Alpha 9: an acetylcholine receptor with novel pharmacological properties expressed in rat cochlear hair cells

ARIA is heavily expressed in rat peripheral auditory and vestibular ganglia

The ARIA (acetylcholine receptor inducing activity) polypeptide is a member of the neuregulin gene family. It was originally purified on the basis of its ability to induce skeletal muscle nicotinic acetylcholine receptors (nAChRs). ARIA mRNA is expressed in ventral horn motor neurons and brain cholinergic neurons. We report here that ARIA mRNA is heavily expressed in the embryonic, developing, and adult peripheral auditory and vestibular ganglia, the spiral ganglion and Scarpa's ganglion. Neither ganglion is cholinergic, but both express mRNAs for nicotinic and muscarinic receptors. The expression of ARIA in these ganglia may be related to the regulation of cholinergic receptors or a more general role for ARIA in growth and development.

Identification of the subunits of the nicotinic cholinergic receptors in the rat cochlea using RT-PCR and in situ hybridization

There are two tissues in the adult mammalian cochlea that are post-synaptic to cholinergic efferent fibers: The outer hair cells (OHCs) and the dendrites of the afferent fibers of the type I spiral ganglion cells. The unusual nicotinic-like pharmacology of cochlear cholinergic responses and the unique embryonic development of cochlear tissues suggest that the inner-ear nicotinic cholinergic receptor (nAChR) may be different from nAChRs described previously at synapses in the mammalian brain, autonomic ganglia, or skeletal muscle. In this study, we determined the mRNA expression of the alpha2-7, alpha9, and beta2-4 subunits of the nicotinic acetylcholine receptor (nAChR) family in the rat cochlea. In micro-dissected tissue from the organ of Corti, spiral ganglion, and the membranous lateral wall, we found mRNA expression of the alpha7 and alpha9 subunits in the organ of Corti and alpha5-7, and beta2 and beta3 in the spiral ganglion using RT-PCR. Employing in situ hybridization with 35S-riboprobes, we localized alpha9 in hair cells regions and alpha6, alpha7 and beta2 in the type I cells of the spiral ganglion. No evidence of nAChR subunit mRNA expression was found in supporting cells, but beta2 was expressed in type II spiral ganglion cells, which are neither cholinergic nor cholinoceptive.

Differential expression of alpha2-7, alpha9 and beta2-4 nicotinic acetylcholine receptor subunit mRNA in the vestibular end-organs and Scarpa's ganglia of the rat

To further characterize the pattern of expression of the nicotinic acetylcholine receptor (nAChR) subunits in the peripheral vestibular system, we conducted RT-PCR of all known mammalian nAChR alpha and beta subunits in mRNA extracted from adult rat vestibular primary afferent neurons (Scarpa's ganglia) and vestibular end-organs. Transcripts encoding the alpha2-7 and beta2-4 nAChR subunits were found in the vestibular ganglia, while alpha3, alpha5-7, alpha9 and beta2-4 nAChR subunits were expressed in the vestibular end-organs. These results support previous electrophysiological, immunocytochemical and molecular biological data, and also provide a more complete understanding of the role of nAChRs in the neurochemical transmission subserving the efferent-afferent interaction in the vestibular periphery.

Mice deficient in the alpha7 neuronal nicotinic acetylcholine receptor lack alpha-bungarotoxin binding sites and hippocampal fast nicotinic currents

The alpha7 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) is abundantly expressed in hippocampus and is implicated in modulating neurotransmitter release and in binding alpha-bungarotoxin (alpha-BGT). A null mutation for the alpha7 subunit was prepared by deleting the last three exons of the gene. Mice homozygous for the null mutation lack detectable mRNA, but the mice are viable and anatomically normal. Neuropathological examination of the brain revealed normal structure and cell layering, including normal cortical barrel fields; histochemical assessment of the hippocampus was also normal. Autoradiography with [3H]nicotine revealed no detectable abnormalities of high-affinity nicotine binding sites, but there was an absence of high-affinity [125I]alpha-BGT sites. Null mice also lack rapidly desensitizing, methyllycaconitine-sensitive, nicotinic currents that are present in hippocampal neurons. The results of this study indicate that the alpha-BGT binding sites are equivalent to the alpha7-containing nAChRs that mediate fast, desensitizing nicotinic currents in the hippocampus. These mice demonstrate that the alpha7 subunit is not essential for normal development or for apparently normal neurological function, but the mice may prove to have subtle phenotypic abnormalities and will be valuable in defining the functional role of this gene product in vivo.

Frequency-dependent enhancement of basilar membrane velocity during olivocochlear bundle stimulation

Basilar membrane (BM) velocity responses were measured in the presence of olivocochlear bundle (OCB) stimulation. Frequency threshold tuning curves (FTCs) were derived from tone-evoked input-output (I/O) functions. Efferent nerve activation produced decreases in velocity amplitude for frequencies around best frequency (BF) at low stimulus levels with little or no effect for stimuli well below the BF. A level-dependent efferent reduction/enhancement of BM velocity was found for certain stimulus frequencies above the BF. Efferent activation either had no effect or caused small reductions in the velocity response produced by low level sound, whereas, at higher stimulus levels, efferent activation increased the velocity response. The derived FTCs, therefore, showed criterion-dependent changes with efferent activation. For low BM criterion velocities, FTCs showed the classic desensitization of the tip region without a shift of BF. Some BM velocity criterion values showed FTCs with an expanded high-frequency response area, also without a shift of BF. The results suggest that the effect of OCB activation changes the gain of the voltage-dependent outer hair cell motility such that BM velocity response near BF is decreased while increasing the response for tones well above BF.

Heterogeneity of nicotinic receptor class and subunit mRNA expression among individual parasympathetic neurons from rat intracardiac ganglia

Neurons have the potential to form thousands of distinct neuronal nicotinic receptors from the eight alpha and three beta subunits that currently are known. In an effort to determine how much of this potential complexity is realized among individual neurons, we examined the nicotinic pharmacological and biophysical properties and receptor subunit mRNA expression patterns in individual neurons cultured from rat epicardial ganglia. Analysis of the whole-cell pharmacology of these neurons showed a diversity of responses to the agonists acetylcholine, nicotine, cytisine, and 1,1-dimethyl-4-phenylpiperazinium, suggesting that a heterogeneous population of nicotinic receptor classes, or subtypes, is expressed by individual neurons. Single-channel analysis demonstrated three distinct conductances (18, 24, and 31 pS), with patches from different neurons containing different combinations of these channel classes. We used single-cell RT-PCR to examine nicotinic acetylcholine receptor (nAChR) subunit mRNA expression by individual neurons. Although mRNAs encoding all eight neuronal nAChR subunits for which we probed (alpha 2-alpha 5, alpha 7, beta 2-beta 4) were present in multicellular cultures, we found that individual epicardial neurons express distinct subsets of these nAChR subunit mRNAs. These results suggest that individual epicardial neurons express distinct arrays of nAChR subunits and that these subunits may assemble into functional receptors with distinct and variable subunit composition. This variable receptor subunit expression provides an explanation for the diversity of pharmacological and single-channel responses we have observed in individual neurons.

