Effects of dopamine on prolactin secretion and cyclic AMP accumulation in the rat anterior pituitary gland

The effects of dopamine on pituitary prolactin secretion and pituitary cyclic AMP accumulation were studied by using anterior pituitary glands from adult female rats, incubated in vitro. During 2h incubations, significant inhibition of prolactin secretion was achieved at concentrations between 1 and 10nm-dopamine. However, 0.1–1μm-dopamine was required before a significant decrease in pituitary cyclic AMP content was observed. In the presence of 1μm-dopamine, pituitary cyclic AMP content decreased rapidly to reach about 75% of the control value within 20min and there was no further decrease for at least 2h. Incubation with the phosphodiesterase inhibitors theophylline (8mm) or isobutylmethylxanthine (2mm) increased pituitary cyclic AMP concentrations 3- and 6-fold respectively. Dopamine (1μm) had no effect on the cyclic AMP accumulation measured in the presence of theophylline, but inhibited the isobutylmethylxanthine-induced increase by 50%. The dopamine inhibition of prolactin secretion was not affected by either inhibitor. Two derivatives of cyclic AMP (dibutyryl cyclic AMP and 8-bromo cyclic AMP) were unable to block the dopamine (1μm) inhibition of prolactin secretion, although 8-bromo cyclic AMP (2mm) significantly stimulated prolactin secretion and both compounds increased somatotropin (growth hormone) release. Cholera toxin (3μg/ml for 4h) increased pituitary cyclic AMP concentrations 4–5-fold, but had no effect on prolactin secretion. The inhibition of prolactin secretion by dopamine was unaffected by cholera toxin, despite the fact that dopamine had no effect on the raised pituitary cyclic AMP concentration caused by this factor. Dopamine had no significant effect on either basal or stimulated somatotropin secretion under any of the conditions tested. We conclude that the inhibitory effects of dopamine on prolactin secretion are probably not mediated by lowering of cyclic AMP concentration, although modulation of the concentration of this nucleotide in some other circumstances may alter the secretion of the hormone.     

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.     

Ovine prolactin potentiates the action of adrenocorticotropic hormone on the secretion of dehydroepiandrosterone sulfate and dehydroepiandrosterone from cultured bovine adrenal cells

To determine the direct effect of prolactin on adrenal androgen secretion, the daily secretions of dehydroepiandrosterone sulfate (DHEA-S), dehydroepiandrosterone (DHEA), androstenedione and cortisol were determined in monolayer culture of bovine adrenal cells in the presence or absence of adrenocorticotropic hormone (ACTH) and/or prolactin. In the absence of ACTH ovine prolactin alone had no effect on steroid secretion during seven-day culture. Ovine prolactin, when administered in combination with ACTH, significantly potentiated the stimulatory effect of ACTH on DHEA-S and DHEA but not androstenedione secretion on the seventh day in culture. On the first day in culture prolactin showed no synergistic effect with ACTH on DHEA and DHEA-S secretion, although ACTH significantly increased DHEA and cortisol secretion. DHEA-S secretion increased as a function of prolactin concentration in the presence of ACTH. These results indicated that long-term treatment by ovine prolactin with ACTH caused the increase in adrenal androgen secretion from bovine adrenal cells. The site of action of prolactin was suggested to be the partial inhibition of adrenal 3 beta-hydroxysteroid dehydrogenase by the result of increases in DHEA-S and DHEA but not androstenedione secretion.     

Effect of cysteamine on suckling-induced prolactin secretion in the rat

We have examined the effects of the thiol agent cysteamine on physiological prolactin secretion in the female rat. Administration of cysteamine completely abolishes suckling-induced prolactin secretion in a dose-dependent manner. Cysteamine treatment does not alter nursing behavior of the mothers. Further, we have found that the prolactin-depleting ability of cysteamine is not altered by a prior suckling stimulus. These results indicate that cysteamine administration inhibits physiologically-induced prolactin secretion with similar potency and efficacy as previously reported for cysteamine effects on basal and pharmacologically-induced prolactin secretion. Furthermore, the effect of cysteamine is not compromised by a previous suckling stimulus, suggesting that "depletion-transformation" of pituitary prolactin stores does not protect against the effect of cysteamine.     

