Pentameric structure and subunit stoichiometry of a neuronal nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors in separate brain regions exhibit different affinities for methyllycaconitine

The family of nicotinic acetylcholine receptors contains numerous subtypes. Since the subunit compositions of most native neuronal nicotinic receptors are unknown, an important method for distinguishing subtypes of functional neuronal receptors is based on pharmacological criteria, such as affinity for snake toxins. We have now examined the affinities of native chick nicotinic receptors for methyllycaconitine, a toxin purified from Delphinium. We find that methyllycaconitine is a potent antagonist at central nicotinic receptors located on Edinger-Westphal neurons, producing nearly complete functional blockade of nicotinic responses at 10 nM. In marked contrast, methyllycaconitine is 1000-fold less potent at blocking nicotinic responses in the lateral spiriform nucleus. Methyllycaconitine inhibits kappa-bungarotoxin-sensitive nicotinic receptors in ciliary ganglia at 0.5-1.0 microM. Radioligand binding studies also reveal heterogeneity in the affinity of the toxin for nicotinic receptors. Methyllycaconitine binds most avidly to [125I] alpha-bungarotoxin sites in brain (Ki = 5.4 nM), and is 200-fold less potent at muscle nicotinic receptors (IC50 = 1.1 microM). The least potent binding of the toxin is to [3H]nicotine sites in brain (Ki = 3.7 microM). Methyllycaconitine is thus a useful pharmacological tool for distinguishing certain subtypes of native nicotinic receptors. The relatively low affinity of the toxin for nicotinic receptors in the lateral spiriform nucleus is consistent with the known properties of these receptors, which include a high affinity for [3H]nicotine and a lack of sensitivity to alpha- and kappa-bungarotoxin. On the basis of high affinity for methyllycaconitine and insensitivity to alpha-bungarotoxin, the nicotinic receptors in the Edinger-Westphal nucleus are unlike any previously described nicotinic receptor subtype.

Characterization of nicotinic receptors in immortalized hippocampal neurons

Nicotinic acetylcholine receptors, particularly nicotinic alpha-bungarotoxin (alpha-BGT) receptors, are present in relatively high concentrations in rat hippocampus. Because of the difficulties encountered in studying receptors using primary cells in culture, especially for biochemical work, we investigated the possibility of using an immortalized cell line from embryonic rat hippocampus (H19-7). RNase protection assays show that alpha 4, alpha 7 and beta 2 neuronal nicotinic receptor subunit mRNAs are present in differentiated but not undifferentiated H19-7 cells, while alpha 2, alpha 3, alpha 5 and beta 3 subunit mRNAs were not detectable under either condition. In line with these results, the present data demonstrate that the H19-7 cells express cell surface nicotinic alpha-BGT binding sites, which were maximal after seven days of differentiation in culture. The receptors were saturable, of high affinity (Kd = 1.30 nM and Bmax = 11.70 fmol/10(5) cells) and had a pharmacological profile similar to that observed for CNS alpha-BGT receptors. On the other hand, although alpha 4 and beta 2 neuronal nicotinic subunit mRNAs were present in differentiated H19-7 cells, no [3H]cytisine binding was observed. Because immortalized cell lines have the advantage that they provide a limitless supply of cells as compared to primary cell cultures, but yet are not malignant in origin, the present results may suggest that the H19-7 immortalized hippocampal cell line represent a useful CNS model system for examining alpha-BGT nicotinic receptors.

Pentameric subunit stoichiometry of a neuronal glutamate receptor

Ionotropic glutamate receptors, neurotransmitter-activated ion channels that mediate excitatory synaptic transmission in the central nervous system, are oligomeric membrane proteins of unknown subunit stoichiometry. To determine the subunit stoichiometry we have used a functional assay based on the blockade of two alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate receptor subunit 1 (GluR1) mutant subunits selectively engineered to exhibit differential sensitivity to the open channel blockers phencyclidine and dizolcipine (MK-801). Coinjection into amphibian oocytes of weakly sensitive with highly sensitive subunit complementary RNAs produces functional heteromeric channels with mixed blocker sensitivities. Increasing the fraction of the highly sensitive subunit augmented the proportion of drug-sensitive receptors. Analysis of the data using a model based on random aggregation of receptor subunits allowed us to determine a pentameric stoichiometry for GluR1. This finding supports the view that a pentameric subunit organization underlies the structure of the neuronal ionotropic glutamate receptor gene family.

Functional contributions of alpha5 subunit to neuronal acetylcholine receptor channels

Ligand-gated ion channels are multi-subunit complexes where each subunit-type is encoded by several related genes. Heterologous expression of any one of the neuronal nicotinic acetylcholine receptors (nAChR) alpha-type subunits, either alone or with any beta-type subunit, typically yields functional nAChR channels. A striking exception is the nAChR alpha5 subunit: although apparently complexed with beta2 and beta4 nAChR subunits in neurons, and expressed in a subset of neurons within the central and peripheral nervous systems, heterologous expression of alpha5, either alone or with any beta-type subunit has failed to yield functional channels. We demonstrate here that alpha5 does participate in nAChRs expressed in hetrologous systems and in primary neurons, and further that alpha5 contributes to the lining of functionally unique nAChR channels, but only if coexpressed with both another alpha- and beta-type subunit. Furthermore, channels containing the alpha5 subunit are potently activated and desensitized by nanomolar concentrations of nicotine.

