Mixed nicotinic-muscarinic properties of the alpha9 nicotinic cholinergic receptor

Functional significance of non-neuronal acetylcholine in skin epithelia

In recent years, the physiological role of non-neuronal acetylcholine (ACh) and its receptors (AChR) in epidermal physiology has been under intense investigation. However, little is known about the role of the non-neuronal cholinergic system in inflammatory skin diseases. We chose the clinically nicotine-dependent skin disease hidradenitis suppurativa (HS) as model to study the influence of long term nicotine ingestion on epidermal morphology and AChR expression. HS is a chronic inflammatory, disabling disease of unknown pathogenesis emerging from the pilosebaceous unit of the intertriginous areas. In order to correlate our findings to specific nicotine effects, we used the organotypical coculture system (OTC) and raised artificial epidermis in the presence of nicotine. After 12 days in culture control OTC showed a mature epithelium, while nicotine treated OTCs were significantly thicker. Using immunofluorescence analysis, nicotine treated OTCs produced significantly stronger immunoreactivity (IR) for the alpha3, M(3) and M(5) AChR antisera than control. In contrast, the alpha7 nAChR antiserum showed a slightly reduced IR in the granular layer and the alpha9 nAChR IR retracted to the lower suprabasal layers. In HS epidermis we found the strongest IR for all AChR around the follicular infundibulum while in the sinus epithelia it was only weak. In contrast to the nicotine treated OTC, the alpha7 nAChR IR in the hyperplastic HS epidermis was clearly extended to all living layers. Altogether we provide first hints for a causative role of the non-neuronal cholinergic system in the pathogenesis of HS by promoting infundibular epithelial hyperplasia and thus follicular plugging.

Mechanisms of efferent-mediated responses in the turtle posterior crista

To study the cellular mechanisms of efferent actions, we recorded from vestibular-nerve afferents close to the turtle posterior crista while efferent fibers were electrically stimulated. Efferent-mediated responses were obtained from calyx-bearing (CD, calyx and dimorphic) afferents and from bouton (B) afferents distinguished by their neuroepithelial locations into BT units near the torus and BM units at intermediate sites. The spike discharge of CD units is strongly excited by efferent stimulation, whereas BT and BM units are inhibited, with BM units also showing a postinhibitory excitation. Synaptic activity was recorded intracellularly after spikes were blocked. Responses of BT/BM units to single efferent shocks consist of a brief depolarization followed by a prolonged hyperpolarization. Both components reflect variations in hair-cell quantal release rates and are eliminated by pharmacological antagonists of alpha9/alpha10 nicotinic receptors. Blocking calcium-dependent SK potassium channels converts the biphasic response into a prolonged depolarization. Results can be explained, as in other hair-cell systems, by the sequential activation of alpha9/alpha10 and SK channels. In BM units, the postinhibitory excitation is based on an increased rate of hair-cell quanta and depends on the preceding inhibition. There is, in addition, an efferent-mediated, direct depolarization of BT/BM and CD fibers. In CD units, it is the exclusive efferent response. Nicotinic antagonists have different effects on hair-cell efferent actions and on the direct depolarization of CD and BT/BM units. Ultrastructural studies, besides confirming the efferent innervation of type II hair cells and calyx endings, show that turtle efferents commonly contact afferent boutons terminating on type II hair cells.

Functional Characterization of the Epidermal Cholinergic System In Vitro

The aim of this study was to analyze the influence of cholinergic and anticholinergic drugs on epidermal physiology using organotypic cocultures (OTCs). Blocking of all acetylcholine receptors (AChRs) by combined treatment with mecamylamine and atropine or treatment with strychnine (blocking alpha9nAChR) for 7-14 days resulted in a complete inhibition of epidermal differentiation and proliferation. Blockage of nicotinic (n)AChR with mecamylamine led to a less pronounced delay in epidermal differentiation and proliferation than blockage of muscarinic (m)AChR with atropine, evidenced by reduced epithelial thickness and expression of terminal differentiation markers like cytokeratin 2e or filaggrin. In OTCs treated with atropine, mecamylamine, or strychnine, we could demonstrate intracellular lipid accumulation in the lower epidermal layers, indicating a severely disturbed epidermal barrier. In addition, we observed prominent acantholysis in the basal and lower suprabasal layers in mecamylamine-, atropine-, and strychnine-treated cultures, accompanied by a decreased expression of cell adhesion proteins. This globally reduced cell adhesion led to cell death via intrinsic activation of apoptosis. In contrast, stimulation of nAChR and mAChR with cholinergic drugs resulted in a significantly thickened epithelium, accompanied by an improved epithelial maturation. In summary, we show that epidermal AChR are crucially involved in the regulation of epidermal homeostasis.

Nicotine activates cell-signaling pathways through muscle-type and neuronal nicotinic acetylcholine receptors in non-small cell lung cancer cells

Nicotinic acetylcholine receptors (nAChR) are expressed on non-neuronal cell types, including normal bronchial epithelial cells, and nicotine has been reported to cause Akt activation in cultured normal airway cells. This study documents mRNA and protein expression of subunits known to form a muscle-type nAChR in non-small cell lung cancer (NSCLC) cell lines. In one NSCLC examined, mRNA and protein for a heteropentamer neuronal-type alpha3beta2 nAChR was detected in addition to a muscle-type receptor. Protein for the alpha5 nAChR was also detected in NSCLC cells. Although, mRNA for the alpha7 nAChR subunit was observed in all cell lines, alpha7 protein was not detectable by immunoblot in NSCLC cell extracts. Immunohistochemistry (IHC) of NSCLC primary tissues from 18 patients demonstrated protein expression of nAChR alpha1 and beta1 subunits, but not alpha7 subunit, in lung tumors, indicating preferential expression of the muscle-type receptor. In addition, the beta1 subunit showed significantly increased expression in lung tumors as compared to non-tumor bronchial tissue. The alpha1 subunit also showed evidence of high expression in lung tumors. Nicotine at a concentration of 10 microM caused phosphorylation of mitogen-activated protein kinase (MAPK) (p44/42) that could be inhibited using nAChR antagonists. Inhibition was observed at 100 nM alpha-bungarotoxin (alpha-BTX) or 10 microM hexamethonium (HEX); maximal inhibition was achieved using a combination of alpha-BTX and HEX. Akt was also phosphorylated in NSCLC cells after exposure to nicotine; this effect was inhibited by the PI3K inhibitor LY294002 and antagonists to the neuronal-type nAChR, but not to the muscle-type receptor. Nicotine triggered influx of calcium in the 273T NSCLC cell line, suggesting that L-type calcium channels were activated. 273T cells also showed greater activation of p44/42 MAPK than of Akt in response to nicotine. Cultures treated with nicotine and the EGFR tyrosine kinase inhibitor gefitinib showed a significant increase in the number of surviving cells compared to gefitinib alone. These data indicate that the muscle-type nAChR, rather than the alpha7 type, is highly expressed in NSCLC and leads to downstream activation of the p44/42 MAPK pathway. Neuronal-type receptors are also present and functional, as evidenced by antagonist studies, although, the expression levels are lower than muscle-type nAChR. They also lead to downstream activation of MAPK and Akt. Nicotine may play a role in regulating survival of NSCLC cells and endogenous acetylcholine released locally in the lung and/or chronic nicotine exposure might play a role in NSCLC development. In addition, exposure of NSCLC patients to nicotine through use of nicotine replacement products or use of tobacco products may alter the efficacy of therapy with EGFR inhibitors.

