Localization of mRNA for the muscarinic M1 receptor in rat stomach

Activation of Muscarinic Acetylcholine Receptor Subtype 4 Is Essential for Cholinergic Stimulation of Gastric Acid Secretion: Relation to D Cell/Somatostatin

Background/Aim: Muscarinic acetylcholine receptors exist in five subtypes (M₁∼M₅), and they are widely expressed in various tissues to mediate diverse autonomic functions, including gastric secretion. In the present study, we demonstrated, using M1∼M5 KO mice, the importance of M4 receptors in carbachol (CCh) stimulation of acid secretion and investigated how the secretion is modulated by the activation of M4 receptors.

Methods: C57BL/6J mice of wild-type (WT) and M1–M5 KO were used. Under urethane anesthesia, acid secretion was measured in the stomach equipped with an acute fistula. CCh (30 μg/kg) was given subcutaneously (s.c.) to stimulate acid secretion. Atropine or octreotide (a somatostatin analog) was given s.c. 20 min before the administration of CCh. CYN154806 (a somatostatin SST2 receptor antagonist) was given i.p. 20 min before the administration of octreotide or CCh.

Results: CCh caused an increase of acid secretion in WT mice, and the effect was totally inhibited by prior administration of atropine. The effect of CCh was similarly observed in the animals lacking M1, M2 or M5 receptors but significantly decreased in M3 or M4 KO mice. CYN154806, the SST2 receptor antagonist, dose-dependently and significantly reversed the decreased acid response to CCh in M4 but not M3 KO mice. Octreotide, the somatostatin analog, inhibited the secretion of acid under CCh-stimulated conditions in WT mice. The immunohistochemical study showed the localization of M₄ receptors on D cells in the stomach. Serum somatostatin levels in M4 KO mice were higher than WT mice under basal conditions, while those in WT mice were significantly decreased in response to CCh.

Conclusions: These results suggest that under cholinergic stimulation the acid secretion is directly mediated by M3 receptors and indirectly modified by M4 receptors. It is assumed that the activation of M4 receptors inhibits the release of somatostatin from D cells and minimizes the acid inhibitory effect of somatostatin through SST2 receptors, resulting in enhancement of the acid response mediated by M3 receptors on parietal cells.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Muscarinic acetylcholine receptors as effector sites for present and future therapeutic applications: focus on non-neural cholinergic systems

This review summarises the expression and function of muscarinic acetylcholine receptors (mAChR) in the organism, and the therapeutic implications of recent patents on cholinergic agents. Aside from the well known therapeutic applications of parasympathomimetic and -lytic drugs for diseases of the heart, eye, circulation and airways, new possible fields for mAChR agonists and antagonists in clinical therapy have begun to emerge. In particular, the role of non-neural cholinergic mechanisms in tissue regeneration, development and even carcinogenesis has become increasingly studied by a number of research teams within the last decade. This review exemplifies and contrasts experimental findings of mAChR drug action, and discusses these with regard to patents from the past 4 years.

Cholinergic agonist-induced pepsinogen secretion from murine gastric chief cells is mediated by M1 and M3 muscarinic receptors

Muscarinic cholinergic mechanisms play a key role in stimulating gastric pepsinogen secretion. Studies using antagonists suggested that the M3 receptor subtype (M3R) plays a prominent role in mediating pepsinogen secretion, but in situ hybridization indicated expression of M1 receptor (M1R) in rat chief cells. We used mice that were deficient in either the M1 (M1R-/-) or M3 (M3R-/-) receptor or that lacked both receptors (M(1/3)R-/-) to determine the role of M1R and M3R in mediating cholinergic agonist-induced pepsinogen secretion. Pepsinogen secretion from murine gastric glands was determined by adapting methods used for rabbit and rat stomach. In wild-type (WT) mice, maximal concentrations of carbachol and CCK caused a 3.0- and 2.5-fold increase in pepsinogen secretion, respectively. Maximal carbachol-induced secretion from M1R-/- mouse gastric glands was decreased by 25%. In contrast, there was only a slight decrease in carbachol potency and no change in efficacy when comparing M3R-/- with WT glands. To explore the possibility that both M1R and M3R are involved in carbachol-mediated pepsinogen secretion, we examined secretion from glands prepared from M(1/3)R-/- double-knockout mice. Strikingly, carbachol-induced pepsinogen secretion was nearly abolished in glands from M(1/3)R-/- mice, whereas CCK-induced secretion was not altered. In situ hybridization for murine M1R and M3R mRNA in gastric mucosa from WT mice revealed abundant signals for both receptor subtypes in the cytoplasm of chief cells. These data clearly indicate that, in gastric chief cells, a mixture of M1 and M3 receptors mediates cholinergic stimulation of pepsinogen secretion and that no other muscarinic receptor subtypes are involved in this activity. The development of a murine secretory model facilitates use of transgenic mice to investigate the regulation of pepsinogen secretion.

