The stimulatory effect of beta-endorphin on the plasma prolactin levels was diminished in the rats treated with 6-hydroxydopamine

Mechanisms for the stimulatory effects of opioidergic and serotonergic input signals on prolactin in pregnant rats

Dopamine (DA) neurons participate in tonic inhibition of prolactin (PRL), whereas beta-endorphin (beta-End) and serotonin (5-HT) neurons appear to be important stimulatory links for nocturnal PRL surges that occur throughout the first half of pregnancy in the rat. The purpose of this study was to determine how these neuronal components might be organized within the pathway controlling PRL release during gestation. Maximal stimulation of DA receptors with the agonist bromocriptine mesylate (Bromo) completely blocked the PRL response to beta-End (100 ng/microliters/min for 15 min) given intracerebroventricularly (i.c.v.) on day 8 of pregnancy. DA receptor blockade, produced by implanting a 25 mg pellet of haloperidol (Hal) on day 7 of pregnancy, resulted in PRL levels of 500-600 ng/ml by the following morning. beta-End i.c.v. or 250 mg/ml/kg BW of the DA synthesis inhibitor, alpha-methyl-p-tyrosine (alpha-MPT), given during the intersurge period, were equally effective in significantly increasing PRL (p less than 0.01) above pretreatment levels. beta-End and alpha-MPT evoked similar increases in rats pretreated with Hal, suggesting the stimulatory effect of beta-End on nocturnal PRL surges may primarily be due to DA inhibition. The next objective was to determine how beta-End and 5-HT might interact to stimulate the nocturnal surge. Day 8 pregnant rats were infused continuously with the opioid receptor blocker, naloxone hydrochloride (Nal), at a rate of 2.0 mg/10 min from 1000-1300 h. The PRL response to an injection of 20 mg/kg BW 5-hydroxytryptophan (5-HTP) at 1200 h was greatly attenuated, compared to controls infused with saline instead of Nal. This suggests that 5-HT stimulates PRL, at least in part, by an action at opioid receptors. Distilled H2O or 10 mg/kg BW of the selective S2 receptor blocker, ketanserin tartrate (Ket), was given intraperitoneally (i.p.) during the intersurge period on day 8 of pregnancy. All animals demonstrated an identical response to beta-End given 2 hours later, regardless of the type of pretreatment. It appears that beta-End does not stimulate PRL by way of an S2 receptor. Although beta-End induced a significant increase in PRL on day 16 of pregnancy, the response was attenuated by more than 60% compared to the response on day 8 of pregnancy. This attenuation may involve placental lactogens, shown to be secreted during this time and to inhibit PRL secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of taurine on growth hormone and prolactin secretion in rats: possible interaction with opioid peptidergic system

The effect of taurine on growth hormone (GH) and prolactin (PRL) secretion was investigated in the urethane-alpha-chloralose anesthetized rats, considering the interaction with endogenous opioid peptidergic system. Intraventricular injection of taurine (0.25 and 1.0 mumol) stimulated GH and PRL secretion in a dose-dependent manner. However, 4.0 mumol taurine failed to show these effect. The intravenous infusion of naloxone (4 mg/kg b.w.) completely inhibited both the GH and PRL secretion induced by taurine (1.0 mumol). The combined treatment of taurine (1.0 mumol) and FK33-824 (Met-enkephalin derivative, 100 micrograms/kg b.w., i.v.) significantly increased GH and PRL responses induced by taurine or FK33-824 alone. These results indicate that taurine is an effective stimulator of GH and PRL secretion in rats, and that the mechanism of this action involves the opioid peptidergic system in the hypothalamus.

Influences of water deprivation and fasting on hypothalamic, pituitary and plasma opioid peptides and prolactin in rats

To study the physiological roles of endogenous opioid peptides in drinking and feeding behaviors, the effects of water deprivation and fasting on plasma immunoreactive (IR) beta-endorphin (beta-end), IR-Antidiuretic hormone (ADH) and IR-Prolactin (Prl), pituitary IR-beta-end and IR-methionine-enkephalin (IR-Met-enk) and IR-ADH, and hypothalamic IR-beta-end and IR-Met-enk were observed in rats. The effects of water deprivation on hypothalamic dopaminergic system was also studied. In water deprived rats, plasma IR-beta-end and Prl were decreased significantly. In the neurointermediate lobe, IR-Met-enk, but not IR-beta-end, was decreased, although these peptides did not change in the anterior lobe and hypothalamus. Intraperitoneal injection of haloperidol reversed the decrease in plasma IR-beta-end in water deprived rats but did not change it in control rats. Subcutaneous injection of CB-154, on the other hand, decreased the plasma IR-beta-end in control rats but not in water deprived rats. The dopamine (DA) turnover rate in hypothalamus, in addition, was increased in water deprived rats as compared with controls. In fasted rats, IR-beta-end in plasma, but not in pituitary lobes and hypothalamus, was increased. The present results suggest that the increase of hypothalamic dopaminergic activity, in part, is related to the suppressed secretions of pituitary IR-beta-end and Prl in water deprivation, and plasma IR-beta-end play some roles in feeding behavior in rats.

