Agonist pharmacology of the neuronal alpha 7 nicotinic receptor expressed in Xenopus oocytes

Role of Alpha-7-Nicotinic Acetylcholine Receptor in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder affecting millions worldwide. One of the leading hypotheses for the underlying cause of AD is a reduction in nicotinic receptor levels in the brain. Among the nicotinic receptors, the alpha-7-nicotinic acetylcholine receptor (α7nAChR) has received particular attention due to its involvement in cognitive function.α7nAChR is a ligand-gated ion channel that is primarily found in the hippocampus and prefrontal cortex, areas of the brain responsible for learning, memory, and attention. Studies have shown that α7nAChR dysfunction is a key contributor to the pathogenesis of AD. The receptor is involved in regulating amyloidbeta (Aβ) production, a hallmark of AD pathology. Many drugs have been investigated as α7nAChR agonists or allosteric modulators to improve cognitive deficits in AD. Clinical studies have shown promising results with α7nAChR agonists, including improved memory and cognitive function. Although several studies have shown the significance of the α7 nAChR in AD, little is known about its function in AD pathogenesis. As a result, in this review, we have outlined the basic information of the α7 nAChR's structure, functions, cellular responses to its activation, and its role in AD's pathogenesis.

Pharmacological profiles and anti-inflammatory activity of pCN-diEPP and mCN-diEPP, new alpha9alpha10 nicotinic receptor ligands

Pain due to inflammation can be reduced by targeting the noncanonical nicotinic receptors (NCNR) in cells of the immune system that regulate the synthesis and release of pro- and anti-inflammatory cytokines. Although NCNR do not generate ion channel currents, the pharmacology of ion-channel forms of the receptors can predict drugs which may be effective regulators of the cholinergic anti-inflammatory system (CAS). Agonists of α7 type receptors have been definitively associated with CAS. Receptors containing α9 and α10 subunits have also been implicated. We have recently characterized two small molecules, pCN-diEPP and mCN-diEPP, as selective α9α10 agonists and antagonists, respectively. We used these drugs, along with nicotine, an α7 agonist and α9α10 antagonist, to probe the mixed populations of receptors that are formed when α7, α9, and α10 are all expressed together in Xenopus oocytes. We also evaluated the effects of the CN-diEPP compounds on regulating the ATP-induced release of interleukin-1β from monocytic THP-1 cells, which express NCNR. The compounds successfully identified separate populations of receptors when all three subunits were co-expressed, including a potential population of homomeric α10 receptors. The α9α10 agonist pCN-diEPP was the more effective regulator of interleukin-1β release in THP-1 cells. pCN-diEPP was also fully effective in a mouse model of inflammatory pain, while mCN-diEPP had only partial effects, requiring a higher dosage. The analgetic effects of pCN-diEPP and mCN-diEPP were retained in α7 knockout mice. Taken together, our results suggest that drugs that selectively activate α9α10 receptors may useful to reduce inflammatory pain through the CAS.

Review of Cyanotoxicity Studies Based on Cell Cultures

Cyanotoxins (CTs) are a large and diverse group of toxins produced by the peculiar photosynthetic prokaryotes of the domain Cyanoprokaryota. Toxin-producing aquatic cyanoprokaryotes can develop in mass, causing “water blooms” or “cyanoblooms,” which may lead to environmental disaster—water poisoning, extinction of aquatic life, and even to human death. CT studies on single cells and cells in culture are an important stage of toxicological studies with increasing impact for their further use for scientific and clinical purposes, and for policies of environmental protection. The higher cost of animal use and continuous resistance to the use of animals for scientific and toxicological studies lead to a progressive increase of cell lines use. This review aims to present (1) the important results of the effects of CT on human and animal cell lines, (2) the methods and concentrations used to obtain these results, (3) the studied cell lines and their tissues of origin, and (4) the intracellular targets of CT. CTs reviewed are presented in alphabetical order as follows: aeruginosins, anatoxins, BMAA (β-N-methylamino-L-alanine), cylindrospermopsins, depsipeptides, lipopolysaccharides, lyngbyatoxins, microcystins, nodularins, cyanobacterial retinoids, and saxitoxins. The presence of all these data in a review allows in one look to advance the research on CT using cell cultures by facilitating the selection of the most appropriate methods, conditions, and cell lines for future toxicological, pharmacological, and physiological studies.

Anatoxin-a: Overview on a harmful cyanobacterial neurotoxin from the environmental scale to the molecular target

Anatoxin-a (ATX-a) is a neurotoxic alkaloid, produced by several freshwater planktonic and benthic cyanobacteria (CB). Such CB have posed human and animal health issues for several years, as this toxin is able to cause neurologic symptoms in humans following food poisoning and death in wild and domestic animals. Different episodes of animal intoxication have incriminated ATX-a worldwide, as confirmed by the presence of ATX-a-producing CB in the consumed water or biofilm, or the observation of neurotoxic symptoms, which match experimental toxicity in vivo. Regarding toxicity parameters, toxicokinetics knowledge is currently incomplete and needs to be improved. The toxin can passively cross biological membranes and act rapidly on nicotinic receptors, its main molecular target. In vivo and in vitro acute effects of ATX-a have been studied and make possible to draw its mode of action, highlighting its deleterious effects on the nervous systems and its effectors, namely muscles, heart and vessels, and the respiratory apparatus. However, very little is known about its putative chronic toxicity. This review updates available data on ATX-a, from the ecodynamic of the toxin to its physiological and molecular targets.

(−)-Cytisine: Access to a stereochemically defined and functionally flexible piperidine scaffold

Cytisine undergoes ready fragmentation to provide a highly flexible (and “privileged”) piperidine scaffold capable of exploring a diversity of chemical space.

