Membrane excitability and dissipative instabilities

Electrical excitation is interpreted in terms of a cooperative structural transition of membrane protomers coupled with the translocation of a permeant molecule in a non-equilibrium environment. Equations for flow of permeant and for membrane conformation are derived for the simple case of a single non-charged permeant. On the basis of a few simple physical assumptions, the theory predicts several important properties of electrically excitable membranes: the steepness of the relation between membrane conductance and potential, the presence of a negative conductance, and the occurrence of instabilities following rapid perturbations of membrane environment, giving rise to some simple cases of action potentials. Several experimental tests of the membrane with its changes of electrical properties are proposed. From a thermodynamic point of view, an electrically excitable membrane, in its resting state, lies beyond a dissipative instability and consequently is in a non-equilibrium state but with stable organization, a "dissipative structure" of Prigogine. Membrane excitation following a small perturbation of the environment would correspond to a jump from such an organization to another stable organization but close to thermodynamic equilibrium. It is shown how the cooperative molecular properties of the membrane are amplified by energy dissipation at the macroscopic level.

An in vitro study of the sub-cellular distribution of nicotinic receptors

Nicotinic and serotoninergic 5HT3 receptors share important sequence identities except for their cytoplasmic loop. Both ends of this loop display conserved 3D helical structures with distinct primary sequences. We decided to check whether these two helices named F and G play a role in the sub-cellular distribution of different nicotinic receptors. We systematically exchanged each helix with the equivalent sequence of neuronal nicotinic and alpha4, beta2 and alpha7 subunits in the functional chimeric alpha7-5HT3 receptor used as a model system. The new chimeras were expressed in vitro in polarized epithelial cells from pig kidney. We quantified synthesis and export of the receptors to the cell surface by measuring alpha-bungarotoxin binding sites. Immunogold labelling was used, at the electron microscope level, to determine the amount of each chimera present at either domain, apical and/or basolateral, of these cells. We noticed that in epithelial cells the majority of alpha-bungarotoxin binding sites remained sequestered in the cytoplasm as already observed in neurons in vivo. The majority of the pentamers present at the cell surface were located at the apical domain. Our results suggest that helix F and G differently regulate assembly and export to the cell surface of alpha-bungarotoxin binding receptors.

On Some Structural Analogies between Acetylcholinesterase and the Macromolecular Receptor of Acetylcholine

Several properties of the enzyme acetylcholinesterase (AChE) isolated in vitro are compared with those of the membrane receptor(s) of acetylcholine expressed by the in vivo electrical response of the electroplax membrane. AChE strongly binds in vitro effectors of the electroplax: agonists e.g., decamethonium or antagonists, e.g., d-tubocurarine and flaxedil. It also reacts covalently with an affinity labeling reagent of the acetylcholine receptor site(s) in vivo (TDF). Two classes of sites on AChE molecule account for the binding of these quaternary nitrogen containing compounds: (1) the anionic site of the active center and (2) noncatalytic "peripheral anionic centers" located outside the active center. A disulfide bond breaking agent, dithiothreitol (DTT) alters in a parallel manner the reaction of AChE and the excitable membrane of the electroplax to TDF. The irreversibility of TDF action is lost in both cases, after exposure to DTT. Both AChE and the acetylcholine receptor thus contain disulfide bonds—they are closely related but not necessarily identical proteins.

Differential role of nicotinic acetylcholine receptor subunits in physical and affective nicotine withdrawal signs

It has been suggested that the negative effects associated with nicotine withdrawal promote continued tobacco use and contribute to the high relapse rate of smoking behaviors. Thus, it is important to understand the receptor-mediated mechanisms underlying nicotine withdrawal to aid in the development of more successful smoking cessation therapies. The effects of nicotine withdrawal are mediated through nicotinic acetylcholine receptors (nAChRs); however, the role of nAChRs in nicotine withdrawal remains unclear. Therefore, we used mecamylamine-precipitated, spontaneous, and conditioned place aversion (CPA) withdrawal models to measure physical and affective signs of nicotine withdrawal in various nAChR knockout (KO) mice. beta2, alpha7, and alpha5 nAChR KO mice were chronically exposed to nicotine through surgically implanted osmotic minipumps. Our results show a loss of anxiety-related behavior and a loss of aversion in the CPA model in beta2 KO mice, whereas alpha7 and alpha5 KO mice displayed a loss of nicotine withdrawal-induced hyperalgesia and a reduction in somatic signs, respectively. These results suggest that beta2-containing nAChRs are involved in the affective signs of nicotine withdrawal, whereas non-beta2-containing nAChRs are more closely associated with physical signs of nicotine withdrawal; thus, the nAChR subtype composition may play an important role in the involvement of specific subtypes in nicotine withdrawal.

The acetylcholine receptor: a model of an allosteric membrane protein mediating intercellular communication

Over the past 20 years the nicotinic acetylcholine receptor has become the prototype of a superfamily of ligand-gated ion channels. As a single macromolecular entity of M(r) about 300,000, the receptor protein mediates, altogether, the activation and the desensitization of the associated ion channel and the regulation of these processes by extracellular and intracellular signals. The notion is discussed that the acetylcholine receptor is a membrane-bound allosteric protein which possesses several categories of specific sites for neurotransmitters and for regulatory ligands, and undergoes conformational transitions which link these diverse sites together. At this elementary molecular level, interactions between signalling pathways may be mediated by membrane-bound allosteric receptors and/or by other categories of cytoplasmic allosteric proteins.

Critical role of the C-terminal segment in the maturation and export to the cell surface of the homopentameric alpha7-5HT3A receptor

Many neurological pathologies are related to misfolded proteins. During folding and assembly in the endoplasmic reticulum, the nicotinic acetylcholine receptor (nAChR) subunits undergo several conformational changes to acquire the ability to bind ligands. After folding and maturation, by mechanisms largely unknown, receptors are exported to the cell surface. We investigated the maturational role of the extracellular C-terminal segment located at the boundary between the extracellular and the transmembrane domains. In the functional chimeric alpha7-5HT3A receptor used as a model system, amino acids from the C-terminal segment were successively deleted or mutated. Upon progressive shortening of the peptide we observed less and less alpha-bungarotoxin binding sites until no sites could be detected when the entire peptide had been deleted (chimera Del 5). Protein synthesis and pentameric assembly were not altered. In Del 5 transfected cells, pentameric receptors present in the endoplasmic reticulum were not detected on the cell surface where Del 5 proteins appeared as patches. With the Del 5 chimera, export of proteins to the cell surface diminished to about half that of wild-type. We propose that the C-terminal segment plays a double role: (i) through an interaction between the penultimate tyrosine residue of the C-terminal segment and the Cys loop of the N-terminal domain, it locks the receptor in a mature alpha-bungarotoxin binding conformation; (ii) this mature conformation, in turn, masks a retention signal present in the first transmembrane segment allowing properly assembled and matured receptors to escape to the cell surface.

