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 similar to 70-and similar to 45-kD of unknown amino acid sequences have been isolated from chromaffin tells and-rat brain, respectively. Other proteins of similar to 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 similar to 70-, similar to 45-, and similar to 29-kD IR proteins, but not MAO, was used to quantitate these proteins in the rat brain cortex. Treatments (7 days) with the I-2-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 similar to 29-kD IR were increased by idazoxan:and LSL 60101 (23%), levels of the similar to 45-kD protein only by cirazoline (44%), and those of the similar to 66-kD protein only by idazoxan (50%). These treatments also increased the densities of [H-3]-idazoxan (I-2) binding sites (32-42%). Chronic treatment with efaroxan, RX821002,and yohimbine (10 mg/kg), which possess: very low affinity for I-2-IRs, did not alter either their immnnoreactivities or the density of I-2 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 I-2 shes (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 I-2 sites after treatment with the foregoing drug (r = 0.92, r = 0.69, and r = 0.75 for the similar to 29-,similar to 45-, and similar to 66-kD proteins, respectively). These results indicate that in the rat cerebral cortex, the I2 sites labeled by [H-3]idazoxan are heterogeneous and that the related immunoreactive IR proteins contribute differently to the modulation of I-2 sites after drug treatment.
