Nicotinic acetylcholine receptors in autonomic ganglia

Label-Free Assessment of Neuronal Activity Using Raman Micro-Spectroscopy

Given the pivotal role of neuronal populations in various biological processes, assessing their collective output is crucial for understanding the nervous system's complex functions. Building on our prior development of a spiral scanning mechanism for the rapid acquisition of Raman spectra from single cells and incorporating machine learning for label-free evaluation of cell states, we investigated whether the Paint Raman Express Spectroscopy System (PRESS) can assess neuronal activities. We tested this hypothesis by examining the chemical responses of glutamatergic neurons as individual neurons and autonomic neuron ganglia as neuronal populations derived from human-induced pluripotent stem cells. The PRESS successfully acquired Raman spectra from both individual neurons and ganglia within a few seconds, achieving a signal-to-noise ratio sufficient for detailed analysis. To evaluate the ligand responsiveness of the induced neurons and ganglia, the Raman spectra were subjected to principal component and partial least squares discriminant analyses. The PRESS detected neuronal activity in response to glutamate and nicotine, which were absent in the absence of calcium. Additionally, the PRESS induced dose-dependent neuronal activity changes. These findings underscore the capability of the PRESS to assess individual neuronal activity and elucidate neuronal population dynamics and pharmacological responses, heralding new opportunities for drug discovery and regenerative medicine advancement.

Activation of the nonneuronal cholinergic cardiac system by hypoxic preconditioning protects isolated adult cardiomyocytes from hypoxia/reoxygenation injury

Activation of the vagus nerve mediates cardioprotection and attenuates myocardial ischemia/reperfusion (I/R) injury. In response to vagal activation, acetylcholine (ACh) is released from the intracardiac nervous system (ICNS) and activates intracellular cardioprotective signaling cascades. Recently, however, a nonneuronal cholinergic cardiac system (NNCCS) in cardiomyocytes has been described as an additional source of ACh. To investigate whether the NNCCS mediates cardioprotection in the absence of vagal and ICNS activation, we used a reductionist approach of isolated adult rat ventricular cardiomyocytes without neuronal cells, using hypoxic preconditioning (HPC) as a protective stimulus. Adult rat ventricular cardiomyocytes were isolated, the absence of neuronal cells was confirmed, and HPC was induced by 10/20 min hypoxia/reoxygenation (H/R) before subjection to 30/5 min H/R to simulate I/R injury. Cardiomyocyte viability was assessed by trypan blue staining at baseline and after HPC+H/R or H/R. Intra- and extracellular ACh was quantified using liquid chromatography-coupled mass spectrometry at baseline, after HPC, after hypoxia, and after reoxygenation, respectively. In a subset of experiments, muscarinic and nicotinic ACh receptor (m- and nAChR) antagonists were added during HPC or during H/R. Cardiomyocyte viability at baseline (69 ± 4%) was reduced by H/R (10 ± 3%). With HPC, cardiomyocyte viability was preserved after H/R (25 ± 6%). Intra- and extracellular ACh increased during hypoxia; HPC further increased both intra- and extracellular ACh (from 0.9 ± 0.7 to 1.5 ± 1.0 nmol/mg; from 0.7 ± 0.6 to 1.1 ± 0.7 nmol/mg, respectively). The addition of mAChR and nAChR antagonists during HPC had no impact on HPC's protection; however, protection was abrogated when antagonists were added during H/R (cardiomyocyte viability after H/R: 23 ± 5%; 13 ± 4%). In conclusion, activation of the NNCCS is involved in cardiomyocyte protection; HPC increases intra- and extracellular ACh during H/R, and m- and nAChRs are causally involved in HPC's cardiomyocyte protection during H/R. The interplay between upstream ICNS activation and NNCCS activation in myocardial cholinergic metabolism and cardioprotection needs to be investigated in future studies.NEW & NOTEWORTHY The intracardiac nervous system is considered to be involved in ischemic conditioning's cardioprotection through the release of acetylcholine (ACh). However, we demonstrate that hypoxic preconditioning (HPC) protects from hypoxia/reoxygenation injury and increases intra- and extracellular ACh during hypoxia in isolated adult ventricular rat cardiomyocytes. HPC's protection involves cardiomyocyte muscarinic and nicotinic ACh receptor activation. Thus, besides the intracardiac nervous system, a nonneuronal cholinergic cardiac system may also be causally involved in cardiomyocyte protection by ischemic conditioning.

Current Progress on Central Cholinergic Receptors as Therapeutic Targets for Alzheimer's Disease

Acetylcholine (ACh) is ubiquitously present in the nervous system and has been involved in the regulation of various brain functions. By modulating synaptic transmission and promoting synaptic plasticity, particularly in the hippocampus and cortex, ACh plays a pivotal role in the regulation of learning and memory. These procognitive actions of ACh are mediated by the neuronal muscarinic and nicotinic cholinergic receptors. The impairment of cholinergic transmission leads to cognitive decline associated with aging and dementia. Therefore, the cholinergic system has been of prime focus when concerned with Alzheimer's disease (AD), the most common cause of dementia. In AD, the extensive destruction of cholinergic neurons occurs by amyloid-β plaques and tau protein-rich neurofibrillary tangles. Amyloid-β also blocks cholinergic receptors and obstructs neuronal signaling. This makes the central cholinergic system an important target for the development of drugs for AD. In fact, centrally acting cholinesterase inhibitors like donepezil and rivastigmine are approved for the treatment of AD, although the outcome is not satisfactory. Therefore, identification of specific subtypes of cholinergic receptors involved in the pathogenesis of AD is essential to develop future drugs. Also, the identification of endogenous rescue mechanisms to the cholinergic system can pave the way for new drug development. In this article, we discussed the neuroanatomy of the central cholinergic system. Further, various subtypes of muscarinic and nicotinic receptors involved in the cognition and pathophysiology of AD are described in detail. The article also reviewed primary neurotransmitters that regulate cognitive processes by modulating basal forebrain cholinergic projection neurons.

The many enigmas of nicotine

This review discusses the diverse effects of nicotine on the various nicotinic acetylcholine receptors of the central and peripheral nervous system and how those effects may promote the usage and addiction to tobacco products.

A brief history of Russian research on the autonomic nervous system

The first information on the structure and function of the autonomic nervous system dates back to the time of Galen (second century), while the beginning of the study of the autonomic nervous system in Russia can be traced back to the mid-19th century. This review is devoted to the professional achievements of Russian researchers in the 19th and 20th centuries who were active in the field of the autonomic nervous system at different stages of the development of neuromorphology and neurophysiology. In addition, recent achievements of modern Russian researchers active in this domain are also highlighted. This review is mainly devoted to research on the autonomic nervous system in Russia, but it would be unfair not to mention the scientists who made a significant contribution to this field of science and worked in the republics of the former USSR. Russian morphology and physiology developed under the significant influence of well-known western scientific schools. I sincerely hope that cooperation between Russian and foreign colleagues will continue and will be fruitful for global science.

Spinal cord injury is associated with changes in synaptic properties of the mouse major pelvic ganglion

Spinal cord injury (SCI) has substantial impacts on autonomic function. In part, SCI results in loss of normal autonomic activity that contributes to injury-associated pathology such as neurogenic bladder, bowel, and sexual dysfunction. Yet little is known of the impacts of SCI on peripheral autonomic neurons that directly innervate these target organs. In this study, we measured changes in synaptic properties of neurons of the mouse major pelvic ganglion (MPG) associated with acute and chronic SCI. Our data show that functional and physiological properties of synapses onto MPG neurons are altered after SCI, and differ between acute and chronic injury. After acute injury, excitatory post-synaptic potentials (EPSPs) show increased rise and decay time constants leading to overall broader and longer EPSPs, while in chronic injured animals EPSPs are reduced in amplitude and show faster rise and decay leading to shorter EPSPs. Synaptic depression and low pass filtering are also altered in injured animals. Lastly, cholinergic currents are smaller in acute injured animals, but larger in chronic injured animals relative to controls. These changes in synaptic properties are associated with differences in nicotinic receptor subunit expression as well. MPG CHRNA3 mRNA levels decreased after injury, while CHRNA4 mRNAs increased. Further, changes in the correlations of alpha- and beta-subunit mRNAs suggests that nicotinic receptor subtype composition is altered after injury. Taken together, our data demonstrate that peripheral autonomic neurons are fundamentally altered after SCI, suggesting that longer-term therapeutic approaches could target these neurons directly to potentially help ameliorate neurogenic target organ dysfunction.

