Activation Of Muscarinic Acetylcholine Receptors Research Articles

New Trends and Perspectives in the Function of Non-neuronal Acetylcholine in Crypt-Villus Organoids in Mice.

Acetylcholine (ACh) is a neurotransmitter that is present in central, parasympathetic, and neuromuscular synapses of mammals. However, non-neuronal ACh is also predicted to function as a local cell signaling molecule. The physiological significance of the presence of non-neuronal ACh in the intestine remains unclear. Here, experiments using cultured crypt-villus organoids that lack nerve and immune cells led us to suggest that endogenous ACh is synthesized in the intestinal epithelium to evoke growth and differentiation of the organoids through activation of muscarinic ACh receptors (mAChRs). Extracts of cultured organoids exhibited a noticeable capacity for ACh synthesis that was sensitive to a potent inhibitor of choline acetyltransferase. Treatment of organoids with carbachol downregulated growth of organoids and expression of marker genes for each epithelial cell type. On the other hand, mAChR antagonists enhanced growth and differentiation of Lgr5-positive stem cells. Collectively, our data provide evidence that endogenous ACh released from mouse intestinal epithelium maintains the homeostasis of intestinal epithelial cell growth and differentiation via mAChRs.

What Is the “Areca” in “Areca Nuts”? Extraction and Neuroactive Bioassay of Arecoline

A series of three practical sessions are designed to give students firsthand experience with the preparation of natural product extracts and assay using a live tissue preparation. Areca or betel nuts are the seeds from the fruit of the Areca catechu palm tree that is known to contain a number of pharmacologically active alkaloids. The principal of these is arecoline that makes up to 1% of the dry nut. Arecoline is a potent spasmogenic agent, causing smooth muscle contraction via muscarinic acetylcholine receptor (mAChR) activation. The first session involves the preparation of methanolic extracts from whole areca nuts and TLC for the qualitative identification of arecoline present in the extract. The second session utilizes the spasmodic effects of arecoline on smooth muscle to allow students to perform a live tissue bioassay using guinea-pig ileum. This response is subsequently blocked by the mAChR antagonist atropine to investigate the mechanism underlying the measured response. The final session gives students the opportunity to construct arecoline dose–response curves based on their experimentally derived data. From this curve and the obtained antagonist results, they are able to calculate an estimate of the arecoline content in the extracts they prepared and the original betel nut samples.

Role of Muscarinic Acetylcholine Receptor-2 in the Cerebellar Cortex in Cardiovascular Modulation in Anaesthetized Rats.

Our previous investigations have demonstrated that microinjection of acetylcholine (ACh) or muscarinic ACh receptor activation in the cerebellar cortex induces a systemic blood pressure depressor response. This study aimed to determine the role of muscarinic ACh receptor-2 (M2 receptor) in the cerebellar cortex in cardiovascular function regulation in rats. A nonselective muscarinic receptor agonist (oxotremorine M, OXO; 30 mM), a selective M2 receptor agonist (arecaidine but-2-ynyl ester tosylate, ABET; 3, 10, and 30 mM), 30 mM OXO mixed with a selective M2 receptor antagonist (methoctramine hydrate, MCT; 0.3, 1, and 3 mM), and normal saline (0.9 % NaCl) were separately microinjected (0.5 µl/5 s) into the cerebellar cortex (lobule VI) of anaesthetized rats. We measured the mean arterial pressure (MAP), maximum change in MAP, and reactive time (RT; the duration required for the blood pressure to return to basal levels), heart rate (HR) and the maximum change in HR during the RT in response to drug activation. The results demonstrated that ABET dose-dependently decreased MAP and HR, increased the maximum change in MAP and the maximum change in HR, and prolonged the RT. Furthermore, MCT dose-dependently blocked the OXO-mediated cardiovascular depressor response. This study provides the first evidence that M2 receptors in the cerebellar cortex are involved in cardiovascular regulation, the activation of which evokes significant depressor and bradycardic responses.

