One of the hallmarks of type 2 diabetes is that pancreatic beta cells fail to release sufficient amounts of insulin in the presence of elevated blood glucose levels. Insulin secretion is modulated by many hormones and neurotransmitters including acetylcholine, the major neurotransmitter of the peripheral parasympathetic nervous system. The physiological role of muscarinic acetylcholine receptors expressed by pancreatic beta cells remains unclear at present. Here, we demonstrate that mutant mice selectively lacking the M3 muscarinic acetylcholine receptor subtype in pancreatic beta cells display impaired glucose tolerance and greatly reduced insulin release. In contrast, transgenic mice selectively overexpressing M3 receptors in pancreatic beta cells show a profound increase in glucose tolerance and insulin release. Moreover, these mutant mice are resistant to diet-induced glucose intolerance and hyperglycemia. These findings indicate that beta cell M3 muscarinic receptors play a key role in maintaining proper insulin release and glucose homeostasis.

The M5 muscarinic receptor is the most recent member of the muscarinic acetylcholine receptor family (M1-M5) 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. M olecular cloning studies have revealed the existence of five molecularly distinct muscarinic acetylcholine receptor subtypes (M 1 -M 5 ) (1, 2). During the past decade, considerable progress has been made in delineating the physiological roles of the M 1 -M 4 muscarinic receptors (3, 4). In contrast, the physiological relevance of the M 5 receptor subtype, which is the most recent member of the muscarinic receptor family to be cloned (5, 6), remains unknown at present (7,8). However, expression of the cloned M 5 muscarinic receptor gene in cultured mammalian cells has shown that the encoded receptor protein is functional and efficiently couples to G proteins of the G q family, similar to the M 1 and M 3 receptor subtypes (5-8).Immunoprecipitation and in situ mRNA hybridization studies have demonstrated the presence of M 5 receptor protein͞mRNA in different areas of the brain including hippocampus, hypothalamus, and distinct midbrain regions (substantia nigra pars compacta and ventral tegmental area) (9-11). However, M 5 receptors are expressed at very low levels, representing less than 2% of the total muscarinic receptor population (M 1 -M 5 ) expressed in the brain (9). Interestingly, the use of highly sensitive reverse transcriptase-PCR (RT-PCR) techniques suggests that M 5 receptors are expressed in all major brain regions (12). More recently, M 5 receptors also have been detected in several peripheral tissues or cells including peripheral blood l...

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