Effect of norepinephrine synthesis inhibitors and a dopamine agonist on hypothalamic LH-RH serum gonadotrophin and prolactin levels in gonadal steroid treated rats

Noradrenergic regulation of prolactin secretion at the level of the paraventricular nucleus of the hypothalamus: functional significance of the alpha-1b and beta-adrenergic receptor subtypes

Previous research has demonstrated that in the Siberian hamster, both photoperiod and estrous cyclicity alter the profile of noradrenergic activity with the paraventricular nucleus of the hypothalamus (PVN), and that noradrenergic activity is correlated with changes in circulating levels of prolactin. Work from our laboratory has demonstrated an inhibitory role for norepinephrine (NE) acting at the alpha-2 receptor subtype within the PVN on serum prolactin levels; however, the functional significance of other adrenergic receptor subtypes on this system is unknown. The purpose of this study was to investigate the functional significance of the alpha-1b and beta-adrenergic receptor subtypes at the level of the PVN on circulating levels of prolactin. These experiments were performed in male Siberian hamsters using reverse microdialysis coupled with serial blood sampling. In Experiment 1, infusion of l-phenylephrine hydrochloride (alpha-1b agonist) initiated a dose-dependent increase in circulating prolactin, whereas infusion of chloroethylclonidine (alpha-1b antagonist) induced a significant dose-dependent decline in prolactin. In Experiment 2, intraparaventricular administration of propranolol (beta antagonist) initiated a significant increase in prolactin levels in a dose-dependent manner, whereas isoproterenol (beta agonist) induced a dose-dependent decline in prolactin. The results of this study indicate that both the alpha-1b and beta-adrenergic receptor subtypes have a significant role in regulating circulating levels of prolactin at the level of the PVN in the Siberian hamster.

Interactions between opioid peptides and adrenaline-containing neurons modulate luteinizing hormone secretion in male rats

Abstract There is increasing evidence that the opioid inhibition of luteinizing hormone (LH) secretion is mediated, at least in part, by catecholaminergic mechanisms. This study determined the effects of selective manipulation of noradrenergic and adrenergic systems on the ability of opiate receptor blockade to induce the release of LH in adult male rats. Selective depletion of hypothalamic noradrenaline levels by 80% following 6-hydroxydopamine infusions into the central tegmental tract did not alter the 2- to 3-fold increase in serum LH levels following opiate receptor blockade with naloxone (2.5 mg/kg). In contrast, both selective depletion of hypothalamic adrenaline by prior treatment with the phenylethanolamine N-methyltransferase inhibitor, LY134046 (2 x 50 mg/kg) and non-selective depletion of all three catecholamines with alpha-methyl-p-tyrosine (250 mg/kg), abolished the naloxone-induced increase in LH. These results suggest that the inhibition of LH secretion by endogenous opioid peptides is influenced by catecholaminergic neurotransmission and further support the view that adrenaline rather than noradrenaline or dopamine is of importance in this context.

Dynamic changes in neuropeptide y concentrations in the median eminence in association with preovulatory luteinizing hormone release in the rat

Abstract Neuropeptide Y (NPY) stimulates luteinizing hormone (LH) release in the rat by a dual action, one in the hypothalamus to excite LH-releasing hormone (LHRH) release and the other at the level of pituitary gonadotrophs to activate and/or potentiate LH release induced by LHRH. Because NPY produces effects similar to norepinephrine with which it may comprise an excitatory hypothalamic circuit, it was hypothesized that NPY concentrations in the hypothalamus would change in a time- and site-specific manner in association with the preovulatory LH surge on proestrus. Concentrations of NPY in individual nuclei of the preoptic-tuberal pathway and serum LH levels were estimated by radioimmunoassays in rats during diestrus 2 and proestrus. On proestrus, serum LH levels were basal between 1000 and 1400 h, rose significantly at 1500 h and plateaued between 1600 and 1800 h. Of the five neural sites examined, only NPY in the median eminence displayed marked fluctuations in close association with the LH surge. NPY concentrations were low between 1000 and 1300 h, and rose abruptly at 1400 h (P<0.05) preceding the onset of LH rise at 1500 h. These elevated levels were maintained until 1600 h, during which time serum LH rose to a plateau and then fell at 1800 h to the low range seen between 1000 and 1300 h. In contrast, the pattern of changes in NPY levels in the arcuate nucleus, suprachiasmatic nucleus and medial preoptic area, three additional sites in the preoptic-tuberal pathway known to participate in the preovulatory LH surge, were markedly different from that seen in the median eminence. In each of these three sites, NPY levels rose significantly at 1800 h from the values at 1000 to 1200 h with a slightly different time-course of increment. None of these areas exhibited changes in NPY concentrations on diestrus 2; NPY concentrations also were unaltered on diestrus 2 or proestrus in the ventromedial nucleus. The present observations of site-specific, dynamic changes in NPY levels on proestrus, in a manner previously documented for LHRH, support the hypothesis that a subset of NPY neurons terminating in the median eminence may be a component of excitatory neural circuitry that either independently or in co-action with the adrenergic system is responsible for the induction of preovulatory LH release.

