Heterogeneity of nicotinic cholinergic receptors in rat superior cervical and nodose Ganglia

Optimization of an autaptic culture system for studying cholinergic synapses in sympathetic ganglia

An autaptic synapse (or 'autapse') is a functional connection between a neuron and itself, commonly used in studying the molecular mechanisms underlying synaptic transmission and plasticity in central neurons. Most previous studies on autonomic synaptic functions have relied on spontaneous connections among neurons in mass cultures. However, growing evidence supports the utility of microcultures cultivating autaptic neurons for examining cholinergic transmission within sympathetic ganglia. Despite these advancements, standardized protocols for culturing autaptic sympathetic neurons have yet to be established. Drawing on historical literature, this study delineates optimal experimental conditions to efficiently and reliably produce cholinergic synapses in sympathetic neurons within a short time frame. Our research emphasizes five key factors: (i) the generation of uniformly sized microislands of growth permissive substrates; (ii) the addition of nerve growth factor, ciliary neurotrophic factor (CNTF), and serum to the culture medium; (iii) independence from specific serum and neuronal medium types; (iv) the reciprocal roles of CNTF and glial cells; and (v) the promotion of cholinergic synaptogenesis in SCG neurons through indirect glia co-cultures, rather than direct glial feeder layer cultures. In conclusion, glia-free monocultures of SCG neurons are relatively simple to prepare and yield robust and reliable synaptic currents. This makes them an effective model system for straightforwardly addressing fundamental questions about neurogenic mechanisms involved in cholinergic synaptic transmission in autonomic ganglia. Furthermore, autaptic culture experiments could eventually be implemented to investigate the roles of functional neuron-satellite glia units in regulating cholinergic functions under physiological and pathological conditions.

Understanding the Pivotal Role of the Vagus Nerve in Health from Pandemics

The COVID-19 pandemic seems endless with the regular emergence of new variants. Is the SARS-CoV-2 virus particularly evasive to the immune system, or is it merely disrupting communication between the body and the brain, thus pre-empting homeostasis? Retrospective analysis of the COVID-19 and AIDS pandemics, as well as prion disease, emphasizes the pivotal but little-known role of the 10th cranial nerve in health. Considering neuroimmunometabolism from the point of view of the vagus nerve, non-invasive bioengineering solutions aiming at monitoring and stimulating the vagal tone are subsequently discussed as the next optimal and global preventive treatments, far beyond pandemics.

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.

Comparative Analysis of Dorsal Root, Nodose and Sympathetic Ganglia for the Development of New Analgesics

Interoceptive and exteroceptive signals, and the corresponding coordinated control of internal organs and sensory functions, including pain, are received and orchestrated by multiple neurons within the peripheral, central and autonomic nervous systems. A central aim of the present report is to obtain a molecularly informed basis for analgesic drug development aimed at peripheral rather than central targets. We compare three key peripheral ganglia: nodose, sympathetic (superior cervical), and dorsal root ganglia in the rat, and focus on their molecular composition using next-gen RNA-Seq, as well as their neuroanatomy using immunocytochemistry and in situ hybridization. We obtained quantitative and anatomical assessments of transmitters, receptors, enzymes and signaling pathways mediating ganglion-specific functions. Distinct ganglionic patterns of expression were observed spanning ion channels, neurotransmitters, neuropeptides, G-protein coupled receptors (GPCRs), transporters, and biosynthetic enzymes. The relationship between ganglionic transcript levels and the corresponding protein was examined using immunohistochemistry for select, highly expressed, ganglion-specific genes. Transcriptomic analyses of spinal dorsal horn and intermediolateral cell column (IML), which form the termination of primary afferent neurons and the origin of preganglionic innervation to the SCG, respectively, disclosed pre- and post-ganglionic molecular-level circuits. These multimodal investigations provide insight into autonomic regulation, nodose transcripts related to pain and satiety, and DRG-spinal cord and IML-SCG communication. Multiple neurobiological and pharmacological contexts can be addressed, such as discriminating drug targets and predicting potential side effects, in analgesic drug development efforts directed at the peripheral nervous system.

α-Conotoxin TxID and [S9K]TxID, α3β4 nAChR Antagonists, Attenuate Expression and Reinstatement of Nicotine-Induced Conditioned Place Preference in Mice

Tobacco smoking has become a prominent health problem faced around the world. The α3β4 nicotinic acetylcholine receptor (nAChR) is strongly associated with nicotine reward and withdrawal symptom. α-Conotoxin TxID, cloned from Conus textile, is a strong α3β4 nAChR antagonist, which has weak inhibition activity of α6/α3β4 nAChR. Meanwhile, its analogue [S9K]TxID only inhibits α3β4 nAChR (IC₅₀ = 6.9 nM), and has no inhibitory activity to other nAChRs. The present experiment investigates the effect of α3β4 nAChR antagonists (TxID and [S9K]TxID) on the expression and reinstatement of nicotine-induced conditioned place preference (CPP) and explores the behaviors of acute nicotine in mice. The animal experimental results showed that TxID and [S9K] TxID could inhibit the expression and reinstatement of CPP, respectively. Moreover, both had no effect in acute nicotine experiment and the locomotor activity in mice. Therefore, these findings reveal that the α3β4 nAChR may be a potential target for anti-nicotine addiction treatment. [S9K]TxID, α3β4 nAChR antagonist, exhibit a superior effect for anti-nicotine addiction, which is promising to develop a novel smoking cessation drug.

The α5 Nicotinic Acetylcholine Receptor Subunit Differentially Modulates α4β2* and α3β4* Receptors

Nicotine, the principal reinforcing compound in tobacco, acts in the brain by activating neuronal nicotinic acetylcholine receptors (nAChRs). This review summarizes our current knowledge regarding how the α5 accessory nAChR subunit, encoded by the CHRNA5 gene, differentially modulates α4β2^* and α3β4^* receptors at the cellular level. Genome-wide association studies have linked a gene cluster in chromosomal region 15q25 to increased susceptibility to nicotine addiction, lung cancer, chronic obstructive pulmonary disease, and peripheral arterial disease. Interestingly, this gene cluster contains a non-synonymous single-nucleotide polymorphism (SNP) in the human CHRNA5 gene, causing an aspartic acid (D) to asparagine (N) substitution at amino acid position 398 in the α5 nAChR subunit. Although other SNPs have been associated with tobacco smoking behavior, efforts have focused predominantly on the D398 and N398 variants in the α5 subunit. In recent years, significant progress has been made toward understanding the role that the α5 nAChR subunit—and the role of the D398 and N398 variants—plays on nAChR function at the cellular level. These insights stem primarily from a wide range of experimental models, including receptors expressed heterologously in Xenopus oocytes, various cell lines, and neurons derived from human induced pluripotent stem cells (iPSCs), as well as endogenous receptors in genetically engineered mice and—more recently—rats. Despite providing a wealth of available data, however, these studies have yielded conflicting results, and our understanding of the modulatory role that the α5 subunit plays remains incomplete. Here, we review these reports and the various techniques used for expression and analysis in order to examine how the α5 subunit modulates key functions in α4β2^* and α3β4^* receptors, including receptor trafficking, sensitivity, efficacy, and desensitization. In addition, we highlight the strikingly different role that the α5 subunit plays in Ca²⁺ signaling between α4β2^* and α3β4^* receptors, and we discuss whether the N398 α5 subunit variant can partially replace the D398 variant.

