Possible involvement of endogenous opioid peptides in prolactin secretion induced by alpha 2-adrenergic stimulation in rats

Noradrenergic mechanisms have a stimulatory role in regulating prolactin (PRL) secretion in the rat. We investigated the mechanism by which the alpha 2-adrenergic system stimulates PRL release in urethane-anesthetized male rats. Intracerebroventricular injection of norepinephrine (2 micrograms/rat) or epinephrine (100 ng and 1 microgram/rat) caused an increase in plasma PRL levels. The PRL increase induced by epinephrine was much greater than that by norepinephrine. Intracerebroventricular injection of phentolamine (1 microgram/rat), an alpha-antagonist, blunted the plasma PRL increase induced by epinephrine (100 ng intracerebroventricularly). Plasma PRL levels were increased by intravenous injection of alpha 2-agonists, clonidine (15 micrograms/100 g of body wt), and xylazine (200 micrograms/100 g of body wt). Plasma PRL increase induced by clonidine or xylazine was suppressed by intravenous injection of naloxone (125 micrograms/100 g of body wt), an opiate antagonist. These findings suggest that alpha 2-adrenergic mechanisms stimulate pituitary PRL secretion, at least partly, by activating endogenous opioid peptides in the rat.     

Involvement of peptide histidine isoleucine (PHI) in prolactin secretion induced by serotonin in rats

The possible role of hypothalamic peptide histidine isoleucine (PHI) in prolactin (PRL) secretion induced by serotoninergic mechanisms was investigated in male rats using a passive immunization technique. Intracerebroventricular injection of serotonin (5HT, 10 micrograms/rat) raised plasma PRL levels both in urethane-anesthetized rats and in conscious rats pretreated with normal rabbit serum (0.5 ml/rat, iv, 30 min before). Plasma PRL responses to 5HT were blunted in these animals when they were pretreated with rabbit antiserum specific for PHI (0.5 ml/rat, iv, 30 min before) (mean +/- SE peak plasma PRL: anesthetized rats 271.3 +/- 38.3 ng/ml vs 150.0 +/- 12.6 ng/ml, p less than 0.01, conscious rats 54.3 +/- 6.8 ng/ml vs 30.7 +/- 4.1 ng/ml, p less than 0.025). These results suggest that hypothalamic PHI is involved, at least in part, in PRL secretion induced by central serotoninergic stimulation in the rat.     

Inhibition of pituitary bioactive prolactin secretion in the male rat by the glucocorticoid agonist decadron phosphate.

In the present studies, the soluble glucocorticoid agonist, decadron phosphate (DEC), was administered i.v. to intact adult male rats in order to evaluate the effects of glucocorticoid receptor stimulation on circulating levels of immunoreactive (ir-) and bioactive (bio-) prolactin (PRL). In light of reports that glucocorticoid-specific receptors exist within the rat brain, additional experiments investigated the effects of intracerebroventricular (i.c.v.) administration of graded doses of the same drug on pituitary ir- and bioPRL secretion. Concentrations of ir- and bioPRL in samples obtained before and after drug treatments were determined by standard PRL radioimmunoassay and the Nb2 rat node lymphoma bioassay, respectively. Rats injected i.v. with 0.5 mg DEC/kg body weight, but not those treated with a tenfold lesser dose, exhibited decreased plasma irPRL concentrations. However, both doses promoted a decline in circulating levels of bioPRL compared to vehicle-treated controls, along with an overall reduction in the plasma bio/irPRL ratio. The magnitude and duration of this drug-induced decline in biopotency of secreted hormone was dose-dependent. While the plasma bio/irPRL ratio was diminished only transiently in rats injected with 0.05 mg DEC/kg, treatment with the higher dose led to a sustained decrease in the plasma bio/irPRL ratio for the duration of the experiment. The current studies also show that intracerebral administration of DEC resulted in dose-dependent alterations in pituitary PRL release. Circulating levels of ir- and bioPRL were not altered in rats injected i.c.v. with 10 ng of DEC, the lowest dose tested.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcitonin gene-related peptide: antinociceptive activity in rats, comparison with calcitonin

