CHRONIC MORPHINE ALTERS DOPAMINE TRANSPORTER DENSITY IN THE RAT-BRAIN - POSSIBLE ROLE IN THE MECHANISM OF DRUG-ADDICTION

The effect of acute or chronic morphine treatment on dopamine transporters was studied with the selective transporter blocker [H-3]GBR12935. Chronic, but not acute treatment of rats with morphine significantly decreased the B-max of [H-3]GBR12935 binding to membranes from the anterior basal forebrain, that includes the nucleus accumbens, but had no such effect on binding to striatum membranes. No effect on the affinity (K-d) of the radioligand to either one of the two brain regions was observed. The selectivity of morphine interaction with the dopamine system was tested with a ligand that binds selectively to serotonin transporters, [H-3]citalopram. Neither acute nor chronic morphine altered [H-3]citalopram binding to the anterior basal forebrain, or the striatum membranes. It is suggested, therefore, that chronic morphine treatment has a long-lasting and selective effect on the activity of dopamine transporters in the dopaminergic reward pathway, but not in the striatum.