Alokesh Duttaroy

Muscarinic Induction of Hippocampal Gamma Oscillations Requires Coupling of the M1 Receptor to Two Mixed Cation Currents

Oscillatory network activity at gamma frequencies is assumed to be of major importance in cortical information processing. Whereas the synaptic mechanisms of gamma oscillations have been studied in detail, the ionic currents involved at the cellular level remain to be elucidated. Here we show that in vitro gamma oscillations induced by muscarine require activation of M1 receptors on hippocampal CA3 pyramidal neurons and are absent in M1 receptor-deficient mice. M1 receptor activation depolarizes pyramidal neurons by increasing the mixed Na(+)/K(+) current I(h) and the Ca(2+)-dependent nonspecific cation current I(cat), but not by modulation of I(M). Our data provide important insight into the molecular basis of gamma oscillations by unequivocally establishing a novel role for muscarinic modulation of I(h) and I(cat) in rhythmic network activity.

Cholinergic dilation of cerebral blood vessels is abolished in M 5 muscarinic acetylcholine receptor knockout mice

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 M5 receptor-selective ligands. To circumvent these difficulties, we used gene targeting technology to generate M5 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 M5 receptors play a role in facilitating muscarinic agonist-induced dopamine release in the striatum. Because M5 receptor mRNA has been detected in several blood vessels, we also investigated whether the lack of M5 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 extra-cerebral arteries remained fully intact in M5R−/− mice. Our findings provide direct evidence that M5 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 Alzheimers disease and focal cerebral ischemia, cerebrovascular M5 receptors may represent an attractive therapeutic target.

Deletion of the M5 muscarinic acetylcholine receptor attenuates morphine reinforcement and withdrawal but not morphine analgesia

Little is known about the physiological roles of the M5 muscarinic receptor, the last member of the muscarinic receptor family (M1–M5) to be cloned. In the brain, the M5 receptor subtype is preferentially expressed by dopaminergic neurons of the substantia nigra and the ventral tegmental area. Dopaminergic neurons located in the ventral tegmental area are known to play important roles in mediating both the rewarding effects of opiates and other drugs of abuse and the manifestations of opiate/drug withdrawal symptoms. We therefore speculated that acetylcholine-dependent activation of M5 receptors might modulate the manifestations of opiate reward and withdrawal. This hypothesis was tested in a series of behavioral, biochemical, and neurochemical studies using M5 receptor-deficient mice (M5−/− mice) as novel experimental tools. We found that the rewarding effects of morphine, as measured in the conditioned place preference paradigm, were substantially reduced in M5−/− mice. Furthermore, both the somatic and affective components of naloxone-induced morphine withdrawal symptoms were significantly attenuated in M5−/− mice. In contrast, the analgesic efficacy of morphine and the development of tolerance to the analgesic effects of morphine remained unaltered by the lack of M5 receptors. The finding that M5 receptor activity modulates both morphine reward and withdrawal processes suggests that M5 receptors may represent a novel target for the treatment of opiate addiction.

Muscarinic receptor subtypes mediating central and peripheral antinociception studied with muscarinic receptor knockout mice: A review

To gain new insight into the physiological and pathophysiological roles of the muscarinic cholinergic system, we generated mutant mouse strains deficient in each of the five muscarinic acetylcholine receptor subtypes (M(1)-M(5)). In this chapter, we review a set of recent studies dealing with the identification of the muscarinic receptor subtypes mediating muscarinic agonist-dependent analgesic effects by central and peripheral mechanisms. Most of these studies were carried out with mutant mouse strains lacking M(2) or/and M(4) muscarinic receptors. It is well known that administration of centrally active muscarinic agonists induces pronounced analgesic effects. To identify the muscarinic receptors mediating this activity, wild-type and muscarinic receptor mutant mice were injected with the non-subtype-selective muscarinic agonist, oxotremorine (s.c., i.t., and i.c.v.), and analgesic effects were assessed in the tail-flick and hot-plate tests. These studies showed that M(2) receptors play a key role in mediating the analgesic effects of oxotremorine, both at the spinal and supraspinal level. However, studies with M(2)/M(4) receptor double KO mice indicated that M(4) receptors also contribute to this activity. Recent evidence suggests that activation of muscarinic receptors located in the skin can reduce the sensitivity of peripheral nociceptors. Electrophysiological and neurochemical studies with skin preparations from muscarinic receptor mutant mice indicated that muscarine-induced peripheral antinociception is mediated by M(2) receptors. Since acetylcholine is synthesized and released by different cell types of the skin, it is possible that non-neuronally released acetylcholine plays a role in modulating peripheral nociception. Our results highlight the usefulness of muscarinic receptor mutant mice to shed light on the functional roles of acetylcholine released from both neuronal and non-neuronal cells.

