Activation Of Muscarinic Acetylcholine Receptors Research Articles

The decline in the clinical relevance of pilocarpine and physostigmine monitored in pharmacology textbooks from 1878 to 2023: nine take-home messages for future (pharmacology) textbook authors.

In a recent study, using hydrogen cyanide as paradigm, we have shown that pharmacological knowledge evolves non-linearly ( https://pubmed.ncbi.nlm.nih.gov/38900251/ ). The aim of this study was to investigate the changes in the presentation of the drugs pilocarpine and physostigmine in textbooks from 1878 to 2023. The categories of structure, molecular mechanism of action, pharmacokinetics, effects, indications, adverse drug reactions, interactions, and contraindications were evaluated. The pharmacological knowledge on the molecular mechanism, chemical structure, and pharmacokinetics of pilocarpine and physostigmine changed the most during the period of 150 years. Until 1944, textbooks did not mention a molecular mechanism of action of pilocarpine and from 1951 onwards they described the activation of muscarinic acetylcholine receptors as the molecular basis of pilocarpine's effect. Until 1944, most textbooks on physostigmine also did not mention the molecular mechanism of action. From 1951 onwards, the reversible inhibition of acetylcholinesterase is mentioned as the mechanism of action of physostigmine. In contrast, in the categories effects, indications, adverse drug reactions, interactions, and contraindications, the detected changes in the pharmacological knowledge presented were comparatively smaller. Older pharmacology textbooks were better than newer ones at discussing changes in knowledge and scientific errors. We noted substantial differences in the presentation of pilocarpine and physostigmine among German and US pharmacology textbooks. We show a decline of the clinical relevance of both drugs and their presentation in pharmacological textbooks with physostigmine being virtually irrelevant. But modern textbooks still discuss physostigmine substantially, fitting to studies on the obsolete drug reserpine ( https://pubmed.ncbi.nlm.nih.gov/38103060/ ). Thus, textbooks often far lag clinical practice. Google Scholar conveys the incorrect impression that physostigmine is clinically more relevant than it is. An exponential decline in prescription numbers is a robust indicator of clinical obsolescence. From our study, we extract nine easily implementable take-home messages for future (pharmacology) textbook authors to ensure that this traditional teaching format will prevail against the competition of allegedly more "modern" teaching media.

The Activation of Muscarinic Acetylcholine Receptors Protects against Neuroinflammation in a Mouse Model through Attenuating Microglial Inflammation.

Neuroinflammation is a critical factor that contributes to neurological impairment and is closely associated with the onset and progression of neurodegenerative diseases. In the central nervous system (CNS), microglia play a pivotal role in the regulation of inflammation through various signaling pathways. Therefore, mitigating microglial inflammation is considered a promising strategy for restraining neuroinflammation. Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and exhibit clear neuroprotective effects in various disease models. However, whether the activation of mAChRs can harness benefits in neuroinflammation remains largely unexplored. In this study, the anti-inflammatory effects of mAChRs were found in a neuroinflammation mouse model. The expression of various cytokines and chemokines was regulated in the brains and spinal cords after the administration of mAChR agonists. Microglia were the primary target cells through which mAChRs exerted their anti-inflammatory effects. The results showed that the activation of mAChRs decreased the pro-inflammatory phenotypes of microglia, including the expression of inflammatory cytokines, morphological characteristics, and distribution density. Such anti-inflammatory modulation further exerted neuroprotection, which was found to be even more significant by the direct activation of neuronal mAChRs. This study elucidates the dual mechanisms through which mAChRs exert neuroprotective effects in central inflammatory responses, providing evidence for their application in inflammation-related neurological disorders.

Reactivation-dependent transfer of fear memory between contexts requires M1 muscarinic receptor stimulation in dorsal hippocampus of male rats.

Memory updating is essential for integrating new information into existing representations. However, this process could become maladaptive in conditions like post-traumatic stress disorder (PTSD), when fear memories generalize to neutral contexts. Previously, we have shown that contextual fear memory malleability in rats requires activation of M1 muscarinic acetylcholine receptors in the dorsal hippocampus. Here, we investigated the involvement of this mechanism in the transfer of contextual fear memories to other contexts using a novel fear memory updating paradigm. Following brief reexposure to a previously fear conditioned context, male rats (n = 8-10/group) were placed into a neutral context to evaluate the transfer of fear memory. We also infused the selective M1 receptor antagonist pirenzepine into the dorsal hippocampus before memory reactivation to try to block this effect. Results support the hypothesis that fear memory can be updated with novel contextual information, but only if rats are reexposed to the originally trained context relatively recently before the neutral context; evidence for transfer was not seen if the fear memory reactivation was omitted or if it occurred 6 h before neutral context exposure. The transferred fear persisted for 4 weeks, and the effect was blocked by M1 antagonism. These findings strongly suggest that fear transfer requires reactivation and destabilization of the original fear memory. The novel preclinical model introduced here, and its implication of muscarinic receptors in this process, could therefore inform therapeutic strategies for PTSD and similar conditions.

