title: Regulation of endogenous dopamine release in amphibian retina by gamma-aminobutyric acid and glycine.
abstract: Endogenous dopamine release in the retina of the African clawed frog (Xenopus laevis) increases in light and decreases in darkness. The roles of the inhibitory amino acid transmitters gamma-aminobutyric acid (GABA) and glycine in regulating this light/dark difference in dopamine release were explored in the present study. Exogenous GABA, the GABA-A receptor agonist muscimol, the GABA-B receptor agonist baclofen, and the GABA-C receptor agonist cis-aminocrotonic acid (CACA) suppressed light-evoked dopamine overflow from eyecups. The effects of GABA-A and -B receptor agonists were selectively reversed by their respective receptor-specific antagonists, whereas the effect of CACA was reversed by the competitive GABA-A receptor antagonist bicuculline. The benzodiazepine diazepam enhanced the effect of muscimol on light-evoked dopamine release. Both GABA-A and -B receptor antagonists stimulated dopamine release in light or darkness. Bicuculline was more potent in light than in darkness. These data suggest that retinal dopaminergic neurons are inhibited by GABA-A and -B receptor activation in both light and darkness but that GABA-mediated inhibitory tone may be greater in darkness than in light. Exogenous glycine inhibited light-stimulated dopamine release in a concentration-dependent and strychnine-sensitive manner. However, strychnine alone did not increase dopamine release in light or darkness, nor did it augment bicuculline-stimulated release in darkness. Additionally, both strychnine and 7-chlorokynurenate, an antagonist of the strychnine-insensitive glycine-binding site of the N-methyl-D-aspartate subtype of glutamate receptor, suppressed light-evoked dopamine release. Thus, the role of endogenous glycine in the regulation of dopamine release remains unclear.
authors.affiliation: Department of Pharmacology, Emory University School of Medicine, Atlanta, GA 30322-3090.
authors.forename: J H
authors.initials: JH
authors.lastname: Boatright
authors.forename: N M
authors.initials: NM
authors.lastname: Rubim
authors.forename: P M
authors.initials: PM
authors.lastname: Iuvone
mesh_terms: D000818:Animals; D004298:Dopamine / Q000096:biosynthesis*; D004305:Dose-Response Relationship, Drug; D018755:GABA Agonists / Q000494:pharmacology; D005998:Glycine / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D008027:Light; D008297:Male; D012160:Retina / Q000187:drug effects / Q000378:metabolism*; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
chemical_list: D018755:GABA Agonists; D005680:gamma-Aminobutyric Acid; D005998:Glycine; D004298:Dopamine