Presynaptic M3 muscarinic cholinoceptors mediate inhibition of excitatory synaptic transmission in area CA1 of rat hippocampus

Acetylcholine can modulate hippocampal network function through activation of both nicotinic and muscarinic acetylcholine receptors (mAChRs). All five mAChR subtypes have been identified in the hippocampus. Besides by their involvement in excitability of hippocampal cells, synaptic plasticity and memory, a large body of research has demonstrated the involvement of presynaptic mAChRs in the inhibition of glutamatergic transmission in the hippocampus. Over the years, however, pharmacological and molecular genetic studies have yielded quite contradictory results regarding the mAChR subtype(s) involved. In this study, multi-electrode array technology was used for the pharmacological elucidation of the subtype of mAChR mediating the depression of excitatory synaptic transmission at the SC-CA1 synapse. Using selective antagonists (VU0255035, MT7, tripinamide, MT3) and allosteric potentiators (VU 10010, VU 0238429) the involvement of M1, M2, M4, and M5 subtypes was ruled out thereby implying a major modulatory role for M3 receptors in the inhibition of excitatory synaptic transmission in area CA1 of rat hippocampus.

Differential Muscarinic Modulation in the Olfactory Bulb.

State-dependent cholinergic modulation of brain circuits is critical for several high-level cognitive functions, including attention and memory. Here, we provide new evidence that cholinergic modulation differentially regulates two parallel circuits that process chemosensory information, the accessory and main olfactory bulb (AOB and MOB, respectively). These circuits consist of remarkably similar synaptic arrangement and neuronal types, yet cholinergic regulation produced strikingly opposing effects in output and intrinsic neurons. Despite these differences, the chemogenetic reduction of cholinergic activity in freely behaving animals disrupted odor discrimination of simple odors, and the investigation of social odors associated with behaviors signaled by the Vomeronasal system.

Endogenous acetylcholine increases alveolar epithelial fluid transport via activation of alveolar epithelial Na,K-ATPase in mice

The contribution of endogenous acetylcholine to alveolar fluid clearance (AFC) and related molecular mechanisms were explored. AFC was measured in Balb/c mice after vagotomy and vagus nerve stimulation. Effects of acetylcholine chloride on AFC in Kunming mice and Na,K-ATPase function in A549 alveolar epithelial cells also were determined. AFC significantly decreased in mice with left cervical vagus nerve transection compared with controls (48.69±2.57 vs. 66.88±2.64, P≤0.01), which was reversed by stimulation of the peripheral (60.81±1.96, P≤0.01). Compared with control, acetylcholine chloride dose-dependently increased AFC and elevated Na,K-ATPase activity, and these increases were blocked or reversed by atropine. These effects were accompanied by recruitment of Na,K-ATPase α1 to the cell membrane. Thus, vagus nerves participate in alveolar epithelial fluid transport by releasing endogenous acetylcholine in the infusion-induced pulmonary edema mouse model. Effects of endogenous acetylcholine on AFC are likely mediated by Na,K-ATPase function through activation of muscarinic acetylcholine receptors on alveolar epithelia.

Regulation of the transient receptor potential channel TRPM3 by phosphoinositides.

The transient receptor potential (TRP) channel TRPM3 is a calcium-permeable cation channel activated by heat and by the neurosteroid pregnenolone sulfate (PregS). TRPM3 is highly expressed in sensory neurons, where it plays a key role in heat sensing and inflammatory hyperalgesia, and in pancreatic β cells, where its activation enhances glucose-induced insulin release. However, despite its functional importance, little is known about the cellular mechanisms that regulate TRPM3 activity. Here, we provide evidence for a dynamic regulation of TRPM3 by membrane phosphatidylinositol phosphates (PIPs). Phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2) and ATP applied to the intracellular side of excised membrane patches promote recovery of TRPM3 from desensitization. The stimulatory effect of cytosolic ATP on TRPM3 reflects activation of phosphatidylinositol kinases (PI-Ks), leading to resynthesis of PIPs in the plasma membrane. Various PIPs directly enhance TRPM3 activity in cell-free inside-out patches, with a potency order PI(3,4,5)P3 > PI(3,5)P2 > PI(4,5)P2 ≈ PI(3,4)P2 >> PI(4)P. Conversely, TRPM3 activity is rapidly and reversibly inhibited by activation of phosphatases that remove the 5-phosphate from PIPs. Finally, we show that recombinant TRPM3, as well as the endogenous TRPM3 in insuloma cells, is rapidly and reversibly inhibited by activation of phospholipase C-coupled muscarinic acetylcholine receptors. Our results reveal basic cellular mechanisms whereby membrane receptors can regulate TRPM3 activity.