Characterizing Muscarinic Receptor Subtype Roles in Inspiratory Burst Modulation of Hypoglossal Motoneurons in Mice

Loss of tongue muscle (innervated by hypoglossal (XII) motoneurons) tone is responsible for airway obstruction during sleep. Contributing factors include loss of noradrenergic drive and activation of muscarinic acetylcholine receptors (mAChRs). While noradrenergic effects are well studied, less is known about muscarinic effects. In mice, XII motoneurons express four mAChRs; M1, M3, and M5 are excitatory while M2 is inhibitory. We hypothesize that M2 is upregulated with postnatal maturation whereas M1, M3, and M5 are downregulated. We therefore expect that activation of M1, M3, and M5 receptors will potentiate inspiratory bursting early in postnatal development whereas activation of M2 receptors will inhibit inspiratory bursting later in postnatal development. To test this, we utilized rhythmic medullary slice preparations from mice at postnatal days 0-5 (P0-5) and P9-14, which contained preBötzinger Complex (site of inspiratory rhythm generation) and XII motoneurons. Blocking M1 receptors locally with pirenzepine (10 μM & 100 μM) decreased the muscarinic potentiation of inspiratory bursting in P0-5 mice at 100 μM (168±23% at 10 μM and 129±16% at 100 μM in muscarine [p<0.05] vs 183±34% muscarine (100 μM) control response, n=9). Activating M1 receptors locally with cevimeline (1 mM) for 30s or 60s did not mediate potentiation of inspiratory bursting as seen with muscarine (116±7% at 30s and 117±7% at 60s vs 181±40% control muscarine response [n=6, p<0.05]). Bath application of M2 antagonist methoctramine (MTH) in P0-5 mice at 1 μM had no effect on muscarine-potentiation of inspiratory bursting (207±109% at 1 μM MTH in muscarine vs 212±84% muscarine control response, n=9). Preliminary analysis of the P9-14 age group using 1 μM MTH suggests there may be an increase in muscarine-mediated potentiation of inspiratory bursting with M2 blockade (170±56% at 1 μM MTH in muscarine vs 152±56% muscarine control response, n=6, p>0.5). Bath application of M3 receptor antagonist 4-DAMP in P0-5 mice at 10 nM had no effect on muscarine-potentiated inspiratory bursting (172±12% vs 178±30% muscarine control), whereas application at 100 nM decreased muscarine-mediated inspiratory burst potentiation compared to control (133±12%, n=4, p<0.05). However, bath application with another M3 receptor antagonist, J Fumarate 104129, had no significant effect on muscarinic modulation of inspiratory bursting in P0-5 mice (188±46% at 0.6 nM [n=6], 185±32% at 6 nM [n=6], and 144±24% at 60 nM [n=4] vs 176±34% muscarine control [n=6]). Overall, our data partially supports a role of M1 receptors contributing to muscarinic potentiation of inspiratory bursting in XII motoneurons, with the contributions of M2 and M3 appearing insignificant early in postnatal life. Preliminary analysis suggests that M2 has a larger contribution later in maturation. Future research will evaluate the contribution of muscarinic receptor subtypes to inspiratory burst modulation later in postnatal life. NIH/NHLBI Funding: R15HL148870. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Distribution of muscarinic acetylcholine receptors in hypoglossal motoneurons across postnatal maturation

Upper airway patency is decreased during rapid eye movement (REM) sleep due to loss of genioglossus (primary tongue protruder) tongue muscle tone. During REM sleep, hypoglossal motoneurons (XII MNs), which innervate genioglossus, receive less excitatory noradrenergic drive and may be inhibited by activation of muscarinic acetylcholine receptors. Data from intact adult rats during natural sleep implicates an inhibitory effect of muscarinic acetylcholine receptors on XII MNs. However, data using the rhythmic slice preparation from neonatal mice indicate that muscarine has a net excitatory effect on inspiratory burst amplitude at XII MNs. Our project examines the changes to distribution of muscarinic acetylcholine receptors (mAChRs) at XII MNs across postnatal maturation. We hypothesize there is an increase in inhibitory (M2) and decrease in excitatory (M1, M3, M5) mAChRs at XII MNs with postnatal maturation. We performed double-labeled immunofluorescence experiments on perfused transverse brainstem slices (20μm) under identical conditions across six postnatal age groups: P0-P2, P3-P5, P6-P10, P11-P13, P14-P17, and adult against mAChR targets M1, M2, M3, M5. Slices were co-labeled with choline acetyltransferase (ChAT) and 4’,6-diamidino-2-phenylindole (DAPI). Images were collected using a confocal microscope. 10 individual XII MNs were identified in each slice using positive ChAT labeling as well as anatomical location. ImageJ was used to determine the average XII MN mAChR subtype intensity. Preliminary data (n = 3) indicate that M1 receptors showed a transitory increase in labeling intensity in XII MNs followed by a decrease across development (P1=89±11%, P4-5=100%, P9=76±18%, P12-13=67±8%, P17=67±4%, adult=56±7%, p = 0.13). M3 (n = 3) showed a trend for a transient increase in labeling intensity at P17 (P1=78±41%, P4-5=87±3%, P9=61±13%, P12-13=86±34%, P17=100%, adult=80±23, p = 0.60). M5 (n = 3) expression intensity decreased consistently over maturation (P1=100%, P4-5=92±34%, P9=75±23%, P12-13=70±10%, P17=66±14%, adult=39±12%, p = 0.0257). In contrast, M2 (n = 3) receptor expression intensity remained relatively consistent across maturation (P1 = 88±23%, P4-5 = 100%, P9=88±10%, P12-13 = 75±15%, P17 = 86±18%, adult = 75±10%). These data partly support our hypothesis of a decrease in expression intensity of excitatory M1, M3, and M5 receptor subtypes into adulthood. Contrary to our hypothesis, we observed minimal change in the expression intensity of inhibitory M2 receptors across maturation. The decrease in labeling intensity of excitatory muscarinic receptor subtypes may support the observed shift in muscarinic modulation from excitation to inhibition with postnatal maturation. Arizona Alzheimer's Consortium, NIH. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Endogenous Acetylcholine Contributes to Flow-Mediated Dilation in Humans

