a second series of sham-operated rats during the different stages of the estrous cycle or from AMPO- or SCN-lesioned rats during persistent estrus. After 2 months the animals were killed between 0830 and 0930 h, and the preoptic area and the basal hypothalamus were microdissected from the brain sections. After extraction and purification, proGnRH and GnRH levels were measured by RIA. ProGnRH levels in the preoptic area were significantly reduced in AMPO- or in SCN-lesioned rats, compared to proestrous controls (P less than 0.01). In contrast, GnRH levels in either area did not differ in AMPO- or in SCN-lesioned animals compared to sham-operated, proestrous controls. Therefore, lesions of the AMPO or the SCN produce PE and reduce proGnRH without reducing GnRH levels. These data would suggest that the AMPO and the SCN participate in the control of the estrous cycle and are necessary for preovulatory surges of PRL and LH to occur and that the AMPO and the SCN form part of the neural circuit that regulates GnRH synthesis and/or release.     

Dorsal raphe lesion alters the estrous cycle and the preovulatory gonadotropin release

The purpose of this study was to examine the role that the dorsal raphe (DR)-median eminence (ME) serotonergic projection may have in the proestrous gonadotropin and prolactin (PRL) release. DR electrolytic lesions were performed in cycling rats during the first day of diestrus. The effect of DR lesions after 48-72 h of survival (short-term lesioned animals) or after 35-40 days of survival (long-term lesioned animals) on estrous cyclicity, preovulatory gonadotropin and PRL releasing pattern, ovulation and serotonin (5-HT) content of the ME were studied. Following DR lesions the estrous cycle became irregular, remaining in the diestrus phase for several days. Preovulatory gonadotropin release in short-term lesioned animals was increased; on the contrary, in long-term lesioned rats a delay in the surge of these two hormones and a decrease in LH secretion were detected. Long-term lesioned animals also showed a diminished secretion of PRL. The number of ova did not differ between control and lesioned animals. DR lesions in both short- and long-term lesioned rats reduced 5-HT levels in the ME by about 50% and nullified the normal 5-HT decline during the afternoon of proestrus. Our results suggest that the DR exerts a stimulatory influence on the preovulatory gonadotropin release by means of its 5-HT projection to the ME. As the pattern of hormonal secretion in lesioned animals remains similar to that of controls, it may be suggested that this pathway acts as a fine modulator of the mechanisms involved in the regulation of LH and FSH release in cycling rats.(ABSTRACT TRUNCATED AT 250 WORDS)