Effect of morphine, beta-endorphin and leu-enkephalin on 3H-spiroperidol binding to bovine pituitary membranes.

The ability of morphine, leu-enkephalin and β-endorphin to antagonize the binding of ³H-spiroperidol to bovine anterior pituitary membranes was studied. All three drugs were virtually inactive despite their ability to stimulate prolactin secretion $\text{in}$$\text{vivo}$ and the reported ability of morphine to antagonize the inhibitory effect of dopamine on prolactin release from rat hemi-pituitaries. These results suggest that opiates do not produce their direct effect on prolactin secreation at the pituitary level through an effect on the ³H-spiroperidol binding site. The opiates may antagonize the effect of dopamine at a component of the dopamine receptor which is independent of the ³H-spiroperidol binding site, or the opiates may stimulate prolactin secretion by an effect on the lactotrophes which is independent of dopamine.     

Role of estrogens on striatal dopaminergic activity

Studies were undertaken to evaluate the effects of estradiol and prolactin on striatal dopamine receptor activity. Dopamine receptors were quantified in partially purified striatal membranes by equilibrium binding using [3H]spiroperidol. When we investigated whether the D-2 dopamine receptor activity changes during the estrous cycle, the results suggest an increase in dopamine receptor density in diestrous, without modifications in the affinity. The finding that in ovariectomized rats the dopamine receptor binding parameters remained unchanged, suggests that gonadal steroids are not essential in the mechanism of action of this receptor. Results of activity of D-2 dopamine receptors showing that hyperprolactinemia fails to increase the number of these receptors do not support the hypothesis that circulating prolactin regulates the activity of these striatal dopamine receptors. Administration of estradiol benzoate (250 micrograms/kg per day) to hyperprolactinemic rats, by s.c. injection, significantly decreased both the density and the affinity of the striatal dopamine receptors. The present data indicate that, although prolactin does not seem to modify the activity of striatal dopamine receptors, it could modulate the estrogen-induced hypersensitivity of these receptors.     

Comparison of the effects of substituted benzamides and standard neuroleptics on the binding of 3H-spiroperidol in the rat pituitary and striatum with in vivo effects on rat prolactin secretion.

The abilities of sulpiride, metoclopramide, clozapine, loxapine, chlorpromazine, thioridazine, fluphenazine, haloperidol, (+)-butaclamol and RMI 81,582 to displace ³H-spiroperidol from rat pituitary and striatal membranes in vitro were compared to their abilities to stimulate rat prolactin secretion in vivo. There was a significant correlation between the abilities of clozapine, chlorpromazine, thioridazine, fluphenazine, RMI 81,582, haloperidol and (+)-butaclamol to bind to pituitary and striatal spiroperidol binding sites and to stimulate rat prolactin secretion. Loxapine was somewhat more potent and sulpiride and metoclopramide were markedly more potent in their abilities to stimulate prolactin secretion than would be predicted on the basis of their abilities to bind to pituitary dopamine receptors as measured by antagonism of ³H-spiroperidol binding. The abilities of metoclopramide and sulpiride to increase prolactin secretion and to produce anti-psychotic and extrapyramidal effects may be mediated by action at dopamine receptors which differ from those at which classical neuroleptics act, and they may also be mediated by non-dopaminergic mechanisms. Potency as inhibitors of ³H-neuroleptic binding in the rat pituitary or striatum appears to have heretofore unappreciated limitations to predict physiological functions such as prolactin stimulation and anti-psychotic activity.     

