Influences of dopamine agonists and antagonists of baclofen antinociception in mice.

The effects of dopamine agonists and antagonists on baclofen antinociception were examined in mice using the tail-flick test. Injection of different doses of baclofen (5-10 mg/kg, i.p.) produced dose-related antinociception. This effect was decreased in animals treated with SKF 38393 (8 mg/kg, i.p.) or apomorphine (2 mg/kg, s.c.). The inhibitory responses of both SKF 38393 and apomorphine were decreased in animals pretreated with SCH 23390 (0.1 mg/kg, s.c.). Bromocriptine (1 mg/kg, i.p.), or lower doses of apomorphine (0.1 mg/kg, s.c.), did not alter the baclofen-induced antinociception. Antinociception induced by baclofen (5-10 mg/kg, i.p.) was decreased in animals pretreated with sulpiride (10 mg/kg, i.p.) but not by SCH 23390 (0.1 mg/kg, s.c.). Administration of apomorphine, SKF 38393, bromocriptine or sulpiride alone did not change the latency in the tail-flick test, but SCH 23390 induced a slight but significant antinociception. On the basis of the above findings, it may be concluded that D-1 receptor activation can decrease the baclofen antinociceptive effect.