Effects of fentanyl and droperidol on the dopamine metabolism of the rat striatum.

Fentanyl in low doses (0.02-0.05 mg/kg) had no significant effect on the dopamine turnover 20 min after intraperitoneal (i.p.) injection and raised the homovanillic acid (HVA) concentration in the rat striatum with a maximum at 60 min after i.p. injection. Higher doses (0.1-0.2 mg/kg)reduced the HVA content after 20 min and raised it after 60 min. Naloxone (1 mg/kg) given shortly (5min) after high doses of fentanyl (0.2 mg/kg) was able to eliminate the early (i.e., 20 min after i.p. injection) decrease and the late (i.e., 60 min after i.p. injection)increase of HVA content in the striatum. Naloxone (1mg/kg) alone slightly increased the HVA concentration, after 20 as well as after 60 min. Droperidol (3-12 mg/kg) and haloperidol (6-12 mg/kg) markedly increased the HVA content in the rat striatum 90 min after i.p. injection. These effects were dose-dependent. Naloxone (1 mg/kg) did not significantly inhibit the rise of HVA induced by droperidol.