Therapeutic potential of α7 nicotinic acetylcholine receptor agonists to combat obesity, diabetes, and inflammation

The cholinergic anti-inflammatory reflex (CAIR) represents an important homeostatic regulatory mechanism for sensing and controlling the body's response to inflammatory stimuli. Vagovagal reflexes are an integral component of CAIR whose anti-inflammatory effects are mediated by acetylcholine (ACh) acting at α7 nicotinic acetylcholine receptors (α7nAChR) located on cells of the immune system. Recently, it is appreciated that CAIR and α7nAChR also participate in the control of metabolic homeostasis. This has led to the understanding that defective vagovagal reflex circuitry underlying CAIR might explain the coexistence of obesity, diabetes, and inflammation in the metabolic syndrome. Thus, there is renewed interest in the α7nAChR that mediates CAIR, particularly from the standpoint of therapeutics. Of special note is the recent finding that α7nAChR agonist GTS-21 acts at L-cells of the distal intestine to stimulate the release of two glucoregulatory and anorexigenic hormones: glucagon-like peptide-1 (GLP-1) and peptide YY (PYY). Furthermore, α7nAChR agonist PNU 282987 exerts trophic factor-like actions to support pancreatic β-cell survival under conditions of stress resembling diabetes. This review provides an overview of α7nAChR function as it pertains to CAIR, vagovagal reflexes, and metabolic homeostasis. We also consider the possible usefulness of α7nAChR agonists for treatment of obesity, diabetes, and inflammation.

Activation of a nerve injury transcriptional signature in airway-innervating sensory neurons after lipopolysaccharide-induced lung inflammation

The lungs and the immune and nervous systems functionally interact to respond to respiratory environmental exposures and infections. The lungs are innervated by vagal sensory neurons of the jugular and nodose ganglia, fused together in smaller mammals as the jugular-nodose complex (JNC). Whereas the JNC shares properties with the other sensory ganglia, the trigeminal (TG) and dorsal root ganglia (DRG), these sensory structures express differential sets of genes that reflect their unique functionalities. Here, we used RNA sequencing (RNA-seq) in mice to identify the differential transcriptomes of the three sensory ganglia types. Using a fluorescent retrograde tracer and fluorescence-activated cell sorting, we isolated a defined population of airway-innervating JNC neurons and determined their differential transcriptional map after pulmonary exposure to lipopolysaccharide (LPS), a major mediator of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) after infection with gram-negative bacteria or inhalation of organic dust. JNC neurons activated an injury response program, leading to increased expression of gene products such as the G protein-coupled receptor Cckbr, inducing functional changes in neuronal sensitivity to peptides, and Gpr151, also rapidly induced upon neuropathic nerve injury in pain models. Unique JNC-specific transcripts, present at only minimal levels in TG, DRG, and other organs, were identified. These included TMC3, encoding for a putative mechanosensor, and urotensin 2B, a hypertensive peptide. These findings highlight the unique properties of the JNC and reveal that ALI/ARDS rapidly induces a nerve injury-related state, changing vagal excitability.

Expression of α3β2β4 nicotinic acetylcholine receptors by rat adrenal chromaffin cells determined using novel conopeptide antagonists

Adrenal chromaffin cells release neurotransmitters in response to stress and may be involved in conditions such as post-traumatic stress and anxiety disorders. Neurotransmitter release is triggered, in part, by activation of nicotinic acetylcholine receptors (nAChRs). However, despite decades of use as a model system for studying exocytosis, the nAChR subtypes involved have not been pharmacologically identified. Quantitative real-time PCR of rat adrenal medulla revealed an abundance of mRNAs for α3, α7, β2, and β4 subunits. Whole-cell patch-clamp electrophysiology of chromaffin cells and subtype-selective ligands were used to probe for nAChRs derived from the mRNAs found in adrenal medulla. A novel conopeptide antagonist, PeIA-5469, was created that is highly selective for α3β2 over other nAChR subtypes heterologously expressed in Xenopus laevis oocytes. Experiments using PeIA-5469 and the α3β4-selective α-conotoxin TxID revealed that rat adrenal medulla contain two populations of chromaffin cells that express either α3β4 nAChRs alone or α3β4 together with the α3β2β4 subtype. Conclusions were derived from observations that acetylcholine-gated currents in some cells were sensitive to inhibition by PeIA-5469 and TxID, while in other cells, currents were sensitive only to TxID. Expression of functional α7 nAChRs was determined using three α7-selective ligands: the agonist PNU282987, the positive allosteric modulator PNU120596, and the antagonist α-conotoxin [V11L,V16D]ArIB. The results of these studies identify for the first time the expression of α3β2β4 nAChRs as well as functional α7 nAChRs by rat adrenal chromaffin cells.

Impaired Expression of Neuregulin 1 and Nicotinic Acetylcholine Receptor β4 Subunit in Diverticular Disease

Neuregulin 1 (NRG1) regulates the expression of the nicotinic acetylcholine receptor (nAChR) and is suggested to promote the survival and maintenance of the enteric nervous system (ENS), since deficiency of its corresponding receptor complex ErbB2/ErbB3 leads to postnatal colonic aganglionosis. As diverticular disease (DD) is associated with intestinal hypoganglionosis, the NRG1-ErbB2/ErbB3 system and the nAChR were studied in patients with DD and controls. Samples of tunica muscularis of the sigmoid colon from patients with DD (n = 8) and controls (n = 11) were assessed for mRNA expression of NRG1, ErbB2, and ErbB3 and the nAChR subunits α3, α5, α7, β2, and β4. Site-specific gene expression levels of the NRG1-ErbB2/3 system were determined in myenteric ganglia harvested by laser microdissection (LMD). Localization studies were performed by immunohistochemistry for the NRG1-ErbB2/3 system and nAChR subunit β4. Rat enteric nerve cell cultures were stimulated with NRG1 or glial-cell line derived neurotrophic factor (GDNF) for 6 days and mRNA expression of the aforementioned nAchR was measured. NRG1, ErbB3, and nAChR subunit β4 expression was significantly down-regulated in both the tunica muscularis and myenteric ganglia of patients with DD compared to controls, whereas mRNA expression of ErbB3 and nAChR subunits β2, α3, α5, and α7 remained unaltered. NRG1, ErbB3, and nAChR subunit β4 immunoreactive signals were reduced in neuronal somata and the neuropil of myenteric ganglia from patients with DD compared to control. nAChR subunit β4 exhibited also weaker immunoreactive signals in the tunica muscularis of patients with DD. NRG1 treatment but not GDNF treatment of enteric nerve cell cultures significantly enhanced mRNA expression of nAchR β4. The down-regulation of NRG1 and ErbB3 in myenteric ganglia of patients with DD supports the hypothesis that intestinal hypoganglionosis observed in DD may be attributed to a lack of neurotrophic factors. Regulation of nAChR subunit β4 by NRG1 and decreased nAChR β4 in patients with DD provide evidence that a lack of NRG1 may affect the composition of enteric neurotransmitter receptor subunits thus contributing to the intestinal motility disorders previously reported in DD.

Differential blockade by huperzine A and donepezil of sympathetic nicotinic acetylcholine receptor-mediated nitrergic neurogenic dilations in porcine basilar arteries

Nicotinic acetylcholine receptor activation on the perivascular sympathetic nerves via axo-axonal interaction mechanism causes norepinephrine release, which triggers the neurogenic nitrergic relaxation in basilar arteries to meet the need of a brain. Donepezil and huperzine A, which are the cholinesterase inhibitors used for Alzheimer's disease therapy, exert controversial effects on nicotinic acetylcholine receptors. Therefore, we investigated how donepezil and huperzine A via the axo-axonal interaction regulate the neurogenic vasodilation of isolated porcine basilar arteries and define their action on different subtypes of the nicotinic acetylcholine receptor by using blood vessel myography, calcium imaging, and electrophysiological techniques. Both nicotine (100 μM) and transmural nerve stimulation (TNS, 8 Hz) induce NO-mediated dilation in the arteries. Nicotine-induced vasodilations were concentration-dependently inhibited by huperzine A and donepezil, with the former being 30 fold less potent than the latter. Both cholinesterase inhibitors weakly and equally decreased TNS-elicited nitrergic vasodilations. Neither huperzine A nor donepezil affected isoproterenol (a β adrenoceptor-agonist)- or sodium nitroprusside (a NO donor)-induced vasodilation. Further, huperzine A was less potent than donepezil in inhibiting nicotine-elicited calcium influxes in rodent superior cervical ganglionic neurons and inward currents in α7- and α3β2-nicotinic acetylcholine receptor-expressing Xenopus oocytes. In conclusion, huperzine A may exert less harmful effect over donepezil on maintaining brainstem circulation and on the nicotinic acetylcholine receptor-associated cognition deficits during treatment for Alzheimer's disease.