Alpha-7 nicotinic agonists for cognitive deficits in neuropsychiatric disorders: A translational meta-analysis of rodent and human studies

Cognitive dysfunction in schizophrenia (SCZ) and Alzheimer's disease (AD) is a major driver of functional disability but is largely unresponsive to current therapeutics. Animal models of cognitive dysfunction relevant to both disorders suggest the α7 nicotinic acetylcholine receptor (nAChR) may be a promising drug development target, with multiple clinical trials subsequently testing this hypothesis in individuals with SCZ and AD. However, the translational value of rodent cognitive tasks for predicting the overall efficacy of this therapeutic target in clinical trials is unknown. To compare effect sizes between rodent and human studies, we searched PubMed and the Cochrane Library for all randomized, placebo-controlled trials of compounds with pharmacological activity at the α7 nAChR for treatment of cognitive dysfunction in SCZ and AD and identified 18 studies comprising 2670 subjects treated with eight different compounds acting as full or partial agonists. Cognitive outcomes were standardized, and random-effects meta-analyses revealed no statistically significant effects of α7 nAChR agonists on overall cognition or any of eight cognitive subdomains when all doses were included (Range of all cognitive outcomes: Cohen's d=-0.077 to 0.12, negative favoring drug). In contrast, analysis of 29 rodent studies testing the same α7 agonists revealed large effect sizes in multiple commonly used preclinical behavioral tests of cognition (Range: d=-1.18 to - 0.73). Our results suggest that targeting the α7 nAChR with agonists is not a robust treatment for cognitive dysfunction in SCZ or AD and necessitate a better understanding of the translational gap for therapeutics targeting the α7 nAChR.

Colorimetric microtiter plate receptor-binding assay for the detection of freshwater and marine neurotoxins targeting the nicotinic acetylcholine receptors

Anatoxin-a and homoanatoxin-a, produced by cyanobacteria, are agonists of nicotinic acetylcholine receptors (nAChRs). Pinnatoxins, spirolides, and gymnodimines, produced by dinoflagellates, are antagonists of nAChRs. In this study we describe the development and validation of a competitive colorimetric, high throughput functional assay based on the mechanism of action of freshwater and marine toxins against nAChRs. Torpedo electrocyte membranes (rich in muscle-type nAChR) were immobilized and stabilized on the surface of 96-well microtiter plates. Biotinylated α-bungarotoxin (the tracer) and streptavidin-horseradish peroxidase (the detector) enabled the detection and quantitation of anatoxin-a in surface waters and cyclic imine toxins in shellfish extracts that were obtained from different locations across the US. The method compares favorably to LC/MS/MS and provides accurate results for anatoxin-a and cyclic imine toxins monitoring. Study of common constituents at the concentrations normally found in drinking and environmental waters, as well as the tolerance to pH, salt, solvents, organic and inorganic compounds did not significantly affect toxin detection. The assay allowed the simultaneous analysis of up to 25 samples within 3.5 h and it is well suited for on-site or laboratory monitoring of low levels of toxins in drinking, surface, and ground water as well as in shellfish extracts.

Cellular, molecular, and genetic substrates underlying the impact of nicotine on learning

Addiction is a chronic disorder marked by long-lasting maladaptive changes in behavior and in reward system function. However, the factors that contribute to the behavioral and biological changes that occur with addiction are complex and go beyond reward. Addiction involves changes in cognitive control and the development of disruptive drug-stimuli associations that can drive behavior. A reason for the strong influence drugs of abuse can exert on cognition may be the striking overlap between the neurobiological substrates of addiction and of learning and memory, especially areas involved in declarative memory. Declarative memories are critically involved in the formation of autobiographical memories, and the ability of drugs of abuse to alter these memories could be particularly detrimental. A key structure in this memory system is the hippocampus, which is critically involved in binding multimodal stimuli together to form complex long-term memories. While all drugs of abuse can alter hippocampal function, this review focuses on nicotine. Addiction to tobacco products is insidious, with the majority of smokers wanting to quit; yet the majority of those that attempt to quit fail. Nicotine addiction is associated with the presence of drug-context and drug-cue associations that trigger drug seeking behavior and altered cognition during periods of abstinence, which contributes to relapse. This suggests that understanding the effects of nicotine on learning and memory will advance understanding and potentially facilitate treating nicotine addiction. The following sections examine: (1) how the effects of nicotine on hippocampus-dependent learning change as nicotine administration transitions from acute to chronic and then to withdrawal from chronic treatment and the potential impact of these changes on addiction, (2) how nicotine usurps the cellular mechanisms of synaptic plasticity, (3) the physiological changes in the hippocampus that may contribute to nicotine withdrawal deficits in learning, and (4) the role of genetics and developmental stage (i.e., adolescence) in these effects.

Effects of weekly exposure to anatoxin-a and nicotine on operant performance of rats

This study examined the effects of acute and weekly administration of anatoxin-a and nicotine on operant performance. Anatoxin-a is a potent nicotinic receptor agonist produced by cyanobacteria, which are found in fresh waters throughout the world. Anatoxin-a is a potential human health hazard and has been responsible for numerous deaths of wildlife, livestock and domestic animals. Remarkably little is known, however, about the effects of anatoxin-a on behavior. Nicotine, the psychomotor stimulant in tobacco, has many well-documented behavioral effects, which often diminish (i.e. tolerance develops) when it is given daily. Male Long Evans rats initially were trained to respond under a multiple variable-ratio 30-response variable-interval 60-s (mult VR-30 VI 60-s) schedule of food reinforcement. They were then divided into 12 groups of 8 that received four weekly subcutaneous injections of anatoxin-a (0.05-0.2 mg/kg), nicotine (0.125-1.8 mg/kg), or vehicle 5-min prior to testing. When initially administered, each compound decreased response rates and reinforcement rates in both components of the multiple schedule. Substantial tolerance developed to the disruptive effects of nicotine with weekly administration. Tolerance also developed to the effects of anatoxin-a, although to a lesser degree; the highest dose severely decreased performance with little evidence of recovery. In conjunction with prior findings, these results suggest the behavioral effects of anatoxin-a and nicotine are similar, but not identical, and that relatively infrequent (episodic) administration can produce tolerance.

Pharmacological and immunological identification of native alpha7 nicotinic receptors: evidence for homomeric and heteromeric alpha7 receptors

Controversy surrounds the expression of alpha7 nicotinic acetylcholine receptors (nAChRs) in adrenal chromaffin cells. In these studies, alpha7 nAChRs expressed in bovine adrenal chromaffin cells are investigated. Using radiolabeled ligand binding techniques, [(125)I]alpha-bungarotoxin (alphaBGT) binding reaches equilibrium within 4 h and is saturable with a K(d) value of 4.2 nM. Using homologous competition experiments, the K(i) for binding of alphaBGT was 1.9 nM. These data are consistent with the expression of homomeric alpha7 nAChRs. Methyllycaconatine (MLA), which binds alpha7 nAChRs with high affinity, inhibits [(125)I]alphaBGT binding in a concentration-dependent manner with a K(i) of 30.6 nM; this value is approximately 10 fold higher than the reported affinity of MLA for alpha7 nAChRs. We also document the ability of bromoacetylcholine (brACh) to alkylate alpha7 nAChRs, as has been previous demonstrated for bovine adrenal alpha3beta4 nAChRs. When adrenal nAChRs are immunoprecipated with mAb319, an antibody which recognizes alpha7 nAChR protein, and then probed with mAb319 using Western blot analysis, a single band of approximately 53 kDa is identified. When adrenal nAChRs are immunoprecipated with mAb35, an antibody which recognizes alpha3 and alpha5 nAChR proteins, and then probed with mAb319 using Western blot analysis, a single band of approximately 53 kDa is identified. Together, these results support the expression of alpha7 nAChRs in bovine adrenal chromaffin cells. However, these data suggest that the subunit composition of some of these receptors may include heteromeric alpha7 nAChRs.

