M₁ Acetylcholine Receptor-Mediated Phosphatidylinositol Turnover in Adult and Senescent Rat Brain Slices

Abstract

Phosphatidylinositol (PI) turnover via muscarinic acetylcholine (mACh) receptor was investigated using the cerebral cortex from adult rats. Activities in the cerebral cortex, hippocampus and striatum from senescent rats were compared with those from adult rat. Carbachol (1 mM)-stimulated [³H]IP accumulation in the presence of 10 mM LiCl was inhibited by pirenzepine more potently than by AF-DX 116. Although the displacing activity of carbachol for [³H]pirenzepine binding was decreased by 50 μM GTPγS, pretreatment of slices with pertussis toxin (PTX, 0.01-1.0 μg/ml) did not affect the carbachol-induced [³H]IP accumulation. In the slices from all 3 tissues, cerebral cortex, hippocampus and striatum, both incorporation of [³H]inositol and carbachol-stimulated [³H]IP accumulation were reduced at 28 months compared to those at 2 months. Furthermore, the B_max values of [³H]pirenzepine binding in membranes from these three regions were diminished at the senescent stage. Taken together, the results suggest that an M₁-subtype of muscarinic acetylcholine receptor could be involved in PI turnover via GTP-binding proteins insensitive to PTX. Age-related changes in M₁-receptor mediated PI turnover seem to be in part due to the decreased number of M₁-receptors with increasing age in the cerebral cortex, hippocampus and striatum; and some qualitative changes also seem to have occurred in the hippocampus of senescent rats in the mACh receptor-PI turnover system.