Characterization of Binding Site Interactions and Selectivity Principles in the α3β4 Nicotinic Acetylcholine Receptor

Nicotinic acetylcholine receptors (nAChRs) play an important role in neurotransmission and are also involved in addiction and several disease states. There is significant interest in therapeutic targeting of nAChRs; however, achieving selectivity for one subtype over others has been a longstanding challenge, given the close structural similarities across the family. Here, we characterize binding interactions in the α3β4 nAChR subtype via structure-function studies involving noncanonical amino acid mutagenesis and two-electrode voltage clamp electrophysiology. We establish comprehensive binding models for both the endogenous neurotransmitter ACh and the smoking cessation drug cytisine. We also use a panel of C(10)-substituted cytisine derivatives to probe the effects of subtle changes in the ligand structure on binding. By comparing our results to those obtained for the well-studied α4β2 subtype, we identify several features of both the receptor and agonist structure that can be utilized to enhance selectivity for either α3β4 or α4β2. Finally, we characterize binding interactions of the α3β4-selective partial agonist AT-1001 to determine factors that contribute to its selectivity. These results shed new light on the design of selective nAChR-targeted ligands and can be used to inform the design of improved therapies with minimized off-target effects.

Comparison of the Anti-inflammatory Properties of Two Nicotinic Acetylcholine Receptor Ligands, Phosphocholine and pCF3-diEPP

Activation of nicotinic acetylcholine receptors (nAChRs) expressed by innate immune cells can attenuate pro-inflammatory responses. Silent nAChR agonists, which down-modulate inflammation but have little or no ionotropic activity, are of outstanding clinical interest for the prevention and therapy of numerous inflammatory diseases. Here, we compare two silent nAChR agonists, phosphocholine, which is known to interact with nAChR subunits α7, α9, and α10, and pCF3-N,N-diethyl-N′-phenyl-piperazine (pCF3-diEPP), a previously identified α7 nAChR silent agonist, regarding their anti-inflammatory properties and their effects on ionotropic nAChR functions. The lipopolysaccharide (LPS)-induced release of interleukin (IL)-6 by primary murine macrophages was inhibited by pCF3-diEPP, while phosphocholine was ineffective presumably because of instability. In human whole blood cultures pCF3-diEPP inhibited the LPS-induced secretion of IL-6, TNF-α and IL-1β. The ATP-mediated release of IL-1β by LPS-primed human peripheral blood mononuclear leukocytes, monocytic THP-1 cells and THP-1-derived M1-like macrophages was reduced by both phosphocholine and femtomolar concentrations of pCF3-diEPP. These effects were sensitive to mecamylamine and to conopeptides RgIA4 and [V11L; V16D]ArIB, suggesting the involvement of nAChR subunits α7, α9 and/or α10. In two-electrode voltage-clamp measurements pCF3-diEPP functioned as a partial agonist and a strong desensitizer of classical human α9 and α9α10 nAChRs. Interestingly, pCF3-diEPP was more effective as an ionotropic agonist at these nAChRs than at α7 nAChR. In conclusion, phosphocholine and pCF3-diEPP are potent agonists at unconventional nAChRs expressed by monocytic and macrophage-like cells. pCF3-diEPP inhibits the LPS-induced release of pro-inflammatory cytokines, while phosphocholine is ineffective. However, both agonists signal via nAChR subunits α7, α9 and/or α10 to efficiently down-modulate the ATP-induced release of IL-1β. Compared to phosphocholine, pCF3-diEPP is expected to have better pharmacological properties. Thus, low concentrations of pCF3-diEPP may be a therapeutic option for the treatment of inflammatory diseases including trauma-induced sterile inflammation.

Evidence for Alpha7 Nicotinic Receptor Activation During the Cough Suppressing Effects Induced by Nicotine and Identification of ATA-101 as a Potential Novel Therapy for the Treatment of Chronic Cough

Studies performed in healthy smokers have documented a diminished responsiveness to tussive challenges, and several lines of experimental evidence implicate nicotine as an antitussive component in both cigarette smoke and the vapors generated by electronic cigarettes (eCigs). We set out to identify the nicotinic receptor subtype involved in the antitussive actions of nicotine and to further evaluate the potential of nicotinic receptor-selective agonists as cough-suppressing therapeutics. We confirmed an antitussive effect of nicotine in guinea pigs. We additionally observed that the alpha-4 beta-2 (α ₄ β ₂)-selective agonist Tc-6683 was without effect on evoked cough responses in guinea pigs, while the α ₇-selective agonist PHA 543613 dose-dependently inhibited evoked coughing. We subsequently describe the preclinical evidence in support of ATA-101, a potent and highly selective (α ₇) selective nicotinic receptor agonist, as a potential candidate for antitussive therapy in humans. ATA-101, formerly known as Tc-5619, was orally bioavailable and moderately central nervous system (CNS) penetrant and dose-dependently inhibited coughing in guinea pigs evoked by citric acid and bradykinin. Comparing the effects of airway targeted administration versus systemic dosing and the effects of repeated dosing at various times prior to tussive challenge, our data suggest that the antitussive actions of ATA-101 require continued engagement of α ₇ nicotinic receptors, likely in the CNS. Collectively, the data provide the preclinical rationale for α ₇ nicotinic receptor engagement as a novel therapeutic strategy for cough suppression. The data also suggest that α ₇ nicotinic acetylcholine receptor (nAChR) activation by nicotine may be permissive to nicotine delivery in a way that may promote addiction. SIGNIFICANCE STATEMENT: This study documents the antitussive actions of nicotine and identifies the α₇ nicotinic receptor subtype as the target for nicotine during cough suppression described in humans. We additionally present evidence suggesting that ATA-101 and other α₇ nicotinic receptor-selective agonists may be promising candidates for the treatment of chronic refractory cough.

Universal nature of cholinergic regulation demonstrated with nicotinic acetylcholine receptors

Mammalian nicotinic acetylcholine receptors (nAChRs) were initially discovered as ligand-gated ion channels mediating fast synaptic transmission in the neuro-muscular junctions and autonomic ganglia. They were further found to be involved in a wide range of basic biological processes within the brain and in non-excitable tissues. The present review summarizes the data obtained in our laboratory during last two decades. Investigation of autonomic ganglia with the nAChR subunit-specific antibodies was followed by identification of nAChRs in B lymphocytes, discovery of mitochondrial nAChRs and their role in mitochondrial apoptosis pathway, and revealing the role of α7 nAChRs and α7-specific antibodies in neuroinflammation-related Alzheimer disease and COVID-19. The data obtained demonstrate the involvement of nAChRs in cell survival, proliferation, cell-to-cell communication and inflammatory reaction. Together with the ability of nAChRs to function in both ionotropic and metabotropic way, these data illustrate the universal nature of cholinergic regulation mediated by nAChRs.

Social Isolation: How Can the Effects on the Cholinergic System Be Isolated?

