Neurochem Int 1989 Jan 01;154:549-54. doi: 10.1016/0197-0186(89)90175-7.

GABA and the regulation of serotonin N-acetyltransferase activity in amphibian retina-II. The role of dopamine.

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Retinal melatonin biosynthesis is regulated in part by the activity of serotonin N-acetyltransferase (NAT), which increases in dark-adapted, but not light-exposed, retinas at night. Using an in vitro eye cup preparation from the African clawed frog (Xenopus laevis), we have obtained evidence indicating that dopamine and gamma-aminobutyric acid (GABA) interact in the regulation of the nocturnal rise in NAT activity. Increases of NAT activity induced by the GABA agonist muscimol were suppressed by dopamine. Spiperone, a D2 dopamine receptor antagonist, and muscimol separately increased NAT activity, but were not additive in their effects. Inhibition of NAT activity by the GABA antagonist picrotoxin was blocked by spiperone. Additionally, muscimol decreased concentrations of dopamine and its principle metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), in light exposed retinas, while picrotoxin increased retinal DOPAC levels in darkness. These data suggest that in darkness, activation of GABA receptors inhibits dopamine secretion, consequently releasing NAT-synthesizing cells from a tonic inhibitory influence.

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