The effects of synthetic human calcitonin gene-related peptide (CGRP) on nociceptive response were evaluated in rats by two behavioral tests (tail-flick and hot-plate) and by electrophysiological recording of the firing of thalamic neurons evoked by peripheral noxious mechanical stimuli. CGRP was administered intracerebroventricularly (i.c.v.) and its effects were compared with that of salmon calcitonin (sCT). In the tail-flick test, CGRP (0.25, 2.5 and 5 micrograms/rat) dose-dependently increased response latencies, whereas sCT (0.125, 2.5, 5 and 10 micrograms/rat) did not. Conversely, in the hot-plate test CGRP was effective in enhancing response latencies only at the highest dose of 10 micrograms/rat, while sCT (0.125, 0.25 and 2.5 micrograms/rat) inhibited the hot-plate response dose-dependently. In electrophysiological studies, CGRP (2.5 micrograms/rat, i.c.v.) completely inhibited the evoked neuronal thalamic firing and the same dose of sCT induced only a partial reduction. Furthermore, the antinociceptive effects of CGRP in the tail-flick test and in the electrophysiological studies were not prevented by naloxone. These results demonstrate that central administration of CGRP is effective in inhibiting nociceptive responses and its action like that of sCT does not involve an opioid mechanism. The differences in the antinociceptive profiles of CGRP and sCT suggest that the inhibitory effects of these peptides may involve different neuronal pathways.     

Stimulation of prolactin release by prolactin-releasing peptide in rats.

We have previously reported a hypothalamic peptide that shows specific prolactin (PRL)-releasing activity in vitro, named prolactin-releasing peptide (PrRP). However, its activity in vivo has not yet been shown. In this study, we examined whether PrRP could induce specific PRL release in vivo using normal cycling female and male rats. Intravenous injection of PrRP31 increased plasma PRL levels in rats in a dose-dependent manner. PrRP31 (50 nmol/kg i.v.) significantly (P < 0.05) stimulated plasma PRL levels within 25 min after injection in rats in proestrus, estrus, and metestrus. A higher dose of PrRP31 (500 nmol/kg i.v.) was necessary for a significant increase in plasma PRL levels in male rats. These results clearly indicate that female rats, especially at proestrus, are more sensitive to PrRP-induced PRL secretion than male rats. The effect of PrRP on PRL release is affected considerably by the estrous cycle and sex, which suggests that PrRP sensitivity is controlled by the endogenous hormonal milieu, such as estrogen levels. PrRP31 did not affect other pituitary hormone secretions. The results indicate that PrRP shows specific PRL-releasing activity in vivo as well as in vitro and suggest that it plays an important role in the regulation of PRL release under certain physiological conditions.     

Effects of prolactin-releasing peptide on tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in estrogen-treated female rats

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 microg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D2 receptor antagonist, sulpiride (1 microg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1-1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.     

Inhibition by estrogen of autofeedback regulation of prolactin secretion

The effect of estradiol-17 beta (E2) on autofeedback regulation of prolactin (PRL) secretion was tested in ovariectomized rats after s.c. implantation of an (E2)-containing or empty silastic capsule, followed by i.v. injection of bovine PRL (b-PRL) or bovine serum albumin (BSA; 500 micrograms/100 g B.W.). Implantation of an E2 capsule (day 0), 2.5 mm or 5.0 mm in length, produced plasma E2 concentrations of 79 +/- 6 (9) and 140 +/- 8 pg/ml (8), respectively. Assay of PRL in plasma samples collected at 1 h intervals between 1100-1800 h on days 3, 4 and 5, after E2 capsule implantation showed a daily afternoon PRL surge. Empty capsule-treated rats did not show any afternoon PRL surge. Injection of b-PRL, but not BSA, at 1200 h on day 3 reduced basal PRL release both on days 3 and 4 in empty capsule-treated rats. In ovariectomized rats treated with a smaller E2 capsule (2.5 mm), b-PRL injection at 1200 h on day 3 reduced the amplitude of the afternoon surge of PRL and the total amount of PRL released on day 4. b-PRL, however, was ineffective in reducing PRL release in rats bearing the large E2 capsule (5.0 mm). These results suggest that high E2 levels in the blood can block the negative feedback action of PRL on PRL release.     

