OPPOSITE INFLUENCES OF DIFFERENT ADRENOCEPTORS ON BACLOFEN-INDUCED ANTINOCICEPTION IN MICE

In the present study, the effects of adrenoceptor agonists and antagon ists on baclofen-induced antinociception was investigated. Intraperito neal administration of different doses of baclofen (2.5, 5 and 10 mg/k g) induced antinociception in the tail-flick test. The response was do se-dependent. The alpha(2)-adrenoceptor agonist, clonidine, increased, while the alpha(1)-adrenoceptor agonist, phenylephrine, decreased the baclofen response. In reserpine-treated animals, alpha(2)-adrenocepto r, clonidine, induced antinociception and increased that of baclofen. Yohambine and propranolol but not prazosin decreased the baclofen effe ct. Although phenoxybenzamine itself induced antinociception it did no t alter the baclofen response significantly. Reserpine treatment decre ased the response induced by single administration of baclofen or phen oxybenzamine and that induced by a combination of baclofen with either phenoxybenzamine or prazosin. It may be concluded that alpha(2)-adren oceptor stimulation increases, while alpha(1)-adrenoceptor activation decreases the baclofen-induced antinociception, and aminergic mechanis m(s) may have a positive influence on baclofen response.