Insights into the activation mechanism of rho1 GABA receptors obtained by coexpression of wild type and activation-impaired subunits

To gain insight into the activation mechanism of homomeric ligand-gated receptor-channels, we examined human homomeric rho1 GABA receptors with fewer than the normal number of agonist binding sites. This was accomplished by coexpressing different ratios of wild type and activation-impaired rho1 subunits. Dose-response relations from oocytes coexpressing wild type and mutant subunits were comprised of two components in terms of GABA sensitivity; one 'wild type'-like and the other 'mutant'-like. Applying the binomial hypothesis to subunit coassembly enabled use to correlate these two components of the GABA dose-response relations to the underlying chimaeric receptor subtypes. We demonstrate that the receptors activate near normal provided that they are comprised of at least three wild type subunits. Our data are consistent with five equivalent and independent GABA binding sites of which only three need bind GABA to open the pore. The two additional binding sites may increase the GABA sensitivity of the rho1 receptor and, when bound by agonist, stabilize the open state.

A molecular scheme for the reaction between gamma-aminobutyric acid and the most abundant chloride channel on crayfish deep extensor abdominal muscle

Single-channel measurements were performed with the aim of constructing a detailed molecular scheme for the reaction between gamma-aminobutyric acid (GABA) and a chloride channel of crayfish deep extensor abdominal muscle (DEAM). GABA was applied in pulses to outside-out patches of muscle membrane, and, based on the dose-response of the peak currents and of their rise times, a linear model with five binding steps has been proposed. Evaluation of the single-channel kinetics indicated at least three open states. Two of them originate most probably from the fully liganded receptor state and are grouped in mixed bursts due to their different life times. The third one appears independently, outside the bursts, and originates from a lower liganded receptor state. Simulations of the dose-responses and the open time distributions with this model led to a set of rate constants which generated relatively optimal fits.

Neuronal nicotinic alpha 7 receptor expressed in Xenopus oocytes presents five putative binding sites for methyllycaconitine

1. The recently isolated compound methyllycaconitine (MLA) is a plant toxin which is a competitive inhibitor of nicotinic acetylcholine receptors (nAChRs). We found that homomeric alpha 7 receptors display a very high sensitivity to MLA with an IC50 in the picomolar range. 2. The competitive nature of the alpha 7 MLA blockade was reinforced by the observation that this compound has no action on wild-type serotoninergic receptors (5-HT3), whereas it is a powerful antagonist of chimaeric receptors alpha 7-5-HT3. 3. The time course of MLA inhibition of the wild-type (WT) alpha 7 follows a monotonic exponential decay whose time constant is proportional to the MLA concentration and could be described by a bimolecular mechanism with a forward rate constant (k+) of 2.7 x 10(7) S-1 M-1. In contrast, recovery from MLA inhibition displays an S-shaped time course that is incompatible with a simple bimolecular reaction. 4. Given the pentameric nature of the neuronal nicotinic receptors, a linear chain model, including five putative MLA binding sites corresponding to the homomeric nature of alpha 7, is proposed. 5. Both onset and recovery data obtained on the alpha 7 wild-type receptor are adequately described by this model assuming that a single MLA molecule is sufficient to block receptor function. 6. Analysis of MLA blockade and recovery of reconstituted heteromeric alpha 4 beta 2 receptors reveals, as expected, a time course compatible with only two binding sites for the toxin and, thus, further supports the validity of our model.

Post-translational regulation of neuronal acetylcholine receptors stably expressed in a mouse fibroblast cell line

Second messenger regulation of neuronal acetylcholine receptors (AChRs) was investigated in a mouse fibroblast cell line, M10, stably transfected with chicken alpha 4 and beta 2 cDNAs. Both forskolin and 8-bromo-cyclic adenosine 3',5'-monophosphate (cAMP) induced large increases in the numbers of AChRs. The increases were due in part to increased transcription and translation of the alpha 4 and beta 2 genes. Blockade of protein synthesis with cycloheximide, however, revealed that forskolin also exerts a post-translational effect, increasing the number of surface receptors by twofold. Immunoblot analysis of sucrose gradient fractions confirmed that the cells had a large fraction of unassembled subunits potentially available for receptor assembly. The post-translational effect of forskolin was blocked by H-89, an inhibitor of cAMP-dependent protein kinase, and by okadaic acid, an inhibitor of phosphatases 1 and 2A. Nicotine also acted post-translationally to induce a twofold increase in the number of surface receptors, but the mechanism differed from that utilized by forskolin, since the effects of the two agents were additive and were differentially affected by okadaic acid. The results suggest that protein phosphorylation-dephosphorylation mechanisms act post-translationally to increase the number of neuronal AChRs maintained on the cell surface. This could be achieved by increasing the efficiency of receptor assembly, transport, or stabilization on the cell surface.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

Subunit stoichiometry of oligomeric membrane proteins: GABAA receptors isolated by selective immunoprecipitation from the cell surface

GABAA receptors are hetero-oligomeric proteins of unknown subunit stoichiometry. In this study alpha 1 beta 3 GABAA receptor channels were functionally expressed in Xenopus oocytes. Direct immunoprecipitation from the oocyte surface was used to exclusively isolate mature GABAA receptors. The subunit ratio was determined by quantitation of the amount of [35S]methionine incorporated into individual receptor subunits. Antibody released from the antigen or antibody not reacted was prevented from reassociation with labeled antigen by addition of excess unlabeled antigen. Variation of the alpha 1 beta 3 ratio of injected cRNAs only slightly affected the subunit ratio in mature receptors. This indicates that the subunit stoichiometry generated is independent of the pools of newly synthesized subunit monomers and supports the view that the receptor assembly is a regulated process. The ratio of alpha 1/beta 3 subunits was found to be 1.1 +/- 0.1 (SEM, n = 6). Our data are in best agreement with a tetrameric receptor with the composition 2 alpha 2 beta. For a pentameric receptor the ratio found slightly favors a receptor with the composition 3 alpha 2 beta. The method developed here is applicable to the determination of the subunit stoichiometry of other recombinant oligomeric membrane proteins.