Knock-out and knock-in mice to investigate the role of nicotinic receptors in the central nervous system

Pharmacological manipulations of nicotinic transmission have long been the only way to investigate the role of nicotinic acetylcholine receptors (nAChRs) in the brain. More recently, however, the use of genetically engineered knock-out (Ko) and knock-in (Kin) mice has provided a powerful alternative to the classical pharmacological approach. These animal models are not only useful in order to re-examine and refine the results derived from pharmacological studies, but they also provide a unique opportunity to determine the subunit composition of native receptors involved in various aspects of nicotinic transmission. Ultimately, this knowledge will be extremely valuable in the process of designing new drugs that can mimic the beneficial effects of nicotine for the treatment of certain neuropathologies or that may be useful in smoking cessation therapies. In this review, we present recent data obtained from studies of mutant mice that have contributed to our understanding of the role and composition of nAChRs in the central nervous system (CNS). The advantages and pitfalls of Ko models will also be discussed.

Acute and long-term changes in the mesolimbic dopamine pathway after systemic or local single nicotine injections

We have examined several neurochemical and behavioural parameters related to the function of the mesolimbic dopamine (DA) pathway in animals treated with nicotine following three modes of drug administration, i.e. systemic intraperitoneal injection, intra-accumbens (Acb) infusion or intraventral tegmental area (intra-VTA) microinjection. The present modes of systemic, intra-Acb and intra-VTA nicotine administration elicited comparable acute increases in dialysate DA levels from the Acb. The increase in extracellular DA levels was paralleled by a significant enhancement of locomotion in a habituated environment in the case of systemic or intra-VTA nicotine administration, whereas unilateral or bilateral intra-Acb nicotine infusion was ineffective, showing that accumbal DA increase is not sufficient to elicit locomotion in this experimental paradigm. Intra-VTA, but not systemic or intra-Acb, nicotine administration caused a long-term (at least 24-h) increase in basal dialysate DA levels from the Acb. In addition, significant increases in tyrosine hydroxylase (TH) and GluR1 (but not dopamine transporter or NR1) mRNA levels in the VTA were detected 24 h after intra-VTA nicotine administration. Systemic nicotine injection caused only an increase in TH mRNA levels while intra-Acb infusion did not modify any of the mRNAs tested. The long-term increase in basal DA levels in the Acb and TH, and GluR1 mRNA levels in the VTA upon intra-VTA nicotine microinjection indicates that even a single nicotine injection can induce plastic changes of the mesolimbic DA pathway.

Use of knock-out mice to determine the molecular basis for the actions of nicotine

Recombinant DNA techniques have been used to identify the family of molecules that mediate nicotine's effects on the brain. Nicotine binds and activates nicotinic acetylcholine receptors (nAChRs) which are made up of combinations of individual nicotinic subunits. It is important to determine which of the many possible subunit combinations are responsible for the physiological and behavioral effects of nicotine that lead to addiction. Molecular genetic tools such as antisense strategies have been useful in elucidating the electrophysiological properties of nAChRs in different tissues. Use of knock-out mice lacking individual nAChR subunits has also begun to elucidate how nicotine exerts its actions from the molecular level to the behavioral level. Experiments using mice lacking the beta2 subunit of the nAChR have shown that binding of nicotine to receptors containing this subunit is the first step in a pathway leading to increased dopamine levels in the mesolimbic dopamine system, and ultimately to the behavioral effects of nicotine in a test of nicotine reinforcement. Mice deficient in various alpha subunits of the nAChR will identify the partners of beta2 mediating the addictive properties of nicotine. In addition, more data needs to be gathered on the electrophysiological properties of different subunit combinations, the effects of nicotine on different neurotransmitter systems and the links between the molecular biology of nicotine receptors, their physiology and the ultimate role of individual receptor subtypes in complex behaviors. Multidisciplinary approaches to nAChR function will be essential to answering these questions.

The Ligand Gated Ion Channel database: an example of a sequence database in neuroscience

Multiple comparisons of receptor sequences, or receptor subunit sequences, has proved to be an invaluable tool in modern pharmacological investigations. Although of outstanding importance, general sequence databases suffer from several imperfections due to their size and their non-specificity. Room therefore exists for expert-maintained databases of restricted focus, where knowledge of the research field helps to filter the huge amount of data generated. Accordingly, neuroscientists have designed databases covering several types of proteins, in particular receptors for neurotransmitters. Ligand-gated ion channels are oligomeric transmembrane proteins involved in the fast response to neurotransmitters. All these receptors are formed by the assembly of homologous subunits, and an unexpected wealth of genes coding for these subunits has been revealed during the last two decades. The Ligand Gated Ion Channel database (LGICdb) has been developed to handle this growing body of information. The database aims to provide only one entry for each gene, containing annotated nucleic acid and protein sequences.

Targeting transcription to the neuromuscular synapse

Concomitant with innervation, genes coding for components of the neuromuscular junction become exclusively expressed in subsynaptic nuclei. A six-base pair element, the N box, can confer synapse-specific transcription to the acetylcholine nicotinic receptor delta and epsilon subunit, utrophin, and acetylcholine esterase genes. N box-dependent synaptic expression is stimulated by the nerve-derived signal agrin and the trophic factor neuregulin, which triggers the MAPK and JNK signaling pathways, to ultimately allow activation by the N box binding Ets transcription factor GABP.

Cajal on neurons, molecules, and consciousness

The discovery of chemical synaptic transmission together with the asymmetric distribution of neurotransmitter release sites and receptors offer an explanation for Cajal's theory on dynamic polarization.

Requirement of the nicotinic acetylcholine receptor beta 2 subunit for the anatomical and functional development of the visual system

In the mammalian visual system the formation of eye-specific layers at the thalamic level depends on retinal waves of spontaneous activity, which rely on nicotinic acetylcholine receptor activation. We found that in mutant mice lacking the beta2 subunit of the neuronal nicotinic receptor, but not in mice lacking the alpha4 subunit, retinofugal projections do not segregate into eye-specific areas, both in the dorso-lateral geniculate nucleus and in the superior colliculus. Moreover, beta2-/- mice show an expansion of the binocular subfield of the primary visual cortex and a decrease in visual acuity at the cortical level but not in the retina. We conclude that the beta2 subunit of the nicotinic acetylcholine receptor is necessary for the anatomical and functional development of the visual system.