Cholinergically stimulated gastric acid secretion is mediated by M(3) and M(5) but not M(1) muscarinic acetylcholine receptors in mice

Muscarinic acetylcholine receptors play an important role in the regulation of gastric acid secretion stimulated by acetylcholine; nonetheless, the precise role of each receptor subtype (M(1)-M(5)) remains unclear. This study examined the involvement of M(1), M(3), and M(5) receptors in cholinergic regulation of acid secretion using muscarinic receptor knockout (KO) mice. Gastric acid secretion was measured in both mice subjected to acute gastric fistula production under urethane anesthesia and conscious mice that had previously undergone pylorus ligation. M(3) KO mice exhibited impaired gastric acid secretion in response to carbachol. Unexpectedly, M(1) KO mice exhibited normal intragastric pH, serum gastrin and mucosal histamine levels, and gastric acid secretion stimulated by carbachol, histamine, and gastrin. Pirenzepine, known as an M(1)-receptor antagonist, inhibited carbachol-stimulated gastric acid secretion in a dose-dependent manner in M(1) KO mice as well as in wild-type (WT) mice, suggesting that the inhibitory effect of pirenzepine on gastric acid secretion is independent of M(1)-receptor antagonism. Notably, M(5) KO mice exhibited both significantly lower carbachol-stimulated gastric acid secretion and histamine-secretory responses to carbachol compared with WT mice. RT-PCR analysis revealed M(5)-mRNA expression in the stomach, but not in either the fundic or antral mucosa. Consequently, cholinergic stimulation of gastric acid secretion is clearly mediated by M(3) (on parietal cells) and M(5) receptors (conceivably in the submucosal plexus), but not M(1) receptors.

Impaired gastric secretion and lack of trophic responses to hypergastrinemia in M3 muscarinic receptor knockout mice

BACKGROUND & AIMS

The physiologic significance of the M(3) muscarinic receptor is unclear due to an absence of specific ligand. In the present study, M(3) receptor knockout (KO) mice were used to elucidate the role of M(3) receptors in gastric acid secretion and gastric mucosal integrity.

METHODS

M(3) KO versus wild-type mice aged 1 month to 2 years were included. Gastric acid secretion was assessed by both direct intragastric pH measurement and pylorus ligation. Serum gastrin and gastric mucosal histamine levels were determined by radioimmunoassay and enzyme-linked immunosorbent assay, respectively. Morphologic analysis was performed by both immunohistochemistry and transmission electron microscopy.

RESULTS

Fasted M(3) KO mice exhibited higher intragastric pH, lower acid output after pylorus ligation, a lower proportion of active parietal cells, and higher serum gastrin levels than fasted wild-type mice. Acid secretion in response to carbachol, histamine, gastrin 17, and 2-deoxy-D-glucose was impaired in the mutant mice. Although carbachol was still able to cause approximately 30% acid output in M(3) KO mice, the acid secretion was inhibited by pirenzepine or famotidine. Despite remarkable hypergastrinemia in M(3) KO mice, there were no trophic responses in the oxyntic mucosa with respect to the mucosal thickness, proliferation rate, and numbers of parietal and enterochromaffin-like cells. Cholecystokinin type 2 receptor antagonist YM022 was without the effect in M(3) KO mice.

CONCLUSIONS

The present study shows that M(3) receptors are essential for basal acid secretion, a fully acid secretory response to histamine and gastrin, and the trophic responses of oxyntic mucosa to gastrin.

Pharmacological control of gastric acid secretion for the treatment of acid-related peptic disease: past, present, and future

Pharmacological agents, such as histamine H(2) receptor antagonists and acid pump inhibitors, are now the most frequently used treatment for such acid-related diseases as gastroduodenal ulcers and reflux esophagitis. Based on increased understanding of the precise mechanisms of gastric acid secretion at the level of receptors, enzymes, and cytoplasmic signal transduction systems, further possibilities exist for the development of effective antisecretory pharmacotherapy. Gastrin CCK(2) receptor antagonists and locally active agents appear to represent promising therapies for the future. Development of gene targeting techniques has allowed production of genetically engineered transgenic and knockout mice. Such genetic technology has increased the investigative power for pharmacotherapy for not only antisecretory agents, but also treatment of mucosal diseases, such as atrophy, hyperplasia, and cancer. Elucidation of the origin of gastric parietal cells also represents an interesting investigative target that should allow a better understanding of not only acid-related diseases, but also the evolution of the stomach as an acid-secreting organ.

Identification and mapping of keratinocyte muscarinic acetylcholine receptor subtypes in human epidermis

Acetylcholine mediates cell-to-cell communications in the skin. Human epidermal keratinocytes respond to acetylcholine via two classes of cell-surface receptors, the nicotinic and the muscarinic cholinergic receptors. High affinity muscarinic acetylcholine receptors (mAChR) have been found on keratinocyte cell surfaces at high density. These receptors mediate effects of muscarinic drugs on keratinocyte viability, proliferation, adhesion, lateral migration, and differentiation. In this study, we investigated the molecular structure of keratinocyte mAChR and their location in human epidermis. Polymerase chain reaction amplification of cDNA sequences uniquely present within the third cytoplasmic loop of each subtype demonstrated the expression of the m1, m3, m4, and m5 mAChR subtypes. To visualize these mAChR, we raised rabbit anti-sera to synthetic peptide analogs of the carboxyl terminal regions of each subtype. The antibodies selectively bound to keratinocyte mAChR subtypes in immunoblotting membranes and epidermis, both of which could be abolished by preincubating the anti-serum with the peptide used for immunization. The immunofluorescent staining patterns produced by each antibody in the epidermis suggested that the profile of keratinocyte mAChR changes during epidermal turnover. The semiquantitative analysis of fluorescence revealed that basal cells predominantly expressed m3, prickle cells had equally high levels of m4 and m5, and granular cells mostly possessed m1. Thus, the results of this study demonstrate for the first time the presence of m1, m3, m4, and m5 mAChR in epidermal keratinocytes. Because keratinocytes express a unique combination of mAChR subtypes at each stage of their development in the epidermis, each receptor may regulate a specific cell function. Hence, a single cytotransmitter, acetylcholine, and muscarinic drugs may exert different biologic effects on keratinocytes at different stages of their maturation.