Neurochemical Evidence that Postsynaptic Nucleus Accumbens D3 Receptor Stimulation Enhances Cocaine Reinforcement

The mechanism by which two D 3 receptor-preferring agonists, 7-hydroxydipropylaminotetralin (7-OH-DPAT) and quinelorane, modulate cocaine reinforcement was examined by monitoring nucleus accumbens dopamine levels with in vivo microdialysis while rats intravenously self-administered the following four different drug solutions consecutively : (1) cocaine ; (2) a combination of cocaine plus a low dose of either agonist ; (3) either agonist alone ; and finally, (4) a physiological saline solution. Both 7-OH-DPAT (4 μg/infusion) and quinelorane (0.25 μg/infusion) decreased cocaine (0.25 mg/infusion) intake in a manner indicating an enhancement of cocaine reinforcement and simultaneously decreased the cocaine-induced elevations in nucleus accumbens dopamine levels by >50%. Subsequent self-administration of either 7-OH-DPAT (4 μg/infusion) or quinelorane (0.25 μg/infusion) alone resulted in significant, but stable, increases in drug intake, with a concurrent decrease in nucleus accumbens dopamine levels to ∼50% below nondrug baseline levels. These findings indicate that postsynaptic D 3 receptor stimulation in the nucleus accumbens enhances the reinforcing properties of cocaine. In a second experiment, local application of 7-OH-DPAT via reverse dialysis (30 and 100 nM perfusate concentrations) dose-dependently decreased nucleus accumbens dopamine efflux to 76 ± 3.9 and 61 ± 6.3% of baseline, respectively, whereas there was no effect of this agonist on dopamine efflux in the ipsilateral striatum of these same animals. Coperfusion with the D 3 receptor-preferring antagonist nafadotride dose-dependently blocked the effect of 7-OH-DPAT on nucleus accumbens dopamine efflux. These results suggest that, at low concentrations, 7-OH-DPAT selectively activates D 3 receptors in vivo.