| Abstract: | Abstract: Contralateral circling behaviour induced by intranigral injection of morphine or an enkephalin analogue, FK 33–824, was studied in rats. Both morphine(0.625–10.0μg)and FK 33–824(1.25–40.0 ng) evoked the same kind of contralateral circling behaviour when injected into the substantia nigra. FK 33–824 was about 1000 times more potent than morphine and its effect lasted a little longer. Pretreatment with naloxone (5 mg/kg subcutaneously or 1 μg intranigrally) almost completely blocked the circling induced by intranigral morphine or FK 33–824. The lesion of the ipsilateral nigrostriatal dopaminergic pathway with 6-hydroxydopamine significantly antagonized both morphine- and FK 33–824-induced circling. Pretreatment with d-amphetamine (l mg/kg intraperitoneally) significantly increased the intensity of the circling induced by morphine or FK 33–824, and pretreatment with haloperidol (0.5 mg/kg subcutaneously) attenuated this intensity. Pretreatment with pilocarpine (10 mg/kg intraperitoneally) or mecamylamine (2 mg/kg intraperitoneally) inhibited the circling, whereas pretreatment with atropine (10 mg/kg intraperitoneally) or nicotine (0.2 mg/kg subcutaneously) increased morphine- and FK 33–824-induced circling. Pretreatment with muscimol (0.5 mg/kg subcutaneously) did not significantly increase morphine-induced circling but tended to increase FK 33–824-induced circling. Pretreatment with bicuculline (3 mg/kg intraperitoneally) slightly but significantly inhibited both morphine- and FK 33–824-induced circling. Pretreatment with strychnine (0.25 mg/kg intraperitoneally) did not modify the circling induced by morphine or FK 33–824. The results show that the contralateral circling behaviour induced by intranigral injection of morphine and FK 33–824 seems to be mediated by the activation of the dopaminergic nigrostriatal system. |
