Pharmacologic characterization of imidazoline receptor proteins identified by immunologic techniques and other methods

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.