PeIA-5466: A Novel Peptide Antagonist Containing Non-natural Amino Acids That Selectively Targets α3β2 Nicotinic Acetylcholine Receptors

Pharmacologically distinguishing α3β2 nicotinic acetylcholine receptors (nAChRs) from closely related subtypes, particularly α6β2, has been challenging due to the lack of subtype-selective ligands. We created analogs of α-conotoxin (α-Ctx) PeIA to identify ligand-receptor interactions that could be exploited to selectively increase potency and selectivity for α3β2 nAChRs. A series of PeIA analogs were synthesized by replacing amino acid residues in the second disulfide loop with standard or nonstandard residues and assessing their activity on α3β2 and α6/α3β2β3 nAChRs heterologously expressed in Xenopus laevis oocytes. Asparagine¹¹ was found to occupy a pivotal position, and when replaced with negatively charged amino acids, selectivity for α3β2 over α6/α3β2β3 nAChRs was substantially increased. Second generation peptides were then designed to further improve both potency and selectivity. One peptide, PeIA-5466, was ∼300-fold more potent on α3β2 than α6/α3β2β3 and is the most α3β2-selective antagonist heretofore reported.

The role of the nAChR subunits α5, β2, and β4 on synaptic transmission in the mouse superior cervical ganglion

Our previous immunoprecipitation analysis of nicotinic acetylcholine receptors (nAChRs) in the mouse superior cervical ganglion (SCG) revealed that approximately 55%, 24%, and 21% of receptors are comprised of α3β4, α3β4α5, and α3β4β2 subunits, respectively. Moreover, mice lacking β4 subunits do not express α5-containing receptors but still express a small number of α3β2 receptors. Here, we investigated how synaptic transmission is affected in the SCG of α5β4-KO and α5β2-KO mice. Using an ex vivo SCG preparation, we stimulated the preganglionic cervical sympathetic trunk and measured compound action potentials (CAPs) in the postganglionic internal carotid nerve. We found that CAP amplitude was unaffected in α5β4-KO and α5β2-KO ganglia, whereas the stimulation threshold for eliciting CAPs was significantly higher in α5β4-KO ganglia. Moreover, intracellular recordings in SCG neurons revealed no difference in EPSP amplitude. We also found that the ganglionic blocking agent hexamethonium was the most potent in α5β4-KO ganglia (IC50 : 22.1 μmol/L), followed by α5β2-KO (IC50 : 126.7 μmol/L) and WT ganglia (IC50 : 389.2 μmol/L). Based on these data, we estimated an IC50 of 568.6 μmol/L for a receptor population consisting solely of α3β4α5 receptors; and we estimated that α3β4α5 receptors comprise 72% of nAChRs expressed in the mouse SCG. Similarly, by measuring the effects of hexamethonium on ACh-induced currents in cultured SCG neurons, we found that α3β4α5 receptors comprise 63% of nAChRs. Thus, in contrast to our results obtained using immunoprecipitation, these data indicate that the majority of receptors at the cell surface of SCG neurons consist of α3β4α5.

Alteration in behavior of rat after chronic exposure to acetamiprid

Background and Aim

Acetamiprid is a chemical of neonicotinoid group which binds with nicotinic acetylcholine receptor (nAChR) and alters the brain function. The present study was taken up to enlight the understanding of nociception behavior in Sprague Dawley (SD) rat after multiple exposures to acetamiprid.

Materials and Methods

For experiment purpose, a total of 48 SD rats were divided into four dose groups having 12 animals each. Group I was control group received only distilled water. Group II, Group III, and Group IV were treated with acetamiprid at a dose rate of 5, 20, and 40 mg/kg body weight, respectively. Rats were tested in induced pain by formalin injection and tail flick test.

Results

The flinch counts in formalin-induced pain in acetamiprid-treated rat were reduced in a dose-dependent manner, whereas, in tail flick test, no such altered pain behavior was observed in treated group compared to control animals.

Conclusion

Acetamiprid alters the centralized nociception through nAChR but could not trigger the associated signal to inhibit the nociception peripherally.

Peripheral N-methyl-D-aspartate receptor localization and role in gastric acid secretion regulation: immunofluorescence and pharmacological studies

The enteric nervous system (ENS) and a glutamate receptor (GluR), N-methyl-D-aspartate receptor (NMDAR), participate in gastric acid secretion (GAS) regulation. NMDARs are localized in different stomach cells; however, knowledge of NMDAR expression and function in the ENS is limited. In the present study, we clarified the types of stomach cells that express the NMDARs that are involved in GAS regulation. The pharmacological method of isolated stomach perfusion by Ghosh and Shild combined with direct mapping of NMDARs by fluorescence microscopy in the rat stomach was employed. By immunofluorescence labeling with an anti-NMDA-NR1 antibody, NMDARs were found to be highly expressed in nerve cells of the submucosal and myenteric plexuses in the stomach. The exact localization of the NMDARs relevant to GAS and its mechanism of action were determined by stimulating different receptors of neuronal and stomach cells using specific secretagogues for NMDA and by selectively blocking those receptors. NMDARs relevant to GAS stimulation are mainly localized in cholinergic interneurons; however, all of the nerve cells of the submucosal ganglia are involved in the stimulating process. In addition, the NMDARs in parietal cells are involved in gastric acid inhibition via influencing H₂-histamine receptors.

α-Conotoxins to explore the molecular, physiological and pathophysiological functions of neuronal nicotinic acetylcholine receptors

The vast diversity of neuronal nicotinic acetylcholine subunits expressed in the central and peripheral nervous systems, as well as in non-neuronal tissues, constitutes a formidable challenge for researchers and clinicians to decipher the role of particular subtypes, including complex subunit associations, in physiological and pathophysiological functions. Many natural products target the nAChRs, but there is no richer source of nicotinic ligands than the venom of predatory gastropods known as cone snails. Indeed, every single species of cone snail was shown to produce at least one type of such α-conotoxins. These tiny peptides (10-25 amino acids), constrained by disulfide bridges, proved to be unvaluable tools to investigate the structure and function of nAChRs, some of them having also therapeutic potential. In this review, we provide a recent update on the pharmacology and subtype specificity of several major α-conotoxins.

Nicotinic acetylcholine receptors in neuropathic and inflammatory pain

Nicotinic acetylcholine receptors (nAChRs) are actively being investigated as therapeutic targets for the treatment of pain and inflammation, but despite more than 30 years of research, there are currently no FDA-approved analgesics that are specific for these receptors. Much of the initial research effort focused on the α4β2 nAChR subtype, but more recently, additional subtypes have been identified as promising new leads and include α6β4, α7, and α9-containing nAChRs. This Review will focus on the distribution of these nAChRs in the cell types involved in neuropathic pain and inflammation and the activity of currently available nicotinic ligands.