Neurotoxicity of channel mutations in heterologously expressed alpha7-nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChR) composed of chick alpha7 subunits mutated to threonine at amino acid valine-251 in the putative channel-lining M2 domain were expressed heterologously in several neuron-like and non-neuronal mammalian cell lines. Expression of mutant alpha7-nAChR is toxic to neuron-like cells of the human neuroblastoma cell lines SH-SY5Y and IMR-32, but not to several other cell types. Growth in the presence of the alpha7-nAChR antagonist methyllycaconitine (MLA) protects against neurotoxicity, as does gradual downregulation of functional, mutant alpha7-nAChR in surviving transfected SH-SY5Y cells. Relative to wild-type alpha7-nAChR, functional alpha7-nAChR mutants show a higher affinity for agonists, slower rates of desensitization, and sensitivity to dihydro-beta-erythroidine (DHbetaE) as an agonist, but they retain sensitivity to MLA as a competitive antagonist. These findings demonstrate that expression of hyperfunctional, mutant forms of Ca2+-permeable alpha7-nAChR is toxic to neuron-like cells.

Nicotine receptor inactivation decreases sensitivity to cocaine

The reinforcing properties of nicotine and psychomotor stimulants are thought to be mediated through the mesolimbic dopamine (DA) system. This study investigates the role of high affinity nicotinic acetylcholine receptors (nAChRs) in cocaine place preference and examines some neurochemical changes in the mesolimbic DA system that might account for the interaction between nicotine and cocaine. 5 mg/kg is the lowest dose of cocaine able to condition a place preference in C57Bl/6 mice. Co-treatment with the nicotinic antagonist mecamylamine (1.0 mg/kg) disrupted place preference to 5 mg/kg cocaine. In addition, mice lacking the high affinity nAChR containing the beta2 subunit showed decreased place preference to 5 mg/kg cocaine, although higher doses of cocaine could condition a place preference in these knock out animals. In contrast, co-administration of a low dose of nicotine (0.2 mg/kg) potentiated place preference to a subthreshold dose of cocaine (3 mg/kg). DA turnover was monitored in several brain regions using tissue levels of DA and its primary metabolite DOPAC as an indication of DA release. Wild type mice showed decreased DA turnover following treatment with 5 mg/kg cocaine; whereas, this response was not seen in mice lacking the beta2 subunit of the nAChR. Induction of chronic fos-related antigens by cocaine was also reduced in mutant mice as compared to their wild type siblings, implying that downstream actions of cocaine were also affected by inactivation of the high affinity nAChR. These data indicate that activation of the high affinity nAChR may contribute to cocaine reinforcement.

Altered neuroadaptation in opiate dependence and neurogenic inflammatory nociception in alpha CGRP-deficient mice

The neuropeptide alpha CGRP (calcitonin gene-related peptide) is involved in the complex process of pain signaling, but the precise contribution of alpha CGRP remains unclear. Here we show that mice lacking alpha CGRP display an attenuated response to chemical pain and inflammation. Furthermore, alpha CGRP(-/-) mice do not show changes in heroin self-administration or morphine tolerance, but display a marked decrease in morphine withdrawal signs, suggesting an important contribution of alpha CGRP to opiate withdrawal.

A method for soluble overexpression of the alpha7 nicotinic acetylcholine receptor extracellular domain

We describe the construction of a soluble protein carrying the N-terminal extracellular domain (ECD) of the alpha7 subunit of the nicotinic acetylcholine receptor. The approach was to fuse the alpha7 ECD at the C and N termini of several monomeric and pentameric soluble carrier proteins and to investigate the soluble expression of the product in Escherichia coli. An initial screening of six carrier proteins resulted in the selection of a fusion protein comprising, from the N to the C terminus, the maltose binding protein, a 17-aa linker containing an enterokinase binding site, and the alpha7 ECD. This protein is soluble upon expression in bacteria and is purified by affinity chromatography. It binds the competitive nicotinic antagonist alpha-bungarotoxin with 2.5 microM affinity and displays a CD spectrum corresponding to a folded protein. The method might be suitable to produce large quantities of protein for crystallization and immunochemical experiments.

Molecular and physiological diversity of nicotinic acetylcholine receptors in the midbrain dopaminergic nuclei

Nicotinic acetylcholine receptors (nAChRs) on dopaminergic (DA) and GABAergic (Gaba) projection neurons of the substantia nigra (SN) and ventral tegmental area (VTA) are characterized by single-cell RT-PCR and patch-clamp recordings in slices of rat and wild-type, beta2-/-, alpha4-/-, and alpha7-/- mice. The eight nAChR subunits expressed in these nuclei, alpha3-7 and beta2-4, contribute to four different types of nAChR-mediated currents. Most DA neurons in the SN and VTA express two nAChR subtypes. One is inhibited by dihydro-beta-erythroidine (2 microm), alpha-conotoxin MII (10 nm), and methyllycaconitine (1 nm) but does not contain the alpha7 subunit; it possesses a putative alpha4alpha6alpha5(beta2)(2) composition. The other subtype is inhibited by dihydro-beta-erythroidine (2 microm) and has a putative alpha4alpha5(beta2)(2) composition. Gaba neurons in the VTA exhibit a third subtype with a putative (alpha4)(2)(beta2)(3) composition, whereas Gaba neurons in the SN have either the putative (alpha4)(2)(beta2)(3) oligomer or the putative alpha4alpha6alpha5(beta2)(2) oligomer. The fourth subtype, a putative (alpha7)(5) homomer, is encountered in less than half of DA and Gaba neurons, in the SN as well as in the VTA. Neurons in the DA nuclei thus exhibit a diversity of nAChRs that might differentially modulate reinforcement and motor behavior.

