Characterization of serotonin transporter in goldfish retina by the binding of [3H]paroxetine and the uptake of [3H]serotonin: modulation by light.

The serotonin (5-HT) uptake system of goldfish retina was evaluated by the binding of [3H]paroxetine to membrane preparations and the uptake of [3H]5-HT into isolated cells from goldfish retina. The order of potency of inhibitors of [3H]paroxetine binding was imipramine > 5-methoxy-N,N- dimethyltryptamine > desipramine > fluoxetine > citalopram > 5-HT. The saturation experiments indicated a high-affinity binding site, and positive cooperativity with Hill coefficient higher than unity. The association reached equilibrium at about one hour of incubation and was efficiently displaced by imipramine. The equilibrium dissociation constants calculated by the antilog of the log concentration of ligand giving 50% of occupation, and by the ratio of dissociation/association constants, were similar: 5.84 and 2.34 nM, respectively. The binding was not significantly reduced by decreasing the temperature of incubation and was sodium dependent. The lesion with 5,7-dihydroxytryptamine reduced the binding to 60%. The uptake of [3H]5-HT into isolated cells also showed positive cooperativity. The order of potency of inhibitors was similar to the one obtained for the binding of [3H]paroxetine. Darkness increased the uptake of 5-HT. The allosteric regulation of the 5-HT transporter and the modulation by light could be related to the physiological role of the monoamine, as a neurotransmitter and as a precursor of melatonin synthesis in the retina.