Release of melanophore stimulating hormone (MSH) from the vertebrate pars intermedia is under inhibitory control by the hypothalamus. Removal of the rat pituitary or the neurointermediate lobe of the frog (Rana pipiens) to in vitro incubation medium results in rapid uninhibited release of MSH. This secretion is inhibited by norepinephrine, epinephrine, phenylephrine, and dopamine, and the inhibition is antagonized by α-adrenergic receptor blocking agents. Isoproterenol stimulation of MSH secretion from isolated glands is blocked by propranolol, a β-adrenergic receptor antagonist. These results implicate dopaminergic or classical α-adrenergic receptors (or both) in inhibition of MSH release by catecholamines, and implicate β-adrenergic by the bioamines.
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