Fast cholinergic efferent inhibition in guinea pig outer hair cells

Hair cells of inner ear are suggested to be inhibited by the activation of the alpha9-containing nicotinic acetylcholine (ACh) receptors (alpha9-containing nAChRs). Several studies have suggested that the native nicotinic-like ACh receptors (nAChRs) in hair cells display a significant permeability of Ca(2+) ions and unusual pharmacological properties. The activation of native nAChRs will initiate the hyperpolarization of hair cells by activation of the small conductance, Ca(2+)-activated K(+) channels (SK). In this work, the properties of the ACh-sensitive potassium current (IK(ACh)) in outer hair cells (OHCs) of guinea pigs were investigated by employing whole-cell patch-clamp. Followed by perfusion of ACh, OHCs displayed a rapid desensitized current with an N-shaped current-voltage curve (I-V) and a reversal potential of - 66 +/- 7 mV. The IK(ACh) was still present during perfusion of either iberiotoxin (IBTX, 200 nM) or TEA (5 mM) but was potently inhibited by apamin (1 muM), TEA (30 mM). The IK(ACh) demonstrated a strong sensitivity to alpha-bungarotoxin (alpha-BgTx), bicuculline and strychnine. These results suggested that OHCs display the well-known SK current, which might be gated by the alpha9-containing nAChRs. Two important changes were present after lowering the Ca(2+) concentration in the external conditions from 2 mM to 0.2 mM: one was a flattened N-shape I-V relationship with a maximum shifted toward hyperpolarized potentials from -20 approximately -30 mV approximately -40 to -50 mV, the other was a significant reduction in the agonist maximal response (percentage of maximal response 10.5 +/- 5.4). These results indicated that native nAChRs are both permeable to and modulated by extracellular Ca(2+) ions. Taken together, this work provides direct evidences that SK channels in OHCs of guinea pigs are gated by alpha9-containing nAChRs, which play an important role in the fast cholinergic efferent inhibition. This fast inhibition is both potently dependent on the permeability of Ca(2+) ions through the native nAChRs and modulated by Ca(2+) ions.

An autoradiographic analysis of rat brain nicotinic receptor plasticity following dietary choline modification

Choline is known to be involved with numerous physiological functions of the nervous system and also acts as a direct acting agonist of alpha7 nicotinic acetylcholine receptors (nAChRs). The purpose of this study was to conduct a brain region-specific evaluation of changes in nAChR subtype expression following dietary choline modification. In addition, we assessed changes in body weight, food/water intake, as well as changes in spatial learning (Morris Water Maze) in response to dietary choline modification. Male Sprague-Dawley rats were exposed to standard, choline supplemented or choline deficient diets for periods of 14 or 28 days. Choline supplemented animals gained significantly less weight over the course of the experiment, in spite of the fact that there were minimal differences in food consumption between the dietary regimens. Spatial memory did not differ between animals maintained on a standard rat diet, and the choline supplemented food. Brains of the animals kept on the diets for 14 and 28 days were used for quantitative autoradiographic analysis of nicotinic receptor subtypes using 125I-Bungarotoxin (alpha7) and 125I-Epibatidine (non-alpha7). There were no significant differences in nicotinic receptor binding or physiologic parameters measured between animals fed standard and choline deficient diets. However 2 weeks of dietary choline supplementation caused significant up-regulation of alpha7 receptors without significant effect on the density of non-alpha7 nAChRs. Increases in BTX binding predominantly occurred in cortical and hippocampal brain regions and ranged between 14 and 30% depending on the brain region. The results of our study suggest that choline acts as a selective agonist at alpha7 nicotinic cholinergic receptors in the rat central nervous system.

SLURP-2: A novel cholinergic signaling peptide in human mucocutaneous epithelium

The biologic role of novel cholinergic toxin-like signaling peptides termed SLURP (secreted mammalian Ly-6/uPAR-related protein) in the mucocutaneous epithelium is a subject of intense research. Previous studies demonstrated that SLURP-1 activates the alpha7 subtype of keratinocyte nicotinic acetylcholine receptors (nAChRs) and facilitates keratinization and programmed cell death, and that the level of SLURP-2 was found to be upregulated several fold in the hyperproliferative skin of patients with psoriasis. In this study, we demonstrated for the first time that human epidermal and oral keratinocytes secrete SLURP-2. We cloned human SLURP-2 and produced the mouse monoclonal antibody 341F10-1F12 that visualized SLURP-2 in the cytoplasm of normal human epidermal and oral keratinocytes grown in culture. In epidermis, SLURP-2 was found predominantly in the suprabasal compartment, whereas in the attached gingiva-in the lowermost epithelial layers. Recombinant SLURP-2 (rSLURP-2) competed with nicotinic radioligands for binding to keratinocytes, showing a higher affinity to the [3H]epibatidine- than [3H]nicotine-labeled sites. Treatment with rSLURP-2 significantly (P < 0.05) increased the number of keratinocytes in culture and their resistance to apoptosis, which could be abolished by mecamylamine more efficiently than alpha-bungarotoxin. By real-time PCR and in-cell western, rSLURP-2 significantly (P < 0.05) downregulated gene expression of the differentiation markers loricrin, filaggrin, and cytokeratins 1 and 10, and pro-apoptotic Bax, Bad, and caspase 3 which were elevated by high extracellular calcium, and rSLURP-2 also abolished activation of caspases 3 and 8 caused by camptothecin. These results indicated that SLURP-2 competes with acetylcholine predominantly at the alpha3 nAChR, and that receptor ligation with SLURP-2 delays keratinocyte differentiation and prevents apoptosis. Thus, the different effects observed for SLURP-1 and -2 can be explained by their differential binding to the nAChR subtypes expressed in keratinocytes. These findings present a novel paradigm of the physiologic regulation of mucocutaneous epithelial cells by locally produced small hormone-like peptide molecules, and open novel directions toward better understanding and treating of skin and mucosal diseases.

Identification of molluscan nicotinic acetylcholine receptor (nAChR) subunits involved in formation of cation- and anion-selective nAChRs

Acetylcholine (ACh) is a neurotransmitter commonly found in all animal species. It was shown to mediate fast excitatory and inhibitory neurotransmission in the molluscan CNS. Since early intracellular recordings, it was shown that the receptors mediating these currents belong to the family of neuronal nicotinic acetylcholine receptors and that they can be distinguished on the basis of their pharmacology. We previously identified 12 Lymnaea cDNAs that were predicted to encode ion channel subunits of the family of the neuronal nicotinic acetylcholine receptors. These Lymnaea nAChRs can be subdivided in groups according to the residues supposedly contributing to the selectivity of ion conductance. Functional analysis in Xenopus oocytes revealed that two types of subunits with predicted distinct ion selectivities form homopentameric nicotinic ACh receptor (nAChR) subtypes conducting either cations or anions. Phylogenetic analysis of the nAChR gene sequences suggests that molluscan anionic nAChRs probably evolved from cationic ancestors through amino acid substitutions in the ion channel pore, a mechanism different from acetylcholine-gated channels in other invertebrates.