[125I]Azidonicotinoid photoaffinity labeling of insecticide-binding subunit of Drosophila nicotinic acetylcholine receptor

The novel synthetic nicotinoid insecticide imidacloprid is a high affinity ligand for the insect nicotinic acetylcholine receptor (nAChR). The goal of this study is to identify the ligand- and insecticide-binding subunit of Drosophila nAChR with a novel [125I]azidonicotinoid ([125I]AN) photoaffinity probe modeled on imidacloprid. [125I]AN photoaffinity labels a single polypeptide in Drosophila head membranes corresponding in molecular mass to 66 kDa at a specific site inhibited by various cholinergic ligands including (-)-nicotine, cytisine, carbachol, alpha-bungarotoxin and d-tubocurarine as well as the insecticides imidacloprid and acetamiprid, pharmacologically consistent with the ligand- and insecticide-binding subunit. The Drosophila nAChR, isolated with three putative subunits (69, 66 and 61 kDa) using a nicotinoid-agarose affinity column, is labeled by [125I]AN primarily at the 66 kDa subunit and secondarily at the 61 kDa subunit. Clearly, the novel synthetic nicotinoid insecticides are valuable contributors in exploring the structure and function of the Drosophila nAChR.

Neuronal alpha-bungarotoxin receptors differ structurally from other nicotinic acetylcholine receptors

We have characterized the alpha-bungarotoxin receptors (BgtRs) found on the cell surface of undifferentiated pheochromocytoma (PC12) cells. The PC12 cells express a homogeneous population of alpha7-containing receptors that bind alpha-Bgt with high affinity (Kd = 94 pM). The BgtRs mediate most of the response elicited by nicotine, because the BgtR-specific antagonists methyllycaconitine and alpha-Bgt block approximately 90% of the whole-cell current. The binding of nicotinic agonists to cell-surface BgtRs was highly cooperative with four different agonists showing Hill coefficients in the range of 2.3-2.4. A similar agonist binding cooperativity was observed for BgtR homomers formed from chimeric alpha7/5HT3 subunits expressed in tsA 201 cells. Two classes of agonist binding sites, in the ratio of 4:1 for PC12 cell BgtRs and 3:1 for alpha7/5HT3 BgtRs, were revealed by bromoacetylcholine alkylation of the reduced sites on both PC12 BgtRs and alpha7/5HT3 BgtRs. We conclude from this data that PC12 BgtRs and alpha7/5HT3 homomers contain at least three distinguishable agonist binding sites and thus are different from other nicotinic receptors.

Fast synaptic signaling by nicotinic acetylcholine and serotonin 5-HT3 receptors in developing visual cortex

Cholinergic and serotonergic fiber systems invade the developing visual cortex several weeks before eye opening; both transmitters have been implicated in plasticity of neocortical circuits. These transmitters have been presumed to act predominantly through second messenger-coupled receptors, because fast cholinergic or serotonergic neurotransmission has never been observed in neocortex. However, acetylcholine and serotonin also act on ligand-gated ion channels; the nicotinic acetylcholine receptor and the serotonin 5-HT3 receptor, respectively. Here, using whole-cell patch-clamp techniques in developing ferret visual cortex, we pharmacologically isolated fast, spontaneous, and evoked cholinergic and serotonergic synaptic events in pyramidal cells and interneurons of all cortical layers. The number of cells receiving such inputs increased with the ingrowth of thalamic afferents, and the frequencies of the spontaneous events increased at eye opening. Thus, both acetylcholine and serotonin can mediate fast synaptic transmission in the visual cortex; the early onset of these mechanisms suggests a role during initial stages of circuit formation and during subsequent experience-dependent remodeling of cortical connections.

Potentiation and inhibition of neuronal nicotinic receptors by atropine: competitive and noncompetitive effects

Atropine, the classic muscarinic receptor antagonist, inhibits ion currents mediated by neuronal nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes. At the holding potential of -80 mV, 1 microM atropine inhibits 1 mM acetylcholine-induced inward currents mediated by rat alpha2beta2, alpha2beta4, alpha3beta2, alpha3beta4, alpha4beta2, alpha4beta4, and alpha7 nicotinic receptors by 12-56%. Inward currents induced with a low agonist concentration are equally inhibited (alpha3beta2, alpha3beta4), less inhibited (alpha2beta4, alpha7), or potentiated (alpha4beta2, alpha4beta4) by 1 microM atropine. Effects on the more sensitive alpha4beta4 nicotinic receptors were investigated in detail by systematic variation of acetylcholine and atropine concentrations and of membrane potential. At high agonist concentration, atropine inhibits alpha4beta4 nicotinic receptor-mediated ion current in a noncompetitive, voltage-dependent way with IC50 values of 655 nM at -80 mV and of 4.5 microM at -40 mV. At low agonist concentration, 1 microM atropine potentiates alpha4beta4 nicotinic receptor-mediated ion current. This potentiating effect is surmounted by high concentrations of acetylcholine, indicating a competitive interaction of atropine with the nicotinic receptor, and potentiation is also reversed at high atropine concentrations. Steady state effects of acetylcholine and atropine are accounted for by a model for combined receptor occupation and channel block, in which atropine acts on two distinct sites. The first site is associated with noncompetitive ion channel block. The second site is associated with competitive potentiation, which appears to occur when the agonist recognition sites of the receptor are occupied by acetylcholine and atropine. The apparent affinity of atropine for the agonist recognition sites of the alpha4beta4 nicotinic acetylcholine receptor is estimated to be 29.9 microM.

Effects of potassium channel blockers on the acetylcholine-induced currents in dissociated outer hair cells of guinea pig cochlea

Much physiological evidence is available to show that acetylcholine (ACh) hyperpolarizes the outer hair cells (OHCs) of guinea pig cochlea and induces Ca2+-activated K+ currents. In this study, using the nystatin perforated patch-clamp technique, we investigated the effects of various K+ channel blockers on the ACh-induced currents (I[ACh]) in dissociated OHCs of guinea pig cochlea. The I(ACh) were inhibited by apamin in a concentration-dependent manner. The half-maximal inhibitory concentration for apamin on the ACh-induced response was 1.59 x 10(-9) M. Charybdotoxin and iberiotoxin had no inhibitory effect on the I(ACh) The inhibitory potency of the various K+ channel blockers on the I(ACh) was as follows: apamin >> quinine approximately quinidine approximately d-tubocurarine > tetraethylammonium chloride > 4-aminopyridine approximately Ba2+ > Cs2+. It is proposed that the Ca2+-activated K+ channels of mammalian cochlear OHCs should be classified as small conductance Ca2+-activated K+ (SK) channels according to their pharmacological properties.

Comparison of the regional expression of nicotinic acetylcholine receptor alpha7 mRNA and [125I]-alpha-bungarotoxin binding in human postmortem brain

Neuronal nicotinic acetylcholine receptors are expressed in the human central nervous system. A specific subtype of this receptor family, the alpha7 nicotinic acetylcholine receptor, is thought to be the principal alpha-bungarotoxin (alphaBTX)-binding protein in mammalian brain. Although the expression of this receptor subtype has been characterized in rat, no study has specifically compared the expression of both the alpha7 gene and the localization of BTX binding sites in human brain. Expression of alpha7 mRNA and receptor protein in human postmortem brain tissue was examined by in situ hybridization and [125I]-alpha-bungarotoxin autoradiography, respectively, with particular emphasis on regions associated with sensory processing. Regions with high levels of both alpha7 gene expression and [125I]-alphaBTX binding include the nucleus reticularis of the thalamus, the lateral and medial geniculate bodies, the basilar pontine nucleus, the horizontal limb of the diagonal band of Broca, the nucleus basalis of Meynert, and the inferior olivary nucleus. High-to-moderate levels of alpha7 probe hybridization were also seen in the hippocampus and the cerebral cortex; however, there was a reduced or variable degree of [125I]-alphaBTX binding in these regions compared with the level of probe hybridization. In most brain regions, [125I]-alphaBTX binding was localized to neuronal cell bodies similar in morphology to those that exhibited alpha7 hybridization, suggesting that the high-affinity [125I]-alphaBTX binding sites in the human brain are likely to be principally composed of alpha7 receptor subtypes.