Reward-dependent learning in neuronal networks for planning and decision making

Neuronal network models have been proposed for the organization of evaluation and decision processes in prefrontal circuitry and their putative neuronal and molecular bases. The models all include an implementation and simulation of an elementary reward mechanism. Their central hypothesis is that tentative rules of behavior, which are coded by clusters of active neurons in prefrontal cortex, are selected or rejected based on an evaluation by this reward signal, which may be conveyed, for instance, by the mesencephalic dopaminergic neurons with which the prefrontal cortex is densely interconnected. At the molecular level, the reward signal is postulated to be a neurotransmitter such as dopamine, which exerts a global modulatory action on prefrontal synaptic efficacies, either via volume transmission or via targeted synaptic triads. Negative reinforcement has the effect of destabilizing the currently active rule-coding clusters; subsequently, spontaneous activity varies again from one cluster to another, giving the organism the chance to discover and learn a new rule. Thus, reward signals function as effective selection signals that either maintain or suppress currently active prefrontal representations as a function of their current adequacy. Simulations of this variation-selection have successfully accounted for the main features of several major tasks that depend on prefrontal cortex integrity, such as the delayed-response test, the Wisconsin card sorting test, the Tower of London test and the Stroop test. For the more complex tasks, we have found it necessary to supplement the external reward input with a second mechanism that supplies an internal reward; it consists of an auto-evaluation loop which short-circuits the reward input from the exterior. This allows for an internal evaluation of covert motor intentions without actualizing them as behaviors, by simply testing them covertly by comparison with memorized former experiences. This element of architecture gives access to enhanced rates of learning via an elementary process of internal or covert mental simulation. We have recently applied these ideas to a new model, developed with M. Kerszberg, which hypothesizes that prefrontal cortex and its reward-related connections contribute crucially to conscious effortful tasks. This model distinguishes two main computational spaces within the human brain: a unique global workspace composed of distributed and heavily interconnected neurons with long-range axons, and a set of specialized and modular perceptual, motor, memory, evaluative and attentional processors. We postulate that workspace neurons are mobilized in effortful tasks for which the specialized processors do not suffice; they selectively mobilize or suppress, through descending connections, the contribution of specific processor neurons. In the course of task performance, workspace neurons become spontaneously co-activated, forming discrete though variable spatio-temporal patterns subject to modulation by vigilance signals and to selection by reward signals. A computer simulation of the Stroop task shows workspace activation to increase during acquisition of a novel task, effortful execution, and after errors. This model makes predictions concerning the spatio-temporal activation patterns during brain imaging of cognitive tasks, particularly concerning the conditions of activation of dorsolateral prefrontal cortex and anterior cingulate, their relation to reward mechanisms, and their specific reaction during error processing.

Tonic nicotinic modulation of serotoninergic transmission in the spinal cord

The spinal serotoninergic projection from the raphe magnus has been shown to modulate nociceptive inputs, and activation of this projection mediates nicotine-elicited analgesia. Here, we investigate the interactions between cholinergic and serotoninergic systems in the spinal cord, by conducting serotonin [5-hydroxytryptamine (5-HT)] efflux experiments on mouse spinal slices. At least three spinal populations of nicotinic receptors are distinguished that affect 5-HT release. The first could be directly located on serotoninergic terminals, is insensitive to nanomolar concentrations of methyllicaconitine (MLA), and may be subjected to a basal (not maximal) cholinergic tone. The second is tonically and maximally activated by endogenous acetylcholine, insensitive to nanomolar concentrations of MLA, and present on inhibitory neurons. The last is also present on inhibitory neurons but is sensitive to nanomolar concentrations of MLA and not tonically activated by acetylcholine. Multiple nicotinic acetylcholine receptor populations thus differentially exert tonic or not tonic control on 5-HT transmission in the spinal cord. These receptors may be major targets for nicotine effects on antinociception. In addition, the presence of a tonic nicotinic modulation of 5-HT release indicates that endogenous acetylcholine plays a role in the physiological regulation of descending 5-HT pathways to the spinal cord.

Desensitization of neuronal nicotinic acetylcholine receptors conferred by N-terminal segments of the beta 2 subunit

Desensitization is a general property of ligand-gated ion channels. Because of a wide array of available subunit combinations, it generates different time constants for channel closure, thereby modulating the processing of information in the brain. Within the family of neuronal nicotinic acetylcholine receptors (nAChRs), alpha 3 beta 2 and alpha 3 beta 4 receptors display contrasting properties of desensitization. When measured using two-electrode voltage-clamp in Xenopus oocytes, desensitization results in current decreases 2 s after initiation of acetylcholine application by 94% for alpha 3 beta 2 receptors, but only by 6% in the case of alpha 3 beta 4 receptors. Desensitization was analyzed by inserting different portions of the beta2 into the beta 4 subunit. Residues 1--212 of the beta2 subunit were able to confer 78% desensitization in 2 s, while smaller chimeras revealed desensitization in 2 s conferred by residues 1--42 alone to a level of 50%, by residues 72--89 to a level of 74%, and by residues 96--212 to a level of 77%. Some long-term (25 min) effects of desensitization driven by acetylcholine were found to rely partially on the same elements, including an enhancement mediated by residues 1--95 and 96--212 of the beta 2 subunit individually. Our results reveal that desensitization relies independently on diverse portions of the extracellular domain of the beta 2 subunit. Phenotype of alpha 3 beta 4 involves, in contrast, complex structural requirements involving residues dispersed throughout the entire N-terminal domain of the beta 4 subunit.

Electrical activity regulates AChR gene expression via JNK, PKCzeta and Sp1 in skeletal chick muscle

Electrical activity of myotubes represses nicotinic acetylcholine receptor (AChR) gene expression. This effect is mimicked by okadaic acid and blocked by tetrodotoxin (TTX) or staurosporine in cultured myocytes [Altiok et al., EMBO J. 16 (1997) 717-725]. In this study, we investigated the mechanism of this repression. We show that addition of exogenous phospholipase D (PLD) and C inhibits AChR expression in a manner which parallels that of okadaic acid. Furthermore, okadaic acid caused an increase of the threonine phosphorylation of protein kinase Czeta (PKCzeta) and activator of transcription factor (ATF2) and a decrease of the phosphorylation of Sp1. All these effects were reversed by staurosporine, and TTX also abolished ATF2 phosphorylation. These data reveal a possible involvement of PLD, c-jun N-terminal kinase, PKCzeta and Sp1 in the repression of AChR genes by electrical activity.

Nicotinic agonists stimulate acetylcholine release from mouse interpeduncular nucleus: a function mediated by a different nAChR than dopamine release from striatum

Acetylcholine release stimulated by nicotinic agonists was measured as radioactivity released from perfused synaptosomes prepared from mouse interpeduncular nucleus (IPN) that had been loaded with [(3)H]choline. Agonist-stimulated release was dependent upon external calcium and over 90% of released radioactivity was acetylcholine. The release process was characterized by dose response curves for 13 agonists and inhibition curves for six antagonists. alpha-Conotoxin MII did not inhibit this release, while alpha-conotoxin AuIB inhibited 50% of agonist-stimulated release. Comparison of this process with [(3)H]dopamine release from mouse striatal synaptosomes indicated that different forms of nicotinic acetylcholine receptors (nAChRs) may mediate these processes. This was confirmed by assays using mice homozygous for the beta 2 subunit null mutation. The deletion of the beta 2 subunit had no effect on agonist-stimulated acetylcholine release, but abolished agonist-stimulated release of dopamine from striatal synaptosomes. Mice heterozygous for the beta 2 subunit null mutation showed decreased dopamine release evoked by L-nicotine with no apparent change in EC(50) value, as well as similar decreases in both transient and persistent phases of release with no changes in desensitization rates.

