Effect of naloxone on luteinizing hormone, follicle-stimulating hormone, and prolactin secretion in the different phases of the estrous cycle

Role of steroid hormones and morphine treatment in the modulation of opioid receptor gene expression in brain structures in the female rat

This study determined the effects of acute treatment with morphine on the expression of the Oprm1, Oprk1, and Oprd1 genes (which encode μ, κ, and δ receptors, respectively) in the striatum, hypothalamus, and periaqueductal gray (PAG) in ovariectomized female rats treated with estrogen. Ovariectomized female rats were divided into five equal groups. Two groups received estrogen (50 µg/kg, 54 h before testing) and saline (ES group) or 3.5 mg/kg morphine (EM group) 2 h before euthanasia. The SS group received saline solution 54 and 2 h before the experiments. The SM group received saline 54 h and 3.5 mg/kg morphine 2 h before the experiments. The W group remained undisturbed. The genes expression were evaluated. Oprm1 and Oprk1 expression were activated, respectively, in the hypothalamus and PAG and in the striatum and PAG by morphine only in estrogen-treated animals. Oprd1 expression in the hypothalamus and PAG was activated by morphine in both estrogen-treated and -nontreated animals. The Oprm1 and Oprk1 gene response to morphine might depend on estrogen, whereas the Oprd1 gene response to morphine might not depend on estrogen, supporting the hypothesis of a functional role for ovarian hormones in opioid receptor-mediated functional adaptations in the female brain.

Effects of SCH 486757, a nociceptin-1 receptor agonist, on fertility and reproductive hormone levels in female CRL:CD®[SD] rats

BACKGROUND

SCH 486757 is a nociceptin-1 receptor agonist that was in development as an antitussive. Studies were conducted to characterize its effects on female fertility and to examine its potential modes of action.

METHODS

Female rats were administered up to 20 mg/kg SCH 486757 before/during mating through gestation day (GD) 7; female fertility and embryonic development were assessed on GD 14. In a subsequent study, pregnant rats were dosed up to 50 mg/kg SCH 486757 from GD 0 to 7. Reproductive hormones were assessed on GD 1, 3, 5, and 7, and embryonic development was assessed on GD 14. A subset of dosed dams were allowed to deliver, were subsequently re-mated, and reproductive hormones and fertility were assessed on GD 7 and 14, respectively. To determine the effects of SCH 486757 on nonpregnant rats, doses of up to 50 mg/kg SCH 486757 were administered for 4 days beginning on the day of estrus; reproductive hormones were assessed after the final dose.

RESULTS

Female rats administered ≥20 mg/kg SCH 486757 exhibited abnormal estrous cycles; decreased fertility, number of corpora lutea, and implantation sites; and increased pre- and postimplantation loss. In general, administration of SCH486757 was associated with lower luteinizing hormone (LH) progesterone (P4), and estradiol (E2) levels in pregnant rats. These effects on fertility/embryonic development and reproductive hormones exhibited reversibility post dosing. Nonpregnant rats in the 50-mg/kg group exhibited apparent decreases in P4 and E2 levels, with no apparent effects on LH values.

CONCLUSIONS

The SCH 486757-related effects on fertility and embryonic development were likely the result of decreases in P4, E2, and/or LH, rather than being due to decreased prolactin levels.

Hirsutism, virilism, polycystic ovarian disease, and the steroid-gonadotropin-feedback system: a career retrospective

This career retrospective describes how the initial work on the mechanism of hormone action provided the tools for the study of hirsutism, virilism, and polycystic ovarian disease. After excessive ovarian and or adrenal androgen secretion in polycystic ovarian disease had been established, the question whether the disease was genetic or acquired, methods to manage hirsutism and methods for the induction of ovulation were addressed. Recognizing that steroid gonadotropin feedback was an important regulatory factor, initial studies were done on the secretion of LH and FSH in the ovulatory cycle. This was followed by the study of basic mechanisms of steroid-gonadotropin feedback system, using castration and steroid replacement and the events surrounding the natural onset of puberty. Studies in ovariectomized rats showed that progesterone was a pivotal enhancer of estrogen-induced gonadotropin release, thus accounting for the preovulatory gonadotropin surge. The effects of progesterone were manifested by depletion of the occupied estrogen receptors of the anterior pituitary, release of hypothalamic LHRH, and inhibition of enzymes that degrade LHRH. Progesterone also promoted the synthesis of FSH in the pituitary. The 3α,5α-reduced metabolite of progesterone brought about selective LH release and acted using the GABA(A) receptor system. The 5α-reduced metabolite of progesterone brought about selective FSH release; the ability of progesterone to bring about FSH release was dependent on its 5α-reduction. The GnRH neuron does not have steroid receptors; the steroid effect was shown to be mediated through the excitatory amino acid glutamate, which in turn stimulated nitric oxide. These observations led to the replacement of the long-accepted belief that ovarian steroids acted directly on the GnRH neuron by the novel concept that the steroid feedback effect was exerted at the glutamatergic neuron, which in turn regulated the GnRH neuron. The neuroprotective effects of estrogens on brain neurons are of considerable interest.

Presence of delta opioid receptors on a subset of hypothalamic gonadotropin releasing hormone (GnRH) neurons

Opioid peptides exert an inhibitory effect on hypothalamic gonadotropin releasing hormone (GnRH) secretion mainly by interacting with mu-opioid receptors. Although a direct role for opioids via delta-opioid receptors (DORs) has been suggested, the presence of these receptors on GnRH neurons has never been demonstrated. In the present study, we determined the distribution of DORs in the basal hypothalamus of rat with special focus on their relation to GnRH neurons. Double-labelling immunofluorescence and confocal microscopy revealed that DORs are exclusively present in a subpopulation of GnRH nerve terminals, with the highest density in the external layer of the median eminence. We then studied the functional characteristics of DORs in an immortalized GnRH-secreting neuronal cell line (GT1-1) known to endogenously express this receptor. Here, pertussis toxin pretreatment abolished the delta-agonist (DPDPE) inhibitory effect on cAMP accumulation. We also analyzed the type of G proteins involved in the signal transduced by the DOR and showed that GT1-1 cells express the inhibitory Go and Gi2 alpha-subunits. However, only Go was down-regulated under chronic DPDPE exposure. Finally, since DOR is expressed postnatally in brain, we compared GnRH neuronal cells immortalized at different developmental stages (the more mature GT1-1 and GT1-7 cells, versus the more immature GN11 cells), evidencing that only mature neurons express DOR. In conclusion, our study indicates that a direct control of opioids via delta-receptors occurs on GnRH neurons and validates the use of GT1 cells to further investigate the nature of the DOR present on GnRH neurons.

