Expression of nicotinic acetylcholine receptor subtypes in brain and retina

Nicotinic acetylcholine receptors in a songbird brain

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that mediate fast synaptic transmission and cell signaling, which contribute to learning, memory, and the execution of motor skills. Birdsong is a complex learned motor skill in songbirds. Although the existence of 15 nAChR subunits has been predicted in the avian genome, their expression patterns and potential contributions to song learning and production have not been comprehensively investigated. Here, we cloned all the 15 nAChR subunits (ChrnA1-10, B2-4, D, and G) from the zebra finch brain and investigated the mRNA expression patterns in the neural pathways responsible for the learning and production of birdsong during a critical period of song learning. Although there were no detectable hybridization signals for ChrnA1, A6, A9, and A10, the other 11 nAChR subunits were uniquely expressed in one or more major subdivisions in the song nuclei of the songbird brain. Of these 11 subunits, ChrnA3-5, A7, and B2 were differentially regulated in the song nuclei compared with the surrounding anatomically related regions. ChrnA5 was upregulated during the critical period of song learning in the lateral magnocellular nucleus of the anterior nidopallium. Furthermore, single-cell RNA sequencing revealed ChrnA7 and B2 to be the major subunits expressed in neurons of the vocal motor nuclei HVC and robust nucleus of the arcopallium, indicating the potential existence of ChrnA7-homomeric and ChrnB2-heteromeric nAChRs in limited cell populations. These results suggest that relatively limited types of nAChR subunits provide functional contributions to song learning and production in songbirds.

Nicotinic Acetylcholine Receptors of PC12 Cells

Nicotinic acetylcholine receptors (nAChRs) have gained much attention in the scientific community since they play a significant role in multiple physiological and pathophysiological processes. Multiple approaches to study the receptors exist, with characterization of the receptors' functionality at a single cellular level using cell culturing being one of them. Derived from an adrenal medulla tumor, PC12 cells express nicotinic receptor subunits and form functional nicotinic receptors. Thus, the cells offer a convenient environment to address questions related to the functionality of the receptors. The review summarizes the findings on nicotinic receptors' expression and functions which were conducted using PC12 cells. Specific focus is given to α3-containing receptors as well as α7 receptor. Critical evaluation of findings is provided alongside insights into what can still be learned about nAChRs, using PC12 cells.

Heterogeneous organization and connectivity of the chicken auditory thalamus (Gallus gallus)

The auditory ascending system contains parallel pathways in vertebrate brains. In chickens (Gallus gallus), three pathways arise from nucleus laminaris (NL), nucleus angularis (NA), and regio intermedius (RI) in the brainstem, innervating three subdivisions of the nucleus mesencephalicus lateralis pars dorsalis (MLd) in the midbrain. The current study reveals the segregation of these pathways in their subsequent projections to the nucleus ovoidalis (Ov) in the thalamus. Based on cytoarchitecture and myelin distribution, we identified seven Ov subregions, including five neuronal clusters within the Ov proper, the nucleus semilunaris parovoidalis (SPO), and the circum-ovoidalis (cOv). Immunocytochemistry further revealed that a ventromedial cluster of the Ov proper (Ovvm) contains unique cell types expressing α8 subunit nicotinic acetylcholine receptor, while SPO and cOv are characterized with expression of calcitonin-gene-related peptide and cholecystokinin. Tract tracing studies demonstrated that Ovvm is a major target of the NL-recipient zone of MLd, while the RI-recipient zone of MLd predominantly projects to a ventrolateral cluster of the Ov proper. Afferent inputs to the remaining regions of the Ov proper mostly arise from the NA-recipient zone of MLd. SPO and cOv receive a projection from the surrounding areas of MLd, named the nucleus intercollicularis. Importantly, the Ov proper, SPO and cOv all project to the Field L2 in the forebrain, the avian auditory cortex. Taken together, these results demonstrate that the avian auditory thalamus is a structurally and functionally heterogeneous structure, implicating an important role in generating novel representations for specific acoustic features.

Expression of Nicotinic Acetylcholine Receptor α4 and β2 Subunits on Direction-Selective Retinal Ganglion Cells in the Rabbit

The direction selectivity of the retina is a distinct mechanism that is critical function of eyes for survival. The direction-selective retinal ganglion cells (DS RGCs) strongly respond to a preferred direction, but rarely respond to opposite direction or null directional visual stimuli. The DS RGCs are sensitive to acetylcholine, which is secreted from starburst amacrine cells (SACs) to the DS RGCs. Here, we investigated the existence and distribution of the nicotinic acetylcholine receptor (nAChR) α4 and β2 subunits on the dendritic arbors of the DS RGCs in adult rabbit retina using immunocytochemistry. The DS RGCs were injected with Lucifer yellow to identify their dendritic morphology. The double-labeled images of dendrites and nAChR subunits were visualized for reconstruction using high-resolution confocal microscopy. Although our results revealed that the distributional pattern of the nAChR subunits on the dendritic arbors of the DS RGCs was not asymmetric in the adult rabbit retina, the distribution of nAChR α4 and β2 subunits and molecular profiles of cholinergic inputs to DS RGCs in adult rabbit retina provide anatomical evidence for direction selectivity.

Retinal ganglion cell neuroprotection induced by activation of alpha7 nicotinic acetylcholine receptors

The α7nAChR agonist, PNU-282987, has previously been shown to have a neuroprotective effect against loss of retinal ganglion cells (RGCs) in an in vivo glaucoma model when the agent was injected into the vitreous chamber of adult Long Evans rat eyes. Here, we characterized the neuroprotective effect of PNU-282987 at the nerve fiber and retinal ganglion cell layer, determined that neuroprotection occurred when the agonist was applied as eye drops and verified detection of the agonist in the retina, using LC/MS/MS. To induce glaucoma-like conditions in adult Long Evans rats, hypertonic saline was injected into the episcleral veins to induce scar tissue and increase intraocular pressure. Within one month, this procedure produced significant loss of RGCs compared to untreated conditions. RGCs were quantified after immunostaining with an antibody against Thy 1.1 and imaged using a confocal microscope. In dose-response studies, concentrations of PNU-282987 were applied to the animal's right eye two times each day, while the left eye acted as an internal control. Eye drops of PNU-282987 resulted in neuroprotection against RGC loss in a dose-dependent manner using concentrations between 100 μM and 2 mM PNU-282987. LC/MS/MS results demonstrated that PNU-282987 was detected in the retina when applied as eye drops, relatively small amounts of PNU-282987 were measured in blood plasma and no PNU-282987 was detected in cardiac tissue. These results support the hypothesis that eye drop application of PNU-282987 can prevent loss of RGCs associated with glaucoma, which can lead to neuroprotective treatments for diseases that involve α7nAChRs.

Classification of nAChRβ2-immunoreactive retinal ganglion cells and their tectal projections in chicks

The relationship between the type of retinal ganglion cell (RGC) and the retinoreceptive layer of the tectum is investigated by the immunostaining of RGCs with nicotinic acetylcholine receptorβ2 (nAChRβ2) antibody and intracellular staining by DiI and also by anterograde degeneration and biotinylated dextran amine labeling of retinotectal fibers in chicks. The results strongly suggest that many of the RGCs that express immunoreactivity to nAChRβ2 send axons to tectal layer 7 and are mainly classified into the simple-type of Groups II and III, which contain the cells providing middle-sized to large dendritic fields with simple dendritic arborization. These nAChRβ2-immunoreactive RGCs receive visual information via the multiple sublayers of the inner plexiform layer.

