The peripheral noradrenergic terminal as possible site of action of salsolinol as prolactoliberin

Salsolinol, an endogenous isoquinoline, induces selective prolactin release in rats [Tóth, B.E., Homicskó, K., Radnai, B., Maruyama, W., DeMaria, J.E., Vecsernyés, M., Fekete, M.I.K., Fül?p, F., Naoi, M., Freeman, M.E., Nagy, G.M., 2001. Salsolinol is a putative neurointermediate lobe prolactin releasing factor. J. Neuroendocrinol. 13, 1042-1050]. The possible role of dopaminergic and adrenergic signal transduction was investigated to learn the mechanism of this action. The effect of salsolinol (10mg/kg i.v.) was inhibited by reserpine treatment (2.5mg/kg i.p.) and reinstated by pretreatment with monoamine oxidase inhibitor (pargyline 75 mg/kg i.p.). Salsolinol did not affect the in vitro release of dopamine (DA) in the median eminence, and did not inhibit the L-DOPA induced increase of DA level in the median eminence. 1-Methyl dihydroisoquinoline (1MeDIQ) is an antagonist of salsolinol induced prolactin release and causes increase in plasma NE level [Mravec, B., Bodnár, I., Fekete, M.I.K., Nagy, G.M., Kvetnansky, R., 2004. An antagonist of prolactoliberine induces an increase in plasma catecholamine levels in the rat. Autonom. Neurosci. 115, 35-40]. Using tissue catecholamine contents as indicators of the interaction between salsolinol and 1MeDIQ we found no interaction between these two agents to explain the changes in prolactin release in the median eminence, lobes of the pituitary, superior cervical and stellate ganglion. Increasing doses of salsolinol caused a dose dependent decrease of tissue dopamine concentration and increase of NE/DA ratio in the salivary gland, atrium and spleen. These changes of DA level and NE/DA ratio run parallel in time with the increase of prolactin release. 1MeDIQ antagonized the increase of prolactin release and decrease of tissue DA content caused by salsolinol. Neither this increase of prolactin secretion nor the decrease of DA level in spleen could be demonstrated in NE transporter (NET) knock out mice. The results presented argue for the possible role of peripheral norepinephrine release as a target for salsolinol in its action releasing prolactin. The dominant role of norepinephrine transporter may be suggested.     

Influence of adrenergic antagonists and apomorphine on prolactin release induced by serotonergic antagonists in the monkey.

The influence of adrenergic receptor blockers on the prolactin releasing effect of methysergide and cyproheptadine was examined in sexually mature female monkeys under ketamine anesthesia. Propranolol, a β-adrenergic blocker, at a dose of 1 mg/kg did not alter the prolactin releasing action of 0.1 mg/kg of methysergide but significantly potentiated (P < 0.025) the prolactin releasing action of 0.5 mg/kg of cyproheptadine. Phentolamine and phenoxybenzamine, both α-adrenergic blockers, at 1 mg/kg blunted the prolactin releasing effect of methysergide and cyproheptadine, but the pattern of prolactin blockade was different between the two putative antiserotonergic drugs. The prior administration of apomorphine, 4 mg/kg, a dopamine receptor stimulator, blocked the prolactin releasing effect of methysergide and cyproheptadine. Evidence presented here and from the literature indicate that the prolactin releasing action of methysergide and cyproheptadine is mediated by an antidopaminergic action directly on the pituitary.     

Control of prolactin secretion in mammals

Evidence describing the neuroendocrine regulation of prolactin secretion in mammals is reviewed, with focus on catecholamines, serotonin, and polypeptides. Dopamine may be a physiological prolactin inhibiting factor (PIF), while norepinephrine and possibly epinephrine regulate prolactin release at the level of the hypothalamus. Serotonin may participate in the regulation of prolactin secretion by stimulating the release of prolactin releasing factor (PRF). The identity of PRF is not known, but two polypeptides--thyrotropin releasing hormone and vasoactive intestinal polypeptide--can act directly on the adenohypophysis to stimulate prolactin release.     

