Circling behavior induced by intranigral injections of GABA and muscimol in rats.

Circling behavior induced by unilateral intranigral injections of GABA or muscimol was studied in rats. Both GABA (10--400 micrograms) and muscimol (1--100 ng) evoked the same kind of contralateral circling behavior dose-dependently when injected into the substantia nigra. Muscinol was much more potent that GABA and its effect lasted longer. Neither GABA nor muscimol induced any ipsilateral circling. Pretreatment with bicuculline (3 mg/kg IP) significantly attenuated the intensity of circling behavior induced by intranigral injections of GABA or muscimol. Pretreatment with haloperidol (0.5 and 1.0 mg/kg SC) also significantly antagonized the circling behavior induced by GABA and muscimol. Pretreatment with atropine (10 mg/kg IP) significantly increased the intensity of circling behavior induced by intranigral injections of muscimol and tended to increase the intensity of circling behavior induced by intranigral injections of GABA. Pretreatment with struchnine (0.25 mg/gk IP) did not modify circling behavior induced by GABA, but did to some extent increase that induced by muscimol. These results indicate that the contralateral circling behavior induced by intranigral injections of GAB and muscimol seems to be dependent both on the activation of the dopaminergic nigrostriatal system and on the nondopaminergic nigral output system.