Paradoxical effect of neuroleptic drugs on prolactin secretion by rat pituitary cell cultures.

Several antipsychotic drugs reverse the dopamine-induced inhibition of prolactin release by rat pituitary cell cultures. Paradoxically, at high doses and without dopamine, antipsychotic drugs can also inhibit prolactin secretion. The mechanism underlying this phenomenon is unclear. Some evidence suggests that these drugs have an agonistic action. We sought to verify whether clozapine and fluphenazine, at doses higher than those reversing dopamine-induced inhibition of prolactin secretion in vitro, show this paradoxical effect and eventually a partial agonistic action. Both antipsychotics inhibited prolactin secretion, clozapine at doses starting from 10(-6) M and fluphenazine from 10(-7) M. Haloperidol reversed clozapine-induced prolactin inhibition but left fluphenazine-induced inhibition unchanged. These in vitro findings suggest that clozapine has a partial agonistic action on dopaminergic receptors but fluphenazine does not.     

Effects of enkephalin in analogues on prolactin release in the rat.

The effects of nineteen enkephalin analogues on the circulating levels of prolactin in the male rat following intraventricular injection of the peptides were determined and compared with that of Met- and Leu-enkephalin. Eleven of the 19 analogues stimulated prolactin secretion. It was found, in general, that the structure activity relationship for enkephalin stimulation of prolactin secretion was similar to that for opiate receptor activity. Analogues which contained a [DAla²] substitution were generally effective in stimulating prolonged prolactin release. Some, but not all analogues containing [DTrp²] or [DLeu⁵] were active. Analogues containing the [DTrp¹], [DPhe⁴] or [DMet⁵] substitutions were ineffective. The prolactin releasing effect of intravenous Tyr-DAla-Gly-Phe-DLeu was reversed by naloxone. Naloxone had no effect on the haloperidol- and alpha-methylparatyrosine induced increases in plasma prolactin levels. The results of these studies are discussed in the light of the suggestion that the enkephalins may function as neuroendocrine modulators.     

Multiple coupling of neurohormone receptors with cyclic AMP and inositol phosphate production in anterior pituitary cells

Regulation of adenohypophyseal hormone secretions has been shown to involve cyclic AMP production, modulation of phosphatidyl inositol diphosphate breakdown and Ca2+ mobilization. Various neurohormone receptors are positively or negatively coupled to adenylate cyclase activity in anterior pituitary cells. The effects of these neurohormones on adenylate cyclase activity are consistent with the effect on hormone secretions, suggesting that modulation of the enzyme activity is actually involved in the regulation of adenohypophyseal secretions. Thus DA inhibits, whereas VIP stimulates adenylate cyclase activity of the same cell type, which, according to the effect of these neurohormones on prolactin secretion, appear to be lactotrophs. On the other hand, SRIF inhibits, whereas GRF stimulates the adenylate cyclase activity of another cell type, namely somatotrophs, whereas CRF appears to act on a third cell type, corticotrophs. Peripheral hormones have been shown to modulate the sensitivity of anterior pituitary cells to these neurohormones. Estradiol long-term treatment has an anti-dopaminergic effect on prolactin secretion. The steroid also suppresses the dopamine inhibition of adenylate cyclase. This effect appears selective to the DA inhibition, since AII inhibition of the enzyme is only partially reduced, whereas the somatostatin inhibition is markedly increased. Peripheral hormones seem to affect the sensitivity of adenohypophyseal cells not only by modulating the number of receptors for a given neurohormone but also by interfering with the coupling mechanisms of these receptors. AII and DA inhibit the adenylate cyclase activity of lactotroph cells. The prolactin stimulation induced by angiotensin is not consistent with the effect of the peptide on adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that histamine-stimulated prolactin secretion is not mediated by an inhibition of tuberoinfundibular dopaminergic neurons.

The effects of histamine on prolactin secretion and the activity of tuberoinfundibular dopaminergic (DA) neurons were examined in male rats. Tuberoinfundibular DA neuronal activity was estimated in situ by measuring the metabolism [concentration of 3,4-dihydroxyphenylacetic acid (DOPAC)] and synthesis [accumulation of 3,4-dihydroxyphenylalanine (DOPA) after administration of a decarboxylase inhibitor] of dopamine in the median eminence. Intracerebroventricular (icv) injection of histamine produced a dose- and time-dependent increase in plasma prolactin levels but had no effect on DOPA accumulation or DOPAC concentrations in the median eminence. These results indicate that the stimulation of prolactin secretion following icv histamine is not mediated by an inhibition of tuberoinfundibular DA neurons.     

