Pharmacological characterization of the receptors involved in the apomorphine-induced polyphasic modifications of locomotor activity in mice.

Increasing doses of apomorphine elicited polyphasic changes in locomotor activity in mice, consisting of: (i) hypokinesia with a peak effect at 25 micrograms/kg, which was not antagonized by various neuroleptics used at the highest doses not changing spontaneous locomotor activity, except for haloperidol; (ii) a relative restoration of locomotor activity at 75 micrograms/kg; this effect was antagonized by all the neuroleptics tested (except clozapine) and was hardly affected by domperidone and sulpiride; this apomorphine-induced effect was markedly increased 12 days after ICV 6-hydroxydopamine, (iii) a further hypokinetic effect with a peak effect at 150 micrograms/kg-1 (occurring simultaneously with hypothermia) which was antagonized by all the neuroleptics tested (including sulpiride at low doses) except levomepromazine and clozapine, and was again hardly affected by domperidone; this hypokinesia was reduced after prior (24 h) administration of 5 mg/kg apomorphine, (iv) finally another restoration of locomotor activity was observed at doses of apomorphine above 200 micrograms/kg.