Cholinergic and serotonergic stimulation of phosphoinositide hydrolysis is decreased in Alzheimer's disease.

Agonist-stimulated phosphoinositide (PPI) hydrolysis is a major signal transduction pathway in brain. These studies investigated neurotransmitter stimulated PPI hydrolysis in postmortem human brain. Preliminary studies using rat brain suggested that moderate postmortem delay has little effect on PPI hydrolysis and that human tissue might be reliably studied for differences in receptor-PLC coupling. Studies in human brain 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 [3H]inositol phosphate (InsP) formation was enhanced by carbachol (232%) and 5-Hydroxytryptamine (5HT-147%). SAX-HPLC separation of [3H]inositol polyphosphates indicated that the major isomer of inositol trisphosphate (InsP3) was Ins(1.4.5)P3, the expected product of PtdIns(4,5)P2 hydrolysis. Ca2+ increased PPI hydrolysis progressively from 100 nM through 50 microM and synergistically enhanced carbachol/GTP gamma S stimulation. Comparisons of age-matched controls with Alzheimer's patients indicated that GTP gamma S, carbachol/GTP gamma S, and 5HT/GTP gamma S-stimulation of PPI hydrolysis is reduced approximately 50% in membranes prepared from Alzheimer's patients. Ca2+ of PPI hydrolysis was not different between controls and Alzheimer's patients suggesting that muscarinic cholinergic and serotonergic receptors are uncoupled from PLC in Alzheimer's disease. These studies indicate that there are changes in cholinergic and serotonergic signal transduction in Alzheimer's disease. Further, this method can be used to study signal transduction events in postmortem human brain.