Effects of nicotine on homeostatic and hedonic components of food intake

Chronic tobacco use leads to nicotine addiction that is characterized by exaggerated urges to use the drug despite the accompanying negative health and socioeconomic burdens. Interestingly, nicotine users are found to be leaner than the general population. Review of the existing literature revealed that nicotine affects energy homeostasis and food consumption via altering the activity of neurons containing orexigenic and anorexigenic peptides in the brain. Hypothalamus is one of the critical brain areas that regulates energy balance via the action of these neuropeptides. The equilibrium between these two groups of peptides can be shifted by nicotine leading to decreased food intake and weight loss. The aim of this article is to review the existing literature on the effect of nicotine on food intake and energy homeostasis and report on the changes that nicotine brings about in the level of these peptides and their receptors that may explain changes in food intake and body weight induced by nicotine. Furthermore, we review the effect of nicotine on the hedonic aspect of food intake. Finally, we discuss the involvement of different subtypes of nicotinic acetylcholine receptors in the regulatory action of nicotine on food intake and energy homeostasis.

α-Conotoxins active at α3-containing nicotinic acetylcholine receptors and their molecular determinants for selective inhibition

Neuronal α3-containing nicotinic acetylcholine receptors (nAChRs) in the peripheral nervous system (PNS) and non-neuronal tissues are implicated in a number of severe disease conditions ranging from cancer to cardiovascular diseases and chronic pain. However, despite the physiological characterization of mouse models and cell lines, the precise pathophysiology of nAChRs outside the CNS remains not well understood, in part because there is a lack of subtype-selective antagonists. α-Conotoxins isolated from cone snail venom exhibit characteristic individual selectivity profiles for nAChRs and, therefore, are excellent tools to study the determinants for nAChR-antagonist interactions. Given that human α3β4 subtype selective α-conotoxins are scarce and this is a major nAChR subtype in the PNS, the design of new peptides targeting this nAChR subtype is desirable. Recent studies using α-conotoxins RegIIA and AuIB, in combination with nAChR site-directed mutagenesis and computational modelling, have shed light onto specific nAChR residues, which determine the selectivity of the α-conotoxins for the human α3β2 and α3β4 subtypes. Publications describing the selectivity profile and binding sites of other α-conotoxins confirm that subtype-selective nAChR antagonists often work through common mechanisms by interacting with the same structural components and sites on the receptor.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Neurotrophic Factors NGF, GDNF and NTN Selectively Modulate HSV1 and HSV2 Lytic Infection and Reactivation in Primary Adult Sensory and Autonomic Neurons

Herpes simplex viruses (HSV1 and HSV2) establish latency in peripheral ganglia after ocular or genital infection, and can reactivate to produce different patterns and frequencies of recurrent disease. Previous studies showed that nerve growth factor (NGF) maintains HSV1 latency in embryonic sympathetic and sensory neurons. However, adult sensory neurons are no longer dependent on NGF for survival, some populations cease expression of NGF receptors postnatally, and the viruses preferentially establish latency in different populations of sensory neurons responsive to other neurotrophic factors (NTFs). Thus, NGF may not maintain latency in adult sensory neurons. To identify NTFs important for maintaining HSV1 and HSV2 latency in adult neurons, we investigated acute and latently-infected primary adult sensory trigeminal (TG) and sympathetic superior cervical ganglia (SCG) after NTF removal. NGF and glial cell line-derived neurotrophic factor (GDNF) deprivation induced HSV1 reactivation in adult sympathetic neurons. In adult sensory neurons, however, neurturin (NTN) and GDNF deprivation induced HSV1 and HSV2 reactivation, respectively, while NGF deprivation had no effects. Furthermore, HSV1 and HSV2 preferentially reactivated from neurons expressing GFRα2 and GFRα1, the high affinity receptors for NTN and GDNF, respectively. Thus, NTN and GDNF play a critical role in selective maintenance of HSV1 and HSV2 latency in primary adult sensory neurons.

Therapeutic concentrations of varenicline in the presence of nicotine increase action potential firing in human adrenal chromaffin cells

Varenicline is a nicotinic acetylcholine receptor (nAChR) agonist used to treat nicotine addiction, but a live debate persists concerning its mechanism of action in reducing nicotine consumption. Although initially reported as α4β2 selective, varenicline was subsequently shown to activate other nAChR subtypes implicated in nicotine addiction including α3β4. However, it remains unclear whether activation of α3β4 nAChRs by therapeutically relevant concentrations of varenicline is sufficient to affect the behavior of cells that express this subtype. We used patch-clamp electrophysiology to assess the effects of varenicline on native α3β4* nAChRs (asterisk denotes the possible presence of other subunits) expressed in human adrenal chromaffin cells and compared its effects to those of nicotine. Varenicline and nicotine activated α3β4* nAChRs with EC₅₀ values of 1.8 (1.2-2.7) μM and 19.4 (11.1-33.9) μM, respectively. Stimulation of adrenal chromaffin cells with 10 ms pulses of 300 μM acetylcholine (ACh) in current-clamp mode evoked sodium channel-dependent action potentials (APs). Under these conditions, perfusion of 50 or 100 nM varenicline showed very little effect on AP firing compared to control conditions (ACh stimulation alone), but at higher concentrations (250 nM) varenicline increased the number of APs fired up to 436 ± 150%. These results demonstrate that therapeutic concentrations of varenicline are unlikely to alter AP firing in chromaffin cells. In contrast, nicotine showed no effect on AP firing at any of the concentrations tested (50, 100, 250, and 500 nM). However, perfusion of 50 nM nicotine simultaneously with 100 nM varenicline increased AP firing by 290 ± 104% indicating that exposure to varenicline and nicotine concurrently may alter cellular behavior such as excitability and neurotransmitter release.

Both pre- and post-synaptic alterations contribute to aberrant cholinergic transmission in superior cervical ganglia of APP(-/-) mice

Though amyloid precursor protein (APP) can potentially be cleaved to generate the pathological amyloid β peptide (Aβ), APP itself plays an important role in regulating neuronal activity. APP deficiency causes functional impairment in cholinergic synaptic transmission and cognitive performance. However, the mechanisms underlying altered cholinergic synaptic transmission in APP knock-out mice (APP(-/-)) are poorly understood. In this study, we conducted in vivo extracellular recording to investigate cholinergic compound action potentials (CAPs) of the superior cervical ganglion (SCG) in APP(-/-) and littermate wild-type (WT) mice. Our results demonstrate that APP not only regulates presynaptic activity, but also affects postsynaptic function at cholinergic synapses in SCG. APP deficiency reduces the number of vesicles in presynaptic terminalsand attenuatesthe amplitude of CAPs, likely due to dysfunction of high-affinity choline transporters. Pharmacological and biochemical examination showed that postsynaptic responsesmediated by α4β2 and α7 nicotinic acetylcholine receptors are reduced in the absence of APP. Our research provides evidences on how APP regulates cholinergic function and therefore may help to identify potential therapeutic targets to treat cholinergic dysfunction associated with Alzheimer's disease pathogenesis.