Catecholamine mechanisms in the feedback effects of estradiol benzoate on the release of LH and prolactin

The role of hypothalamic catecholamines and luteinizing hormone releasing hormone (LHRH) in the negative feedback effect of estradiol benzoate (EB) on luteinizing hormone (LH) release was studied in chronic ovariectomized rats. Administration of 10 micrograms EB decreased plasma LH levels and increased LHRH content in the medial basal hypothalamus (MBH) 1 day after injection. Inhibition of dopamine and norepinephrine synthesis with alpha-methyl-p-tyrosine (alpha-MT) reduced the LHRH content in the MBH in both oil- and EB-treated animals and partially reversed the decrease in plasma LH levels. Inhibition of norepinephrine synthesis with fusaric acid decreased LHRH content in both oil- and EB-treated rats but had no effect on plasma LH levels. The results suggest that at least a portion of the inhibitory effect of EB on LH release is due to the stimulation of an inhibitory dopaminergic mechanism which reduces LHRH release from the MBH. This feedback mechanism is apparently not susceptible to dopaminergic receptor blockade since administration of pimozide had no effect on LH levels. The stimulatory feedback effect of EB on prolactin release was studied in the same animals. alpha-MT and EB produced additive effects on plasma prolactin levels whereas fusaric acid blocked the EB-induced increase in plasma prolactin levels. Pimozide appeared to potentiate the effect of EB on prolactin release. The results reconfirm the possible role of noradrenergic neurons in the release of prolactin induced by EB and also suggest that EB stimulates a dopaminergic mechanism which is inhibitory to prolactin release but is normally masked by increased noradrenergic activity.     

Ca2+ channel activating function of maitotoxin, the most potent marine toxin known, in clonal rat pheochromocytoma cells

Actions of maitotoxin, the most potent marine toxin known obtained from toxic dinoflagellate, Gambier-discus toxicus, were studied using clonal rat pheochromocytoma cells (PC12), rat liver mitochondria and liposomes. Maitotoxin induced a profound release of norepinephrine and dopamine from PC12 cells and the molar ratio of norepinephrine to dopamine was almost the same as that stored in the cells. This releasing action was apparent at a concentration of 5 X 10(-10) g/ml or more, the releasing rate increased with an increase in the concentration of applied maitotoxin and attained maximum at about 10(-6) g/ml. The [3H]norepinephrine release induced by maitotoxin was abolished in the absence of external Ca2+ and increased with increasing concentration of external Ca2+ up to 10 mM. The release gradually decreased as the external Na+ concentrations were reduced from 130 to 20 mM, but maitotoxin is still able to induce a profound release in the absence of external Na+. The releasing action of maitotoxin was markedly suppressed by various Ca2+ channel blockers, such as Mn2+, verapamil, and nicardipine, and by a local anesthetic, tetracaine. The inhibitory actions of Ca2+ channel blockers were antagonized by external Ca2+ and became less obvious in the higher Ca2+ concentration range. Maitotoxin did not exhibit any ionophoretic activities on rat mitochondrial and liposomal membranes. These results suggest that maitotoxin has the ability to activate voltage-dependent Ca2+ channels of PC12 cells.     

The dopamine of the rat mammotroph in cell culture as a model for drug action

Dopamine can act directly on pituitary cells to inhibit prolactin release. This action can be blocked by dopamine receptor blocking drugs such as haloperidol, sulpiride and other neuroleptic agents. Comparison of the properties of the mammotroph dopamine receptor with the adenylate cyclase linked dopamine receptor of the limbic forebrain reveals some obvious differences. For example, dopamine receptor stimulants such as S-584 and lergotrile mesylate are inactive in stimulating the adenylate cyclase preparations but are potent in inhibiting pituitary prolactin secretion. Such inhibition of prolactin secretion can be reversed by haloperidol or sulpiride. In contrast to these observations, sulpiride does not block dopamine stimulation of cAMP formation. In addition, dopamine, apomorphine or lergotrile mesylate have no effect on a pituitary adenylate cyclase preparation and dopamine fails to elevate cAMP in the intact cells in culture. Despite the similarity between these two dopamine sensitive systems with respect to a number of agonists and antagonists, the exceptions described suggest that the pituitary system with further study may offer some greater reliability as a predictive test for clinically useful agents. These results also suggest that the receptors for dopamine, like that for norepinephrine, are of two types, only one of which is coupled to adenylate cyclase.     