Effects of pimozide and domperidone administration on prolactin levels in neonatally estrogenized female rats

The effects of two dopaminergic blockers, pimozide and domperidone, on the prolactin secretion were investigated in adult female rats treated neonatally with estrogens (100 micrograms of estradiol benzoate s.c. on day 1). These rats showed hyperprolactinemia (556 micrograms/l vs 57.7 in oil-injected) and treatment with pimozide or domperidone failed to increase prolactin levels in the adult age. These results suggest that the hyperprolactinemia in neonatally estrogenized female rats is produced by loss of the dopaminergic inhibition on prolactin secretion, so that the pharmacological blockade of dopaminergic receptors is uneffective. The dopamine levels in hypothalamus were similar in control and estrogenized females suggesting that failure in dopaminergic inhibition is due to a decrease in dopamine secretion to portal vessels.     

Angiotensin II and dopamine modulate both cAMP and inositol phosphate productions in anterior pituitary cells. Involvement in prolactin secretion

Despite their opposite effects on prolactin secretion, both dopamine and angiotensin II inhibit adenylate cyclase activity in homogenates of anterior pituitary cells in primary culture. Dopamine and angiotensin II inhibition of adenylate cyclase was not additive, suggesting that both neurohormones inhibit the adenylate cyclase of the lactotroph cells. Pretreatment with Bordetella pertussis toxin (islet activator protein) completely suppressed the dopamine-induced inhibition of both adenylate cyclase and prolactin secretion. The islet activator protein also reversed the angiotensin II-induced inhibition of the adenylate cyclase activity. In contrast, angiotensin II stimulation of prolactin release was not affected by the toxin. Angiotensin II also induced a dose-dependent stimulation of inositol phosphates (250%) with an EC50 of 0.1 nM, close to that observed for prolactin secretion. Islet activator protein pretreatment did not block the stimulation of inositol phosphate production. Dopamine inhibited the angiotensin II-stimulated prolactin release and the production of inositol phosphates induced by angiotensin II. It is concluded that angiotensin II and dopamine receptors of lactotroph cells are able to modulate both cAMP and inositol phosphate production. The dopamine receptor of lactotrophs appears to be the first example of a receptor which is negatively coupled to the production of inositol phosphates.     

Effect of serotonin depletion by p-chlorophenylalanine on serum prolactin levels in estrogen-treated ovariectomized rats: insights concerning the serotoninergic, dopaminergic and opioid systems.

The stimulatory effect of serotonin on prolactin secretion is well documented, and the administration of an inhibitor of serotonin synthesis (p-chlorophenylalanine - pCPA) has the expected inhibitory action on prolactin release in most experimental situations. However, there is evidence that in certain physiological or experimental conditions, activation of the serotoninergic system can also determine inhibition of prolactin secretion. The aim of the present study was to investigate the ability of estrogen to modify the effect of pCPA on prolactin secretion and to evaluate the participation of opioid and/or dopaminergic systems in regulating pCPA-induced prolactin secretion in estradiol-treated rats. We observed that pCPA administration (200 mg/kg/day, s.c., 2 days) to ovariectomized (OVX) female rats treated with estradiol benzoate (300 microg/week for 2 weeks, or 50 microg/week for 4 weeks, s.c.) causes a significant increase in serum prolactin, whereas no effect is observed in intact rats or in OVX rats without treatment. Bromocriptine administration completely reversed prolactin values previously increased by estradiol and by pCPA [OVX rats + estradiol = 86.50 ng/ml (68.90-175.02), OVX + estradiol + pCPA = 211.30 ng/ml (142.03-311.00), OVX + estradiol + pCPA + bromocriptine = 29.35 ng/ml (23.01 - 48.74), p<0.05. Naloxone administration partially reduced estrogen-induced high prolactin concentrations, but did not affect prolactin secretion stimulation determined by pCPA. Overall, the data from this report confirm the involvement of the dopaminergic system and, to a lesser degree, of endogenous opioids in prolactin secretion stimulation determined by estradiol. Furthermore, our results suggest that the stimulatory action of pCPA on prolactin secretion in estradiol-treated OVX rats is mediated by serotonin, which may also act indirectly on dopamine neurons.     