AT-1001 Is a Partial Agonist with High Affinity and Selectivity at Human and Rat α3β4 Nicotinic Cholinergic Receptors

AT-1001 [N-(2-bromophenyl)-9-methyl-9-azabicyclo[3.3.1] nonan-3-amine] is a high-affinity and highly selective ligand at α3β4 nicotinic cholinergic receptors (nAChRs) that was reported to decrease nicotine self-administration in rats. It was initially reported to be an antagonist at rat α3β4 nAChRs heterologously expressed in HEK293 cells. Here we compared AT-1001 actions at rat and human α3β4 and α4β2 nAChRs similarly expressed in HEK 293 cells. We found that, as originally reported, AT-1001 is highly selective for α3β4 receptors over α4β2 receptors, but its binding selectivity is much greater at human than at rat receptors, because of a higher affinity at human than at rat α3β4 nAChRs. Binding studies in human and rat brain and pineal gland confirmed the selectivity of AT-1001 for α3β4 nAChRs and its higher affinity for human compared with rat receptors. In patch-clamp electrophysiology studies, AT-1001 was a potent partial agonist with 65-70% efficacy at both human and rat α3β4 nAChRs. It was also a less potent and weaker (18%) partial agonist at α4β2 nAChRs. Both α3β4 and α4β2 nAChRs are upregulated by exposure of cells to AT-1001 for 3 days. Similarly, AT-1001 desensitized both receptor subtypes in a concentration-dependent manner, but it was 10 and 30 times more potent to desensitize human α3β4 receptors than rat α3β4 and human α4β2 receptors, respectively. After exposure to AT-1001, the time to recovery from desensitization was longest for the human α3β4 nAChR and shortest for the human α4β2 receptor, suggesting that recovery from desensitization is primarily related to the dissociation of the ligand from the receptor.

Anxiolytic- and antidepressant-like effects of the methadone metabolite 2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline (EMDP)

The enhancement of GABAergic and monoaminergic neurotransmission has been the mainstay of pharmacotherapy and the focus of drug-discovery for anxiety and depressive disorders for several decades. However, the significant limitations of drugs used for these disorders underscores the need for novel therapeutic targets. Neuronal nicotinic acetylcholine receptors (nAChRs) may represent one such target. For example, mecamylamine, a non-competitive antagonist of nAChRs, displays positive effects in preclinical tests for anxiolytic and antidepressant activity in rodents. In addition, nicotine elicits similar effects in rodent models, possibly by receptor desensitization. Previous studies (Xiao et al., 2001) have identified two metabolites of methadone, EMDP (2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline) and EDDP (2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine), which are considered to be inactive at opiate receptors, as relatively potent noncompetitive channel blockers of rat α3β4 nAChRs. Here, we show that these compounds are likewise highly effective blockers of human α3β4 and α4β2 nAChRs. Moreover, we show that they display relatively low affinity for opiate binding sites labeled by [(3)H]-naloxone. We then evaluated these compounds in rats and mice in preclinical behavioral models predictive of potential anxiolytic and antidepressant efficacy. We found that EMDP, but not EDDP, displayed robust effects predictive of anxiolytic and antidepressant efficacy without significant effects on locomotor activity. Moreover, EMDP at behaviorally active doses, unlike mecamylamine, did not produce eyelid ptosis, suggesting it may produce fewer autonomic side effects than mecamylamine. Thus, the methadone metabolite EMDP may represent a novel therapeutic avenue for the treatment of some affective disorders.

Menthol Enhances the Desensitization of Human α3β4 Nicotinic Acetylcholine Receptors

The α3β4 nicotinic acetylcholine receptor (nAChR) subtype is widely expressed in the peripheral and central nervous systems, including in airway sensory nerves. The nAChR subtype transduces the irritant effects of nicotine in tobacco smoke and, in certain brain areas, may be involved in nicotine addiction and/or withdrawal. Menthol, a widely used additive in cigarettes, is a potential analgesic and/or counterirritant at sensory nerves and may also influence nicotine's actions in the brain. We examined menthol's effects on recombinant human α3β4 nAChRs and native nAChRs in mouse sensory neurons. Menthol markedly decreased nAChR activity as assessed by Ca(2+) imaging, (86)Rb(+) efflux, and voltage-clamp measurements. Coapplication of menthol with acetylcholine or nicotine increased desensitization, demonstrated by an increase in the rate and magnitude of the current decay and a reduction of the current integral. These effects increased with agonist concentration. Pretreatment with menthol followed by its washout did not affect agonist-induced desensitization, suggesting that menthol must be present during the application of agonist to augment desensitization. Notably, menthol acted in a voltage-independent manner and reduced the mean open time of single channels without affecting their conductance, arguing against a simple channel-blocking effect. Further, menthol slowed or prevented the recovery of nAChRs from desensitization, indicating that it probably stabilizes a desensitized state. Moreover, menthol at concentrations up to 1 mM did not compete for the orthosteric nAChR binding site labeled by [(3)H]epibatidine. Taken together, these data indicate that menthol promotes desensitization of α3β4 nAChRs by an allosteric action.

Role of α5-containing nicotinic receptors in neuropathic pain and response to nicotine

Nicotinic receptors in the central nervous system (nAChRs) are known to play important roles in pain processing and modulate behavioral responses to analgesic drugs, including nicotine. The presence of the α5-neuronal nicotinic accessory subunit in the nicotinic receptor complex is increasingly understood to modulate reward and aversive states, addiction, and possibly pathological pain. In the current study, using α5-knockout (KO) mice and subunit-specific antibodies, we assess the role of α5-containing neuronal nicotinic receptors in neuropathic pain and in the analgesic response to nicotine. After chronic constriction injury (CCI) or partial sciatic nerve ligation (PSNL), no differences in mechanical, heat, or cold hyperalgesia were found in wild-type (WT) versus α5-KO littermate mice. The number of α5-containing nAChRs was decreased (rather than increased) after CCI in the spinal cord and in the thalamus. Nevertheless, thermal analgesic response to nicotine was marginally reduced in CCI α5-KO mice at 4 days after CCI, but not at later timepoints or after PSNL. Interestingly, upon daily intermittent nicotine injections in unoperated mice, WT animals developed tolerance to nicotine-induced analgesia to a larger extent than α5-KO mice. Our results suggest that α5-containing nAChRs mediate analgesic tolerance to nicotine but do not play a major role in neuropathic pain.

Differential expression of the beta4 neuronal nicotinic receptor subunit affects tolerance development and nicotinic binding sites following chronic nicotine treatment

The role of neuronal nicotinic acetylcholine receptors (nAChR) containing the β4 subunit in tolerance development and nicotinic binding site levels following chronic nicotine treatment was investigated. Mice differing in expression of the β4-nAChR subunit [wild-type (β4(++)), heterozygote (β4(+-)) and null mutant (β4(--))] were chronically treated for 10 days with nicotine (0, 0.5, 1.0, 2.0 or 4.0mg/kg/h) by constant intravenous infusion. Chronic nicotine treatment elicited dose-dependent tolerance development. β4(--) mice developed significantly more tolerance than either β4(++) or β4(+-) mice which was most evident following treatment with 4.0mg/kg/h nicotine. Subsets of [(125)I]-epibatidine binding were measured in several brain regions. Deletion of the β4 subunit had little effect on initial levels of cytisine-sensitive [(125)I]-epibatidine binding (primarily α4β2-nAChR sites) or their response (generally increased binding) to chronic nicotine treatment. In contrast, β4 gene-dose-dependent decreases in expression 5IA-85380 resistant [(125)I]-epibatidine binding sites (primarily β4*-nAChR) were observed. While these β4*-nAChR sites were generally resistant to regulation by chronic nicotine treatment, significant increases in binding were noted for habenula and hindbrain. Comparison of previously published tolerance development in β2(--) mice (less tolerance) to that of β4(--) mice (more tolerance) supports a differential role for these receptor subtypes in regulating tolerance following chronic nicotine treatment.

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.