Mediation of glutamatergic receptors and nitric oxide on striatal dopamine release evoked by anatoxin-a. An in vivo microdialysis study

In this work, the involvement of ionotropic glutamatergic receptors and nitric oxide on striatal dopamine release induced by anatoxin-a was investigated in conscious and freely-moving rats. To study the participation of glutamatergic receptors, the effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine (MK-801) and d(-)-2-amino-5-phosphonopentanoic acid (APV), were examined. The perfusion of 3.5 mM anatoxin-a increased the extracellular dopamine levels to 701% relative to the basal. When CNQX was administered with 3.5 mM anatoxin-a, the increase of dopamine levels was 29% smaller than that observed with anatoxin-a alone. When MK-801 and APV were administered, the effect of anatoxin-a was attenuated 26% and 25% respectively in terms of that observed with anatoxin-a alone. And with CNQX plus MK-801, the effect of anatoxin-a was 53% inhibited in terms of the effect of anatoxin-a alone. These results suggest that the striatal dopamine release induced by anatoxin-a is partly mediated by activation of both ionotropic glutamatergic receptors. Since the neuronal form of nitric oxide synthase (nNOS) produces nitric oxide (NO) primarily in response to activation of NMDA receptors, it was tested if NO could play any role in the effect of anatoxin-a. Treatment with NOS inhibitors, L-nitro-arginine methyl ester (L-NAME) and d(-)-2-amino-5-phosphonopentanoic acid (7-NI), induced decreased anatoxin-a effects of 22% and 26% respectively. In conclusion, the present in vivo results demonstrate that anatoxin-a induced an indirect activation of ionotropic glutamatergic receptors (NMDA and AMPA/kainite receptors), which stimulate striatal dopamine release. On the other hand, activation of NMDA receptors may elicit NO increased levels enhancing dopamine release.

In vivo Effects of the Anatoxin-a on Striatal Dopamine Release

Anatoxin-a is an important neurotoxin that acts a potent nicotinic acetylcholine receptor agonist. This characteristic makes anatoxin-a an important tool for the study of nicotinic receptors. Anatoxin-a has been used extensively in vitro experiments, however anatoxin-a has never been studied by in vivo microdialysis studies. This study test the effect of anatoxin-a on striatal in vivo dopamine release by microdialysis.The results of this work show that anatoxin-a evoked dopamine release in a concentration-dependent way. Atropine had not any effect on dopamine release evoked by 3.5 mM anatoxin-a. However, perfusion of nicotinic antagonists mecamylamine and alpha-bungarotoxin induced a total inhibition of the striatal dopamine release. Perfusion of alpha7*-receptors antagonists, metillycaconitine or alpha-bungarotoxin, partially inhibits the release of dopamine stimulated by anatoxin-a. These results show that anatoxin-a can be used as an important nicotinic agonist in the study of nicotinic receptor by in vivo microdialysis technique and also support further in vivo evidences that alpha7*nicotinic AChRs are implicated in the regulation of striatal dopamine release.

Nicotine effects on the activity of mice exposed prenatally to the nicotinic agonist anatoxin-a

Anatoxin-a is a nicotinic agonist produced by several genera of cyanobacteria, and has caused numerous deaths of wildlife, livestock and domestic animals world-wide. Several studies in the literature have shown that exposure of mice and rats to nicotine early in development alters its effects when the rodents are subsequently challenged with nicotine. We therefore determined the effect of nicotine on the motor activity of adult mice that had been exposed prenatally to anatoxin-a. Pregnant CD-1 mice received either saline vehicle or one of two doses of (+/-) anatoxin-a (125, 200 microg/kg), i.p., on GD13-17. As adults (8 months), control mice of both genders were used to determine the effect of nicotine (0, 0.1, 0.3, 1.0 or 3.0 mg/kg, s.c.) on motor activity measured for 30-min in a photocell device. Under these conditions, nicotine produced dose-related decreases in both horizontal and vertical activity, with an ED50 estimated to be 0.65 mg/kg. Next, additional control mice and mice exposed prenatally to anatoxin-a received the nicotine ED50 and saline vehicle, in a counterbalanced fashion, with one week separating treatments. Nicotine decreased both horizontal and vertical activity in all mice, regardless of prenatal anatoxin-a treatment. Thus, no enduring effects of prenatal anatoxin-a were obtained in adult mice following nicotine challenge.

Neurotoxins in axenic oscillatorian cyanobacteria: coexistence of anatoxin-a and homoanatoxin-a determined by ligand-binding assay and GC/MS

Two neurotoxic alkaloids, anatoxin-a and its homologue homoanatoxin-a, were purified from the filamentous cyanobacteria Oscillatoria sp. strain 193 (PCC 9240) and Oscillatoria formosa NIVA CYA-92 (PCC 10111), respectively, and characterized by mass spectrometry. Biological activity was determined by examining the capacity of the toxins to competitively inhibit the binding of radiolabelled bungarotoxin to acetylcholine receptors, using post-synaptic membrane fractions of Torpedo electric tissue. Inhibition was concentration dependent, with a K(i) of 5.4+/-1.1x10(-8) M for anatoxin-a and 7.4+/-0.9x10(-8) M for homoanatoxin-a. Their high affinities for the nicotinic cholinergic receptors were exploited to adapt the radioligand-binding assay for routine detection of this class of neurotoxins directly in low-molecular-mass cell extracts of cyanobacteria. Confirmation of the results and toxin identification were achieved by coupled gas chromatography-mass spectrometry (GC/MS). Seventy-six axenic strains, representative of 13 genera, were analysed. Five strains of the genus Oscillatoria, hitherto unknown for their toxicity, inhibited bungarotoxin binding. GC/MS revealed that Oscillatoria sp. strains PCC 6407, PCC 6412 and PCC 9107 synthesized exclusively anatoxin-a, whereas both anatoxin-a and homoanatoxin-a were produced by strain PCC 9029. Oscillatoria sp. strain PCC 6506, an isolate co-identic with strain PCC 9029, also produced both neurotoxins, but their respective presence depended upon growth conditions. The latter results suggest that regulatory differences in at least some of the cyanobacterial strains may account for the preferential synthesis of only one of the two neurotoxins or for their simultaneous occurrence.