Ca2+ permeability through rat cloned alpha9-containing nicotinic acetylcholine receptors

We investigated the functional properties of rat alpha9 and alpha9alpha10 nicotinic acetylcholine receptors (nAChRs) expressed by transient transfection in the rat GH4C1 cell line, using both Ca(2+) imaging and whole-cell recording. Acute applications of ACh generated short-delay fast-rising and quick-decaying Ca(2+) transients, suppressed in Ca(2+)-free medium and invariably accompanied by the activation of whole-cell inward currents. The mean amplitude of ACh-induced currents was as small as -16 pA in alpha9 subunit cDNA-transfected GH4C1 cells (alpha9-GH4C1), while they were much larger (range: -150 to -300 pA) in alpha9alpha10 subunit cDNAs-transfected GH4C1 cells (alpha9alpha10-GH4C1). Currents were not activated by nicotine, were blocked by methyllycaconitine and were ACh concentration-dependent. Because the Ca(2+) permeability of alpha9-containing nAChRs has been estimated in immortalized cochlear UB/OC-2 mouse cells, we also characterized the ACh-induced responses in these cells. Unlike alpha9- and alpha9alpha10-GH4C1 cells, UB/OC-2 cells responded to ACh with both long-delay methyllycaconitine-insensitive whole-cell currents and long-lasting Ca(2+) transients, the latter being detected in the absence of Ca(2+) in the extracellular medium and being suppressed by the Ca(2+)-ATPase inhibitor thapsigargin, known to deplete IP(3)-sensitive stores. These results indicated the involvement of muscarinic nAChRs and the lack of functional ACh-gated receptor channels in UB/OC-2 cells. Thus, we measured the fractional Ca(2+) current (P(f), i.e. the percentage of total current carried by Ca(2+) ions) in alpha9alpha10-GH4C1, obtaining a P(f) value of 22 +/- 4%; this is the largest value estimated to date for a ligand-gated receptor channel. The physiological role played by Ca(2+) entry through alpha9-containing nAChRs gated by ACh is discussed.

Acetylcholine causes an increase of intracellular calcium in human sperm

Sperm nicotinic acetylcholine receptors (nAChRs) can influence motility and the initiation of acrosome reaction (AR). We report that AR initiation by acetylcholine (ACh) in capacitated human sperm requires both Na+ and Ca2+ in the external medium. Pre-incubation with 50 microM 3-quinuclidinyl benzilate (QNB) or 50 nM strychnine failed to inhibit the ACh-initiated AR, demonstrating that muscarinic AChRs and nAChRs containing alpha9 subunits do not mediate this event. Choline (2.5, 5 and 10 mM), a highly specific but low potency agonist of the alpha7 nAChR initiated AR, with its effect blocked by the nAChR antagonist methyllycaconitine (MLA). ACh (50-400 microM) stimulated a small transient rise in the intracellular Ca2+ in sperm populations loaded with FURA-2, with 200 microM ACh being maximal (146 nM +/- 23 SEM). The nAChR antagonists, alpha-bungarotoxin (alpha-BTX) and MLA, reduced the ACh-initiated Ca2+ rise by 75 and 78%, respectively, demonstrating the majority of the rise is mediated through nAChRs containing alpha7 or alpha9 subunits. Single cell imaging studies using FLUO-3 resolved two patterns of ACh-stimulated Ca2+ increase in the sperm head: 94% of responding sperm displayed a rise (59.6% +/- 5.7 SEM increase from resting fluorescence intensity), returning to resting levels over a period of 2-3 min. The remaining sperm (6%) displayed a sharp spike of Ca2+ ( approximately 1 min; 86% +/- 4.3 SEM change in fluorescence intensity), followed by abrupt loss of fluorescence, a pattern suggestive of AR. A Ca2+ influx in the sperm midpiece appeared to accompany the Ca2+ influx seen in the head. These observations confirm an ionotropic role for nAChRs in sperm function.

Key roles of hydrophobic rings of TM2 in gating of the alpha9alpha10 nicotinic cholinergic receptor

We have performed a systematic mutagenesis of three hydrophobic rings (17', 13' and 9') within transmembrane region (TM) 2 of the alpha9alpha10 nicotinic cholinergic receptor (nAChR) to a hydrophilic (threonine) residue and compared the properties of mutant receptors reconstituted in Xenopus laevis oocytes. Phenotypic changes in alpha9alpha10 mutant receptors were evidenced by a decrease in the desensitization rate, an increase in both the EC(50) for ACh as well as the efficacy of partial agonists and the reduction of the allosteric modulation by extracellular Ca(2+). Mutated receptors exhibited spontaneous openings and, at the single-channel level, an increased apparent mean open time with no major changes in channel conductance, thus suggesting an increase in gating of the channel as the underlying mechanism. Overall, the degrees of the phenotypes of mutant receptors were more overt in the case of the centrally located V13'T mutant. Based on the atomic model of the pore of the electric organ of the Torpedo ray, we can propose that the interactions of side chains at positions 13' and 9' are key ones in creating an energetic barrier to ion permeation. In spite of the fact that the roles of the TM2 residues are mostly conserved in the distant alpha9alpha10 member of the nAChR family, their mechanistic contributions to channel gating show significant differences when compared to other nAChRs. These differences might be originated from slight differential intramolecular rearrangements during gating for the different receptors and might lead each nAChR to be in tune with their physiological roles.

Alpha-9 nicotinic acetylcholine receptor immunoreactivity in the rodent vestibular labyrinth

Vestibular tissues (cristae ampullares, macular otolithic organs, and Scarpa's ganglia) in chinchilla, rat, and guinea pig were examined for immunoreactivity to the alpha9 nicotinic acetylcholine receptor (nAChR) subunit. The alpha9 antibody was generated against a conserved peptide present in the intracellular loop of the predicted protein sequence of the guinea pig alpha9 nAChR subunit. In the vestibular periphery, staining was observed in calyces around type I hair cells, at the synaptic pole of type II hair cells, and in varying levels in Scarpa's ganglion cells. Ganglion cells were also triply labeled to detect alpha9, calretinin, and peripherin. Calretinin labels calyx-only afferents. Peripherin labels bouton-only afferents. Dimorphic afferents, which have both calyx and bouton endings, are not labeled by calretinin or peripherin. In these experiments, alpha9 was expressed in both calyx and dimorphic afferents. A subpopulation of small ganglion cells did not contain the alpha9 nAChR but did stain for peripherin. We surmise that these are bouton-only afferents. Bouton (regularly discharging) afferents also show efferent responses, although they are qualitatively different from those in irregularly discharging (calyx and dimorphic) afferents, much slower and longer lasting. Thus, regular afferents are probably more affected via a muscarinic cholinergic or a peptidergic mechanism, with a much smaller superimposed fast nicotinic-type response. This latter response could be due to one of the other nicotinic receptors that have been described in studies from other laboratories.

