Distribution of 2A- and 2C-Adrenergic Receptor Immunoreactivity in the Central Nervous System

α₂ -Adrenergic receptors (α₂ -ARs) are G-protein-coupled receptors that mediate the inhibitory effects of norepinephrine and epinephrine in the peripheral and central nervous system (CNS). In the CNS, these receptors have been implicated in the regulation of analgesia, memory, and central cardiovascular control (1 ). Three subtypes of the receptor have been identified (α_2A -, α_2B -, and α_2C -ARs; for review, see ref. 2 ). Comparisons of the three subtypes show a high degree of sequence similarity and no apparent differences in their intracellular effectors. However, clues to the discrete functions of the subtypes have been provided by recent transgenic studies in which targeted mutation of the α_2A -AR or overexpression of the α_2C -AR in mice reveal specific effects on cardiovascular function, locomotor behavior, and thermoregulation (3 –6 ) (for reviews, see refs. 7 and 8 ).