Functional and non-functional isoforms of the human muscle acetylcholine receptor

1. The properties of a recently identified isoform of the human muscle nicotinic acetylcholine receptor (AChR) alpha subunit (alpha +), which in muscle is expressed at similar levels to the alpha subunit, were investigated by both electrophysiological and biochemical approaches following expression in Xenopus laevis oocytes. The single-channel properties of adult (alpha 2 beta delta epsilon) and fetal (alpha 2 beta delta gamma) forms of the human AChR were also investigated. 2. The mean burst duration of adult channels (4.1 +/- 0.3 ms, mean +/- S.E.M., n = 5) is half that of fetal channels (7.9 +/- 0.6 ms, n = 4), while the single-channel conductance is larger (62.2 +/- 0.8 and 37.9 +/- 1.6 pS for adult and fetal channels, respectively), comparable to the developmental changes in single-channel properties observed for other mammalian species. 3. In contrast to the alpha isoform, the alpha + subunit does not bind 125I-labelled alpha-bungarotoxin or monoclonal antibodies directed against the AChR 'main immunogenic region' (MIR), illustrating why the alpha + subunit was first detected through screening of cDNA libraries. 4. By using site-directed mutagenesis to produce subunits that conferred different single-channel conductances on the AChR, we demonstrate that the alpha + isoform is not integrated into functional AChRs. 5. The mutagenesis experiments also revealed that the two alpha subunits within an AChR pentamer are not equivalent within the pore lining region.

Determination of the subunit stoichiometry of an inwardly rectifying potassium channel

Inwardly rectifying K+ channels are distantly related to their voltage-gated counterparts and possess a structural motif of only two putative transmembrane segments in each subunit. They are formed by the assembly of an unknown number of subunits. We have examined the subunit stoichiometry of a strongly rectifying K+ channel, IRK1, by linking together the coding sequence of three or four subunits and distinguishing channels with different numbers of subunits carrying a double mutation that alters inward rectification and single-channel properties. We find that IRK1 channels, like voltage-gated K+ channels, are tetrameric channels. Interestingly, the high sensitivity to Mg2+ and polyamines, cations that produce inward rectification by blocking the channel pore from the cytoplasmic side is largely retained in a channel containing only one wild-type subunit and three subunits bearing mutations that abolish high affinity Mg2+ and polyamine block.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

The GABAA receptors

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The ionic channel of neural nicotinic acetylcholine receptors is funnel-shaped

Membrane currents evoked by iontophoretically applied acetylcholine were recorded from non-dissociated neurons of rat superior cervical ganglion using the whole-cell patch-clamp recording method. Blocking effects produced by a series of specially synthesized organic compounds, the blockers of the open channel of ganglionic nicotinic acetylcholine receptors, used as tools, were studied, and dimensions of the channel were deduced from correlation between the sizes of the blocking molecules and their blocking activities. Two channel cross-profiles were found, small and large, as approximated by the rectangles with the most probable dimensions 5.8 x 8.0 A and 7.0 x (8.4 - 9.0) A, at intra- and extracellular sides of the channel, correspondingly, in addition to the 6.1 x 8.3 A (medium) cross-profile found in our earlier work and localized between the two above. These findings indicate that the channel is funnel-shaped in the area where the open channel blockers are bound. The binding site is localized at the level of the medium cross-profile or between it and small cross-profile. The small cross-profile is probably a selectivity filter.

Expression of neuronal acetylcholine nicotinic receptor alpha 4 and beta 2 subunits during postnatal development of the rat brain

The expression of the alpha 4 and beta 2 subunits of neuronal nicotinic acetylcholine receptors (nAChRs) was studied in developing rat brain using in situ hybridization. The levels of both transcripts were already high at birth in cerebral cortex, medial habenula, CA1/CA3 regions of the hippocampus and several thalamic nuclei. In general, the beta 2 subunit showed a higher density of hybrids than the alpha 4. Beta 2 expression did not change with age in the medial habenula, medial geniculate nucleus or in the hippocampus whereas it decreased in the cortex. The developmental pattern of the hybridization signal for alpha 4 was different according to the brain area considered. The expression of the two transcripts showed a biphasic pattern in some thalamic nuclei: the lowest levels occurring during the first and second postnatal weeks respectively, and the highest levels during the second and fourth postnatal weeks. The ontogenetic profile of the expression of the alpha 4 subunit in the thalamic nuclei coincided with that of [3H]-L-nicotine binding sites. These findings suggest that the two subunits of nAChRs are independently regulated in most of the brain areas examined, and that in some regions, such as the thalamus, the ontogenetic variations reported for the alpha 4 subunit correlate with those observed for the [3H]-L-nicotine binding sites.

Monovalent and divalent cation permeability and block of neuronal nicotinic receptor channels in rat parasympathetic ganglia