Muscarinic ACh Receptor Activation Causes Transmitter Release From Isolated Frog Vestibular Hair Cells

In the frog, vestibular efferent fibers innervate only type-II vestibular hair cells. Through this direct contact with hair cells, efferent neurons are capable of modifying transmitter release from hair cells onto primary vestibular afferents. The major efferent transmitter, acetylcholine (ACh), is known to produce distinct pharmacological actions involving several ACh receptors. Previous studies have implicated the presence of muscarinic ACh receptors on vestibular hair cells, although, surprisingly, a muscarinic-mediated electrical response has not been demonstrated in solitary vestibular hair cells. This study demonstrates that muscarinic receptors can evoke transmitter release from vestibular hair cells. Detection of this release was obtained through patch-clamp recordings from catfish cone horizontal cells, serving as glutamate detectors after pairing them with isolated frog semicircular canal hair cells in a two-cell preparation. Although horizontal cells alone failed to respond to carbachol, application of 20 μM carbachol to the two-cell preparation resulted in a horizontal cell response that could be mimicked by exogenous application of glutamate. All of the horizontal cells in the two-cell preparation responded to 20 μM CCh. Furthermore, this presumed transmitter release persisted in the presence of d-tubocurarine at concentrations that block all known hair cell nicotinic ACh receptors. The effect on the detector cell, imparted by the carbachol application to the hair cell-horizontal cell preparation, was blocked both by 2-amino-5-phosphonopentanoic acid, a selective N-methyl-d-aspartate antagonist, and the muscarinic antagonist, atropine. Thus vestibular hair cells from the frog semicircular canal can be stimulated to release transmitter by activating their muscarinic receptors.

The properties of ACh-induced BK currents in guinea pig type II vestibular hair cells

Molecular biological studies have demonstrated that both muscarinic receptor subtypes and nicotinic receptor subunits were located in mammalian vestibular sensorineural epithelium. However, the functional roles are still unclear, with the exception of the well-known alpha9-containing nicotinic ACh receptor (alpha9nAChR). In this study, the properties of acetylcholine (ACh)-induced currents were investigated by whole-cell patch clamp technique in isolated type II vestibular hair cells (VHCs II) of guinea pig. VHCs II displayed a sustained, non-inactivating current when extracellular application of ACh. ACh-induced currents restored gradually and it took about 60 s to get a complete recovery. ACh-induced current was not affected by extracellular Na(+), but strongly affected by extracellular K(+) and Ca(2+). Depletion of the intracellular Ca(2+) stores by intracellular application of inositol 1,4,5-trisphosphate (IP3) or blocking of the release of intracellular Ca(2+) stores by intracellular application of heparin failed to inhibit this current. ACh-induced currents were inhibited by nifedipine, Cd(2+), tetraethylammonium (TEA), charybdotoxin (CTX), iberiotoxin (IBTX), atropine and d-tubocurarine (DTC), respectively, but not by apamin. In conclusion, ACh stimulates a large conductance, Ca(2+)-activated K(+) current (BK) in guinea pig VHCs II by activation of the influx of Ca(2+) ions, which is mediated by an ACh receptor that could not be defined to be the odd-number muscarinic receptor.

Facilitating efferent inhibition of inner hair cells in the cochlea of the neonatal rat

Cholinergic brainstem neurones make inhibitory synapses on outer hair cells (OHCs) in the mature mammalian cochlea and on inner hair cells (IHCs) prior to the onset of hearing. We used electrical stimulation in an excised organ of Corti preparation to examine evoked release of acetylcholine (ACh) onto neonatal IHCs from these efferent fibres. Whole-cell voltage-clamp recording revealed that low frequency (0.25-1 Hz) electrical stimulation produced evoked inhibitory postsynaptic currents (IPSCs) at a relatively high fraction of failures (65%) and with mean amplitudes of about -20 pA at -90 mV, corresponding to a quantum content of approximately 1. Evoked IPSCs had biphasic waveforms at -60 mV, were blocked reversibly by alpha-bungarotoxin and strychnine and are most likely mediated by the alpha9/alpha10 acetylcholine receptor, with subsequent activation of calcium-dependent potassium (SK2) channels. Paired pulse stimulation with intervals of 10-100 ms caused facilitation of 200-300% in the mean IPSC amplitude. A train of 10 pulses with an interpulse interval of 25 ms produced increasingly larger IPSCs with maximum amplitudes greater than -100 pA due to facilitation and summation throughout the train. Repetitive efferent stimulation at 5 Hz or higher hyperpolarized IHCs by 5-10 mV and could completely prevent the generation of calcium action potentials normally evoked by depolarizing current injection.

Biophysical and pharmacological characterization of nicotinic cholinergic receptors in rat cochlear inner hair cells

Before the onset of hearing, a transient efferent innervation is found on inner hair cells (IHCs). This synapse is inhibitory and mediated by a nicotinic cholinergic receptor (nAChR) probably formed by the alpha9 and alpha10 subunits. We analysed the pharmacological and biophysical characteristics of the native nAChR using whole-cell recordings from IHCs in acutely excised apical turns of the rat organ of Corti. Nicotine did not activate but rather blocked the acetylcholine (ACh)-evoked currents with an IC50 of 1 +/- 0.1 microM. Antagonists of non-cholinergic receptors such as strychnine, bicuculline and ICS-205930 blocked ACh-evoked responses with an IC50 of 8.6 +/- 0.8 nM, 59 +/- 4 nM and 0.30 +/- 0.02 microM, respectively. The IHC nAChR was both permeable to (P(Ca)/P(Na) = 8 +/- 0.9) and modulated by external Ca2+. ACh-evoked currents were potentiated by Ca2+ up to 500 microM but were reduced by higher concentrations of this cation. Ba2+ mimicked the effects of Ca2+ whereas Mg2+ only blocked these currents. In addition, elevation of extracellular Ca2+ reduced the amplitude of spontaneous synaptic currents without affecting their time course. The receptor had an EC50 for ACh of 60.7 +/- 2.8 microM in 0.5 mM Ca2+. In the absence of Ca2+, the EC50 for ACh increased, suggesting that potentiation by Ca2+ involves changes in the apparent affinity for the agonist. These pharmacological and biophysical characteristics of the IHC nAChR closely resemble those of the recombinant alpha9alpha10 nAChR, reinforcing the hypothesis that the functional nAChR at the olivocochlear efferent-IHC synapse is composed of both the alpha9 and alpha10 subunits.

Developmental regulation of nicotinic synapses on cochlear inner hair cells

In the mature cochlea, inner hair cells (IHCs) transduce acoustic signals into receptor potentials, communicating to the brain by synaptic contacts with afferent fibers. Before the onset of hearing, a transient efferent innervation is found on IHCs, mediated by a nicotinic cholinergic receptor that may contain both alpha9 and alpha10 subunits. Calcium influx through that receptor activates calcium-dependent (SK2-containing) potassium channels. This inhibitory synapse is thought to disappear after the onset of hearing [after postnatal day 12 (P12)]. We documented this developmental transition using whole-cell recordings from IHCs in apical turns of the rat organ of Corti. Acetylcholine elicited ionic currents in 88-100% of IHCs between P3 and P14, but in only 1 of 11 IHCs at P16-P22. Potassium depolarization of efferent terminals caused IPSCs in 67% of IHCs at P3, in 100% at P7-P9, in 93% at P10-P12, but in only 40% at P13-P14 and in none of the IHCs tested between P16 and P22. Earlier work had shown by in situ hybridization that alpha9 mRNA is expressed in adult IHCs but that alpha10 mRNA disappears after the onset of hearing. In the present study, antibodies to alpha10 and to the associated calcium-dependent (SK2) potassium channel showed a similar developmental loss. The correlated expression of these gene products with functional innervation suggests that Alpha10 and SK2, but not Alpha9, are regulated by synaptic activity. Furthermore, this developmental knock-out of alpha10, but not alpha9, supports the hypothesis that functional nicotinic acetylcholine receptors in hair cells are heteromers containing both these subunits.