Social species form organizations that support individuals because the consequent social behaviors help these organisms survive. The isolation of these individuals may be a stressor. We reviewed the potential mechanisms of the effects of social isolation on cholinergic signaling and vice versa how changes in cholinergic signaling affect changes due to social isolation.There are two important problems regarding this topic. First, isolation schemes differ in their duration (1–165 days) and initiation (immediately after birth to adulthood). Second, there is an important problem that is generally not considered when studying the role of the cholinergic system in neurobehavioral correlates: muscarinic and nicotinic receptor subtypes do not differ sufficiently in their affinity for orthosteric site agonists and antagonists. Some potential cholinesterase inhibitors also affect other targets, such as receptors or other neurotransmitter systems. Therefore, the role of the cholinergic system in social isolation should be carefully considered, and multiple receptor systems may be involved in the central nervous system response, although some subtypes are involved in specific functions. To determine the role of a specific receptor subtype, the presence of a specific subtype in the central nervous system should be determined using search in knockout studies with the careful application of specific agonists/antagonists.

The Hair Cell α9α10 Nicotinic Acetylcholine Receptor: Odd Cousin in an Old Family

Nicotinic acetylcholine receptors (nAChRs) are a subfamily of pentameric ligand-gated ion channels with members identified in most eumetazoan clades. In vertebrates, they are divided into three subgroups, according to their main tissue of expression: neuronal, muscle and hair cell nAChRs. Each receptor subtype is composed of different subunits, encoded by paralogous genes. The latest to be identified are the α9 and α10 subunits, expressed in the mechanosensory hair cells of the inner ear and the lateral line, where they mediate efferent modulation. α9α10 nAChRs are the most divergent amongst all nicotinic receptors, showing marked differences in their degree of sequence conservation, their expression pattern, their subunit co-assembly rules and, most importantly, their functional properties. Here, we review recent advances in the understanding of the structure and evolution of nAChRs. We discuss the functional consequences of sequence divergence and conservation, with special emphasis on the hair cell α9α10 receptor, a seemingly distant cousin of neuronal and muscle nicotinic receptors. Finally, we highlight potential links between the evolution of the octavolateral system and the extreme divergence of vertebrate α9α10 receptors.

Ethanol interaction with α3β4 nicotinic acetylcholine receptors in neurons of the laterodorsal tegmentum

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) play a key role in the rewarding effects of ethanol (EtOH), and while several nAChR subtypes have been implicated, attention has recently shifted to a role for the α3β4 nAChR. The laterodorsal tegmental nucleus (LDTg), a brainstem cholinergic nucleus that sends excitatory projections to the ventral tegmental area, is an Integral part of the brain reward pathway. Here we investigate a potential role for LDTg α3β4 nAChRs in EtOH self-administration and reward.

METHODS

Sprague-Dawley rats were given ad libitum access to a 20% EtOH solution, as part of a two-bottle choice paradigm. Approximately 1 week after removal of EtOH access, we measured LDTg α3β4 nAChR current responses to focal application of acetylcholine (ACh), using whole-cell patch clamp electrophysiology recordings in acute brain slices. In addition, we used whole-cell electrophysiology to assess the acute effects of EtOH on the sensitivity of LDTg α3β4 nAChRs.

RESULTS

Focal application of ACh onto LDTg neurons resulted in large α3β4 nAChR-mediated inward currents, the magnitude of which showed a positive correlation with levels of EtOH self-administration. In addition, using brain slices taken from EtOH-naïve rats, bath application of EtOH resulted in a moderate potentiation of LDTg α3β4 nAChR sensitivity.

CONCLUSIONS

Using a rat model, increased α3β4 nAChR function was associated with greater EtOH self-administration, with α3β4 nAChR function also acutely potentiated by EtOH. Assuming that similar findings apply to humans, the α3β4 nAChR could be a therapeutic target in the treatment of EtOH use disorder.

Molecular insights into the benefits of nicotine on memory and cognition (Review)

The health risks of nicotine are well known, but there is some evidence of its beneficial effects on cognitive function. The present review focused on the reported benefits of nicotine in the brain and summarizes the associated underlying mechanisms. Nicotine administration can improve cognitive impairment in Alzheimer's disease (AD), and dyskinesia and memory impairment in Parkinson's disease (PD). In terms of its mechanism of action, nicotine slows the progression of PD by inhibiting Sirtuin 6, a stress‑responsive protein deacetylase, thereby decreasing neuronal apoptosis and improving neuronal survival. In AD, nicotine improves cognitive impairment by enhancing protein kinase B (also referred to as Akt) activity and stimulating phosphoinositide 3‑kinase/Akt signaling, which regulates learning and memory processes. Nicotine may also activate thyroid receptor signaling pathways to improve memory impairment caused by hypothyroidism. In healthy individuals, nicotine improves memory impairment caused by sleep deprivation by enhancing the phosphorylation of calmodulin‑dependent protein kinase II, an essential regulator of cell proliferation and synaptic plasticity. Furthermore, nicotine may improve memory function through its effect on chromatin modification via the inhibition of histone deacetylases, which causes transcriptional changes in memory‑related genes. Finally, nicotine administration has been demonstrated to rescue long‑term potentiation in individuals with sleep deprivation, AD, chronic stress and hypothyroidism, primarily by desensitizing α7 nicotinic acetylcholine receptors. To conclude, nicotine has several cognitive benefits in healthy individuals, as well as in those with cognitive dysfunction associated with various diseases. However, further research is required to shed light on the effect of acute and chronic nicotine treatment on memory function.

Familial Autonomic Ganglionopathy Caused by Rare CHRNA3 Genetic Variants

OBJECTIVE

To determine the molecular basis of a new monogenetic recessive disorder that results in familial autonomic ganglionopathy with diffuse autonomic failure.

METHODS

Two adult siblings from one family (I-4 and I-5) and another participant from a second family (II-3) presented with severe neurogenic orthostatic hypotension (nOH), small nonreactive pupils, and constipation. All 3 affected members had low norepinephrine levels and diffuse panautonomic failure.

RESULTS

Whole exome sequencing of DNA from I-4 and I-5 showed compound heterozygosity for c.907_908delCT (p.L303Dfs*115)/c.688 G>A (p.D230N) pathologic variants in the acetylcholine receptor, neuronal nicotinic, α3 subunit gene (CHRNA3). II-3 from the second family was homozygous for the same frameshift (fs) variant (p.L303Dfs*115//p.L303Dfs*115). CHRNA3 encodes a critical subunit of the nicotinic acetylcholine receptors (nAChRs) responsible for fast synaptic transmission in the autonomic ganglia. The fs variant is clearly pathogenic and the p.D230N variant is predicted to be damaging (SIFT)/probably damaging (PolyPhen2). The p.D230N variant lies on the interface between CHRNA3 and other nAChR subunits based on structural modeling and is predicted to destabilize the nAChR pentameric complex.

CONCLUSIONS

We report a novel genetic disease that affected 3 individuals from 2 unrelated families who presented with severe nOH, miosis, and constipation. These patients had rare pathologic variants in the CHRNA3 gene that cosegregate with and are predicted to be the likely cause of their diffuse panautonomic failure.