Evidence that serotonin neurons stimulate secretion of prolaction releasing factor.

The purpose of the present study was to determine if serotonin was stimulatory to prolactin release by inhibition of the dopaminergic system or by stimulating release of a prolactin releasing factor (PRF). We measured the amount of prolactin secreted after administration of 30 mg/kg of 5-hydroxytryptophan (5-HTP) to male rats pretreated with fluoxetine (10 mg/kg) and compared it with the amount of prolactin released in male rats treated with αmethyl-p-tyrosine methyl ester (αMT) or various dopamine receptor blocking agents. In every experiment the serotonergic stimulus provided by 5-HTP in fluoxetine-pretreated rats released considerably more prolactin than did treatment with αMT or dopaminergic blockers. We conclude that serotonin releases prolactin not by inhibiting dopaminergic neurons but rather by stimulating the release of PRF.     

Current status of the rat prolactin releasing factor

The release of prolactin is governed by both inhibiting and releasing factors. Basal plasma concentration of prolactin is controlled mainly through inhibition by a prolactin release-inhibiting factor (PIF), while acute stimulation of prolactin release is believed to be caused by a prolactin-releasing factor (PRF). It is the general consensus that PIF is dopamine. The PRF plays an important role in stimulation of prolactin release, and there are promising putative PRFs.     

Reexamination of dopamine as the prolactin-release inhibiting factor (PIF): supplementary agent may be required for dopamine to function as the physiological PIF

A large number of studies have been performed concerning dopamine's inhibitory effect on prolactin release, but many of these studies have examined the effect of dopamine dissolved in a solution containing ascorbic acid. Ascorbic acid, routinely used to protect dopamine from oxidation, alone does not stimulate or inhibit prolactin release, but it can potentiate the inhibitory effect of dopamine in a static monolayer culture system by approximately 100 times. We have closely examined the inhibitory effect of dopamine on prolactin release in the absence of ascorbic acid using a perifusion system. Male rat adenohypophyses were dispersed with trypsin and cultured in a Petri dish to form cell clusters. Inhibition of prolactin release by dopamine (1 mumol/L) in the absence of ascorbic acid was sustained for only 63 min during the 2-h perifusion period. Following a 2-h period of incubation of dopamine in the same experimental solution, the dopamine concentration was reduced from 1 to 0.18 mumol/L, yet this "2-h-old dopamine" was still effective in inhibiting prolactin release (approximately 30 min). This result suggests that the lactotrophs may be desensitized by chronic exposure to a high concentration of dopamine in the absence of ascorbic acid. In contrast, when a low concentration of dopamine (3 nmol/L) containing ascorbic acid (0.1 mmol/L) was perifused, inhibition of prolactin release was sustained for the entire 2-h perifusion period. Although there may be a large number of explanations for dopamine's transient inhibitory effect on prolactin release, the present results suggest that dopamine may require supplementary agent(s) to effectively inhibit prolactin release and thus function as the prolactin release inhibitory factor (PIF).(ABSTRACT TRUNCATED AT 250 WORDS)     

Further studies on the effect of cimetidine and other neurotropic drugs on rat serum prolactin