Ca2+ permeability of nicotinic acetylcholine receptors from rat dorsal root ganglion neurones

Ca2+ entry through neuronal nicotinic ACh receptors (nAChRs) modulates many biological processes in nervous tissue. In order to study the functional role of nAChRs in peripheral sensory signalling, we measured their Ca2+ permeability in rat dorsal root ganglion (DRG) neurones, and analysed the effects of nAChR-mediated Ca2+ influx on the function of the vanilloid receptor TRPV1. The fractional Ca2+ current (P(f), i.e. the percentage of current carried by Ca2+ ions) flowing through nAChR channels was measured by Ca2+ imaging fluorescence microscopy in combination with the patch-clamp technique. Functional nAChRs were expressed in a subset of adult DRG neurones (about 24% of the cells), typically with small to medium size as measured by their capacitance (40 +/- 3 pF). In most cells, ACh evoked slowly desensitizing currents, insensitive to methyllycaconitine (MLA, 10 nm), a potent antagonist of homomeric nAChRs. Fast decaying currents, probably mediated by alpha7*-nAChRs (i.e. native alpha7-containing nAChRs), were observed in 15% of ACh-responsive cells, in which slowly decaying currents, mediated by heteromeric nAChRs, were simultaneously present. The nAChRs of adult DRG neurones exhibited a P(f) value of 2.2 +/- 0.6% in the presence of MLA and 1.9 +/- 0.6% (P > 0.1) in the absence of MLA, indicating that homomeric MLA-sensitive nAChRs do not contribute to Ca2+ entry into adult DRG neurones. Conversely, 10% of neonatal DRG neurones showed ACh-evoked currents completely blocked by MLA. In these neurones, nAChRs showed a larger P(f) value (9.5 +/- 1.5%), indicating the expression of bona fide alpha7*-nAChRs. Finally, we report that Ca2+ influx through nAChRs in adult DRG neurones negatively modulated the TRPV1-mediated responses, representing a possible mechanism underlying the analgesic properties of nicotinic agonists on sensory neurones.

A Family of Acetylcholine-gated Chloride Channel Subunits in Caenorhabditis elegans

The genome of the nematode Caenorhabditis elegans encodes a surprisingly large and diverse superfamily of genes encoding Cys loop ligand-gated ion channels. Here we report the first cloning, expression, and pharmacological characterization of members of a family of anion-selective acetylcholine receptor subunits. Two subunits, ACC-1 and ACC-2, form homomeric channels for which acetylcholine and arecoline, but not nicotine, are efficient agonists. These channels are blocked by d-tubocurarine but not by alpha-bungarotoxin. We provide evidence that two additional subunits, ACC-3 and ACC-4, interact with ACC-1 and ACC-2. The acetylcholine-binding domain of these channels appears to have diverged substantially from the acetylcholine-binding domain of nicotinic receptors.

Nicotine signals through muscle-type and neuronal nicotinic acetylcholine receptors in both human bronchial epithelial cells and airway fibroblasts

BACKGROUND

Non-neuronal cells, including those derived from lung, are reported to express nicotinic acetylcholine receptors (nAChR). We examined nAChR subunit expression in short-term cultures of human airway cells derived from a series of never smokers, ex-smokers, and active smokers.

METHODS AND RESULTS

At the mRNA level, human bronchial epithelial (HBE) cells and airway fibroblasts expressed a range of nAChR subunits. In multiple cultures of both cell types, mRNA was detected for subunits that constitute functional muscle-type and neuronal-type pentomeric receptors. Two immortalized cell lines derived from HBE cells also expressed muscle-type and neuronal-type nAChR subunits. Airway fibroblasts expressed mRNA for three muscle-type subunits (alpha1, delta, and epsilon) significantly more often than HBE cells. Immunoblotting of HBE cell and airway fibroblast extracts confirmed that mRNA for many nAChR subunits is translated into detectable levels of protein, and evidence of glycosylation of nAChRs was observed. Some minor differences in nAChR expression were found based on smoking status in fibroblasts or HBE cells. Nicotine triggered calcium influx in the immortalized HBE cell line BEAS2B, which was blocked by alpha-bungarotoxin and to a lesser extent by hexamethonium. Activation of PKC and MAPK p38, but not MAPK p42/44, was observed in BEAS2B cells exposed to nicotine. In contrast, nicotine could activate p42/44 in airway fibroblasts within five minutes of exposure.

CONCLUSIONS

These results suggest that muscle-type and neuronal-type nAChRs are functional in airway fibroblasts and HBE cells, that prior tobacco exposure does not appear to be an important variable in nAChR expression, and that distinct signaling pathways are observed in response to nicotine.

Differential regulation of keratinocyte chemokinesis and chemotaxis through distinct nicotinic receptor subtypes

Nicotinergic agents can act as both chemokines and chemoattractants for cell migration. Epidermal keratinocytes both synthesize acetylcholine and use it as a paracrine and autocrine regulator of cell motility. To gain a mechanistic insight into nicotinergic control of keratinocyte motility, we determined types of nicotinic acetylcholine receptors and signaling pathways regulating keratinocyte chemokinesis and chemotaxis, using respective modifications of the agarose gel keratinocyte outgrowth assay. Random migration of keratinocytes was significantly (P<0.05) inhibited by hemicholinum-3, a metabolic inhibitor of acetylcholine synthesis, as well as by the alpha-conotoxins MII and AuIB, preferentially blocking alpha3-containing nicotinic acetylcholine receptors. The use of antisense oligonucleotides specific for nicotinic-acetylcholine-receptor subunits and knockout mice demonstrated pivotal role for the alpha3beta2 channel in mediating acetylcholine-dependent chemokinesis. Signaling pathways downstream of alpha3beta2 included activation of the protein-kinase-C isoform delta and RhoA-dependent events. The nicotinergic chemotaxis of keratinocytes was most pronounced towards the concentration gradient of choline, a potent agonist of alpha7 nicotinic acetylcholine receptor. The alpha7-preferring antagonist alpha-bungarotoxin significantly (P<0.05) diminished keratinocyte chemotaxis, further suggesting a central role for the alpha7 nicotinic acetylcholine receptor. This hypothesis was confirmed in experiments with anti-alpha7 antisense oligonucleotides and alpha7-knockout mice. The signaling pathway mediating alpha7-dependent keratinocyte chemotaxis included intracellular calcium, activation of calcium/calmodulin-dependent protein-kinase II, conventional isoforms of protein-kinase C, phosphatidylinositol-3-kinase and engagement of Rac/Cdc42. Redistribution of alpha7 immunoreactivity to the leading edge of keratinocytes upon exposure to a chemoattractant preceded crescent shape formation and directional migration. Application of high-resolution deconvolution microscopy demonstrated that, on the cell membrane of keratinocytes, the nicotinic acetylcholine receptor subunits localize with the integrin beta1. The obtained results demonstrate for the first time that alpha3 and alpha7 nicotinic acetylcholine receptors regulate keratinocyte chemokinesis and chemotaxis, respectively, and identify signaling pathways mediating these functions, which has clinical implications for wound healing and control of cancer metastases.