Acetylcholine-evoked currents mediated by activation of nicotinic receptors in rat parasympathetic neurons were examined using whole-cell voltage clamp. The relative permeability of the neuronal nicotinic acetylcholine (nACh) receptor channel to monovalent and divalent inorganic and organic cations was determined from reversal potential measurements. The channel exhibited weak selectivity among the alkali metals with a selectivity sequence of Cs+ > K+ > Rb+ > Na+ > Li+, and permeability ratios relative to Na+ (Px/PNa) ranging from 1.27 to 0.75. The selectivity of the alkaline earths was also weak, with the sequence of Mg2+ > Sr2+ > Ba2+ > Ca2+, and relative permeabilities of 1.10 to 0.65. The relative Ca2+ permeability (PCa/PNa) of the neuronal nACh receptor channel is approximately fivefold higher than that of the motor endplate channel (Adams, D. J., T. M. Dwyer, and B. Hille. 1980. Journal of General Physiology. 75:493-510). The transition metal cation, Mn2+ was permeant (Px/PNa = 0.67), whereas Ni2+, Zn2+, and Cd2+ blocked ACh-evoked currents with half-maximal inhibition (IC50) occurring at approximately 500 microM, 5 microM and 1 mM, respectively. In contrast to the muscle endplate AChR channel, that at least 56 organic cations which are permeable to (Dwyer et al., 1980), the majority of organic cations tested were found to completely inhibit ACh-evoked currents in rat parasympathetic neurons. Concentration-response curves for guanidinium, ethylammonium, diethanolammonium and arginine inhibition of ACh-evoked currents yielded IC50's of approximately 2.5-6.0 mM. The organic cations, hydrazinium, methylammonium, ethanolammonium and Tris, were measureably permeant, and permeability ratios varied inversely with the molecular size of the cation. Modeling suggests that the pore has a minimum diameter of 7.6 A. Thus, there are substantial differences in ion permeation and block between the nACh receptor channels of mammalian parasympathetic neurons and amphibian skeletal muscle which represent functional consequences of differences in the primary structure of the subunits of the ACh receptor channel.

The characterization of alpha-bungarotoxin receptors on the surface of parasympathetic neurons in the frog heart

Receptors for alpha-bungarotoxin are found on the surface of parasympathetic neurons in the frog cardiac ganglion by light microscopic autoradiography. Competition studies suggest that these receptors are cholinergic and indicate that they are also recognized by neuronal bungarotoxin (kappa-bungarotoxin). These receptors are outnumbered by those recognized exclusively by neuronal bungarotoxin. Unlike neuronal bungarotoxin receptors, alpha-bungarotoxin receptors are not concentrated at synaptic sites. Fluorescence techniques fail to find evidence for clusters of alpha-bungarotoxin receptors anywhere on the neuronal surface. The possible function of these receptors, which apparently do not play a role in fast synaptic transmission, is discussed.

alpha-Bungarotoxin-sensitive nicotinic receptors on bovine chromaffin cells: molecular cloning, functional expression and alternative splicing of the alpha 7 subunit

Chromaffin cells from the bovine adrenal medulla express alpha-bungarotoxin-sensitive acetylcholine receptors whose subunit composition is unknown. Northern blot analysis showed that the alpha 7 subunit, a main component of these alpha-bungarotoxin-sensitive acetylcholine receptors in avian and rat brain, is expressed in chromaffin cells. The cDNA of this bovine alpha 7 subunit was cloned by polymerase chain reaction amplification of adrenal medulla RNA for detailed characterization of structure and function. The protein-coding region revealed 92% amino acid sequence identity to rat alpha 7 and 89% to chicken alpha 7 subunits. The alpha-bungarotoxin affinity of alpha 7 homomers expressed in Xenopus oocytes was similar to that observed previously with native chromaffin alpha-bungarotoxin-sensitive acetylcholine receptors. Cross-linking and sucrose gradient experiments suggested that, like the muscular and neuronal acetylcholine receptors; the alpha 7 receptor has a pentameric structure. Upon activation with nicotinic agonists the alpha 7 receptor exhibited rapidly desensitizing cation currents that were blocked by nicotinic antagonists and showed inward rectification. The amplification of adrenal medulla RNA by reverse transcription-polymerase chain reaction methods revealed an alternatively spliced isoform of the bovine alpha 7 subunit, where the exon that codes for the M2 transmembrane segment was skipped during mRNA processing. Oocyte expression of this isoform does not yield functional channels. However, this alternative mRNA exhibits dose-dependent inhibition of alpha 7 homomer expression when coinjected with the undeleted isoform.

GABAergic inhibition of crayfish deep extensor abdominal muscle exhibits a steep dose-response relationship and a high degree of cooperativity

A patch-clamp study was done to characterize the recently found GABAergic (i.e. gamma-aminobutyric acid) inhibitory synaptic channels of crayfish deep extensor abdominal muscle. Outside-out patches were rapidly activated by GABA to measure the dose/response curves for the open probability of the channels, Po, and the rise time, tr, (time from Po = 0.1 to Po = 0.9). In some of the patches the GABA-activated currents decayed due to desensitization and such patches were not studied further. Rare channel openings were elicited with 0.1 mM GABA. The Po at this low concentration of GABA was 0.0005 to 0.01. Application of 10 mM GABA was necessary to reach the maximal Po of 0.9. The slope of the dose/response relationship in the double logarithmic plot was 5.4 +/- 1.1 (mean +/- SD; n = 9) between 0.1 mM and 0.2 mM GABA. The plot of tr versus GABA concentration had a peculiar shape, recently found to be characteristic for positive cooperativity of the binding sites. tr increased from a minimum at 10 mM GABA with declining concentrations of GABA and reached a peak at 0.4 mM GABA. Below 0.4 mM GABA, tr decreased again. With 0.2 mM GABA tr was 0.40 +/- 0.1 (mean +/- SD; n = 4) of the peak value measured at 0.4 mM GABA. Simulations were compared with the experimental results and a linear reaction scheme with five binding sites for GABA was established to describe the dose/response curves for Po and tr.

