Developmental and Regional Studies of the Metabolism of Inositol 1,4,5‐Trisphosphate in Rat Brain

Abstract

Coupling of CNS receptors to phosphoinositide turnover has previously been found to vary with both age and brain region. To determine whether the metabolism of the second messenger inositol 1,4,5‐trisphosphate also displays such variations, activities of inositol 1,4,5‐trisphosphate 5′‐phosphatase and 3′‐kinase were measured in developing rat cerebral cortex and adult rat brain regions. The 5′‐phosphatase activity was relatively high at birth (∼50% of adult values) and increased to adult levels by 2 weeks postnatal. In contrast, the 3′‐kinase activity was low at birth and reached ∼50% of adult levels by 2 weeks postnatal. In the adult rat, activities of the 3′‐kinase were comparable in the cerebral cortex, hippocampus, and cerebellum, whereas much lower activities were found in hypothalamus and pons/medulla. The 5′‐phosphatase activities were similar in cerebral cortex, hippocampus, hypothalamus, and pons/medulla, whereas 5‐to 10‐fold higher activity was present in the cerebellum. The cerebellum is estimated to contain 50–60% of the total inositol 1,4,5‐trisphosphate 5′‐phosphatase activity present in whole adult rat brain. The localization of the enriched 5′‐phosphatase activity within the cerebellum was examined. Application of a histochemical lead‐trapping technique for phosphatase indicated a concentration of inositol 1,4,5‐trisphosphate 5′‐phosphatase activity in the cerebellar molecular layer. Further support for this conclusion was obtained from studies of Purkinje cell‐deficient mutant mice, in which a marked decrement of cerebellar 5′‐phosphatase was observed. These results suggest that the metabolic fate of inositol 1,4,5‐trisphosphate depends on both brain region and stage of development.