Effects of dopamine D 1 and D 2 receptor antagonists on oral activity in rats

Abstract

Two experiments were performed to investigate the actions of the selective D 1 blocker SCH 23390 and the selective D 2 blocker sulpiride, on oral movements in rats; these were quantified by a human observer scoring vacuous chewing movements (VCMs), jaw tremor and head movements, as well as a computer analysis which measured the amplitude and slope of each movement. In the first experiment it was found that both SCH 23390 and sulpiride decreased VCMs and head movements in a dose-dependent manner, with SCH 23390 more effectively decreasing head movements and sulpiride more effectively decreasing VCMs. In a second experiment, the effectiveness of these two drugs in blocking the actions of selective D 1 (SKF 38393) and D 2 (LY 171555) agonists was studied. The SKF 38393-induced increase in computer-scored movement was attenuated by both sulpiride and SCH 23390, whereas the LY 171555-induced decrease in VCMs was attenuated by sulpiride, while SCH 23390 exacerbated it. These findings, together with our earlier results (12), suggest a simple relationship of D 1 receptors to oral movement, with increased activation resulting in increased oral movement and decreased activation resulting in decreased oral movement. The relationship of D 2 receptors to oral movement shows a more complex pattern, with both stimulation and blockade decreasing oral movement. One possibility may be the existence of more than one subpopulation of D 2 receptors mediating these effects.