3H-apomorphine labels both dopamine postsynaptic receptors and autoreceptors

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Abstract
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 neurons themselves, and believed to mediate an inhibition of these neurons'' activity, either at nerve endings or on cell bodies. The pharmacology of autoreceptors remains to be established because attempts to characterize autoreceptors by 3H-ligand binding techniques have produced controversial data. Seeman and co-workers stated that 3H-apomorphine selectively labels autoreceptors, whereas Creese concluded that this ligand selectively labels postsynaptic DA receptors. Large differences in the capacity and drug specificity of 3H-apomorphine receptor sites in rat striatum were reported. 3H-Apmorphine labels 2 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.