Action of harmaline and diazepam on the cerebellar content of cyclic GMP and on the activities of two endogenous inhibitors of protein kinase.

The rat cerebellum contains a significant amount of cGMP-dependent protein kinase, cAMP-dependent and cyclic nucleotide-independent protein kinase, and a large concentration of protein kinase inhibitors. These inhibitors are thermostable proteins which can be separated by gel chromatography into two molecular forms: the type 1 and type 2 inhibitors of protein kinase (14). The type 1 inhibitor blocks the rat cerebellar cAMP-dependent protein kinase activity while the type 2 inhibitor blocks the cGMP-dependent protein kinase, the cAMP-dependent protein kinase, and the cyclic nucleotide-independent protein kinases. The activity of the type 2 inhibitor increased or decreased in opposite direction to changes of cerebellar cGMP content generated by injection of 10 mg/kg harmaline 2.5 mg diazepam. No changes of type 1 inhibitor were observed under these conditions. The drug-induced shift of type 2 inhibitor of protein kinase was not mediated by changes in protein synthesis because it persisted after pretreatment with cycloheximide. These results are compatible with the hypothesis that cGMP modulates phosphorylation in cerebellum by changing the relationship between cGMP-dependent protein kinase and type 2 inhibitor content.