Effect of prolactin and estradiol on rat striatal dopamine receptors

Chronic estrogen treatment has been found to increase the level of rat striatal dopamine receptors. Since it is well known that estrogen treatment increases circulating prolactin levels, we have investigated the possibility that the stimulatory effect of estrogens on dopamine receptors is exerted via prolactin. Ovariectomized female or intact male rats were implanted with three adenohypophyses under the kidney capsule or treated with 17 β-estradiol (10 μg, twice daily) for 2 weeks. In animals of both sexes, the pituitary-implanted and estradiol-treated rats showed higher levels of [³H]spiperone binding to striatal dopamine receptors. This effect of estradiol or pituitary implants on dopamine receptors was further investigated in ovariectomized rats. The pituitary-implanted and estradiol-treated rats had elevated plasma prolactin levels and an increased density of striatal dopamine receptors without alteration of their affinity. The role of the pituitary in the effect of estradiol was next investigated using hypophysectomized female rats treated with 17 β-estradiol (10 μg, twice daily), o-prolactin (500 μg, twice daily) or bearing three anterior pituitary implants. The implants as well as the treatment with estradiol or prolactin increased the level of striatal dopamine receptors in hypophysectomized rats while, as expected, the estradiol-treated animals did not have elevated plasma prolactin levels. The present data indicate that high prolactin levels lead, as observed with chronic estradiol treatment, to an increased density of striatal dopamine receptors. However, the effect of estradiol may not be explained exclusively by increased prolactin levels since a similar stimulatory effect is observed in hypophysectomized animals.     

Effect of 3-PPP,a putative dopamine autoreceptor agonist,on rat serum prolactin levels

3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) has been reported to be a relatively selective agonist for dopamine (DA) auto-receptors in the striatal and limbic region. We have examined the effect of 3-PPP on rat plasma or serum prolactin levels. 3-PPP produced a non-significant decrease in baseline plasma prolactin levels. It produced a dose-dependent inhibition of the increase in serum prolactin levels produced by gamma-butyrolactone. Both doses of 3-PPP tested completely reversed the increase in serum prolactin levels.produced by reserpine and alpha-methylparatyrosine. These results strongly indicate that 3-PPP directly stimulated DA receptors on pituitary lactotrophes. 3-PPP only weakly inhibited the ability of ³H-spiroperidol to bind to pituitary or striatal membranes, suggesting that it may act at a different DA receptor than classical DA receptor blocking drugs. This DA receptor could have properties in common with the autoreceptors of the mesolimbic and nigrostriatal DA neurons.     

Prolactin inhibits the development of stress-induced ulcers in the rat

Hyperprolactinaemia, as induced by pituitary homografts under the kidney capsule, was accompanied by an inhibition of development of gastric ulcers following the application of cold-plus-restraint stress in male rats. This effect was mimicked by intracisternal administration of a low dose of the hormone. Peripheral injection of the dopamine receptor antagonist, domperidone, also inhibited the development of stress-induced ulcers. However, no effect was found after peripheral injection of another dopamine receptor antagonist, haloperidol. This latter drug appeared to antagonize the cytoprotective effect of prolactin (PRL) on stress-induced ulcers. Furthermore, peripheral injection of the prostaglandin synthesis inhibitor, indomethacin, increased the incidence of gastric ulcers in hyperprolactinaemic rats subjected to cold -plus-restraint stress. These data suggest that the cytoprotective effect of PRL on development of gastric ulcers in stressed animals may involve both central (i.e. dopamine transmission) and peripheral (i.e. prostaglandin synthesis) mechanisms.     

Inhibition of prolactin secretion by a direct effect of methysergide on the pituitary lactotrophs in the rat

Methysergide administered i.p. caused a dose dependent decrease of serum prolactin levels in rats of both sexes bearing large bilateral electrolytic lesions in the median eminence. This prolactin release inhibiting action of methysergide was prevented by pretreatment of the animals with dopamine receptor blockers pimozide or spiroperidol, which by themselves had no effect on serum prolactin levels. Similar results were observed when the dopamine receptor agonist piribedil was used instead of methysergide. It is concluded that methysergide is capable of inhibiting prolactin secretion by activation of dopamine receptors of the pituitary lactotrophs.     

