Cellular signaling mechanisms for muscarinic acetylcholine receptors

Signaling pathways for muscarinic acetylcholine receptors (mAChRs) include several enzymes and ion channels. Recent studies have revealed the importance of various isoforms of both alpha and betagamma subunits of G proteins in initiation of signaling as well as the role of the small monomeric G protein, Rho, in the activation of phospholipase D. Modulation of adenylyl cyclase activity by mAChRs appears more diverse as the interaction of various receptor subtypes with the many isoforms of the enzyme are studied. Both alpha and beta subunits of G(i/o) may be involved. Some mAChR responses arise through release of nitric oxide from nitrergic nerves, including salivary gland secretion and hippocampal slow wave activity. mAChRs utilize a variety of intracellular pathways to activate various mitogen-activated protein kinases. The kinases are involved in cholinergic regulation of kidney epithelial function, catabolism of amyloid precursor protein, hippocampal long-term potentiation, activation of phospholipase A(2), and gene induction. mAChR activation can also stimulate or inhibit cellular growth and apoptosis, dependent on prior levels of cellular activity. Modulation of ion channels by mAChR agonists appears increasingly complex, based on recent studies. K+ channels may be activated by M-2 and M-4 mAChR stimulation, although in the rat superior cervical ganglion topographical constraints appear to limit the effect to the M-2 mAChR. Another ganglionic K+ current, the M current, is inhibited by M-1 mAChR activation, but in murine hippocampus inhibition involves another receptor subtype. R-type Ca2+ channels are both facilitated and inhibited by M-1 and M-2 mAChRs; facilitation being more pronounced with activation of M-1 mAChRs and inhibition with M-2 mAChRs.