Neuroprotection of rat retinal ganglion cells mediated through alpha7 nicotinic acetylcholine receptors

Glutamate-induced excitotoxicity is thought to play an important role in several neurodegenerative diseases in the central nervous system (CNS). In this study, neuroprotection against glutamate-induced excitotoxicity was analyzed using acetylcholine (ACh), nicotine and the α7 specific nicotinic acetylcholine receptor (α7 nAChR) agonist, N-[(3R)-1-azabicyclo[2.2.2]oct-3-yl]-4-chlorobenzamide hydrochloride (PNU-282987), in cultured adult rat retinal neurons. Adult Long Evans rat retinas were dissociated and retinal ganglion cells (RGCs) were isolated from all other retinal tissue using a two-step panning technique. Once isolated, RGCs were cultured under various pharmacological conditions to demonstrate excitotoxicity and neuroprotection against excitotoxicity. After 3 days, RGCs were immunostained with antibodies against the glycoprotein, Thy 1.1, counted and cell survival was assessed relative to control untreated conditions. 500 μM glutamate induced excitotoxicity in large and small RGCs in an adult rat dissociated culture. After 3 days in culture with glutamate, the cell survival of large RGCs decreased by an average of 48.16% while the cell survival of small RGCs decreased by an average of 42.03%. Using specific glutamate receptor agonists and antagonists, we provide evidence that the excitotoxic response was mediated through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainic acid (KA) and N-methyl-d-aspartate (NMDA) glutamate receptors through an apoptotic mechanism. However, the excitotoxic effect of glutamate on all RGCs was eliminated if cells were cultured for an hour with 10 μM ACh, 100 μM nicotine or 100 nM of the α7 nAChR agonist, PNU-282987, before the glutamate insult. Inhibition studies using 10nM methyllycaconitine (MLA) or α-bungarotoxin (α-Bgt) supported the hypothesis that neuroprotection against glutamate-induced excitotoxicity on rat RGCs was mediated through α7 nAChRs. In immunocytochemical studies, double-labeled experiments using antibodies against Thy 1.1 and α7 nAChR subunits demonstrated that both large and small RGCs contained α7 nAChR subunits. The data presented in this study support the hypothesis that ACh and nicotinic acetylcholine receptor (nAChR) agonists provide neuroprotection against glutamate-induced excitotoxicity in adult rat RGCs through activation of α7 nAChR subunits. These studies lay the groundwork required for analyzing the effect of specific α7 nAChR agonists using in vivo models of excitotoxicity. Understanding the type of ACh receptors involved in neuroprotection in the rat retina could ultimately lead to therapeutic treatment for any CNS disease that involves excitotoxicity.

Brain regional heterogeneity and toxicological mechanisms of organophosphates and carbamates

The brain is a well-organized, yet highly complex, organ in the mammalian system. Most investigators use the whole brain, instead of a selected brain region(s), for biochemical analytes as toxicological endpoints. As a result, the obtained data is often of limited value, since their significance is compromised due to a reduced effect, and the investigators often arrive at an erroneous conclusion(s). By now, a plethora of knowledge reveals the brain regional variability for various biochemical/neurochemical determinants. This review describes the importance of brain regional heterogeneity in relation to cholinergic and noncholinergic determinants with particular reference to organophosphate (OP) and carbamate pesticides and OP nerve agents.

RIC-3: a nicotinic acetylcholine receptor chaperone

RIC-3 is a transmembrane protein which acts as a molecular chaperone of nicotinic acetylcholine receptors (nAChRs). For some nAChR subtypes (such as homomeric alpha7 neuronal nAChRs), RIC-3 is required for efficient receptor folding, assembly and functional expression. In contrast, for other nAChR subtypes (such as heteromeric alpha4beta2 neuronal nAChRs) there have been reports that RIC-3 can both enhance and reduce levels of functional expression. There is also evidence that RIC-3 can modulate maturation of the closely related 5-hydroxytryptamine (5-HT) receptor (5-HT(3)R). As with heteromeric nAChRs, apparently contradictory results have been reported for the influence of RIC-3 on 5-HT(3)R maturation in different expression systems. Recent evidence indicates that these differences in RIC-3 chaperone activity may be influenced by the host cell, suggesting that other proteins may play an important role in modulating the effects of RIC-3 as a chaperone. RIC-3 was originally identified in the nematode Caenorhabditis elegans as the protein encoded by the gene ric-3 (resistance to inhibitors of cholinesterase) and has subsequently been cloned and characterized from mammalian and insect species. This review provides a brief history of RIC-3; from the identification of the ric-3 gene in C. elegans in 1995 to the more recent demonstration of its activity as a nAChR chaperone.

Structural and functional heterogeneity of nicotinic receptors

Three gene families of the ligand-gated ion channel gene superfamily encode proteins which bind cholinergic ligands: (1) nicotinic acetylcholine receptors (AChRs) from skeletal muscle, (2) AChRs from neurons, and (3) neuronal alpha-bungarotoxin-binding proteins (alpha BgtBPs). AChRs from muscles and nerves function as ACh-gated cation channels, but alpha BgtBPs do not appear to function in this way. A family of neuronal AChR subtypes has been characterized using monoclonal antibodies and cDNA probes. Neuronal AChRs exhibit sequence homologies with muscle AChRs, but differ in subunit composition, pharmacological and electrophysiological properties, and, in some cases, apparent functional roles. The genes that encode the subunits of the various purified AChR subtypes have been determined in several cases. Histological localization of AChR subunit mRNAs by in situ hybridization and of subunit proteins by immunohistochemistry is being conducted with increasing resolution. The subunit structure of alpha BgtBP is uncertain, but cDNAs have been identified for two subunits. Sequences of these cDNAs reveal that alpha BgtBPs are members of the ligand-gated ion channel gene family, and suggest that they could function as gated cation channels. Biochemical and molecular genetic approaches to studies of neuronal AChRs and related proteins are merging to provide a detailed description of a complex family of AChRs widely dispersed throughout the nervous system, which are probably important to many activities of the nervous system, but whose functional roles are not yet well characterized.

Optical studies of nicotinic acetylcholine receptor subtypes in the guinea-pig enteric nervous system

Nicotinic transmission in the enteric nervous system (ENS) is extensive, but the role of individual nicotinic acetylcholine receptor (nAChR) subtypes in the functional connectivity of its plexuses has been elusive. Using monoclonal antibodies (mAbs) against neuronal alpha3-, alpha4-, alpha3/alpha5-, beta2-, beta4- and alpha7-subunits, combined with radioimmunoassays and immunocytochemistry, we demonstrate that guinea-pig enteric ganglia contain all of these nAChR-subunits with the exception of alpha4, and so, differ from mammalian brain. This information alone, however, is insufficient to establish the functional role of the identified nAChR-subtypes within the enteric networks and, ultimately, their specific contributions to gastrointestinal physiology. We have used voltage-sensitive dyes and a high-speed CCD camera, in conjunction with specific antagonists to various nAChRs, to elucidate some of the distinct contributions of the individual subtypes to the behaviour of enteric networks. In the guinea-pig, the submucous plexus has the extraordinary advantage that it is virtually two-dimensional, permitting optical recording, with single cell resolution, of the electrical activity of all of its neurones. In this plexus, the block of alpha3beta2-, alpha3beta4- and/or alpha7-nAChRs always results in a decrease in the magnitude of the synaptic response. However, the magnitude of the fast excitatory post-synaptic potentials (epsps) evoked by electrical stimulation of a neighbouring ganglion varies from cell to cell, reflecting the differential expression of subunits already observed using mAbs, as well as the strengths of the activated synaptic inputs. At the same time, we observe that submucous neurones have a substantial mecamylamine (Mec)-insensitive (non-nicotinic) component to their fast epsps, which may point to the presence of purinergic or serotonergic fast epsps in this system. In the myenteric plexus, on the other hand, the antagonist-induced changes in the evoked synaptic response vary depending upon the location of the stimulating electrode with respect to the ganglion under study. The range of activity patterns that follows sequential pharmacological elimination of individual subtypes suggests that nAChRs may be capable of regulating the activity of both excitatory and inhibitory pathways, in a manner similar to that described in the central nervous system.

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.