Nicotinic AChR in subclassified capsaicin-sensitive and -insensitive nociceptors of the rat DRG

Nociceptive cells of the dorsal root ganglion (DRG) were subclassified, in vitro, according to patterns of voltage-activated currents. The distribution and form of nicotinic ACh receptors (nAChRs) were determined. nAChRs were present on both capsaicin-sensitive and -insensitive nociceptors but were not universally present in unmyelinated nociceptors. In contrast, all A delta nociceptors (types 4, 6, and 9) expressed slowly decaying nAChR. Three major forms of nicotinic currents were identified. Specific agonists and antagonists were used to demonstrate the presence of alpha7 in two classes of capsaicin-sensitive, unmyelinated nociceptors (types 2 and 8). In type 2 cells, alpha7-mediated currents were found in isolation. Whereas alpha7 was co-expressed with other nAChR in type 8 cells. These were the only classes in which alpha7 was identified. Other nociceptive classes expressed slowly decaying currents with beta4 pharmacology. Based on concentration response curves formed by nicotinic agonists [ACh, nicotine, dimethyl phenyl piperazinium (DMPP), cytisine] evidence emerged of two distinct nAChR differentially expressed in type 4 (alpha3beta4) and types 5 and 8 (alpha3beta4 alpha5). Although identification could not be made with absolute certainty, patterns of potency (type 4: DMPP > cytisine > nicotine = ACh; type 5 and type 8: DMPP = cytisine > nicotine = ACh) and efficacy provided strong support for the presence of two distinct channels based on an alpha3beta4 platform. Studies conducted on one nonnociceptive class (type 3) failed to reveal any nAChR. After multiple injections of Di-I (1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) into the hairy skin of the hindlimb, we identified cell types 2, 4, 6, 8, and 9 as skin nociceptors that expressed nicotinic receptors. We conclude that at least three nicotinic AChR are diversely distributed into discrete subclasses of nociceptors that innervate hairy skin.

Pharmacology of the neuronal nicotinic acetylcholine receptor of cultured Kenyon cells of the honeybee, Apis mellifera

We investigated the pharmacology of the nicotinic acetylcholine receptor of honeybee Kenyon cells, a subset of olfactory interneurons, which are crucial for olfactory learning and memory. Whole-cell currents were recorded using patch-clamp techniques. Pressure application of agonists induced inward currents in cultured Kenyon cells at holding potentials of -110 mV. Acetylcholine or carbamylcholine were full agonists, nicotine, epibatidine and cytisine were only partial agonists. Coapplications of these partial agonists with acetylcholine reduced the current amplitude. The most efficient antagonists were dihydroxy-beta-erythroidine (EC(50)=0.5 pmol x l(-1)) and methyllycaconitine (EC(50)=24 pmol x l(-1)). The open channel blocker mecamylamine, d-tubocurarine and hexamethonium were rather weak blockers of the honeybee nicotinic response. Bath applications of the muscarinic antagonist atropine inhibited nicotinic currents dependent on concentration (EC(50)=24.3 micromol x l(-1)). Muscarine, pilocarpine or oxotremorine (1 mmol x l(-1)) did not induce any measurable currents. The non-cholinergic drugs strychnine, bicuculline and picrotoxin partially and reversibly blocked the acetylcholine-induced currents. Our results indicate the expression of only one nicotinic acetylcholine receptor subtype in cultured Kenyon cells. Muscarinic as well as non-cholinergic antagonists also inhibit the receptor function, distinguishing the honeybee nicotinic receptor from the "typical" nicotinic receptor of vertebrates and from many described insects receptors.

Nicotinic acetylcholine receptor subtypes in nociceptive dorsal root ganglion neurons of the adult rat

Stimulation of nicotinic acetylcholine receptors (nAChR) excites peripheral sensory nerve fibres, but also exert antinociceptive effects. The differences in these nAChR-mediated effects could be related to the expression of different nAChR subtypes located on nociceptive neurons. In the present study, we focused on the recently described alpha 10-nAChR subunit, and on alpha 4 and alpha 7 subunits, which are the most abundant subunits in the central nervous system. In nociceptive neurons from thoracic and lumbar dorsal root ganglia (DRG), nAChR subunits were found at transcriptional (RT-PCR), translational (immunohistochemistry) and functional levels. Cultured DRG neurons express mRNA for the subunits alpha 2-7 and alpha 10. The alpha-subunit proteins 4, 7 and 10 were colocalised in virtually all nociceptive neurons that were identified by immunoreactivity for the vanilloid receptor TRPV-1. These findings were corroborated by current recordings and calcium measurements, which revealed excitatory inward currents and calcium responses in capsaicin sensitive neurons.

Synergistic control of keratinocyte adhesion through muscarinic and nicotinic acetylcholine receptor subtypes

The biological mechanisms involved in initiating, coordinating, and ultimately terminating cell-cell adhesion in the stratified epithelium are not well understood at present. This study was designed to elucidate the roles of the muscarinic M3, the nicotinic alpha3, and the mixed muscarinic-nicotinic alpha9 acetylcholine receptors in physiologic control of keratinocyte adhesion. Both muscarinic and nicotinic antagonists caused keratinocyte detachment and reversibly increased the permeability of keratinocyte monolayers, indicative of the involvement of both muscarinic and nicotinic pathways in the cholinergic control of keratinocyte adhesion. Since phosphorylation of adhesion proteins plays an important role in rapid assembly and disassembly of intercellular junctions, we measured muscarinic and nicotinic effects on phosphorylation of keratinocyte adhesion molecules. The phosphorylation levels of E-cadherin, beta-catenin, and gamma-catenin increased following pharmacological blockage of muscarinic receptors. Long-term blocking of alpha3, alpha9, and M3 receptor signaling pathways with antisense oligonucleotides resulted in cell-cell detachment and changes in the expression levels of E-cadherin, beta-catenin, and gamma-catenin in cultured human keratinocytes. Simultaneous inhibition of several receptor subtypes with a mixture of antisense oligonucleotides produced intensified abnormalities with cell adhesion. Moreover, altered cell-cell adhesion was found in the stratified epithelium of alpha3, alpha9, and M3 receptor knockout mice. Keratinocytes from these mice exhibited abnormal expression of adhesion molecules at both the protein and the mRNA levels. Thus, our data indicate that the alpha3, alpha9, and M3 acetylcholine receptors play key roles in regulating in a synergistic mode keratinocyte adhesion, most probably by modulating cadherin and catenin levels and activities. These findings may aid in the development of novel methods useful for the treatment of skin adhesion diseases and tumor metastasis.