Presence of kappa-opioid tone at the onset of the ovulatory luteinizing hormone surge in the proestrous rat

A decrease in endogenous opioid peptide inhibitory tone on the afternoon of proestrus is one event underlying generation of the ovulatory luteinizing hormone (LH) surge. Whether this disinhibition involves a complete loss of opioid suppression at the time of the LH surge is controversial. The objective of the present study was to determine whether a total loss specifically of the kappa-opioid inhibitory component suppressing LH secretion occurs on proestrus at the onset of the LH surge. Proestrous rats were infused intraventricularly with either artificial cerebrospinal fluid (aCSF) or aCSF containing nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist, from 15:30 or 16:30 h (the approximate onset time of the spontaneous LH surge) to 18:50 h. The LH surge in rats treated with nor-BNI beginning at 15:30 h started 0.5 h earlier than the spontaneous surge in aCSF controls, and had significantly higher plasma LH levels from 16:30 to 17:30 h. Nor-BNI administration begun at 16:30 h also produced an LH surge with more elevated plasma LH levels at 17:30 and 18:00 h than in aCSF-treated controls. These results demonstrate that significant amounts of kappa-opioid tone are still present during the hours when the LH surge is initiated. Thus, a complete loss of kappa-opioid inhibition is not required for the onset of the LH surge on proestrus.

Pentobarbital stimulates the activity of the GnRH pulse generator interacting with opioid neurons in rats in proestrus

In order to investigate the possibility that i.p. injection of pentobarbital sodium (PB, 32 mg/kg bw) potentiates the GnRH pulse generator activity, effects of i.v. infusions of an opiate receptor antagonist naloxone (NAL, 2 mg/h) on the pulsatile LH secretion were compared in saline (SAL)- and PB-injected rats in proestrus and diestrus 1. In SAL-injected rats in proestrus, NAL infusions significantly increased both the frequency and amplitude of LH pulses, and also the overall mean LH concentration. In PB-injected rats in proestrus, all the parameters of the pulsatile LH secretion were similar to those in SAL-injected rats in proestrus. The NAL infusion in PB-injected rats caused an increase in the frequency, but it was similar to that in SAL-injected rats. But, increases in the amplitude and the overall mean LH observed during NAL infusions in PB-injected rats were greater than in SAL-injected rats. In SAL-injected rats in diestrus 1, NAL infusions increased all the parameters, as in rats in proestrus. In PB-injected rats in diestrus 1, LH secretion was severely suppressed. NAL infusions recovered the pulsatile LH secretion, but the frequency and the overall mean LH of the secretion were smaller than those obtained during NAL infusions in SAL-injected rats. In addition, characteristic increases in the MUA (volleys), which occur in association with the initiation of an LH pulse and thus are considered to represent an increased activity of the GnRH pulse generator, appeared more frequently during NAL infusions in PB-injected rats in proestrus than in SAL-injected rats. These results suggest that the GnRH pulse generator in rats in proestrus, but not in rats in diestrus 1, is refractory to PB and further is potentiated by PB in the response to NAL. Together with the fact that this dosage of PB blocks the surge of LH secretion in rats in proestrus, the concept of the existence of separate neuronal mechanisms responsible for the surge and pulsatile secretion of LH are supported.

Regulation of the preovulatory gonadotropin surge by endogenous steroids

Estradiol secreted by growing ovarian follicle(s) has been considered classically to be the neural trigger for the preovulatory surge of gonadotropins. The observation that the estradiol-induced gonadotropin surge in ovariectomized rats is of lesser magnitude and duration than that found in the cycling rat at proestrus has resulted in a search for other steroid regulators. Progesterone is a major regulator of the preovulatory gonadotropin surge. It can only act in the presence of an estrogen background, which is necessary for the synthesis of progesterone receptors. In the estrogen-primed ovariectomized rat, progesterone is able to initiate and enhance the gonadotropin surge to the magnitude observed on the day of proestrus and limit it to 1 day. The physiological role of progresterone in the induction of the preovulatory gonadotropin surge has been demonstrated by the attenuation of the progesterone-induced surge and the endogenous proestrus surge by progesterone receptor antagonist RU486 and the progesterone synthesis inhibitor trilostane. The promoter region of the follicle-stimulating hormone (FHS)-beta gene contains multiple progesterone response elements and progesterone brings about FSH release as well. The reduction of progesterone in the 5 alpha-position appears to be important for the regulation of progesterone secretion. Corticosteroids appear to play a significant role in the secondary FSH surge on late proestrus and early estrus.

The role of endogenous opioids in the luteinizing hormone surge in rats: studies with clocinnamox, a long-lasting opioid receptor antagonist

Endogenous opioid peptides have been demonstrated to regulate luteinizing hormone (LH) secretion in a variety of species. Studies in rodents suggest a role of opioid peptide systems in controlling the timing of the LH surge, which is entrained to the circadian rhythm. The current studies utilize clocinnamox, a novel long-lasting opioid receptor antagonist that is capable of occupying mu-opioid receptors for periods of one week or more, to examine the role of endogenous opioid systems on the LH surge. Administration of clocinnamox [14b-(p-chlorocinamoylamino)-7,8-dihydro-N-cyclopropylmethyl normophineone mesylate]) on the morning of proestrus advanced the LH surge by several hours. Despite the blockade of opioid receptors and analgesia for more than one week, administration of clocinnamox on the evening of diestrus II had no effect on the timing of the LH surge but significantly increased plasma LH levels throughout the day of proestrus. These data suggest that removal of opioid tone is unlikely to be the critical signal controlling the initiation of the LH surge in rodents, although it does appear to be permissive for the surge. Furthermore, the mu-opioid receptor appears to be the receptor involved in the regulation of the LH surge.

