Regulation of melanotropin release from the pars intermedia of the amphibian Xenopus laevis: evaluation of the involvement of serotonergic, cholinergic, or adrenergic receptor mechanisms.

Melanophore-stimulating hormone (MSH) release from the pars intermedia of the pituitary gland is probably regulated by multiple factors of hypothalamic origin. We have examined a number of potential regulatory factors for their effects on MSH release from the amphibian Xenopus laevis. Serotonin and acetylcholine have no effect on MSH release. Both adrenaline and noradrenaline inhibit release of MSH in a dose-dependent manner. Studies with specific receptor agonists and antagonists reveal that these neurotransmitters exert their in vitro effects primarily through a dopamine D-2 receptor, although an alpha-adrenergic receptor could not be excluded. We further conclude that the pars intermedia of X. laevis lacks a beta-adrenergic receptor for the regulation of MSH secretion from the pars intermedia. In mammals, this receptor activates the adenylate cyclase system. Our studies reveal that despite the lack of beta-adrenergic receptors, cyclic-AMP is likely an intracellular factor involved in the stimulation of MSH release.