Acetylcholine enhances keratocyte proliferation through muscarinic receptor activation

Acetylcholine (ACh), a classical neurotransmitter, has been shown to be present in various non-neuronal cells, including cells of the eye, such as corneal epithelium and endothelium, and to have widespread physiological effects such as cytoskeleton reorganization, cellular proliferation, differentiation, and apoptosis. The aim of this study was to investigate the effect of ACh on corneal keratocyte proliferation, and the underlying mechanisms, in order to explore its possible effect in corneal wound healing. Primary culture of human keratocytes was established from donated corneas. Cell viability and fraction of proliferating cells were detected by MTS assay and BrdU incorporation ELISA, respectively. Expression of proliferative markers, PCNA and Ki-67, was detected by western blot and immunocytochemistry. Activation of the MAPK/Erk signaling pathway and its involvement in ACh-enhanced proliferation was determined by western blot analysis, MTS, and BrdU ELISA. We found that ACh enhanced keratocyte proliferation even at low concentrations. Stimulation of proliferation was mediated through activation of muscarinic ACh receptors (mAChRs). Western blot analysis revealed that ACh stimulation of keratocytes upregulated the expression of PCNA and Ki-67, and Ki-67 immunocytochemistry showed that ACh-treated cells were in an active phase of the cell cycle. ACh activated MAPK signaling, and this step was crucial for the ACh-enhanced proliferation, as inhibition of the MAPK pathway resulted in ACh having no proliferative effect. In conclusion, ACh enhances keratocyte proliferation and might thus play a role in proper corneal wound healing.

Activation of muscarinic receptors in rat parotid acinar cells induces AQP5 trafficking to nuclei and apical plasma membrane

BackgroundThe subcellular distribution of aquaporin-5 (AQP5) in rat parotid acinar cells in response to muscarinic acetylcholine receptor (mAChR) activation remains unclear. MethodsImmunoconfocal and immunoelectron microscopy were used to visualize the distribution of AQP5 in parotid acinar cells. Western blotting was used to analyze AQP5 levels in membranes. To clarify the characteristics of membrane domains associated with AQP5, detergent solubility and sucrose-density flotation experiments were performed. ResultsUnder control conditions, AQP5 was diffusely distributed on the apical plasma membrane (APM) and apical plasmalemmal region and throughout the cytoplasm. Upon mAChR activation, AQP5 was predominantly located in the nucleus, APM and lateral plasma membrane (LPM). Subsequently, localization of AQP5 in the nucleus, APM and LPM was decreased. Prolonged atropine treatment inhibited mAChR agonist-induced translocation of AQP5 to the nucleus, APM and LPM. AQP5 levels were enhanced in isolated nuclei and nuclear membranes prepared from parotid tissues incubated with mAChR agonist. mAChR agonist induced AQP5 levels in both soluble and insoluble nuclear fractions solubilized with Triton X-100 or Lubrol WX. Small amounts of AQP5 in nuclei were detected using low-density sucrose gradient. When AQP5 was present in the nuclear membrane, nuclear size decreased. ConclusionThe activation of mAChR induced AQP5 translocation to the nucleus, APM and LPM, and AQP5 may trigger water transport across the nuclear membrane and plasma membrane in rat parotid acinar cells. General significanceAQP5 translocates to the nuclear membrane and may trigger the movement of water, inducing shrinkage of the nucleus and the start of nuclear functions.