Estrogen-induced alteration of mu-opioid receptor immunoreactivity in the medial preoptic nucleus and medial amygdala

The mu-opioid receptor (mu-OR), like most G-protein-coupled receptors, is rapidly internalized after agonist binding. Although opioid peptides induce internalization in vivo, there are no studies that demonstrate mu-OR internalization in response to natural stimuli. In this study, we used laser-scanning microscopy to demonstrate that estrogen treatment induces the translocation of mu-OR immunoreactivity (mu-ORi) from the membrane to an internal location in steroid-sensitive cell groups of the limbic system and hypothalamus. Estrogen-induced internalization was prevented by the opioid antagonist naltrexone, suggesting that translocation was largely dependent on release of endogenous agonists. Estrogen treatment also altered the pattern of mu-ORi at the bright-field light microscopic level. In the absence of stimulation, the majority of immunoreactivity is diffuse, with few definable mu-OR+ cell bodies or processes. After stimulation, the density of distinct processes filled with mu-ORi was significantly increased. We interpreted the increase in the number of mu-OR+ processes as indicating increased levels of internalization. Using this increase in the density of mu-OR+ fibers, we showed that treatment of ovariectomized rats with estradiol benzoate induced a rapid and reversible increase in the number of fibers. Significant internalization was noted within 30 min and lasted for >24 hr after estrogen treatment in the medial preoptic nucleus, the principal part of the bed nucleus, and the posterodorsal medial amygdala. Naltrexone prevented the increase of mu-OR+ processes. These data imply that estrogen treatment stimulates the release of endogenous opioids that activate mu-OR in the limbic system and hypothalamus providing a "neurochemical signature" of steroid activation of these circuits.

mu-Opioid agonist-stimulated [35S]GTPgammaS binding in guinea pig hypothalamus: effects of estrogen

mu-Opioid receptors play a critical role in the regulation of the female reproductive cycle, and estrogen modulates the coupling of mu-opioid receptors to a potassium channel in the basal hypothalamus (BH) of the female guinea pig. Therefore, we ascertained the distribution of mu-opioid receptors in the BH with autoradiography using the mu-opioid selective agonist [3H]DAMGO. In addition, we investigated the effects of estrogen on DAMGO- or the GABAB receptor agonist baclofen-stimulated [35S]GTPgammaS binding in the BH. Based on the high density of mu-opioid receptors, but the lack of effects of estrogen on [35S]GTPgammaS binding, we conclude that mu-opioid receptor interaction with its G-protein is not the target of estrogen's actions.

Opioid-glutamate-nitric oxide connection in the regulation of luteinizing hormone secretion in the rat

Opioid neurons are recognized to be an important component of the inhibitory "brake" in the CNS that restrains LHRH secretion. Opioid inhibition could be exerted directly on LHRH neurons, or it could be achieved via indirect mechanisms involving restrainment of excitatory "accelerator" neurons that facilitate LHRH release. The purpose of the present study was to explore the second hypothesis by investigating whether removal of opioid inhibition by administering the opioid antagonist, naloxone leads to enhanced activation of glutamate and nitric oxide (NO) neurons, which are known to be important excitatory "accelerator" components for the control of LHRH secretion. Naloxone administration (2.5 mg/kg) to adult male rats induced a significant elevation of serum LH levels at 20 min post injection. NOS activity in preoptic area (POA) and medial basal hypothalamic (MBH) fragments was demonstrated to be significantly elevated 20 min post naloxone injection. Administration of a glutamate (NMDA) receptor antagonist (MK-801, 0.2 mg/kg) abolished the naloxone-induced increase in NOS activity in the POA and MBH, with a corresponding block of the naloxone-induced LH release. Glutamate appears to only be involved in LH surge generation and not to regulate basal LH levels, as MK-801 had no effect on basal LH release. Because previous work by our laboratory and others have provided evidence that NO is a mediator of glutamate effects in the hypothalamus, these findings are interpreted to mean that opioid inhibition is mediated on glutamate neurons that are upstream of NO neurons. In support of this contention, we found that NMDA treatment enhanced NOS activity in the male rat POA and MBH fragments in vitro, an effect that was specific as it was completely blocked by the NMDA receptor antagonist, MK-801. Additionally, in vivo microdialysis studies revealed that naloxone treatment significantly enhances glutamate release in the preoptic area (POA) at 15 min post injection in conscious, unanesthetized, freely moving male rats. Release rates of the control amino acid, serine did not change significantly following naloxone injection. Taken as a whole, these findings provide evidence for an opioid-glutamate-NO pathway in the control of LHRH secretion, and they demonstrate the importance of "brake-accelerator" interactions in the control of LHRH and LH secretion.

Membrane-bound tyrosine aminopeptidase activities in the rat brain throughout the estrous cycle

Puromycin sensitive and insensitive membrane-bound aminopeptidase activity levels during the estrous cycle in several brain areas have been described in this research. We have found the highest aminopeptidase M activity levels during the proetrous stage in the hypothalamus, the amygdala and the pituitary gland. Since this enzyme has been involved in opioid peptide metabolism, it is suggested that aminopeptidase M could play a part in the decrease in the inhibitory influence of the endogenous opioids peptides that participate in the LH surge.

Antiprogestins inhibit the binding of opioids to mu-opioid receptors in nervous membrane preparations

The present study showed that the glucocorticoid/progesterone antagonists, 17 beta-hydroxy-1 1 beta-(4-dimethylamino-phenyl-1)-17-(prop-1-ynyl)estra-4,9-dien+ ++-3-one (RU486) and 17 beta-hydroxy-11 beta-(4-dimethylamino-phenyl-1)-17-(propan-3-ol)estra-4,9-dien-3-o ne (ZK 98299), inhibit the binding of labeled dihydromorphine to mu-opioid receptors present on membrane preparations derived from rat and mouse brain, as well as from human neuroblastoma cells. The inhibitory effect of RU486 was dose-dependent and linked to a decrease of the affinity of labeled dihydromorphine to the mu-opioid receptors. Kinetic experiments have shown that RU486 induces a decrease of the association rate constant (k + 1) of dihydromorphine. RU486 also proved able to dissociate the dihydromorphine-mu-opioid receptor complex, although at a rate slower than that exhibited by unlabeled dihydromorphine. Finally, the addition of NaCl (100 mM) to the incubation buffer induced a 50% decrease of the inhibitory effect of RU486. A 6-day treatment of neuroblastoma cells with RU486 eliminated the inhibitory effect morphine exerts on the intracellular accumulation of cyclic AMP induced by prostaglandin E1. These results indicate that RU-486 may interact with brain mu-opioid receptors in vitro, by decreasing the affinity of opioid ligands.

