Fig. 2.

Systemic activation of D1-like receptors produces no consistent effect on the activity of SN DA cells in rats pretreated with low doses of a D2 agonist. A, Graph showing lack of a consistent effect of the D1 agonist SKF38393 on SN DA cells in rats pretreated with low doses of the D2 agonist quinpirole (5–20 μg/kg, i.v.). Open and filled circles represent, respectively, the firing rate of individual cells immediately before SKF38393 administration (10–20 mg/kg, i.v.) and after SKF38393 had produced a maximum effect. Straight lines link the data from individual cells. Of nine cells tested, seven were slightly excited or showed no response to SKF38393 (<10% change in baseline) and two were inhibited. Overall, the firing rate was decreased nonsignificantly from 56.5 ± 7.3 to 52.8 ± 9.4% of baseline (p = 0.757; n = 9; open and filed bars).B, Typical rate histogram showing lack of an effect of SKF38393 on the activity of an SN DA cell pretreated with a low dose of quinpirole. A single dose of quinpirole (QUIN, 10 μg/kg, i.v.) inhibited the firing of the cell to ∼40% of baseline. Subsequent administration of SKF38393 (SKF, 10 mg/kg, i.v.) did not significantly affect the remaining activity. In this and following figures, arrows indicate the times of drug injection, and the numbers above thearrows represent the doses of the drug injected.