Jongrye Jeon

A Subpopulation of Neuronal M4 Muscarinic Acetylcholine Receptors Plays a Critical Role in Modulating Dopamine-Dependent Behaviors

Acetylcholine (ACh) regulates many key functions of the CNS by activating cell surface receptors referred to as muscarinic ACh receptors (M1–M5 mAChRs). Like other mAChR subtypes, the M4 mAChR is widely expressed in different regions of the forebrain. Interestingly, M4 mAChRs are coexpressed with D1 dopamine receptors in a specific subset of striatal projection neurons. To investigate the physiological relevance of this M4 mAChR subpopulation in modulating dopamine-dependent behaviors, we used Cre/loxP technology to generate mutant mice that lack M4 mAChRs only in D1 dopamine receptor-expressing cells. The newly generated mutant mice displayed several striking behavioral phenotypes, including enhanced hyperlocomotor activity and increased behavioral sensitization following treatment with psychostimulants. These behavioral changes were accompanied by a lack of muscarinic inhibition of D1 dopamine receptor-mediated cAMP stimulation in the striatum and an increase in dopamine efflux in the nucleus accumbens. These novel findings demonstrate that a distinct subpopulation of neuronal M4 mAChRs plays a critical role in modulating several important dopamine-dependent behaviors. Since enhanced central dopaminergic neurotransmission is a hallmark of several severe disorders of the CNS, including schizophrenia and drug addiction, our findings have substantial clinical relevance.

Role of the M3 muscarinic acetylcholine receptor in β‐cell function and glucose homeostasis

The release of insufficient amounts of insulin in the presence of elevated blood glucose levels is one of the key features of type 2 diabetes. Various lines of evidence indicate that acetylcholine (ACh), the major neurotransmitter of the parasympathetic nervous system, can enhance glucose‐stimulated insulin secretion from pancreatic β‐cells. Studies with isolated islets prepared from whole body M3 muscarinic ACh receptor knockout mice showed that cholinergic amplification of glucose‐dependent insulin secretion is exclusively mediated by the M3 muscarinic receptor subtype. To investigate the physiological relevance of this muscarinic pathway, we used Cre/loxP technology to generate mutant mice that lack M3 receptors only in pancreatic β‐cells. These mutant mice displayed impaired glucose tolerance and significantly reduced insulin secretion. In contrast, transgenic mice overexpressing M3 receptors in pancreatic β‐cells showed a pronounced increase in glucose tolerance and insulin secretion and were resistant to diet‐induced glucose intolerance and hyperglycaemia. These findings indicate that β‐cell M3 muscarinic receptors are essential for maintaining proper insulin secretion and glucose homeostasis. Moreover, our data suggest that enhancing signalling through β‐cell M3 muscarinic receptors may represent a new avenue in the treatment of glucose intolerance and type 2 diabetes.

Metabolic Roles of the M3 Muscarinic Acetylcholine Receptor Studied with M3 Receptor Mutant Mice: A Review

The M3 muscarinic acetylcholine (ACh) receptor (M3 mAChR) is expressed in many central and peripheral tissues. It is a prototypic member of the superfamily of G protein-coupled receptors and preferentially activates G proteins of the Gq family. Recent studies involving the use of newly generated mAChR mutant mice have revealed that the M3 mAChR plays a key role in regulating many important metabolic functions. Phenotypic analyses of mutant mice that either selectively lacked or overexpressed M3 receptors in pancreatic β -cells indicated that β -cell M3 mAChRs are essential for maintaining proper insulin release and glucose homeostasis. The experimental data also suggested that strategies aimed at enhancing signaling through β -cell M3 mAChRs might be beneficial for the treatment of type 2 diabetes. Recent studies with whole body M3 mAChR knockout mice showed that the absence of M3 receptors protected mice against various forms of experimentally or genetically induced obesity and obesity-associated metabolic deficits. Under all experimental conditions tested, M3 receptor-deficient mice showed greatly ameliorated impairments in glucose homeostasis and insulin sensitivity, reduced food intake, and a significant elevation in basal and total energy expenditure, most likely due to increased central sympathetic outflow and increased rate of fatty acid oxidation. These findings are of potential interest for the development of novel therapeutic approaches for the treatment of obesity and associated metabolic disorders.

Involvement of a Subpopulation of Neuronal M4 Muscarinic Acetylcholine Receptors in the Antipsychotic-like Effects of the M1/M4 Preferring Muscarinic Receptor Agonist Xanomeline

Disturbances in central dopaminergic neurotransmission are believed to be centrally involved in the pathogenesis of schizophrenia. Central dopaminergic and cholinergic systems interact and the cholinergic muscarinic agonist xanomeline has shown antipsychotic effects in clinical studies. Preclinical studies indicate that the M4 muscarinic cholinergic receptor subtype (mAChR) modulates the activity of the dopaminergic system and that this specific mAChR subtype is involved in mediating the antipsychotic-like effects of xanomeline. A specific neuronal subpopulation that expresses M4 mAChRs together with D1 dopamine receptors seems to be especially important in modulating dopamine-dependent behaviors. Using mutant mice that lack the M4 mAChR only in D1 dopamine receptor-expressing cells (D1-M4-KO), we investigated the role of this neuronal population in the antipsychotic-like effects of xanomeline in amphetamine-induced hyperactivity and apomorphine-induced climbing. Interestingly, the antipsychotic-like effects of xanomeline in the two models were almost completely abolished in D1-M4-KO mice, suggesting that M4 mAChRs colocalized with D1 dopamine receptors are centrally involved in mediating the antipsychotic-like effects of xanomeline. This is consistent with the hypothesis that activation of the M4 mAChR represents a potential target for the future medical treatment of psychosis.

Neuronal M3 muscarinic acetylcholine receptors are essential for somatotroph proliferation and normal somatic growth

The molecular pathways that promote the proliferation and maintenance of pituitary somatotrophs and other cell types of the anterior pituitary gland are not well understood at present. However, such knowledge is likely to lead to the development of novel drugs useful for the treatment of various human growth disorders. Although muscarinic cholinergic pathways have been implicated in regulating somatotroph function, the physiological relevance of this effect and the localization and nature of the receptor subtypes involved in this activity remain unclear. We report the surprising observation that mutant mice that selectively lack the M3 muscarinic acetylcholine receptor subtype in the brain (neurons and glial cells; Br-M3-KO mice) showed a dwarf phenotype associated with a pronounced hypoplasia of the anterior pituitary gland and a marked decrease in pituitary and serum growth hormone (GH) and prolactin. Remarkably, treatment of Br-M3-KO mice with CJC-1295, a synthetic GH-releasing hormone (GHRH) analog, rescued the growth deficit displayed by Br-M3-KO mice by restoring normal pituitary size and normal serum GH and IGF-1 levels. These findings, together with results from M3 receptor/GHRH colocalization studies and hypothalamic hormone measurements, support a model in which central (hypothalamic) M3 receptors are required for the proper function of hypothalamic GHRH neurons. Our data reveal an unexpected and critical role for central M3 receptors in regulating longitudinal growth by promoting the proliferation of pituitary somatotroph cells.