Naloxone prevents dark-background adaptation in amphibians.

Pituitary intermediate lobe melanocyte-stimulating hormone (MSH) is responsible for normal skin darkening in amphibians. Light-background adapted frogs (Rana pipiens) injected with naloxone and placed on black backgrounds maintain melanophore indices and pituitary cytology characteristic of light-background adaptation. In vitro and in vivo experiments showed that naloxone hydrochloride did not have a direct effect on skin melanophores or on the neurointermediate lobe. These data suggest that naloxone acts at the level of the central nervous system to inhibit the mechanism(s) responsible for release of MSH when light-background adapted frogs are placed on a dark background. Release of MSH, known to be tonically inhibited by the hypothalamus, may be modulated by opiate receptor-dependent mechanisms.