Cardiac nicotinic receptors show β-subunit-dependent compensatory changes

Nicotinic receptors (NRs) play an important role in the cholinergic regulation of heart functions, and converging evidence suggests a diverse repertoire of NR subunits in the heart. A recent hypothesis about the plasticity of β NR subunits suggests that β2-subunits and β4-subunits may substitute for each other. In our study, we assessed the hypothetical β-subunit interchangeability in the heart at the level of mRNA. Using two mutant mice strains lacking β2 or β4 NR subunits, we examined the relative expression of NR subunits and other key cholinergic molecules. We investigated the physiology of isolated hearts perfused by Langendorff's method at basal conditions and after cholinergic and/or adrenergic stimulation. Lack of β2 NR subunit was accompanied with decreased relative expression of β4-subunits and α3-subunits. No other cholinergic changes were observed at the level of mRNA, except for increased M3 and decreased M4 muscarinic receptors. Isolated hearts lacking β2 NR subunit showed different dynamics in heart rate response to indirect cholinergic stimulation. In hearts lacking β4 NR subunit, increased levels of β2-subunits were observed together with decreased mRNA for acetylcholine-synthetizing enzyme and M1 and M4 muscarinic receptors. Changes in the expression levels in β4^-/- hearts were associated with increased basal heart rate and impaired response to a high dose of acetylcholine upon adrenergic stimulation. In support of the proposed plasticity of cardiac NRs, our results confirmed subunit-dependent compensatory changes to missing cardiac NRs subunits with consequences on isolated heart physiology.NEW & NOTEWORTHY In the present study, we observed an increase in mRNA levels of the β2 NR subunit in β4^-/- hearts but not vice versa, thus supporting the hypothesis of β NR subunit plasticity that depends on the specific type of missing β-subunit. This was accompanied with specific cholinergic adaptations. Nevertheless, isolated hearts of β4^-/- mice showed increased basal heart rate and a higher sensitivity to a high dose of acetylcholine upon adrenergic stimulation.

Nicotinic Acetylcholine Receptor Involvement in Inflammatory Bowel Disease and Interactions with Gut Microbiota

The gut-brain axis describes a complex interplay between the central nervous system and organs of the gastrointestinal tract. Sensory neurons of dorsal root and nodose ganglia, neurons of the autonomic nervous system, and immune cells collect and relay information about the status of the gut to the brain. A critical component in this bi-directional communication system is the vagus nerve which is essential for coordinating the immune system’s response to the activities of commensal bacteria in the gut and to pathogenic strains and their toxins. Local control of gut function is provided by networks of neurons in the enteric nervous system also called the ‘gut-brain’. One element common to all of these gut-brain systems is the expression of nicotinic acetylcholine receptors. These ligand-gated ion channels serve myriad roles in the gut-brain axis including mediating fast synaptic transmission between autonomic pre- and postganglionic neurons, modulation of neurotransmitter release from peripheral sensory and enteric neurons, and modulation of cytokine release from immune cells. Here we review the role of nicotinic receptors in the gut-brain axis with a focus on the interplay of these receptors with the gut microbiome and their involvement in dysregulation of gut function and inflammatory bowel diseases.

Mitochondrial α7 nicotinic acetylcholine receptors are displaced from complexes with VDAC1 to form complexes with Bax upon apoptosis induction

Nicotinic acetylcholine receptors (nAChRs) mediate fast synaptic transmission in muscles and autonomic ganglia and regulate cytokine and neurotransmitter release in the brain and non-excitable cells. The α7 nAChRs localized in the outer membrane of mitochondria regulate cytochrome c release stimulated by apoptosis-inducing agents. However, the mechanisms through which nAChRs influence mitochondrial permeability remain obscure. Here we put an aim to explore the interaction of nAChRs with voltage-dependent anion channels (VDAC1) and pro-apoptotic protein Bax in the course of apoptosis induction. By using molecular modeling in silico, it was shown that both Bax and VDAC1 can bind within the 4th transmembrane portion (M4) of nAChR subunits. Experimentally, α7 nAChR-Bax and α7 nAChR-VDAC1 complexes were identified by sandwich ELISA in mitochondria isolated from astrocytoma U373 cells. Stimulating apoptosis of U373 cells by H₂O₂ disrupted α7-VDAC complexes and favored formation of α7-Bax complexes accompanied by cytochrome c release from mitochondria. α7-selective agonist PNU282987 or type 2 positive allosteric modulator PNU120596 disrupted α7-Bax and returned α7 nAChR to complex with VDAC1 resulting in attenuation of cytochrome c release. It is concluded that mitochondrial nAChRs regulate apoptosis-induced mitochondrial channel formation by modulating the interplay of apoptosis-related proteins in mitochondria outer membrane.

Progress in nicotinic receptor structural biology

Here we begin by briefly reviewing landmark structural studies on the nicotinic acetylcholine receptor. We highlight challenges that had to be overcome to push through resolution barriers, then focus on what has been gleaned in the past few years from crystallographic and single particle cryo-EM studies of different nicotinic receptor subunit assemblies and ligand complexes. We discuss insights into ligand recognition, ion permeation, and allosteric gating. We then highlight some foundational aspects of nicotinic receptor structural biology that remain unresolved and are areas ripe for future exploration. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Activation of α7nAChR via vagus nerve prevents obesity-induced insulin resistance via suppressing endoplasmic reticulum stress-induced inflammation in Kupffer cells

Obesity is a major risk factor for type 2 diabetes mellitus and insulin resistance (IR). In the state of obesity, excess fat accumulates in the liver, a key organ in systemic metabolism, altering the inflammatory and metabolic signals contributing substantially to the development of hepatic IR. Current therapies for these metabolic disorders have not been able to reverse their rapidly rising prevalence. One of the reasons is that the effects of existing drugs are predominantly non-lasting [1,2]. The vagus nerve (VN) is known to play an essential role in maintaining metabolic homeostasis while decreased VN activity has been suggested to contribute to obesity associated metabolic syndrome [3,4]. Several studies have reported that activation of α7 nicotinic acetylcholine receptor (α7nAChR) cholinergic signaling with or without VN intervention has protective effects against obesity-related inflammation and other metabolic complications [5]. However, the molecular mechanisms are still not elucidated. Exaggerated endoplasmic reticulum (ER) stress and consequent dysregulated inflammation has been implicated in the development of lipid accumulation and IR [6]. Whether targeting α7nAChR can regulate IR through these pathways is rarely reported. Accordingly, the present proposal posits that activation of the α7nAChR by VNS attenuates ER stress induced inflammation, thus ameliorating hepatic IR in Kupffer cell. We will focus on the specific interaction between vagal cholinergic activity and the modulation of ER stress induced inflammation via the α7nAChR associated pathway during IR development. Recently, the Endocrine Society has emphasized the absence of specific evidence from basic science, clinical, and epidemiological literature to assess current knowledge regarding underlying mechanisms of obesity [7]. In this proposal, we assign a significant role to α7nAChR in obesity-induced hepatic IR, and suggest a possible therapeutic strategy with VNS intervention.

Different Effects of Nicotine and N-Stearoyl-ethanolamine on Episodic Memory and Brain Mitochondria of α7 Nicotinic Acetylcholine Receptor Knockout Mice

Nicotinic acetylcholine receptors of α7 subtype (α7 nAChRs) are involved in regulating neuroinflammation and cognitive functions. Correspondingly, α7-/- mice demonstrate pro-inflammatory phenotype and impaired episodic memory. In addition, nAChRs expressed in mitochondria regulate the release of pro-apoptotic factors like cytochrome c. Here we studied whether the cognitive deficiency of α7-/- mice can be cured by oral consumption of either nicotine or N-stearoylethanolamine (NSE), a lipid possessing anti-inflammatory, cannabimimetic and membrane-stabilizing activity. Mice were examined in Novel Object Recognition behavioral test, their blood, brains and brain mitochondria were tested for the levels of interleukin-6, various nAChR subtypes and cytochrome c released by ELISA. The data presented demonstrate that both substances stimulated the raise of interleukin-6 in the blood and improved episodic memory of α7-/- mice. However, NSE improved, while nicotine worsened the brain mitochondria sustainability to apoptogenic stimuli, as shown by either decreased or increased amounts of cytochrome c released. Both nicotine and NSE up-regulated α4β2 nAChRs in the brain; NSE up-regulated, while nicotine down-regulated α9-containing nAChRs in the brain mitochondria. It is concluded that the level of alternative nAChR subtypes in the brain is critically important for memory and mitochondria sustainability in the absence of α7 nAChRs.

