UNCOUPLING OF MUSCARINIC CHOLINERGIC PHOSPHOINOSITIDE SIGNALS IN SENESCENT CEREBRAL CORTICAL AND HIPPOCAMPAL MEMBRANES

Muscarinic-cholinergic signals in brain are mediated in part through t he hydrolysis of phosphoinositides (PtdIns) by phospholipase C (PLC). To test the hypothesis that muscarinic PtdIns signals change during ag ing, membranes were prepared from the cerebral cortex and hippocampus of young (4-6 months old), middle aged (8-10 months old) and senescent (24-26 months old) Fisher 344 rats. Carbachol dose-dependently increa sed [H-3]-PtdIns hydrolysis in both brain regions in all three age gro ups, however, in senescent rats the maximal response was decreased to 69.26 +/- 4.33% (p<0.01) in cortex and to 48.29 +/- 2.55% (p<0.01) in hippocampus of young rat values. In contrast to the decrease in carbac hol-stimulated phosphoinositide hydrolysis, calcium-stimulated phospho inositide hydrolysis was not altered. GTP gamma S also dose-dependentl y increased [H-3]-PtdIns hydrolysis in membranes from all three age gr oups through G-protein-PLC activation. Similar to carbachol, GTP gamma S-activated [H-3]-PtdIns hydrolysis was reduced approximately 40% in senescent rats membranes. Muscarinic receptor (mAChR) density, as dete rmined by [H-3]-QNB binding decreased slightly in cortical membranes, but not in hippocampal membranes. These data suggest that muscarinic s timulated [H-3]-PtdIns responses are decreased in senescent brain prim arily due to an uncoupling of the receptor-G-protein and/or G-protein- PLC link, although decreases in receptor density may also contribute t o reduced muscarinic [H-3]-PtdIns signaling. (C) 1998 Elsevier Science Ltd. All rights reserved.