CHARACTERIZATION OF SEROTONIN TRANSPORTER IN GOLDFISH RETINA BY THE BINDING OF [H-3] PAROXETINE AND THE UPTAKE OF [H-3] SEROTONIN - MODULATION BY LIGHT

The serotonin (5-HT) uptake system of goldfish retina was evaluated by the binding of [H-3]paroxetine to membrane preparations and the uptak e of [H-3]5-HT into isolated cells from goldfish retina. The order of potency of inhibitors of [H-3]paroxetine binding was imipramine > 5-me thoxy-N,N-dimethyltryptamine > desipramine > fluoxetine > citalopram > 5-HT. The saturation experiments indicated a high-affinity binding si te, 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 dissociat ion constants calculated by the antilog of the log concentration of li gand giving 50% of occupation, and by the ratio of dissociation/associ ation constants, were similar: 5.84 and 2.34 nM, respectively. The bin ding was not significantly reduced by decreasing the temperature of in cubation and was sodium dependent. The lesion with 5,7-dihydroxytrypta mine reduced the binding to 60%. The uptake of [H-3]5-HT into isolated cells also showed positive cooperativity. The order of potency of inh ibitors was similar to the one obtained for the binding of [H-3]paroxe tine. 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.