A comprehensive analysis of the clinical characteristics and laboratory features in 179 patients with autoimmune autonomic ganglionopathy

The clinical importance of autoantibodies against the ganglionic acetylcholine receptor (gAChR) remains to be fully elucidated. We aimed to identify the clinical characteristics of autoimmune autonomic ganglionopathy (AAG) in patients with gAChR autoantibodies. For this cohort investigation, serum samples were obtained from patients with AAG between 2012 and 2018 in Japan. We measured the levels of autoantibodies against gAChRα3 and gAChRβ4 and evaluated clinical features, as well as assessing the laboratory investigation results among the included patients. A total of 179 patients tested positive for antibodies, including 116 gAChRα3-positive, 13 gAChRβ4-positive, and 50 double antibody-positive patients. Seropositive AAG patients exhibited widespread autonomic dysfunction. Extra-autonomic manifestations including sensory disturbance, central nervous system involvement, endocrine disorders, autoimmune diseases, and tumours were present in 118 patients (83%). We observed significant differences in the frequencies of several autonomic and extra-autonomic symptoms among the three groups. Our ¹²³I-metaiodobenzylguanidine myocardial scintigraphy analysis of the entire cohort revealed that the heart-to-mediastinum ratio had decreased by 80%. The present study is the first to demonstrate that patients with AAG who are seropositive for anti-gAChRβ4 autoantibodies exhibit unique autonomic and extra-autonomic signs. Decreased cardiac uptake occurred in most cases, indicating that ¹²³I- metaiodobenzylguanidine myocardial scintigraphy may be useful for monitoring AAG. Therefore, our findings indicate that gAChRα3 and gAChRβ4 autoantibodies cause functional changes in postganglionic fibres in the autonomic nervous system and extra-autonomic manifestations in seropositive patients with AAG.

Cholinergic Receptors and Addiction

Human behavior can be controlled by physical or psychological dependencies associated with addiction. One of the most insidious addictions in our society is the use of tobacco products which contain nicotine. This addiction can be associated with specific receptors in the brain that respond to the natural neurotransmitter acetylcholine. These nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels formed by the assembly of one or multiple types of nAChR receptor subunits. In this paper, we review the structure and diversity of nAChR subunits and our understanding for how different nAChR subtypes play specific roles in the phenomenon of nicotine addiction. We focus on receptors containing β2 and/or α6 subunits and the special significance of α5-containing receptors. These subtypes all have roles in regulating dopamine-mediated neurotransmission in the mesolimbic reward pathways of the brain. We also discuss the unique roles of homomeric α7 nAChR in behavioral responses to nicotine and how our knowledge of nAChR functional diversity may help guide pharmacotherapeutic approaches for treating nicotine addiction. While nicotine addiction is a truly global problem, the use of areca nut (betel) products is also a serious addiction associated with public health issues across most of South Asia, impacting as many as 600 million people. We discuss how cholinergic receptors of the brain are also involved with areca addiction and the unique challenges for dealing with addiction to this substance.

CAKUT and Autonomic Dysfunction Caused by Acetylcholine Receptor Mutations

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common cause of chronic kidney disease in the first three decades of life, and in utero obstruction to urine flow is a frequent cause of secondary upper urinary tract malformations. Here, using whole-exome sequencing, we identified three different biallelic mutations in CHRNA3, which encodes the α3 subunit of the nicotinic acetylcholine receptor, in five affected individuals from three unrelated families with functional lower urinary tract obstruction and secondary CAKUT. Four individuals from two families have additional dysautonomic features, including impaired pupillary light reflexes. Functional studies in vitro demonstrated that the mutant nicotinic acetylcholine receptors were unable to generate current following stimulation with acetylcholine. Moreover, the truncating mutations p.Thr337Asnfs∗81 and p.Ser340∗ led to impaired plasma membrane localization of CHRNA3. Although the importance of acetylcholine signaling in normal bladder function has been recognized, we demonstrate for the first time that mutations in CHRNA3 can cause bladder dysfunction, urinary tract malformations, and dysautonomia. These data point to a pathophysiologic sequence by which monogenic mutations in genes that regulate bladder innervation may secondarily cause CAKUT.

Agonist Selectivity and Ion Permeation in the α3β4 Ganglionic Nicotinic Receptor

Nicotinic acetylcholine receptors are pentameric ion channels that mediate fast chemical neurotransmission. The α3β4 nicotinic receptor subtype forms the principal relay between the central and peripheral nervous systems in the autonomic ganglia. This receptor is also expressed focally in brain areas that affect reward circuits and addiction. Here, we present structures of the α3β4 nicotinic receptor in lipidic and detergent environments, using functional reconstitution to define lipids appropriate for structural analysis. The structures of the receptor in complex with nicotine, as well as the α3β4-selective ligand AT-1001, complemented by molecular dynamics, suggest principles of agonist selectivity. The structures further reveal much of the architecture of the intracellular domain, where mutagenesis experiments and simulations define residues governing ion conductance.

Interaction of neurotransmitters and neurochemicals with lymphocytes

Neurotransmitters and neurochemicals can act on lymphocytes by binding to receptors expressed by lymphocytes. This review describes lymphocyte expression of receptors for a selection of neurotransmitters and neurochemicals, the anatomical locations where lymphocytes can interact with neurotransmitters, and the effects of the neurotransmitters on lymphocyte function. Implications for health and disease are also discussed.

Mitochondrial Nicotinic Acetylcholine Receptors Support Liver Cells Viability After Partial Hepatectomy

Nicotinic acetylcholine receptors (nAChRs) expressed on the cell plasma membrane are ligand-gated ion channels mediating fast synaptic transmission, regulating neurotransmitter and cytokine release and supporting the viability of many cell types. The nAChRs expressed in mitochondria regulate the release of pro-apoptotic factors, like cytochrome c, in ion channel-independent manner. Here we show that α3β2, α7β2, and α9α10 nAChR subtypes are up-regulated in rat liver mitochondria 3–6 h after partial hepatectomy resulting in increased sustainability of mitochondria to apoptogenic effects of Ca²⁺ and H₂O₂. In contrast, laparotomy resulted in down-regulation of all nAChR subunits, except α9, and decreased mitochondria sustainability to apoptogenic effects of Ca²⁺ and H₂O₂. Experiments performed in liver mitochondria from α3+/-, α7-/-, β4-/-, α7β2-/-, or wild-type C57Bl/6J mice demonstrated that the decrease of α3 or absence of α7 or α7/β2 subunits in mitochondria is compensated with β4 and α9 subunits, which could be found in α3β4, α4β4, α9β4, and α9α10 combinations. Mitochondria from knockout mice maintained their sustainability to Ca²⁺ but were differently regulated by nAChR subtype-specific ligands: PNU-282987, methyllycaconitine, dihydro-β-erythroidine, α-conotoxin MII, and α-conotoxin PeIA. It is concluded that mitochondrial nAChRs play an important role in supporting the viability of hepatic cells and, therefore, may be a pharmacological target for pro-survival therapy. The concerted action of multiple nAChR subtypes controlling either CaKMII- or Src-dependent signaling pathways in mitochondria ensures a reliable protection against apoptogenic factors of different nature.

α7 nicotinic acetylcholine receptors are involved in suppression of the antibody immune response

This study demonstrates that α7 nicotinic acetylcholine receptors (nAChRs) regulate mouse B lymphocyte proliferation and IgM production in ion-independent manner. The high α7 nAChR levels were found in CD5+ and Foxp3+ B cells; induction of Foxp3+ cells in vitro was attenuated in the absence or upon inhibition of α7 nAChRs. The adoptively transferred B lymphocytes, stimulated in presence of methyllicaconitine, decreased the IgM response and abolished the IgG response in the host. The data obtained demonstrate the importance of cholinergic regulation for the antibody immune response and immunosuppression.