LGICdb: the ligand-gated ion channel database

Ligand-Gated Ion Channels (LGIC) are polymeric transmembrane proteins involved in the fast response to numerous neurotransmitters. All these receptors are formed by homologous subunits and the last two decades revealed an unexpected wealth of genes coding for these subunits. The Ligand-Gated Ion Channel database (LGICdb) has been developed to handle this increasing amount of data. The database aims to provide only one entry for each gene, containing annotated nucleic acid and protein sequences. The repository is carefully structured and the entries can be retrieved by various criteria. In addition to the sequences, the LGICdb provides multiple sequence alignments, phylogenetic analyses and atomic coordinates when available. The database is accessible via the World Wide Web (http://www.pasteur.fr/recherche/banques/LGIC /LGIC.html), where it is continuously updated. The version 16 (September 2000) available for download contained 333 entries covering 34 species.

Localization of nAChR subunit mRNAs in the brain of Macaca mulatta

We present here a systematic mapping of nAChR subunit mRNAs in Macaca mulatta brain. A fragment, from the transmembrane segments MIII to MIV of Macaca neuronal nAChR subunits was cloned, and shown to exhibit high identity (around 95%) to the corresponding human subunits. Then, specific oligodeoxynucleotides were synthesized for in situ hybridization experiments. Both alpha4 and beta2 mRNA signals were widely distributed in the brain, being stronger in the thalamus and in the dopaminergic cells of the mesencephalon. Most brain nuclei displayed both alpha4 and beta2 signals with the exception of some basal ganglia regions and the reticular thalamic nucleus which were devoid of alpha4 signal. alpha6 and beta3 mRNA signals were selectively concentrated in the substantia nigra and the medial habenula. The strongest signals for alpha3 or beta4 mRNAs were found in the epithalamus (medial habenula and pineal gland), whereas there were no specific alpha3 or beta4 signals in mesencephalic dopaminergic nuclei. alpha5 and alpha7 mRNA signals were found in several brain areas, including cerebral cortex, thalamus and substantia nigra, although at a lower level than alpha4 and beta2. The distribution of alpha3, alpha4, alpha5, alpha6, alpha7, beta2, beta3 and beta4 subunit mRNAs in the monkey is substantially similar to that observed in rodent brain. Surprisingly, alpha2 mRNA signal was largely distributed in the Macaca brain, at levels comparable with those of alpha4 and beta2. This observation represents the main difference between rodent and Macaca subunit mRNA distribution and suggests that, besides alpha4beta2*, alpha2beta2* nAChRs constitute a main nAChR isoform in primate brain.

Nicotinic-agonist stimulated (86)Rb(+) efflux and [(3)H]epibatidine binding of mice differing in beta2 genotype

Nicotinic acetylcholine receptor function and binding was measured in 12 brain regions from mice differing in beta2 subunit expression. Function was measured by on-line detection of (86)Rb(+) efflux stimulated under conditions that measure two pharmacologically distinct nicotinic responses: (1) stimulation with 10 microM nicotine, a response that is relatively sensitive to inhibition by the antagonist, dihydro-beta-erythroidine (DHbetaE); and (2) stimulation with 10 microM epibatidine in the presence of 2 microM DHbetaE, a response that is relatively resistant to inhibition by DHbetaE. Deletion of the beta2 subunit profoundly reduced both DHbetaE-sensitive and -resistant (86)Rb(+) efflux in each brain region and essentially eliminated activity in regions such as cerebral cortex and thalamus. However, residual activity was observed in regions such as olfactory bulbs and inferior colliculus. [(3)H]Epibatidine binding was measured under conditions that allow estimation of both high- and low-affinity sites. High-affinity sites sensitive to inhibition by the nicotinic agonist, cytisine, were virtually eliminated in every region by the beta2 null mutation. In contrast, only a subset of the high-affinity sites insensitive to inhibition by cytisine were eliminated in beta2 null mutants, suggesting receptor heterogeniety. Similarly, low affinity [(3)H]epibatidine binding was heterogeneous in that a fraction of the sites required the beta2 subunit. Many remaining sites were sensitive to inhibition by alpha-bungarotoxin indicating that a subset of the low affinity [(3)H]epibatidine binding are of the alpha7* subtype. Distinct regional variation was observed among the 12 brain regions. These studies confirm important roles for beta2-containing receptors in mediating pharmacologically distinct functions and as components of several identifiable binding sites.

Molecular determinants by which a long chain toxin from snake venom interacts with the neuronal alpha 7-nicotinic acetylcholine receptor

Long chain curarimimetic toxins from snake venom bind with high affinities to both muscular type nicotinic acetylcholine receptors (AChRs) (K(d) in the pm range) and neuronal alpha 7-AChRs (K(d) in the nm range). To understand the molecular basis of this dual function, we submitted alpha-cobratoxin (alpha-Cbtx), a typical long chain curarimimetic toxin, to an extensive mutational analysis. By exploring 36 toxin mutants, we found that Trp-25, Asp-27, Phe-29, Arg-33, Arg-36, and Phe-65 are involved in binding to both neuronal and Torpedo (Antil, S., Servent, D., and Ménez, A. (1999) J. Biol. Chem. 274, 34851-34858) AChRs and that some of them (Trp-25, Asp-27, and Arg-33) have similar binding energy contributions for the two receptors. In contrast, Ala-28, Lys-35, and Cys-26-Cys-30 selectively bind to the alpha 7-AChR, whereas Lys-23 and Lys-49 bind solely to the Torpedo AChR. Therefore, alpha-Cbtx binds to two AChR subtypes using both common and specific residues. Double mutant cycle analyses suggested that Arg-33 in alpha-Cbtx is close to Tyr-187 and Pro-193 in the alpha 7 receptor. Since Arg-33 of another curarimimetic toxin is close to the homologous alpha Tyr-190 of the muscular receptor (Ackermann, E. J., Ang, E. T. H., Kanter, J. R., Tsigelny, I., and Taylor, P. (1998) J. Biol. Chem. 273, 10958-10964), toxin binding probably occurs in homologous regions of neuronal and muscular AChRs. However, no coupling was seen between alpha-Cbtx Arg-33 and alpha 7 receptor Trp-54, Leu-118, and Asp-163, in contrast to what was observed in a homologous situation involving another toxin and a muscular receptor (Osaka, H., Malany, S., Molles, B. E., Sine, S. M., and Taylor, P. (2000) J. Biol. Chem. 275, 5478-5484). Therefore, although occurring in homologous regions, the detailed modes of toxin binding to alpha 7 and muscular receptors are likely to be different. These data offer a molecular basis for the design of toxins with predetermined specificities for various members of the AChR family.

