Dopaminergic control of locomotion, mouthing, snout contact, and grooming: opposing roles of D₁ and D₂ receptors

The study compares the behavioral profiles induced in rats (N=118) by the D₂-dopaminergic receptor agonist quinpirole (0.03 and 0.5 mg/kg), and the D₁-agonist SKF38393 (1.25-40 mg/kg), and both agonists administered together. Locomotion and snout contact frequency were reduced by the low but increased by the high dose of quinpirole; SKF38393 also reduced these behaviors and attenuated the effect of the high quinpirole dose. Only the high dose of quinpirole increased the duration of snout contact bouts and the frequency of mouthing; SKF38393 had no effect but in combination with the high dose of quinpirole, it enhanced the performance of these behaviors greatly. The duration of mouthing bouts was not affected by either agonist but was greatly extended when SKF38393 was administered together with the high dose of quinpirole. Grooming was inhibited by both the low and the high dose of quinpirole, and stimulated by the injection of SKF38393 or its addition to the low dose of quinpirole. These findings suggest that snout contact is controlled by modulating the frequency of episodes whereas mouthing is controlled by modulating the duration of episodes. Moreover, although they do not disprove the prevailing notion of D₁-D₂ receptor synergism, the present data are consistent also with an oppositional model of D₁-D₂ receptor interaction in the regulation of locomotion, snout contact, mouthing, and grooming in intact animals.