Rabbit retinal ganglion cell responses to nicotine can be mediated by beta2-containing nicotinic acetylcholine receptors

Acetylcholine (ACh) affects the response properties of many retinal ganglion cells (GCs) through the activation of nicotinic acetylcholine receptors (nAChRs). To date there have been few studies directly correlating the expression of specific nAChR subtypes with the physiological and morphological characteristics of specific retinal GCs. This study was designed to correlate responses to nicotine application with immunohistochemical evidence of nAChR expression in physiologically and morphologically identified ganglion cells. Extracellular recordings were used to physiologically identify rabbit retinal GCs, based on responses to light stimulation. Cells were then tested for responses to nicotine application and/or for expression of nAChRs, as judged by immunoreactivity to mAb210, an nAChR antibody. The morphologies of many physiologically identified cells were also determined by dye injection. More than three-fourths of ganglion cells tested responded to nicotine application under cobalt-induced synaptic blockade. The nicotine sensitivity was consistent with nAChR immunoreactivity and was also correlated with specific morphological subgroups of GCs. Overall, approximately two-thirds of all physiologically identified GCs that were processed for immunohistochemistry displayed immunoreactivity. In total, 18 of 22 physiologically identified cells demonstrated both sensitivity to nicotine application under synaptic blockade and mAb210 immunoreactivity (mAb210-IR). Thus, mAb210-IR is likely to represent functional nAChRs that can modulate retinal information processing and visual functioning via direct excitation of a number of GC classes.

Selective effects of carbamate pesticides on rat neuronal nicotinic acetylcholine receptors and rat brain acetylcholinesterase

Effects of commonly used carbamate pesticides on rat neuronal nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes have been investigated using the two-electrode voltage clamp technique. The potencies of these effects have been compared to the potencies of the carbamates to inhibit rat brain acetylcholinesterase. The potency order of six carbamates to inhibit alpha4beta4 nicotinic receptors is fenoxycarb > EPTC > carbaryl, bendiocarb > propoxur > aldicarb with IC50 values ranging from 3 microM for fenoxycarb to 165 microM for propoxur and >1 mM for aldicarb. Conversely, the potency order of these carbamates to inhibit rat brain acetylcholinesterase is bendiocarb > propoxur, aldicarb > carbaryl > EPTC, fenoxycarb with IC50 values ranging from 1 microM for bendiocarb to 17 microM for carbaryl and > mM for EPTC and fenoxycarb. The alpha4beta2, alpha3beta4, and alpha3beta2 nicotinic acetylcholine receptors are inhibited by fenoxycarb, EPTC, and carbaryl with potency orders similar to that for alpha4beta4 receptors. Comparing the potencies of inhibition of the distinct subtypes of nicotinic acetylcholine receptors shows that the alpha3beta2 receptor is less sensitive to inhibition by fenoxycarb and EPTC. The potency of inhibition depends on the carbamate as well as on a combination of alpha and beta subunit properties. It is concluded that carbamate pesticides affect different subtypes of neuronal nicotinic receptors independently of acetylcholinesterase inhibition. This implicates that neuronal nicotinic receptors are additional targets for some carbamate pesticides and that these receptors may contribute to carbamate pesticide toxicology, especially after long-term exposure.

Identification of residues that confer alpha-conotoxin-PnIA sensitivity on the alpha 3 subunit of neuronal nicotinic acetylcholine receptors

Neuronal nicotinic receptors composed of the alpha3 and beta2 subunits are at least 1000-fold more sensitive to blockade by alpha-conotoxin-PnIA than are alpha2beta2 receptors. A series of chimeric subunits, formed from portions of alpha2 and alpha3, were coexpressed with beta2 in Xenopus oocytes and tested for toxin sensitivity. We found determinants of toxin sensitivity to be widely distributed in the extracellular domain of alpha3. Analysis of receptors formed by a series of mutant alpha3 subunits, in which residues that differ between alpha3 and alpha2 were changed from what occurs in alpha3 to what occurs in alpha2, allowed identification of three determinants of alpha-conotoxin-PnIA sensitivity: proline 182, isoleucine 188, and glutamine 198. Comparison with determinants of alpha-conotoxin-MII and kappa-bungarotoxin sensitivity on the alpha3 subunit revealed overlapping, but distinct, arrays of determinants for each of these three toxins. When tested against an EC50 concentration of acetylcholine, the IC50 for alpha-conotoxin-PnIA blockade was 25 +/- 4 nM for alpha3beta2, 84 +/- 7 nM for alpha3P182Tbeta2, 700 +/- 92 nM for alpha3I188Kbeta2, and 870 +/- 61 nM for alpha3Q198Pbeta2. To examine the location of these residues within the receptor structure, we generated a homology model of the alpha3beta2 receptor extracellular domain using the structure of the acetylcholine binding protein as a template. All three residues are located on the C-loop of the alpha3 subunit, with isoleucine 188 nearest the acetylcholine-binding pocket.

Phosphorylation sites within alpha4 subunits of alpha4beta2 neuronal nicotinic receptors: a comparison of substrate specificities for cAMP-dependent protein kinase (PKA) and protein kinase C (PKC)

The present study determined whether putative phosphorylation sites within the M3/M4 cytoplasmic domain of the human alpha4 subunit of alpha4beta2 neuronal nicotinic receptors are substrates for cAMP-dependent protein kinase (PKA) or protein kinase C (PKC). Five peptides corresponding to predicted phosphorylation sequences were synthesized, and phosphorylation was compared with standard peptide substrates for each kinase, that is, Kemptide for PKA and glycogen synthase (GS) 1-8 for PKC. VRCRSRSI had the highest affinity for PKA, with a Km of 44.5 microM; Kemptide had a Km of 7.7 microM. LMKRPSVVK and KARSLSVQH were also phosphorylated by PKA, but had lower affinities of 593 microM and 2896 microM, respectively. LMKRPSVVK had the highest affinity for PKC with a Km of 182 microM; GS 1-8 had a Km of 2.1 microM. VRCRSRSI had a comparative affinity for PKC with a Km of 327 microM. PCKCTCKK was not phosphorylated by PKA, but was a substrate for PKC with a Km of 1392 microM, whereas PGPSCKSP was not phosphorylated by either kinase. Based on these findings, results suggest that Ser-362 and Ser-486 on the human alpha4 subunit may be phosphorylated by either PKA or PKC, Ser-467 is a putative PKA site, and Thr-532 represents a likely PKC substrate; Ser-421 does not appear to be phosphorylated by either kinase.

Nitrous oxide and xenon inhibit the human (alpha 7)5 nicotinic acetylcholine receptor expressed in Xenopus oocyte

The neuronal nicotinic acetylcholine (nACh) receptor is one of the ligand-gated ion channels that regulate the synaptic release of neurotransmitters in the central nervous system. Recently, neuronal nACh receptors have received attention as a potential target for general anesthetics because many general anesthetics inhibit their functions at clinical concentrations. Several general anesthetics are known to inhibit the homomeric (alpha(7))(5) nACh receptor, a subtype of neuronal nACh receptors, but the effects of two gaseous anesthetics, nitrous oxide (N(2)O) and xenon (Xe), remain unknown. Using the two-electrode voltage-clamping technique, we investigated the effects of N(2)O and Xe at the human (alpha(7))(5) nACh receptor expressed in Xenopus oocytes. At clinically relevant concentrations, N(2)O and Xe reversibly inhibited the ACh-induced currents of the (alpha(7))(5) nACh receptor in a concentration-dependent manner. The inhibitory actions of both anesthetics at the (alpha(7))(5) nACh receptor were noncompetitive and voltage-independent. Our results suggest that inhibition of the (alpha(7))(5) nACh receptor by N(2)O and Xe may play a role in their anesthetic effects.