Background & Objective. Flow-mediated dilation (FMD) is a crucial function of the vascular endothelium wherein intraluminal shear stress stimulates endothelium-dependent vasodilation mediated primarily by nitric oxide. Brachial artery FMD (baFMD) is widely evaluated in clinical research as an index of endothelial function, and impaired baFMD is an independent predictor of cardiovascular risk. Despite the physiological and clinical relevance of FMD, the mechanisms by which shear stress is transduced to endothelium-dependent vasodilation (i.e., upstream of nitric oxide production) remain unclear. baFMD corresponds closely with sensitivity to the endothelium-dependent vasodilator acetylcholine (ACh), and preclinical models suggest ACh is released from endothelial cells in response to shear stress. Therefore, the present study tested the hypothesis that activation of muscarinic ACh receptors facilitates baFMD in humans. Methods. baFMD was evaluated using Doppler ultrasound in healthy participants (n=11; 5F/6M; 23±2 y; means ± SE) under control conditions and following local inhibition of muscarinic ACh receptors via intra-arterial atropine infused proximal to the measurement site. baFMD was assessed in response to transient and sustained shear stimuli in separate trials: 1) reactive hyperemia following 5 min of arterial cuff occlusion (RH-FMD) and 2) steady-state rhythmic handgrip exercise at graded intensities (10, 15, 20, & 25% maximal voluntary contraction, MVC) to produce step increases in shear rate (EX-FMD). Blockade effcacy was confirmed via a dose-response to intra-arterial ACh infusion pre and post atropine. Results. Baseline brachial artery diameter and forearm blood flow were similar in all trials (all P>0.05). Atropine reduced peak brachial artery dilation following cuff release (RH-FMD, Δ diameter, atropine: 4.6±0.8 vs. control: 6.5±0.8%, P=0.04) and increased the shear stimulus (shear rate area under the curve, SRAUC: atropine, 29±3 vs. control: 21±3 s−1×103, P=0.01) such that the percent change in brachial diameter was diminished by 35±16% when normalized to SRAUC (baFMD:SRAUC ratio, atropine: 1.7±0.3 vs. control: 3.9±1.0, P=0.02). Similarly, in the EX-FMD trial, atropine reduced brachial artery diameter at all exercise intensities (25% MVC, percent change in diameter: atropine, 10.8±1.4 vs. control, 15.5±1.7%, P=0.04) despite a greater shear stimulus (shear rate: atropine, 10.9±1.1 vs. control, 9.3±0.9 s−1×102, P=0.01). Thus, atropine reduced the slope describing the relationship between brachial artery vasodilation (percent change) and the change in shear rate by 38±8% across all exercise intensities (slope, atropine: 1.3±0.1 vs. control: 2.4±0.4% Δ×s×10−2, P=0.007). As expected, atropine reduced brachial artery vasodilation in response to upstream infusion of ACh (1.5 μg ACh/dl forearm volume/min, Δ diameter, atropine: 2±1 vs. control: 23±4%, P=0.001). Conclusions. The results suggest endogenous ACh facilitates baFMD in response to both transient and sustained shear stimuli, which highlights a novel physiological role of endogenous ACh in vascular regulation in humans and provides insight regarding the basic mechanisms of flow-mediated vasodilation. NIH R01 HL119337 and F31 HL142240. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

M2 muscarinic receptors negatively modulate cell migration in human glioblastoma cells

Glioblastoma (GB) is a very aggressive human brain tumor. The high growth potential and invasiveness make this tumor surgically and pharmacologically untreatable. Our previous work demonstrated that the activation of the M2 muscarinic acetylcholine receptors (M2 mAChRs) inhibited cell proliferation and survival in GB cell lines and in the cancer stem cells derived from human biopsies. The aim of the present study was to investigate the ability of M2 mAChR to modulate cell migration in two different GB cell lines: U87 and U251. By wound healing assay and single cell migration analysis performed by time-lapse microscopy, we demonstrated the ability of M2 mAChRs to negatively modulate cell migration in U251 but not in the U87 cell line. In order to explain the different effects observed in the two cell lines we have evaluated the possible involvement of the intermediate conductance calcium-activated potassium (IKCa) channel. IKCa channel is present in the GB cells, and it has been demonstrated to modulate cell migration. Using the perforated patch-clamp technique we have found that selective activation of M2 mAChR significantly reduced functional density of the IKCa current in U251 but not in U87 cells. To understand whether the M2 mAChR mediated reduction of ion channel density in the U251 cell line was relevant for the cell migration impairment, we tested the effects of TRAM-34, a selective inhibitor of the IKCa channel, in wound healing assay. We found that it was able to markedly reduce U251 cell migration and significantly decrease the number of invadopodia-like structure formations. These results suggest that only in U251 cells the reduced cell migration M2 mAChR-mediated might involve, at least in part, the IKCa channel.