Design, synthesis and discovery of picomolar selective α4β2 nicotinic acetylcholine receptor ligands

Developing novel and selective compounds that desensitize α4β2 nicotinic acetylcholine receptors (nAChRs) could provide new effective treatments for nicotine addiction, as well as other disorders. Here we report a new class of nAChR ligands that display high selectivity and picomolar binding affinity for α4β2 nicotinic receptors. The novel compounds have Ki values in the range of 0.031-0.26 nM and properties that should make them good candidates as drugs acting in the CNS. The selected lead compound 1 (VMY-2-95) binds with high affinity and potently desensitizes α4β2 nAChRs. At a dose of 3 mg/kg, compound 1 significantly reduced rat nicotine self-administration. The overall results support further characterizations of compound 1 and its analogues in preclinical models of nicotine addiction and perhaps other disorders involving nAChRs.

Patterns of nicotinic receptor antagonism II: cardiovascular effects in rats

BACKGROUND

Tobacco cessation pharmacotherapies currently are limited to nicotine itself, the partial nicotine agonists varenicline and cytisine, and the antidepressant bupropion. Compared with agonists, nicotinic antagonists such as the noncompetitive, nonselective compound mecamylamine, and the competitive, α4β2-preferring antagonist dihydro-β-erythroidine (DHβE) may be a novel approach to the treatment of tobacco smoking as both are effective antagonists of nicotine's central effects. Considering nicotinic acetylcholine receptors mediate critical peripheral effects of acetylcholine, such as cardiovascular effects, it is important to study how nicotinic antagonists would alter the cardiovascular system and the cardiovascular changes induced by nicotine.

METHODS

The effects of several nicotinic agonists and antagonists on blood pressure and heart rate were measured in conscious, unrestrained rats following parenteral administration using a telemetry system.

RESULTS

Nicotine and other nicotinic receptor agonists (epibatidine, varenicline, and cytisine) produced similar increases in blood pressure, whereas their effects on heart rate were biphasic. The cardiovascular changes were attenuated by the nonselective nicotine antagonist, mecamylamine, but the peripherally restricted antagonist hexamethonium blocked only the agonist-induced changes in blood pressure. The α7-preferring antagonist, MLA, and the α4β2-preferring antagonist, DHβE, were much less effective in blocking the agonist-induced cardiovascular changes, indicating that nicotine's cardiovascular effects, are due to activation at autonomic ganglia involving nicotinic receptor subtypes other than α4, α7, or β2.

CONCLUSIONS

The data indicate that the cardiovascular effects of nicotine and nicotine-like agents are mediated through receptor mechanisms that are distinct from those that mediate the central effects of nicotine.

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.

α-Conotoxin PeIA[S9H,V10A,E14N] potently and selectively blocks α6β2β3 versus α6β4 nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) containing α6 and β2 subunits modulate dopamine release in the basal ganglia and are therapeutically relevant targets for treatment of neurological and psychiatric disorders including Parkinson's disease and nicotine dependence. However, the expression profile of β2 and β4 subunits overlap in a variety of tissues including locus ceruleus, retina, hippocampus, dorsal root ganglia, and adrenal chromaffin cells. Ligands that bind α6β2 nAChRs also potently bind the closely related α6β4 subtype. To distinguish between these two subtypes, we synthesized novel analogs of a recently described α-conotoxin, PeIA. PeIA is a peptide antagonist that blocks several nAChR subtypes, including α6/α3β2β3 and α6/α3β4 nAChRs, with low nanomolar potency. We systematically mutated PeIA and evaluated the resulting analogs for enhanced potency and/or selectivity for α6/α3β2β3 nAChRs expressed in Xenopus oocytes (α6/α3 is a subunit chimera that contains the N-terminal ligand-binding domain of the α6 subunit). On the basis of these results, second-generation analogs were then synthesized. The final analog, PeIA[S9H,V10A,E14N], potently blocked acetylcholine-gated currents mediated by α6/α3β2β3 and α6/α3β4 nAChRs with IC(50) values of 223 pM and 65 nM, respectively, yielding a >290-fold separation between the two subtypes. Kinetic studies of ligand binding to α6/α3β2β3 nAChRs yielded a k(off) of 0.096 ± 0.001 min(-1) and a k(on) of 0.23 ± 0.019 min(-1) M(-9). The synthesis of PeIA[S9H,V10A,E14N] demonstrates that ligands can be developed to discriminate between α6β2 and α6β4 nAChRs.

Partial activation of α7 nicotinic acetylcholine receptors: insights from molecular dynamics simulations

Nicotinic acetylcholine receptors (nAChRs) are drug targets for neuronal disorders and diseases. Partial agonists for nAChRs are currently being developed as drugs for the treatment of neurological diseases for their relative safety originated from reduced excessive stimulation. In the current study, molecular docking, molecular dynamics simulations and binding energy calculations were performed to theoretically investigate the interactions between the partial agonists, 4-OH-DMXBA and tropisetron with α7-nAChR. The results suggest that the partial agonists 4-OH-DMXBA and tropisetron bind with α7-nAChR in a binding mode similar to that with AChBP. The non-conserved residues in the binding sites contribute to the orientation deviation of these partial agonists from their orientation in AChBP. Energy calculation and decomposition using MM-GB/SA suggests that the van der Waals term (ΔE(VDW)) is the main driving force for the binding of the partial agonists to α7-nAChR. The molecular dynamics simulations showed that the opening of the C-loop binding with the partial agonists is in-between the openings for the binding with the full agonist and in the apo state. This conformation difference for the C-loop sheds light on the partial agonism of nAChR.

Nicotinic regulation of energy homeostasis

INTRODUCTION

The ability of nicotine, the primary psychoactive substance in tobacco smoke, to regulate appetite and body weight is one of the factors cited by smokers that prevents them from quitting and is the primary reason for smoking initiation in teenage girls. The regulation of feeding and metabolism by nicotine is complex, and recent studies have begun to identify nicotinic acetylcholine receptor (nAChR) subtypes and circuits or cell types involved in this regulation.

DISCUSSION

We will briefly describe the primary anatomical and functional features of the input, output, and central integration structures of the neuroendocrine systems that regulate energy homeostasis. Then, we will describe the nAChR subtypes expressed in these structures in mammals to identify the possible molecular targets for nicotine. Finally, we will review the effects of nicotine and its withdrawal on feeding and energy metabolism and attribute them to potential central and peripheral cellular targets.

The antinociceptive effects of nicotinic partial agonists varenicline and sazetidine-A in murine acute and tonic pain models

Nicotinic agonists display a wide-range profile of antinociceptive activity in acute, tonic, and chronic pain models. However, their effectiveness is limited by their unacceptable side effects. We investigated the antinociceptive effects of two new α4β2* nicotinic partial agonists, varenicline and sazetidine-A, in acute thermal and tonic pain mouse models. Both drugs failed to induce significant effects in the tail-flick and hot-plate tests after subcutaneous administration. However, they blocked nicotine's effects in these tests at very low doses. In contrast to acute pain tests, varenicline and sazetidine-A dose-dependently induced an analgesic effect in the mouse formalin test after systemic administration. Their antinociceptive effects were mediated, however, by different nicotinic acetylcholine receptor (nAChR) subtypes. Sazetidine-A effects were mediated by β2* nAChR subtypes, whereas varenicline actions were attributed to α3β4 nAChRs. Moreover, low inactive doses of varenicline blocked nicotine's actions in phase II of the formalin test. Overall, our results suggest that the antagonistic actions of varenicline at low doses are mediated by β2*-nAChRs and at higher doses as an agonist by α3β4*-nAChRs. In contrast, both actions of sazetidine-A are mediated by β2*-nAChR subtypes. These results suggest that nicotinic partial agonists possess analgesic effects in a rodent tonic pain model and may provide a potential treatment for the treatment of chronic pain disorders.