The hyperprolactinemic effect of H2 histamine receptor antagonists has been described in the rat and in humans. The present study was undertaken to explore more fully the hyperprolactinemic action of cimetidine, and its interrelationship with other neurotropic agents. Adult ovariectomized estrogen-primed rats were injected with mepiramine, diphenhydramine, pilocarpine, atropine, dopamine, cimetidine or saline as control, at different sequences and the effect on serum prolactin was determined. The effect of cimetidine on prolactin release "in vitro" by hemipituitaries of estrogenized male rats during a short time incubation period, was also investigated. Our results indicate that cimetidine is able to release prolactin and that this effect is not prevented by atropine or the classical antihistaminergic agents mepiramine and diphenhydramine. Both pilocarpine and dopamine inhibit the prolactin release due to cimetidine. The hypoprolactinemic action of pilocarpine was completely blocked by atropine, but not by mepiramine or diphenhydramine. Finally, cimetidine was unable to modify significantly the prolactin release by incubated pituitaries. It is postulated that cimetidine acts mainly at the brain and that the hyperprolactinemic effect is not mediated by muscarinic or H1 histaminergic receptors. This action can be prevented by drugs such as dopamine that are able to act on the lactotroph, and also by pilocarpine, probably by stimulating a prolactin inhibiting pathway.     

Prolactin secretion in acute stress is controlled by prolactin releasing factor.

Experiments were carried out to demonstrate that the surge of prolactin release with ether stress is due to the release of a prolactin releasing factor rather than an inhibition of release of prolactin inhibiting factor (PIF). When the normal male rat was exposed to ether dopamine (30 ng/10 μl/min), a putative PIF, was infused through the right carotid artery, the prolactin surge still occurred. The elevated circulating prolactin level induced by estradiol implantation was lowered by the infusion of dopamine (30 ng/10 μl/min), indicating that the infused dopamine was reaching the adenohypophysis. The lowered prolactin concentration caused by the infusion of dopamine was elevated by ether stress. The hypothesis that the prolactin surge following ether stress is due to the inhibition of PIF is unlikely since the surge subsequent to ether stress occurred during a constant infusion of the putative PIF, dopamine. We concluded that the prolactin surge is due to the stimulation of PRF secretion rather than an inhibition of PIF secretion.     

Effect of dopamine agonists or antagonists, TRH, stress and piglet removal on plasma prolactin concentrations in lactating gilts

The effect of dopamine agonists (ergocryptine), antagonists (chlorpromazine, haloperidol, reserpine, pimozide), thyrotropin releasing hormone or stress (restraint, piglet removal) on prolactin release was studied in primiparous lactating gilts. All animals were fitted with surgically implanted jugular catheters before farrowing. The only drug treatments which resulted in a significant change in PRL concentrations in blood were thyrotropin releasing hormone (increase) and ergocryptine (decrease). The results suggest that dopamine may not be the only regulator of prolactin in lactating pigs. Further studies are needed to identify drugs which would be useful in clinical situations for treatment of lactation failure due to low prolactin secretion. In the two stress-exposed groups, there was a gradual, steady decline in the plasma concentration of prolactin which resulted from loss of suckling contact with the piglets. Thus, snare restraint does not increase prolactin secretion in lactating sows confirming the results of other studies on pigs in different physiologic states.     

Release of phosphodiesterase activator from particulate fractions of cerebellum and striatum by putative neurotransmitters

The endogenous phosphodiesterase activator (PDEA) described by Cheung (1,2) is, in part, stored as a membrane-bound protein (12,13). PDEA can be released from the membranes by a cAMP-dependent phosphorylation of a protein that may function as PDEA binding site (13). We found that PDEA can be released from brain particulate fraction by 1 M norepinephrine, dopamine, adenosine, and histamine in the presence of ATP and a purified cAMP-dependent protein kinase; in similar conditions, serotonin is ineffective in concentrations up to 0.1 mM. Norepinephrine and dopamine activate the adenylate cyclase activity of those preparations from which they release the PDEA. Norepinephrine is more potent than dopamine in releasing PDEA from the particulate fraction of cerebellum, whereas dopamine is more active than norepinephrine in releasing PDEA from the particulate fraction of striatum. The release of PDEA elicited by both neurotransmitters is concentration-dependent; increasing the transmitter concentrations above a certain limit decreases the rate of PDEA release.     