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

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

Nitrogen substitution modifies the activity of cytisine on neuronal nicotinic receptor subtypes

Cytisine very potently binds and activates the alpha 3 beta 4 and alpha 7 nicotinic subtypes, but only partially agonises the alpha 4 beta 2 subtype. Although with a lower affinity than cytisine, new cytisine derivatives with different substituents on the basic nitrogen (CC1-CC8) bind to both the heteromeric and homomeric subtypes, with higher affinity for brain [3H]epibatidine receptors. The cytisine derivatives were tested on the Ca(2+) flux of native or transfected cell lines expressing the rat alpha 7, or human alpha 3 beta 4 or alpha 4 beta 2 subtypes using Ca(2+) dynamics in conjunction with a fluorescent image plate reader. None elicited any response at doses of up to 30-100 microM, but all inhibited agonist-induced responses. Compounds CC5 and CC7 were also electrophysiologically tested on oocyte-expressed rat alpha 4 beta 2, alpha 3 beta 4 and alpha 7 subtypes. CC5 competitively antagonised the alpha 4 beta 2 and alpha 3 beta 4 subtypes with similar potency, whereas CC7 only partially agonised them with maximum responses of respectively 3% and 11% of those of 1 mM acetylcholine. Neither compound induced any current in the oocyte-expressed alpha 7 subtype, and both weakly inhibited acetylcholine-induced currents. Adding chemical groups of a different class or size to the basic nitrogen of cytisine leads to compounds that lose full agonist activity on the alpha 3 beta 4 and alpha 7 subtypes.

Comparative pharmacology of rat and human alpha7 nAChR conducted with net charge analysis

1. Pharmacological studies of alpha7 nicotinic acetylcholine receptors are confounded by the fact that rapid desensitization to high agonist concentration causes alpha7 peak responses to occur well in advance of complete solution exchange. For this reason, peak currents are an invalid measure of response to applied agonist concentrations. We show that results comparable to those that have been corrected for instantaneous concentration are obtained if net charge is used as the measure of receptor response. 2. Dose response curves obtained with these methods indicate that alpha7 receptors are approximately 10 fold more sensitive to agonist than previously reported. The agonists, ACh, choline, cytisine, GTS-21, 4OH-GTS-21 and 4-MeO-CA have the same rank order potency for both human and rat receptors: 4-MeO-CA > 4OH-GTS-21 > GTS-21 > cytisine > ACh > choline. However, differences in efficacy exist between rat and human receptors. GTS-21 is more efficacious for rat than human alpha7 receptors and cytosine more efficacious for human than rat alpha7 receptors. 3. Choline is the least potent agonist for both human and rat alpha7, with a potency approximately 10 fold lower than that of ACh. While the EC50 for the activation of alpha7 receptors by choline (400-500 microM) is outside the normal physiological range (10-100 microM), choline can nonetheless produce detectable levels of channel activation in the physiological concentration range. Since these concentrations are relatively non-desensitizing, the contribution of choline-activated alpha7 receptor current may play a significant role in the regulation of calcium homeostasis in alpha7-expressing neurons.

Role of the large cytoplasmic loop of the alpha 7 neuronal nicotinic acetylcholine receptor subunit in receptor expression and function

The role of the large intracellular loop of the nicotinic acetylcholine receptor (nAChR) alpha7 subunit in the expression of functional channels was studied. For this purpose, systematic deletions and substitutions were made throughout the loop and the ability of the mutated alpha7 subunits to support expression of functional nAChRs at the Xenopus oocyte membrane was tested. Surface nAChR expression was abolished upon removal of sequences at two regions, a 29-amino acid segment close to the N-terminus of the loop (amino acids 297-325) and adjacent to the third transmembrane region and an 11-amino acid segment near the fourth transmembrane region. Some residues (amino acids 317-322) within the 29 amino acids N-terminal segment could be substituted by others but not deleted without loss of expression, suggesting that a certain structure, determined by the number of amino acids rather than by their identity, has to be maintained in this region. The contiguous sequence M323 K324 R325 did not tolerate deletions and substitutions. Removal of the rest of the cytoplasmic loop was not deleterious; even higher expression levels (2-4-fold) were obtained upon large deletions of the loop (Delta399-432 and Delta339-370). High expression levels were observed provided that a minimal sequence of three amino acids (E371, G372, and M373) was present. In addition, some electrophysiological properties of mutant nAChRs were modified. Substitution of the EGM sequence by other protein segments produced a variety of effects, but, in general, insertions were not well tolerated, suggesting the existence of tight structural restrictions in the large cytoplasmic region of the rat alpha7 subunit.

Prolonged stimulation of presynaptic nicotinic acetylcholine receptors in the rat interpeduncular nucleus has differential effects on transmitter release

Alterations in nicotinic acetylcholine (nAChR) receptor number can be induced by chronic exposure to nicotine possibly by stabilization of the desensitized state(s) of the receptor. Since within the central nervous system (CNS), many nAChRs are localized presynaptically, we have investigated the physiological consequences of prolonged nicotine applications on spontaneous transmitter release. In the presence of glutamate receptor antagonists, bicuculline-sensitive spontaneous GABA inhibitory synaptic currents (IPSCs) could be readily resolved in whole-cell recordings from neurons in the interpeduncular nucleus (IPN) maintained as brain slices. Nicotine (300nM) caused a marked enhancement in the frequency of spontaneous events. During a 15min exposure to nicotine, the time course of changes in IPSC frequency could be divided into two groups. In most neurons, there was a fast increase in event frequency followed by a decline to a lower steady-state level that remained above baseline. In the remaining neurons, the effect of nicotine was more slowly developing and outlasted the application. Interestingly, the rapid effect was associated with a shift to higher amplitude events, whereas, no change in the IPSC amplitude histogram was observed during the slow onset effect. These data show that prolonged stimulation of presynaptic nicotinic receptors can have different outcomes that could potentially contribute to the diverse effects of nicotine on central information processing.

