Catecholaminergic Control of α-Melanocyte-Stimulating Hormone (αMSH) Release by Frog Neurointermediate Lobe in Vitro: Evidence for Direct Stimulation of αMSH Release by Thyrotropin-Releasing Hormone*

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Abstract
The role of dopaminergic and adrenergic innervation of the intermediate lobe of amphibian pituitary in the release of αMSH has been studied in vitro. Neurointermediate lobes of frog (Rana ridibunda Pallas) have been perifused in amphibian culture medium (ACM) for 5–7 h. αMSH released in the effluent perifusate was measured by means of a sensitive and specific RIA. No significant morphological alteration of neurointermediate lobe cells was observed during the perifusion experiment, even at the electron microscopic level. The existence of dopaminergic receptors, responsible for an inhibition of frog melanotrophs, was shown using the dopmainergic agonists apomorphine (10-6 M) and bromo-2-ergocryptine (10-8 and 10-7 M), which initiated a marked reduction of αMSH secretion. The effect of apomorphine was obliterated by the dopaminergic antagonist haloperidol. Haloperidol itself induced a dose-related stimulation, and the monoamine oxidase inhibitor nialamide (4 × 10-3 M) inhibited αMSH secretion. In addition, haloperidol led to a complete reversal of the inhibitory effect of nialamide on αMSH secretion. These results demonstrate the existence, in the parenchyme of the intermediate lobe, of dopaminergic nerve fibers that are functionally active. The β-adrenergic agonist isoproterenol was responsible for a dose-related stimulation of αMSH secretion; the stimulatory effect was reversed by the β- adrenergic antagonist propranolol. TRH is a potent stimulator of αMSH secretion in amphibians. Since haloperidol and propranolol did not abolish the stimulation of αMSH release induced by TRH, it appeared that TRH action was not mediated via an inhibition of dopamine release or via a stimulation of adrenergic nerve fibers.