Claudin-4 is required for modulation of paracellular permeability by muscarinic acetylcholine receptor in epithelial cells

The epithelial cholinergic system plays an important role in water, ion and solute transport. Previous studies have shown that activation of muscarinic acetylcholine receptors (mAChRs) regulates paracellular transport of epithelial cells; however, the underlying mechanism is still largely unknown. Here, we found that mAChR activation by carbachol and cevimeline reduced the transepithelial electrical resistance (TER) and increased the permeability of paracellular tracers in rat salivary epithelial SMG-C6 cells. Carbachol induced downregulation and redistribution of claudin-4, but not occludin or ZO-1 (also known as TJP1). Small hairpin RNA (shRNA)-mediated claudin-4 knockdown suppressed, whereas claudin-4 overexpression retained, the TER response to carbachol. Mechanistically, the mAChR-modulated claudin-4 properties and paracellular permeability were triggered by claudin-4 phosphorylation through ERK1/2 (also known as MAPK3 and MAPK1, respectively). Mutagenesis assay demonstrated that S195, but not S199, S203 or S207, of claudin-4, was the target for carbachol. Subsequently, the phosphorylated claudin-4 interacted with β-arrestin2 and triggered claudin-4 internalization through the clathrin-dependent pathway. The internalized claudin-4 was further degraded by ubiquitylation. Taken together, these findings suggested that claudin-4 is required for mAChR-modulated paracellular permeability of epithelial cells through an ERK1/2, β-arrestin2, clathrin and ubiquitin-dependent signaling pathway.

Activation of m1 muscarinic acetylcholine receptor induces surface transport of KCNQ channels through a CRMP-2-mediated pathway.

Neuronal excitability is strictly regulated by various mechanisms, including modulation of ion channel activity and trafficking. Stimulation of m1 muscarinic acetylcholine receptor (also known as CHRM1) increases neuronal excitability by suppressing the M-current generated by the Kv7/KCNQ channel family. We found that m1 muscarinic acetylcholine receptor stimulation also triggers surface transport of KCNQ subunits. This receptor-induced surface transport was observed with KCNQ2 as well as KCNQ3 homomeric channels, but not with Kv3.1 channels. Deletion analyses identified that a conserved domain in a proximal region of the N-terminal tail of KCNQ protein is crucial for this surface transport--the translocation domain. Proteins that bind to this domain were identified as α- and β-tubulin and collapsin response mediator protein 2 (CRMP-2; also known as DPYSL2). An inhibitor of casein kinase 2 (CK2) reduced tubulin binding to the translocation domain, whereas an inhibitor of glycogen synthase kinase 3 (GSK3) facilitated CRMP-2 binding to the translocation domain. Consistently, treatment with the GSK3 inhibitor enhanced receptor-induced KCNQ2 surface transport. M-current recordings from neurons showed that treatment with a GSK3 inhibitor shortened the duration of muscarinic suppression and led to over-recovery of the M-current. These results suggest that m1 muscarinic acetylcholine receptor stimulates surface transport of KCNQ channels through a CRMP-2-mediated pathway.

Activation of muscarinic acetylcholine receptors elevates intracellular Ca(2+) concentrations in accessory lobe neurons of the chick.

Accessory lobes are protrusions located at the lateral sides of the spinal cord of chicks and it has been proposed that they play a role as a sensory organ for equilibrium during walking. We have reported that functional neurons exist in the accessory lobe. As there is histological evidence that synaptic terminals of cholinergic nerves exist near the somata of accessory lobe neurons, we examined the effects of acetylcholine on changes in intracellular Ca2+ concentrations ([Ca2+]i), as an index of cellular activities. Acetylcholine (0.1-100 µM) caused a transient rise in the [Ca2+]i. Acetylcholine-evoked [Ca2+]i rises were observed in the absence of extracellular Ca2+, and they were abolished in the presence of cyclopiazonic acid, an inhibitor of Ca2+-ATPase of intracellular Ca2+ stores or atropine, a muscarinic receptor antagonist. mRNAs coding M3 and M5 isoforms of the muscarinic receptors were detected in accessory lobes by the RT-PCR. These results indicate that chick accessory lobe neurons express functional muscarinic acetylcholine receptors, and that acetylcholine stimulates Ca2+ mobilization from intracellular Ca2+ stores, which elevates the [Ca2+]i in the somata of accessory lobe neurons, through activation of these receptors. Cholinergic synaptic transmission to the accessory lobe neurons may regulate some cellular functions through muscarinic receptors.

Muscarinic M1 receptor and cannabinoid CB1 receptor do not modulate paraoxon-induced seizures.

