Supersensitivity to apomorphine in experimentally induced hypokinesia and drug-induced modifications of the apomorphine response.

The development and degree of supersensitivity to the locomotor stimulant effect of apomorphine were studied in rats which had been rendered hypokinetic by bilateral injections of 6-hydroxydopamine into the anterolateral hypothalamus. Up to 2 days after surgery the effect of apomorphine was comparable in lesioned and normal rats, indicating that dopaminergic supersensitivity did not develop over this short period. As the duration between the 6-hydroxydopamine injections and time of testing with apomorphine increased, the animals became progressively more sensitive to the stimulant effects of apomorphine. Pretreatment with butaclamol reduced the effect of apomorphine in a dose-dependent manner. A high dose of clozapine also antagonized the effect of apomorphine, but a low dose potentiated it. No inhibition was observed following administration of the alpha-adrenergic antagonist, phenoxybenzamine, or the beta-adrenergic antagonist, propranolol. The 5HT antagonist methysergide and the anticholinergic drug, scopolamine potentiated the effects of apomorphine.l These studies suggest that the apomorphine-induced ambulation in hypokinetic rats is primarily mediated through dopaminergic mechanisms but both serotonergic and cholinergic mechanisms exert modulating influences.