Pertussis toxin blocks the inhibitory effect of muscarinic cholinergic agonists on cyclic AMP accumulation and prolactin secretion in GH3 anterior-pituitary tumour cells.

The inhibition of prolactin secretion and cyclic AMP accumulation in GH3 cells by muscarinic agonists was blocked by preincubation of the cells with pertussis toxin (islet-activating protein). There was a lag of approx. 80 min in the onset of the effect on secretion. These results suggest that muscarinic agonists decrease prolactin secretion by inhibiting adenylate cyclase activity.     

Prolactin does not mediate the suppressive effect of the suckling stimulus on luteinizing hormone secretion in ovariectomized lactating rats

The plasma LH concentration in ovariectomized lactating rats is low for 14 days postpartum, while the prolactin concentration is high during this period. We examined the effect of the inhibition of increased prolactin secretion with bromocriptine (CB-154) on the LH secretion in lactating rats ovariectomized on day 2 (day 0 = day of parturition). Blood samples were collected through an indwelling atrial cannula every day. LH levels were kept low until day 9 in lactating rats injected daily with CB-154 (0.6 mg/day, s.c.). The duration of the period during which LH secretion was suppressed was shorter in lactating rats treated with CB-154 than in saline-injected controls. The replacement with ovine prolactin by means of a mini-osmotic pump (0.3 mg/day, s.c.) in CB-154-treated lactating rats restored the duration of LH suppression. In rats deprived of their pups on day 2, the LH concentration rose immediately after removal of the pups and the LH level was not significantly different between rats treated with CB-154, ovine prolactin and saline, indicating that neither the CB-154 treatment nor the high level of prolactin alone has any effect on LH secretion in rats deprived of their pups. The present results clearly demonstrate that prolactin does not mediate the suppressing effect of the suckling stimulus on LH secretion in early lactation and support our theory that the suckling stimulus controls the LH and prolactin secretion independently at the hypothalamic level.     

Opioid regulation of luteinising hormone and prolactin release in the horse—identical or independent endocrine pathways?

In order to determine if endogenous opioids regulate luteinising hormone (LH) and prolactin secretion via a common, gonadotropin-releasing hormone (GnRH) dependent pathway in the horse, effects of the opioid antagonist naloxone (300 mg) and the GnRH agonist buserelin (20 μg) on prolactin and LH secretion were investigated in stallions (n = 22), long-term castrated geldings (n = 15) and non-lactating mares during the luteal phase of the oestrous cycle (n = 16). Blood samples for determination of LH and prolactin concentrations were withdrawn at 15 min intervals for 120 min. After 60 min of sampling, animals were treated with either naloxone, buserelin or saline. In stallions, naloxone significantly increased LH as well as prolactin release (P < 0.05), indicating an opioid inhibition of both hormones, whereas in mares, naloxone stimulated only LH secretion (P < 0.05). No changes in plasma LH or prolactin concentrations after injection of naloxone were found in geldings. In all animal groups, buserelin induced a significant release of LH (P < 0.05) without affecting prolactin. We conclude that endogenous opioids inhibit LH and prolactin release in the horse but the regulation of these two hormones involves independent opioid pathways. These are activated differentially in stallions, geldings and mares. The opioid regulation of prolactin secretion is not mediated via GnRH.     

Prolactin release and milk removal induced by suckling and milking in lactating ewes is prevented by L-dopa treatment