Opioidergic regulation of thyroid activity: possible interference with the serotonergic system

Acute systemic administration of (D-Met2, pro5-NH2)-enkephalin (ENKamide), a very potent enkephalin analog, and of morphine not only diminished the basal levels of serum TSH under resting conditions, but also significantly reduced the enhanced serum TSH concentrations induced by goitrogen treatment or by bilateral thyroidectomy. Acute administration of opiates failed to inhibit the pituitary TSH response to exogenous TRH administration. The TSH release-inhibiting effect of ENKamide was reversed by pretreatment with the serotonin synthesis inhibitor, para-chlorophenylalanine (pCPA) or with the central serotonin receptor blocker, metergoline. These results furnish evidence in favour of the following concepts: (a) opioid compounds equally influence the tonic release of the TRH-TSH system under resting conditions, and also suppress the reactive changes of this circuit following specific loads, leading to an activation of this system; (b) opioids do not act directly at the pituitary level in inhibiting TSH secretion, but rather seem to suppress the release of TRH from the hypothalamus; (c) they may exert their inhibitory effect on the activity of the TRH-TSH-thyroid system by increasing the activity of the central nervous serotonergic system.

beta-Endorphin alters dopamine uptake by the dopamine neurons of the hypothalamus and striatum

Opioid peptides have well-documented modulatory effects on the synaptic transmission of several neurotransmitters. In both hypothalamus and striatum there is dense innervation by the beta-endorphin and/or enkephalin neuronal systems, and physiologically relevant neuroregulatory interactions of these neurons occur with other important neurotransmitter neuronal systems, such as the dopaminergic tuberoinfundibular and nigroneostriatal systems. Previous reports have examined the effects of opioid peptides on release, synthesis and degradation of dopamine in these brain regions. In this report, we describe the effects of the intracerebral administration of beta-endorphin to increase dopamine (re)uptake by dopaminergic nerve terminals of the hypothalamus and striatum. The specific, high affinity uptake of [3H]dopamine by dopaminergic nerve terminals was studied in a synaptosomal preparation by pharmacological exclusion, using desmethylimipramine, of dopamine uptake into other monoaminergic nerve terminals. The augmentation of in vitro dopamine uptake in both hypothalamus and striatum following intracisternal administration of beta-endorphin is specifically mediated by opiate receptors, since it could be prevented by pretreatment with an opiate receptor antagonist, naltrexone. In hypothalamus, the increased dopamine uptake represents a primary effect of beta-endorphin on hypothalamic dopamine neurons and is not secondary to the opioid peptide-induced stimulation of prolactin secretion, since identical effects of beta-endorphin administration are seen in hypophysectomized and intact animals. The effect of beta-endorphin to increase hypothalamic dopamine uptake was not reflected by a change in the affinity constant for dopamine, but involved an increase in maximal initial velocity of uptake. Naltrexone blocked the effect of beta-endorphin to increase the Vmax for [3H]dopamine uptake by hypothalamic dopamine neurons in both intact and hypophysectomized rats. In vitro exposure of hypothalamic and striatal dopaminergic nerve terminals to a wide range of concentrations of beta-endorphin failed to reproduce the in vivo results; some concentrations of beta-endorphin produced small decreases in [3H]dopamine uptake which were not reversed by naloxone. The data of this study provide evidence for a further mechanism by which beta-endorphin may alter dopaminergic neurotransmission, namely by increasing dopamine reuptake into dopaminergic nerve endings.

The endocrinology of the opiates and opioid peptides

Since the isolation of the enkephalins five yr ago, there has been an explosive increase in knowledge concerning the effects of the opiates and opioid peptides. This review deals with the interactions of opiates with the endocrine system in rat and man. The opioid peptides have been demonstrated to exert a variety of effects on pituitary hormone secretion in rat and man. In the rat, opiates stimulate growth hormone, prolactin and ACTH release and inhibit the release of the glycoprotein hormones. In man, the physiologic role of the endogenous opiates appears to be involved predominantly in ACTH and gonadotrophin regulation. Opiate effects are mainly exerted at the level of the hypothalamus but further modulating effects may occur at the pituitary and at end-organs. Opiate-induced hormonal effects appear to be mediated through dopaminergic and/or serotonergic mechanisms. Recent studies have also suggested a possible local neuromodulatory role for the opioid peptides in the control of carbohydrate metabolism and reproductive processes.

Endogenous opiates: 1980

Vast amounts of research have been done that have attempted to delineate the pharmacological and physiological effects of the endogenous opiate peptides. A great deal of knowledge has also been accumulated in a limited time span concerning the types and locations of the opiate receptors and peptides, as well as their functions. In 1980, reports were made concerning the effects of these peptides on analgesia, on tolerance and dependence, on activity, on learning and memory, on schizophrenia and other types of emotional disturbances, and on physiological responses such as eating and drinking, cardiovascular responses, and sexual function. Additional understanding was also gained concerning their interactions with neurotransmitters, other neuropeptides, and hormones. These and other studies published only in 1980 are reviewed in this paper, which is the third of an annual series.