Muscarinic receptor activation promotes destabilization and updating of object location memories in mice

The storage of long-term memories is a dynamic process. Reminder cues can destabilize previously consolidated memories, rendering them labile and modifiable. However, memories that are strongly encoded or relatively remote at the time of reactivation can resist destabilization only being rendered labile under conditions that favour memory updating. Using the object location recognition task, here we show in male C57BL/6 mice that novelty-induced destabilization of strongly-encoded memories requires muscarinic acetylcholine receptor (mAChR) activation. Furthermore, we use the objects-in-updated locations task to show that updating of object location memories is mAChR-dependent. Thus, mAChR stimulation appears to be critical for spatial memory destabilization and related memory updating. Enhancing our understanding of the role of ACh in memory updating should inform future research into the underlying causes of behavioural disorders that are characterized by persistent maladaptive memories, such as age-related cognitive inflexibility and post-traumatic stress disorder.

Activation of M1 muscarinic acetylcholine receptors by proline-rich oligopeptide 7a (

The bioactive peptides derived from snake venoms of the Viperidae family species have been promising as therapeutic candidates for neuroprotection due to their ability to prevent neuronal cell loss, injury, and death. Therefore, this study aimed to evaluate the cytoprotective effects of a synthetic proline-rich oligopeptide 7a (PRO-7a; <EDGPIPP) from Bothrops jararaca snake, on oxidative stress-induced toxicity in neuronal PC12 cells and astrocyte-like C6 cells. Both cells were pre-treated for four hours with different concentrations of PRO-7a, submitted to H2O2-induced damage for 20 h, and then the oxidative stress markers were analyzed. Also, two independent neuroprotective mechanisms were investigated: a) L-arginine metabolite generation via argininosuccinate synthetase (AsS) activity regulation to produce agmatine or polyamines with neuroprotective properties; b) M1 mAChR receptor subtype activation pathway to reduce oxidative stress and neuron injury. PRO-7a was not cytoprotective in C6 cells, but potentiated the H2O2-induced damage to cell integrity at a concentration lower than 0.38 μM. However, PRO-7a at 1.56 µM, on the other hand, modified H2O2-induced toxicity in PC12 cells by restoring cell integrity, mitochondrial metabolism, ROS generation, and arginase indirect activity. The α-Methyl-DL-aspartic acid (MDLA) and L-NΩ-Nitroarginine methyl ester (L-Name), specific inhibitors of AsS and nitric oxide synthase (NOS), which catalyzes the synthesis of polyamines and NO from L-arginine, did not suppress PRO-7a-mediated cytoprotection against oxidative stress. It suggested that its mechanism is independent of the production of L-arginine metabolites with neuroprotective properties by increased AsS activity. On the other hand, the neuroprotective effect of PRO-7a was blocked in the presence of dicyclomine hydrochloride (DCH), an M1 mAChR antagonist. For the first time, this work provides evidence that PRO-7a-induced neuroprotection seems to be mediated through M1 mAChR activation in PC12 cells, which reduces oxidative stress independently of AsS activity and L-arginine bioavailability.

Increased GIRK channel activity prevents arrhythmia in mice with heart failure by enhancing ventricular repolarization

Ventricular arrhythmia causing sudden cardiac death is the leading mode of death in patients with heart failure. Yet, the mechanisms that prevent ventricular arrhythmias in heart failure are not well characterized. Using a mouse model of heart failure created by transverse aorta constriction, we show that GIRK channel, an important regulator of cardiac action potentials, is constitutively active in failing ventricles in contrast to normal cells. Evidence is presented indicating that the tonic activation of M2 muscarinic acetylcholine receptors by endogenously released acetylcholine contributes to the constitutive GIRK activity. This constitutive GIRK activity prevents the action potential prolongation in heart failure ventricles. Consistently, GIRK channel blockade with tertiapin-Q induces QT interval prolongation and increases the incidence of arrhythmia in heart failure, but not in control mice. These results suggest that constitutive GIRK channels comprise a key mechanism to protect against arrhythmia by providing repolarizing currents in heart failure ventricles.

