Adrenergic control of alpha-melanocyte-stimulating hormone release in frog pituitary is mediated by both beta- and a nonconventional alpha 2-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 beta-adrenergic agonists. In the present study, we have examined the possible involvement of alpha-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 microM) to perifused frog neurointermediate lobes induced a dose-dependent inhibition of alpha-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 alpha 2-adrenergic antagonist). Conversely, in the presence of 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 catecholamines may exert a complex regulatory action on frog melanotrope cells through activation of dopaminergic D2, alpha 2- and beta-adrenergic receptors.
Lamacz
M
M
European Institute for Peptide Research, INSERM U-413, UA CNRS, University of Rouen, Mont-Saint-Aignan, France.
Garnier
M
M
Héry
F
F
Tonon
M C
MC
Vaudry
H
H
D018663:Adrenergic Agents / Q000494:pharmacology*; D018674:Adrenergic Antagonists / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D002395:Catecholamines / Q000378:metabolism; D018492:Dopamine Antagonists / Q000494:pharmacology; D004837:Epinephrine / Q000494:pharmacology; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D010902:Pituitary Gland / Q000378:metabolism*; D011895:Rana ridibunda; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D000521:alpha-MSH / Q000378:metabolism*
D018663:Adrenergic Agents; D018674:Adrenergic Antagonists; D000316:Adrenergic alpha-Agonists; D002395:Catecholamines; D018492:Dopamine Antagonists; D011942:Receptors, Adrenergic, alpha; D011943:Receptors, Adrenergic, beta; D000521:alpha-MSH; D009638:Norepinephrine; D004837:Epinephrine