Muscarinic Acetylcholine Receptor Knockout Mice

Muscarinic acetylcholine receptors (mAChRs) play critical roles in regulating the activity of many important functions of the central and peripheral nervous system. However, identification of the physiological and pathophysiological roles of the individual mAChR subtypes (M1-M5) has proven a difficult task, primarily due to the lack of ligands endowed with a high degree of receptor subtype selectivity and the fact that most tissues and organs express multiple mAChRs. To circumvent these difficulties, we and others have used gene targeting strategies to generate mutant mouse lines containing inactivating mutations of the M1-M5 mAChR genes. The different mAChR mutant mice and the corresponding wild-type control animals were subjected to a battery of physiological, pharmacological, behavioral, biochemical, and neurochemical tests. The M1-M5 mAChR mutant mice (MXR-/- mice) were all viable and reproduced normally. However, each mutant mouse line displayed distinct phenotypical changes. For example, M1R-/- mice showed a pronounced increase in locomotor activity, probably due to the increase in dopamine release in the striatum. In addition, pilocarpine-induced epileptic seizures were absent in M1R-/- mice. Pharmacological analysis of M2R-/- mice indicated that the M2 subtype plays a key role in mediating three of the most striking central muscarinic effects: tremor, hypothermia, and analgesia. As expected, muscarinic agonist-mediated bradycardia was abolished in M2R-/- mice. M3R-/- mice displayed a significant decrease in food intake, reduced body weight and peripheral fat deposits, and very low serum leptin and insulin levels. Additional studies showed that the M3 receptor subtype also plays a key role in mediating smooth muscle contraction and glandular secretion. Behavioral analysis of M4R-/- mice suggested that M4 receptors mediate inhibition of D1 dopamine receptor-mediated locomotor stimulation, probably at the level of striatal projection neurons. Studies with M5R-/- mice indicated that vascular M5 receptors mediate cholinergic relaxation of cerebral arteries and arterioles. Behavioral and neurochemical studies showed that M5 receptor activity modulates both morphine reward and withdrawal processes, probably through activation of M5 receptors located on midbrain dopaminergic neurons. These results offer promising new perspectives for the rational development of novel muscarinic drugs.

Generation and pharmacological analysis of M2 and M4 muscarinic receptor knockout mice.

Muscarinic acetylcholine receptors (M1-M5) play important roles in the modulation of many key functions of the central and peripheral nervous system. To explore the physiological roles of the two Gi-coupled muscarinic receptors, we disrupted the M2 and M4 receptor genes in mice by using a gene targeting strategy. Pharmacological and behavioral analysis of the resulting mutant mice showed that the M2 receptor subtype is critically involved in mediating three of the most striking central muscarinic effects, tremor, hypothermia, and analgesia. These studies also indicated that M4 receptors are not critically involved in these central muscarinic responses. However, M4 receptor-deficient mice showed an increase in basal locomotor activity and greatly enhanced locomotor responses following drug-induced activation of D1 dopamine receptors. This observation is consistent with the concept that M4 receptors exert inhibitory control over D1 receptor-mediated locomotor stimulation, probably at the level of striatal projection neurons where the two receptors are known to be coexpressed. These findings emphasize the usefulness of gene targeting approaches to shed light on the physiological and pathophysiological roles of the individual muscarinic receptor subtypes.

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.

M1-M3 muscarinic acetylcholine receptor-deficient mice: novel phenotypes.

The five muscarinic acetylcholine receptors (M1-M5 mAChRs) mediate a very large number of important physiological functions (Caulfield, 1993; Caulfield and Birdsall, 1998; Wess, 2004). Because of the lack of small molecule ligands endowed with a high degree of receptor subtype selectivity and the fact that most tissues or cell types express two or more mAChR subtypes, identification of the physiological and pathophysiological roles of the individual mAChR subtypes has proved to be a challenging task. To overcome these difficulties, we recently generated mutant mouse lines deficient in each of the five mAChR genes (M1R-/- mice, M2R-/- mice, M3R-/- mice, etc. [Wess, 2004]). Phenotyping studies showed that each of the five mutant mouse lines displayed characteristic physiological, pharmacological, behavioral, biochemical, or neurochemical deficits (Wess, 2004). This chapter summarizes recent findings dealing with the importance of the M2mAChR for cognitive processes and the roles of the M1 and M3 mAChRs in mediating stimulation of glandular secretion.

Muscarinic acetylcholine receptor knockout mice: Phenotypical analysis and clinical implications

Publisher Summary This chapter discusses the phenotypical analysis and clinical implications of muscarinic acetylcholine (ACh) receptor knockout mice. Molecular cloning studies have revealed the existence of five molecularly distinct muscarinic acetylcholine (ACh) receptor (mAChR) subtypes (M 1 –M 5 ). At a molecular level, the M 1 , M 3 , and M 5 receptors preferentially couple to G proteins of the G q /G 11 family, whereas the M 2 and M 4 receptors are primarily linked to G proteins of the G i /G o class. Studies with mAChR agonists and antagonists have shown that mAChRs are involved in the control of numerous fundamental physiological processes. Central mAChRs are known to regulate a large number of vegetative, sensory, and motor functions. Moreover, central muscarinic mechanisms play important roles in arousal, attention, rapid eye movement (REM) sleep, emotional responses, stress modulation, and higher cognitive processes such as memory and learning. The chapter reviews the major phenotypes displayed by mutant mice lacking M 2 , M 3 , or M 4 mAChRs. The M 2 and M 4 mAChR genes were inactivated via homologous recombination in mouse embryonic stem (ES) cells. Homozygous M 2 -/- (M 2 R -/- ) and M 4 -/- receptor (M 4 R -/- ) mutant mice were obtained with the expected Mendelian frequency, indicating that there was no increase in embryonic or postnatal mortality. Moreover, wild-type (WT) and M 2 and M 4 receptor mutant mice did not differ in overall health, were fertile, and bred normally.