Acetylcholine Receptor Agonists Exerted Neuroprotection Against Doxorubicin‐Induced Chemobrain

AbstractBackgroundAlthough doxorubicin has been broadly used in treating a wide range of cancers, it frequently causes a serious neurological consequences referred to as chemobrain in cancer patients. Chemobrain shares several key pathological features with Alzheimer’s disease (AD) including neuroinflammation and Tau pathology. In addition to the inhibition of acetylcholinesterase, the activation of a7nicotinic and muscarinic acetylcholine receptors (a7nAChR and mAChR) using their agonists has been considered an effective therapeutic target against dementia. However, the benefit of intervention with nAChR and mAChR agonists in DOX‐induced chemobrain has never been investigated.MethodTwenty‐five male Wistar rats were assigned to received either (0.9% NSS) or doxorubicin (3 mg/kg; 6 doses) via intraperitoneal injection. Then, doxorubicin‐treated rats were divided into 4 intervention groups to treat with either vehicle, an a7nAChR agonist (PNU‐282987; 3 mg/kg), a mAChR agonist (bethanechol; 12 mg/kg), or a combined treatment of PNU‐282987 and bethanechol via intraperitoneal injection as a concomitant treatment with doxorubicin for a period of 30 days. After completion of the treatment paradigm, cognitive function was evaluated using the novel object location task (NOLT). The hippocampal tissues were dissected to enable further investigation.ResultSystemic administration of doxorubicin impaired long‐term cognitive function in rats as shown by a decreased preference index in the NOLT (Figure A). Furthermore, doxorubicin‐treated rats exhibited neuroinflammation in the hippocampus as demonstrated by an increase in TNF‐a protein expression (Figure B) and an alteration in microglial morphology to a pro‐inflammatory amoeboid‐liked phenotype (Figure C). Notably, injection of doxorubicin markedly reduced the phosphorylation of GSK3b at Ser9 (Figure D) and upregulated the phosphorylation of Tau at Thr181 (Figure E). Also, a decline in the dendritic spine density in the CA1 region was observed in doxorubicin‐treated rats compared with the controls (Figure F). Interestingly, all AChR agonists restored cognitive function in rats in a similar manner by attenuating neuroinflammation and Tau phosphorylation while preserving dendritic spine density (Figure A‐F).ConclusionTogether, activation of AChRs by their agonists protected against the neuropathology of doxorubicin‐induced chemobrain, indicating their potential role as a novel strategy to prevent chemobrain in patients receiving chemotherapy.

A muscarinic, GIRK channel-mediated inhibition of inspiratory-related XII nerve motor output emerges in early postnatal development in mice.

In neonatal rhythmic medullary slices, muscarinic acetylcholine receptor (mAChR) activation of hypoglossal (XII) motoneurons that innervate the tongue has a net excitatory effect on XII inspiratory motor output. Conversely, during rapid eye movement sleep in adult rodents, XII motoneurons experience a loss of excitability partly due to activation of mAChRs. This may be mediated by activation of G-protein-coupled inwardly rectifying potassium (GIRK) channels. Therefore, this study was designed to evaluate whether muscarinic modulation of XII inspiratory motor output in mouse rhythmic medullary slices includes GIRK channel-mediated inhibition and, if so, when this inhibitory mechanism emerges. Local pressure injection of the mAChR agonist muscarine potentiated inspiratory bursting by 150 ± 28% in postnatal day (P)0-P5 rhythmic medullary slice preparations. In the absence of muscarine, pharmacological GIRK channel block by Tertiapin-Q did not affect inspiratory burst parameters, whereas activation with ML297 decreased inspiratory burst area. Blocking GIRK channels by local preapplication of Tertiapin-Q revealed a developmental change in muscarinic modulation of inspiratory bursting. In P0-P2 rhythmic medullary slices, Tertiapin-Q preapplication had no significant effect on muscarinic potentiation of inspiratory bursting (a negligible 6% decrease). However, preapplication of Tertiapin-Q to P3-P5 rhythmic medullary slices caused a 19% increase in muscarinic potentiation of XII inspiratory burst amplitude. Immunofluorescence experiments revealed expression of GIRK 1 and 2 subunits and M1, M2, M3, and M5 mAChRs from P0 to P5. Overall, these data support that mechanisms underlying muscarinic modulation of inspiratory burst activity change postnatally and that potent GIRK-mediated inhibition described in adults emerges early in postnatal life.NEW & NOTEWORTHY Muscarinic modulation of inspiratory bursting at hypoglossal motoneurons has a net excitatory effect in neonatal rhythmic medullary slice preparations and a net inhibitory effect in adult animals. We demonstrate that muscarinic modulation of inspiratory bursting undergoes maturational changes from postnatal days 0 to 5 that include emergence of an inhibitory component mediated by G-protein-coupled inwardly rectifying potassium channels after postnatal day 3 in neonatal mouse rhythmic medullary slice preparations.

Role of Conserved Tyrosine Lid Residues in the Activation of the M2 Muscarinic Acetylcholine Receptor.