Effects of serotonergic agents on neuronal nicotinic acetylcholine receptors

In Xenopus oocytes expressing neuronal nicotinic acetylcholine receptors (nAcChoRs), made up of alpha 2 and beta 4 subunits, acetylcholine (AcCho) elicited ionic membrane currents (AcCho currents) that were modulated by serotonergic agents. Both agonists and antagonists specific for various serotonin (5-hydroxytryptamine, 5HT) receptor subtypes interacted directly with alpha 2 beta 4 nAcChoRs: 5HT, (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin, methysergide, spiperone, and ketanserin reversibly reduced the amplitude of AcCho currents and accelerated their decay. The AcCho-current time course decayed with two exponential functions. In the presence of 5HT, the fast time constant of current decay (tau f) was not greatly modified, but the slow time constant (tau s) was reduced. With AcCho and 5HT both at 100 microM, tau s was reduced from 140 s to 85 s. The order of potency for inhibition of AcCho current amplitudes was (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin > methysergide > spiperone > ketanserin > 5HT. The inhibition was voltage-dependent but the magnitude of the voltage dependence for the different blockers did not correspond to their blocking potency: e.g., the block with spiperone was stronger than with 5HT, but it was less voltage-dependent. Our results suggest that serotonergic agents block neuronal nAcChoRs in a noncompetitive manner, similar to the block of muscle nAcChoR by curare and other substances. These results show that neuronal nAcChoR channels that have been activated by their specific neurotransmitter may be modulated by nonspecific neurotransmitters and their antagonists. These effects may help to better understand brain functions as well as the mode of action of the many serotonergic agents that are used in medical practice.

Expression of recombinant homo-oligomeric 5-hydroxytryptamine3 receptors provides new insights into their maturation and structure

A recombinant baculovirus containing a mouse 5-hydroxytryptamine3 (5-HT3) receptor subunit cDNA under the control of the polyhedrin promoter was shown to direct the production of large amounts of functional 5-HT3 receptor in insect cells, as assayed by Western blotting and ligand binding. After solubilization, the receptor was purified to homogeneity by affinity chromatography and characterized pharmacologically. The ligand binding characteristics of the recombinant receptor were essentially identical to those of the native receptor, both before and after purification. Only fully glycosylated receptors bound to the ligand affinity resin, although subsequent removal of the sugar did not affect ligand binding. Visualization of the purified receptor using electron microscopy showed that the receptor preparation contained a homogeneous population of pentameric doughnut-shaped particles. The general appearance of the recombinant homo-oligomeric channels was indistinguishable from that of native 5-HT3 receptors. Yields of purified receptors were of the order of 200 micrograms/3 liters of original culture. The amount and homogeneity of the purified receptor are sufficient to begin preliminary crystallization trials.

Neurons can maintain multiple classes of nicotinic acetylcholine receptors distinguished by different subunit compositions

Although 10 genes have been cloned encoding putative subunits of neuronal nicotinic acetylcholine receptors, little is known about the variety or subunit composition of such receptors expressed by individual neurons. Chick ciliary ganglion neurons express five of the known genes and assemble a class of synaptic-type receptors collectively containing gene products from three of them: alpha 3, beta 4, and alpha 5. Using subunit-specific monoclonal antibodies, we show here that all of the synaptic-type acetylcholine receptors having alpha 3 also have beta 4 subunits and vice versa. In addition, most, if not all, of the alpha 5 gene product present in fully assembled receptors is associated with both alpha 3 and beta 4 subunits. Although the receptors may be homogeneous in these respects, only about 20% of them also contain the fourth gene product, beta 2, newly identified in the ganglion; essentially all of the neurons express the beta 2 gene. No beta 2 subunits are found coassembled with the fifth acetylcholine receptor gene product expressed by the neurons, alpha 7, which has been shown previously to comprise a class of abundant, nonsynaptic receptors on the cells. The identification of three acetylcholine receptor subtypes distinguished by subunit composition on the same neurons provokes questions about their individual physiological roles.

Identification of the human neuronal nicotinic cholinergic alpha 2 receptor locus, (CHRNA2), within an 8p21 mapped locus, by sequence homology with rat DNA

We have identified a cosmid, at the D8S131 locus, that shows sequence homology with exon 2 of the rat gene for the neuronal nicotinic acetylcholine receptor alpha 2 subunit. A 357-bp sequence surrounding a rare cutter AscI site contains a 152-bp region of homology. The human CHRNA2 gene is therefore positioned at the D8S131 locus, which has been mapped to 8p21.

Characterization of the nicotinic acetylcholine receptor beta 3 gene. Its regulation within the avian nervous system is effected by a promoter 143 base pairs in length

Genomic and cDNA clones encoding the chicken neuronal nicotinic acetylcholine receptor beta 3 subunit were isolated and sequenced. The beta 3 gene consists of six protein-encoding exons and the deduced protein has the structural features found in all other members of the neuronal nicotinic acetylcholine receptor subunit family. Although they are undetectable in most brain compartments, beta 3 mRNAs are relatively abundant in the developing retina and in the trigeminal ganglion. In situ hybridization and immunohistochemical analysis demonstrated that in retina, beta 3 transcripts and protein are confined to subpopulations of cells in the inner nuclear and ganglion cell layers. Beta 3 is expressed in the proximal and distal regions of the developing trigeminal ganglion, i.e. in both placode- and neural crest-derived neurons. Transient transfection assays in cells freshly dissociated from selected regions of the central nervous system at different developmental stages allowed the identification of genetic elements involved in the neuronal-selective expression of the beta 3 gene. A promoter fragment 143 base pairs in length and containing TATA, CAAT, and other consensus sequences is sufficient to restrict reporter gene expression to a subpopulation of retinal neurons. This promoter is totally inactive upon transfection into neuronal and non-neuronal cells from other regions of the central nervous system.