Pharmacological Properties of α9α10 Nicotinic Acetylcholine Receptors Revealed by Heterologous Expression of Subunit Chimeras

The nicotinic acetylcholine receptor (nAChR) alpha9 and alpha10 subunits are expressed primarily within hair cells of the inner ear and have been implicated in auditory processing. Although functional recombinant nAChRs generated by the coexpression of alpha9 and alpha10 in Xenopus laevis oocytes have been described previously, there have been no reports of the successful heterologous expression of alpha9alpha10 nAChRs in cultured cell lines. In this study, subunit chimeras (alpha9chi and alpha10chi) have been constructed that contain the extracellular, ligand binding domain of the alpha9 or alpha10 subunits fused to the C-terminal domain of the 5-hydroxytryptamine type 3A (5HT3A) subunit. Specific high-affinity binding of the nicotinic radioligand [3H]methyllycaconitine was detected in membrane preparations of mammalian cells transfected with alpha9chi or alpha10chi alone, but significantly higher levels of binding were detected when alpha9chi and alpha10chi were cotransfected, providing evidence of a requirement for coassembly of alpha9 and alpha10 for the efficient formation of a nicotinic binding site. The pharmacological profile of alpha9chialpha10chi receptors, determined by equilibrium radioligand binding studies, is broadly similar to that determined previously by electrophysiological studies conducted with native and recombinant alpha9alpha10 nAChRs. In agreement with evidence that alpha9alpha10 nAChRs exhibit an atypical pharmacological profile, we have identified specific high-affinity binding of several non-nicotinic ligands including strychnine (a glycine receptor antagonist), bicuculline (a GABAA receptor antagonist), and atropine (a muscarinic acetylcholine receptor antagonist). Results have also been compared with radioligand binding data conducted with a previously described alpha7/5HT3A (alpha7chi) subunit chimera.

Functional expression and properties of a nicotinic α9/5-HT3A chimeric receptor

We describe the functional properties of a nicotinic alpha9/serotonin subtype 3A (5HT3A) chimeric receptor expressed in Xenopus laevis oocytes. The chimera preserved ligand-binding properties of alpha9 and channel properties of 5HT3A. Thus, it responded to acetylcholine in a concentration-dependent manner with an EC50 of 70 microM but not to serotonin. It was blocked by methyllycaconitine, strychnine, atropine and nicotine, with the same rank order of potency as alpha9 receptors. The current-voltage relationship of currents through the alpha9/5HT3A chimera was similar to that of the 5HT3A receptors. These results are an evidence of functional coupling between the ligand-binding and the channel domains of the chimeric receptor.

A pharmacologically distinct nicotinic ACh receptor is found in a subset of frog semicircular canal hair cells

Frog vestibular organs are endowed with a prominent cholinergic efferent innervation whose stimulation results in several different effects, thereby suggesting diversity in the expression of postsynaptic acetylcholine (ACh) receptors. The application of ACh can mimic efferent stimulation in producing both an inhibition and a facilitation of afferent discharge which are thought to be mediated by at least two distinct ACh receptors present on vestibular hair cells, i.e., alpha9-containing nicotinic receptors (alpha9nAChR) and muscarinic receptors (mAChR), respectively. Using patch-clamp and multiunit vestibular afferent recordings, we demonstrate the presence of an additional excitatory hair cell nicotinic ACh receptor pharmacologically distinct from both alpha9nAChR and mAChR. In order of increasing potency, this distinct receptor was activated by ACh, carbachol, and particularly by the selective nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP). This DMPP-sensitive nicotinic receptor (RDMPP) was antagonized by the classic nicotinic antagonist d-tubocurarine, but refractory to strychnine, atropine, and propylbenzilylcholine mustard, at concentrations that completely block alpha9nAChR and/or mAChR. Activation of RDMPP on application of ACh or DMPP to a subpopulation of isolated posterior semicircular canal (SCC) hair cells resulted in a large depolarization (18.0 +/- 1.2 mV). The current underlying this depolarization was typically small (80.1 +/- 21.6 pA) and showed an inward rectification starting around -45 mV. Given their respective EC50s (47 nM vs. 20 microM), RDMPP was nearly 400 times more sensitive to ACh than alpha9nAChR and thus responded to concentrations of ACh considered too low to be effective at stimulating alpha9nAChR. Despite this remarkable sensitivity, exogenous ACh readily stimulated the mAChR in the intact posterior SCC preparation but failed to activate RDMPP unless the acetylcholinesterase inhibitor physostigmine was present, or high concentrations of ACh were used (>3 mM). In frog, RDMPP most likely underlies the rapid excitatory response seen during efferent stimulation.

Interactions of atropine with heterologously expressed and native alpha 3 subunit-containing nicotinic acetylcholine receptors

(1) Atropine, a classical muscarinic antagonist, has been reported previously to inhibit neuronal nicotinic acetylcholine receptors (nAChRs). In the present study, the action of atropine has been examined on alpha3beta4 receptors expressed heterologously in Xenopus oocytes and native nAChRs in medial habenula neurons. (2) At concentrations of atropine often used to inhibit muscarinic receptors (1 micro M), responses induced by near-maximal nicotine concentrations (100 micro M) at negative holding potentials (-65 mV) are inhibited (14-30%) in a reversible manner in both alpha4 and alpha3 subunit-containing heteromeric nAChRs. Half-maximal effective concentrations (IC(50) values) for atropine inhibition are similar for the four classes of heteromeric receptors studied (4-13 micro M). (3) For alpha3beta4 nAChRs in oocytes, inhibition by atropine (10 micro M) is not overcome at higher concentrations of agonist, and is increased with membrane hyperpolarization. These results are consistent with non-competitive antagonism--possibly ion channel block. (4) At low concentrations of both nicotine (10 micro M) and atropine (<10 micro M), potentiation ( approximately 25%) of alpha3beta4 nAChR responses in oocytes is observed. The relative balance between potentiation and inhibition is dependent upon membrane potential. (5) In rat medial habenula (MHb) neurons, atropine (0.3-3.0 micro M) inhibited nicotine-induced responses in both a concentration and membrane potential-dependent manner (at -40 mV, IC(50)=4 micro M), similar to the effects on alpha3beta4-nAChRs in oocytes. However, unlike heterologously expressed receptors, potentiation was barely detectable at depolarized membrane potentials using low concentrations of nicotine (3-10 micro M). Conversely, the weak agonist, choline (1-3 mM) was observed to augment responses of MHb nAChRs.

Coexpression of alpha 9 and alpha 10 nicotinic acetylcholine receptors in rat dorsal root ganglion neurons

Previous binding studies have suggested the presence of a so far unknown nicotinic acetylcholine receptor subunit in dorsal root ganglia (Pugh et al., 1995). Here, we investigated whether the most recently identified subunit, alpha10, and its potential interaction partner, alpha9 (Elgoyhen et al., 2001), are expressed in these ganglia. All neurons of rat dorsal root ganglia, but no glial cells, expressed both alpha9 and alpha10 mRNA in in situ hybridization, and exhibited alpha10 immunoreactivity using a newly raised antibody. These findings were confirmed by RT-PCR and western blotting. The data show that dorsal root ganglion neurons coexpress alpha9 and alpha10 nicotinic receptor subunits, thereby providing the first example of neuronal expression of this receptor subunit pair.

Unconventional ligands and modulators of nicotinic receptors

Evidence gathered from epidemiologic and behavioral studies have indicated that neuronal nicotinic receptors (nAChRs) are intimately involved in the pathogenesis of a number of neurologic disorders, including Alzheimer's disease, Parkinson's disease, and schizophrenia. In the mammalian brain, neuronal nAChRs, in addition to mediating fast synaptic transmission, modulate fast synaptic transmission mediated by the major excitatory and inhibitory neurotransmitters glutamate and GABA, respectively. Of major interest, however, is the fact that the activity of the different subtypes of neuronal nAChR is also subject to modulation by substances of endogenous origin such as choline, the tryptophan metabolite kynurenic acid, neurosteroids, and beta-amyloid peptides and by exogenous substances, including the so-called nicotinic allosteric potentiating ligands, of which galantamine is the prototype, and psychotomimetic drugs such as phencyclidine and ketamine. The present article reviews and discusses the effects of unconventional ligands on nAChR activity and briefly describes the potential benefits of using some of these compounds in the treatment of neuropathologic conditions in which nAChR function/expression is known to be altered.

