COINCIDENCE DETECTION AT THE LEVEL OF PHOSPHOLIPASE-C ACTIVATION MEDIATED BY THE M4 MUSCARINIC ACETYLCHOLINE-RECEPTOR

Background: One of the principal mechanisms by which G-protein-coupled receptors evoke cellular responses is through the activation of phosp holipase C (PLC) and the subsequent release of Ca2+ from intracellular stores. Receptors that couple to pertussis toxin (PTX)-insensitive G proteins typically evoke large increases in PLC activity and intracell ular Ca2+ release. In contrast, receptors that use only PTX-sensitive G proteins usually generate weak PLC-dependent responses, but efficien tly regulate a second effector enzyme, adenylyl cyclase. For example, in many cell types, agonist binding by the m4 muscarinic acetylcholine receptor (m4 receptor) results in a strong inhibition of adenylyl cyc lase and very little stimulation of PLC activity or release of intrace llular Ca2+. We have investigated whether the weak, PTX-sensitive stim ulation of PLC activity by the m4 receptor can play a significant role in the generation of cellular responses. Results: We report here that PTX-sensitive Ca2+ release mediated by the m4 receptor in transfected Chinese hamster ovary cells is greatly enhanced when endogenous purin ergic receptors simultaneously activate a PTX-insensitive signaling pa thway. Furthermore, m4-receptor-induced transcription of the c-fos gen e (a Ca2+-sensitive response) is similarly potentiated when purinergic receptors are coactivated. These enhanced m4-receptor-dependent Ca2responses do not require an influx of external Ca2+, and occur in the absence of detectable purinergic-receptor-stimulated Ca2+ release; the y apparently require the activation of both PTX-sensitive and PTX-inse nsitive G-protein pathways. Measurements of phosphoinositide hydrolysi s indicate that the enhancement of m4-receptor-mediated Ca2+ signaling by purinergic receptors is due to a synergistic increase in agonist-s timulated PLC activity. Conclusions: These studies demonstrate that th e potency of m4-receptor-mediated PLC signaling is highly dependent up on the presence or absence of other PLC-activating agonists. The abili ty of the m4 receptor to evoke a strong, but conditional, activation o f PLC may allow this type of receptor to participate in a coincidence- detection system that amplifies simultaneous PLC-activating signals th rough a mechanism involving crosstalk between PTX-sensitive and PTX-in sensitive G-protein pathways.