Circling behaviour induced by intranigral injection of baclofen in rats

Baclofen (10 ng-1 micrograms) induced a dose-dependent contral ateral circling behaviour when injected unilaterally into the pars reticulata of the rat substantia nigra (SN). Pretreatment with bicuculline (3 mg/kg i.p.) significantly attenuated the intensity of the circling behaviour induced by intranigral baclofen but failed to prevent it completely. Pretreatment with strychnine (0.25 mg/kg i.p.) did not modify the circling behaviour induced by baclofen. Pretreatment with haloperidol (0.5 and 1.0 mg/kg s.c.) significantly but not completely antagonized the circling response to baclofen. Pretreatment with atropine (10 mg/kg i.p.) signficantly increased the intensity of the circling behaviour within the first 30 min after intranigral injection of baclofen. The results show that baclofen, when injected uniaterally into the SN, causes contralateral circling behaviour similar to that described for GABA and muscimol. This suggests that baclofen may have some GABA agonist activity in the SN. The contralateral circling behaviour induced by baclofen seems to be mediated by both dopaminergic and non-dopaminergic systems.