Prenatal exposure to morphine alters brain mu opioid receptor characteristics in rats

Prenatal morphine exposure alters neither the binding capacity nor the affinity of ligand binding to mu opioid receptors of adult male brains. However, males have significantly higher Bmax in the hypothalamus than ovariectomized females. In females, prenatal exposure to morphine reduces the Bmax of mu opioid receptors 25% in the hypothalamus and preoptic area. Estrogen treatment increases the Bmax of mu opioid receptors in the striatum of all ovariectomized females but in the hypothalamus only of morphine-exposed females, thereby eliminating the sex difference observed in control animals.

Effects of steroids on the brain opioid system

The experiments reported here add further evidence in support of the view that sex steroids may influence the binding characteristics of brain opioid receptors. In particular, it has been shown that: (a) the number of mu-opioid receptors varies in the hypothalamus of regularly cycling female rats according to the different phases of the estrous cycle, which are characterized by fluctuations of circulating levels of sex steroids; (b) the number of mu-opioid receptors decreases in the hypothalamus and in the corpus striatum when ovariectomized rats are submitted to treatments with estradiol and progesterone able to induce a "positive" feedback effect on LH release. A treatment with estrogen alone able to induce a "negative" feedback effect on LH release brings about an increase of the number of mu-opioid receptors in the thalamus and in the hippocampus; (c) in addition to the mu-receptors, receptors of the delta type may also be involved in the control of gonadotropin secretion; recent results here presented indicate that a line of immortalized hypothalamic cells (GT1 cells), which synthesize and secrete LHRH, present delta opioid receptors on their membranes; these are apparently involved in the control of LHRH release from these cells.

Evidence that hypothalamic neuropeptide Y gene expression increases before the onset of the preovulatory LH surge

Neuropeptide Y (NPY), a 36 amino acid residue peptide, is involved in stimulation of LHRH and LH surges on proestrus and those induced by ovarian steroids in ovariectomized (ovx) rats. Recently, we observed that NPY gene expression in the medial basal hypothalamus (MBH) was increased before the onset of the LH surge in the ovarian steroid-primed ovx rats. Since the ovarian steroidal milieu during the estrous cycle is markedly different from that prevailing after ovarian steroid injections in ovx rats, we evaluated in cycling rats the temporal relationship between MBH preproNPY mRNA levels and the preovulatory LH surge on the day of proestrus and compared that with diestrus II, concomitant with basal LH levels. PreproNPY mRNA levels in the MBH were measured by solution hybridization/RNAse protection assay, using a cRNA probe. On the day of diestrus II, preproNPY mRNA levels changed little between 1000 and 1800 h. Quite unexpectedly, preproNPY mRNA levels at 1000 h on proestrus were similar to diestrus II levels, despite additional exposure to ovarian steroids during this interval. However, from these low levels at 1000 h, the preproNPY mRNA profile displayed a biphasic rise. During the first phase, preproNPY mRNA rose significantly at 1200 h and remained elevated at 1300 and 1400 h concomitant with basal serum LH levels. Thereafter, a second rise in preproNPY mRNA began at 1500 h, peaked rapidly at 1600 h and declined significantly at 1800 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen augments hypothalamicβ-endorphin secretion and activates an inhibitoryβ-endorphin short-loop feedback system

The role of estrogen in the regulation of hypothalamicβ-endorphin hormone secretion is studied by determiningβ-endorphin concentration in pituitary portal plasma of ovariectomized rats in the presence or absence of this steroid and/or the opioid antagonist naloxone. Twenty-six hours following s.c. injection of 10 /µg estradiol benzoate (estrogen) or oil, rats anesthetized with Saffan (alphaxolone/alphadolone) underwent pituitary stalk exposure and hypophysectomy, after which pituitary portal blood was continuously collected and stored in 15 min aliquots from 1100-1400 h. At 1100 h, animals were given an initial bolus iv injection of naloxone or saline (naloxone, 2 mg/ kg, or saline, 0.1 ml) and then infused (iv) continuously with naloxone (2 mg/kg/h) or saline (0.8 ml/h) until 1400 h. Plasma samples were extracted and assayed by radioimmunoassay forβ-endorphin. Treatment with estrogen increased the meanβ-endorphin levels twofold as compared to oil-treated controls. Naloxone potentiated estrogen action ofβ-endorphin secretion, but did not affect basalβ-endorphin secretion. These results suggest that estrogen enhancedβ-endorphin secretion from the hypothalamus. Furthermore, the hypersecretion ofβ-endorphin induced by naloxone with, but not without, estrogen supports the existence of an estrogen-activated short-loop negative feedback mechanism regulatingβ-endorphin secretion.

Effects of estrogen on the number of neurons expressing beta-endorphin in the medial basal hypothalamus of the female guinea pig

The distribution pattern of immunoreactive beta-endorphin neurons was studied in female guinea pigs that were ovariectomized, and one week later were injected with 25 micrograms estradiol benzoate or oil. The animals (5 from each group) were perfused after 24 hours with 4% paraformaldehyde. The locations of beta-endorphin cells and fibers were determined using avidin-biotin immunohistochemistry on free-floating vibratome sections. beta-endorphin-immunoreactive fibers were distributed widely throughout specific regions of the rostral forebrain, similar to what has been described in other species. beta-endorphin cell bodies were found in the arcuate nucleus and in adjacent ventrolateral areas throughout the rostrocaudal extent of the basal hypothalamus. Cells immunoreactive to beta-endorphin were also present in the caudal part of the ventromedial nucleus of the hypothalamus. The number of beta-endorphin neurons was quantified in anatomically matched sections through the rostral, medial and caudal basal hypothalamus of estradiol benzoate- and oil-treated guinea pigs. Analysis of variance revealed that the number of immunoreactive beta-endorphin cells was significantly increased in all regions of the basal hypothalamus of estrogen-treated guinea pigs as compared to vehicle-treated animals (P < 0.01). These data indicate that in the guinea pig, the number of neurons expressing beta-endorphin is increased in the arcuate nucleus 24 hours after estrogen treatment.