Modulation of neuronal nicotinic acetylcholine receptors by mercury

Mercuric chloride exerted a biphasic modulatory effect on rat neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus laevis oocytes as heteromers of the alpha3 or alpha4 and beta2 or beta4 subunits. The degree of modulation was subunit-dependent, with beta4-containing receptors displaying greater potentiation and alpha4-containing receptors displaying greater inhibition. Thus, alpha4beta4 receptors displayed both robust potentiation and robust inhibition. During prolonged coapplication of HgCl(2), first potentiation then inhibition of the acetylcholine (ACh) response was observed. Upon coapplication of 1 microM HgCl(2), a 2-fold increase in ACh-induced current was achieved in 55 +/- 1 s. With continued HgCl(2) application, the ACh response was slowly inhibited until, after 5 min, less than 10% of the initial response remained. By measuring potentiation at its peak and inhibition 5 min after the start of HgCl(2) coapplication, we obtained EC(50) and IC(50) values of 262 +/- 75 and 430 +/- 72 nM, respectively. HgCl(2) potentiation was voltage-dependent, increasing at more positive holding potentials. Upon washout of mercury chloride, potentiation reversed with a t(1/2) of 4.6 min. Inhibition reversed more slowly, with less than half the initial response recovered after 15 min of wash. Although free cysteine residues are common targets for mercury, elimination of all free cysteines located in the extracellular domains of the alpha4 and beta4 subunits did not alter the effects of mercuric chloride. Potentiation and inhibition of neuronal nAChRs may occur through action at a transmembrane or cytoplasmic location after passive diffusion of mercuric chloride across the plasma membrane.

Effects of P-Aminophenyl Dichloroarsine on Reduced High-affinity [3H]Nicotine Binding Sites from Chick Brain: A Covalent, Yet Reversible, Agent for Neuronal Nicotinic Receptors

Neuronal nicotinic acetylcholine receptor (nAChR) alpha-subunits contain a conserved disulphide that is essential for function. Here, we have examined the effects of sulphydryl redox reagents on [3H]nicotine binding to chick brain nAChR immunoisolated with the monoclonal antibody mAb35. The disulphide reducing agent, dithiothreitol (DTT), inhibited [3H]nicotine binding [50% inhibitory concentration (IC50)=146 microM] but this effect was reversed (93 +/- 1.5%) by subsequent reoxidation with 1 mM dithio-bis(nitrobenzoic acid) (DTNB). The trivalent arsenical, p-aminophenyl dichloroarsine (APA), which reacts with pairs of spatially close sulphydryls, was a potent inhibitor of reoxidation by DTNB (IC50=35 nM). However, application of the 'anti-arsenical', 2,3-dimercaptopropane sulphonic acid (DMPS), restored agonist binding after APA treatment (50% effective concentration=120 microM). Paradoxically, DMPS was also found to be a potent oxidizing agent of these receptors. Affinity alkylation of reduced nAChRs with bromoacetylcholine (BAC; 100 microM) irreversibly blocked nicotine binding (>90%). We propose (but have not proven) that APA interacts with the cysteines homologous to Cys192 - 193 in Torpedo AChRs, since APA pretreatment of reduced neuronal receptors protected against irreversible BAC alkylation, as shown by subsequent reversal of DMPS (2 mM; 20 min). This study illustrates the potent and reversible nature of the arsenical's covalent interaction with an isolated nAChR and suggests that modified arsenicals could be useful nAChR probes.

Identification of three cAMP-dependent protein kinase (PKA) phosphorylation sites within the major intracellular domain of neuronal nicotinic receptor alpha4 subunits

This study determined whether all protein kinase A (PKA) and protein kinase C (PKC) phosphorylation sites on the alpha4 subunit of rat alpha4beta2 neuronal nicotinic receptors could be localized to the M3/M4 cytoplasmic domain of the protein, and investigated specific amino acid substrates for the kinases through two-dimensional phosphopeptide mapping and site-directed mutagenesis. Experiments were conducted using alpha4beta2 receptors expressed in Xenopus oocytes and a fusion protein corresponding to the M3/M4 cytoplasmic domain of alpha4 (alpha4(333-594) ). When oocytes expressing alpha4beta2 receptors were incubated with [(32) P]orthophosphate in order to label endogenous ATP stores, phosphorylation of alpha4 subunits was evident. Incubation of either immunoprecipitated receptors or the fusion protein with [(32) P]ATP and either PKA or PKC followed by trypsinization of the samples demonstrated that the kinases phosphorylated alpha4 subunits on multiple phosphopeptides, and that the phosphorylated full-length alpha4 protein and fusion protein produced identical phosphopeptide maps. Site-directed mutagenesis of Ser365, Ser472 and Ser491 to alanines in the fusion protein eliminated phosphopeptides phosphorylated by PKA, but not by PKC. Other mutations investigated, Ser470, Ser493, Ser517 and Ser590, did not alter the phosphopeptide maps. Results indicate that Ser365, Ser472 and Ser491 on neuronal nicotinic receptor alpha4 subunits are phosphorylated by PKA and are likely to represent post-translational regulatory sites on the receptor.

Development of the visual system of the chick. II. Mechanisms of axonal guidance

The quest to understand axonal guidance mechanisms requires exact and multidisciplinary analyses of axon navigation. This review is the second part of an attempt to synthesise experimental data with theoretical models of the development of the topographic connection of the chick retina with the tectum. The first part included classic ideas from developmental biology and recent achievements on the molecular level in understanding cytodifferentiation and histogenesis [J. Mey, S. Thanos, Development of the visual system of the chick. (I) Cell differentiation and histogenesis, Brain Res. Rev. 32 (2000) 343-379]. The present part deals with the question of how millions of fibres exit from the eye, traverse over several millimetres and spread over the optic tectum to assemble a topographic map, whose precision accounts for the sensory performance of the visual system. The following topics gained special attention in this review. (i) A remarkable conceptual continuity between classic embryology and recent molecular biology has revealed that positional cellular specification precedes and determines the formation of the retinotectal map. (ii) Graded expression of asymmetric genes, transcriptional factors and receptors for signal transduction during early development seem to play a crucial role in determining the spatial identity of neurons within surface areas of retina and optic tectum. (iii) The chemoaffinity hypothesis constitutes the conceptual framework for development of the retinotopic organisation of the primary visual pathway. Studies of repulsive factors in vitro developed the original hypothesis from a theoretical postulate of chemoattraction to an empirically supported concept based on chemorepulsion. (iv) The independent but synchronous development of retina and optic tectum in topo-chronologically corresponding patterns ensures that ingrowing retinal axons encounter receptive target tissue at appropriate locations, and at the time when connections are due to be formed. (v) The growth cones of the retino-fugal axons seem to be guided both by local cues on glial endfeet and within the extracellular matrix. On the molecular level, the ephrins and their receptors have emerged as the most likely candidates for the material substrate of a topographic projection along the anterior-posterior axis of the optic tectum. Yet, since a number of alternative molecules have been proposed for the same function, it remains the challenge for the near future to define the proportional contribution of each one of the individual mechanisms proposed by matching theoretical predictions with the experimental evidence.

Inefficient cell-surface expression of hybrid complexes formed by the co-assembly of neuronal nicotinic acetylcholine receptor and serotonin receptor subunits

Previous studies have demonstrated that relatively low levels of alpha4beta2 neuronal nicotinic acetylcholine receptors (nAChRs) are expressed on the cell surface of transfected mammalian cell lines but that surface expression levels can be dramatically up-regulated by co-expression of these subunits with chimeric subunits containing the N-terminal portion of the neuronal nAChR alpha4 or beta2 subunits together with the C-terminal domain of the 5-HT(3A) subunit. Recent work has also suggested that the nAChR alpha4 subunit can co-assemble in a "promiscuous" manner with the serotonin receptor 5-HT(3A) subunit to form functional hybrid receptors. In this study we have examined whether co-assembly of either alpha4 or beta2 with 5-HT(3A) itself (rather than with the alpha4/5-HT(3A) or beta2/5-HT(3A) subunit chimeras) can also facilitate cell surface expression of alpha4 and beta2 subunits in transfected mammalian cells. Evidence has been obtained by immunoprecipitation, cell-surface antibody binding and radioligand binding which indicates that the 5-HT(3A) can co-assemble with both the alpha4 and beta2 nAChR subunits. We conclude, however, that co-assembly of 5-HT(3A) with either alpha4 or beta2 does not result in efficient cell surface expression of the nAChR subunits and that co-assembled hybrid (nAChR subunit + 5-HT(3)R subunit) receptor complexes are largely retained within the cell.

