ADRENERGIC CONTROL OF ALPHA-MELANOCYTE-STIMULATING HORMONE-RELEASE INFROG PITUITARY IS MEDIATED BY BOTH BETA-SUBTYPE AND A NONCONVENTIONALALPHA(2)-SUBTYPE OF ADRENOCEPTORS

Previous studies have shown that melanotrope cells of the pars interme dia of Rana ridibunda are inhibited by dopaminergic D-2 agonists and s timulated by beta-adrenergic agonists. In the present study, we have e xamined the possible involvement of alpha-adrenoreceptors in the regul ation of frog melanotrope cells. Reversed-phase HPLC analysis combined with electrochemical detection revealed the presence of both dopamine and noradrenaline in pars intermedia extracts (74.1 and 3.2 ng/mg pro tein, respectively), while adrenaline was undetectable. Administration of graded doses of noradrenaline and adrenaline (from 0.1 to 10 mu M) to perifused frog neurointermediate lobes induced a dose-dependent in hibition of alpha-MSH release. The inhibitory effect of adrenaline was partially blocked by the D-2-dopaminergic antagonist sulpiride and to tally suppressed by concomitant administration of sulpiride and yohimb ine (an alpha(2)-adrenergic antagonist). Conversely, in the presence o f sulpiride, noradrenaline provoked a strong stimulation of alpha-MSH secretion which was totally blocked by the beta-adrenergic antagonist propranolol. Taken together, our results indicate that endogenous cate cholamines may exert a complex regulatory action on frog melanotrope c ells through activation of dopaminergic D-2, alpha(2)- and beta-adrene rgic receptors.