Molecular evolution of the nicotinic acetylcholine receptor: an example of multigene family in excitable cells

An extensive phylogenetic analysis of the nicotinic-acetylcholine-receptor subunit gene family has been performed by cladistic and phenetic methods. The conserved parts of amino acid sequences have been analyzed by CLUSTAL V and PHYLIP software. The structure of the genes was also taken in consideration. The results show that a first gene duplication may have occurred before the appearance of Bilateria. Three subfamilies then appeared: I--the neuronal alpha-bungarotoxin binding-site subunits (alpha 7, alpha 8); III--the neuronal nicotinic subunits (alpha 2-alpha 6, beta 2-beta 4), which also contain the muscle acetylcholine-binding subunit (alpha 1); and IV--the muscle non-alpha subunits (beta 1, gamma, delta, epsilon). The Insecta subunits (subfamily II) could be orthologous to family III and IV. Several tissular switches of expression from neuron to muscle and the converse can be inferred from the extant expression of subunits and the reconstructed trees. The diversification of the neuronal nicotinic subfamily begins in the stem lineage of chordates, the last duplications occurring shortly before the onset of the mammalian lineage. Such evolution parallels the increase in complexity of the cholinergic systems.

Pharmacological and physiological properties of a putative ganglionic nicotinic receptor, alpha 3 beta 4, expressed in transfected eucaryotic cells

Neuronal nicotinic acetylcholine receptor subunits alpha 3 (PCA48E) and beta 4S (ZPC13) were expressed in human embryonic kidney (HEK)-293 cells by calcium phosphate transfection. In the presence of atropine, acetylcholine (ACh) induced fast activating currents which exhibited desensitization and inward rectification. The EC50 for ACh was 202 +/- 32 microM with a Hill coefficient of 1.9 +/- 0.4. The rank order of nicotinic agonist potency was 1,1-dimethyl-4-phenylpiperozinium (DMPP) > cytisine = nicotine approximately equal to ACh. The maximal response elicited by DMPP was substantially less than that elicited by other agonists, suggesting that DMPP is a partial agonist. ACh (500 microM) responses were very effectively blocked by equimolar concentrations (100 microM) of the ganglionic antagonists d-tubocurarine, mecamylamine and hexamethonium. Equal concentrations of the potent muscle receptor antagonist decamethonium and the competitive antagonist dihydro-beta-erythroidine were much less effective. alpha bungaro-toxin (1 microM) had little effect on ACh-induced responses. This physiological and pharmacological profile is consistent with a ganglionic nicotinic response.

Distribution of nicotinic receptors in the human hippocampus and thalamus

Neuronal nicotinic acetylcholine receptors consist of different subunits, alpha and beta, with different subtype arrangement corresponding to distinct pharmacological and functional properties. The expression of alpha 3, alpha 7 and beta 2 mRNA in the human brain was studied by in situ hybridization and compared to [3H]nicotine, [3H]cytisine and [125I]alpha-bungarotoxin binding in contiguous sections. The beta 2 probe showed a strong hybridization signal in the granular layer of the dentate gyrus and in the CA2/CA3 region of the hippocampus and in the insular cortex, and a signal of lower intensity in the subicular complex and entorhinal cortex. The alpha 3 probe showed strong hybridization in the dorsomedial, lateral posterior, ventroposteromedial and reticular nuclei of the thalamus, and a weak signal in the hippocampal region and in the entorhinal, insular and cingular cortex. The amount of alpha 7 mRNA was high at the level of the dentate granular layer and the CA2/CA3 region of the hippocampus, in the caudate nucleus and in the pulvinar and ventroposterolateral nuclei of the thalamus. [3H]Nicotine and [3H]cytisine binding appeared to be identical in anatomical distribution and relative intensity. It was high in the thalamic nuclei, the putamen and in the hippocampal formation in the subicular complex and the stratum lacunosum moleculare. The level of [125I]alpha-bungarotoxin binding was particularly high in the hippocampus and in the pyramidal cells of the CA1 region, but was relatively low in the subicular complex. Our data indicate that in the human brain nicotinic receptor subtypes have discrete distributions, which are in part different from those of other species.

alpha 4 beta 2 subunit combination specific pharmacology of neuronal nicotinic acetylcholine receptors in N1E-115 neuroblastoma cells

Pharmacological characteristics of native neuronal nicotinic acetylcholine receptor-mediated ion currents in mouse N1E-115 neuroblastoma cells have been investigated by superfusion of voltage clamped cells with known concentrations of the agonists acetylcholine, nicotine and cytisine, and the antagonists alpha-bungarotoxin and neuronal bungarotoxin. The sensitivity of the nicotinic acetylcholine receptor for agonists followed the agonist potency rank-order: nicotine approximately acetylcholine >> cytisine. The EC50 values of acetylcholine and nicotine are 78 microM and 76 microM, respectively. Equal concentrations of acetylcholine and nicotine induce inward currents with approximately the same peak amplitude whereas cytisine induces much smaller inward currents. Acetylcholine-induced currents are unaffected by high concentrations of alpha-bungarotoxin. Conversely, at 10 and 90 nM neuronal bungarotoxin reduces the amplitude of the 1 mM acetylcholine-induced inward current to 47% and 11% of control values, respectively. Both the agonist potency rank-order and the differential sensitivity to snake toxins of nicotinic receptors in N1E-115 cells are consistent with the known pharmacological profile of alpha 4 beta 2 nicotinic receptors expressed in Xenopus oocytes and distinct from those of all other nicotinic acetylcholine receptors of known functional subunit compositions. All data indicate that the native nicotinic acetylcholine receptor in N1E-115 cells is an assembly of alpha 4 and beta 2 subunits, the putative major subtype of nicotinic acetylcholine receptor in the brain.