Tyrosine administration decreases serum prolactin levels in chronically reserpinized rats

The injection of tyrosine, 200 mg/kg, decreased serum prolactin levels and elevated hypothalamic (and striatal) concentrations of two dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, in chronically reserpinized rats. Tyrosine administration had none of these effects in otherwise untreated rats, and did not block the increase in serum prolactin that occurred 4 hours after a single injection of reserpine. As anticipated, the injection of dopa decreased serum prolactin in all rats. Valine, another large neutral amino acid, did not modify serum prolactin in chronically reserpinized animals. Since prolactin secretion is normally inhibited by dopamine released from the hypothalamus, reserpine treatment probably elevates serum prolactin by depleting the hypothalamus of dopamine. Our data suggest that tyrosine injection suppresses serum prolactin levels in chronically reserpinized rats by enhancing the synthesis and release of hypothalamic dopamine. Thus, administration of tyrosine, dopamine's dietary precursor, can alter physiologic functions that depend on dopamine.     

Effect of chronic haloperidol and quinacrine coadministration on striatal HVA levels and stereotypic behaviors in response to apomorphine in the rat

The effects of chronic administration of quinacrine, a phospholipase A2 inhibitor, on striatal homovanillic acid (HVA) levels and behavioral sensitivity to challenge with a dopamine agonist were examined in rats. Moreover, the ability of chronic phospholipase A2 inhibition to modulate the behavioral supersensitivity and striatal HVA reduction induced by chronic haloperidol administration was also examined. Daily intraperitoneal injection of quinacrine resulted in a significant reduction of striatal HVA levels. Coadministration of haloperidol with quinacrine in this paradigm caused a more profound reduction of striatal HVA levels than either drug administered alone. That this effect of combined administration is not simply due to postsynaptic effects of quinacrine on dopamine receptor sensitivity is suggested by the fact that behavioral supersensitivity was not induced by quinacrine alone nor was the behavioral supersensitivity induced by the quinacrinehaloperidol combination greater than that induced by chronic haloperidol administration alone. There were no effects of any treatment condition on striatal levels of serotonin (5-HT) or 5-hydroxyindoleacetic acid (5-HIAA). These data implicate phospholipase A2 activity in the regulation of dopaminergic transmission.     

The regulation of adenohypophyseal prolactin secretion: effect of triiodothyronine and methylene blue on estrogenized rat adenohypophysis

Estrogens and thyroid hormones contribute importantly to cell proliferation and tumor transformation in the pituitary gland. We found that methylene blue antagonized estrogen-promoted adenohypophyseal enlargement and the enhancement of prolactin secretion. The purpose of the present article is to provide a review about neurotransmitters and their receptors involved in estrogen-induced anterior pituitary growth and in the antagonistic effects of triiodothyronine (T3) and methylene blue (MB). Central dopaminergic and noradrenergic systems are the most important factors regulating pituitary growth and function. Recently nitric oxide (NO) was added to the list of the neurotransmitters and neuropeptides involved in the control of the anterior pituitary secretion. Our data suggest that estrogen-induced anterior pituitary growth is associated with decreased synthesis and metabolism of central catecholamines, reduction of adenohypophyseal beta-adrenergic receptors and increase of dopamine DA-2 receptors. We found that the treatment with T3 or MB prevented both estrogen-induced catecholaminergic inhibition and dopamine DA-2 receptor increment in the anterior pituitary. In contrast to T3, MB given alone also slightly decreased the anterior pituitary weight. Serum levels and anterior pituitary content of prolactin were increased after treatment with estradiol benzoate (EB), whereas T3 or MB partially attenuated prolactin hypersecretion after estrogen administration. This is in accord with the attenuation of EB-induced inhibition of dopaminergic system by T3 and MB. MB given in combination with EB also partially attenuated EB-promoted rise of adenohypohyseal NO synthase activity which plays an important role in the regulation of prolactin secretion. Further studies on central catecholaminergic systems, pituitary receptors, the nitrergic system and mechanisms of intracellular signal transduction are necessary for better understanding of pituitary tumor transformation and possibly for the discovery of new approaches towards treating patients with these diseases.     