Effects of ovarian hormones on brain opioid binding sites in castrated female rats

These experiments were performed to analyze whether treatments of ovariectomized female rats with ovarian steroid regimens able to induce either an increase (positive feedback effect) or a decrease (negative feedback effect) of serum levels of luteinizing hormone (LH) have some impact on the characteristics of mu-opioid binding sites in circumscribed areas of the brain. The increase of serum levels of LH elicited by a treatment with estradiol benzoate (EB) plus progesterone (P; positive feedback effect) was accompanied by a significant decrease in the number of mu-binding sites in the hypothalamus and in the corpus striatum. The decrease in serum levels of LH induced by a treatment with EB alone (negative feedback effect) brought about a significant increase of the number of mu-binding sites in the thalamus and in the hippocampus. These results seem to suggest that the release of LH induced by EB plus P may involve a decrease of hypothalamic mu-binding sites. Apparently, the inhibitory effect on LH release exerted by EB alone does not involve any change of the density of these binding sites in the hypothalamus.

Interaction between ovarian and adrenal steroids in the regulation of gonadotropin secretion

Recent work from our laboratory suggests that a complex interaction exists between ovarian and adrenal steroids in the regulation of preovulatory gonadotropin secretion. Ovarian estradiol serves to set the neutral trigger for the preovulatory gonadotropin surge, while progesterone from both the adrenal and the ovary serves to (1) initiate, (2) synchronize, (3) potentiate and (4) limit the preovulatory LH surge to a single day. Administration of RU486 or the progesterone synthesis inhibitor, trilostane, on proestrous morning attenuated the preovulatory LH surge. Adrenal progesterone appears to play a role in potentiating the LH surge since RU486 still effectively decreased the LH surge even in animals ovariectomized at 0800 h on proestrus. The administration of ACTH to estrogen-primed ovariectomized (ovx) immature rats caused a LH and FSH surge 6 h later, demonstrating that upon proper stimulation, the adrenal can induce gonadotropin surges. The effect was specific for ACTH, required estrogen priming, and was blocked by adrenalectomy or RU486, but not by ovariectomy. Certain corticosteroids, most notably deoxycorticosterone and triamcinolone acetonide, were found to possess "progestin-like" activity in the induction of LH and FSH surges in estrogen-primed ovx rats. In contrast, corticosterone and dexamethasone caused a preferential release of FSH, but not LH. Progesterone-induced surges of LH and FSH appear to require an intact N-methyl-D-aspartate (NMDA) neurotransmission line, since administration of the NMDA receptor antagonist, MK801, blocked the ability of progesterone to induce LH and FSH surges. Similarly, NMDA neurotransmission appears to be a critical component in the expression of the preovulatory gonadotropin surge since administration of MK801 during the critical period significantly diminished the LH and PRL surge in the cycling adult rat. FSH levels were lowered by MK801 treatment, but the effect was not statistically significant. The progesterone-induced gonadotropin surge appears to also involve mediation through NPY and catecholamine systems. Immediately preceding the onset of the LH and FSH surge in progesterone-treated estrogen-primed ovx. rats, there was a significant elevation of MBH and POA GnRH and NPY levels, which was followed by a significant fall at the onset of the LH surge. The effect of progesterone on inducing LH and FSH surges also appears to involve alpha 1 and alpha 2 adrenergic neuron activation since prazosin and yohimbine (alpha 1 and 2 blockers, respectively) but not propranolol (a beta-blocker) abolished the ability of progesterone to induce LH and FSH surges. Progesterone also caused a dose-dependent decrease in occupied nuclear estradiol receptors in the pituitary.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of naloxone and beta-casomorphin on the hypothalamic-pituitary-luteinizing hormone axis in vitro

The effect of naloxone and beta-casomorphin on luteinizing hormone (LH) release from pituitary cell aggregates, obtained by three-dimensional culture, with or without mediobasal hypothalamic fragments was studied in vitro. Short-term naloxone perifusion at a concentration of 10(-5)M did not modify either basal or LHRH-stimulated LH release from the pituitary cell aggregates. In contrast, a 12-min naloxone perifusion at the same concentration caused an increase in LH release in the mediobasal hypothalamic-pituitary cell aggregate axis. This increase was rapid (12-16 min after time pulse), marked [up to 10 times (p less than 0.004) the initial base line], short (return to the base line secretion 32-40 min after the beginning of the time pulse) and dose-dependent, with a rise greater than 1000% at a concentration of 10(-4) (p less than 0.006). The same effect was observed when a second pulse was applied 48 min after the first one. LH release induced by naloxone was antagonized 56 +/- 2% (p less than 0.03) by beta-casomorphin (an exogenous opiate) at a concentration of 10(-5) M. beta-casomorphin alone did not modify LH basal secretion, but inhibited 25.1 +/- 2.4% (p less than 0.008) LH release enhanced by LHRH. These results indicate that naloxone, an opiate antagonist, markedly increases LH release via a mu-type opioid receptor mechanism at the hypothalamic level only, during short-term exposure.