The alpha9alpha10 nicotinic acetylcholine receptor is permeable to and is modulated by divalent cations

The native cholinergic receptor that mediates synaptic transmission between olivocochlear fibers and outer hair cells of the cochlea is permeable to Ca(2+) and is thought to be composed of both the alpha 9 and the alpha 10 cholinergic nicotinic subunits. The aim of the present work was to study the permeability of the recombinant alpha 9 alpha 10 nicotinic acetylcholine receptor to Ca(2+), Ba(2+) and Mg(2+) and its modulation by these divalent cations. Experiments were performed, by the two-electrode voltage-clamp technique, in Xenopus laevis oocytes injected with alpha 9 and alpha 10 cRNA. The relative divalent to monovalent cation permeability was high ( approximately 10) for Ca(2+), Ba(2+) and Mg(2+). Currents evoked by acetylcholine (ACh) were potentiated by either Ca(2+) or Ba(2+) up to 500 microM but were blocked by higher concentrations of these cations. Potentiation by Ca(2+) was voltage-independent, whereas blockage was stronger at hyperpolarized than at depolarized potentials. Mg(2+) did not potentiate but it blocked ACh-evoked currents (IC(50)=0.38 mM). In the absence of Ca(2+), the EC(50) for ACh was higher (48 microM) than that obtained with 1.8 mM Ca(2+) (14.3 microM), suggesting that potentiation by Ca(2+) involves changes in the apparent affinity of the alpha 9 alpha 10 receptor for ACh.

Human bronchial epithelial and endothelial cells express alpha7 nicotinic acetylcholine receptors

The epithelial or endothelial cells that line the human bronchi and the aorta express nicotinic acetylcholine receptors (nAChRs) of alpha3 subtypes. We report here that human bronchial epithelial cells (BEC) and aortic endothelial cells (AEC) express also the nAChR alpha7 subunit, which forms functional nAChRs. Polymerase chain reaction and in situ hybridization experiments detected alpha7 subunit mRNA in cultured human BEC and AEC and in sections of rat trachea. The binding of radiolabeled alpha-bungarotoxin revealed a few thousand binding sites per cell in cultured human BEC and human and bovine AEC. Western blot and immunohistochemistry experiments demonstrated that cultured BEC and AEC express a protein(s) recognized by anti-alpha7 antibodies. Whole-cell patch-clamp studies of cultured human BEC demonstrated the presence of fast-desensitizing currents activated by choline and nicotine that were blocked reversibly by methyllycaconitine (1 nM) and irreversibly by alpha-bungarotoxin (100 nM), consistent with the expression of functional alpha7 nAChRs. In some cells, choline activated also slowly decaying currents, confirming previous reports that BEC express functional alpha3beta4 nAChRs. Exposure of cultured BEC to nicotine (1 microM) for 3 days up-regulated functional alpha7 and alpha3 nAChRs, as indicated by the increased number of cells responding to acetylcholine and choline, with both fast-desensitizing currents, which were blocked irreversibly by alpha-bungarotoxin, and with slowly desensitizing currents, which are alpha-bungarotoxin-insensitive currents. The presence of alpha7 nAChRs in BEC and AEC suggests that some toxic effects of tobacco smoke could be mediated through these nicotine-sensitive receptors.

A glia-derived acetylcholine-binding protein that modulates synaptic transmission

There is accumulating evidence that glial cells actively modulate neuronal synaptic transmission. We identified a glia-derived soluble acetylcholine-binding protein (AChBP), which is a naturally occurring analogue of the ligand-binding domains of the nicotinic acetylcholine receptors (nAChRs). Like the nAChRs, it assembles into a homopentamer with ligand-binding characteristics that are typical for a nicotinic receptor; unlike the nAChRs, however, it lacks the domains to form a transmembrane ion channel. Presynaptic release of acetylcholine induces the secretion of AChBP through the glial secretory pathway. We describe a molecular and cellular mechanism by which glial cells release AChBP in the synaptic cleft, and propose a model for how they actively regulate cholinergic transmission between neurons in the central nervous system.

Programmed cell death of keratinocytes culminates in apoptotic secretion of a humectant upon secretagogue action of acetylcholine

The programmed cell death of the stratified squamous epithelial cells comprising human epidermis culminates in abrupt transition of viable granular keratinocytes (KC) into dead corneocytes sloughed by the skin. The granular cell-corneocyte transition is associated with a loss in volume and dry cell weight but the mechanism for and biological significance of this form of keratinocyte apoptosis remain obscure. We show that terminally differentiated KC extrude into the intercellular spaces of living epidermis the cytoplasmic buds containing randomly congregated components of the cytosol as well as filaggrin, a precursor of the natural moisturizing factor. The discharge of secretory product is reminiscent of holocrine secretion, suggesting the term ‘apoptotic secretion’ for this novel, essential step in the process of cornification. The secretory product may become a part of the glycocalyx (a.k.a. ‘intercellular cement substance’ of epidermis) and serve as a humectant that counterbalances the osmotic pressure imposed by the natural moisturizing factor located in the stratum corneum comprised by corneocytes. The apoptotic secretion commences upon secretagouge action of acetylcholine which is synthesized and released by KC. A combination of a cholinergic nicotinic agonist and a muscarinic antagonist which increases intracellular calcium levels is required to trigger the apoptotic secretion. Analysis of the relative amounts of cholinergic enzymes and receptors expressed by KC capable of secretion and the pharmacological profiles of secretion regulation revealed an upward concentration gradient of free acetylcholine in epidermis which may provide for its unopposed secretagogue action via the m1 muscarinic and the (α)7, and (α)9 nicotinic receptor types expressed by KC at the latest stage of their development in the epidermis.

alpha10: a determinant of nicotinic cholinergic receptor function in mammalian vestibular and cochlear mechanosensory hair cells

We report the cloning and characterization of rat alpha10, a previously unidentified member of the nicotinic acetylcholine receptor (nAChR) subunit gene family. The protein encoded by the alpha10 nAChR subunit gene is most similar to the rat alpha9 nAChR, and both alpha9 and alpha10 subunit genes are transcribed in adult rat mechanosensory hair cells. Injection of Xenopus laevis oocytes with alpha10 cRNA alone or in pairwise combinations with either alpha2-alpha6 or beta2-beta4 subunit cRNAs yielded no detectable ACh-gated currents. However, coinjection of alpha9 and alpha10 cRNAs resulted in the appearance of an unusual nAChR subtype. Compared with homomeric alpha9 channels, the alpha9alpha10 nAChR subtype displays faster and more extensive agonist-mediated desensitization, a distinct current-voltage relationship, and a biphasic response to changes in extracellular Ca(2+) ions. The pharmacological profiles of homomeric alpha9 and heteromeric alpha9alpha10 nAChRs are essentially indistinguishable and closely resemble those reported for endogenous cholinergic eceptors found in vertebrate hair cells. Our data suggest that efferent modulation of hair cell function occurs, at least in part, through heteromeric nAChRs assembled from both alpha9 and alpha10 subunits.