Distribution of alpha 2-adrenergic receptors in the human brainstem: an autoradiographic study using [3H]p-aminoclonidine.

The distribution of alpha 2-adrenergic receptors in the human brainstem and some forebrain areas was examined by means of an in vitro autoradiographic technique using [3H]p-aminoclonidine as a ligand. High densities of alpha 2-adrenergic receptors were observed in the dorsal motor nucleus of the Xth nerve, the nucleus of the solitary tract, the locus coeruleus and the substantia grisea centralis. Other brainstem areas presenting significant densities of alpha 2-adrenergic receptors were the substantia gelantinosa of the Vth nerve, the nucleus cuneiformis, the superficial gray layer of the superior colliculus, the cortical mantle of the inferior colliculus and the inferior olivary nuclei. Of the forebrain areas studied, the highest density was seen in the primary visual cortex. The distribution observed presents some significant differences with that previously described for the rat. The localization of alpha 2-adrenergic receptors in the human brain correlates well with the distribution of noradrenergic neurons in the human brainstem and suggests sites of action of alpha 2-adrenergic drugs in the human brain.