[3H]Epibatidine binding to bovine adrenal medulla: evidence for alpha3beta4* nicotinic receptors

In these studies, [3H]epibatidine is used as the radioligand to characterize nicotinic acetylcholine receptors (nAChRs) from bovine adrenal medulla. Specific binding reaches equilibrium within 30 min, and is saturable with a Kd value of 0.5 nM. The affinities of several cholinergic agents were determined, including nicotine (Ki, 0.2 microM), cytisine (Ki, 0.4 microM), carbachol (Ki, 4.7 microM), dihydro-beta-erythrodine (Ki, 33.6 microM), D-tubocurarine (Ki, 0.4 microM), 1,1-dimethyl-4-phenyl-piperazinium (Ki, 0.8 microM), decamethonium (Ki, 234 microM) and methyllycaconitine (Ki, 1.3 microM). These values are similar to reported values for recombinant alpha3beta4 nAChRs in transfected cell lines. These studies demonstrate [3H]epibatidine binding to an easily obtainable adrenal membrane preparation and support the characterization of adrenal nAChRs as alpha3beta4* nAChRs.

Relationship between the increased cell surface alpha7 nicotinic receptor expression and neuroprotection induced by several nicotinic receptor agonists

Nicotine and other nicotinic acetylcholine receptor agonists have been shown to exert neuroprotective actions in vivo and in vitro by an as yet unknown mechanism. Even the identification of the subtype of nicotinic receptor(s) mediating this action has not been determined. In neural cell lines, the induction of cytoprotection often requires exposure to nicotine for up to 24 hr to produce a full protective effect. One phenomenon associated with chronic exposure of neural cells to nAChR agonists is the increased expression of nAChRs (upregulation), possibly as a response to desensitization. Because nicotinic receptors desensitize rapidly in the continuous presence of agonist, we investigated whether the neuroprotective actions produced by different nicotinic receptor agonists was related to their ability to induce nicotinic receptor upregulation. Differentiated PC12 cells were preincubated for 24 hr with various nAChR ligands, and the cells were subsequently deprived of both NGF and serum to induce cytotoxicity. Under control conditions cell viability was reduced to 66.5 +/- 5.4% of control by trophic factor withdrawal. For those cells pretreated with nicotine (1 nM-100 microM) cell viability increased from 74.2 +/- 1.5 to 97.3 +/- 4%. The neuroprotective action of nicotine was blocked by co-treatment with either 5 microM mecamylamine or 10 nM methyllycaconitine (MLA). The high potency blockade by MLA suggested that neuroprotection was mediated through the alpha7 nicotinic receptor subtype. For the seven agonists examined for neuroprotective activity, only nicotine was capable of evoking a near maximal (near 100% cell viability) neuroprotective action. The next most effective group included epibatidine, 4OHGTS-21, methycarbamylcholine, and 1,1-dimethyl-4-phenyl-piperazinium iodide. These least effective group included cytisine and tetraethylammonium. Incubation of differentiated PC12 cells with 10 microM nicotine increased the number of [(125)I]alpha bungarotoxin ([(125)I]alphaBGTbinding sites by 41% from 82.6 +/- 3.67 to 117 +/- 10.3 fmol/mg protein). Under similar conditions of incubation, the nicotinic receptor agonist cytisine (that was least effective in terms of neuroprotection) failed to increase the number of [(125)I]alphaBGT binding sites. Cells expressing increased levels of cell surface [(125)I]alphaBGT binding sites received added neuroprotective benefit from nicotine. Thus the induced upregulation of the alpha7 subtype of nicotinic receptors during chronic exposure to nicotine may be responsible for the drug's neuroprotective action.

Distributions of nicotinic acetylcholine receptor alpha7 and beta2 subunits on cultured hippocampal neurons

The hippocampus receives cholinergic afferents and expresses neuronal nicotinic acetylcholine receptors. In particular, the alpha7 and beta2 nicotinic subunits are highly expressed in the hippocampus. There has been controversy about the location, distribution and roles of neuronal nicotinic acetylcholine receptors [Role L. W. and Berg D. K. (1996) Neuron 16, 1077-1085; Wonnacott S. (1997) Trends Neurosci. 20, 92-98]. Using immunocytochemistry and patch-clamp techniques, we examined the density and distribution of nicotinic receptors on rat hippocampal neurons in primary tissue culture. The density and distribution of alpha7 subunits change with days in culture. Before 10 days in culture, alpha7 expression, monitored immunocytochemically, is low and nicotinic currents are small or absent. In older cultures, about two-thirds of the neurons express nicotinic currents, and alpha7 appears in small patches on the soma and out along the neuronal processes. These patches of alpha7 subunits on the surface of the neuronal processes often co-localize with the presynaptic marker, synaptotagmin. The other most common nicotinic subunit, beta2, stays confined mainly to the soma and proximal processes, and beta2 is distributed more uniformly and is not specifically localized at presynaptic areas. The two subunits, alpha7 and beta2, have different expression patterns on the surface of the cultured hippocampal neurons. Taken together with previous physiological studies, the results indicate that alpha7 subunits can be found at presynaptic terminals, and at these locations, these calcium-permeable channels may influence transmitter release.

Two distinct nicotinic receptors, one pharmacologically similar to the vertebrate alpha7-containing receptor, mediate Cl currents in aplysia neurons

Ionotropic, nicotinic receptors have previously been shown to mediate both inhibitory (Cl-dependent) and excitatory (cationic) cholinergic responses in Aplysia neurons. We have used fast perfusion methods of agonist and antagonist application to reevaluate the effects on these receptors of a wide variety of cholinergic compounds, including a number of recently isolated and/or synthesized alpha toxins [alpha-conotoxin (alphaCTx)] from Conus snails. These toxins have been shown in previous studies to discriminate between the many types of nicotinic receptors now known to be expressed in vertebrate muscle, neuroendocrine, and neuronal cells. One of these toxins (alphaCTx ImI from the worm-eating snail Conus imperialis) revealed that two kinetically and pharmacologically distinct elements underlie the ACh-induced Cl-dependent response in Aplysia neurons: one element is a rapidly desensitizing current that is blocked by the toxin; the other is a slowly desensitizing current that is unaffected by the toxin. The two kinetically defined elements were also found to be differentially sensitive to different agonists. Finally, the proportion of the rapidly desensitizing element to the sustained element was found to be cell-specific. These observations led to the conclusion that two distinct nicotinic receptors mediate Cl currents in Aplysia neurons. The receptor mediating the rapidly desensitizing Cl-dependent response shows a strong pharmacological resemblance to the vertebrate alpha-bungarotoxin-sensitive, alpha7-containing receptor, which is permeable to calcium and mediates a rapidly desensitizing excitatory response.

