Use of M1-M5 muscarinic receptor knockout mice as novel tools to delineate the physiological roles of the muscarinic cholinergic system

In this review we report recent findings on the physiological role of the five known muscarinic acetylcholine receptors (mAChRs) as shown by gene targeting technology. Using knockout mice for each mAChRs subtype, the role of mAChRs subtypes in a number of physiological functions was confirmed and new activities were discovered. The M, mAChRs modulate neurotransmitter signaling in cortex and hippocampus. The M-3 mAChRs are involved in exocrine gland secretion, smooth muscle contractility, pupil dilation, food intake, and weight gain. The role of the M-5 mAChRs involves modulation of central dopamine function and the tone of cerebral blood vessels. mAChRs of the M-2 subtype mediate muscarinic agonist-induced bradycardia, tremor, hypothermia, and autoinhibition of release in several brain regions. M-4 mAChRs modulate dopamine activity in motor tracts and act as inhibitory autoreceptors in striatum. Thus, as elucidated by gene targeting technology, mAChRs have widespread and manifold functions in the periphery and brain.