One of the major signs of severe organophosphate poisoning is seizures. Previous studies have shown that both muscarinic agonist- and organophosphate-induced seizures require activation of muscarinic acetylcholine receptors in the central nervous system. Seizures induced by the muscarinic agonist pilocarpine require the M1 receptor and are modulated by cannabinoid CB1 receptors. In this study, we determined whether M1 and CB1 receptors also regulated seizures induced by the organophosphate paraoxon. We found no differences in seizures induced by paraoxon in wild-type (WT) and M1 knockout (KO) mice, indicating that in contrast to pilocarpine seizures, M1 receptors are not required for paraoxon seizures. Furthermore, we found that pilocarpine administration resulted in seizure-independent activation of ERK in the hippocampus in a M1 receptor-dependent manner, while paraoxon did not induce seizure-independent activation of ERK in the mouse hippocampus. This shows that pilocarpine and paraoxon activated M1 receptors in the hippocampus to different extents. There were no differences in seizures induced by paraoxon in WT and CB1 KO mice, and neither CB1 agonist nor antagonist administration had significant effects on paraoxon seizures, indicating that, in contrast to pilocarpine seizures, paraoxon seizures are not modulated by CB1 receptors. These results demonstrate that there are fundamental molecular differences in the regulation of seizures induced by pilocarpine and paraoxon.

Non-neuronal acetylcholine as an endogenous regulator of proliferation and differentiation of Lgr5-positive stem cells in mice.

Non-neuronal acetylcholine (ACh) is predicted to function as a local cell signaling molecule. However, the physiological significance of the synthesis of non-neuronal ACh in the intestine remains unclear. Here, experiments using crypt-villus organoids that lack nerve and immune cells in culture led us to suggest that endogenous ACh is synthesized in the intestinal epithelium to evoke growth and differentiation of the organoids through activation of muscarinic ACh receptors (mAChRs). The extracts of the cultured organoids showed a noticeable capacity for ACh synthesis that was sensitive to a potent inhibitor of choline acetyltransferase. Imaging MS revealed endogenous ACh localized in the epithelial layer in mouse small intestinal epithelium in vivo, suggesting that there are non-neuronal resources of ACh. Treatment of organoids with carbachol downregulated the growth of organoids and the expression of marker genes for epithelial cells. On the other hand, antagonists for mAChRs enhanced the growth and differentiation of organoids, indicating the involvement of mAChRs in regulating the proliferation and differentiation of Lgr5-positive stem cells. Collectively, our data provide evidence that endogenous ACh released from intestinal epithelium maintains homeostasis of intestinal epithelial cell growth and differentiation via mAChRs in mice.

Muscarinic cholinergic receptors modulate inhibitory synaptic rhythms in hippocampus and neocortex.

Activation of muscarinic acetylcholine (ACh) receptors (mAChRs) powerfully affects many neuronal properties as well as numerous cognitive behaviors. Small neuronal circuits constitute an intermediate level of organization between neurons and behaviors, and mAChRs affect interactions among cells that compose these circuits. Circuit activity is often assessed by extracellular recordings of the local field potentials (LFPs), which are analogous to in vivo EEGs, generated by coordinated neuronal interactions. Coherent forms of physiologically relevant circuit activity manifest themselves as rhythmic oscillations in the LFPs. Frequencies of rhythmic oscillations that are most closely associated with animal behavior are in the range of 4–80 Hz, which is subdivided into theta (4–14 Hz), beta (15–29 Hz) and gamma (30–80 Hz) bands. Activation of mAChRs triggers rhythmic oscillations in these bands in the hippocampus and neocortex. Inhibitory responses mediated by GABAergic interneurons constitute a prominent feature of these oscillations, and indeed, appear to be their major underlying factor in many cases. An important issue is which interneurons are involved in rhythm generation. Besides affecting cellular and network properties directly, mAChRs can cause the mobilization of endogenous cannabinoids (endocannabinoids, eCBs) that, by acting on the principal cannabinoid receptor of the brain, CB1R, regulate the release of certain neurotransmitters, including GABA. CB1Rs are heavily expressed on only a subset of interneurons and, at lower density, on glutamatergic neurons. Exogenous cannabinoids typically disrupt oscillations in the theta (θ) and gamma (γ) ranges, which probably contributes to the behavioral effects of these drugs. It is important to understand how neuronal circuit activity is affected by mAChR-driven eCBs, as this information will provide deeper insight into the actions of ACh itself, as well as into the effects of eCBs and exogenous cannabinoids in animal behavior. After covering some basic aspects of the mAChR system, this review will focus on recent findings concerning the mechanisms and circuitry that generate θ and γ rhythms in hippocampus and neocortex. The ability of optogenetic methods to probe the many roles of ACh in rhythm generation is highlighted.