Nicotinic receptors at the amino acid level

nAChRs are pentameric transmembrane proteins into the superfamily of ligand-gated ion channels that includes the 5HT3, glycine, GABAA, and GABAC receptors. Electron microscopy, affinity labeling, and mutagenesis experiments, together with secondary structure predictions and measurements, suggest an all-beta folding of the N-terminal extracellular domain, with the connecting loops contributing to the ACh binding pocket and to the subunit interfaces that mediate the allosteric transitions between conformational states. The ion channel consists of two distinct elements symmetrically organized along the fivefold axis of the molecule: a barrel of five M2 helices, and on the cytoplasmic side five loops contributing to the selectivity filter. The allosteric transitions of the protein underlying the physiological ACh-evoked activation and desensitization possibly involve rigid body motion of the extracellular domain of each subunit, linked to a global reorganization of the transmembrane domain responsible for channel gating.

Control of neurulation by the nucleosome assembly protein-1-like 2

Neurulation is a complex process of histogenesis involving the precise temporal and spatial organization of gene expression. Genes influencing neurulation include proneural genes determining primary cell fate, neurogenic genes involved in lateral inhibition pathways and genes controlling the frequency of mitotic events. This is reflected in the aetiology and genetics of human and mouse neural tube defects, which are of both multifactorial and multigenic origin. The X-linked gene Nap1l2, specifically expressed in neurons, encodes a protein that is highly similar to the nucleosome assembly (NAP) and SET proteins. We inactivated Nap1l2 in mice by gene targeting, leading to embryonic lethality from mid-gestation onwards. Surviving mutant chimaeric embryos showed extensive surface ectoderm defects as well as the presence of open neural tubes and exposed brains similar to those observed in human spina bifida and anencephaly. These defects correlated with an overproduction of neuronal precursor cells. Protein expression studies showed that the Nap1l2 protein binds to condensing chromatin during S phase and in apoptotic cells, but remained cytoplasmic during G1 phase. Nap1l2 therefore likely represents a class of tissue-specific factors interacting with chromatin to regulate neuronal cell proliferation.

Calcium mobilization elicited by two types of nicotinic acetylcholine receptors in mouse substantia nigra pars compacta

Nicotinic acetylcholine receptors (nAChRs) are expressed in the midbrain ascending dopaminergic system, a target of many addictive drugs. Here we assessed the intracellular Ca2+ level by imaging fura-2-loaded cells in substantia nigra pars compacta in mouse brain slices, and we examined the influence on this level of prolonged exposures to nicotine using mice lacking the nAChR beta2-subunit. In control cells, superfusion with nicotine (10-100 microM) caused a long-lasting rise of intracellular Ca2+ level which depended on extracellular Ca2+. This nicotinic response was almost completely absent in beta2-/- mutant mice, leaving a small residual response to a high concentration (100 microM) of nicotine which was inhibited by the alpha7-subunit-selective antagonist, methyllycaconitine. Conversely, the alpha7-subunit-selective agonist choline (10 mM) caused a methyllycaconitine-sensitive increase in intracellular Ca2+ level both in wild-type and beta2-/- mutant mice. Nicotine-elicited Ca2+ mobilization was reduced by the Na+ channel blocker tetrodotoxin (TTX) and by T-type Ca2+ channel blocking agents, whereas the choline-elicited Ca2+ increase was insensitive to TTX. Neither nicotine nor choline produced Ca2+ increase following inhibition of the release of Ca2+ from intracellular stores by dantrolene. These results demonstrate that in nigral dopaminergic neurons, nicotine can elicit Ca2+ mobilization via activation of two distinct nAChR subtypes: that of beta2-subunit-containing nAChR followed by activation of Na+ channel and T-type Ca2+ channels, and/or activation of alpha7-subunit-containing nAChR. The Ca2+ influx due to nAChR activation is subsequently amplified by the recruitment of intracellular Ca2+ stores. This Ca2+ mobilization may possibly contribute to the long-term effects of nicotine on the dopaminergic system.

Structural differences in the two agonist binding sites of the Torpedo nicotinic acetylcholine receptor revealed by time-resolved fluorescence spectroscopy

The nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata carries two nonequivalent agonist binding sites at the alphadelta and alphagamma subunit interfaces. These sites have been characterized by time-resolved fluorescence with the partial nicotinic agonist dansyl-C(6)-choline (Dnscho). When bound to the detergent-solubilized receptor, the fluorescence lifetime distribution of Dnscho displays a characteristic signature, with four separable components at 0.2, 1.8, 7.2, and 18.3 ns, respectively. Competition experiments with the antagonist d-tubocurarine (dTC), known to bind preferentially to the alphagamma site, result in substantial changes of this signature, associated with a strong decrease in average fluorescence lifetime. Comparisons with two other competitive antagonists, alpha-conotoxin M1 and alpha-bungarotoxin, demonstrate that Dnscho binds with a similar affinity to the two sites but that the microenvironment of the probe is different for each site. Using a two-site binding model together with published equilibrium constants to describe the competitive binding of dTC and Dnscho, we reach a satisfactory description of the changes in fluorescence lifetimes and propose characteristic fluorescence parameters of the probe bound to each type of site. This analysis indicates that Dnscho at the alphadelta site is principally associated with a 8.7 ns lifetime, while it has a 20.2 ns major lifetime at the alphagamma site. Therefore, the observed fluorescence heterogeneity arises in large part from the structural differences of the two binding sites. As a result, this signal can be used to identify the binding preferences of competitive ligands of unknown pharmacology.

Nicotinic receptor function: new perspectives from knockout mice

Knockout mice, in which one or more genes of interest are silenced, provide unique opportunities to analyse diverse aspects of gene function in vivo. In particular, the contribution of the encoded protein(s) in complex behaviours can be assessed. Since the first targeted disruption in 1995 of the gene encoding the beta2-subunit of the nicotinic acetylcholine receptor (nAChR), all but a few of the mammalian nAChR subunits have been disrupted (i.e. alpha7, alpha4, alpha3, alpha9, beta4 and beta3). Recent advances brought by genetically modified mice to our understanding of the endogenous composition and role of nAChRs in the nervous system, and of the diverse pharmacological actions of nicotine regarding learning, analgesia, reinforcement, development and aging in the brain will be discussed.