The glutamate receptor subunit delta1 is highly expressed in hair cells of the auditory and vestibular systems

In the inner ear, fast excitatory synaptic transmission is mediated by ionotropic glutamate receptors, including AMPA, kainate, and NMDA receptors. The recently identified delta1 and delta2 glutamate receptors share low homology with the other three types, and no clear response or ligand binding has been obtained from cells transfected with delta alone or in combination with other ionotropic receptors. Studies of mice lacking expression of delta2 show that this subunit plays a crucial role in plasticity of cerebellar glutamatergic synapses. In addition, these mice show a deficit in vestibular compensation. These findings and the nature of glutamatergic synapses between vestibulocochlear hair cells and primary afferent dendrites suggest that delta receptors may be functionally important in the inner ear and prompted us to investigate the expression of delta receptors in the cochlea and peripheral vestibular system. Reverse transcription and DNA amplification by PCR combined with immunocytochemistry and in situ hybridization were used. Our results show that the expression of delta1 in the organ of Corti is intense and restricted to the inner hair cells, whereas delta1 is expressed in all spiral ganglion neurons as well as in their satellite glial cells. In the vestibular end organ, delta1 was highly expressed in both hair cell types and also was expressed in the vestibular ganglion neurons. The prominent expression of delta1 in inner hair cells and in type I and type II vestibular hair cells suggests a functional role in hair cell neurotransmission.

Sp1 and Sp3 regulate expression of the neuronal nicotinic acetylcholine receptor beta4 subunit gene

Neuronal nicotinic acetylcholine receptors play important roles in signal transduction within the nervous system. The receptors exist in a variety of functionally distinct subtypes that are determined by their subunit structures. The subunits are encoded by 11 genes, alpha2-alpha9 and beta2-beta4. Three of the genes, alpha3, alpha5, and beta4, are tightly clustered, and their encoded proteins make up the predominant receptor subtype in the peripheral nervous system. The tight linkage of the genes suggests there may be a common regulatory mechanism underlying their expression. However, although their expression patterns significantly overlap, they are not identical, indicating that independent regulatory mechanisms must also exist. Our studies have focused upon the gene encoding the beta4 subunit for which we have identified several transcriptional regulatory elements. One of these elements, E2, specifically interacts with the general transcription factor Sp1. Here we show that another member of the Sp family of factors, Sp3, can specifically interact with E2 whereas two other members, Sp2 and Sp4, cannot. Co-transfection experiments indicate that Sp3 can transactivate a beta4 promoter/reporter gene construct and, furthermore, that Sp1 and Sp3 can transactivate the beta4 reporter construct synergistically. The transactivation is dependent upon an intact E2 and may involve direct interactions between Sp1 and Sp3.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

In vitro approaches to species and receptor diversity in cellular neurotoxicology

Effects of selective and non-selective neurotoxic compounds on membrane currents mediated by nicotinic acetylcholine receptors (nAChR), natively expressed in cultured cells and artificially expressed in Xenopus oocytes, have been investigated in vitro using voltage clamp techniques. Mammalian neuronal nAChR in cultured mouse N1E-115 cells, muscle type nAChR in cultured mouse BC3H1 cells and insect neuronal nAChR in dissociated locust thoracic ganglion neurons show interspecies differences in sensitivity to neurotoxic compounds. The nitromethylene heterocyclic insecticide WL145004 and physostigmine selectively agonize the insect type nAChR. The mouse neuronal and muscle types of nAChR are much less sensitive to and are partially inhibited by WL 145004. Intraspecies differences have been investigated for the effects of Pb(2+) on subtypes of nAChR expressed in Xenopus oocytes. The nature of the effect of the heavy metal Pb(2+) depends on the combination of mammalian neuronal a and beta nAChR subunits. Ion currents mediated by alpha4beta2 and alpha3beta4 nAChR are inhibited and those mediated by alpha3beta2 nAChR are potentiated by Pb(2+). Distinct sensitivities of subtypes of mammalian neuronal nAChR to agonists and antagonists are employed to characterize native nAChR in N 1E-115 cells. Implications of receptor diversity for neurotoxicology are discussed.

Nicotinic acetylcholine receptors in health and disease

Nicotinic acetylcholine receptors (AChRs) are a family of acetylcholine-gated cation channels that form the predominant excitatory neurotransmitter receptors on muscles and nerves in the peripheral nervous system. AChRs are also expressed on neurons in lower amounts throughout the central nervous system. AChRs are even being reported on unexpected cell types such as keratinocytes. Structures of these AChRs are being determined with increasing precision, but functions of some orphan subunits are just beginning to be established. Functional roles for postsynaptic AChRs in muscle are well known, but in neurons the post-, peri-, extra-, and presynaptic roles of AChRs are just being revealed. Pathogenic roles of AChRs are being discovered in many diseases involving mechanisms ranging from mutations, to autoimmune responses, to the unknown; involving cell types ranging from muscles, to neurons, to keratinocytes; and involving signs and symptoms ranging from muscle weakness to epilepsy, to neurodegenerative disease, to psychiatric disease, to nicotine addiction. Awareness of AChR involvement in some of these diseases has provoked new interests in development of therapeutic agonists for specific AChR subtypes and the use of expressed cloned AChR subunits as possible immunotherapeutic agents. Highlights of recent developments in these areas will be briefly reviewed.

A role for chloride in the suppressive effect of acetylcholine on afferent vestibular activity

Afferents of the frog semicircular canal (SCC) respond to acetylcholine (ACh) application (0.3-1.0 mM) with a facilitation of their activity while frog saccular afferents respond with suppression (Guth et al., 1994). All recordings are of resting (i.e., non-stimulated) multiunit activity as previously reported (Guth et al., 1994). Substitution of 80% of external chloride (Cl-) by large, poorly permeant anions of different structures (isethionate, methanesulfonate, methylsulfate, and gluconate) reduced the suppressive effect of ACh in the frog saccular afferents. This substitution did not affect the facilitatory response of SCC afferents to ACh. Chloride channel blockers were also used to test further whether Cl- is involved in the ACh suppressive effect. These included: niflumic and flufenamic acids, picrotoxin, 5-nitro-2-(-3-phenylpropylamino)benzoic acid (NPPB), and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). As with the Cl- substitutions, all of these agents reduced the suppressive response to ACh in the saccule, but not the facilitatory response seen in the SCC. The suppressive effect of ACh on saccular afferents is considered to be due to activation of a nicotinic-like receptor (Guth et al., 1994; Guth and Norris, 1996). Taking into account the effects of both Cl- substitutions and Cl- channel blockers, we conclude that changes in Cl- availability influence the suppressive effect of ACh and that therefore Cl- may be involved in this effect.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Expression of the nicotinic acetylcholine receptor subunit, alpha9, in the guinea pig cochlea

