No abstract

Two atypical neuroleptic agents, clozapine and fluperlapine, produced rapid elevations in plasma PRL concentrations that were similar in magnitude to those produced by haloperidol. However, the PRL response to clozapine or fluperlapine was of much shorter duration than that elicited by haloperidol. Clozapine, but neither fluperlapine nor haloperidol, produced a rapid increase in the activity of tuberoinfundibular dopamine (TIDA) neurons, as evidenced by an enhanced accumulation of dihydroxyphenylalanine (DOPA) in the median eminence after the inhibition of DOPA decarboxylase. The clozapine-induced increase in DOPA accumulation was evident within 30 minutes after its administration and persisted for at least 4 hours. The clozapine-induced increase in the activity of TIDA neurons may account, in part, for the abbreviated PRL response to this neuroleptic. In addition, ability to produce a short-lived increase in PRL secretion in the rat appears to be common to the atypical neuroleptic drugs.

8-OH- DPAT (8-hydroxy-2-[di-n-propylamino] tetralin) is a novel aminotetralin derivative which has been proposed to be a serotonin (5-HT) agonist devoid of dopamine agonist effects. We now report that the administration of 8-OH- DPAT , like known 5-HT agonists, produced a rapid elevation of serum prolactin concentrations in male rats. The prolactin response to 8-OH- DPAT , like that induced by other 5-HT agonists, was greatly potentiated in animals pretreated with the tryptophan hydroxylase inhibitor, para-chlorophenylalanine. However, the 8-OH- DPAT -induced elevation of serum prolactin concentrations in untreated rats was not dose-dependent and was modest in magnitude compared to that produced by known 5-HT agonists. In contrast to the stimulatory effects of 8-OH- DPAT on prolactin secretion in vivo 8-OH- DPAT suppressed the secretion of prolactin from anterior pituitary tissue in vitro, and this effect was blocked by haloperidol. The results of the present study are supportive of the view that 8-OH- DPAT has dopamine agonist, as well as 5-HT agonist, properties.

The ability of antidepressant drugs, electroconvulsive treatment (ECT), or lithium chloride (LiCl), to modify prolactin secretion in the rat was studied. Chlorimipramine, citalopram, fluoxetine, imipramine and zimelidine potentiated the low dose 5-hydroxytryptophan (5-HTP)-induced increase in prolactin secretion, suggesting inhibition of serotonin (5-HT) uptake by these drugs. Amitriptyline, doxepin, iprindole, mianserin and trazadone inhibited the prolactin stimulating effects of high doses 5-HTP and quipazine, suggesting that these drugs have 5-HT receptor blocking properties. Tandamine inhibited only 5-HTP-induced increase in prolactin secretion. Chronic administration of imipramine, potentiated the effect of low dose 5-HTP significantly more than an acute dose. Amitriptyline, produced similar inhibition of the 5-HTP-induced increase in prolactin secretion after both acute and chronic administration. The ability of bupropion and mazindol to inhibit alpha-methylparatyrosine-induced prolactin secretion, and of nomifensine to inhibit reserpine-induced prolactin secretion, is consistent with other evidence that these agents are indirect dopamine (DA) agonists. Desipramine, acutely, had no effect on any of the above paradigms but after chronic administration, potentiated the effect of low dose 5-HTP on prolactin secretion. Nortriptyline had no effect on prolactin secretion after acute or chronic treatment. ECT for 10 days did not affect the ability of a 5-HT agonist or d-amphetamine to modify prolactin secretion. However, chronic, but not acute, treatment with LiCl markedly enhanced the prolactin response to 5-HT agonists and reserpine while shifting the dose response curve for d-amphetamine and apomorphine to the right. These results are discussed in light of current theories of the role of 5-HT and DA in depression.