Computational models of association cortex

Recent computational models, or mathematical realizations of neurobiological theories, are providing insights into the organization and workings of the association cortex. Such models concern the construction of cortical maps, the neural basis of cognitive functions such as visual perception, reward-motivated learning and some aspects of consciousness.

Pharmacological and null mutation approaches reveal nicotinic receptor diversity

We have developed an array of assays for nicotinic acetylcholine receptor binding and function. [125I]alpha-Bungarotoxin-, (-)-[3H]nicotine-, and [3H]epibatidine-binding nicotinic acetylcholine receptors were assayed in mouse brain membranes and sections. Nicotinic acetylcholine receptor function was quantified using synaptosomal [3H]dopamine, [3H]gamma-aminobutyric acid ([3H]GABA), and 86Rb(+) efflux techniques. Additionally, the effects of beta2 subunit deletion on each of the measures were assessed. Detailed pharmacological comparison revealed minimally six nicotinic binding subtypes: [125I]alpha-bungarotoxin-binding nicotinic acetylcholine receptors; beta2-subunit-dependent and -independent high-affinity (-)-[3H]nicotine-binding sites; beta2-dependent and -independent cytisine-resistant [3H]epibatidine-binding sites; and a beta2-dependent low-affinity [3H]epibatidine binding site. Comparative pharmacology suggested that [3H]GABA and dihydro-beta-erythroidine (DHbetaE)-sensitive 86Rb(+) efflux are mediated by the same (probably alpha4beta2) nicotinic acetylcholine receptor subtype, while other nicotinic acetylcholine receptor subtypes evoke [3H]dopamine and DHbetaE-resistant 86Rb(+) efflux. In whole-brain preparations, each measure of nicotinic acetylcholine receptor function was beta2 dependent. The majority of beta2-independent [3H]epibatidine binding was located in small, scattered brain nuclei, suggesting that individual nuclei may prove suitable for identification of novel, native nicotinic acetylcholine receptors.

Molecular characterization of the specificity of interactions of various neurotoxins on two distinct nicotinic acetylcholine receptors

Snake curaremimetic toxins are currently classified as short-chain and long-chain toxins according to their size and their number of disulfide bonds. All these toxins bind with high affinity to muscular-type nicotinic acetylcholine receptor, whereas only long toxins recognize the alpha7 receptor with high affinity. On the basis of binding experiments with Torpedo or neuronal alpha7 receptors using wild-type and mutated neurotoxins, we characterized the molecular determinants involved in these different recognition processes. The functional sites by which long and short toxins interact with the muscular-type receptor include a common core of highly conserved residues and residues that are specific to each of toxin families. Furthermore, the functional sites through which alpha-cobratoxin, a long-chain toxin, interacts with muscular and alpha7 receptors share similarities but also marked differences. Our results reveal that the three-finger fold toxins have evolved toward various specificities by displaying distinct functional sites.

Hierarchical neuronal modeling of cognitive functions: from synaptic transmission to the Tower of London

Recent progress in the molecular biology of synaptic transmission, in particular of neurotransmitter receptors, offers novel information relevant to 'realistic' modeling of neural processes at the single cell and network level. Sophisticated computer analyses of two-dimensional crystals by high resolution electron microscopy yield images of single neurotransmitter receptor molecules with tentative identifications of ligand binding sites and of conformational transitions. The dynamics of conformational changes can be accounted for by a 'multistate allosteric network' model. Allosteric receptors also possess the structural and functional properties required to serve as coincidence detectors between pre- and post-synaptic signals and, therefore, can be used as building blocks for a chemical Hebb synapse. These properties were introduced into networks of formal neurons capable of producing and detecting temporal sequences. In more elaborate models of pre-frontal cortex functions, allosteric receptors control the selection of transient 'pre-representations' and their stabilization by external or internal reward signals. We apply this scheme to Shallice's Tower of London test, and we show how a hierarchical neuronal architecture can implement executive or planning functions associated with frontal areas. (Académie des sciences/Elsevier, Paris.)

Specific phosphorylation of Torpedo 43K rapsyn by endogenous kinase(s) with thiamine triphosphate as the phosphate donor

43K rapsyn is a peripheral protein specifically associated with the nicotinic acetylcholine receptor (nAChR) present in the postsynaptic membrane of the neuromuscular junction and of the electrocyte, and is essential for its clustering. Here, we demonstrate a novel specific phosphorylation of 43K rapsyn by endogenous protein kinase(s) present in Torpedo electrocyte nAChR-rich membranes and identify thiamine triphosphate (TTP) as the phosphate donor. In the presence of Mg(2+) and [gamma-(32)P]-TTP, 43K rapsyn is specifically phosphorylated with a (32)P-half-maximal incorporation at approximately 5-25 microM TTP. The presence of TTP in the cytosol and of 43K rapsyn at the cytoplasmic face of the postsynaptic membrane, together with TTP-dependent phosphorylation of 43K rapsyn without added exokinases, suggests that TTP-dependent-43K-rapsyn phosphorylation may occur in vivo. In addition, phosphoamino acid and chemical stability analysis suggests that the residues phosphorylated are predominantly histidines. Inhibition of phosphorylation by Zn(2+) suggests a possible control of 43K rapsyn phosphorylation state by its zinc finger domain. Endogenous kinase(s) present in rodent brain membranes can also use [gamma-(32)P]-TTP as a phosphodonor. The use of a phosphodonor (TTP) belonging to the thiamine family but not to the classical (ATP, GTP) purine triphosphate family represents a novel phosphorylation pathway possibly important for synaptic proteins.

How well can molecular modelling predict the crystal structure: the case of the ligand-binding domain of glutamate receptors

The concept that the ligand-binding domain of vertebrate glutamate receptor channels and bacterial periplasmic substrate-binding proteins (PBPs) share similar three-dimensional (3D) structures has gained increasing support in recent years. On the basis of a dual approach that included computer-assisted molecular modelling and functional studies of site-specific mutants, theoretical 3D models of this domain have been proposed. This article reviews to what extent these models could predict the crystal structure of the ligand-binding domain of an ionotropic glutamate receptor subunit recently determined at high resolution by X-ray diffraction studies.

