Adrenergic Control of α-Melanocyte-Stimulating Hormone Release in Frog Pituitary Is Mediated by both β- and a Nonconventional α2-Subtype of Adrenoreceptors

Previous studies have shown that melanotrope cells of the pars intermedia of Rana ridibunda are inhibited by dopaminergic D2 agonists and stimulated by β-adrenergic agonists. In the present study, we have examined the possible involvement of α-adrenoreceptors in the regulation 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 protein, respectively), while adrenaline was undetectable. Administration of graded doses of noradrenaline and adrenaline (from 0.1 to 10 µM) to perifused frog neurointermediate lobes induced a dose-dependent inhibition of α-MSH release. The inhibitory effect of adrenaline was partially blocked by the D2-dopaminergic antagonist sulpiride and totally suppressed by concomitant administration of sulpiride and yohimbine (an α2-adrenergic antagonist). Conversely, in the presence of sulpiride, noradrenaline provoked a strong stimulation of α-MSH secretion which was totally blocked by the β-adrenergic antagonist propranolol. Taken together, our results indicate that endogenous catecholamines may exert a complex regulatory action on frog melanotrope cells through activation of dopaminergic D2, α2- and β-adrenergic receptors.