Repeated apomorphine treatment causes behavioural supersensitivity and dopamine D2 receptor hyposensitivity

This study reports the changes in the behavioural responsiveness of rats in the "Ungerstedt Model' following repeated injections of apomorphine. The changes in striatal [3H]haloperidol binding coincident with the behavioural responses are also estimated. Following unilateral lesions of the substantia nigra, the number of [3H]haloperidol binding sites in the ipsilateral striatum increase, and this effect correlates with the behavioural effect following a single apomorphine injection. Repeated apomorphine treatment, however, results in a decrease of neuroleptic binding sites in the ipsilateral and contralateral striatum accompanied by increased behavioural responses. Thus repeated apomorphine treatment discriminates the receptor mechanisms involved in the behavioural response from those receptors labelled by neuroleptics.