Subunit-dependent modulation of neuronal nicotinic receptors by zinc

We examined the effect of zinc on rat neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes as simple heteromers of alpha2, alpha3, or alpha4 and beta2 or beta4. Coapplication of zinc with low concentrations of acetylcholine (</=EC(10)) resulted in differential effects depending on receptor subunit composition. The alpha2beta2, alpha2beta4, alpha3beta4, alpha4beta2, and alpha4beta4 receptors exhibited biphasic modulation by zinc, with potentiation of the acetylcholine response occurring at 1-100 micrometer zinc and inhibition occurring at higher zinc concentrations. In contrast, alpha3beta2 receptors were only inhibited by zinc (IC(50) = 97 +/- 16 micrometer). The greatest potentiating effect of zinc was seen with alpha4beta4 receptors that were potentiated to 560 +/- 17% of the response to ACh alone, with an EC(50) of 22 +/- 4 micrometer zinc. Cadmium, but not nickel, was also able to potentiate alpha4beta4 receptors. Both zinc potentiation of alpha4beta4 receptors and zinc inhibition of alpha3beta2 receptors were voltage independent. The sensitivity of zinc potentiation of alpha4beta4 to diethylpyrocarbonate treatment and alterations in pH suggested the involvement of histidine residues. Zinc continued to inhibit alpha4beta4 and alpha3beta2 after diethylpyrocarbonate treatment. Application of a potentiating zinc concentration increased the response of alpha4beta2 and alpha4beta4 receptors to saturating ACh concentrations. The rate of Ach-induced desensitization of these receptors was unaffected by zinc. Our results reveal zinc potentiation as a new mode of neuronal nAChR modulation.

Ion channel diseases of the central nervous system

In the last decade, advances in molecular genetics and cellular electrophysiology have increased our understanding of ion channel function. A number of diseases termed "channelopathies" have been discovered that are caused by ion channel dysfunction. Channelopathies can be caused by autoimmune, iatrogenic, toxic or genetic mechanisms. Mutations in genes encoding ion channel proteins that disrupt channel function are now the most commonly identified cause of channelopathies, perhaps because gene disruption is readily detected by the methods of molecular genetics. Ion channels are abundant in the central nervous system (CNS), but CNS channelopathies are rare; however, they overlap with some important neurological disorders, such as epilepsy, ataxia, migraine, schizophrenia, Alzheimer's disease and other neurodegenerative diseases. It is possible that more CNS channelopathies will be discovered when additional ion channels are characterized and the complex mechanisms of brain function are better understood. At present, increased knowledge of the identity, structure and function of ion channels is facilitating diagnosis and treatment of many channelopathies.

Characterization of human alpha4beta2 neuronal nicotinic receptors stably expressed in SH-EP1 cells

These studies characterized human alpha4beta2 neuronal nicotinic receptors stably expressed in a human epithelial cell line (SH-EP1). Receptors in transfected SH-EPI-halpha4beta2 cells were functional, as determined by increases in intracellular Ca2+ in response to a nicotine stimulus. Nicotine increased Fura-2 fluorescence in a concentration-dependent manner with an apparent EC50 of 2.4 microM, a response that was blocked by the specific antagonist mecamylamine. When cells were incubated in 50 nM nicotine for 24 hours, the Ca2+ response inactivated by 44%, an effect that recovered within 24 hours. SH-EP1-halpha4beta2 cells expressed a single class of high affinity binding sites for [3H]cytisine with a Kd of 0.63 +/- 0.08 nM and a Bmax of 6,797 +/- 732 femtomoles/mg protein. Incubation of cells with 50 nM nicotine for 24 hours increased the Bmax by 45% without changing affinity, a concentration-dependent effect with an EC50, of 58.6 nM. The nicotine-induced up regulation was reversible, and control values were achieved within 24 hours. Results indicate that SH-EPI-halpha4beta2 cells may be a good model system to study regulation of human alpha4beta2 receptors, the most abundant nicotinic receptor subtype in brain.

Nicotinic acetylcholine receptor subunit proteins alpha7 and beta4 decrease in the superior cervical ganglion after axotomy

Synaptic transmission in the superior cervical ganglion (SCG) is mediated by nicotinic acetylcholine receptors (nAChR). After transection of the postganglionic nerves of the SCG in the adult rat, the transcript levels of four of the five nAChR subunits present in the ganglion, alpha3, alpha5, alpha7, and beta4, decrease dramatically. In the present study, the effect of axotomy on nAChR subunit expression was examined at the protein level, focusing on the alpha7 and beta4 subunits. Immunohistochemistry with monoclonal antibody mAb306 (for the alpha7 subunit) and polyclonal antibody 4886 (for the beta4 subunit) showed that immunoreactivities for both alpha7 and beta4 subunits were concentrated in neurons in the intact ganglion. Results from double staining with antibodies to these subunits and to tyrosine hydroxylase, the enzyme that catalyzes the rate-limiting step in the biosynthesis of the sympathetic neurotransmitter norepinephrine, demonstrated that most neurons in the SCG express both the alpha7 and beta4 subunits. Three days after axotomy, the number of immunolabeled neurons and the intensity of the immunostaining per labeled neuron were decreased for both subunits. Decreases in subunit levels were also observed by Western blot analysis. Observing changes in these subunits over time after surgery revealed that, while the protein level of the alpha7 subunit recovered substantially within 2 weeks after the lesion, that of the beta4 subunit stayed low. These data demonstrate that decreases in nicotinic receptor subunits are among the changes in proteins that occur in axotomized sympathetic neurons, and suggest that these decreases may contribute to the depression in ganglionic synaptic transmission observed in axotomized ganglia.

Expression of cholinergic system molecules during development of the chick nervous system

There are suggestions of the participation of nicotinic acetylcholine receptors (nAChRs), the acetylcholine degradation enzyme, acetylcholinesterase (AChE), and the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT), in the development of the nervous system. In this study, we aimed at comparing the development of some subunits of the nAChRs, AChE, and ChAT in the chick nervous system by standard immunohistochemical methods. The expression of all molecules investigated here appeared very early in ganglia (embryonic day 3.5-4), persisting into posthatching, except for ChAT, which is not detected after hatching in ganglia. A differential development was observed for nAChR subunits, with these receptors appearing around embryonic day 6 in some sites. The time-course of development of different nAChR subunits revealed several instances of transient expression (such as in the cerebellum), increasing expression (such as in the nucleus spiriformis lateralis), and diminishing expression into posthatching stages (such as in the oculomotor and throclear nuclei). Expression of AChE and ChAT also starts around embryonic day 6 in some structures and follows mainly increasing time-courses in the chick brain. The results of this study reveal a developmentally regulated expression of cholinergic system-related molecules in the chick nervous system and characterize differential time-courses of expression for nAChR subunits, AChE, and ChAT during development.

Neuronal nicotinic acetylcholine receptors: from the gene to the disease

The neuronal nicotinic acetylcholine receptors are excitatory ligand-gated channels. Widely expressed throughout the peripheral and central nervous system, their properties depend upon their subunit composition. Furthermore, genetic studies have revealed a high degree of variation at the genomic level and alternative splicing of the mRNAs coding for these integral membrane proteins. In particular, genes coding for alpha4 and alpha7 subunits harbour a high degree of polymorphisms. Although well characterised at their molecular and functional level, the role of these receptors in the central nervous system remains obscure. Despite accumulating evidence for the participation of nicotinic receptors in disorders of the central nervous system including nicotinic addiction, Parkinson's disease, Alzheimer's disease and Tourette's syndrome, the exact role of these receptors is still speculative. Because most of these phenotypes are complex and genetically heterogeneous, the investigation is difficult. However, in the past few years, significant progress has been made in understanding the contribution of nicotinic acetylcholine receptors to the origin of epilepsies and schizophrenia. By concentrating on the latest results gained for these diseases, we discuss in this review the possible relationships between neuronal nicotinic receptors and neurological and psychiatric disorders.