The development of subtype selective small molecule drugs for the muscarinic acetylcholine receptor (mAChR) family has been challenging. The design of more selective ligands can be improved by understanding the structure and function of key amino acid residues that line ligand binding sites. Here we study the role of three conserved key tyrosine residues [Y1043.33, Y4036.51, and Y4267.39 (Ballesteros and Weinstein numbers in superscript)] at the human M2 mAChR, located at the interface between the orthosteric and allosteric binding sites of the receptor. We specifically focused on the role of the three tyrosine hydroxyl groups in the transition between the inactive and active conformations of the receptor by making phenylalanine point mutants. Single-point mutation at either of the three positions was sufficient to reduce the affinity of agonists by ∼100-fold for the M2 mAChR, whereas the affinity of antagonists remained largely unaffected. In contrast, neither of the mutations affected the efficacy of orthosteric agonists. When mutations were combined into double and triple M2 mAChR mutants, the affinity of antagonists was reduced by more than 100-fold compared with the wild-type M2 receptor. In contrast, the affinity of allosteric modulators, either negative or positive, was retained at all single and multiple mutations, but the degree of allosteric effect exerted on the endogenous ligand acetylcholine was affected at all mutants containing Y4267.39F. These findings will provide insights to consider when designing future mAChR ligands. SIGNIFICANCE STATEMENT: Structural studies demonstrated that three tyrosine residues between the orthosteric and allosteric sites of the M2 muscarinic acetylcholine receptor (mAChR) had different hydrogen bonding networks in the inactive and active conformations. The role of hydroxyl groups of the tyrosine residues on orthosteric and allosteric ligand pharmacology was unknown. We found that hydroxyl groups of the tyrosine residues differentially affected the molecular pharmacology of orthosteric and allosteric ligands. These results provide insights to consider when designing future mAChR ligands.

Vagal-mAChR4 signaling promotes Friend virus complex (FV)-induced acute erythroleukemia

Erythroleukemia belongs to acute myeloid leukemia (AML) type 6 (M6), and treatment remains difficult due to the poor prognosis of the disease. Friend virus (FV) is a complex of two viruses: Friend murine leukemia virus (F-MuLV) strain along with a defective spleen focus-forming virus (SFFV), which can induce acute erythroleukemia in mice. We have previously reported that activation of vagal α7 nicotinic acetylcholine receptor (nAChR) signaling promotes HIV-1 transcription. Whether vagal muscarinic signaling mediates FV-induced erythroleukemia and the underlying mechanisms remain unclear. In this study, sham and vagotomized mice were intraperitoneally injected with FV. FV infection caused anemia in sham mice, and vagotomy reversed this change. FV infection increased erythroblasts ProE, EryA, and EryB cells in the spleen, and these changes were blocked by vagotomy. In bone marrow, FV infection reduced EryC cells in sham mice, an effect that was counteracted by vagotomy. FV infection increased choline acetyltransferase (ChAT) expression in splenic CD4+ and CD8+ T cells, and this change was reversed by vagotomy. Furthermore, the increase of EryA and EryB cells in spleen of FV-infected wild-type mice was reversed after deletion of ChAT in CD4+ T cells. In bone marrow, FV infection reduced EryB and EryC cells in sham mice, whereas lack of ChAT in CD4+ T cells did not affect this change. Activation of muscarinic acetylcholine receptor 4 (mAChR4) by clozapine N-oxide (CNO) significantly increased EryB in the spleen but decreased the EryC cell population in the bone marrow of FV-infected mice. Thus, vagal-mAChR4 signaling in the spleen and bone marrow synergistically promotes the pathogenesis of acute erythroleukemia. We uncover an unrecognized mechanism of neuromodulation in erythroleukemia.

Muscarinic modulation increases excitability inspiratory and non-inspiratory hypoglossal motoneurons in mice postnatal days 0-5