Nicotinic Acetylcholine Receptor α9 and α10 Subunits Are Expressed in the Brain of Mice

The α9 and α10 nicotinic acetylcholine receptor (nAChR) subunits are likely to be the evolutionary precursors to the entire cys-loop superfamily of ligand-gated ion channels, which includes acetylcholine, GABA, glycine and serotonin ionotropic receptors. nAChRs containing α9 and α10 subunits are found in the inner ear, dorsal root ganglia and many non-excitable tissues, but their expression in the central nervous system has not been definitely demonstrated. Here we show the presence of both α9 and α10 nAChR subunits in the mouse brain by RT-PCR and immunochemical approaches with a range of nAChR subunit-selective antibodies, which selectivity was demonstrated in the brain preparations of α7−/−, α9−/− and α10−/− mice. The α9 and α10 RNA transcripts were found in medulla oblongata (MO), cerebellum, midbrain (MB), thalamus and putamen (TP), somatosensory cortex (SC), frontal cortex (FC) and hippocampus. High α9-selective signal in ELISA was observed in the FC, SC, MO, TP and hippocampus and α10-selective signal was the highest in MO and FC. The α9 and α10 proteins were found in the brain mitochondria, while their presence on the plasma membrane has not been definitely confirmed The α7-, α9- and α10-selective antibodies stained mainly neurons and hypertrophied astrocytes, but not microglia. The α9- and α10-positive cells formed ordered structures or zones in cerebellum and superior olive (SO) and were randomly distributed among α7-positive cells in the FC; they were found in CA1, CA3 and CA4, but not in CA2 region of the hippocampus. The α9 and α10 subunits were up-regulated in α7−/− mice and both α7 and α9 subunits were down-regulated in α10−/− mice. We conclude that α9 and α10 nAChR subunits are expressed in distinct neurons of the mouse brain and in the brain mitochondria and are compensatory up-regulated in the absence of α7 subunits.

Nicotinic Cholinergic Mechanisms in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative condition characterized by increased accumulation of Aβ and degeneration of cholinergic signaling between basal forebrain and hippocampus. Nicotinic acetylcholine receptors (nAChRs) are important mediators of cholinergic signaling in modulation of learning and memory function. Accumulating lines of evidence indicate that a nAChR subtype, α7 receptor (α7-nAChR), plays an important role in modulations of excitatory neurotransmitter release, improvement of learning and memory ability, and enhancement of cognitive function. Importantly, the expression and function of α7-nAChRs is altered in the brain of AD animal models and AD patients, suggesting that this nAChR subtype participates in AD pathogenesis and may serve as a novel therapeutic target for AD treatment. However, the mechanisms underlying the role of α7-nAChRs in AD pathogenesis are very complex, and either neuroprotective effects or neurotoxic effects may occur through the α7-nAChRs. These effects depend on the levels of α7-nAChR expression and function, disease stages, or the use of α7-nAChR agonists, antagonists, or allosteric modulators. In this chapter, we summarize recent progresses in the roles of α7-nAChRs played in AD pathogenesis and therapy.

Alanine scan of α-conotoxin RegIIA reveals a selective α3β4 nicotinic acetylcholine receptor antagonist

Activation of the α3β4 nicotinic acetylcholine receptor (nAChR) subtype has recently been implicated in the pathophysiology of various conditions, including development and progression of lung cancer and in nicotine addiction. As selective α3β4 nAChR antagonists, α-conotoxins are valuable tools to evaluate the functional roles of this receptor subtype. We previously reported the discovery of a new α4/7-conotoxin, RegIIA. RegIIA was isolated from Conus regius and inhibits acetylcholine (ACh)-evoked currents mediated by α3β4, α3β2, and α7 nAChR subtypes. The current study used alanine scanning mutagenesis to understand the selectivity profile of RegIIA at the α3β4 nAChR subtype. [N11A] and [N12A] RegIIA analogs exhibited 3-fold more selectivity for the α3β4 than the α3β2 nAChR subtype. We also report synthesis of [N11A,N12A]RegIIA, a selective α3β4 nAChR antagonist (IC50 of 370 nM) that could potentially be used in the treatment of lung cancer and nicotine addiction. Molecular dynamics simulations of RegIIA and [N11A,N12A]RegIIA bound to α3β4 and α3β2 suggest that destabilization of toxin contacts with residues at the principal and complementary faces of α3β2 (α3-Tyr(92), Ser(149), Tyr(189), Cys(192), and Tyr(196); β2-Trp(57), Arg(81), and Phe(119)) may form the molecular basis for the selectivity shift.

Autonomic neural control of intrathoracic airways

Autonomic neural control of the intrathoracic airways aids in optimizing air flow and gas exchange. In addition, and perhaps more importantly, the autonomic nervous system contributes to host defense of the respiratory tract. These functions are accomplished by tightly regulating airway caliber, blood flow, and secretions. Although both the sympathetic and parasympathetic branches of the autonomic nervous system innervate the airways, it is the later that dominates, especially with respect to control of airway smooth muscle and secretions. Parasympathetic tone in the airways is regulated by reflex activity often initiated by activation of airway stretch receptors and polymodal nociceptors. This review discusses the preganglionic, ganglionic, and postganglionic mechanisms of airway autonomic innervation. Additionally, it provides a brief overview of how dysregulation of the airway autonomic nervous system may contribute to respiratory diseases.

Structure activity relationship of synaptic and junctional neurotransmission

Chemical neurotransmission may include transmission to local or remote sites. Locally, contact between 'bare' portions of the bulbous nerve terminal termed a varicosity and the effector cell may be in the form of either synapse or non-synaptic contact. Traditionally, all local transmissions between nerves and effector cells are considered synaptic in nature. This is particularly true for communication between neurons. However, communication between nerves and other effectors such as smooth muscles has been described as nonsynaptic or junctional in nature. Nonsynaptic neurotransmission is now also increasingly recognized in the CNS. This review focuses on the relationship between structure and function that orchestrate synaptic and junctional neurotransmissions. A synapse is a specialized focal contact between the presynaptic active zone capable of ultrafast release of soluble transmitters and the postsynaptic density that cluster ionotropic receptors. The presynaptic and the postsynaptic areas are separated by the 'closed' synaptic cavity. The physiological hallmark of the synapse is ultrafast postsynaptic potentials lasting milliseconds. In contrast, junctions are juxtapositions of nerve terminals and the effector cells without clear synaptic specializations and the junctional space is 'open' to the extracellular space. Based on the nature of the transmitters, postjunctional receptors and their separation from the release sites, the junctions can be divided into 'close' and 'wide' junctions. Functionally, the 'close' and the 'wide' junctions can be distinguished by postjunctional potentials lasting ~1s and tens of seconds, respectively. Both synaptic and junctional communications are common between neurons; however, junctional transmission is the rule at many neuro-non-neural effectors.

