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*

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, h...

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