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The I₁-imidazoline receptor is a novel brainstem modulator of sympathetic outflow that is elevated on platelets and in brains of depressed patients. A positive correlation has been reported (accompanying manuscript) between plasma norepinephrine (NE) concentrations and the densities (B_max) of platelet I₁ binding sites (I₁ sites). I₁-candidate proteins of 33 kDa and 85 kDa are now identified on Western blots probed with anti-imidazoline receptor antiserum (IRBP antiserum), that correlate with B_max values for I₁ sites. Furthermore, a human megakaryoblastoma cell line (MEG-01) has been used to study the regulation of these proteins on megakaryocytic cells, while bovine adrenal chromaffin cells provide a standard I₁ cell type for comparison. Both the 33 kDa and 85 kDa IRBP-immunoreactive bands were enriched in plasma membrane fractions. IRBP antiserum did not cross-react with I₂ imidazoline binding sites …