Testosterone and postnatal ontogenesis of hypothalamic mu ([3H]dihydromorphine) opioid receptors in the rat

Brain opioids modulate the activity of the hypothalamo-pituitary complex by binding to specific receptors which have been subdivided at least into 3 subclasses (mu, kappa, delta, etc). mu-Receptors and their ligands seem to be particularly involved in the control of gonadotropin and prolactin release. It is known that the neuroendocrine system, as well as the brain opioid systems and their receptors, are not fully mature at birth; it is also known that the postnatal maturation of many brain machineries is under the control of androgens secreted by the developing testes. Consequently, it has been investigated whether the presence or the absence of testosterone at time of birth may induce changes of the binding characteristics of hypothalamic mu-opioid receptors. The experiments have been performed by evaluating the maximal binding capacity (Bmax, an index of the number of receptors), and the affinity constant (Ka) of the specific mu-ligand dihydromorphine in hypothalamic plasma membrane preparations derived from normal male rats, normal female rats, male rats orchidectomized 2 days after birth and female rats treated 2 days after birth with 1.25 mg of testosterone propionate. Animals belonging to the 4 groups were killed at days 16, 26 and 60 of age. The results obtained show that, at 16 days of age, in the 4 groups of rats the number of hypothalamic mu receptors is identical. At 26 days a significant increase in the number of mu-receptors occurs in normal female animals, while their levels remain similar to those found at 16 days in the other 3 groups of animals. At 60 days of age, the number of mu-receptors in normal females remains elevated, while the number of mu-receptors increases to reach normal female levels in the hypothalamus of neonatally castrated males. At 60 days, there were no changes in normal males or in androgenized females. The variations here reported took place without any change of the Ka of dihydromorphine for the mu-receptors. These data show a sexual dimorphism of hypothalamic mu-receptors and suggest that their ontogenetic development may be linked to the presence or the absence of androgens at time of birth.

Effects of Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) on GH, LH, prolactin, FSH, and TSH secretion in rats with and without morphine

Plasma concentrations of GH, LH, prolactin, FSH, and TSH were evaluated in adult rats after administration of Tyr-MIF-1. Male rats were killed 0, 15, 30, and 60 min after Tyr-MIF-1 (0.02, 0.2 and 2.0 mg/kg) and ovariectomized females 15 min after injection of the peptide (0.2 and 2.0 mg/kg). The effect of Tyr-MIF-1 on pituitary hormonal secretion in morphine-treated ovariectomized rats also was studied. After 15 min, Tyr-MIF-1 (0.2 mg/kg) increased plasma concentrations of LH in males (p less than 0.05) and, at 2.0 mg/kg, in ovariectomized rats (p less than 0.05). Tyr-MIF-1 (0.2 mg/kg) decreased plasma concentrations of GH as compared with diluent at 15 min in males (p less than 0.05) but was ineffective in ovariectomized females not receiving morphine. Plasma concentrations of prolactin, FSH, and TSH remained unchanged both in males and in ovariectomized females by any of the administered doses of the peptide at any of the times tested. When administered to ovariectomized rats injected earlier with morphine sulfate, Tyr-MIF-1 (0.2 and 2.0 mg/kg) reduced (p less than 0.05) the effect of morphine (5 mg/kg) on GH secretion and tended (p = 0.061) to partially inhibit the effect of morphine (10 mg/kg) on prolactin secretion at a dose of 2.0 mg/kg. The decrease in plasma concentrations of TSH after morphine at a dose of 10 mg/kg (p less than 0.001) remained unaffected by any of administered doses of Tyr-MIF-1. The results suggest that Tyr-MIF-1 may affect the regulation of the secretion of some anterior pituitary hormones.

Modulation of the binding characteristics of hypothalamic mu opioid receptors in rats by gonadal steroids

Recent evidence suggests that the effects of the opioids on gonadotropin release may depend on the endocrine status existing in the experimental animal. In the brain, the effects of the opioids are exerted through the interaction with different classes of opioid receptors (mu, delta, kappa, etc.). Among these, the mu receptors appear to be particularly relevant to the control of gonadotropin secretion. Different groups of experiments have been performed in the rat in order to analyze whether changes of circulating levels of sex steroids may have an impact on the binding characteristics of hypothalamic mu opioid receptors, as evaluated by a receptor binding assay performed on plasma membrane preparations, using [3H]dihydromorphine as a mu ligand. In a first series of experiments, it has been observed that the ontogenesis of hypothalamic mu opioid receptors is different in male and in female rats: the concentration of mu sites, similar in animals of the two sexes at 16 days of age, increases in females, but not in males, between day 16 and day 26 of life. This sexual difference persists in 60-day old animals, when the brain is fully mature. It has also been observed that the pattern of maturation of hypothalamic mu receptors can be reversed by neonatal castration of males and by neonatal testosterone treatment of females. In a second series of experiments, it has been shown that in the hypothalamus of regularly cycling female rats the concentration of mu receptors varies during the different phases of the estrous cycle. In particular, a rather high density of mu sites during diestrus day 2 and the morning of the day of proestrus was found; this is followed by a progressive decline during the afternoon of the day of proestrus and the day of estrus, with a minimum value of the concentration of mu receptors being recorded in the first day of diestrus. These fluctuations seem to be linked to the physiological changes of serum levels of ovarian steroids: in fact, in a third series of experiments, it has been found that the positive feedback effect on LH release, exerted by the treatment of ovariectomized female rats with estrogens plus progesterone, is accompanied by a significant decrease of the concentration of hypothalamic mu opioid receptors; treatments with estrogens alone, able to induce a negative feedback effect on LH secretion, are not associated with modifications of hypothalamic mu receptors. These data seem to indicate that hypothalamic mu receptors may be involved in the positive but not in the negative feedback control of LH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Ontogenic studies of the neural control of adenohypophyseal hormones in the rat: gonadotropins

1. Serotonergic, dopaminergic, and opioid systems controlling luteinizing hormone (LH) and follicle stimulating hormone (FSH) secretion develop with particular characteristics in the male and female prepubertal rats. 2. Serotonergic pathways evoke a maximal release of LH and FSH in female rats from day 12 to day 20 of age, but not in males of the same age. 3. Antidopaminergic drugs increase LH and FSH levels only in the female infantile rats. This effect is absent at birth and disappears after 20 days of age. 4. Naloxone markedly increases gonadotropins in 12-day-old females. 5. On the other hand, in 12-day-old male rats some neurotropic drugs such as diazepam could enhance LH levels, the effect being absent at other ages or in female littermates. 6. A period of high sensitivity of gonadotropins to neurotropic drugs is present during the second and third weeks of life of the rat and it is related to the sexual differentiation of the brain.

