Monoaminergic control of MSH release in the lizard Anolis carolinensis

Abstract The release of MSH, in vivo or in vitro , was investigated in the lizard Anolis carolinensis . A single intraperitoneal (ip) injection of α-methyl- p -tyrosine (400 μg/g body wt) produced an intense and long-lasting darkening response in animals adapted to a white illuminated background; α-methyl- m -tyrosine (400 μg/g body wt) and α-methyl-DOPA (100 μg/g body wt) also produced a darkening response, but the effect was slighter and shorter. Pimozide (10 μg/g body wt) also gave rise to a darkening response and para -chlorophenylalanine (400 μg/g body wt) completely abolished the darkening produced by pimozide or α-methyl- p -tyrosine. Hypophysectomized animals did not change from green color when injected with any of the darkening drugs. Incubation of neurointermediate halves with serotonin (5.2 × 10 −7 M ), dopamine (2.6 × 10 −6 M ), or serotonin plus dopamine showed that dopamine appreciably inhibits the serotonin-stimulated release of this hormone. These results suggest that a serotoninergic stimulatory, and a dopaminergic inhibitory, mechanism controls the release of MSH from the pars intermedia of the Anolis pituitary gland.