Enzymatic regulation of the concentration of cyclic GMP in mouse brain

Abstract

There are marked differences in the concentration of cyclic GMP in various regions of mouse brain. For example, in cerebellum, the concentration of cyclic GMP is about ten times higher than that of cerebral cortex, corpus striatum, hippocampus or brain stem. In an attempt to determine the reason for this variation in cyclic GMP levels, the concentration of cyclic GMP and the activities of guanylate cyclase and cyclic GMP phosphodiesterase in several areas of mouse brain have been compared. Of the brain areas examined, the total guanylate cyclase activity found in whole homogenates was highest in the corpus striatum. Further studies showed that this enzyme is present in both the soluble and particulate fractions and that the ratio of the specific activities of guanylate cyclase between the soluble and particulate fractions varied with the brain area. Thus, in the corpus striatum, the specific activity of guanylate cyclase in the soluble fraction was about five times that of the particulate fraction. By contrast, in the hippocampus, the specific activity of the enzyme was similar in both fractions. The activity of cyclic GMP phosphodiesterase also varied with the brain area. The corpus striatum had the highest enzyme activity when measured either at low (2 μM) or high (200 μM) substrate concentrations; the cerebellum and brain stem had the lowest activities of phosphodiesterase. Cyclic GMP phosphodiesterase activity also was measured in the presence of the endogenous protein activator of phosphodiesterase and in the presence of EGTA, which prevents the calcium-dependent activation of the enzyme. These studies indicated that the corpus striatum and the brain stem had the highest, and the cerebellum, the lowest proportion of the activator-dependent form of phosphodiesterase. The results suggest that the activity of cyclic GMP phosphodiesterase is more important than that of guanylate cyclase in controlling the basal concentration of cyclic GMP in mouse brain. They also suggest that the high basal concentration of cyclic GMP in the cerebellum and the exaggerated increases in the nucleotide that occur in this tissue in response to various experimental manipulations may be due to a relatively low activity of cyclic GMP phosphodiesterase and, in particular, to a relatively low activity of the activator-dependent form of this enzyme.