Cholinergic dilation of cerebral blood vessels is abolished in M5 muscarinic acetylcholine receptor knockout mice

The M-5 muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M-1-M-5) to be cloned. At present, the physiological relevance of this receptor subtype remains unknown, primarily because of its low expression levels and the lack of M-5 receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M-5 receptor-deficient mice (M5R(-/-) mice). M5R(-/-) mice did not differ from their wild-type littermates in various behavioral and pharmacologic tests. However, in vitro neurotransmitter release experiments showed that M-5 receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M-5 receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M-5 receptors led to changes in vascular tone by using several in vivo and in vitro vascular preparations. Strikingly, acetylcholine, a powerful dilator of most vascular beds, virtually lost the ability to dilate cerebral arteries and arterioles in M5R(-/-) mice. This effect was specific for cerebral blood vessels, because acetylcholine-mediated dilation of extracerebral arteries remained fully intact in M5R(-/-) mice. Our findings provide direct evidence that M-5 muscarinic receptors are physiologically relevant. Because it has been suggested that impaired cholinergic dilation of cerebral blood vessels may play a role in the pathophysiology of Alzheimer's disease and focal cerebral ischemia, cerebrovascular M-5 receptors may represent an attractive therapeutic target.