Ladislav Krulich

Site of action for β-endorphin-induced changes in plasma luteinizing hormone and prolactin in the ovariectomized rat

A number of sites have been hypothesized as loci at which opioid substances act to alter the secretion of luteinizing hormone (LH) and prolactin (PRL) (1-8). The aim of the present study was to determine the site(s) at which the opioid peptide beta-endorphin (beta-END) acts to influence plasma LH and PRL levels in the ovariectomized (OVX) rat. beta-END, administered into the third ventricle of conscious OVX rats fitted with jugular catheters, significantly decreased plasma LH in doses greater than or equal to 50 ng and increased PRL levels at all doses administered (10, 50, 100 and 250 ng) in a dose dependent fashion. To identify possible central nervous system sites of action, 250 ng beta-END was unilaterally infused into various brain sites. Plasma LH was significantly decreased and plasma PRL significantly increased by infusions into the ventromedial hypothalamic area, the anterior hypothalamic area, and the preoptic-septal area. There was no significant effect of beta-END infusions into the lateral hypothalamic area, amygdala, midbrain central gray, or caudate nucleus. When hemipituitaries of OVX rats were incubated in vitro with beta-END (10(-7)M to 10(-5)M), there was no suppression of basal or LHRH-induced LH release, nor was there any alteration of basal PRL release. It is concluded that beta-END acts at a medial hypothalamic and/or preoptic-septal site and not the pituitary, to alter secretion of LH and PRL.

Neurotransmitter Control of Thyrotropin Secretion

The central dopaminergic system seems to have an inhibitory influence on the secretion of thyrotropin (TSH) both in humans and rats. This is documented by observations that in humans, especially in subjects with primary hypothyroidism, the dopamine precursor l-Dopa, dopamine receptor agonist bromocryptine, and dopamine itself decrease plasma levels of TSH and in some instances inhibit TSH secretory response to thyrotropin-releasing hormone (TRH). Conversely blockade of dopamine receptors leads to an elevation of circulating TSH. It is probable the dopaminergic drugs act on the pituitary thyrotrops rather than on the release of hypothalamic TRH. Analogous results were obtained in rats. In contrast, the noradrenergic system, studied mainly in rats, has a stimulatory influence on the secretion of TSH. This view is mainly supported by findings that acute interruption of noradrenergic neurotransmission causes a decrease of serum TSH levels and blocks the cold-induced stimulation of TSH secretion. The role of the central serotonergic system remains undecided because some experimental findings indicate that it has an inhibitory influence but others indicate an opposite function. The remaining transmitter systems have not been studied extensively enough to allow definite conclusions about their roles in the regulation of TSH secretion.

Opioid κ Receptors and the Secretion of Prolactin (PRL) and Growth Hormone (GH) in the Rat

The effects of bremazocine and U-50,488, two selective opioid kappa receptor agonists, and the preferential mu receptor agonist morphine on the secretion of PRL and GH were compared in conscious male rats bearing permanent right atrial cannulae for serial blood sampling and drug delivery. All three opioids stimulated PRL secretion in a dose-related manner, but the kappa agonists differed from morphine in several respects. They were considerably more potent than morphine in triggering a PRL response, but were unable to elevate PRL levels to more than 100 ng/ml, whereas morphine, at the highest dose (4.5 mg/kg), induced an almost twice larger response. Also their PRL-releasing effect was inhibited more strongly by the preferential kappa receptor antagonist Mr-2266 than by naloxone, whereas Mr-2266 and naloxone, which are equipotent as antagonists of the mu receptors, were equipotent in suppressing the PRL-stimulating effect of morphine, a mu agonist. In a complete contrast to morphine, which effectively stimulated GH secretion, the kappa agonists had no effect on GH release at lower doses and suppressed it at higher doses. It is concluded that the PRL-releasing effect of the kappa agonists is mediated by the kappa receptors which may participate with the mu receptors in regulation of PRL secretion by opioids. The GH-inhibiting effect of the kappa agonists requires further clarification.

Stimulation of prolactin secretion by morphine: role of the central serotonergic system.

Abstract Unanesthetized male rats with indwellinh right atrial cannulae were injected with morphine (MOR) i.v. which produced a dose-related increase in plasma prolactin levels (PRL). This effect was blocked partially by naloxone (NAL) at a dose of 0.06 mg/kg and totally by 0.6 mg/kg NAL. Interruption of central serotonergic neurotransmission by receptor blockade, with metergoline (MET) or cyproheptadine (Cypro), inhibition of tryptophan hydroxylase by para-chlorophenylalanine or destruction of serotonin neurons by 5, 7-dihydroxytryptamine antagonized the morphine (3 mg/kg) induced elevation in PRL release. Depression of dopaminergic activity with α-methyl-para-tyrosine elevated the basal PRL levels, but it did not prevent a further increase of prolactin levels by morphine (3 mg/kg). These data are compatible with the hypothesis that morphine stimulates PRL release by activation of the central serotonergic system.