Nicotinic stimulation produces multiple forms of increased glutamatergic synaptic transmission

Synaptic modulation and long-term synaptic changes are thought to be the cellular correlates for learning and memory (Madison et al., 1991; Aiba et al., 1994, Goda and Stevens, 1996). The hippocampus is a center for learning and memory that receives abundant cholinergic innervation and has a high density of nicotinic acetylcholine receptors (nAChRs) (Wada et al., 1989; Woolf, 1991). We report that stro ng, brief stimulation of nAChRs enhanced hippocampal glutamatergic synaptic transmission on two independent time scales and altered the relationship between consecutively evoked synaptic currents. The nicotinic synaptic enhancement required extracellular calcium and was produced by the activation of presynaptic alpha7-containing nAChRs. Although one form of glutamatergic enhancement lasted only for seconds, another form lasted for minutes after the nicotinic stimulation had ceased and the nicotinic agonist had been washed away. The synaptic enhancement lasting minutes suggests that nAChR activity can initiate calcium-dependent mechanisms that are known to induce glutamatergic synaptic plasticity. The results with evoked synaptic currents showed that nAChR activity can alter the relationship between the incoming presynaptic activity and outgoing postsynaptic signaling along glutamatergic fibers. Thus, the same information arriving along the same glutamatergic afferents will be processed differently when properly timed nicotinic activity converges onto the glutamatergic presynaptic terminals. Influencing information processing at glutamatergic synapses may be one way in which nicotinic cholinergic activity influences cognitive processes. Disruption of these nicotinic cholinergic mechanisms may contribute to the deficits associated with the degeneration of cholinergic functions during Alzheimer's disease.

Genomic organization and partial duplication of the human alpha7 neuronal nicotinic acetylcholine receptor gene (CHRNA7)

The human alpha7 neuronal nicotinic acetylcholine receptor gene (HGMW-approved symbol CHRNA7) has been characterized from genomic clones. The gene is similar in structure to the chick alpha7 gene with 10 exons and conserved splice junction positions. The size of the human gene is estimated to be larger than 75 kb. A putative promoter 5' of the translation start in exon 1 has been cloned and sequenced. The promoter region lacks a TATA box and has a high GC content (77%). Consensus Sp1, AP-2, Egr-1, and CREB transcription factor binding sites appear to be conserved between bovine and human genes. The alpha7 nAChR gene was found to be partially duplicated, with both loci mapping to the chromosome 15q13 region. A yeast artificial chromosome contig was constructed over a genetic distance of 5 cM that includes both alpha7 loci and the region between them. Four novel exons are described, located in genomic clones containing the partially duplicated gene. The duplicated sequences, including the novel exons, are expressed in human brain.

Nicotinic agonists competitively antagonize serotonin at mouse 5-HT3 receptors expressed in Xenopus oocytes

The 5-HT3 receptor (5-HT3R) is part of a superfamily of ligand-gated ion channels which includes nicotinic acetylcholine receptors (nAChR). cRNA derived from the long isoform cloned mouse 5-HT3R was used to drive expression of 5-HT3Rs in Xenopus oocytes. 5-HT-induced currents were monitored using two-electrode voltage-clamp. Eight nicotinic agonists, including ACh and nicotine, but not alpha-anatoxin, were found to antagonize 5-HT-induced currents. With the exception of 3-(2,4)-dimethoxybenzylidene-anabaseine (DMXB-anabaseine; GTS-21) this antagonism appeared to be competitive since it could be overcome by increasing concentrations of 5-HT. Potency of 5-HT3 antagonism was comparable to reported values for nAChR alpha7 activation. These results confirm the notion of families of receptors and further indicate that strong similarities can exist in some critical binding domains.

Pharmacological and physiological characterization of murine homomeric beta3 GABA(A) receptors

Gamma-Aminobutyric acid (GABA[A]) receptor beta3 subunits were expressed in Xenopus laevis oocytes and studied using two-electrode voltage clamp. Injected oocytes exhibited an increased resting membrane conductance and more depolarized membrane potentials compared to uninjected control cells. Oocytes expressing beta3 subunits were insensitive to GABA and muscimol, but pentobarbitone increased the membrane conductance in a concentration-dependent manner. The membrane current response to pentobarbitone reversed at the Cl- equilibrium potential and at relatively high concentrations (> 500 microM), a rebound Cl- current was induced following the removal of pentobarbitone. In transfected human embryonic kidney (HEK) cells, the rebound current amplitude was reduced by desensitizing the beta3 receptor with increased durations of ligand application. Both picrotoxin (0.5 nM to 10 microM) and Zn2+ (10 nM to 100 microM) reduced the resting membrane conductance for beta3 cDNA-injected oocytes. These oocytes were insensitive to flurazepam (5 microM) and alphaxalone (10 microM), but responded with increased membrane conductance to propofol (10 microM) and pregnanolone (50 nM to 5 microM). The antagonists, bicuculline (10 microM) and strychnine (50 nM to 100 microM), also induced conductance increases in a concentration dependent manner; however, glycine (1 mM) was inactive. It was concluded that beta3 subunits form spontaneously opening ion channels that can be up-regulated by some allosteric modulators, principally by pentobarbitone and propofol and, surprisingly, by bicuculline and strychnine, whilst picrotoxin and Zn2+ acted as antagonists. Computer modelling of some kinetic schemes was used to describe the rebound current observed in transfected HEK cells. This indicated that pentobarbitone, after modulation of the conductance, is potentially capable of further binding to the beta3 receptor complex 'driving' the receptor into one or more desensitized states. This phenomenon may be of some importance for native neuronal GABA(A) receptors, where pentobarbitone can also evoke rebound current activation.

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.

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

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

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.

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

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

The molecular biology of neuronal nicotinic acetylcholine receptors

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

Presynaptic nicotinic ACh receptors

Nicotinic ACh (nACh) receptors in the CNS are composed of a diverse array of subunits and have a range of pharmacological properties. However, despite the fact that they are ligand-gated cation channels, their physiological functions have not been determined. This has led to increased interest in presynaptic nACh receptors that act to modulate the release of transmitter from presynaptic terminals.