Acetylcholine is a major neurotransmitter of the cochlear efferent system. Based on its high level of expression in hair cells, the recently cloned nicotinic receptor subunit, alpha9 [Elgoyhen et al., Cell 79 (1994) 705-715], is likely to be the postsynaptic receptor for acetylcholine in hair cells either as a homomeric complex or with other subunits yet to be identified. To further study this receptor, we cloned and sequenced alpha9 cDNA from the guinea pig organ of Corti library [Wilcox and Fex, Hear. Res. 62 (1992) 124-126]. The sequence of the guinea pig alpha9 cDNA is similar to that of the rat, with identities of 85% and 89% at the nucleotide and amino acid levels, respectively. Most differences are in the cytoplasmic loop domain between the transmembrane segments 3 and 4. We also observed minor differences in the putative ligand binding regions. Pharmacological differences between acetylcholine receptors on outer hair cells of rat and guinea pig have been reported, and the minor structural changes we observe could account for these differences. Reverse transcription-polymerase chain reaction analysis showed a high expression of alpha9 in the organ of Corti while expression was low or not detected in the spiral ganglion. In situ hybridization histochemistry showed expression of alpha9 mRNA in both inner and outer hair cells, with much higher expression in outer hair cells than in inner hair cells. In the inner hair cell, silver grains were more abundant over the basal part of the cell than over the apical part. Immunocytochemistry showed a pattern of distribution of the alpha9 protein similar to that seen for mRNA with in situ hybridization. Immunolabeling was most intense at the bases of both inner and outer hair cells. To determine the effect of hair cell loss on alpha9 expression, hair cells were destroyed by either systemic or local application of kanamycin. This treatment led to a down regulation of alpha9 in hair cells; this down regulation appeared to precede hair cell degeneration. In the spiral ganglion, a transient up regulation of alpha9, as determined by RT-PCR, was seen 4-6 weeks after kanamycin treatment.

Differential expression of alpha-bungarotoxin-sensitive neuronal nicotinic receptors in adrenergic chromaffin cells: a role for transcription factor Egr-1

Adrenomedullary chromaffin cells express at least two subtypes of acetylcholine nicotinic receptors, which differ in their sensitivity to the snake toxin alpha-bungarotoxin. One subtype is involved in the activation step of the catecholamine secretion process and is not blocked by the toxin. The other is alpha-bungarotoxin-sensitive, and its functional role has not yet been defined. The alpha7 subunit is a component of this subtype. Autoradiography of bovine adrenal gland slices with alpha-bungarotoxin indicates that these receptors are restricted to medullary areas adjacent to the adrenal cortex and colocalize with the enzyme phenylethanolamine N-methyl transferase (PNMT), which confers the adrenergic phenotype to chromaffin cells. Transcripts corresponding to the alpha7 subunit also are localized exclusively to adrenergic cells. To identify possible transcriptional regulatory elements of the alpha7 subunit gene involved in the restricted expression of nicotinic receptors, we isolated and characterized its 5' flanking region, revealing putative binding sites for the immediate early gene transcription factor Egr-1, which is known to activate PNMT expression. In reporter gene transfection experiments, Egr-1 increased alpha7 promoter activity by up to sevenfold. Activation was abolished when the most promoter-proximal of the Egr-1 sites was mutated, whereas modification of a close upstream site produced a partial decrease of the Egr-1 response. Because Egr-1 was found to be expressed exclusively in adrenergic cells, we suggest that this transcription factor may be part of a common mechanism involved in the induction of the adrenergic phenotype and the differential expression of alpha-bungarotoxin-sensitive nicotinic receptors in the adrenal gland.

The alpha-bungarotoxin-binding nicotinic acetylcholine receptor from rat brain contains only the alpha7 subunit

When expressed in Xenopus oocytes, the rat alpha7 subunit forms homo-oligomeric nicotinic acetylcholine receptors, which are blocked by alpha-bungarotoxin. Since the pharmacological and physiological properties of the alpha7 receptor expressed in oocytes are similar to those of the alpha-bungarotoxin-sensitive nicotinic currents recorded from neuronal preparations and the distribution patterns of alpha7 mRNA and alpha-bungarotoxin-binding sites in the rat brain are very similar, alpha7 is thought to be the main component of the alpha-bungarotoxin-binding nicotinic receptor in the mammalian brain. However, while alpha7 is found in purified alpha-bungarotoxin-binding complexes from rat brain or PC12 cells, other proteins copurify with it. Therefore, the question whether alpha7 forms a homo-oligomeric alpha-bungarotoxin-binding nicotinic receptor in the mammalian brain remains. We have developed and characterized affinity-purified polyclonal antibodies and used these antibodies in Western blot analyses of alpha-bungarotoxin-binding proteins purified from rat brains. We report here that our experimental data support the current working hypothesis that the alpha-bungarotoxin-binding nicotinic receptor is a homo-oligomer of alpha7 subunits in the rat brain.

Nicotine protects cultured cortical neurons against glutamate-induced cytotoxicity via alpha7-neuronal receptors and neuronal CNS receptors

We examined the effects of nicotine on glutamate-induced cytotoxicity using primary cultures of rat cortical neurons. The cell viability decreased significantly when cultures were exposed to glutamate for 10 min and then incubated with glutamate-free medium for 1 h. The exposure of cultures to nicotine (10 microM) for 8-24 h prior to glutamate application ameliorated the glutamate-induced cytotoxicity, with no significant effect of nicotine alone on the cell viability. Neuroprotection by nicotine was dependent on the incubation period. alpha-bungarotoxin (alpha-BTX) and methyllycaconitine (MLA), both of which are alpha7-neuronal receptor antagonists, and dihydro-beta-erythroidine (DHbetaE), a neuronal central nervous system (CNS) receptor antagonist, each significantly antagonized the protection by nicotine against glutamate-induced cytotoxicity. Ionomycin, a calcium ionophore, and S-nitrosocysteine (SNOC), a nitric oxide (NO) donor, also induced cytotoxicity in a manner similar to glutamate. Nicotine protected cultures against ionomycin-induced cytotoxicity, but not against SNOC-induced cytotoxicity. These results suggest that nicotine protects cultured cortical neurons against glutamate-induced cytotoxicity via alpha7-neuronal receptors and neuronal CNS receptors by reducing NO-formation triggered by Ca2+ influx.

Nicotine regulates mRNA level of tyrosine hydroxylase gene but not that of nicotinic acetylcholine receptor genes in PC12 cells

To understand the molecular mechanism of nicotine addiction, we examined the mRNA level of the tyrosine hydroxylase (TH) gene and that of the nicotinic acetylcholine receptor (nAChR) genes by long-term nicotine treatment. The transcript levels of the four subunit genes of the nAChR (alpha3, alpha5, alpha7, and beta4) were down-regulated by the treatment with forskolin, whereas the mRNA levels of the TH gene was increased in PC12 cells. By long-term nicotine treatment, the mRNA level of the nAChR genes did not change, but transcript levels of alpha3, alpha5, alpha7, and beta4 nAChR genes were still negatively regulated by forskolin. However, the mRNA level of TH gene did not change by forskolin under long-term nicotine treatment. The TH gene may be regulated by a nicotine-related signaling pathway, whereas alpha3, alpha5, alpha7, and beta4 nAChR genes may be further regulated by a protein kinase A (PKA) pathway under long-term nicotine treatment.

