CHOLINERGIC AND SEROTONERGIC STIMULATION OF PHOSPHOINOSITIDE HYDROLYSIS IS DECREASED IN ALZHEIMERS-DISEASE

Agonist-stimulated phosphoinositide (PPI) hydrolysis is a major signal transduction pathway in brain. These studies Investigated neurotransm itter stimulated PPI hydrolysis in postmortem human brain. Preliminary studies using rat brain suggested that moderate postmortem delay has little effect on Pm hydrolysis and that human tissue might be reliably studied for differences in receptor-PLC coupling. Studies in human br ain membranes (frontal cortex) indicated that the time course for GTP gamma S and carbachol/GTP gamma S-stimulated PPI hydrolysis was linear for at least 20 min. GTP gamma S-stimulated [H-3]inositol phosphate ( InsP) formation was enhanced by carbachol (232%) and 5-Hydroxytryptami ne (5HT-147%). SAX-HPLC separation of [3H]inositol polyphosphates indi cated that the major isomer of inositol trisphosphate (InsP(3)) was In s(1,4,5)P-3, the expected product of PtdIns(4,5)P-2 hydrolysis. Ca2+ i ncreased PPI hydrolysis progressively from 100 nM through 50 mu M and synergistically enhanced carbachol/GTP gamma S stimulation. Comparison s of age-matched controls with Alzheimer's patients indicated that GTP gamma S, carbachol/GTP gamma S, and 5HT/GTP gamma S-stimulation of PP I hydrolysis is reduced approximately 50% in membranes prepared from A lzheimer's patients. Ca2+ stimulation of PPI hydrolysis was not differ ent between controls and Alzheimer's patients suggesting that muscarin ic cholinergic and serotonergic receptors are uncoupled from PLC in Al zheimer's disease. These studies indicate that there are changes in ch olinergic and serotonergic signal transduction in Alzheimer's disease. Further, this method can be used to study signal transduction events in postmortem human brain.