Effect of?-adrenoceptor agonists on apomorphine-induced turning in rats

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Abstract
Theβ-adrenoceptor agonists clenbuterol, salbutamol and formoterol were found to inhibit apomorphine-induced turning behavior in rats with unilateral nigrostriatal lesions. The inhibitory effect of clenbuterol was antagonized by (−)-propranolol, but unaffected by practolol (which does not cross the blood-brain barrier), indicating a central localization of theβ-adrenoceptors mediating the inhibitory effect. A dopamine-releasing activity of clenbuterol did not seem to be responsible for the effect, because known dopamine-releasing agents (amphetamine, methylphenidate) did not antagonize apomorphine-induced turning behavior. Furthermoreα-methyl-p-tyrosine pretreatment did not prevent the inhibitory effect of clenbuterol. Since L-5-hydroxytryptophan did not mimic the inhibitory effect of clenbuterol, it was concluded that the inhibition of turning behavior ist not mediated by the interaction of clenbuterol with the serotonin system. The finding that clenbuterol depressed locomotor activity in the same dose range as it inhibited apomorphine-induced turning tentatively suggests that clenbuterol inhibits apomorphine-induced turning behavior by its centralβ-adrenoceptor mediated sedative action.