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)

Effects of ethanol on ion channels

Ion channels play critical roles in nervous system function, from initiating rapid synaptic activity to propagation of action potentials. Studies have indicated that many of the effects of ethanol on the nervous system are likely caused by the actions of ethanol on ion channels. Ion channels are multimeric structures that gate ions through subtle changes in tertiary structure. Ethanol readily enters molecular sites within multimeric ion channels, modifying intermolecular forces and bonds that are important for the open-close-inactivation kinetic properties of channels. The diversity of channel composition caused by the multimeric structure results in subtypes of channels that have a spectrum of sensitivity to ethanol that translates into brain regional differences in ethanol sensitivity, in part caused by differences in ion channel subunit composition. Ethanol has been shown to affect both receptor-activated ion channels and voltage-gated ion channels. The acute intoxicating and incoordinating effects of ethanol are probably related to inhibition of subtypes of NMDA-glutamate receptor ion channels and potentiation of certain subtypes of GABAA receptor ion channels. Effects on these channels, as well as glycine, nicotinic cholinergic, serotonergic, and other ion channels, likely contribute to the euphoric, sedative, and other acute actions of ethanol. Changes in ion channel subunit composition, density, and properties probably also contribute to ethanol tolerance, dependence, withdrawal hyperexcitability, and neurotoxicity. A substantial number of studies have implicated glutamate NMDA receptor, GABAA, and L-type voltage-gated calcium channels in the adaptive changes in the brain during chronic ethanol exposure. The diversity of ion channels subunits, their prominent role in brain function, and ethanol action are likely to make them important contributors to alcoholism and alcohol abuse.

A nicotinic acetylcholine receptor subunit from insect brain forms a non-desensitising homo-oligomeric nicotinic acetylcholine receptor when expressed in Xenopus oocytes

The locust alpha-like nicotinic receptor subunit alpha L1 was expressed in Xenopus oocytes. Small but reproducible currents were elicited by application of high concentrations of nicotine, demonstrating that alpha L1 is capable of forming homo-oligomeric channels. Nicotine-evoked currents were blocked by alpha-bungarotoxin and methyllycaconitine. Comparison with chick alpha 7 receptors showed that the two receptors differ with respect to nicotine sensitivity and time course of evoked currents. Nicotine dose-response curves gave EC50 values of 24 and 830 microM for alpha 7 and alpha L1 respectively. Whereas alpha 7 responses showed characteristic fast onset and rapid desensitization within 3 s, alpha L1 currents displayed a slow onset and showed no tendency to desensitize during 45 s of agonist application. Thus alpha L1 is a novel nicotine subunit for the further exploration of structure-function relationships of ligand-gated ion channels. The question of the subunit composition of native insect receptors remains open.

Native nicotinic acetylcholine receptors in human Imr32 neuroblastoma cells: functional, immunological and pharmacological properties

IMR32 cells express two classes of surface nicotinic receptors: those labelled with high affinity by [125I]neuronal toxin, and those labelled by [125I]alpha-bungarotoxin. Whole-cell patch-clamp recordings indicate that both classes of receptor are able to elicit inward currents that are totally blocked by d-tubocurarine but only partially blocked by alpha-bungarotoxin. In IMR32 cells, nicotine induces an increase in the intracellular level of free Ca2+. This increase, which is also completely blocked by d-tubocurarine and only partially blocked by alpha-bungarotoxin and Cd2+, is due to extracellular calcium influx through both the nicotinic receptors and the voltage-activated Ca2+ channels. By using subunit-specific polyclonal antibodies, we have demonstrated that the alpha-bungarotoxin receptors contain the alpha 7 subunit, but none of the other subunits whose transcripts are present in IMR32 cells. The pharmacological profile of these human alpha 7-containing alpha-bungarotoxin receptors is similar to that observed in the native chick alpha 7 receptor, but there are also some species-specific differences.

Stable expression and pharmacological properties of the human alpha 7 nicotinic acetylcholine receptor

The alpha 7 neuronal nicotinic acetylcholine receptor subtype forms a Ca(2+)-permeable homooligomeric ion channel sensitive to alpha-bungarotoxin in Xenopus oocytes. In this study, we have stably and functionally expressed the human alpha 7 cDNA in a mammalian cell line, HEK-293 and examined its pharmacologic properties. [125I] alpha-Bungarotoxin bound to transfected cells with a Kd value of 0.7 nM and a Bmax value of 973 pmoL/mg protein. No specific binding was detected in untransfected cells. Specific binding could be displaced by unlabeled alpha-bungarotoxin (Ki = 0.5 nM) and an excellent correlation was observed between binding affinities of a series of nicotinic cholinergic ligands in transfected cells and those in the human neuroblastoma IMR-32 cell line. Additionally, cell surface expression of alpha 7 receptors was detected by fluorescein isothiocyanate-conjugated alpha-bungarotoxin in transfected cells. Whole cell currents sensitive to blockade by alpha-bungarotoxin, and with fast kinetics of activation and inactivation, were recorded from transfected cells upon rapid application of (-)-nicotine or acetylcholine with EC50 values of 49 microM and 155 microM respectively. We conclude that the human alpha 7 subunit when expressed alone can form functional ion channels and that the stably transfected HEK-293 cell line serves as a unique system for studying human alpha 7 nicotinic receptor function and regulation, and for examining ligand interactions.

Developmental expression of alpha 7 neuronal nicotinic receptor messenger RNA in rat sensory cortex and thalamus

The distribution of alpha 7 messenger RNA expression was characterized in developing rat cortex and thalamus. Northern blot analysis of neonatal and adult cortex revealed a single messenger RNA transcript of 5.7 kb. Using in situ hybridization with both full length and short 35S-labeled alpha 7 riboprobes, a distinct transient expression of messenger RNA within sensory cortex and thalamus, during early postnatal development, was observed. alpha 7 transcripts were expressed in low levels as early as embryonic day 13 in the ventricular zone of the neocortex, and as early as embryonic day 15 in the thalamic neuroepithelium. A marked increase in messenger RNA levels was observed during the late prenatal period in both sensory and non-sensory regions of the cortex and thalamus. Moderate to high levels of messenger RNA were maintained into the first postnatal week, followed by a decline into adulthood. alpha 7 messenger RNA expression was significantly higher in the anterodorsal, lateral dorsal, ventral posterior medial and ventral posterior lateral thalamic nuclei of postnatal day 7 pups than in adult brains. Expression of messenger RNA within dorsal lateral geniculate, ventral lateral geniculate and medial geniculate did not show a significant reduction with age. Within the developing cortex, messenger RNA expression delineated the primary somatosensory, auditory and visual cortices in a unique laminar pattern that was consistently and significantly higher than in the adult in superficial layer VI. Higher levels of expression were also observed in retrosplenial cortex at postnatal day 7 than in the adult. Tangential sections through postnatal day 7 cortex revealed low levels of alpha 7 messenger RNA expression delineating the primary sensory areas in layer IV, corresponding to acetylcholinesterase-labeled thalamocortical afferents. However, these sensory areas exhibited higher levels of alpha 7 messenger RNA expression and were more clearly defined in layer VI, but not by acetylcholinesterase staining. The distribution of alpha 7 messenger RNA within the developing thalamocortical system parallels the distribution of alpha-bungarotoxin binding sites and suggests that the receptor is localized on both thalamic cells and their cortical target neurons. This transient and distinct pattern of distribution of the alpha 7 neuronal nicotinic receptor, which coincides with the major phase of thalamocortical development, suggests that it may play a functional role in the development of cortical circuitry.

