3H-apomorphine labels both dopamine postsynaptic receptors and autoreceptors

Evidence points to the existence of multiple classes of dopamine (DA) receptor in mammalian brain which can be distinguished by their pharmacological specificities and localizations, and by the actions they mediate. Among them, dopaminergic autoreceptors are regulatory receptors presumed to be present in the membrane of the DA neurones themselves, and believed to mediate an inhibition of these neurones' activity, either at nerve endings or on cell bodies. However, the pharmacology of autoreceptors remains to be established because attempts to characterize autoreceptors by 3H-ligand binding techniques have produced controversial data. Thus Seeman and co-workers stated that 3H-apomorphine selectively labels autoreceptors, whereas Creese et al. concluded that this ligand selectively labels postsynaptic DA receptors. In addition, large differences in the capacity and drug specificity of 3H-apomorphine receptor sites in rat striatum have been reported. We demonstrate here that 3H-apomorphine labels two classes of DA receptor, distinguishable using domperidone, a selective DA antagonist. Lesion studies indicate that they correspond to a certain class of postsynaptic receptor and to autoreceptors, respectively. Each of these classes displays a clearly distinct pharmacological specificity for antipsychotics and DA agonists.