Hypoglossal motoneurons (XII MNs) innervate the tongue and their activity normally contributes to stiffening of the airway to maintain airway patency. During sleep, there is normal reduction in airway tone. The specific mechanisms that contribute to decreased airway tone during sleep remain to be completely resolved, although research from adult rodents suggests activation of muscarinic acetylcholine receptors (mAChRs) is an important contributor by inhibiting XII MNs. Conversely, data from neonatal rodents, have shown that muscarinic modulation leads to an overall excitatory effect in XII MNs. Therefore, the overarching objective of this research project is to uncover the mechanisms through which muscarinic modulation can influence the excitability of XII MNs across postnatal maturation. Rhythmic medullary slices from neonatal mice (postnatal day 0-5) were used for whole cell electrophysiology experiments to test the effects of activating mAChRs at XII MNs in both non-inspiratory and inspiratory XII MNs. Data are reported as mean±SD. We hypothesized that mAChR activation at XII MNs would increase excitability of non-inspiratory and inspiratory XII MNs, and potentiate inspiratory bursting in inspiratory XII MNs. Starting at -60 mV, muscarinic modulation depolarized non-inspiratory and inspiratory XII MNs similarly (3.6±2.6 vs 4.6±2.1 mV, n=12, p&gt;0.05), and had no effect on input resistance (CTL vs Musc; non-insp: 120±23 vs 128±33 MΩ; inspiratory: 140±28 vs 137±32 MΩ; n =12; p&gt;0.05). Muscarinic modulation can increase the hyperpolarization-activated cation current (I h ), but we observed no difference in the sag potential in response to -150 (CTL vs Musc; non-insp: 2.3±2.3 vs 3.8±3.0 mV; inspiratory: 2.7±2.8 vs 4.9±2.8 mV; n =12; p &gt; 0.05), -300 (CTL vs Musc; non-insp: 9.0±7.8 vs 11.7±8.0 mV; inspiratory: 9.9±6.4 vs 12.9±7.5 mV; n=12; p&gt;0.05), and -500 pA (CTL vs Musc; non-insp: 21.9±15.0 vs 23.6±14.2 mV; inspiratory: 22.0±9.5 vs 23.5±9.5 mV; n=12; p&gt;0.05) current steps from -45 mV in non-inspiratory compared to inspiratory XII MNs. Muscarinic modulation significantly increased the firing frequency after muscarine application for inspiratory cells with the current step injections 100 pA (CTL vs Musc; 14.7±6.8 vs 21.1±6.4 Hz, n=9 and 10; p&lt;0.05) and 175 pA (CTL vs Musc; 19.5±6.4 vs 25.0±9.4 Hz, n=11 and 8; p&lt;0.05). However, muscarinic modulation did not affect the slope of firing rate/injected current relationship for non-inspiratory and inspiratory XII MNs (CTL vs Musc; non-insp: 0.16±0.12 vs 0.15±0.04 Hz/pA, n=9; inspiratory: 0.12±0.02 vs 0.16±0.07 Hz/pA, n=11 and 12; p&gt;0.05). Finally, muscarinic modulation significantly increased inspiratory burst amplitude (CTL vs Musc; 15.2±3.1 vs 18.3±4.5 mV; n=3; p&lt;0.05), and a trend to increase inspiratory burst area (CTL vs Musc; 4260±779 vs 5354±3063 mV ms, n=3; p=0.08). These data indicated that muscarinic modulation has similar effects on non-inspiratory and inspiratory XII MNs early in postnatal growth. Funding: NIH This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

The role and mechanism of tight junctions in the regulation of salivary gland secretion.

Tight junctions (TJs) are cell-cell interactions that localize at the most apical portion of epithelial/endothelial cells. One of the predominant functions of TJs is to regulate material transport through paracellular pathway, which serves as a selective barrier. In recent years, the expression and function of TJs in salivary glands has attracted great interest. The characteristics of multiple salivary gland TJ proteins have been identified. During salivation, the activation of muscarinic acetylcholine receptor and transient receptor potential vanilloid subtype 1, as well as other stimuli, promote the opening of acinar TJs by inducing internalization of TJs, thereby contributing to increased paracellular permeability. Besides, endothelial TJs are also redistributed with leakage of blood vessels in cholinergic-stimulated submandibular glands. Furthermore, under pathological conditions, such as Sjögren's syndrome, diabetes mellitus, immunoglobulin G4-related sialadenitis, and autotransplantation, the integrity and barrier function of TJ complex are impaired and may contribute to hyposalivation. Moreover, in submandibular glands of Sjögren's syndrome mouse model and patients, the endothelial barrier is disrupted and involved in hyposecretion and lymphocytic infiltration. These findings enrich our understanding of the secretory mechanisms that link the importance of epithelial and endothelial TJ functions to salivation under both physiological and pathophysiological conditions.

DNA-modularized construction of bivalent ligands precisely regulates receptor binding and activation

<h2>Summary</h2> Great interest is focused on the construction of bivalent ligands, where the spacer is crucial to coordinate the distance and orientation of two pharmacophores for optimal biological effects. Current strategies rely on polymers as spacers but suffer from the paucity of structural precision and variability with time-consuming ligation procedures. Herein, we originate a DNA-modularized strategy where pharmacophores are modularly modified with the protecting groups used for automatic DNA synthesis while natural deoxynucleotides serve as spacers. By programmably regulating the number and permutation of the bridged DNA spacer, the two pharmacophores are adjusted with a defined distance and versatile orientation. Using this strategy, we successfully constructed a reservoir of bivalent ligands containing an orthosteric agonist and an allosteric modulator, exhibiting a single-nucleotide difference in the selective activation of muscarinic acetylcholine receptors. Our strategy opens a new avenue for the precise construction and efficient screening of bivalent ligands toward a myriad of biomedical applications.

The M1 Muscarinic Acetylcholine Receptor Positive Allosteric Modulator (PAM) VU0453595 and M4 PAM VU0467154 Normalize Sleep‐Wake Architecture Deficits in Aged Mice