Two distinct classes of functional 7-containing nicotinic receptor on rat superior cervical ganglion neurons

Nicotinic acetylcholine receptors (nAChRs) that bind alpha-bungarotoxin (alpha Bgt) were studied on isolated rat superior cervical ganglion (SCG) neurons using whole-cell patch clamp recording techniques. Rapid application of ACh onto the soma of voltage clamped neurons evoked a slowly desensitizing current that was reversibly blocked by alpha Bgt (50 nM). The toxin-sensitive current constituted on average about half of the peak whole-cell response evoked by ACh. Nanomolar concentrations of methyllycaconitine blocked the alpha Bgt-sensitive component of the ACh-evoked current as did intracellular dialysis with an anti-alpha 7 monoclonal antibody. The results indicate that the slowly reversible toxin-sensitive response elicited by ACh arises from activation of an unusual class of alpha 7-containing receptor (alpha 7-nAChR) similar to that reported previously for rat intracardiac ganglion neurons. A second class of functional alpha 7-nAChR was identified on some SCG neurons by using rapid application of choline to elicit responses. In these cases a biphasic response was obtained, which included a rapidly desensitizing component that was blocked by alpha Bgt in a pseudo-irreversible manner. The pharmacology and kinetics of the responses resembled those previously attributed to alpha 7-nAChRs in a number of other neuronal cell types. Experiments measuring the dissociation rate of 125I-labelled alpha Bgt from SCG neurons revealed two classes of toxin-binding site. The times for toxin dissociation were consistent with those required to reverse blockade of the two kinds of alpha Bgt-sensitive response. These results indicate that rat SCG neurons express two types of functional alpha 7-nAChR, differing in pharmacology, desensitization and reversibility of alpha Bgt blockade.

Molecular diversity of neuronal nicotinic acetylcholine receptors

The potent behavioral and cognitive effects of nicotine highlight the physiological importance of nicotinic acetylcholine receptors (nAChRs). These receptors are part of the superfamily of neurotransmitter-gated ion channels that are responsible for rapid intercellular communication. Molecular cloning of the protein subunits that make up these receptors has led to greater understanding of the pharmacology and physiology of nAChRs. This review outlines our current understanding of the molecular constituents of these receptors and some of the recent studies of the structural determinants of receptors function.

Selective effects of a 4-oxystilbene derivative on wild and mutant neuronal chick alpha7 nicotinic receptor

1. We assessed the pharmacological activity of triethyl-(beta-4-stilbenoxy-ethyl) ammonium (MG624), a drug that is active on neuronal nicotinic receptors (nicotinic AChR). Experiments on the major nicotinic AChR subtypes present in chick brain, showed that it inhibits the binding of [125I]-alphaBungarotoxin (alphaBgtx) to the alpha7 subtype, and that of [3H]-epibatidine (Epi) to the alpha4beta2 subtype, with Ki values of respectively 106 nM and 84 microM. 2. MG624 also inhibited ACh elicited currents (I(ACh)) in the oocyte-expressed alpha7 and alpha4beta2 chick subtypes with half-inhibitory concentrations (IC50) of respectively 109 nM and 3.2 microM. 3. When tested on muscle-type AChR, it inhibited [125I]-alphaBgtx binding with a Ki of 32 microM and ACh elicited currents (I(ACh)) in the oocyte-expressed alpha1beta1gammadelta chick subtype with an IC50 of 2.9 microM. 4. The interaction of MG624 with the alpha7 subtype was investigated using an alpha7 homomeric mutant receptor with a threonine-for-leucine 247 substitution (L247T alpha7). MG624 did not induce any current in oocytes expressing the wild type alpha7 receptor, but did induce large currents in the oocyte-expressed L247T alpha7 receptor. The MG624 elicited current (I(MG62)) has an EC50 of 0.2 nM and a Hill coefficient nH of 1.9, and is blocked by the nicotinic receptor antagonist methyllycaconitine (MLA). 5. These binding and electrophysiological studies show that MG624 is a potent antagonist of neuronal chick alpha7 nicotinic AChR, and becomes a competitive agonist following the mutation of the highly conserved leucine residue 247 located in the M2 channel domain.

Postsynaptic nicotinic receptors on dopaminergic neurons in the substantia nigra pars compacta of the rat

Previous studies have shown that application of nicotinic agonists in the substantia nigra pars compacta increases the firing rate of dopaminergic neurons. We have used intracellular recordings to show that the response of these neurons to nicotine is postsynaptic, since it persists in the presence of low-calcium buffer containing tetrodotoxin. Burst firing in the presence of nicotine was not observed. The presence of postsynaptic nicotinic receptors was confirmed by immunohistochemical localization of the alpha4 nicotinic receptor subunit on dendrites in the substantia nigra pars compacta. The majority of tyrosine hydroxylase-immunopositive neurons in the substantia nigra pars compacta were also immunopositive for the alpha4 subunit. Immunohistochemical localization of the alpha4 and beta2 subunits in adjacent brain sections produced similar patterns of staining. Electron micrographs clearly indicated the presence of alpha4 subunit at postsynaptic densities. The predominant role of nicotinic receptors in the central nervous system has been suggested to be the presynaptic modulation of neurotransmitter release [McGehee D. S. and Role L. W. (1995) A. Rev. Physiol. 57, 521-546]. Although several postsynaptic nicotinic responses have also been reported in the literature, it is unclear as to whether the postsynaptic nicotinic receptors mediating responses to exogenously applied agonists are involved in synaptic transmission. From our electrophysiological and immunohistochemical results, we conclude that alpha4-containing nicotinic receptors are found at synapses on dopaminergic neurons. These synapses are similar to the cholinergic synapses described at these neurons, suggesting that nicotinic receptors are important in modulating the excitability of dopaminergic neurons by direct synaptic transmission.

Autosomal dominant nocturnal frontal-lobe epilepsy: genetic heterogeneity and evidence for a second locus at 15q24

Autosomal dominant nocturnal frontal-lobe epilepsy (ADNFLE) is a recently identified partial epilepsy in which two different mutations have been described in the alpha4 subunit of the neuronal nicotinic acetylcholine receptor (CHRNA4). An additional seven families are presented in which ADNFLE is unlinked to the CHRNA4 region on chromosome 20q13.2. Seven additional sporadic cases showed no evidence of defective CHRNA4. One of the families showed evidence of linkage to 15q24, close to the CHRNA3/CHRNA5/CHRNB4 cluster (maximum LOD score of 3.01 with D15S152). Recombination between ADNFLE and CHRNA4, linkage to 15q24 in one family, and exclusion from 15q24 and 20q13.2 in others demonstrate genetic heterogeneity with at least three different genes for ADNFLE. The CHRNA4 gene and the two known CHRNA4 mutations are responsible for only a minority of ADNFLE. Although the ADNFLE phenotype is clinically homogeneous, there appear to be a variety of molecular defects responsible for this disorder, which will provide a challenge to the understanding of the basic mechanism of epileptogenesis.

3-(5-Alkylamino-4-isoxazolyl)-1,2,5,6-tetrahydropyridines: a novel class of central nicotinic receptor ligands

A novel class of central nicotinic acetylcholine receptor ligands, 3-(5-alkylamino-4-isoxazolyl)-1,2,5,6-tetrahydropyridine 4a-f, was synthesized. Several of the compounds showed high affinity for central nicotinic receptors (4c: IC50 = 50 nM), with more than a 100-fold selectivity for nicotinic over muscarinic receptors. The compounds showed up to a 10-fold selectivity for the central nicotinic subtype combination alpha 4 beta 2 (4c: IC50 = 4.6 nM), as compared to the major ganglionic subtype composed of alpha 3 containing subunits (4c: IC50 = 48 nM). The compounds were further evaluated in a dopamine release assay in vitro, and in a drug discrimination assay in vivo. Compound 4a is an effective nicotinic agonist with a potency 50-100 times lower than nicotine. Extending the alkylamino chain beyond one, compound (4b-f), changed the pharmacological profile of the compounds in an antagonistic direction.

