Alpha-2 adrenergic receptor subtypes indicated by [3H]yohimbine binding in human brain

Pharmacologic characterization of mammalian alpha-2 adrenergic receptors in various tissues and species has provided evidence for the existence of two alpha-2 adrenergic receptor subtypes. Prazosin and oxymetazoline have been shown to differentiate between the receptor subtypes as defined in rat tissues. In order to determine the relative proportions of these two receptor subtypes in human brain, the inhibition of the binding of the alpha-2 adrenergic antagonist [3H]yohimbine by oxymetazoline and prazosin was studied in membranes from three brain regions. Inhibition curves in membranes from the cerebral cortex and cerebellum were consistent with a single class of receptor binding sites suggesting that these two brain regions contain only one of the two subtypes. This subtype has the pharmacologic characteristics of the alpha-2A adrenergic subtype (yohimbine greater than oxymetazoline much greater than prazosin). In contrast, inhibition curves for both ligands in the human caudate nucleus were consistent with a model of two classes of binding sites in approximately equal proportions, suggesting that this tissue contains approximately equal densities of the alpha-2A and alpha-2B adrenergic receptor subtypes.