Chronic nicotine treatment up-regulates alpha3 and alpha7 acetylcholine receptor subtypes expressed by the human neuroblastoma cell line SH-SY5Y

Chronic exposure to nicotine has been reported to increase the number of nicotinic acetylcholine receptors (AChRs) in brain. The mechanism of up-regulation for the alpha4beta2 AChR subtype, which accounts for the majority of high affinity nicotine binding in mammalian brain, has previously been shown to involve a decrease in the rate of alpha4beta2 AChR turnover. Here, we report an investigation of the extent and mechanism of nicotine-induced up-regulation of alpha3 AChRs and alpha7 AChR subtypes expressed in the human neuroblastoma cell line SH-SY5Y. Up-regulation of human alpha3 AChRs and alpha7 AChRs, unlike alpha4beta2 AChRs, requires much higher nicotine concentrations than are encountered in smokers; the extent of increase of surface AChRs is much less; and the mechanisms of up-regulation are different than with alpha4beta2 AChRs. The mechanisms of up-regulation may be different for alpha3 AChRs or alpha7 AChRs. Chronic treatment with nicotine or carbamylcholine, but not d-tubocurarine, mecamylamine, or dihydro-beta-erythroidine, induced a 500-600% increase in the number of alpha3 AChRs but only a 30% increase in alpha7 AChRs. Chronic nicotine treatment did not increase affinity for nicotine or increase the amount of RNA for alpha3 or alpha7 subunits. The effect of nicotine on up-regulation of alpha7 AChRs was partially blocked by either d-tubocurarine or mecamylamine. The effect of nicotine treatment on the number of alpha3 AChRs was only slightly blocked by the antagonists d-tubocurarine, mecamylamine, or dihydro-beta-erythroidine at concentrations that efficiently block alpha3 AChR function. Most of the nicotine-induced increase in alpha3 AChRs was found to be intracellular. The alpha3 AChRs, which accumulate intracellularly, were shown to have been previously exposed on the cell surface by their susceptibility to antigenic modulation. The data suggest that chronic exposure to nicotine may induce a conformation of cell surface alpha3 AChRs that at least in this cell line are consequently internalized but not immediately destroyed.

The molecular biology of neuronal nicotinic acetylcholine receptors

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

Acetylcholine, outer hair cell electromotility, and the cochlear amplifier

The dominant efferent innervation of the cochlea terminates on outer hair cells (OHCs), with acetylcholine (ACh) being its principal neurotransmitter. OHCs respond with a somatic shape change to alterations in their membrane potential, and this electromotile response is believed to provide mechanical feedback to the basilar membrane. We examine the effects of ACh on electromotile responses in isolated OHCs and attempt to deduce the mechanism of ACh action. Axial electromotile amplitude and cell compliance increase in the presence of the ligand. This response occurs with a significantly greater latency than membrane current and potential changes attributable to ACh and is contemporaneous with Ca2+ release from intracellular stores. It is likely that increased axial compliance largely accounts for the increase in motility. The mechanical responses are probably related to a recently demonstrated slow efferent effect. The implications of the present findings related to commonly assumed efferent behavior in vivo are considered.

Functional properties of neuronal nicotinic acetylcholine receptor channels expressed in transfected human cells

To study how subunit composition affects the functional properties of neuronal nicotinic acetylcholine receptors (nAChRs), we examined the behaviour of acetylcholine (ACh)-induced single-channel currents in human BOSC 23 cells transiently transfected with various subunit cDNA combinations. For all nAChRs examined (chick and rat alpha 3 beta 4, chick alpha 3 beta 2, alpha 4 beta 2, alpha 7 and alpha 8), expression levels were high enough to allow measurements of acetylcholine-evoked whole-cell currents and nicotine-elicited Ca2+ transients as well as the functional characterization of nAChR channels. Unitary acetylcholine-evoked events of alpha 8 nAChR had a slope conductance of 23 pS, whereas two conductance classes (19-23 and 32-45 pS) were identified for all other nAChR channels. The mean channel open times were significantly longer for homomeric alpha 7 and alpha 8 nAChRs (6-7 ms) than for heteromeric nAChRs (1-3 ms), with the exception of alpha 3 beta 4 nAChRs (8.4 ms for rat, 7 ms for chick). At least two species of heterologously expressed nAChRs (alpha 3 beta 4 and alpha 3 beta 2) exhibited single-channel characteristics similar to those reported for native receptors. The variety of nAChR channel conductance and kinetic properties encountered in human cells transfected with nAChR subunits contributes to the functional diversity of nAChRs in nerve cells.

Neuronal nicotinic receptor expression in sensory neurons of the rat trigeminal ganglion: demonstration of alpha3beta4, a novel subtype in the mammalian nervous system

The identification of a family of neuronal nicotinic receptor subunit genes establishes the potential for multiple subtypes with diverse physiological functions. Virtually all of the high affinity nicotinic receptors measured to date in the rodent CNS are composed of alpha4 and beta2 subunits only. However, the demonstration of other subunit transcripts in a variety of central and peripheral nervous tissues suggests a greater degree of receptor subtype heterogeneity than so far has been elucidated. The purpose of the present studies was to determine at the mRNA and protein levels which neuronal nicotinic receptor subunits are expressed by sensory neurons of the rat trigeminal ganglion and in what combinations these gene products associate to form neuronal nicotinic receptor subtypes in this tissue. Radioreceptor binding analysis indicated that in the adult rat trigeminal ganglion there exist at least two nicotinic receptor binding sites with differing affinities for [3H]-epibatidine. In situ hybridization histochemical studies revealed the existence of mRNA encoding the alpha3, alpha4, alpha5, beta2, and beta4 subunits, but not the alpha2 subunit. Immunoprecipitation with subunit-specific antisera demonstrated that each of the subunits present in the ganglion at the mRNA level is a constituent of nicotinic receptors capable of binding 3H-epibatidine. Various applications of these approaches yielded strong evidence that, in addition to alpha4beta2, which is thought to be the predominant neuronal nicotinic receptor subtype in the rodent CNS, trigeminal sensory neurons express as the principal subtype alpha3beta4, which has not been demonstrated previously in mammalian nervous tissue.

Unique postsynaptic signaling at the hair cell efferent synapse permits calcium to evoke changes on two time scales

The cholinergic efferent fibers to the outer hair cells (OHCs) of the mammalian cochlea suppress sound-evoked activity of the auditory nerve on two time scales via one nicotinic receptor. A rapid action (tens of milliseconds) is responsible for modulating auditory nerve responses to acoustic stimulation. A slower action (tens of seconds) may protect the ear from acoustic overstimulation. The rapid action is likely caused by calcium influx through the nicotinic receptor that leads to opening of calcium-activated potassium (KCa) channels, but the mechanism of the slower action has not been explained. To investigate this mechanism, we perfused the cochlea with agents that alter intracellular calcium release and uptake. Both fast and slow effects were enhanced by perfusion of the cochlea with ryanodine, an agonist of calcium-induced calcium release (CICR). Antagonists of sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA), cyclopiazonic acid, and thapsigargin (1) selectively enhanced the magnitude of slow effects, (2) prevented the diminution of slow effects with continued efferent stimulation, and (3) spread the range of frequencies over which slow effects were observed. We propose that the slow effect is attributable to release of calcium from the subsurface cisterna of the OHC, perhaps triggered by CICR from the synaptic cisterna; the two time scales of efferent action may result from the unique arrangement of the two cisternae in the baso-lateral region of the OHC.