Distribution of neuronal nicotinic receptor subunits in human brain

Neuronal nicotinic acetylcholine receptors (nAchRs) are multimeric proteins constituted of two different subunits, alpha and beta, with different subtypes arrangement and different pharmacological and functional properties. nAchRs mediate neurotransmission in many central and peripheral synapses and appear to be affected in human degenerative disorders. We have studied the distribution of nAchR in human brain, particularly in the hippocampus and thalamus, by binding of 3H-nicotine and 3H-cytisine and by in situ hybridization with human alpha 3 and beta 2 nAchR subunits of mRNA. An alpha 3 probe shows a strong hybridization signal in the thalamus, while a beta 2 probe has a good signal at the level of the enthorinal cortex, hippocampus and in caudate and putamen. The alpha 3 and beta 2 mRNA localization is different from that described in other species. 3H-nicotine and 3H-cytisine binding were very similar in terms of anatomical distribution and comparable to the binding described in other animal species. The binding of the two ligands was distributed over the areas labeled by the alpha 3 and beta 2 probes and did not completely overlap with either of the subunits.

Nicotine-related brain disorders: the neurobiological basis of nicotine dependence

1. This paper was written at a moment when the dependence liability of nicotine, the psychoactive component from tobacco, was the center of a dispute between the tobacco manufacturing companies and the scientific community (Nowak, 1994a-c). Without being comprehensive, it tries to summarize evidence compiled from several disciplines within neuroscience demonstrating that nicotine produces a true psychiatric disease, behaviorally expressed as dependence to the drug (American Psychiatric Association, 1994). Nicotine dependence has a biological substratum defined as "neuroadaptation to nicotine." 2. The first part of the article defines terms such as "abuse," "tolerance," "dependence," and "withdrawal." It discusses clinical and experimental facts at the whole-organism level, showing that animals and humans will seek and self-administer nicotine because of its rewarding properties. 3. The second part discusses the neurobiological basis of neuroadaptation to nicotine. It presents information on neuroanatomical circuits which may be involved in nicotine-related brain disorders, such as the mesocorticolimbic pathway and the basal forebrain-frontal cortex pathway. It also discusses work from several laboratories, including our own, that support the notion of a molecular basis for neuroadaptative changes induced by nicotine in the brain of a chronic smoker. 4. Although still under experimental scrutiny, the hallmark of neuroadaptation to nicotine is up-regulation of nicotinic receptors, possibly due to nicotine-induced desensitization of their function (Marks et al., 1983; Schwartz and Kellar, 1985). A correlation between these plastic changes and the behavioral data obtained from animal and human experiments is still needed to understand dependence to nicotine fully.

Structural basis of ion channel permeation and selectivity

There has been rapid progress in understanding the structural basis of ion selectivity and permeation in both ligand- and voltage-gated channels. Recognition of similarities in overall architecture within a channel class has led to an increasing focus on the specific molecular determinants that endow a channel with its own distinctive character. It has been possible in some cases to identify individual amino acids essential for ion selectivity.

Nicotinic acetylcholine receptors in the ground squirrel retina: localization of the beta 4 subunit by immunohistochemistry and in situ hybridization

Immunohistochemical and in situ hybridization techniques were used to localize the beta 4 subunit of the neuronal nicotinic acetylcholine receptors (nAChRs) in the ground squirrel retina. The beta 4 nAChR subunit was detected in both transverse and horizontal sections of the retina using a subunit-specific antiserum and the avidin-biotin complex technique. Two bands of labeled processes were seen in the inner plexiform layer, corresponding approximately to the laminae where the cholinergic cells arborize. Labeled cells were found in the ganglion cell layer and the inner third of the inner nuclear layer. The cells in the ganglion cell layer were medium- to large-sized and were frequently observed to give rise to axon-like processes. Most of the labeled neurons in the inner nuclear layer were small presumptive amacrine cells, but a few medium-to-large cells were also labeled. These could constitute a different class of amacrine cells or displaced ganglion cells. The latter possibility is supported by the existence of nAChR-containing displaced ganglion cells in the avian retina. In situ hybridization with a 35S-labeled cRNA probe revealed the expression of mRNA coding for the nAChR beta 4 subunit in the ganglion cell layer and the inner third of the inner nuclear layer. This finding confirmed the immunohistochemical data of the cellular localization of beta 4 nAChR subunit. These results indicate that the beta 4 nAChR subunit is expressed by specific subtypes of neurons on the ground squirrel retina.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological properties of neuronal nicotinic receptors reconstituted from the vertebrate beta 2 subunit and Drosophila alpha subunits

Three cDNAs (ALS, D alpha 2 and ARD) isolated from the nervous system of Drosophila and encoding putative nicotinic acetylcholine receptor subunits were expressed in Xenopus oocytes in order to study their functional properties. Functional receptors could not be reconstituted from any of these subunits taken singly or in twos and threes. In contrast, large evoked currents (in the microA range) were consistently observed upon agonist application on oocytes co-injected with ALS or D alpha 2 in combination with the chick beta 2 structural subunit. The ALS/beta 2 and D alpha 2/beta 2 receptors are highly sensitive to acetylcholine and nicotine, and their physiological properties resemble those of native or reconstituted receptors from vertebrates. Although the physiological properties of ALS/beta 2 and D alpha 2/beta 2 receptors are quite similar, clear differences appear in their pharmacological profiles. The ALS/beta 2 receptor is highly sensitive to alpha-bungarotoxin while the D alpha 2/beta 2 receptor is totally insensitive to this agent. These results demonstrate that the Drosophila ALS and D alpha 2 cDNAs encode neuronal nicotinic subunits responding to physiological concentrations of the agonists acetylcholine and nicotine.