Direct excitation of parvalbumin-positive interneurons by M1 muscarinic acetylcholine receptors: roles in cellular excitability, inhibitory transmission and cognition.

Parvalbumin-containing (PV) neurons, a major class of GABAergic interneurons, are essential circuit elements of learning networks. As levels of acetylcholine rise during active learning tasks, PV neurons become increasingly engaged in network dynamics. Conversely, impairment of either cholinergic or PV interneuron function induces learning deficits. Here, we examined PV interneurons in hippocampus (HC) and prefrontal cortex (PFC) and their modulation by muscarinic acetylcholine receptors (mAChRs). HC PV cells, visualized by crossing PV-CRE mice with Rosa26YFP mice, were anatomically identified as basket cells and PV bistratified cells in the stratum pyramidale; in stratum oriens, HC PV cells were electrophysiologically distinct from somatostatin-containing cells. With glutamatergic transmission pharmacologically blocked, mAChR activation enhanced PV cell excitability in both CA1 HC and PFC; however, CA1 HC PV cells exhibited a stronger postsynaptic depolarization than PFC PV cells. To delete M1 mAChRs genetically from PV interneurons, we created PV-M1 knockout mice by crossing PV-CRE and floxed M1 mice. The elimination of M1 mAChRs from PV cells diminished M1 mAChR immunoreactivity and muscarinic excitation of HC PV cells. Selective cholinergic activation of HC PV interneurons using Designer Receptors Exclusively Activated by Designer Drugs technology enhanced the frequency and amplitude of inhibitory synaptic currents in CA1 pyramidal cells. Finally, relative to wild-type controls, PV-M1 knockout mice exhibited impaired novel object recognition and, to a lesser extent, impaired spatial working memory, but reference memory remained intact. Therefore, the direct activation of M1 mAChRs on PV cells contributes to some forms of learning and memory.

Synthesis and pharmacological evaluation of analogues of benzyl quinolone carboxylic acid (BQCA) designed to bind irreversibly to an allosteric site of the M ₁ muscarinic acetylcholine receptor.

Activation of the M1 muscarinic acetylcholine receptor (mAChR) is a prospective treatment for alleviating cognitive decline experienced in central nervous system (CNS) disorders. Current therapeutics indiscriminately enhance the activity of the endogenous neurotransmitter ACh, leading to side effects. BQCA is a positive allosteric modulator and allosteric agonist at the M1 mAChR that has high subtype selectivity and is a promising template from which to generate higher affinity, more pharmacokinetically viable drug candidates. However, to efficiently guide rational drug design, the binding site of BQCA needs to be conclusively elucidated. We report the synthesis and pharmacological validation of BQCA analogues designed to bind irreversibly to the M1 mAChR. One analogue in particular, 11, can serve as a useful structural probe to confirm the location of the BQCA binding site; ideally, by co-crystallization with the M1 mAChR. Furthermore, this ligand may also be used as a pharmacological tool with a range of applications.

Functional link between muscarinic receptors and large-conductance Ca2+ -activated K+ channels in freshly isolated human detrusor smooth muscle cells.

