Antinociceptive effect of clomipramine in monoarthritic rats as revealed by the paw pressure test and the C-fiber-evoked reflex

The antinociceptive effect of clomipramine was studied in monoarthritic rat s by using the paw pressure test and the C-fiber-evoked reflex. Monoarthrit is was produced by intra-articular injection of complete Freund's adjuvant into the tibio-tarsal joint. Joint circumference as well as vocalization th reshold to graded paw pressure were evaluated weekly during a 14-week perio d after the intra-articular injection. At week 8, monoarthritic and vehicle -injected control rats were given either clomipramine or saline and both th e paw pressure threshold and inhibition of the C-fiber-evoked reflex respon se were evaluated. Results showed that (i) 1.5, 3.0, and 6.0 mg/kg, i.v. of clomipramine induced significantly greater dose-dependent antinociception to paw pressure testing in the monoarthritic group, as compared to the cont rol one; and (ii) 0.75, 1.5, 3.0, and 6.0 mg/kg, i.v. of clomipramine exert ed significantly higher dose-dependent inhibition of the C-reflex activity in monoarthritic rats than in controls. Results suggest that the higher sen sitivity to clomipramine in monoarthritic rats could be related to adaptive changes occurring in monoamine metabolism or in other neurotransmitter sys tems during chronic pain. (C) 2001 Published by Elsevier Science B.V.