Effects of dopaminergic agents on visceral pain measured by the mouse writhing test

The present study explored the role of the dopaminergic transmission in the mouse writhing test analgesia by examining the relative analgesic activity of indirect dopaminergic agonists (amphetamine and cocaine), a mixed D1/D2 direct agonist (apomorphine), and a direct D1 (SKF38393) and D2 (bromocriptine) dopaminergic agonist. Amphetamine (1, 3 and 10 mg/kg, s.c.), cocaine (3 and 10 mg/kg, s.c.), apomorphine (0.3, 1 and 3 mg/kg, s.c.) and bromocriptine (30 mg/kg, s.c.) induced a significant decrease of the number of writhes. SKF38393 (1, 3, 10 and 30 mg/kg, s.c.) had no effect on writhing. The antinociceptive effect of amphetamine and cocaine was not reversed by naltrexone, haloperidol or SCH23390. The apomorphine- and bromocriptine-induced analgesia was not reduced by naltrexone or SCH23390 but was attenuated by haloperidol; the apomorphine-induced analgesia was not modified by domperidone. The present results suggest an involvement of the dopaminergic transmission in visceral nociception. This dopaminergic component appears to involve exclusively the central D2 receptor system, and does not seem to be influenced by opioid mechanisms.