Behavioral sensitization and tolerance to cocaine and the occupation of dopamine receptors by dopamine

Data from the authors' laboratory on the neural substrates of Pavlovian conditioning and behavioral sensitization to psychomotor stimulants are reviewed. The findings of a recent experiment on the role of occupation of dopamine receptors by dopamine and its association to behavioral sensitization are reported. Daily intermittent injections of cocaine produced behavioral sensitization to the locomotor response in rats, whereas continuous cocaine infusions produced behavioral tolerance. Behavioral sensitization to cocaine was blocked by coadministration of nimodipine, an L-type calcium channel blocker. The increase in locomotion produced by cocaine was associated with an increase in the occupation of striatal dopamine D1 and D2 receptors, measured as the density of receptors protected from denaturation by N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ). This association was not observed when rats were given a challenge injection of cocaine 10 d after withdrawal from similar treatment regimens. Rats given a cocaine challenge after withdrawal from either intermittent or continuous cocaine treatments regimens exhibited increased occupation of striatal D1 and D2 receptors. This increase was similar in magnitude to that observed in rats without a history of cocaine treatments after a challenge injection of cocaine. This suggests that the differences in occupancy of striatal dopamine receptors by dopamine observed in the prewithdrawal condition are likely the results of differences in brain levels of cocaine achieved by the two treatment regimens. Occupancy of striatal dopamine D1 and D2 receptors does not appear to be related to the development of sensitization to the motor-stimulating effects of cocaine.