POTENT ANTINOCICEPTIVE EFFECTS OF CLONIDINE SYSTEMICALLY ADMINISTERED IN AN EXPERIMENTAL-MODEL OF CLINICAL PAIN, THE ARTHRITIC RAT

The effects of various doses of the alpha-2 adrenoceptor agonist clonidine administered systemically (30, 50 and 100 mug/kg i.v.), were investigated on the vocalization threshold to paw pressure in normal rats and in rals with Freund's adjuvant-induced arthritis. Previous results have suggested that there is an increase in the activity of the bulbospinal noradrenergic systems in these arthritic animals. In the present study, clonidine led to significant antinociceptive effects in both groups of rats. Clonidine was found to be highly effective in arthritic animals, even at the lower concentration: the elevation in threshold produced by 30 mug/kg i.v. was 160% in arthritic vs. 124% in normal rats. The effects of clonidine were prevented dose-dependently by pretreatment with yohimbine or idazoxan 250 to 1000 mug/kg i.v., in the two groups of rats, indicating clearly that the dose-dependent effects of i.v. clonidine are mediated by alpha-2 adrenoceptors.