Direct stimulation of pituitary prolactin release by glutamate

The ability of glutamate and other excitatory amino acids to stimulate prolactin secretion when administered to adult animals is hypothesized to depend on a central site of action in the brain, but there are no data to support this position. An alternative hypothesis was tested that glutamate would stimulate prolactin release when applied directly to primary cultures of dispersed adult female rat anterior pituitary cells studied in a perifusion protocol. Glutamate increased the rate of prolactin release within two minutes in a self-limited manner. Glutamate-stimulated prolactin release was augmented about 4-fold by elimination of magnesium from the perfusate and was associated with stimulation of pituitary calcium flux. Ketamine and MK-801 both reduced the basal rate of prolactin release and abolished the effects of glutamate. Pituitary cells of 10-day-old rats responded similarly to glutamate. Exposure to glutamate did not influence subsequent responses to physiological hypothalamic secretagogues, thus the likelihood of toxicity was minimized. These results suggest that the N-methyl-D-aspartate (NMDA) subclass of the glutamate receptor complex is involved. Prolactin secretion may be regulated physiologically through a functional glutamate receptor on pituitary 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.     

Concurrent treatment with benztropine and haloperidol attenuates development of behavioral hypersensitivity but not dopamine receptor proliferation

Groups of male rats (n = 16 each) were treated with normal saline, haloperidol (0.75 mg/kg), benztropine (1.8 mg/kg) or haloperidol and benztropine once a day for 24 days. Following a 96 hour drug free interval, subsets of these animals were assessed for apomorphine-induced (0.75 mg/kg) stereotypic behavior, sacrificed and analyzed for striatal dopamine biochemistry or sacrificed and analyzed for spiroperidol binding sites. Benztropine cotreatment attenuated the development of behavioral hypersensitivity to haloperidol but did not alter either the dopamine receptor proliferation or the striatal dopamine biochemical changes induced by haloperidol. These results suggest that behavioral hypersensitivity is not an automatic manifestation of dopamine receptor proliferation but must depend, in part, on other factors.     

Failure of 2 Hydroxyestradiol to interact with dopamine inhibition of human prolactin secretion in vitro and with dopamine receptors of prolactin-secreting adenomas

The ability of 2-Hydroxyestradiol, a catecholestrogen, and 17 beta Estradiol to interact with the dopamine inhibition of prolactin and with dopamine receptors has been tested on dispersed human prolactin-secreting cells obtained from ten pituitary adenomas. There is a 80% inhibition of prolactin secretion obtained by addition of dopamine in a superfusion system. This inhibition is not affected by preexposure to the steroids, or by their introduction into the perifusion medium. Moreover 2 Hydroxyestradiol and 17 beta Estradiol do not interact with the binding of 3H Domperidone to DA receptors.     

Differential effects of dopamine antagonists on prolactin secretion from cultured rat pituitary cells.

Various dopamine antagonists, including two novel non-neuroleptic drugs domperidone and halopemide, stimulated apomorphine-suppressed prolactin secretion from cultured rat pituitary cells. The potency of these drugs closely paralleled their rank-order in displacing $\text{in vitro}$ H³-haloperidol binding in rat striatum reported by others (10). Concentration-effect curves were parallel except those of pimozide and clopimozide which were biphasic : prolactin secretion was stimulated at low concentrations but depressed at concentrations above 25nM. When added alone, pimozide and clopimozide, but none of the other drugs tested, also depressed prolactin secretion. The present findings indicate that prolactin secretion from cultured pituitary cells may provide an $\text{in vitro}$ test system suitable to differentiate antagonists of dopamine receptors and possibly to distinguish pure from partial antagonists.     