Involvement of hypothalamic vasoactive intestinal polypeptide (VIP) in prolactin secretion induced by serotonin in rats

To study the possible involvement of hypothalamic vasoactive intestinal polypeptide (VIP) in regulating the secretion of prolactin (PRL), the effect of anti-VIP rabbit serum on serotonin (5-HT)-induced PRL release was examined in urethane-anesthetized male rats. Anti-VIP serum (AVS) or normal rabbit serum (NRS) was infused into a single hypophysial portal vessel of the rat for 40 min at a rate of 2 microliters/min with the aid of a fine glass cannula and 5-HT was injected into a lateral ventricle 10 min after the start of the infusion. Intraventricular injection of 5-HT (10 micrograms/rat) caused an increase in plasma PRL levels in control animals infused with NRS and 5-HT-induced PRL release was blunted in animals infused with AVS (mean +/- SE peak plasma PRL: 118.9 +/- 19.8 ng/ml vs 54.7 +/- 16.2 ng/ml, p less than 0.05). These findings suggest that the secretion of PRL induced by 5-HT is mediated, at least in part, by hypothalamic VIP release into the hypophysial portal blood in the rat.     

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.     

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)     

Prolactin secretion in cattle as affected by haloperidol and α-Methyl-p-Tyrosine

Prolactin (PRL) secretion in response to i.v. injection of different doses of α-Methyl-p-Tyrosine (αMT) and haloperidol (HAL) was studied in one cow and three bulls. HAL was tested at doses of 0.033, 0.1, and 0.33 mg/kg body weight (BW), and αMT was tested at doses of 0.1, 1.0, 10, and 30 mg/kg BW. Blood was collected via an indwelling catheter into the external jugular vein, and plasma PRL was analysed by radioimmunoassay. Dose-related increases in plasma PRL concentrations were observed after administration of both αMT and HAL. Peak PRL concentrations after injection of HAL at a low, medium, and high dose were 22, 45, and 70 ng/ml, respectively. Peak PRL concentrations after injection of increasing doses of αMT were 3.0, 10, 40 and 70 ng/ml. HAL (0.1 mg/kg BW) and αMT (10 mg/kg BW) were administered intravenously to four heifers during summer and winter. Response to both drugs, as measured by changes in PRL secretion, was greater in summer than winter. Peak plasma PRL levels after HAL injection were 327 ± 58 ng/ml in June and 149 ± 27 ng/ml in January, whereas peak levels after αMT were 166±29 and 60±9 ng/ml, respectively. Basal PRL secretion was also greater in summer than winter. The results of this study demonstrate that PRL in peripheral plasma of cattle is increased in response to administration of antidopaminergic drugs and that this increase is greater during the summer than the winter.     

Calcitonin secretion during postnatal development in the rat: beta-adrenergic agents and vitamin D3 metabolites

The possible contribution of catecholamines and vitamin D3 metabolites to the high plasma calcitonin (CT) levels in suckling baby rats is unknown. So, in vivo and in vitro (using a perifusion system) effects of beta-adrenergic agents and vitamin D3 metabolites on CT release were studied in the rat during the postnatal development. In 13-day-old rats, the increase in plasma CT levels induced by isoproterenol injection (0.1 micrograms/kg b.w.) was inhibited by a previous administration of propranolol. A significant decrease in plasma CT levels was observed after propranolol injection in baby rats (0.68 +/- 0.05 ng/ml vs. 0.93 +/- 0.01 ng/ml). A daily injection of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3; 25 pmoles/rat/day during 4 days) induced a marked rise in plasma calcium (16.1 +/- 0.2 mg/dl), and a great decrease in thyroidal CT contents (approximately 70% of control values) in 13-day-old rats while no change was noted with 24,25-dihydroxycholecalciferol (24,25-(OH)2D3). A negative correlation between plasma calcium and thyroidal CT stores was found in suckling and in weaning rats treated with different doses of 1,25-(OH)2D3, suggesting an indirect effect of 1,25-(OH)2D3 on CT secretion. The mobilization of the thyroidal CT content was greater in weaning than in suckling rats in response to a given hypercalcemia. In vitro, 5 X 10(-5) M isoproterenol induced a rapid increase in CT secretion rate while 1,25-(OH)2D3 inhibited the rise in CT release induced by 3.0 mM calcium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of metoclopramide-induced prolactin on aldosterone secretion in normal subjects

