Potency and efficacy of dopamine agonists in mouse strains differing in dopamine cell and receptor number

The potency and efficacy of the selective dopamine D2 receptor agonist quinpirole, the mixed D1/D2 agonist apomorphine, and the selective D1 receptor agonist SKF 38393 in producing hypothermia and changes in locomotor activity were evaluated in four strains of mice: CBA/J, C57BL/6J, ICR Swiss and CF1. CBA/J mice previously have been shown to be deficient in dopamine cell and receptor number relative to other strains such as C57BL/6J mice, whereas ICR Swiss and CF1 are commonly used strains of mice. Quinpirole (0.125 to 1.0 mg/kg) was equiefficacious and equipotent in producing hypothermia in all 4 strains. Apomorphine (0.125 to 16 mg/kg) was equiefficacious in producing hypothermia in all 4 strains, but was approximately four-fold less potent in CBA/J mice than in the other strains. SKF 38393 had little effect on body temperature in any of the 4 strains. Basal motor activity was lowest in CBA/J mice, and tended to be highest in ICR Swiss mice. Quinpirole (0.125 to 32 mg/kg) had no effect on motor activity in CBA/J mice, but decreased motor activity in the other 3 strains. Apomorphine (1 to 16 mg/kg) produced modest increases in motor activity in all 4 strains. The magnitude of the changes produced by apomorphine was comparable in all strains when expressed as change from mean control values. SKF 38393 (8 to 64 mg/kg) also increased motor activity in all 4 strains, with comparable increases when expressed as change from mean control values. The present results are consistent with the interpretation that inherited deficiencies in dopamine cell and receptor number in CBA/J mice produce functional decrements in D2, but not D1, dopamine receptor function.