Local anaesthetic blockade of neuronal nicotinic ACh receptor-channels in rat parasympathetic ganglion cells

1 The effects of the local anaesthetics QX-222 and procaine on nicotinic acetylcholine (ACh)-evoked currents in cultured parasympathetic cardiac neurones of the rat were investigated by use of the whole-cell, perforated-patch, and outside-out recording configurations of the patch clamp method. 2 QX-222 and procaine, applied to the extracellular surface, reversibly inhibited the peak amplitude of the whole-cell nicotinic ACh-evoked current in a concentration-dependent manner, with half-maximal inhibitory concentrations (IC50) of 28 microM and 2.8 microM, respectively, at -80 mV. In these neurones, the sustained inward current mediated by M1 muscarinic receptor activation was unaltered by QX-222, and neither local anaesthetic affected the adenosine 5'-triphosphate (ATP)-evoked current. 3 QX-222 and procaine block of nicotinic ACh-evoked inward current was voltage-dependent and enhanced by hyperpolarization. An e-fold change in their dissociation equilibrium constants (Kd) resulted from a 62 mV and a 122 mV change in membrane potential, respectively. 4 Both local anaesthetics produce a concentration-dependent increase in the half-time of decay of the nicotinic ACh-evoked inward current. 5 Measurements of unitary currents in outside-out patches showed that QX-222 reversibly increased the mean burst duration and closed time and reduced the mean channel open time and open-state probability of the nicotinic ACh receptor-channel (AChR) in a concentration-dependent manner. 6 The Kd and voltage sensitivity of local anaesthetic block of the nicotinic AChR in rat intracardiac neurones suggests that the pore-forming region of this channel differs from that of the AChR in frog and rat skeletal muscle and from the neuronal alpha 4 beta 2 ACh receptor-channel.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Barium permeability of neuronal nicotinic receptor alpha 7 expressed in Xenopus oocytes

The rat alpha 7 neuronal nicotinic acetylcholine receptor was expressed and studied in Xenopus oocytes. The magnitude and reversal potential of instantaneous whole cell currents were examined in solutions containing varying concentrations of either calcium or barium, and in the presence or absence of the intracellular calcium chelator BAPTA. In external barium, application of nicotine elicits an inwardly rectifying response; in calcium the response is larger and has a linear IV relation. Pretreatment of oocytes with BAPTA-AM could not prevent activation of calcium-dependent chloride channels in external Ringer containing calcium. Using an extended GHK equation, the permeability ratio PBa/PNa of the alpha 7 receptor was determined to be about 17. Our results suggest that alpha 7 nicotinic receptors are highly permeable to divalent cations.

Residues 1 to 80 of the N-terminal domain of the beta subunit confer neuronal bungarotoxin sensitivity and agonist selectivity on neuronal nicotinic receptors

Standard two electrode voltage clamp techniques were used to investigate the response of neuronal nicotinic acetylcholine receptors, expressed in Xenopus oocytes, to various agonists and neuronal bungarotoxin (NBT). The beta subunit is an important determinant of the receptor's pharmacological profile. Co-expression of alpha 4 and beta 2 subunits produced a receptor that was relatively insensitive to cytisine and nicotine and inhibited by NBT, whilst the alpha 4 beta 4 combination produced a receptor that was highly sensitive to cytisine and nicotine but resistant to toxin. The first 80 amino acids of the N-terminal domain of the beta subunit are implicated in these characteristics, since the combination of alpha 4 with a hybrid beta subunit comprising amino acids 1-->80 of beta 2 and 81-->416 of beta 4 became relatively insensitive to nicotine and cytisine and resistant to inhibition by neuronal bungarotoxin.

Immunohistochemical localization of a ligand-binding and a structural subunit of nicotinic acetylcholine receptors in the central nervous system of Drosophila melanogaster

The distribution of two subunits of nicotinic acetylcholine receptors in the developing and the differentiated central nervous system of Drosophila melanogaster was studied. With subunit-specific antibodies raised against the ligand-binding alpha-like subunit ALS and the putative non-ligand-binding subunit ARD, we find both ALS-like and ARD-like immunoreactivity widely distributed in most neuropiles of the optic lobes, the protocerebrum, the deutocerebrum and the thoracic ganglion of the adult fly. With a single exception, namely in the lamina of the visual system, the antigens recognized by the two types of antibodies are colocalized. This observation is consistent with previous immunoprecipitation data indicating that the ALS and ARD proteins are integral components of the same hetero-oligomeric receptor that binds the nicotinic antagonist alpha-bungarotoxin with high affinity. During embryonic development ARD-like immunoreactivity is first detectable in approximately 10 hour old embryos. Both subunits are consistently detected in the central nervous system of the late embryo, the three larval stages, and all prepupal and pupal stages. During metamorphosis the optic stalk is transiently immunoreactive with anti-ARD, but not with anti-ALS antiserum. Although in larvae and adults, immunoreactivity with both types of antibodies is most abundant in synaptic regions, in embryos and pupae strong staining of cortical cell body layers is observed, in particular with anti-ARD antisera. As these developmental periods coincide with strong accumulation of ARD transcripts, the cell body staining may reflect newly synthesized and assembled receptors, while the functional ARD- and ALS-containing receptor may be destined for synapses.

Homomeric and native alpha 7 acetylcholine receptors exhibit remarkably similar but non-identical pharmacological properties, suggesting that the native receptor is a heteromeric protein complex

Sucrose gradient analysis of chick acetylcholine receptor (AChR) alpha 7 subunits expressed in oocytes indicates that they form pharmacologically active homomers of the same size as native alpha 7 AChRs, a size compatible with a complex of five alpha 7 subunits. By immunoisolating the [35S]methionine-labeled alpha 7 subunits we also demonstrate that they do not appear to assemble with endogenous Xenopus AChR subunits. Pharmacological characterization of detergent-solubilized brain alpha 7 AChRs and alpha 7 homomers reveals that they have similar but nonidentical properties. The pharmacological difference is most accentuated for cytisine (approximately 50-fold). Thus, at least in E18 chicken brain, most or all of the native alpha 7 AChRs do not appear to be homomeric.