We investigated the role of prolactin (PRL) on modurating the secretion of aldosterone in normal male subjects. Metoclopramide (5mg) which causes a significant rise of PRL was given by intravenous injection. The peak of PRL level at 30 min. after i.v. injection of metoclopramide (20.0 ± 1.6 ng/ml, mean ± S.E.) was significantly higher than the basal level (6.4 ± 2.1 ng/ml, P < 0.01), but plasma aldosterone, serum sodium, potassium and plasma renin activity did not change significantly throughout the period of the study. Cortisol levels, however, reduced significantly after 30 min. and remained significantly low, probably because of diurnal variation. Present results suggest that PRL might at least not play a physiological role on regulating the secretion of aldosterone in man.     

Prostaglandin D2 stimulates vasoactive intestinal polypeptide release into rat hypophysial portal blood

The effect of prostaglandin D2 (PGD2) on vasoactive intestinal polypeptide (VIP) release from the hypothalamus was examined by determining plasma VIP levels in rat hypophysial portal blood. Intraventricular injection of PGD2 (5 micrograms/rat) caused a 3-fold increase in the concentration of plasma VIP in hypophysial portal blood in anesthetized rats. A PGD2 metabolite, 13,14-dihydro-15-keto PGD2, did not affect VIP levels in portal blood. The flow rate of hypophysial portal blood was not changed after the injection of PGD2. The intraventricular injection of PGD2, but not PGD2 metabolite, resulted in an increase in peripheral plasma prolactin (PRL) levels in the rat. These findings suggest that PGD2 plays a stimulatory role in regulating VIP release from the hypothalamus into hypophysial portal blood and causes PRL secretion from the pituitary in rats.     

Growth hormone and prolactin release by substance P in rats

Intravenous injection of synthetic Substance P resulted in a significant and dose-related increase in plasma growth hormone (GH) and prolactin (PRL) in urethane-anesthetized rats. Increases in plasma GH induced by Substance P were significantly suppressed by the simultaneous administration of either ?-dopa or nicotine, whereas plasma PRL responses to Substance P were blunted by ?-dopa but not by nicotine. Substance P also raised plasma GH and PRL in rats with extensive hypothalamic destruction. L-dopa significantly suppressed plasma PRL responses to Substance P in rats with hypothalamic destruction. However, plasma GH responses to Substance P were not significantly affected by ?-dopa nor by nicotine in animals with hypothalamic ablation. These results suggest that Substance P stimulates rat GH and PRL secretion possibly acting on the anterior pituitary and that ?-dopa and nicotine affect GH and PRL release induced by Substance P in different ways.     

Adrenergic and dopaminergic control of growth hormone secretion in urethan anesthetized adult male rats

The effects of drugs which interfere with alpha-adrenergic and dopaminergic mechanisms, involved in GH and PRL secretion, have been analyzed in urethane anesthetized rats. Clonidine induced a dose-dependent release of GH (0.0032--0.1 mg/kg i.v.) as well as of PRL (0.032--1.0 mg/kg i.v.). The lowest dose of clonidine, when given into the third ventricle, provoked a very pronounced release of GH. Phentolamine, given intravenously, inhibited the clonidine-induced GH release in a dose-dependnet manner. L-Dopa administered intravenously and apomorphine administered intravenously or intraventricularly did not affect basal secretion of GH bu- produced a dose-dependnet inhibition of clonidine-induced GH release. Pimozide did not change basal GH secretion. Furthermore pimozide did not attenuate the inhibition of clonidine-induced GH secretion seen after apomorphine administration, however, it completely reversed apomorphine-induced PRL inhibition. These findings demonstrate that an alpha-adrenoceptor-mediated stimulatory mechanism is involved in GH and PRL secretion. An inhibitory dopaminergic mechanism is confirmed for PRL secretion and suggested for GH secretion.     