Differential role of the opioid μ and δ receptors in the activation of prolactin (PRL) and growth hormone (GH) secretion by morphine in the male rat

Administration of naloxazone (50 mg/kg i.v.), an irreversible, selective and long acting antagonist of the mu 1 subclass of the opioid receptors, strongly reduced stimulation of PRL secretion by morphine (5.0 mg/kg i.v.) injected 24 hours later into conscious, unrestrained rats. In contrast, the effect of morphine on PRL release was unimpaired in rats treated 24 hours beforehand with either the reversible opioid antagonist naloxone (50 mg/kg i.v.), or the vehicle for naloxazone. A complete suppression of the PRL response to morphine (3.0 mg/kg i.v.) was observed in animals given intraventricular (IVT) injection of beta- funaltrexamine (beta-FNA, 2.5 micrograms), another selective, irreversible and long acting antagonist of the mu receptors, 24 hours beforehand. Neither naloxazone nor beta-FNA had any effect on the activation of GH secretion by morphine, which, however, was conspicuously reduced by ICI 154, 129, a preferential delta receptor antagonist, injected IVT (50 micrograms) 5 minutes before morphine. ICI 154, 129 had no effect on the PRL response to morphine. It is concluded that the PRL stimulating effect of morphine is mediated by the mu receptors, whereas activation of GH probably involves the delta sites.

THE EFFECTS OF NEUROTENSIN ON ANTERIOR PITUITARY HORMONE SECRETION

The present experiments were conducted to determine the effects of neurotensin on secretion of a variety of anterior pituitary hormones. Conscious rats with indwelling cannulae in the third ventricle and external jugular vein were used and the effects on plasma hormone levels measured by radioimmunoassay. Neurotensin was found to decrease plasma prolactin levels in ovariectomized females, normal males, and males in which prolactin levels had been elevated by ether or by a combination of fluoxetine and 5-hydroxytryptaphane. The prolactin-lowering effect was blocked by alpha-methyl-tyrosine to inhibit catecholamine synthesis and by the specific dopamine receptor blocker, spiroperidol. In ovariectomized females, neurotensin was also capable of suppressing LH and elevating growth hormone following its intraventricular injection. Intravenous injection of the peptide elevated prolactin but had no effect on the release of the other pituitary hormones. When hemipituitaries of ovariectomized rats were incubated in vitro, neurotensin elevated prolactin and TSH release into the medium. The minimal effective dose to elevate prolactin and TSH release was 50 ng/ml. Release of gonadotropins and growth hormone was unaffected. It is concluded that neurotensin inhibits prolactin release by a CNS, presumably hypothalamic action, to stimulate the tuberoinfundibular dopaminergic neurons. The dopamine released then inhibits prolactin release either by a direct action on the pituitary or by release of another prolactin-inhibiting factor. In addition, the peptide has a direct prolactin-releasing action on the pituitary. Neurotensin can inhibit LH and stimulate growth hormone presumably by a hypothalamic action since there was no effect on the release of these pituitary hormones by glands incubated in vitro. Although the peptide had no effect on TSH release following its intraventricular injection, it stimulated prolactin release by pituitaries incubated in vitro. The physiological significance of these results is not yet established; however, the presence of the peptide in regions concerned with pituitary control suggests that it may play a physiological role.

Catecholaminergic Regulation of TSH and Growth Hormone Release in Ovariectomized and Ovariectomized, Steroid-Primed Rats

Third ventricular injection of dopamine (DA), Piribedil (ET-495), a DA receptor stimulator, norepinephrine (NE), epinephrine (E) and systemic administration of larger doses of DA and the receptor stimulant, apomorphine (APM), were used to evaluate their role in the regulation of TSH and GH secretion in ovariectomized (OVX) as well as ovariectomized, estrogen-progesterone treated (OEP) rats. Intraventricular or i.p. injection of DA or its agonists, ET-495, and APM, caused a lowering of plasma TSH and an elevation of plasma GH concentration in OVX as well as in OEP rats. In contrast, intraventricular injection of NE or E increased plasma TSH and GH concentration. On the basis of these results it is concluded that the central dopaminergic system is inhibitory to TSH secretion, as reflected in our exeriments by the significant reduction of TSH levels. On the other hand, the noradrenergic and adrenergic system has a stimulatory role on the release of TRH as evidenced by the increase in plasma TSH levels. Activation of dopaminergic, noradrenergic and adrenergic systems appears to promote release by hypothalamic GH releasing hormone as reflected in the enhanced concentration of plasma GH, but the precise physiological role of these biogenic amines in modulating the release of TSH and GH hormone remains to be elucidated.

Stimulation of growth hormone release by intraventricular administration of 5HT or quipazine in unanesthetized male rats.

Summary Intraventricular injection of 5HT (4 and 20 μg) in unanesthetized, unrestrained male rats fitted with permanent intrajugular cannulas for withdrawal of blood samples caused a dose-related elevation of plasma GH levels. Similar effects were also observed following intraventricular injection of the serotonin receptor agonist, quipazine. The GH-releasing effect of both drugs was abolished by a serotonin receptor blocker, methysergide. It is concluded that activation of the central serotoninergic system stimulates GH secretion in the rat.

On the Prolactin-Inhibiting Effect of Neurotensin

Neurotensin (NT) when injected in a dose of 5 μg into the third cerebral ventricle of conscious, unrestrained male rats, outfitted with chronic third ventricle and jugular cannulae, decreased the rest

TSH in serum and milk of normal, thyroidectomized, and hyperthyroid lactating rats.

Summary TSH was determined by ra-dioimmunoassay in serum and milk of euthyroid, hyperthyroid, and thyroidecto-mized lactating rats. In the normal euthy-roid rats, the average serum TSH concentration was 219 ± 39.8 ng/ml, while the average concentration in milk was 156 ± 9.8 ng/ ml. Five to seven days after thyroidectomy both values were significantly increased to 892 ± 109 and 230 ± 6.5 ng/ml, respectively. In animals treated with a very large dose of T3 for 4 days, serum and milk TSH concentrations were conspicuously decreased to 72 ± 7.1 and 63 ± 6.7 ng/ml, respectively.