Neural circuits involved in the control of LHRH secretion: a model for estrous cycle regulation

These studies have identified a core of hypothalamic neuronal systems in the regulation of episodic basal and preovulatory LH release during the estrous cycle. Steroid concentrating neurons under the direction of ovarian steroid milieu promote LHRH accumulation and either independently or in association with the opioid peptide neurons modulate the episodic LHRH discharge. The sequence of neural events that occur preceding the preovulatory LH surge on the afternoon of proestrus are described. Evidence is presented to support the hypothesis that a transient curtailment in the inhibitory influence of hypothalamic opioid peptides in the morning of proestrus, sets in motion a chain of temporally-related events starting with augmentation of adrenergic tone, leading to accumulation of neuropeptide Y and LHRH in the median eminence and culminating in episodic LHRH and LH hypersecretion in the afternoon of proestrus.

Hypothalamic tyrosine hydroxylase activity and plasma gonadotropin and prolactin levels in ovariectomized-steroid treated rats

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase (TH) activity were evaluated in ovariectomized (OVX) estradiol benzoate (EB) or progesterone (P) treated rats. Single injection of 10 micrograms or daily injection of 5 micrograms EB/rat for 7 days significantly lowered gonadotropin levels in OVX animals and elevated PRL levels. Single injection of 2 mg or daily injection of 200 micrograms P/rat for 7 days increased gonadotropin and PRL levels. Hypothalamic TH activity was significantly elevated by estradiol. Single injection of 2 mg P suppressed TH activity in contrast to the elevation in enzyme activity following chronic treatment. These results indicate that hypothalamic noradrenergic as well as dopaminergic neurons participate in the stimulatory or inhibitory feedback effects of ovarian hormones on gonadotropin and PRL secretion.

Release of gonadotropin-releasing hormone from the Japanese quail hypothalamus in vitro

The regulation of GnRH (gonadotropin-releasing hormone) secretion in Japanese quail was studied by maintaining excised hypothalamic slices containing the median eminence region in a continuous flow superfusion system. GnRH released into the superfusate was measured by radioimmunoassay using an antibody generated against synthetic GnRH. GnRH secretion increased in a dose-response manner when hypothalamic slices from female quail were exposed to superfusion medium containing elevated potassium ion concentrations. The potassium-induced GnRH release was found to be calcium dependent. GnRH was also increased when either 1 X 10(-6) M norepinephrine, epinephrine, or 1 X 10(-5) M of the beta-adrenergic agonist, isoproterenol was added to the superfusion medium. GnRH secretion from hypothalami of castrate and intact male quail was measured on Days 1, 10, and 21 of photostimulation and was elevated on Day 21. No differences were observed between the castrate and intact groups. It is concluded that GnRH is released from the quail hypothalamus in a manner analogous to mammals and that is release is subject to adrenergic stimulation.

beta-Endorphin regulates lordosis in female rats by modulating LH-RH release

Several lines of evidence have implicated the endogenous opioid peptides in the regulation of masculine sexual behaviour. However, although the opioid related peptides alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropin (ACTH) have been shown to affect lordosis behaviour in the female rat, there is as yet no evidence for a role of the endogenous opiates in the regulation of female sexual behaviour. We present here evidence that the endogenous opiates in the mesencephalic central grey (MCG) are involved in the control of lordosis behaviour in the female rat.

Electron microscopic study of immunoreactive LHRH perikarya with special reference to neuronal regulation

In early postnatal rats, immunoreactive LHRH perikarya in the preoptic area were studied by light and electron microscopy. Synaptic junctions were found between the immunoreactive perikaryon or its process, and the immunonegative nerve fibers. The significance of these synapses is discussed in relation to possible mechanisms by which the activities of LHRH neurons are regulated.