AbstractBackgroundDeclining basal forebrain cholinergic integrity has been associated with disturbances in sleep‐wake architecture in Alzheimer’s Disease (AD). Acetylcholinesterase inhibitors (AChEIs) provide symptomatic treatment for the cognitive deficits in AD through nonselective enhancement of cholinergic signaling. Additionally, AChEIs promote wakefulness, but reduce NREM sleep quality and produce dose limiting side effects through nonselective activation of peripheral muscarinic acetylcholine receptors (mAChR). Selectively targeting different mAChRs with positive allosteric modulators (PAMs) provides an alternate approach. In the current study, we assessed the effects of the M1 PAM VU0453595 and the M4 PAM VU0467154 on sleep‐wake architecture and arousal following dosing across the circadian cycle in aged mice.MethodsYoung (3‐4 month‐old, n = 13‐14 per group) and aged (19‐21 month‐old, n = 14 per group) C57/BL6 mice were implanted with HD‐X02 telemetry devices (DSI) for electroencephalography (EEG) recording. The M1 PAM VU0453595 (3‐30mg/kg IP) or the M4 PAM VU0467154 (1‐30mg/kg IP) were dosed 2‐hours into the active or inactive phases of the circadian cycle. EEG was recorded for 24‐hours, with sleep stages scored in 5‐second epochs. Arousal was assessed through changes in gamma power during wake, and sleep quality was measured through changes in delta power during NREM sleep. Statistical comparisons involved repeated measures one‐ or two‐way ANOVAs as appropriate, with Dunnett’s or Sidak’s multiple comparisons applied in all cases.ResultsThe M1 PAM VU0453595 increased wakefulness and arousal with inactive phase dosing in young and aged mice. During the active phase, VU0453595 increased wakefulness and arousal only in aged mice. The M4 PAM VU0467154 increased NREM sleep in young and aged mice when dosed in both phases. In the inactive phase, suppression of REM sleep in young and aged mice is also observed. During NREM sleep, VU0467154 increased delta power in young and aged mice.ConclusionThese findings suggest that M1 and M4 PAMs may be valuable as treatments in AD at different phases of the circadian cycle. M1 PAMs could be beneficial with morning dosing in clinical populations as they enhanced wakefulness and arousal. M4 PAMs could be advantageous with evening dosing in clinical population as they enhanced NREM sleep quantity and quality.

Clearance of small intestinal crypts involves goblet cell mucus secretion by intracellular granule rupture and enterocyte ion transport.

Goblet cells in the small intestinal crypts contain large numbers of mucin granules that are rapidly discharged to clean bacteria from the crypt. Because acetylcholine released by neuronal and nonneuronal cells controls many aspects of intestinal epithelial function, we used tissue explants and organoids to investigate the response of the small intestinal crypt to cholinergic stimulation. The activation of muscarinic acetylcholine receptors initiated a coordinated and rapid emptying of crypt goblet cells that flushed the crypt contents into the intestinal lumen. Cholinergic stimulation induced an expansion of the granule contents followed by intracellular rupture of the mucin granules. The mucus expanded intracellularly before the rupture of the goblet cell apical membrane and continued to expand after its release into the crypt lumen. The goblet cells recovered from membrane rupture and replenished their stores of mucin granules. Mucus secretion from the goblet cells depended on Ca2+ signaling and the expansion of the mucus in the crypt depended on gap junctions and on ion and water transport by enterocytes adjacent to the goblet cells. This distinctive mode of mucus secretion, which we refer to as "expanding secretion," efficiently cleans the small intestine crypt through coordinated mucus, ion, and fluid secretion by goblet cells and enterocytes.

Intrinsic threshold plasticity: cholinergic activation and role in the neuronal recognition of incomplete input patterns.

Activity-dependent changes in membrane excitability are observed in neurons across brain areas and represent a cell-autonomous form of plasticity (intrinsic plasticity; IP) that in itself does not involve alterations in synaptic strength (synaptic plasticity; SP). Non-homeostatic IP may play an essential role in learning, e.g. by changing the action potential threshold near the soma. A computational problem, however, arises from the implication that such amplification does not discriminate between synaptic inputs and therefore may reduce the resolution of input representation. Here, we investigate consequences of IP for the performance of an artificial neural network in (a) the discrimination of unknown input patterns and (b) the recognition of known/learned patterns. While negative changes in threshold potentials in the output layer indeed reduce its ability to discriminate patterns, they benefit the recognition of known but incompletely presented patterns. An analysis of thresholds and IP-induced threshold changes in published sets of physiological data obtained from whole-cell patch-clamp recordings from L2/3 pyramidal neurons in (a) the primary visual cortex (V1) of awake macaques and (b) the primary somatosensory cortex (S1) of mice in vitro, respectively, reveals a difference between resting and threshold potentials of ∼15mV for V1 and ∼25mV for S1, and a total plasticity range of ∼10mV (S1). The most efficient activity pattern to lower threshold is paired cholinergic and electric activation. Our findings show that threshold reduction promotes a shift in neural coding strategies from accurate faithful representation to interpretative assignment of input patterns to learned object categories. KEY POINTS: Intrinsic plasticity may change the action potential threshold near the soma of neurons (threshold plasticity), thus altering the input-output function for all synaptic inputs 'upstream' of the plasticity location. A potential problem arising from this shared amplification is that it may reduce the ability to discriminate between different input patterns. Here, we assess the performance of an artificial neural network in the discrimination of unknown input patterns as well as the recognition of known patterns subsequent to changes in the spike threshold. We observe that negative changes in threshold potentials do reduce discrimination performance, but at the same time improve performance in an object recognition task, in particular when patterns are incompletely presented. Analysis of whole-cell patch-clamp recordings from pyramidal neurons in the primary somatosensory cortex (S1) of mice reveals that negative threshold changes preferentially result from electric stimulation of neurons paired with the activation of muscarinic acetylcholine receptors.