Impact of human D398N single nucleotide polymorphism on intracellular calcium response mediated by α3β4α5 nicotinic acetylcholine receptors

The human CHRNA5 D398N polymorphism (rs16969968) causes an aspartic acid to asparagine change in the nicotinic acetylcholine receptor (nAChR) α5 subunit gene. The N398 variant of CHRNA5 is linked to increased risk for nicotine dependence. In this study, we explored the effect of the CHRNA5 D398N polymorphism on the properties of human α3β4* nicotinic acetylcholine receptors in human embryonic kidney (HEK) cells. Addition of either D398 or N398 variant of α5 subunit in the α3β4* receptor did not affect total [(125)I]-epibatidine binding or surface expression of the receptor. However, addition of α5(D398) into α3β4* receptor decreased the maximal response to agonist without significantly affecting EC(50) in aequorin intracellular calcium assay. α3β4α5(N398) nAChRs showed further decreased maximal response. The differences in agonist efficacy between the receptor subtypes were found to be dependent upon the concentration of external calcium but independent of external sodium. Moreover, activation of α3β4α5 nAChRs led to significantly greater intracellular calcium release from IP(3) stores relative to α3β4 nAChRs although no effect of the α5 polymorphism was observed. Finally, inclusion of the α5 variant caused a small shift to the left in IC(50) for some of the antagonists tested, depending upon α5 variant but did not affect sensitivity of α3β4* receptors to desensitization in response to incubation with nicotine. In conclusion, addition of either variant of α5 into an α3β4α5 receptor similarly effects receptor pharmacology and function. However, the N398 variant exhibits a reduced response to agonists when extracellular calcium is high and it may lead to distinct downstream cellular signaling.

Subunit composition of α5-containing nicotinic receptors in the rodent habenula

Gene association studies in humans have linked the α5 subunit gene CHRNA5 to an increased risk for nicotine dependence. In the CNS, nicotinic acetylcholine receptors (nAChRs) that contain the α5 subunit are expressed at relatively high levels in the habenulo-interpeduncular system. Recent experimental evidence furthermore suggests that α5-containing receptors in the habenula play a key role in controlling the intake of nicotine in rodents. We have now analysed the subunit composition of hetero-oligomeric nAChRs in the habenula of postnatal day 18 (P18) C57Bl/6J control mice and of mice with deletions of the α5, the β2, or the β4 subunit genes. Receptors consisting of α3β4* clearly outnumbered α4β2*-containing receptors not only in P18 but also in adult mice. We found low levels of α5-containing receptors in both mice (6%) and rats (2.5% of overall nAChRs). Observations in β2 and β4 null mice indicate that although α5 requires the presence of the β4 subunit for assembling (but not of β2), α5 in wild-type mice assembles into receptors that also contain the subunits α3, β2, and β4.

Cholinergic chemosensory cells in the auditory tube

The luminal composition of the auditory tube influences its function. The mechanisms involved in the monitoring are currently not known. For the lower respiratory epithelium, such a sentinel role is carried out by cholinergic brush cells. Here, using two different mouse strains expressing eGFP under the control of the promoter of choline acetyltransferase (ChAT), we show the presence of solitary cholinergic villin-positive brush cells also in the mouse auditory tube epithelium. They express the vesicular acetylcholine (ACh) transporter and proteins of the taste transduction pathway such as α-gustducin, phospholipase C beta 2 (PLC(β2)) and transient receptor potential cation channel subfamily M member 5 (TRPM5). Immunoreactivity for TRPM5 and PLCβ2 was found regularly, whereas α-gustducin was absent in approximately 15% of the brush cells. Messenger RNA for the umami taste receptors (TasR), Tas1R1 and 3, and for the bitter receptors, Tas2R105 and Tas2R108, involved in perception of cycloheximide and denatonium were detected in the auditory tube. Using a transgenic mouse that expresses eGFP under the promotor of the nicotinic ACh receptor α3-subunit, we identified cholinoceptive nerve fibers that establish direct contacts to brush cells in the auditory tube. A subpopulation of these fibers displayed also CGRP immunoreactivity. Collectively, we show for the first time the presence of brush cells in the auditory tube. These cells are equipped with all proteins essential for sensing the composition of the luminal microenvironment and for communication of the changes to the CNS via attached sensory nerve fibers.

Bimodal effects of fluoxetine on cerebral nitrergic neurogenic vasodilation in porcine large cerebral arteries

Fluoxetine-induced relaxation of the smooth muscle of small cerebral arteries is thought beneficial in treating mental disorders. The present study was designed to examine effect of fluoxetine on neurogenic nitrergic vasodilation in large cerebral arteries, using in vitro tissue myography, techniques of electrophysiology, calcium imaging and biochemistry. In isolated porcine endothelium-denuded basilar arteries in the presence of U-46619-induced active muscle tone, fluoxetine in low concentration (<0.03 μM) significantly enhanced nicotine- and choline-induced relaxations. The vasorelaxation, however, was blocked by higher concentration of fluoxetine (>0.3 μM) with maximum inhibition at 3 μM. At this concentration, fluoxetine did not affect the basal tone or vasorelaxations induced by transmural nerve stimulation, sodium nitroprusside, or isoproterenol. Furthermore, fluoxetine exclusively blocked nicotine-induced inward currents and calcium influx in cultured neurons of rat superior cervical ganglion and Xenopus oocytes expressing human α7-, α3β2-, or α4β2-nicotinic acetylcholine receptors (nAChRs). In addition, fluoxetine at 0.03 μM and 3 μM significantly enhanced and blocked, respectively, nicotine-induced norepinephrine (NE) release from cerebral perivascular sympathetic nerves. These results indicate that fluoxetine via axo-axonal interaction mechanism exhibits bimodal effects on nAChR-mediated neurogenic nitrergic dilation of basilar arteries. Fluoxetine in high concentrations decreases while in low concentrations it increases neurogenic vasodilation. These results from in vitro experimentation suggest that optimal concentrations of fluoxetine which increase or minimally affect neurogenic vasodilation indicative of regional cerebral blood flow may be important consideration in treating mental disorders.

Nicotinic acetylcholine receptors in dorsal root ganglion neurons include the α6β4* subtype

The α6-containing nicotinic acetylcholine receptors (nAChRs) have recently been implicated in diseases of the central nervous system (CNS), including Parkinson's disease and substance abuse. In contrast, little is known about the role of α6* nAChRs in the peripheral nervous system (where the asterisk denotes the possible presence of additional subunits). Dorsal root ganglia (DRG) neurons are known to express nAChRs with a pharmacology consistent with an α7, α3β4*, and α4β2* composition. Here we present evidence that DRG neurons also express α6* nAChRs. We used RT-PCR to show the presence of α6 subunit transcripts and patch-clamp electrophysiology together with subtype-selective α-conotoxins to pharmacologically characterize the nAChRs in rat DRG neurons. α-Conotoxin BuIA (500 nM) blocked acetylcholine-gated currents (I(ACh)) by 90.3 ± 3.0%; the recovery from blockade was very slow, indicating a predominance of α(x)β4* nAChRs. Perfusion with either 300 nM BuIA[T5A;P6O] or 200 nM MII[E11A], α-conotoxins that target the α6β4* subtype, blocked I(ACh) by 49.3 ± 5 and 46.7 ± 8%, respectively. In these neurons, I(ACh) was relatively insensitive to 200 nM ArIB[V11L;V16D] (9.4±2.0% blockade) or 500 nM PnIA (23.0±4% blockade), α-conotoxins that target α7 and α3β2*/α6β2* nAChRs, respectively. We conclude that α6β4* nAChRs are among the subtypes expressed by DRG, and to our knowledge, this is the first demonstration of α6β4* in neurons outside the CNS.

