Alpha 2A-adrenergic receptors are present in lower brainstem catecholaminergic and serotonergic neurons innervating spinal cord.

A subtype-specific polyclonal antibody was used for the immunohistochemical detection of alpha 2A-adrenergic receptors (alpha 2A-ARs) in the rat lower brainstem (medulla and pons). 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 T3 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.