Nicotinic acetylcholine receptor (nACh-R) agonist-induced changes in brain monoamine turnover in mice

The aim of the present study was to evaluate the effects of nicotinic acetylcholine receptor (nACh-R) agonists such as (-)-nicotine and related compounds on brain monoamine turnover. A single administration of (-)-nicotine (0.04, 0.2, 1.0, and 5.0 mg/kg SC) increased both noradrenaline (NA) and dopamine (DA) turnover in a dose-dependent manner, and the maximum effects were achieved 30 min after treatment with (-)-nicotine (1.0 mg/kg). The effect of (-)-nicotine on serotonin (5-HT) turnover was complicated; 5-HT turnover was increased at a low dose of (-)-nicotine (0.04 mg/kg) but decreased at a high dose (1.0 mg/kg). The (-)-nicotine (1.0 mg/kg)-induced changes in monoamine turnover were blocked by pretreatment with the centrally acting nACh-R channel blocker mecamylamine (2.0 mg/kg i.p.) but not by hexamethonium (2.0 mg/kg i.p.). These findings indicate that systemically administered (-)-nicotine can enhance brain NA and DA turnover and affect 5-HT turnover, both of which are mediated by central nACh-R. The changes in the monoamine turnover induced by (+/-)-anabasine were similar to those induced by (-)-nicotine, while (-)-lobeline and (-)-cytisine had little effect, and 1,1-dimethyl-4-phenyl-piperazinium (DMPP) increased NA and 5-HT turnover but not DA turnover at all doses tested. (S)-3-Methyl-5-(l-methyl-2- pyrrolidinyl)isoxazole (ABT-418), a selective neuronal nACh-R agonist, increased NA, DA and 5-HT turnover, but had a weaker effect on DA turnover than NA and 5-HT turnover. In addition, 9-amino-1,2,3,4-tetrahydroacridine (THA), an acetylcholine esterase inhibitor, also increased monoamine turnover in the brain. Pretreatment with mecamylamine completely blocked the THA-induced increase in NA and 5-HT turnover, but not in DA turnover, suggesting that the nACh-R system is involved in the THA-induced increase in brain NA and 5-HT turnover. On the other hand, (-)-cytisine, a partial agonist for the beta 2 subunit containing nACh-R, completely inhibited the nACh-R agonist- and THA-induced increases in NA turnover, but not in DA turnover, and normalized the changes in 5-HT turnover. In conclusion, the subtypes of nACh-Rs mediating DA turnover may be different from those mediating NA and 5-HT turnover in the CNS.

Determinants of specificity for alpha-conotoxin MII on alpha3beta2 neuronal nicotinic receptors

The competitive antagonist alpha-conotoxin-MII (alpha-CTx-MII) is highly selective for the alpha3beta2 neuronal nicotinic receptor. Other receptor subunit combinations (alpha2beta2, alpha4beta2, alpha3beta4) are >200-fold less sensitive to blockade by this toxin. Using chimeric and mutant subunits, we identified amino acid residues of alpha3 and beta2 that participate in determination of alpha-CTx-MII sensitivity. Chimeric alpha subunits, constructed from the alpha3 and alpha4 subunits, as well as from the alpha3 and alpha2 subunits, were expressed in combination with the beta2 subunit in Xenopus laevis oocytes. Chimeric beta subunits, formed from the beta2 and beta4 subunits, were expressed in combination with alpha3. Determinants of alpha-CTx-MII sensitivity on alpha3 were found to be within sequence segments 121-181 and 181-195. The 181-195 segment accounted for approximately half the difference in toxin sensitivity between receptors formed by alpha2 and alpha3. When this sequence of alpha2 was replaced with the corresponding alpha3 sequence, the resulting chimera formed receptors only 26-fold less sensitive to alpha-CTx-MII than alpha3beta2. Site-directed mutagenesis within segment 181-195 demonstrated that Lys185 and Ile188 are critical in determination of sensitivity to toxin blockade. Determinants of alpha-CTx-MII sensitivity on beta2 were mapped to sequence segments 1-54, 54-63, and 63-80. Site-directed mutagenesis within segment 54-63 of beta2 demonstrated that Thr59 is important in determining alpha-CTx-MII sensitivity.

Expression of alpha 7 neuronal nicotinic receptors during postnatal development of the rate cerebellum

Several lines of evidence suggest that alpha-bungarotoxin-sensitive neuronal nicotinic acetylcholine receptors may play a developmental role by modulating plasticity in neuronal circuits. The alpha 7 subunit, a main component of these receptors, is expressed in most regions of the brain, including the cerebellum, where it is present almost exclusively in Purkinje cells and deep cerebellar nuclei. Purkinje cells constitute the only efferent pathway of the cerebellum and their development involves complex interactions, which have been extensively studied. They therefore provide a potentially useful model for analysis of development plasticity which could be influenced by alpha 7 neuronal nicotinic receptors. In the present study a previously characterized monoclonal antibody (mAb 307) has been used to determine the temporal pattern of expression of the alpha 7 subunit in the developing rat cerebellum. No detectable alpha 7 immunoreactivity is found between P0 and P2. Between P3 and P5, however, the Purkinje cell layer shows moderate immunolabeling. alpha 7 expression in this layer increases rapidly between P8 and P15. This increase in alpha 7 staining, which overlaps in time with important developmental and synaptogenic events, is not uniform throughout the cerebellar cortex. Thus, between P3 and P5 all Purkinje cells are weakly labeled, while at later stages (P8-P15) immunolabeling becomes more intense, but at the same time, disappears from Purkinje cells in rostral lobules. In addition, a very well defined pattern for discontinuous or columnar labeling is detected in regions of the Purkinje cell layer where alpha 7 subunits were being expressed. Finally, at P20, alpha 7 subunit labeling is found again in all Purkinje cells, although with lower intensity. These results suggest that alpha 7 receptor expression is developmentally regulated, with a time course that parallels the final differentiation of Purkinje cells. In addition, the heterogeneous spatial distribution of alpha 7-containing nicotinic receptors indicates that, during cerebellar maturation, these cells may receive different signals that modulate receptor gene expression in a very specific way.

Genetic manipulations of cholinergic communication reveal trans-acting control mechanisms over acetylcholine receptors

Several approaches have been developed for genetic modulations of receptor expression. These initiated with gene cloning and heterologous expression in microinjected Xenopus oocytes, and proceeded through transgenic expression and genomic disruption of receptor genes in mice. In addition, antisense treatments have reduced receptor levels in a transient, reversible manner. Integration of foreign DNA with host genomic sequences yields both cis- and trans-acting responses. These may depend on the DNA integration site, host cells condition and most importantly, the affected signal transduction circuit. For example, acetylcholinesterase (AChE) overexpression in microinjected Xenopus tadpoles has been shown to upregulate alpha-bungarotoxin binding levels, indicating trans-acting control conferring overproduction of muscle nicotinic acetylcholine receptors. In transgenic mice expressing human AChE, the hypothermic response to oxotremorine was suppressed, reflecting modified levels of brain muscarinic receptors. To dissociate the feedback processes occurring in transfected cells from responses related to DNA integration, we examined the endogenous expression of the alpha 7 neuronal nicotinic acetylcholine receptor in PC12 cells transfected with DNA vectors carrying alternative splicing variants of human AChE mRNA. Our findings demonstrate suppression of alpha 7 receptor levels associated with the accumulation of foreign DNA in the transfected cells. Acetylcholine receptor levels thus depend on multiple elements, each of which should be considered when genetic interventions are employed.