Functional expression of human α9* nicotinic acetylcholine receptors in X. laevis oocytes is dependent on the α9 subunit 5' UTR

Nicotinic acetylcholine receptors (nAChRs) containing the α9 subunit are expressed in a wide variety of non-neuronal tissues ranging from immune cells to breast carcinomas. The α9 subunit is able to assemble into a functional homomeric nAChR and also co-assemble with the α10 subunit into functional heteromeric nAChRs. Despite the increasing awareness of the important roles of this subunit in vertebrates, the study of human α9-containing nAChRs has been severely limited by difficulties in its expression in heterologous systems. In Xenopus laevis oocytes, functional expression of human α9α10 nAChRs is very low compared to that of rat α9α10 nAChRs. When oocytes were co-injected with cRNA of α9 and α10 subunits of human versus those of rat, oocytes with the rat α9 human α10 combination had an ∼-fold higher level of acetylcholine-gated currents (I_ACh) than those with the human α9 rat α10 combination, suggesting difficulties with human α9 expression. When the ratio of injected human α9 cRNA to human α10 cRNA was increased from 1∶1 to 5∶1, I_ACh increased 36-fold (from 142±23 nA to 5171±748 nA). Functional expression of human α9-containing receptors in oocytes was markedly improved by appending the 5′-untranslated region of alfalfa mosaic virus RNA4 to the 5′-leader sequence of the α9 subunit cRNA. This increased the functional expression of homomeric human α9 receptors by 70-fold (from 7±1 nA to 475±158 nA) and of human α9α10 heteromeric receptors by 80-fold (from 113±62 nA to 9192±1137 nA). These findings indicate the importance of the composition of the 5′ untranslated leader sequence for expression of α9-containing nAChRs.

Single-channel properties of α3β4, α3β4α5 and α3β4β2 nicotinic acetylcholine receptors in mice lacking specific nicotinic acetylcholine receptor subunits

Previous attempts to measure the functional properties of recombinant nicotinic acetylcholine receptors (nAChRs) composed of known receptor subunits have yielded conflicting results. The use of knockout mice that lack α5, β2, α5β2 or α5β2α7 nAChR subunits enabled us to measure the single-channel properties of distinct α3β4, α3β4α5 and α3β4β2 receptors in superior cervical ganglion (SCG) neurons. Using this approach, we found that α3β4 receptors had a principal conductance level of 32.6 ± 0.8 pS (mean ± SEM) and both higher and lower secondary conductance levels. α3β4α5 receptors had the same conductance as α3β4 receptors, but differed from α3β4 receptors by having an increased channel open time and increased burst duration. By contrast, α3β4β2 receptors differed from α3β4 and α3β4α5 receptors by having a significantly smaller conductance level (13.6 ± 0.5 pS). After dissecting the single-channel properties of these receptors using our knockout models, we then identified these properties – and hence the receptors themselves – in wild-type SCG neurons. This study is the first to identify the single-channel properties of distinct neuronal nicotinic receptors in their native environment.

Varenicline and nicotine enhance GABAergic synaptic transmission in rat CA1 hippocampal and medial septum/diagonal band neurons

AIMS

The FDA approved smoking cessation aid varenicline can effectively attenuate nicotine-stimulated dopamine release. Varenicline may also exert important actions on other transmitter systems that also influence nicotine reinforcement or contribute to the drug's cognitive and affective side effects. In this study, we determined if varenicline, like nicotine, can stimulate presynaptic GABA release.

MAIN METHODS

Using whole-cell patch-clamp techniques, we measured GABA(A)R-mediated asynchronous, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) in acute brain slices from two brain regions important for learning and memory, the hippocampus and basal forebrain.

KEY FINDINGS

Both varenicline (10 μM) and nicotine (10 μM) applications alone resulted in small but significant increases in amplitude, as well as robustly enhanced frequency of mIPSCs in hippocampal CA1 pyramidal neurons and medial septum/diagonal band (MS/DB) neurons. A unique subpopulation of MS/DB neurons showed decreases in frequency. In the presence of nicotine, varenicline effectively attenuated the expected enhancement of hippocampal mIPSC frequency like a competitive antagonist. However, in the MS/DB, varenicline only partially attenuated nicotine's effects. Reversing the order of drug application by adding nicotine to varenicline-exposed slices had little effect.

SIGNIFICANCE

Varenicline, like nicotine, stimulates presynaptic GABA release, and also exerts a partial agonist action by attenuating nicotine-stimulated release in both the hippocampus and basal forebrain. These effects could potentially affect cognitive functions.

Autoantibody-induced internalization of nicotinic acetylcholine receptor α3 subunit exogenously expressed in human embryonic kidney cells

Autoantibody against nicotinic acetylcholine receptor (nAChR) α3 subunit has been implicated in the pathogenesis of paraneoplastic neurological syndrome. To examine the effect of anti-α3 subunit autoantibody on cell-surface nAChRs, we established human embryonic kidney 293 cells stably co-expressing α3 and β4 subunits. Upon incubation with seropositive patient's serum, this cell line showed co-accumulation of patient's IgG and α3 subunits in the cytoplasm. These data support the hypothesis that anti-α3 subunit autoantibody induces internalization of cell-surface nAChRs and thereby impairs synaptic transmission.

Neurotrophins regulate cholinergic synaptic transmission in cultured rat sympathetic neurons through a p75-dependent mechanism

The sympathetic nervous system regulates many essential physiological systems, and its dysfunction is implicated in cardiovascular diseases. Mechanisms that control the strength of sympathetic output are therefore potential targets for the management of these disorders. Here we show that neurotrophins rapidly potentiate cholinergic transmission between cultured rat sympathetic neurons. We found that brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), acting at the p75 receptor, increased the amplitude of excitatory postsynaptic currents (EPSCs). We observed increased amplitude but not frequency of miniature synaptic currents after p75 activation, suggesting that p75 acts postsynaptically to modulate transmission at these synapses. This neurotrophic modulation enhances cholinergic EPSCs via sphingolipid signaling. Application of sphingolactone-24, an inhibitor of neutral sphingomyelinase, blocked the effect of BDNF, implicating a sphingolipid pathway. Furthermore, application of the p75-associated sphingolipid second messengers C(2)-ceramide and d-erythro-sphingosine restricted to the postsynaptic cell mimicked BDNF application. Postsynaptic blockade of ceramide production with fumonisin, a ceramide synthase inhibitor, blocked the effects of BDNF and d-erythro-sphingosine, implicating ceramide or ceramide phosphate as the active signal. Together these data suggest that neurotrophin signaling, which occurs in vivo via release from sympathetic neurons and target tissues such as the heart, acutely regulates the strength of the sympathetic postganglionic response to central cholinergic inputs. This pathway provides a potential mechanism for modulating the strength of sympathetic drive to target organs such as the heart and could play a role in the development of cardiovascular diseases.

Preservation of α-3 neuronal nicotinic acetylcholine receptor expression in sympathetic ganglia after brain death

The goal of this study was to evaluate if the immunohistochemical expression of alpha-3 neuronal nicotinic acetylcholine receptor subunit in sympathetic ganglia remains stable after brain death, determining the possible use of sympathetic thoracic ganglia from subjects after brain death as study group. The third left sympathetic ganglion was resected from patients divided in two groups: BD-organ donors after brain death and CON-patients submitted to sympathectomy for hyperhidrosis (control group). Immunohistochemical staining for alpha-3 neuronal nicotinic acetylcholine receptor subunit was performed; strong and weak expression areas were quantified in both groups. The BD group showed strong alpha-3 neuronal nicotinic acetylcholine receptor expression in 6.55% of the total area, whereas the CON group showed strong expression in 5.91% (p = 0.78). Weak expression was found in 6.47% of brain-dead subjects and in 7.23% of control subjects (p = 0.31). Brain death did not affect the results of the immunohistochemical analysis of sympathetic ganglia, and its use as study group is feasible.