Human alpha 7 nicotinic acetylcholine receptor responses to novel ligands

Responses of the human alpha 7 nicotinic acetylcholine receptor (nAChR) in Xenopus laevis oocytes were quantified using two-electrode voltage clamp in the presence of barium (10 mM) to block secondary activation of Ca(2+)-dependent chloride currents. The effect of (S)-3-methyl-5-(1-methyl-2-pyrrolidinyl) isoxazole (ABT-418) and (2,4)-dimethoxybenzylidene anabaseine (GTS-21), two potential compounds for the treatment of Alzheimer's Disease, and of the natural product (+/-)epibatidine were compared to (-)nicotine. (+/-)Epibatidine acted as an agonist and was 64-fold more potent than (-) nicotine (EC50s = 1.30 +/- 0.11 microM and 83 +/- 10 microM, respectively). ABT-418 also was an agonist, 3-fold less potent and 75% as efficacious as (-)nicotine (EC50 = 264 +/- 34 microM). GTS-21, in contrast, inhibited the response to (-)nicotine at concentrations < or = 10 microM and itself elicited only a small response at higher concentrations (12% of the (-)nicotine response at 1 mM). Reversible blockade by methyllycaconitine (10 nM) corroborated the responses as due to activation of alpha 7 nAChR. This represents the first characterization of human alpha 7 nAChR responses to these novel nicotinic agonists.

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.

alpha-Bungarotoxin binding sites in rat hippocampal and cortical cultures: initial characterisation, colocalisation with alpha 7 subunits and up-regulation by chronic nicotine treatment

High density neuronal cultures from rat E18 hippocampus and cortex have been characterised with respect to cholinergic binding sites. No specific binding of [3H]nicotine or [3H]cytisine to live cells in situ was detected although the limit for detection was estimated to be 30 fmol/mg protein. Muscarinic binding sites labelled with [3H]QNB were present at a density of 0.75 pmol/mg protein. [125I]alpha-Bungarotoxin (alpha Bgt) bound to hippocampal cultures with a Bmax of 128 fmol/mg protein and a Kd of 0.6 nM; cortical cultures expressed five times fewer [125I]alpha-Bgt binding sites. Fluorescence cytochemistry with rhodamine-alpha-Bgt indicated that 95% of hippocampal neurons were labelled, compared with only 36% of cortical neurons. Average densities of 4 x 10(4) and 2 x 10(4) binding sites/cell were calculated for hippocampal and cortical cultures, respectively. Double labelling experiments with mAb307 (which recognises the rat alpha 7 nicotinic receptor subunit) and rhodamine-alpha-Bgt gave coincident labelling patterns, supporting the correlation between the alpha 7 subunit and Bgt-sensitive neuronal nicotinic receptor. Treatment of hippocampal cultures with 10 microM nicotine for 14 days elicited a 40% increase in the numbers of [125I]alpha-Bgt binding sites, mimicking the up-regulation observed in in vivo studies. Primary cultures offer a useful in vitro system for investigating the expression and regulation of brain alpha-Bgt-sensitive receptors.

Presence of alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors in rat olfactory bulb neurons

The presence of a functional nicotinic acetylcholine receptor (nAChR) in rat olfactory bulb (OB) neurons was investigated using the whole-cell patch-clamp technique. Acetylcholine (ACh) or other nicotinic agonists elicited fast-decaying whole-cell currents in cultured OB neurons. ACh-evoked currents could be blocked by the highly selective competitive nicotinic antagonists alpha-bungarotoxin (alpha-BGT, 10 nM) and methyllycaconitine (MLA, 1 nM) and exhibited strong inward rectification in the presence of intracellular Mg2+. The properties of the nicotinic currents in OB neurons were similar to those of type IA currents elicited by nicotinic agonists in rat hippocampal neurons. The present results suggest that the fast-decaying nicotinic currents evoked in OB neurons are subserved by nAChRs containing alpha 7 subunits.

Pharmacology and biophysical properties of alpha 7 and alpha 7-alpha 8 alpha-bungarotoxin receptor subtypes immunopurified from the chick optic lobe

Two chick optic lobe alpha-bungarotoxin receptor subtypes (alpha 7 and alpha 7-alpha 8) were immunopurified using polyclonal antibodies raised against synthetic peptides of chick alpha 7 and alpha 8 alpha-bungarotoxin receptor subunits. The alpha 7 subtype contained the M(r) 57,000 alpha 7 subunit, and represented 60-70% of the alpha-bungarotoxin receptors; the alpha 7-alpha 8 subtype contained the M(r) 57,000 alpha 7 and alpha 8 subunits, and represented only 20-25% of the receptors. Both subtypes also had an additional M(r) 52,000 subunit. The affinity of these subtypes for alpha-bungarotoxin as well as antagonists was similar. However, the alpha 7-alpha 8 subtype displayed consistently higher affinities for agonists. When reconstituted in planar lipid bilayers, the alpha 7-alpha 8 subtype displayed several conductance states of 10-50 pS; the alpha 7 subtype had only one conductance state of 45 pS. The alpha 7-alpha 8 subtype was activated by lower agonist concentrations than the alpha 7 subtype. When expressed in Xenopus oocytes, the alpha 8 subunit formed functional homomeric receptors that desensitized rapidly. These channels were blocked by alpha-bungarotoxin and displayed a higher affinity for agonists than the alpha 7 homomeric receptor. Taken together, these data indicate that at least two alpha-bungarotoxin subtypes are present in the chick optic lobe. They operate as ligand-gated channels and display different agonist sensitivities and kinetics/conductance properties.