Promoter analysis of the neuronal nicotinic acetylcholine receptor alpha4 gene: methylation and expression of the transgene

Neuronal nicotinic acetylcholine receptor (nAChR) subunit genes compose a family of genes. The major isoform of nAChR in the brain is made up of the alpha4 and beta2 subunits and possesses a high affinity for nicotine. To investigate the mechanisms of the regulation of the nAChR alpha4 gene expression in mouse, its genomic DNA was cloned and characterized. The transcription initiation site was mapped by primer extension and RNase protection experiments and localized at about 254 bp upstream of the translation initiation site. The 5' flanking region of this gene did not have typical TATA box but GC-rich sequences were found around the initiation site. Methylation analysis of this region revealed that genomic DNAs from liver and muscle are partially methylated, whereas little methylation was observed in genomic DNA from brain. To characterize the cis-acting elements driving cell-specific expression of the alpha4 subunit gene, we produced lines of transgenic mice which carry a series of fragments of the alpha4 gene fused with bacterial lacZ as a reporter gene. An 11.5-kb DNA fragment containing 9 kb of the region upstream of the transcription initiation site and the first intron was found to confer an expression pattern which coincides rather well with the endogenous gene expression pattern at early embryonic stages, suggesting that the elements necessary for the onset of alpha4 gene expression are located in this region. A DNA fragment containing the 1.8-kb upstream sequence and the first intron drove expression of lacZ in a limited subset of alpha4 expressing cells, whereas the 1.8-kb upstream sequence alone did not elicit any significant expression. These results show that both upstream and intronic sequences are important for cell-specific expression of the nAChR alpha4 gene.

Effects of the alpha subunit on imidacloprid sensitivity of recombinant nicotinic acetylcholine receptors

1. Imidacloprid is a new insecticide with selective toxicity for insects over vertebrates. Recombinant (alpha4beta2) chicken neuronal nicotinic acetylcholine receptors (AChRs) and a hybrid nicotinic AChR formed by co-expression of a Drosophila melanogaster neuronal alpha subunit (SAD) with the chicken beta2 subunit were heterologously expressed in Xenopus oocytes by nuclear injection of cDNAs. The agonist actions of imidacloprid and other nicotinic AChR ligands ((+)-epibatidine, (-)-nicotine and acetylcholine) were compared on both recombinant nicotinic AChRs by use of two-electrode, voltage-clamp electrophysiology. 2. Imidacloprid alone of the 4 agonists behaved as a partial agonist on the alpha4beta2 receptor; (+)-epibatidine, (-)-nicotine and acetylcholine were all full, or near full, agonists. Imidacloprid was also a partial agonist of the hybrid Drosophila SAD chicken beta2 receptor, as was (-)-nicotine, whereas (+)-epibatidine and acetylcholine were full agonists. 3. The EC50 of imidacloprid was decreased by replacing the chicken alpha4 subunit with the Drosophila SAD alpha subunit. This alpha subunit substitution also resulted in an increase in the EC50 for (+)-epibatidine, (-)-nicotine and acetylcholine. Thus, the Drosophila (SAD) alpha subunit contributes to the greater apparent affinity of imidacloprid for recombinant insect/vertebrate nicotinic AChRs. 4. Imidacloprid acted as a weak antagonist of ACh-mediated responses mediated by SADbeta2 hybrid receptors and as a weak potentiator of ACh responses mediated by alpha4beta2 receptors. This suggests that imidacloprid has complex effects upon these recombinant receptors, determined at least in part by the alpha subunit.

The neuronal alpha6 subunit forms functional heteromeric acetylcholine receptors in human transfected cells

We examine some of the biological and physiological properties of the avian alpha6 neuronal nicotinic acetylcholine receptor (nAChR) subunit. We show here that, beginning at embryonic day 5, alpha6 mRNA is abundantly expressed in the developing chick neuroretina, where it coexists with other nicotinic receptor subunit mRNAs such as alpha3, beta2 and beta4. In contrast, alpha6 mRNA is absent from the optic tectum and from the peripheral ganglia. Despite numerous efforts, the alpha6 subunit has long failed the critical test of functional reconstitution. Here we use patch-clamp techniques and confocal laser microscopy to measure ACh-activated currents and nicotine-elicited Ca2+ transients in human BOSC 23 cells transfected with chick alpha6 in combination with other chick nAChR neuronal subunits. Heterologously expressed alpha6 and beta4 subunits form functional heteromeric nAChRs, which are permeable to Ca2+ ions and blocked by the nicotinic antagonist methyllycaconitine (10 microM). Likewise, ACh elicits measurable currents in cells transfected with alpha6 and beta2. Hill analysis of the dose-response curves in cells transfected with alpha3, beta4 and alpha6 cDNAs, suggests the assembly of functional alpha3beta4alpha6 receptor, with an apparent affinity for ACh threefold lower than alpha3beta4. Our results indicate that alpha6-containing nAChRs assemble in heterologous expression systems and are probably present in retinal cells.

Differential co-localization of nicotinic acetylcholine receptor subunits with calcium-binding proteins in retinal ganglion cells

Immunohistochemistry was used to examine the co-occurrence of nicotinic acetylcholine receptor subunits with calcium-binding proteins in ganglion cells of the chick retina. The alpha3 subunit was rarely observed in ganglion cells containing calbindin, calretinin, or parvalbumin. On the other hand, the alpha8 subunit was more often co-localized with all calcium-binding proteins studied. These results may be related to the high calcium permeability of nicotinic receptors that contain the alpha8 subunit.

The triggering of spreading depression in the chick retina by nicotinic receptor agonists

Spreading depression was evoked in vitro in retinas of 3-6 day old chickens by the nicotinic cholinergic agonists nicotine and cytisine. The response was reproducible and inhibited by the nicotinic cholinergic receptor antagonist mecamylamine and by the NMDA receptor antagonist -3-(2 carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP). The response to nicotinic agonists was not inhibited by alpha-bungarotoxin. The data show that spreading depression can be evoked in the chick retina by alpha-bungarotoxin insensitive nicotinic acetylcholine receptor subtypes and that the response is dependent upon NMDA receptor activation. This nicotine evoked spreading depression was inhibited by cadmium chloride indicating the involvement of voltage sensitive calcium channels. It is therefore argued that nicotine evokes spreading depression indirectly, as a result of calcium sensitive glutamate release. The glutamate released thus exerting its effects via NMDA receptors.

Influence of subunit composition on desensitization of neuronal acetylcholine receptors at low concentrations of nicotine

The influence of alpha and beta subunits on the properties of nicotine-induced activation and desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes was examined. Receptors containing alpha4 subunits were more sensitive to activation by nicotine than alpha3-containing receptors. At low concentrations of nicotine, nAChRs containing beta2 subunits reached near-maximal desensitization more rapidly than beta4-containing receptors. The concentration of nicotine producing half-maximal desensitization was influenced by the particular alpha subunit expressed; similar to results for activation, alpha4-containing receptors were more sensitive to desensitizing levels of nicotine than alpha3-containing receptors. The alpha subunit also influenced the rate of recovery from desensitization; this rate was approximately inversely proportional to the apparent nicotine affinity for the desensitized state. The homomeric alpha7 receptor showed the lowest sensitivity to nicotine for both activation and desensitization; alpha7 nAChRs also demonstrated the fastest desensitization kinetics. These subunit-dependent properties remained in the presence of external calcium, although subtle, receptor subtype-specific effects on both the apparent affinities for activation and desensitization and the desensitization kinetics were noted. These data imply that the subunit composition of various nAChRs determines the degree to which receptors are desensitized and/or activated by tobacco-related levels of nicotine. The subtype-specific balance between receptor activation and desensitization should be considered important when the cellular and behavioral actions of nicotine are interpreted.