The effect of L-DOPA on milk removal and on prolactin release during suckling or milking was studied in lactating ewes. Various doses of L-DOPA (25, 50, 100 and 200 mg per animal) were injected iv 30 min before the suckling or milking period. Control ewes were injected with 0.9% NaCl solution only. Milking induced a significant long-lasting release of prolactin. An inhibition of milk removal was obtained with the dose of 200 mg of L-DOPA. An inhibition of prolactin secretion was observed related to the dose of drug administered. The inhibitory effect of 200 mg of L-DOPA on the secretion of prolactin after milking lasted for about 120 min, and thereafter a significant increase in serum prolactin level occurred. This increase in serum prolactin was not due to a "rebound" effect of L-DOPA, since the milking stimulus had to be present to induce the delayed increase in prolactin. Doses of 25 or 50 mg of L-DOPA prevented the surge of prolactin observed immediately after milking, but a long-lasting release of prolactin was obtained thereafter. The inhibitory effect of L-DOPA on prolactin release could be overridden by the suckling or milking stimuli according to the dose administered. The suckling stimulus was more effective than milking in overriding the inhibitory effect of the low dose of L-DOPA. The results indicate that milk removal and prolactin release induced by milking or suckling in lactating ewes is inhibited by an increase in monoamines at the hypothalamic-hypophyseal level.     

Photorefractoriness in European starlings: associated hypothalamic changes and the involvement of thyroid hormones and prolactin

Responsivity to photostimulation in previously photorefractory European starlings is caused by subjection to short daylengths and is characterized by a marked activation of the hypothalamus in terms of synthesis of gonadotropin releasing hormone. This active hypothalamic state is amplified for a time by a subsequent exposure to long days but is soon completely reversed as the birds become photorefractory again. This latter effect of long photoperiods and the concurrent secretion of prolactin are dependent on the presence of thyroid hormones. Conceivably, prolactin causes photorefractoriness by inhibition at a hypothalamic level.     

Effects of pretreatment with tetradecanoyl phorbol acetate on regulation of growth hormone and prolactin secretion from ovine anterior pituitary cells

Tetradecanoyl phorbol acetate (TPA) stimulates growth hormone (GH) and prolactin secretion from ovine anterior pituitary cells. Pretreatment of the cells with TPA abolishes this effect, presumably due to down-regulation of protein kinase C. Such pretreatment did not alter effects of thyrotropin-releasing hormone or dopamine on prolactin secretion, suggesting no involvement of protein kinase C. Pretreatment with TPA attenuated actions of GH-releasing hormone on GH release (but not actions on cyclic AMP levels), possibly due to depletion of cellular stores of GH. Such pretreatment also attenuated inhibition of GH release by somatostatin, possibly due to phosphorylation of receptors or associated proteins by protein kinase C.     

Pulsatile secretion of prolactin in the male rat after pimozide administration is not due to pulsatile inhibition of PIF secretion.

Sequential blood samples were taken every 2 min from intact male rats implanted with a permanent indwelling right atrial cannula. The relationship between pimozide dose and prolactin secreation was established by administering graded doses of pimozide (30–3000 μg/kg) as a single bolus injection through the indwelling cannula. The maximum response of prolactin secretion was achieved with 300 μg/kg pimozide. Higher doses of pimozide did not raise further the circulating prolactin concentration suggesting that the receptors for the presumed prolactin inhibiting factor (PIF) were blocked completely at this dose. Marked pulsatile fluctuations in circulating prolactin concentration were observed after administration of pimozide, at all dosages, or of another ‘specific’ dopaminergic receptor blocking agent, d-butaclamol. Since we assume that PIF receptors are completely blocked by the higher doses of pimozide, we conclude that this pulsatile secretion of prolactin cannot be due to the inhibition of PIF secretion but may be due either to the stimulation of prolactin releasing factor (PRF) secretion, or to an inherent rhythmicity in the prolactin secreting cells.     

Characterization of the inhibitory effects of hyperprolactinaemia on the mechanism controlling LH secretion in chronically ovariectomized rats

The time-course of the inhibitory effect of hyperprolactinaemia on LH secretion was delineated. Hyperprolactinaemia was induced in ovariectomized rats with injections of domperidone or ovine prolactin and circulating LH levels were measured from 1 h to 9 days after the treatment. Inhibition of LH secretion occurred within 2-4 h after treatment, and was maintained (provided that serum prolactin remained elevated) for a period of 6 days only. Thereafter LH levels increased to become insignificantly different from control levels on Day 9. A reduction in pituitary responsiveness was not associated with the acute or sub-chronic inhibition of LH secretion, although a significant fall in responsiveness was observed simultaneously with the return of serum LH levels to control values. No changes in hypothalamic LH-RH content was found. It is concluded that an impairment of pituitary function is not responsible for the inhibitory action of prolactin on LH secretion.