Rapid elevation of rat serum prolactin concentration by cyclosporine, a novel immunosuppressive drug

Within one hr of the administration of cyclosporine to rats, there was a 4-fold elevation in the serum prolactin concentration. Doses of 0.12, 1.2, and 12 micrograms/100 g body weight cyclosporine significantly elevated the serum prolactin level. Higher doses, 120 or 1200 micrograms/100 g body weight cyclosporine resulted in small but insignificant elevations of the serum prolactin concentration. Bromocriptine, a dopamine agonist which inhibits prolactin release from the anterior pituitary, completely blocked the elevation in serum prolactin in response to cyclosporine alone. These data suggest that the ability of cyclosporine to suppress immune function may involve its ability to rapidly produce hyperprolactinemia.     

During one year's neuroleptic treatment in rats striatal dopamine receptor blockade decreases but serum prolactin levels remain elevated

Rats were treated for one year with either trifluoperazine dihydrochloride (2.5–3.5 mg/kg/day) or thioridazine dihydrochloride (30–40 mg/kg/day) when prolactin levels were measured in comparison to animals treated acutely with a single oral bolus of the same drugs in approximately the same dose. Serum prolactin levels at the end of the year of neuroleptic treatment with either drug remained elevated compared to those in control animals, and the elevation was no different from that obtained by administration of an equivalent acute single oral bolus. In contrast, the inhibition of apomorphine-induced stereotypy produced by the acute administration of either drug disappeared during chronic treatment, to be replaced after a year's neuroleptic administration by a supersensitive response. Similarly, the increase in dopamine turnover produced by acute neuroleptic administration, evidenced by raised striatal 3, 4-dihydroxy-phenylacetic acid (DOPAC) levels, also disappeared at the end of a year's treatment, when specific binding of ³H-spiperone to striatal homogenates indicated an increased number of dopamine receptors. The disappearance of evidence of blockade of striatal dopamine receptors, which appeared to become supersensitive during a year's chronic treatment with either trifluoperazine or thioridazine, contrasts with the persistence of the effect of these drugs on serum prolactin levels.     

Increased ability of apomorphine to reduce serum concentrations of prolactin in rats treated chronically with alpha-methyltyrosine

Serum concentrations of prolactin were significantly increased in rats for up to 9 hours after a single i.p. injection of α-methyltyrosine (50 mg/kg); apomorphine caused a dose-dependent reduction of the elevated prolactin concentrations. Doses of apomorphine required to reduce serum prolactin concentrations.were lower in animals previously injected with α-methyltyrosine three times a day for 10 days than in animals which received a single injection of α-methyltyrosine. These results suggest that chronic disruption of the normal release of dopamine from tuberoinfundibular neurons leads to the development of increased sensitivity of dopamine receptors involved with the inhibition of prolactin release from the anterior pituitary.     

A comparison of the effects of anti-psychotic drugs on pituitary, striatal and limbic system post-synaptic dopamine receptors.

Dopamine (DA) antagonists promote the secretion of prolactin (PRL) from the anterior pituitary gland by blocking the effects of DA at receptors in the pituitary itself. Thus, comparison of the properties of these receptors with DA receptors in the striatal, meso-limbic and meso-cortical regions is of interest. Evidence is presented that clozapine, RMI-81, 582 (a morphanthridine derivative), trebenzomine (CI-686, a chromanamine derivative) and sultopride (a benzamide) have much weaker effects on human and rat PRL secretion than would be predicted by their anti-psychotic potency. The reverse is true of two other benzamides, sulpiride and metoclopramide. Classical neuroleptics of the phenothiazine, butyrophenone and thioxanthene types appear to affect rat and human PRL secretion in a manner which is mainly but not entirely consistent with their known effects on striatal and meso-limbic/meso-cortical postsynaptic DA receptors. Preliminary studies indicate presynaptic receptors which affect prolactin secretion are not present in rats. Supersensitivity may develop in the tubero-infundibular (TI) system after chronic neuroleptic treatment but altered sensitivity of these receptors was not found in schizophrenics given apomorphine.