Stimulation of prolactin secretion by L-3,4-dihydroxyphenyl-serine (L-DOPS) via central norepinephrine in the rat

Intracerebroventricular (icv) injection of L-3,4-dihydroxyphenylserine (L-DOPS) (50 and 250 micrograms/rat) raised in a dose-related manner both plasma prolactin (PRL) and CSF norepinephrine (NE) in urethane-anesthetized male rats. Intravenous (iv) injection of larger doses of L-DOPS (5 and 10 mg/100 g BW) slightly but significantly increased plasma PRL and CSF NE. L-DOPS injection (50 micrograms/rat, icv or 5 mg/100 g BW, iv) also raised plasma PRL in conscious rats. There was a good correlation (r = 0.74) between CSF NE and peak plasma PRL in the anesthetized animals. Propranolol (100 micrograms/100 g BW, iv) inhibited plasma PRL responses to L-DOPS (50 micrograms/rat, icv) and NE injection (1 microgram/rat, icv) raised plasma PRL in anesthetized animals. These findings indicate that L-DOPS stimulates PRL secretion via central noradrenergic mechanisms in the rat.     

Further evidence that peptide histidine isoleucine (PHI) may function as a prolactin releasing factor in rats

Intraventricular administration of peptide histidine isoleucine (PHI) (200 ng, 1, 5 and 10 micrograms/rat) resulted in a significant and dose-related increase in plasma prolactin (PRL) levels in urethane-anesthetized rats and in conscious rats with intraatrial and intraventricular catheters. Intravenous injection of PHI (10 micrograms/rat) also raised plasma PRL levels in these animals. In in vitro studies, PRL release from superfused rat anterior pituitary cells was stimulated by PHI (10(-9), 10(-8) and 10(-7) M) in a dose-related manner. The stimulating effect of PHI (10(-7)M) on PRL release in vitro was as potent as that of vasoactive intestinal polypeptide (VIP) (10(-7) M) and was observed even in the presence of dopamine (10(-7) M). These results suggest that PHI plays a stimulating role in regulating PRL secretion by acting, at least in part, directly on the pituitary in the rat.     

Studies on the physiological role and mechanism of action of neuropeptide Y in the regulation of luteinizing hormone secretion in the rat

It has been recently shown that intraventricular or systemic injection of neuropeptide Y (NPY) can produce a decrease in plasma luteinizing hormone (LH) levels in castrated rats of both sexes. In order to evaluate the physiological role of NPY in the regulation of LH secretion in the female rat, we proceeded to immunoneutralization experiments using specific antibodies to NPY. Injection of 0.5 ml antiserum to NPY produce a 20-fold increase of LH plasma levels, whereas injection of preimmune serum did not modify the plasma concentrations of LH. To investigate the possibility that catecholamines or serotonin might be involved in the effect of NPY in LH secretion, castrated rats were treated with alpha-methylparatyrosine (alpha-MPT), an inhibitor of catecholamine biosynthesis, or received an i.c.v. injection of the neurotoxin 5-7-dihydroxytryptamine (5,7-DHT) prior to the intraventricular injections of NPY. The pretreatment with alpha-MPT could not prevent the decrease of plasma LH induced by NPY injection whereas the pretreatment with 5,7-DHT reversed the effect of NPY injection. The anatomical connection between LH-releasing hormone (LHRH) and NPY neuronal systems were also investigated using double immunostaining technique. It appeared that NPY endings are in apposition to LHRH cell bodies in the preoptic area in proximity to the organum vasculosum of the lamina terminalis (OVLT).(ABSTRACT TRUNCATED AT 250 WORDS)