Opioids and the developing organism: a comprehensive bibliography, 1984-1988

A comprehensive bibliography of the literature concerned with opioids and the developing organism for 1984-1988 is presented. Utilized with companion papers (Neurosci. Biobehav. Rev. 6:439-479; 1982; 8:387-403; 1984), these articles cover the clinical and laboratory references beginning in 1875. For the years 1984, 1985, 1986, 1987, and 1988, a total of 877 citations were recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics, and subdivided into such topics as the type of opioid explored and the general area of biological interest (e.g., physiology).

Modulation by sex steroids of brain opioid receptors: implications for the control of gonadotropins and prolactin secretion

Several experiments have been performed in order to analyze whether physiological or experimental changes of the endocrine environment might modify the binding characteristics of brain mu and kappa opioid receptors in the brain of the female and male rat. (a) In a first series of experiments, it has been observed that in the whole brain of regularly cycling female rats the number of mu receptors shows variation during the different phases of the estrous cycle. In particular a significant increase of the number of mu receptors has been observed in the morning of proestrus and in the afternoon of estrus. (b) In a second series of experiments, it has been shown that the administration of estrogens brings about a significant increase in the number of mu receptors in the hippocampus and in the thalamus of ovariectomized rats, while the administration of a regime including estrogen and progesterone induces a significant decrease of the number of mu receptors in the hypothalamus and in the corpus striatum. These data seem to indicate that hypothalamic mu receptors may be involved in the positive but not in the negative feedback control of LH secretion. (c) In a third series of experiments, it has been found that the number of mu receptors in the whole brain of 15- and 22-month-old male rats and in the hypothalamus of 22-month-old male rats is significantly lower than in the same tissues of young animals; moreover, the administration to old animals of testosterone does not modify the number of hypothalamic mu opioid receptors, indicating that the decline of brain mu receptors in old animals is not the consequence of the physiological decline of testosterone secretion but probably represents an autonomous phenomenon. (d) In a fourth series of experiments, it was shown that, in young male rats, the concentration of kappa receptors is extremely variable in different regions of the brain. The highest concentrations have been found in the hypothalamus and in the striatum; also in the mesencephalon and in the amygdala kappa receptors are present in rather elevated quantities; lower concentrations have been found in the thalamus, the frontal poles, the hippocampus and in the anterior and posterior cerebral cortex. These experiments have shown in addition that the process of aging induces an increase of the number of kappa receptors in the amygdala and in the thalamus; no age-linked modifications were observed in the other structures examined.(ABSTRACT TRUNCATED AT 400 WORDS)

Developmental changes in FSH secretion induced by 5-hydroxytryptophan, naloxone and haloperidol in male and female rats

Follicle-stimulating hormone (FSH) secretion is increased in the immature female rat from day 5 to days 17-18 of life, and decreases steadily thereafter until puberty. It has been reported that estradiol negative feedback and inhibin-like peptides are low during this period, while luteinizing hormone (LH) and FSH sensitivity to LH-releasing hormone (LHRH) are maximal. It was therefore of interest to study the effects of some neurotropic drugs on FSH release at 12 days of age, and to compare their effects at 1 and 20 days. Besides, as developmental patterns and regulation of FSH are different in male and female rats, the experiments were carried out using male and female littermates. The drugs chosen were haloperidol, 5-hydroxytryptophan and naloxone. These drugs release LH in the infantile female rat, the effect decreasing or disappearing as the animal matures; no effects of these drugs have been reported on FSH release in infantile rats to the present time. It was found that haloperidol (0.25 mg/kg), naloxone (2 mg/kg) and 5-hydroxytryptophan (50 mg/kg) markedly increased the already high titers of FSH in the 12-day-old female rat. This effect could not be discerned in newborn rats, and had disappeared at 20 days of age. Male littermates failed to respond at any age. When adult male and female rats in diestrus were tested, all drugs at the chosen doses were ineffective in altering FSH release. These data suggest that the infantile female rat represents an interesting physiological model to evaluate the neural regulation of FSH in a situation in which inhibitory signals provided by inhibin and estrogen in later life are diminished.

Delayed suppression of serum luteinizing hormone after naloxone treatment in neonatal female rats

In female neonatal rats, opiate receptor blockade markedly raises serum luteinizing hormone (LH) levels. The LH effect of acute treatment with opiate antagonists is apparently brief in older rats; however, age-related differences in antagonist pharmacokinetics may result in different LH response patterns. The duration of LH response to naloxone (NAL) and naltrexone (NTX) was examined in 5 day-old (d.o.) female rats and compared to the duration of analgesia blockade. The rise in serum LH following opiate receptor blockade in 5 d.o. rats was of similar duration to that previously observed in older animals and much briefer than blockade of analgesia. Furthermore, neonatal rats exhibited a delayed suppression of LH 6 hr following NAL, but not NTX, treatment. Stimulation and later suppression of LH were still observed after five repetitive NAL treatments at 6 hr intervals.

Analysis of estradiol-independent and -dependent endogenous opioid peptide suppression of pulsatile LH release between the mornings of diestrus 2 and proestrus in the rat estrous cycle

The aim of this study was to analyze possible estradiol (E2)-independent and -dependent endogenous opioid peptide (EOP) suppression of pulsatile LH release between the mornings of diestrus 2 (D2) and proestrus by examining the LH response to naloxone infusions in the presence or absence of proestrous levels of E2. Pulsatile LH secretion remained unchanged between D2 and proestrus but mean blood LH levels, pulse amplitude and frequency increased within 24 hr following ovariectomy on D2. This increase was due in large part to the loss of E2 negative feedback, since restoration of physiological proestrous E2 levels returned LH pulse frequency to proestrous a.m. levels and greatly reduced pulse amplitude. In ovariectomized rats lacking E2 negative feedback, continuous infusion of the EOP receptor antagonist naloxone (0.5 and 2 mg/kg/hr) caused a further increase in pulse amplitude and frequency. This naloxone-induced increment in pulsatile LH release was exerted via centrally located EOP receptors since naloxone did not alter pituitary responsiveness to LHRH, and its stimulatory action on pulsatile release was diminished by simultaneous infusion with morphine. Naloxone also increased pulsatile LH release in E2-treated animals. The naloxone-induced increments in LH pulse amplitude were the same in the presence or absence of E2 negative feedback. Moreover, the increments in amplitude produced by naloxone in E2-treated rats were significantly less than those resulting from the combination of ovariectomy plus naloxone infusion in empty capsule-implanted rats. These data indicated that naloxone infusion in E2-implanted animals blocked an E2-independent EOP suppression of this parameter of pulsatile release.(ABSTRACT TRUNCATED AT 250 WORDS)

