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ABSTRACT: Biochemical and pharmacologic evidence supports the heterogeneous nature of imidazoline receptors (IRs). However, only monoamine oxidase (MAO) (55- and 61-kD) isozymes have been identified as imidazoline binding site-containing proteins. Idazoxan-binding proteins of ∼70- and ∼45-kD of unknown amino acid sequences have been isolated from chromaffin cells and rat brain, respectively. Other proteins of ∼27–30 to > 80 kD have been visualized by immunologic and photoaffinity labeling techniques in different tissues and species. The specific antiserum that recognizes the ∼70-, ∼45-, and ∼29-kD IR proteins, but not MAO, was used to quantitate these proteins in the rat brain cortex. Treatments (7 days) with the I2-selective imidazoline drugs idazoxan (10 mg/kg), cirazoline (1 mg/kg), and LSL 60101 ([2-(2-benzofuranyl) imidazole; 10 mg/kg]) induced differential changes in these proteins: levels of the ∼29-kD IR were increased by idazoxan and LSL 60101 (23%), levels of the ∼45-kD protein only by cirazoline (44%), and those of the ∼66-kD protein only by idazoxan (50%). These treatments also increased the densities of [3H]-idazoxan (I2) binding sites (32–42%). Chronic treatment with efaroxan, RX821002, and yohimbine (10 mg/kg), which possess very low affinity for I2-IRs, did not alter either their immunoreactivities or the density of I2 sites. Chronic treatment with MAO inhibitors clorgyline and phenelzine (10 mg/kg) and acute treatment with EEDQ (1.6 mg/kg, 6 h) induced decreases in the levels of these IR proteins (17–47%) and I2 sites (31–57%). Significant correlations were found when the mean percentage changes in immunoreactivity of IR proteins were related to the mean percentage changes in the density of I2 sites after treatment with the foregoing drug (r= 0.92, r= 0.69, and r= 0.75 for the ∼29-, ∼45-, and ∼66-kD proteins, respectively). These results indicate that in the rat cerebral cortex, the I2 sites labeled by [3H]idazoxan are heterogeneous and that the related immunoreactive IR proteins contribute differently to the modulation of I2 sites after drug treatment.