ALPHA(2A)-ADRENERGIC RECEPTORS ARE PRESENT IN LOWER BRAIN-STEM CATECHOLAMINERGIC AND SEROTONERGIC NEURONS INNERVATING SPINAL-CORD

A subtype-specific polyclonal antibody was used for the immunohistochemical detection of alpha(2A)-adrenergic receptors (alpha(A2)-ARs) in the rat lower brainstem (medulla and pens). Using dual-label fluorescence histochemistry, punctate alpha(2A)-AR-like immunoreactivity (alpha(2A)-AR-LIR) was identified in noradrenergic, adrenergic, and serotonergic neurons of the pontomedullary region. Confocal microscopic examination of material simultaneously labeled for TH-LIR and alpha(2A)-LIR revealed that the clusters of alpha(2A)-LIR were located intracellularly. Lower medullary neurons with spinal projections to segment T-3 were retrogradely labeled using FITC-conjugated microbeads and the material was processed for simultaneous detection of alpha(2A)-LIR and either TH-LIR or 5-HT-LIR. Using this triple-label approach, we found that virtually all medullary serotonergic cells (raphe pallidus, raphe obscurus and parapyramidal area) including those with identified spinal projections contain punctate alpha(2A)-AR-LIR. In contrast, fewer than 10% of dorsal raphe serotonergic cells examined for comparison were immunoreactive. The triple labeling approach also indicated that more than 95% of the TH-immunoreactive cells of the dorsal and ventrolateral medulla, including those with demonstrable spinal projections (A5 noradrenergic and C1/C3 adrenergic) had detectable amounts of alpha(2A)-AR-LIR. The presence of alpha(2A)-ARs in a large fraction of bulbospinal pre-sympathetic neurons (noradrenergic A5, adrenergic C1 and C3 and serotonergic raphe cells) could explain the powerful and relatively selective effect of clonidine and other centrally acting alpha(2A)-AR agonists on sympathetic efferent activity and hypertension.