Pharmacology of neuronal nicotinic acetylcholine receptor subtypes

The search for the physiological function of nicotinic receptors on neurons in the brain began with their discovery. It was initially assumed that, as in ganglia and at the neuromuscular junction, nicotinic receptors would gate fast synaptic transmission in the brain. The best functional evidence now, however, points to a role in modifying the release of other transmitters. This does not preclude a postsynaptic role in transmission for nicotinic receptors in the brain, but attempts to locate such a synapse have not been successful. If fast nicotinic synapses are present in the brain, they are probably low in number and may be masked by other more prevalent synapses (such as glutamatergic) so identification will not be easy. The extent of diversity of nicotinic receptors is substantial. At the molecular level this is reflected in the number of different genes that encode receptor subunits and the multiple possible combinations of subunits that function in expression systems. From the cellular level there is a broad diversity of properties of native receptors in neurons. Some useful pharmacological tools allow the limited identification of subunits in native receptors. For example, block by alpha-bungarotoxin identifies alpha 7, alpha 8, or alpha 9 subunits; activation of a receptor by cytisine indicates an alpha 7 or beta 4 subunit; and neuronal bungarotoxin block identifies a beta 2 subunit. Despite the clues to identity gained by careful use of these agents, we have not been able to identify all the components of any native receptor based on pharmacological properties assessed from expression studies. When both pharmacological and biophysical properties of a receptor are taken into consideration, none of the combinations tested in oocytes mimics native receptors exactly. The reason for this discrepancy has been debated at length; it is possible that oocytes do not faithfully manufacture neuronal nicotinic receptors. For example, they may not correctly modify the protein after translation or they may allow a combination of subunits that do not occur in vivo. Another possibility is that correct combinations of subunits have not yet been tested in oocytes. Data from immunoprecipitation experiments suggest that many receptors contain three or more different subunits. Results from further experiments injecting combinations of three or more subunits into oocytes may be enlightening. The diversity of receptors may allow targeting of subtypes to specific locations. Nicotinic receptors are located presynaptically, preterminally, and on the cell soma. The function of the nicotinic receptors located on innervating axons is presumably to modify the release of other neurotransmitters. It is an attractive hypothesis that nicotinic receptors might be involved in modifying the weight of central synapses; however, in none of the regions where this phenomenon has been described is there any evidence for axoaxonal contacts. The presynaptic receptors described so far are pharmacologically unique; therefore, if there are different subtypes of nicotinic receptors modifying the release of different transmitters, they may provide a means of exogenously modifying the release of a particular transmitter with drugs. There are still many basic unanswered questions about nicotinic receptors in the brain. What are the compositions of native nicotinic receptors? What is their purpose on neurons? Although there is clearly a role presynaptically, what is the function of those located on the soma? Neuronal nicotinic receptors are highly permeable to calcium, unlike muscle nicotinic receptors, and this may have important implications for roles in synaptic plasticity and development. Finally, why is there such diversity? (ABSTRACT TRANCATED)

Linkage of strain-specific nicotinic receptor alpha 7 subunit restriction fragment length polymorphisms with levels of alpha-bungarotoxin binding in brain

Inbred mouse strains have been shown to differ in their levels of brain alpha-bungarotoxin binding. These differences in alpha-bungarotoxin receptors have been shown to correlate with an animal's sensitivity to nicotine-induced seizures. Recent studies have shown that the alpha 7 nicotinic acetylcholine receptor subunit is the major alpha-bungarotoxin binding site in rodent brain. In this report, we examined whether mouse strains that differ in levels of alpha-bungarotoxin binding and sensitivity to nicotine-induced convulsions also differ for the alpha 7 subunit. A full-length murine alpha 7 cDNA was cloned and sequenced and found to be identical to that of a mouse alpha 7 cDNA recently reported. Subsequently, a comparison of alpha 7 cDNA sequences and RNA species was performed between two strains (C3H/2 and DBA/2) that differ in levels of brain alpha-bungarotoxin binding and sensitivity to nicotine-induced seizures. The only difference observed was a single nucleotide difference in the open reading frame of alpha 7 that does not affect the primary amino acid sequence. Inbred strains were also surveyed for restriction fragment length polymorphisms at the alpha 7 locus. Strain-specific polymorphisms were identified, and F2 and backcross animals from a classic genetic cross between C3H/2 and DBA/2 mice were compared for the inheritance of alpha 7 genotype and alpha-bungarotoxin receptor levels. A significant association between genotype and receptor levels was observed in both, the F2 and backcross generations. These results indicate that alpha 7 genotype is an important determinant of alpha-bungarotoxin receptor levels.

Transcriptional regulation of neuronal nicotinic acetylcholine receptor genes. Functional interactions between Sp1 and the rat beta4 subunit gene promoter

To date, 11 members (alpha2-alpha9 and beta2-beta4) of the neuronal nicotinic acetylcholine receptor gene family have been identified. These genes encode subunits that form distinct receptors with different pharmacological and physiological profiles in temporally and spatially restricted patterns within the nervous system. Distinct molecular mechanisms probably orchestrate the expression of various receptor subtypes, yet little is known of specific transcriptional regulatory elements and their associated factors that are responsible for this segregated pattern of expression. Here we report the identification of an element, in the 5'-flanking region of the rat beta4 subunit gene, containing a CA box that is necessary for beta4 promoter activity in a transiently transfected cholinergic cell line, SN17. This element was shown to interact with a protein(s) in SN17 nuclear extracts that is antigenically related to the transcriptional activator Sp1. Furthermore, co-transfection experiments confirmed that Sp1 can transactivate a beta4 promoter-reporter gene construct, indicating that Sp1 is necessary, at least in part, for transcriptional activation of the beta4 subunit gene.

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

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

Nicotine increases [Ca2+]i and regulates electrical activity in insect neurosecretory cells (DUM neurons) via an acetylcholine receptor with 'mixed' nicotinic-muscarinic pharmacology

An increase in intracellular free calcium concentration ([Ca2+]i) was observed following the application of nicotine to isolated adult dorsal unpaired median (DUM) neurons of the cockroach (Periplaneta americana) terminal abdominal ganglion (TAG) using Fura-2 fluorescence measurements. Bath-applied nicotine (1 mM) induced a transient increase in [Ca2+]i. Calcium responses to bath-applied nicotine were blocked completely by alpha-bungarotoxin (100 nM) and were reduced by 50% in the presence of pirenzepine (1 microM). The sensitivity of the response to both nicotinic and muscarinic antagonists suggested that it was mediated by an acetylcholine receptor with 'mixed' pharmacology. In whole cell current-clamp experiments, nicotine reduced the frequency of evoked action potentials by decreasing the slope of the predepolarization in the last two-thirds of the pacemaker potential. Voltage-clamp studies revealed that nicotine modified the inactivation properties of the maintained low-voltage-activated (LVA) calcium current increasing the rate of relaxation of this current and transforming a U-shaped voltage dependence of inactivation into a monotonic relationship to voltage. These effects were blocked when isolated DUM neurons were pretreated with 0.5 microM alpha-bungarotoxin. Our findings suggested a novel calcium-dependent regulation of firing behavior in TAG DUM neurons following activation of an acetylcholine receptor with 'mixed' pharmacology, resulting in a rise in [Ca2+]i which reduces firing frequency by modulating a maintained LVA calcium current responsible for the action potential predepolarization.