Oestrogen and progesterone interactions in the control of gonadotrophin and prolactin secretion

Oestrogen and progesterone have marked effects on the secretion of the gonadotrophins and prolactin. During most of the oestrous or menstrual cycle the secretion of gonadotrophin is maintained at a relatively low level by the negative feedback of oestrogen and progesterone on the hypothalamic-pituitary system. The spontaneous ovulatory surge of gonadotrophin is produced by a positive feedback cascade. The cascade is initiated by an increase in the plasma concentration of oestradiol-17 beta which triggers a surge of luteinizing hormone releasing hormone (LHRH) and an increase in pituitary responsiveness to LHRH. The facilitatory action of oestrogen on pituitary responsiveness is reinforced by progesterone and the priming effect of LHRH. How oestrogen and progesterone exert their effects is not clear but the facilitatory effects of oestrogen take about 24 h, and the stimulation of LHRH release is produced by an indirect effect of oestradiol on neurons which are possibly opioid, dopaminergic or noradrenergic and which modulate the activity of LHRH neurons. In the rat, a spontaneous prolactin surge occurs at the same time as the spontaneous ovulatory gonadotrophin surge. The prolactin surge also appears to involve a positive feedback between the brain-pituitary system and the ovary. However, the mechanism of the prolactin surge is poorly understood mainly because the neural control of prolactin release appears to be mediated by prolactin inhibiting as well as releasing factors, and the precise role of these factors has not been established. The control of prolactin release is further complicated by the fact that oestradiol stimulates prolactin synthesis and release by a direct action on the prolactotrophes. Prolactin and gonadotrophin surges also occur simultaneously in several experimental steroid models. A theoretical model is proposed which could explain how oestrogen and progesterone trigger the simultaneous surge of LH and prolactin.     

Actions of releasing factors on isolated secretory granules mediated by calcium.

Synthetic TRH, crude hypothalamic extract and partially purified prolactin releasing factor stimulated prolactin and growth hormone release from isolated secretory granules. Somatostatin and partially purified prolactin release-inhibiting factor inhibited release of both hormones. Calcium promoted hormone release from granules; its releasing action was potentiated by TRH and ionophore A23187 but reduced by somatostatin.     

The role of monoamines in female puberty

The estradiol positive feedback mechanism appears to become mature between days 10 and 20 after birth. Rising serum prolactin levels between day 20 after birth and puberty are correlated with high hypothalamic norepinephrine turnover. High prolactin levels stimulate hypothalamic dopamine (DA) turnover, which may actively inhibit hypothalamic luteinizing hormone-releasing hormone (LHRH) release. Hypothalamic DNA receptor sensitivity is high in 10- to 20-day-old rats and gradually decreases between day 20 after birth and puberty. The reason for this desensitization may be the high hypothalamic DA turnover. This may result in a less strong inhibition of LHRH release allowing the positive feedback action of estradiol to elicit the first preovulatory luteinizing hormone (LH) surge initiating puberty.     

Dopaminergic Inhibition of Prolactin Release and Calcium Influx Induced by Neurotensin in Anterior Pituitary Is Independent of Cyclic AMP System

The present study demonstrates that 3,4-dihydroxyphenylethylamine (DA, dopamine) prevents neurotensin (NT) stimulation of both prolactin (PRL) release and calcium influx by interacting with specific receptors that are functionally linked to calcium channels. As shown by the studies with dispersed cells from rat anterior pituitary, the pharmacology of the control of PRL release and calcium influx, both induced by NT, was found to be typical of a DAergic process. This was demonstrated by the order of potency of agonists in inhibiting PRL release and calcium influx (DA greater than epinephrine greater than norepinephrine much greater than isoproterenol); by the high affinity of antagonists such as haloperidol and fluphenazine for this process; and by the high degree of stereoselectivity of sulpiride. Specific D2 receptor agonists, such as bromocriptine and lisuride, and the specific D2 receptor antagonist (-)-sulpiride were found to be highly potent on the DA receptors negatively coupled with calcium channels and PRL release. DA was found to lack the capacity to change the influx of calcium induced by either the sodium channel activator veratridine or high extracellular potassium levels, thus indicating a specific action of this amine on calcium channels sensitive to NT. In a range of concentrations that are effective in inhibiting either the calcium influx or the PRL release, both induced by NT, DA did not alter the cyclic AMP generating system. DA (from 1.0 nM to 50 nM) did not affect adenylate cyclase activity in rat pituitary gland homogenates and did not modify intracellular cyclic AMP levels in pituitary cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that pimozide is not a partial agonist of dopamine receptors.

