Fig. 6.

The anesthetic chloral hydrate blocks the D1-mediated feedback inhibition of SN DA neurons. A, Graph showing lack of an effect of a D1 agonist on SN DA cells in chloral hydrate-anesthetized animals (400 mg/kg, i.p.). Before testing for the effect of SKF38393, rats were pretreated with a high dose of quinpirole (40–160 μg/kg, i.v.). Open andfilled circles represent the firing rate of individual cells before and after SKF38393 injection (10–20 mg/kg, i.v.), respectively. No significant D1 effect was observed in any of the seven cells tested. The average firing rate of the cells was slightly increased from 33.7 ± 4.0 to 35.8 ± 4.0% of baseline after SKF38393 injection (open and filled bars;p = 0.082). B, Typical rate histogram showing lack of an effect of SKF38393 (20 mg/kg) on an SN DA cell in a chloral hydrate-anesthetized animal (400 mg/kg, i.p.). The firing rate of the cell was reduced to about 30% of baseline after a high dose of quinpirole (QUIN, 80 μg/kg, i.v.). Subsequent administration of SKF38393 (SKF) and SCH23390 (SCH) produced no effect on the remaining activity. Haloperidol (HAL) returned the firing rate to baseline.