Acetylcholine receptor subunit homomer formation requires compatibility between amino acid residues of the M1 and M2 transmembrane segments

The neuronal nicotinic acetylcholine receptor (nAChR) subunits alpha3 and alpha7 have different assembly behavior when expressed in heterologous expression systems: alpha3 subunits require other subunits to assemble functional nAChRs, whereas alpha7 subunits can produce homomeric nAChRs. A previous analysis of alpha7/alpha3 chimeric constructs identified a domain comprising the first putative membrane-spanning segment, M1, as essential to homomeric assembly. The present study dissected further this domain, identifying three amino acid residues, which are located at the most intracellular third of the M1 transmembrane segment, as important in the assembly of homomers. Moreover, formation of homooligomeric complexes seems to require a compatible accommodation between this region and certain residues of the second transmembrane segment, M2. Thus, compatibility between defined domains of the M1 and M2 transmembrane segments appears as a determinant factor governing homomer association of nAChR subunits.

An amino acid exchange in the second transmembrane segment of a neuronal nicotinic receptor causes partial epilepsy by altering its desensitization kinetics

The alpha4 subunit of the neuronal nicotinic acetylcholine receptor is the first gene shown to be involved in a human idiopathic epileptic disease. A missense mutation, leading to the replacement of serine 248 by phenylalanine in the second transmembrane segment, had been detected in patients with autosomal dominant nocturnal frontal lobe epilepsy. The properties of the wild type receptor composed of alpha4 and beta2 subunits and the mutant receptor where alpha4 subunits carried the mutation at serine 248 were compared by means of cDNA manipulation and expression in Xenopus oocytes. The mutant receptor exhibited faster desensitization upon activation by acetylcholine and recovery from the desensitized state was much slower than in the wild type receptor. We conclude that the reported mutation causes seizures via a diminution of the activity of the alpha4beta2 neuronal nicotinic acetylcholine receptor.

Nudicauline and elatine as potent norditerpenoid ligands at rat neuronal alpha-bungarotoxin binding sites: importance of the 2-(methylsuccinimido)benzoyl moiety for neuronal nicotinic acetylcholine receptor binding

Methyllycaconitine (MLA, 1) is a novel, potent probe for mammalian and insect nicotinic acetylcholine receptors (nAChR) and displays remarkable selectivity toward neuronal [125I]-alpha-bungarotoxin (alpha BgTX) binding sites that correspond to alpha 7-type nAChR in mammalian brain. We have shown that, among a number of selected norditerpenoid alkaloids, elatine (2) and nudicauline (3) are equipotent with, or better than, MLA (1) in binding to brain [125I]-alpha BgTX binding sites, with IC50 values of 6.1, 1.7, and 7.6 nM, respectively. The 2-((S)-methylsuccinimido)benzoyl moiety of these ligands is crucial for high-affinity binding, whereas structural modifications to the norditerpenoid core of the ligand can be tolerated without loss of activity or selectivity. In addition to MLA (1), elatine (2), and nudicauline (3), we have examined lycoctonine (4), inuline (6), lappaconitine (7), N-desacetyllappaconitine (8), delsoline (10), delcorine (11), deltaline (12), condelphine (13), and karacoline (14). This study therefore extends the range of norditerpenoids, other than MLA, which can be used to probe this important class of nAChR. All 12 alkaloids were assessed for activity at [3H]nicotine binding sites which are considered to represent alpha 4 beta 2 nAChR. Furthermore, the 1H and 13C NMR spectroscopic data of MLA and elatine have been critically compared.

Expression of nicotinic acetylcholine receptor mRNA in the adult rat peripheral vestibular system

The mRNA expression of the neuronal nicotinic acetylcholine receptor subunits was determined in adult rat vestibular end-organs and in Scarpa's ganglion (SCG) by in situ hybridization with [35S] riboprobes. Neurons in the SCG expressed the alpha 4-7 and beta 2-3 mRNAs, but not alpha 3 or beta 4 mRNAs. Not all SCG neurons expressed every mRNA found in SCG. The alpha 6 and beta 2-3 riboprobes labeled all neurons, but alpha 4, alpha 5, and alpha 7 mRNAs were selectively expressed in one or more subpopulations of SCG neurons. Vestibular sensory hair cells, in contrast, expressed only alpha 9 mRNA.

Acetylcholine response in guinea pig outer hair cells. I. Properties of the response

The properties of the ACh (acetylcholine) response in guinea pig outer hair cells (OHCs) are not well understood. It has been shown that the response to ACh involves the activation of a Ca2+ dependent K+ selective conductance (referred to as Ksub where sub stands for suberyldicholine). In the present study, we examined the voltage dependence, the time dependence, and the desensitization of the ACh response. In addition, we examined the K+ selectivity of K(sub). These properties are important for aiding in the determination of the type of K+ channels activated by ACh. Patch-clamp technique in the whole-cell mode was used to record from single OHCs isolated from adult pigmented guinea pigs. ACh (100 microM) was applied to the voltage-clamped OHCs and the ACh induced currents (IACh) were measured. A voltage dependence of the ACh response was found with the ACh induced currents decaying monoexponentially at potentials positive to -30 mV. The decay of the ACh induced currents was faster soon after establishing the whole-cell mode of recording when compared to the decay of the currents some time later. This effect, referred to as the time dependence, was different from the desensitization of the response upon prolonged application of ACh. The desensitization of the ACh induced currents was about 50% after 2 min of continuous application of 100 microM ACh. The examined characteristics of the ACh response in guinea pig OHCs indicate a voltage and time dependence of the response and strong K+ selectivity of the Ksub.

Neuronal nicotinic receptor alpha 6 subunit mRNA is selectively concentrated in catecholaminergic nuclei of the rat brain

Although the neuronal nicotinic receptor alpha 6 subunit was cloned several years ago, its functional significance remains to be investigated. Here we describe an in situ hybridization study of the mRNA for this subunit in the adult rat central nervous system using oligonucleotide probes. Specific alpha 6 mRNA labelling was restricted to a few nuclei throughout the brain; it was particularly high in several catecholaminergic nuclei [the locus coeruleus (A6), the ventral tegmental area (A10) and the substantia nigra (A9)] at levels significantly higher than those found for any other known nicotinic receptor subunit mRNA. Labelling for alpha 6 mRNA was also detected at lower levels in the reticular thalamic nucleus, the supramammillary nucleus and the mesencephalic V nucleus. Some cells of the medial habenula (medioventral part) and of the interpeduncular nucleus (central and lateral parts) were also labelled. The distribution of alpha 6 mRNA was compared with the distribution of the other known nicotinic acetylcholine receptor subunit mRNAs. In several nuclei, the expression of alpha 6 was complementary to those of other alpha subunits. Moreover, some of the cell groups (such as the substantia nigra, the ventral tegmental area and the locus coeruleus) previously thought to contain mainly alpha 3 mRNA in fact were found to contain high levels of alpha 6 mRNA. Finally, we found extensive colocalization of alpha 6 and beta 3, indicating the possible existence of nicotinic receptor hetero-oligomers containing both subunits. The present results show that alpha 6 is the major nicotinic acetylcholine receptor alpha subunit expressed in dopaminergic cell groups of the mesencephalon and noradrenergic cells of the locus coeruleus. This suggests the involvement of the alpha 6 subunit in some of the major functions of central nicotinic circuits, including the modulation of locomotor behaviour and reward.