[H-3] RACLOPRIDE AND [H-3] SPIROPERIDOL BINDING TO RETINAL MEMBRANES OF THE TELEOST EUGERRES PLUMIERI - EFFECT OF LIGHT AND DARK-ADAPTATION

Monoamine and metabolites were determined in the retina of the teleost Eugerres plumieri after dark and light adaptation. Dopamine, homovani llic acid and 3,4-dihydroxyphenylacetic acid increased after light exp osure. The results indicate an increase in the turnover rate of dopami ne due to light exposure. Dopamine D-2 receptors were studied by deter mining the binding parameters of [H-3]spiroperidol and [H-3]raclopride to retinal membranes. The results were best fitted to a two-site mode l, where the high-affinity site may correspond to D-2 receptors and th e low-affinity site could be D-4 receptors, which have been recently d escribed in the retina, although further research is needed to confirm this suggestion. The number of sites labeled with [H-3]spiroperidol w as lower than with [H-3]raclopride. This may indicate the existence of monomer and dimer conformations of D-2-like receptors in the retina, as has been shown in the brain. Light exposure increased the number of sites labeled with both ligands. Since D-2 receptors are known to mod ulate the production of melatonin, the augmentation in the capacity of these receptors could contribute to the reduction of melantonin durin g light exposure. (C) 1997 Elsevier Science Ltd.