Distribution of the alpha 2, alpha 3, and alpha 5 nicotinic acetylcholine receptor subunits in the chick brain

Nicotinic acetylcholine receptors (nAChRs) are ionotropic receptors comprised of alpha and beta subunits. These receptors are widely distributed in the central nervous system, and previous studies have revealed specific patterns of localization for some nAChR subunits in the vertebrate brain. In the present study we used immunohistochemical methods and monoclonal antibodies to localize the alpha 2, alpha 3, alpha 5 nAChR subunits in the chick mesencephalon and diencephalon. We observed a differential distribution of these three subunits in the chick brain, and showed that the somata and neuropil of many central structures contain the alpha 5 nAChR subunit. The alpha 2 and alpha 3 subunits, on the other hand, exhibited a more restricted distribution than alpha 5 and other subunits previously studies, namely alpha 7, alpha 8 and beta 2. The patterns of distribution of the different nAChR subunits suggest that neurons in many brain structures may contain several subtypes of nAChRs and that in a few regions one particular subtype may determine the cholinergic nicotinic responses.

Detection of functional nicotinic receptors blocked by alpha-bungarotoxin on PC12 cells and dependence of their expression on post-translational events

A major class of nicotinic receptors in the nervous system is one that binds alpha-bungarotoxin and contains the alpha7 gene product. PC12 cells, frequently used to study nicotinic receptors, express the alpha7 gene and have binding sites for the toxin, but previous attempts to elicit currents from the putative receptors have failed. Using whole-cell patch-clamp recording techniques and rapid application of agonist, we find a rapidly desensitizing acetylcholine-induced current in the cells that can be blocked by alpha-bungarotoxin. The current amplitude varies dramatically among three populations of PC12 cells but correlates well with the number of toxin-binding receptors. In contrast, the current shows no correlation with alpha7 transcript; cells with high levels of alpha7 mRNA can be negative for toxin binding and yet have other functional nicotinic receptors. Northern blot analysis and reverse transcription-PCR reveal no defects in alpha7 RNA from the negative cells, and immunoblot analysis demonstrates that they contain full-length alpha7 protein, although at reduced levels. Affinity purification of toxin-binding receptors from cells expressing them confirms that the receptors contain alpha7 protein. Transfection experiments demonstrate that PC12 cells lacking native toxin-binding receptors are deficient at producing receptors from alpha7 gene constructs, although the same cells can produce receptors from other transfected gene constructs. The results indicate that nicotinic receptors that bind alpha-bungarotoxin and contain alpha7 subunits require additional gene products to facilitate assembly and stabilization of the receptors. PC12 cells offer a model system for identifying those gene products.

Alpha7 and alpha8 nicotinic receptor subtypes immunopurified from chick retina have different immunological, pharmacological and functional properties

Nicotinic receptors are present in the chick retina, but their structure and functional characteristics are still unclear. Using anti-alpha7 and anti-alpha8 subunit-specific antibodies, we immunopurified the alpha7 and alpha8 subtypes of chick retina neuronal nicotinic receptors. When analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, the two purified subtypes consistently showed a similar peptide composition characterized by the presence of two major peptides of M(r) 58 +/- 1 and 54 +/- 1 kDa, and two minor peptides of 67 and 61 +/- 1 kDa. In the alpha7 subtype, the 58 kDa peptide was recognized by anti-alpha7 but not by anti-alpha8 antibodies; in the alpha8 subtype, the 58 kDa peptide was recognized only by anti-alpha8 antibodies. The alpha7 subtype had a single class of [125I]alpha-bungarotoxin binding sites with a K(D) value of 1.2 nM, whereas the purified alpha8 subtype had two classes of binding sites, one with a K(D) of 5.5 nM and the other with very high affinity (KD 52 pM), but present in only 8% of the receptors. Competition binding experiments also showed the presence on the alpha8 subtype of high- and low-affinity classes of binding sites; the affinity for cholinergic drugs of the former was greater than that of the single class present on the alpha7 subtype. When reconstituted in planar lipid bilayers, both subtypes formed ligand-gated cation channels with major conductance levels of 42 and 52 pS but with different lifetimes; the two channels were activated by agonists and blocked by d-tubocurarine and the glycinergic antagonist strychnine. In line with the binding data, the reconstituted alpha8 subtype had greater agonist sensitivity than the alpha7 subtype.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Presumptive presynaptic nicotinic acetylcholine receptors in the chick tectum: effects of lesions of the lateral spiriform nucleus

There are indications that nicotinic acetylcholine receptor subunits in the superficial layers of the chick tectum (Cajal's layers 1-7) may be transported from the retina. However, nicotinic receptor subunits are detectable by immunohistochemistry in all layers of the optic tectum. In this study, we performed unilateral electrolytic lesions of the lateral spiriform nucleus, which projects to the deep layers of the tectum and contains high amounts of nicotinic receptors in its perikarya. Following lesions of the lateral spiriform nucleus, both the alpha 5 and the beta 2 subunits were markedly depleted in the neuropil of the deep layers of the ipsilateral optic tectum (layers 8-13). No changes were observed in somata that contain either subunit. The present results suggest that most of the nicotinic acetylcholine receptor subunits in the chick optic tectum occur in axonal systems and could then constitute presynaptic receptors.

Optical recordings of the effects of cholinergic ligands on neurons in the ganglion cell layer of mammalian retina

Cholinergic regulation of the activity of rabbit retinal ganglion cells and displaced amacrine cells was investigated using optical recording of changes in intracellular free calcium ([Ca2+]i). Labeling of neurons in the mature retina was achieved by injecting calcium green-1 dextran (CaGD) into the isolated retina. Nicotine increased ganglion cell [Ca2+]i, affecting every loaded cell in some preparations; the pharmacology of nicotine was consistent with an action at neuronal nicotinic receptors, and specifically it was kappa-(neuronal-)bungarotoxin-sensitive but alpha-bungarotoxin-insensitive. Muscarine also raised [Ca2+]i, but it was less potent than nicotine, affecting only a subpopulation of ganglion cells, with an M1-like muscarinic receptor pharmacology. Neither the nicotine- nor muscarine-induced increases of ganglion cell [Ca2+]i were blocked by the glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and aminophosphonopentanoic acid. Therefore, the effects of cholinergic agonists on ganglion cell [Ca2+]i were not attributable to an indirect effect mediated by glutamatergic bipolar cells. The effects of nicotine and muscarine were abolished in calcium-free solution, indicating that the responses depend on calcium influx. Displaced (Cb) cholinergic amacrine cells were also loaded with CaGD and were identified by selective labeling with the nuclear dye 4',6-diamidino-2-phenyl-indole. Cb amacrine cells did not respond to either nicotine or muscarine, but responded vigorously to the glutamate receptor agonist kainic acid. There is anatomical evidence indicating that cholinergic amacrine cells make synaptic contact with each other, but the present results do not support the hypothesis that communication between these cells is cholinergic.

Neurogenesis of cholinoceptive neurons in the chick retina

Immunocytochemistry and [3H]thymidine autoradiography were combined in this study to determine the neurogenesis of cholinoceptive cells in the chick retina. After injections of [3H]thymidine between embryonic days 1 and 11, the time of birth of retinal neurons containing either the alpha 3 or the alpha 8 subunit of the nicotinic acetylcholine receptors was determined. The results indicate that the alpha 3-positive neurons in the ganglion cell layer leave the cell cycle from E2 through E7, and those in the inner nuclear layer (amacrine and displaced ganglion cells) from E2 through E9. The alpha 8-positive cells in the ganglion cell layer were born from E1 through E7, and those in the inner nuclear layer (amacrine and bipolar cells) from E2 through E11. These data suggest that the time of birth of cholinoceptive neurons in the chick retina follows the general pattern of cell generation in the chick retina, and that alpha 8-positive cells in the ganglion cell layer start to leave the cell cycle almost one day earlier than the alpha 3-positive cells in the same layer.