Abstract

Mechanisms underlying agonist- and antagonist-induced up-regulation in HEK-293 cells transfected to express D2 dopamine receptors were investigated. We have reported previously that exposure of cells to agonists and antagonists led to an increase in the density of receptors. The time course of up-regulation on exposure to (-)-sulpiride, a D2 dopamine-selective antagonist, was measured. A lag in the effect of antagonists not seen with up-regulation induced by exposure to agonists was observed. Effects of N-propylnorapomorphine were synergistic with those of cyclic AMP analogs, however, synergistic effects between (-)-sulpiride and cyclic AMP analogs were not observed. These findings suggest that separate mechanisms may be involved in agonist- and antagonist-induced up-regulation. Changes in mRNA levels did not appear to account for the increase in receptors after agonist or antagonist treatment. Results of experiments with cycloheximide, a protein-synthesis inhibitor, suggest that increased protein synthesis, and not decreased protein degradation, is responsible for up-regulation by both NPA and (-)-sulpiride. Studies monitoring receptor recovery after treatment with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, an irreversible alkylating agent, suggest that rates of receptor incorporation into membranes are increased after treatment with either an agonist or an antagonist.