Activation of muscarinic acetylcholine receptors (mAChRs) constitutes the primary mechanism for enhancing excitability and contractility of human detrusor smooth muscle (DSM). Since the large-conductance Ca(2+)-activated K(+) (KCa1.1) channels are key regulators of human DSM function, we investigated whether mAChR activation increases human DSM excitability by inhibiting KCa1.1 channels. We used the mAChR agonist, carbachol, to determine the changes in KCa1.1 channel activity upon mAChR activation in freshly isolated human DSM cells obtained from open bladder surgeries using the perforated whole cell and single KCa1.1 channel patch-clamp recordings. Human DSM cells were collected from 29 patients (23 males and 6 females, average age of 65.9 ± 1.5 years). Carbachol inhibited the amplitude and frequency of KCa1.1 channel-mediated spontaneous transient outward currents and spontaneous transient hyperpolarizations, which are triggered by the release of Ca(2+) from ryanodine receptors. Carbachol also caused membrane potential depolarization, which was not observed in the presence of iberiotoxin, a KCa1.1 channel inhibitor, indicating the critical role of the KCa1.1 channels. The potential direct carbachol effects on KCa1.1 channels were examined under conditions of removing the major cellular Ca(2+) sources for KCa1.1 channel activation with pharmacological inhibitors (thapsigargin, ryanodine, and nifedipine). In the presence of these inhibitors, carbachol did not affect the single KCa1.1 channel open probability and mean KCa1.1 channel conductance (cell-attached configuration) or depolarization-induced whole cell steady-state KCa1.1 currents. The data support the concept that mAChR activation triggers indirect functional KCa1.1 channel inhibition mediated by intracellular Ca(2+), thus increasing the excitability in human DSM cells.

Galantamine promotes adult hippocampal neurogenesis via M₁ muscarinic and α7 nicotinic receptors in mice.

Galantamine, an inhibitor of acetylcholinesterase, promotes hippocampal neurogenesis, but the exact mechanism for this is not known. In the present study, we examined the mechanisms underlying the effects of acute galantamine on neurogenesis in the mouse hippocampus. Galantamine (3 mg/kg) increased the number of 5-bromo-2'-deoxyuridine (BrdU)-positive cells in the subgranular zone of the dentate gyrus. This effect was blocked by the muscarinic receptor antagonist scopolamine and the preferential M1 muscarinic receptor antagonist telenzepine, but not by the nicotinic receptor antagonists mecamylamine and methyllycaconitine. Galantamine did not alter the ratio of neuronal nuclei (NeuN)- or glial fibrillary acidic protein (GFAP)-positive cells to BrdU-labeled cells in the subgranular zone and granule cell layer. Galantamine (1, 3 mg/kg) promoted the survival of 2-wk-old newly divided cells in mice in the granule cell layer of the dentate gyrus, whereas it did not affect the survival of newly divided cells at 1 and 4 wk. Galantamine-induced increases in cell survival were blocked by the α7 nicotinic receptor antagonist methyllycaconitine, but not by scopolamine. Bilateral injection of recombinant IGF2 into the dentate gyrus of the hippocampus mimicked the effects of galantamine. The effects of galantamine were blocked by direct injection of the IGF1 receptor antagonist JB1. These findings suggest that galantamine promotes neurogenesis via activation of the M1 muscarinic and α7 nicotinic acetylcholine receptors. The present study also suggests that IGF2 is involved in the effects of galantamine on the survival of 2-wk-old immature cells in the granule cell layer.

Allosteric Modulation of M1 Muscarinic Acetylcholine Receptor Internalization and Subcellular Trafficking

Allosteric modulators are an attractive approach to achieve receptor subtype-selective targeting of G protein-coupled receptors. Benzyl quinolone carboxylic acid (BQCA) is an unprecedented example of a highly selective positive allosteric modulator of the M1 muscarinic acetylcholine receptor (mAChR). However, despite favorable pharmacological characteristics of BQCA in vitro and in vivo, there is limited evidence of the impact of allosteric modulation on receptor regulatory mechanisms such as β-arrestin recruitment or receptor internalization and endocytic trafficking. In the present study we investigated the impact of BQCA on M1 mAChR regulation. We show that BQCA potentiates agonist-induced β-arrestin recruitment to M1 mAChRs. Using a bioluminescence resonance energy transfer approach to monitor intracellular trafficking of M1 mAChRs, we show that once internalized, M1 mAChRs traffic to early endosomes, recycling endosomes and late endosomes. We also show that BQCA potentiates agonist-induced subcellular trafficking. M1 mAChR internalization is both β-arrestin and G protein-dependent, with the third intracellular loop playing an important role in the dynamics of β-arrestin recruitment. As the global effect of receptor activation ultimately depends on the levels of receptor expression at the cell surface, these results illustrate the need to extend the characterization of novel allosteric modulators of G protein-coupled receptors to encapsulate the consequences of chronic exposure to this family of ligands.