Central noradrenergic neurones concentrate 3H-oestradiol

Central catecholamines play an important part in the regulation of hormone secretion from the pituitary gland and in the mediation of male and female sexual behaviour. Noradrenaline has been shown to stimulate the release of luteinizing hormone (LH) probably by influencing the secretion of LH-releasing hormone (LHRH). The medial preoptic and hypothalamic areas, including the median eminence, contain noradrenaline-containing terminals which originate from discrete noradrenaline-containing cell groups in the lower brain stem. These cell groups have been identified in the pons and the medulla oblongata by histochemical methods and pharmacological experiments. Recent studies with antiserum to dopamine-beta-hydroxylase (DBH), the enzyme that converts dopamine to noradrenaline, provided evidence for the existence of noradrenergic cells. These include the locus coeruleus (group A6), a ventrally located more diffuse but continuous subcoeruleus group (group A5), a cell group located dorsal to the nucleus (n.) dorsalis motorius nervi vagi (group A2) and a cell group in or near the nucleus reticularis lateralis (group A1). Using the thaw-mount autoradiographic technique, oestradiol-concentrating neurones have been localized in many areas of the lower brain stem, including the locus coeruleus, n. tractus solitarii, n. dorsalis motorius nervi vagi, and reticular formation where catecholamine-containing neurones exist. We report here the simultaneous localization, in the same histological section, of 3H-oestradiol and the enzyme dopamine-beta-hydroxylase in neurones of the rat lower brain stem with a combined technique of thaw-mount autoradiography and immunohistochemistry, demonstrating that noradrenaline- or adrenaline-containing neurones are oestradiol target cells.

The effect of LHRH antagonist analogs and an antibody to LHRH on mating behavior in female rats

The action of two antagonist analogs and an antibody to luteinizing hormone-releasing hormone (LHRH) on sexual receptivity was studied in avariectomized, estrogen-progesterone primed female rats. Small amounts of each LHRH substance or saline was infused through a cannula positioned in either the third ventricle or arcuate-ventromedial (ARC-VMH) area of the hypothalamus. Infusions were carried out at the time of progesterone priming, which was 42 hrs post-estrogen treatment, and sexual receptivity, as denoted by the lordosis-to-mount ratio, was measured six hrs later. One antagonist analog, [D-pGlu1, D-Phe2, D-Trp3,6]-LHRH[1], had little or no effect on sexual receptivity when tested in either site. Similarly, an antibody to LHRH, tested only in the ARC-VMH, had no observable effect on lordotic behavior. However, the second and the most potent antagonist analog, [Ac-dehydro-Pro1, pCl-D-Phe2, D-Trp3,6]-LHRH[2], produced a marked and significant decrement in lordotic behavior when infused into either the third ventricle or ARC-VMH. These results suggest that this potent and long-acting, competitive antagonist analog of LHRH prevented endogenous LHRH from exerting its normal role in the induction of sexual receptivity and provide evidence to support the contention that the role of LHRH in mediating receptivity in the female rat is physiologically relevant.

Progesterone administration in vivo stimulates release of luteinizing hormone-releasing hormone in vitro

The release of luteinizing hormone-releasing hormone (LHRH) from tissue from the mediobasal hypothalamic-anterior hypothalamic-preoptic area of prepuberal female rats was measured in a perfusion system. Measurements were also made of the concentrations of LHRH in these tissue fragments and of luteinizing hormone in serum obtained when the rats were killed. Four groups of immature rats were studied: intact, ovariectomized, ovariectomized and implanted with estradiol-containing capsules, and ovariectomized rats primed with estradiol and injected with progesterone. The release of LHRH from the tissue of ovariectomized animals was significantly less than that of intact females and was not modified when the ovariectomized rats received estradiol. However, there was a four- to fivefold increase in LHRH release from tissue of ovariectomized rats primed with estradiol when they were killed 6 hours after they received an injection of progesterone. The concentrations of LHRH in tissue and of luteinizing hormone in serum varied among groups and with the time of day that the animals were killed. The interactions among luteinizing hormone, gonadal steroids, and the photoperiod seem to set the appropriate conditions for neural processes triggering a complete and normal release of luteinizing hormone.