The dopamine receptor antagonist pimozide, at concentrations up to 10 nM, competitively antagonized the inhibitory action of a pomorphine on prolactin (PRL) secretion by cultured rat pituitary cells. At higher concentrations pimozide as well as the analogues clopimozide and penfluridol suppressed PRL secretion. The latter effect could not be reversed by dopamine antagonists devoid of intrinsic effects on PRL release. Suppression of PRL release was also observed with compounds which were devoid of dopamine receptor agonistic or antagonistic properties such as R 6694 and R 5052, structurally related to pimozide, and also with loperamide. The inhibitory action of pimozide on PRL release resembled that of the calcium antagonist flunarizine. Concentration effect curves showed parallel slopes and the effect of both compounds could be reversed by increasing the concentration of calcium ions (Ca²⁺). Both flunarizine and pimozide were also capable of inhibiting releasing factor-stimulated luteinizing hormone secretion, an effect not shared by apomorphine. Pimozide and the various structurally related compounds used, also antagonized Ca²⁺-induced smooth muscle contractions of the isolated caudal artery of the rat.The present findings indicate that pimozide is a competitive antagonist without partial agonistic activity on apomorphine-sensitive dopamine receptors in the pituitary and that its inhibitory effect on PRL release as well as on vascular smooth muscle contractions is due to interference with a Ca²⁺-dependent mechanism of the stimulus-effect coupling process.     

The role of domperidone in the regulation of prolactin release in rats

Effects of domperidone, a dopamine antagonist, on prolactin release in female rats were studied. Oral administration of domperidone for 14 days caused a significant increase in serum prolactin levels in mature female rats. The routes by which domperidone exerted its effects on prolactin release were studied by a in vitro incubation system using rat pituitary tissues. Pituitary halves were incubated with (1) domperidone, (2) dopamine, (3) dopamine plus domperidone, (4) hypothalamic extracts from rats which had been treated with control meal (control hypothalamic extract), (5) control hypothalamic extract plus domperidone, and with (6) hypothalamic extract from rats which had been treated with domperidone for 14 days (domperidone-treated hypothalamic extract). Pituitary halves, when incubated alone, released a significant amount of prolactin into the incubation medium after 24 hours incubation, which was completely inhibited by dopamine or control hypothalamic extract. The addition of domperidone could not reverse the inhibitory effect of dopamine or control hypothalamic extract. On the other hand, domperidone-treated hypothalamic extract showed no inhibitory effects on prolactin release. These results indicated that domperidone could increase serum prolactin levels in female rats by acting primarily at the hypothalamus.     

Dopamine-inhibited adenylate cyclase in female rat adenohypophysis

A dopamine-inhibited adenylate cyclase has been demonstrated in anterior pituitary gland of adult female rats, lactating and not lactating. This inhibitory effect was completely GTP dependent. In contrast, in the adenohypophysis of male rats, dopamine had no detectable effect on adenylate cyclase activity. In female rats the inhibition of the enzyme appears mediated by specific dopaminergic receptors: the effect of dopamine was mimicked by the dopaminergic agonists apomorphine and the ergot derivative CH 29–717, while norepinephrine was much less potent. On the other hand, the dopaminergic antagonists trifluoperazine and sulpiride competitively antagonized the dopamine inhibition of the adenylate cyclase. The possibility that the dopamine-inhibited enzyme is located in mammotrophs appears supported 1) by its observation in the female rat pituitary, which contains this type of cells in much larger proportion than the male gland (33–38% vs. < 5%); 2) by the pharmacological similarity between the dopaminergic receptors mediating the adenylate cyclase inhibition (this work) and those regulating prolactin release (which have been characterized in previous studies). The well known inhibition of prolactin release brought about by dopamine could therefore be mediated, at least in part, by a decrease in the intracellular level of cAMP.