Oral administration of a dual analog of two myasthenogenic T cell epitopes down-regulates experimental autoimmune myasthenia gravis in mice

Myasthenia gravis (MG) and experimental autoimmune MG (EAMG) are T cell-regulated, antibody-mediated autoimmune diseases. The major autoantigen in MG is the nicotinic acetylcholine receptor (AChR). Two peptides, representing sequences of the human AChR alpha-subunit, p195-212 and p259-271, were previously shown to be immunodominant T cell epitopes in MG patients as well as, respectively, in SJL and BALB/c mice. A dual analog (termed Lys-262-Ala-207) composed of the tandemly arranged two single amino acid analogs of p195-212 and p259-271 was shown to inhibit, in vitro and in vivo, MG-associated autoimmune responses. Furthermore, the dual analog could down-regulate myasthenogenic manifestations in mice with EAMG that was induced by inoculation of a pathogenic T cell line. In the present study, the ability of the dual analog to treat EAMG induced in susceptible C57BL/6 mice by native Torpedo AChR was evaluated. Mice that were diagnosed to have clinical symptoms of EAMG were treated with the dual analog by oral administration, 500 microg per mouse three times a week for 5-8 weeks. Treatment with the dual analog down-regulated the clinical manifestations of the ongoing disease as assessed by the clinical score, grip strength (measured by a grip strength meter), and electromyography. The effects on the clinical EAMG correlated with a reduced production of anti-AChR antibody as well as a decrease in the secretion of interleukin-2 and, more dramatically, interferon-gamma, in response to AChR triggering. Thus, the dual analog is an efficient immunomodulator of EAMG in mice and might be of specific therapeutic potential for MG.

Diversity and distribution of nicotinic acetylcholine receptors in the locus ceruleus neurons

The neurons of the locus ceruleus are responsible for most of the noradrenergic innervation in the brain and nicotine potentiates noradrenaline release from their terminals. Here we investigated the diversity and subcellular distribution of nicotinic acetylcholine receptors (nAChRs) in the locus ceruleus both somatically, by combining single-cell reverse transcription-PCR with electrophysiological characterization, and at the level of nerve terminals, by conducting noradrenaline efflux experiments. The proportion of neurons in the locus ceruleus expressing the nicotinic subunit mRNAs varied from 100% (beta2) to 3% (alpha2). Yet, two populations of neurons could be distinguished on the basis of the pattern of expression of nAChR mRNAs and electrophysiological properties. One population (type A) of small cells systematically expressed alpha3 and beta4 mRNAs (and often alpha6, beta3, alpha5, alpha4), and nicotinic agonists elicited large currents with a potency order of cytisine > nicotine. Another population (type B) of cells with large soma did not contain alpha3 and beta4 mRNAs but, systematically, alpha6 and beta3 (and often alpha4) and responded to nicotinic agonists in the order of nicotine > cytisine. The nicotinic modulation of noradrenaline release in the hippocampus displayed an order of potency nicotine > cytisine, suggesting that noradrenergic terminals in the hippocampus originate largely from type B cells of the locus ceruleus. Accordingly, immunocytochemical labeling showed that beta3 is present in hippocampal terminals. The alpha6beta3beta2(alpha4) heterooligomer thus behaves as the main nicotinic regulator of the ceruleo-hippocampal pathway.

Molecular basis of the charge selectivity of nicotinic acetylcholine receptor and related ligand-gated ion channels

Nicotinic acetylcholine receptors are homo- or heteropentameric proteins belonging to the superfamily of receptor channels including the glycine and GABA-A receptors. Affinity labelling and mutagenesis experiments indicated that the M2 transmembrane segment of each subunit lines the ion channel and is coiled into an alpha-helix. Comparison of the M2 sequence of the cation-selective alpha 7 nicotinic receptor to that of the anion-selective alpha 1 glycine receptor identified amino acids involved in charge selectivity. Mutations of the alpha 7 homo-oligomeric receptor within (or near) M2, namely E237A, V251T and a proline insertion P236' were shown to convert the ionic selectivity of alpha 7 from cationic to anionic. Systematic analysis of each of these three mutations supports the notion that the conversion of ionic selectivity results from a local structural reorganization of the 234-238 loop. The 234-238 coiled loop, previously shown to lie near the narrowest portion of the channel, is thus proposed to contribute directly to the charge selectivity filter. A possible functional analogy with the voltage-gated ion channels and related receptors is discussed.

Involvement of alpha6 nicotinic receptor subunit in nicotine-elicited locomotion, demonstrated by in vivo antisense oligonucleotide infusion

Enhanced locomotion in a habituated environment is a well documented effect of nicotine mediated by the mesotelencephalic dopaminergic system. The nicotinic receptor subunit alpha6 is, among other subunits, strongly expressed in the dopaminergic neurons of the mesencephalon. To examine the functional role of this subunit, we inhibited its expression in vivo using antisense oligonucleotides. In vitro treatments of embryonic mesencephalic neuron cultures demonstrated that the alpha6 antisense oligonucleotides caused a marked decrease in the level of alpha6 subunit protein. In vivo, 1 week infusion of alpha6 antisense oligonucleotides by osmotic mini-pump reduced the effect of nicotine on locomotor activity in habituated environment by 70%. These data support the notion that the effects of nicotine on the dopaminergic system involve alpha6 subunit containing nAChRs.

Ultrastructural localization of the alpha4-subunit of the neuronal acetylcholine nicotinic receptor in the rat substantia nigra

The distribution of the alpha4-subunit of the neuronal nicotinic acetylcholine receptor (nAChR) in the rat brain was examined at light and electron microscopy levels using immunohistochemical staining. In the present study we demonstrate the specificity, in both tissue homogenates and brain sections, of a polyclonal antibody raised against the rat nAChR alpha4-subunit. The characterization of this antibody involved: (1) Western blot analysis of rat brain homogenates and membrane extracts from cells previously transfected with diverse combinations of neuronal nAChR subunits, and (2) immunohistochemistry using transfected cells and rat brain tissue. At the light microscope level, the alpha4-subunit-like-immunoreactivity (LI) was widely distributed in the rat brain and matched the distribution of the alpha4-subunit transcripts observed previously by in situ hybridization. Strong immunohistochemical labeling was detected in the mesencephalic dopaminergic nuclei. The nAChRs in this region are thought to be responsible for the modulation of dopaminergic transmission. The neurotransmitter identity of alpha4-immunolabeled neurons in the substantia nigra pars compacta (SNpc) and the ventral tegmental area was thus assessed by investigating the possible colocalization of the nAChR alpha4-subunit with tyrosine hydroxylase using confocal microscopy. The double labeling experiments unambiguously indicated that the alpha4-subunit-LI is present in dopaminergic neurons. At the electron microscope level, the neurons in the SNpc exhibited alpha4-subunit-LI in association with a minority of postsynaptic densities, suggesting that the alpha4-subunit may be a component of functional nAChRs mediating synaptic transmission between midbrain cholinergic neurons and mesencephalic dopaminergic neurons.