IMMUNOHISTOCHEMICAL LOCALIZATION OF ALPHA-2A-ADRENERGIC RECEPTORS IN CATECHOLAMINERGIC AND OTHER BRAIN-STEM NEURONS IN THE RAT

Alpha2-adrenergic receptors mediate a large portion of the known inhibitory effects of catecholamines on central and peripheral neurons. Molecular cloning studies have established the identity of three alpha2-adrenergic receptor genes from several species that encode the A, B and C subtypes of the receptor. The rat alpha2A-adrenergic receptor, as defined by sequence similarity, is the orthologue of the human alpha2A-adrenergic receptor. In this paper, we report the development of rabbit antisera directed against a portion of the third intracellular loop of the rat alpha2A-adrenergic receptor and the histochemical localization of alpha2A-adrenergic receptor-like immunoreactive material in the brainstem and spinal cord of the adult rat. Our antisera detected alpha2A-adrenergic receptor-specific punctate staining associated with neuronal perikarya. Alpha2A-adrenergic receptor-like immunoreactivity was widely, but heterogeneously, distributed in the brainstem and spinal cord, predominantly in areas involved in the control of autonomic function. Double labelling with antisera to tyrosine hydroxylase or phenylethanolamine-N-methyl-transferase revealed that alpha2A-adrenergic receptor-like immunoreactivity is present in most, perhaps all, noradrenergic and adrenergic cells of the brainstem. Alpha2A-adrenergic receptor-like immunoreactivity was detected in a small percentage of the dopaminergic cells of the A9 and A10 groups. This study provides the first description of the specific immunohistochemical localization of alpha2A-adrenergic receptors using a subtype-specific polyclonal antibody. The results support the view that alpha2-adrenergic receptors are involved in central cardiovascular control and suggest that the catecholaminergic autoreceptors of central noradrenergic and adrenergic neurons are the A subtype of the alpha2-adrenergic receptors.