A selective Akt inhibitor produces hypotension and bradycardia in conscious rats due to inhibition of the autonomic nervous system

Akt is a serine-threonine kinase that is amplified in a variety of human cancers, and as with other anticancer agents, some Akt inhibitors have produced functional cardiovascular effects such as marked hypotension that may limit their clinical benefit. Although identified in preclinical studies, the mechanism(s) responsible for these effects are often not fully characterized; potential targets include Akt signaling disruption in cardiac tissue, vascular smooth muscle, and/or autonomic system signaling. A selective Akt inhibitor was found to produce a rapid and marked hypotension and bradycardia in conscious rats. Isolated right atrial tissue and isolated thoracic aortic rings were used to examine direct effects of Akt inhibition on cardiac and vascular tissues, respectively. In addition, rats surgically prepared with telemetry units for monitoring blood pressure and heart rate were used to investigate potential effects on the autonomic nervous system (ANS). Whereas this Akt inhibitor did not produce any significant effect on atrial tissue, it did cause vasorelaxation of aortic rings. More significantly, in conscious rats, the Akt inhibitor inhibited the neural pressor response to the known nicotinic acetylcholine receptor (nAchR) agonist dimethylphenylpiperazinium (DMPP). In fact, the response observed was comparable to the response observed with the known ganglionic blocker hexamethonium. Thus, the hypotension and bradycardia produced by the Akt inhibitor is primarily due to blockade of nAchRs in autonomic ganglia. This finding highlights the importance of evaluating the ANS for cardiovascular effects associated with new chemical entities as well as suggesting a novel direct effect of an Akt inhibitor on nAchRs.

Changes in cationic selectivity of the nicotinic channel at the rat ganglionic synapse: a role for chloride ions?

The permeability of the nicotinic channel (nAChR) at the ganglionic synapse has been examined, in the intact rat superior cervical ganglion in vitro, by fitting the Goldman current equation to the synaptic current (EPSC) I–V relationship. Subsynaptic nAChRs, activated by neurally-released acetylcholine (ACh), were thus analyzed in an intact environment as natively expressed by the mature sympathetic neuron. Postsynaptic neuron hyperpolarization (from −40 to −90 mV) resulted in a change of the synaptic potassium/sodium permeability ratio (P_K/P_Na) from 1.40 to 0.92, corresponding to a reversible shift of the apparent acetylcholine equilibrium potential, E_ACh, by about +10 mV. The effect was accompanied by a decrease of the peak synaptic conductance (g_syn) and of the EPSC decay time constant. Reduction of [Cl⁻]_o to 18 mM resulted in a change of P_K/P_Na from 1.57 (control) to 2.26, associated with a reversible shift of E_ACh by about −10 mV. Application of 200 nM αBgTx evoked P_K/P_Na and g_syn modifications similar to those observed in reduced [Cl⁻]_o. The two treatments were overlapping and complementary, as if the same site/mechanism were involved. The difference current before and after chloride reduction or toxin application exhibited a strongly positive equilibrium potential, which could not be explained by the block of a calcium component of the EPSC. Observations under current-clamp conditions suggest that the driving force modification of the EPSC due to P_K/P_Na changes represent an additional powerful integrative mechanism of neuron behavior. A possible role for chloride ions is suggested: the nAChR selectivity was actually reduced by increased chloride gradient (membrane hyperpolarization), while it was increased, moving towards a channel preferentially permeable for potassium, when the chloride gradient was reduced.

Analysis of the excitatory motor response evoked by nicotinic and muscarinic blockade of ovine small bowel

It has been reported that the administration of anticholinergic drugs evokes inhibitory and excitatory responses, but the precise character of the latter has not yet been defined. This study was thus devoted to analyzing its occurrence following various doses of hexamethonium (Hx) and atropine (At) administration in the course of different phases of the small-intestinal migrating motor complex (MMC) in fasted and non-fasted sheep and to further characterize the excitatory responses in comparison with individual phases of the MMC. Two basic types of excitatory response were found. In the course of chronic experiments, various doses of Hx and At evoked rebound excitation (RE, i.e., irregular contractions or spike bursts evoked in response to the anticholinergic drug) alternating with phase 3-like activity (not the organized phase 3 of the MMC or its parts). The intensity of these changes varied and was related to the drug dose. Thus intense and non-intense RE activity were distinguished. In non-fasted sheep, these alterations were slightly less pronounced than in fasted animals. When the drug was given during phase 1 of the MMC, RE did not occur or was greatly reduced and its arrival was delayed. Hx triggered RE mostly in the duodenum, while the action of At was most effective in the jejunum. It is concluded that Hx and At initially hamper small-intestinal motility and just after that evoke a secondary stimulatory response, i.e., phase 3-like activity and RE of different intensity, duration, and repeatability in fasted and non-fasted sheep. These stimulatory effects may resemble unorganized phases of the MMC.

Cytisine induces autonomic cardiovascular responses via activations of different nicotinic receptors

Nicotinic cholinergic receptors mediate autonomic transmission at ganglia. However, whether different subtypes of nicotinic cholinergic receptors expressed in autonomic ganglia elicit distinct roles in mediating sympathetic and parasympathetic regulations remain to be defined. In this study, we observed that different subtypes of nicotinic receptors were responsible for the sympathetic and parasympathetic cardiovascular responses. In urethane anesthetized mice, intravenous injection with cytisine, a non-selective nicotinic agonist, induced a brief but pronounced decrease in heart rate, followed by increases in heart rate and arterial blood pressure. The bradycardic response was blocked by atropine, and the pressor response was blocked by prazosin, confirming that these responses were parasympathetic and sympathetic activities, respectively. Hexamethonium, a ganglionic blocker, blocked both sympathetic and parasympathetic responses. Pretreatment with methyllycaconitine citrate, a selective alpha7 nicotinic receptor antagonist, significantly attenuated cytisine-induced sympathetic response with little effect on the parasympathetic response. In contrast, pretreatment with dihydro-beta-erythroidine hydrobromide, a selective alpha4beta2 nicotinic receptor antagonist, blocked cytisine-induced parasympathetic response but not the sympathetic response. Pretreatment with dihydro-beta-erythroidine hydrobromide also blocked baroreflex associated parasympathetic bradycardic response. Moreover, treatment with nicotine induced a bradycardic response without a significant pressor response, which was also attenuated by dihydro-beta-erythroidine hydrobromide. Collectively, these data suggest that different nicotinic receptors play distinct roles in sympathetic and parasympathetic ganglia. Specifically, activations of alpha7 and alpha4beta2 nicotinic receptors are involved in cytisine-induced cardiovascular sympathetic and parasympathetic responses, respectively.

Haeme oxygenase-1 overexpression via nAChRs and the transcription factor Nrf2 has antinociceptive effects in the formalin test

Epibatidine has shown antinociceptive effects in various pain models, being 200-fold more potent than morphine. Previous results from our laboratory demonstrated that HO-1 overexpression has an antinociceptive effect in the formalin test. Furthermore, epibatidine was able to induce haeme oxygenase-1 (HO-1). So, the aim of this study was to investigate the effect of HO-1 overexpression induced by epibatidine in nociception elicited by formalin injection in the mice hindpaw. Administration of epibatidine (4 microg/kg) 24h before the test reduced the nociceptive response during the first phase and second phase of the formalin test. This effect was prevented by treatment with tin protoporphyrin (SnPP, an inhibitor of HO-1 activity) administered via intraplantar 5min before the test, suggesting a main role of HO-1. Western blot analysis revealed that epibatidine treatment increased by 2-fold HO-1 expression in the paw; this effect was lost in knockout mice for nuclear factor-erythroid 2-related factor 2 (Nrf2) and was accompanied by the loss of its antinociceptive effect. Furthermore, the antinociceptive effect of epibatidine was related to the activation of alpha7 and/or alpha9 nAChRs since methyllycaconitine (MLA) and mecamylamine but not dihydro-beta-erythroidine (DHbetaE) reverted this effect. Finally, we showed by flow cytometry and by immunofluorescence that white blood cells of the animals injected with epibatidine expressed more HO-1 than control animals, and this expression was also reverted by MLA pre-treatment. These findings demonstrate that HO-1 induction by epibatidine has antinociceptive and anti-inflammatory effects by the activation of MLA-sensitive nAChRs.