Steroid-independent endogenous opioid peptide suppression of pulsatile luteinizing hormone release between estrus and diestrus in the rat estrous cycle

We have previously demonstrated an absence of ovarian steroid negative feedback on pulsatile luteinizing hormone (LH) release between estrus and early diestrus 1 (D1) in the rat estrous cycle. The object of the present study was to determine if there was a steroid-independent endogenous opioid peptide (EOP) suppression of pulsatile LH release in this same 24-h interval, and if so, which parameter(s) of pulsatile LH release were affected. Rats were bled on estrus, or 24 h following sham ovariectomy (OVX), or OVX at 08.30-10.00 h on estrus. At the time of bleeding all rats were infused i.v. for 4 h either with 0.9% saline (0.5 ml/h) or naloxone (0.005, 0.05, 0.5, or 2 mg/kg/h). At 1 h after the infusion began, rats were bled for 3 h (40 or 50 microliters whole blood/5 min) between 09.30 and 12.30 h. Mean blood LH levels increased between estrus and early D1 due to increases in LH pulse amplitude and frequency. OVX on estrus decreased plasma levels of estradiol and progesterone 24 h later, but did not augment the increase in pulsatile LH release. However, naloxone infusion augmented the increase in pulsatile LH secretion in sham ovariectomized rats in a dose-dependent fashion. While infusion of 0.005 or 0.05 mg/kg/h had no effect, 0.5 or 2 mg/kg/h increased blood LH levels by increasing both LH pulse amplitude and frequency. The stimulatory effect of naloxone on pulsatile LH release was blocked by simultaneous infusion of morphine, demonstrating that the effect was mediated by EOP receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Gonadal steroid modulation of brain opioid systems

It is becoming increasingly clear that the effects of the opioids and their synthetic analogs on anterior pituitary function largely depend on the steroid milieu present in the animal at time of drug administration. However, it is still unclear whether gonadal steroids regulate the opioid-modulated mechanisms by affecting the number of opiate receptors in the brain. To further investigate these issues, the effects of opiate agonists and antagonists on LH, FSH and prolactin (Prl) secretion have been studied in: (a) normal and castrated male rats, and (b) normally cycling female rats. The binding characteristics of the brain subclass of mu opiate receptors have been analyzed in the same group of experimental animals; this type of receptors seems to be particularly involved in the control of gonadotropin and Prl release. When injected intraventricularly into normal male rats, morphine (200 micrograms/rat) induced in a significant elevation of serum LH levels at 10 and 20 min. In long-term castrated animals the administration of the drug significantly reduced LH secretion at 40 and 60 min after the injection, the inhibition lasted up to 180 min. Morphine, when given intraventricularly to normal males, induced a conspicuous and significant elevation of serum Prl levels at 10, 20, 40 and 60 min after treatment. However, when the drug was administered to castrated rats, it did not significantly affect Prl release at any time interval considered. Morphine intraventricular injections did not modify serum FSH levels either in normal or in castrated male rats. The concentration of mu opiate receptors was found to be similar when measured in the whole brain of normal and orchidectomized rats. In adult cycling female rats, s.c. injections of naloxone (2.5 mg/kg) stimulated LH release in every phase of the estrous cycle; the magnitude of the responses was highly variable, being particularly elevated at 16.00 h of the day of proestrous and at 10.00, 12.00 and 14.00 h of the day of estrous. Conversely, LH response to naloxone was totally obliterated at 18.00 and 20.00 h of the day of proestrous, when the preovulatory LH surge was found to occur. The concentration of brain opiate receptors of the mu type showed significant variations during the different phases of the estrous cycle, with higher levels at 12.00 h of the day of proestrous and at 18.00 h of the day of estrous.(ABSTRACT TRUNCATED AT 400 WORDS)

Steroidal modulation of the regulatory neuropeptides: luteinizing hormone releasing hormone, neuropeptide Y and endogenous opioid peptides

Recent studies show that a large number of neuropeptides may play important roles in the hypothalamic control of reproduction and related sexual, feeding and locomotor behaviours. Based on the evidence summarized here we propose that gonadal steroids may exert a "trophic" influence on the regulatory peptides namely LHRH, NPY and EOP locally in the hypothalamus.

Opioid control of LH secretion in humans: menstrual cycle, menopause and aging reduce effect of naloxone but not of morphine

A number of studies have been made on the role played by endogenous opioid peptides in the secretion of LH in humans. However no previous studies have compared the effects of the most potent pharmacological agonist and antagonist, morphine and naloxone, in the same subjects. The present study examined the acute effects of injections of morphine and naloxone on plasma LH levels in 30 healthy subjects (18 women and 12 men). Fertile women were subdivided into follicular (n = 6) and luteal (n = 6) phase groups; the remaining 6 were postmenopausal women. The 12 men were sub-divided in two groups of 6 subjects according to age (24-33 years, and over 60 years). There was a two day interval between injection studies in the same subjects. Morphine significantly decreased plasma LH levels in all groups examined (P less than 0.01). On the other hand, naloxone caused a significant increase in plasma LH levels in fertile women during the luteal phase of the cycle, but not during the follicular phase or in postmenopausal subjects, and in young but not in aged men (P less than 0.01). These results indicate that in humans there is a change in the activity of the opioids regulating LH secretion during the menstrual cycle, after menopause and in aged men and that these may be studied by the use of naloxone. The inability of naloxone under certain conditions to increase LH levels reflects the decreased activity of the endogenous system, while morphine, being active in all the subjects, seems to be less discriminative, at least in physiological conditions.