Behavioral effects of the R-(+)- and S-(−)-enantiomers of the dopamine D1-like partial receptor agonist SKF 83959 in monkeys

Dopamine D1-like partial receptor agonists such as SKF 83959 have been proposed as potential candidates for the treatment of cocaine addiction. The present studies were conducted to further characterize SKF 83959 by pharmacologically evaluating effects of its R-(+)- and S-(−)-enantiomers, MCL 202 and MCL 201, respectively, on overt behavior (eye blinking) and schedule-controlled performance in squirrel monkeys. MCL 202, like the D1 full receptor agonist SKF 82958, produced dose-related increases in eye blinking and decreases in rates of fixed-ratio responding. However, the magnitude of effects of MCL 202 on eye blinking was less than observed with SKF 82958. In contrast to the effects of its R-(+) enantiomer, MCL 201 was relatively devoid of behavioral activity up to doses that were approximately 10-fold greater than MCL 202. Pretreatment with the selective D1-like receptor antagonist SCH 39166 dose-dependently antagonized increases in eye blinking produced by MCL 202, confirming the involvement of D1 mechanisms in its effects. A dose-ratio analysis of the antagonism of effects of MCL 202 by SCH 39166 revealed an apparent pA2 value of 7.675 with a slope of − 0.78 ± 0.04. In further studies, pretreatment with MCL 202 antagonized the effects of SKF 82958 on eye blinking and, like SCH 39166, schedule-controlled behavior in a dose-related manner. A dose-ratio analysis of the antagonist effects of MCL 202 on the SKF 82958-induced increases in eye blinking revealed ratios of 2.7, 4.8 and 31.1 for 0.1, 0.3 and 1.0 mg/kg dose of the antagonist, respectively, indicative of a significant change in the potency of SKF 82958. These results suggest that MCL 202, like its parent compound SKF 83959, has both D1 receptor-mediated agonist and antagonist properties, consistent with its characterization as a partial agonist at the D1-like receptor. In addition, the inactivity of MCL 201, the S-(−)-enantiomer, suggests that the behavioral effects of SKF 83959 can be attributed primarily to the activity of its R-(+)-enantiomer.