Melanocyte stimulating hormone inhibition by acetylcholine and noradrenaline in the frog skin bioassay

The mechanism of aggregation induced in MSH-dispersed dermal melanocytes was studied in Rana pipiens by reflectance photometry in vitro and by microscopy in vivo. Acetylcholine inhibits MSH strongly and irreversibly in one third of all frogs tested in vitro and has almost no effect on the remaining animals. No lightening action occurs in vivo. Different skin samples from the same animal give the same response to acetylcholine. An individual response to acetylcholine implies similar responses to other cholinergics. The lightening action of acetylcholine is inhibited by atropine. Noradrenaline induces a reversible MSH-inhibition in all frogs in vitro as well as in vivo. The lightening action of noradrenaline, inhibited by sympatholytics, is ten times stronger than that of acetylcholine. The l-isomer has only twice the lightening potency as the d-isomer. Both lightening agents work if given at the maximum of MSH-dispersion or before the addition of MSH. Fundamental differences between the mechanisms of dispersion and aggregation, and between the lightening induced by acetylcholine and by noradrenaline, are emphasized.