Endogenously expressed muscarinic receptors in HEK293 cells augment up-regulation of stably expressed α4β2 nicotinic receptors

Nicotine-induced up-regulation of neuronal nicotinic receptors (nAChRs) has been known and studied for more than 25 years. Other nAChR ligands can also up-regulate nAChRs, but it is not known if these ligands induce up-regulation by mechanisms similar to that of nicotine. In this study, we compared up-regulation by three different nicotinic agonists and a competitive antagonist of several different nAChR subtypes expressed in HEK293 cells. Nicotine markedly increased α4β2 nAChR binding site density and β2 subunit protein. Carbachol, a known nAChR and muscarinic receptor agonist, up-regulated both α4β2 nAChR binding sites and subunit protein 2-fold more than did nicotine. This increased up-regulation was shown pharmacologically to involve endogenously expressed muscarinic receptors, and stimulation of these muscarinic receptors also correlated with a 2-fold increase in α4 and β2 mRNA. Muscarinic receptor activation in these cells appears to affect CMV promoter activity only minimally (∼1.2 fold), suggesting that the increase in α4 and β2 nAChR mRNA may not be dependent on enhanced transcription. Instead, other mechanisms may contribute to the increase in mRNA and a consequent increase in receptor subunits and binding site density. These studies demonstrate the possibility of augmenting nAChR expression in a cell model through mechanisms and targets other than the nAChR receptor itself.

α4β2 nicotinic acetylcholine receptors in the early postnatal mouse superior cervical ganglion

Heteropentameric nicotinic acetylcholine receptors (nAChR) mediate fast synaptic transmission in ganglia of the autonomic nervous system. It is undisputed that α3 and β4 are the predominant subunits in the superior cervical ganglion (SCG); however, reports on the presence of receptors that contain α4 have been controversial. Here, we have searched for the presence of α4-containing nAChRs in the postnatal rat and mouse SCG. We now show by immunoprecipitation combined with radioligand binding that α4-containing receptors constitute about 20% of hetero-oligomeric nAChRs in postnatal Day 3 (P3) mice. However, already by P9, the level of α4 approaches zero. In contrast, the number of α4-containing receptors is close to zero in the rat SCG at all times investigated. Deletion of the β2 subunit by using α5β2-double knockout (KO) mice removes all α4-containing receptors, suggesting that in the postnatal mouse SCG, α4 co-assembles only with β2 but not with β4. α4β2 receptors are, on the other hand, up-regulated in the SCG of P3 α5β4-double KO mice, where they make up about 50% of receptors that bind [(3) H]-epibatidine. Nonetheless, receptors on the surface of SCG neurons from α5β4-double KO mice maintained for one to two days in culture comprise <10% of α4β2 and >90% of α3β2, as determined by patch clamp recordings with α4β2- and α3β2-specific ligands. We propose that in the P3 SCG of wild type mice, α3β4 (±α5) represent about 62% of receptors, whereas 17% are α3β2β4, and 21% are α4β2 (±α5) receptors.

Neural regulation of intestinal nutrient absorption

The nervous system and the gastrointestinal (GI) tract share several common features including reciprocal interconnections and several neurotransmitters and peptides known as gut peptides, neuropeptides or hormones. The processes of digestion, secretion of digestive enzymes and then absorption are regulated by the neuro-endocrine system. Luminal glucose enhances its own absorption through a neuronal reflex that involves capsaicin sensitive primary afferent (CSPA) fibres. Absorbed glucose stimulates insulin release that activates hepatoenteric neural pathways leading to an increase in the expression of glucose transporters. Adrenergic innervation increases glucose absorption through α1 and β receptors and decreases absorption through activation of α2 receptors. The vagus nerve plays an important role in the regulation of diurnal variation in transporter expression and in anticipation to food intake. Vagal CSPAs exert tonic inhibitory effects on amino acid absorption. It also plays an important role in the mediation of the inhibitory effect of intestinal amino acids on their own absorption at the level of proximal or distal segment. However, chronic extrinsic denervation leads to a decrease in intestinal amino acid absorption. Conversely, adrenergic agonists as well as activation of CSPA fibres enhance peptides uptake through the peptide transporter PEPT1. Finally, intestinal innervation plays a minimal role in the absorption of fat digestion products. Intestinal absorption of nutrients is a basic vital mechanism that depends essentially on the function of intestinal mucosa. However, intrinsic and extrinsic neural mechanisms that rely on several redundant loops are involved in immediate and long-term control of the outcome of intestinal function.

Targeting peripheral afferent nerve terminals for cough and dyspnea

Chronic unproductive coughing and dyspnea are symptoms that severely diminish the quality of life in a substantial proportion of the population. There are presently few if any drugs that effectively treat these symptoms. Rational drug targets for cough and dyspnea have emerged over the recent years based on developments in our understanding of the innervation of the respiratory tract. These drug targets can be subcategorized into those that target the vagal afferent nerve endings, and those that target neural activity within the CNS. This review focuses on targets presumed to be in the peripheral terminals of afferent nerves within the airways. Conceptually, the activity of peripheral afferent nerves involved with unwanted urge-to-cough or dyspnea sensations can be inhibited by limiting the intensity of the stimulus, inhibiting the amplitude of the stimulus-induced generator potential, or inhibiting the transduction between the generator potential and action potential discharge and conduction. These mechanisms reveal many therapeutic strategies for anti-tussive and anti-dyspnea drug development with peripheral sites of action.

Cholinergic chemosensory cells in the trachea regulate breathing

In the epithelium of the lower airways, a cell type of unknown function has been termed "brush cell" because of a distinctive ultrastructural feature, an apical tuft of microvilli. Morphologically similar cells in the nose have been identified as solitary chemosensory cells responding to taste stimuli and triggering trigeminal reflexes. Here we show that brush cells of the mouse trachea express the receptors (Tas2R105, Tas2R108), the downstream signaling molecules (α-gustducin, phospholipase C(β2)) of bitter taste transduction, the synthesis and packaging machinery for acetylcholine, and are addressed by vagal sensory nerve fibers carrying nicotinic acetylcholine receptors. Tracheal application of an nAChR agonist caused a reduction in breathing frequency. Similarly, cycloheximide, a Tas2R108 agonist, evoked a drop in respiratory rate, being sensitive to nicotinic receptor blockade and epithelium removal. This identifies brush cells as cholinergic sensors of the chemical composition of the lower airway luminal microenvironment that are directly linked to the regulation of respiration.

Quantitative analysis of the heteromeric neuronal nicotinic receptors in the rat hippocampus

The objective of this study was to identify and quantify the heteromeric neuronal nicotinic receptors (nAChRs) in the rat hippocampus. The density of nAChR subtypes was assessed by labeling them with [(3)H]epibatidine ([(3)H]EB) followed by immunoprecipitation with subunit-selective antibodies. Sequential immunoprecipitation assays were used to establish associations between two different subunits, which then allowed the full subunit composition of the receptors to be deduced. Our results show that most of the hippocampal heteromeric nAChRs contain α4 and β2 subunits. In fact, we identified two populations containing these two predominant subunits, the α4β2 and α4β2α5 subtypes which account for ∼ 40% and ∼ 35%, respectively, of the total [(3)H]EB-labeled receptors. An additional heteromeric subtype with the subunit composition of α4β2α3 represented ∼ 10% of the total nAChRs, and another 10% of the immunoprecipitated receptors contained α4 and β4 subunits, with or without the α3 subunit. To determine if α4β2 and α4β2α5 nAChR subtypes differ in their ligand binding affinities, the α3- and β4-containing receptors were first removed by immunoprecipitation and then, competition studies with acetylcholine, nicotine, cytisine and sazetidine-A against [(3)H]EB were carried out on the remaining α4β2 and α4β2α5 subtypes. Results suggested these subtypes have comparable binding affinities for the nicotinic ligands used here.