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Increased sensitivity to nicotine-induced seizures in mice heterozygous for the L250T mutation in the alpha7 nicotinic acetylcholine receptor.

alpha7 Nicotinic acetylcholine receptors (nAChRs) are sparsely distributed throughout the peripheral and central nervous systems. Several studies have suggested that central alpha7 nicotinic receptors may influence sensitivity to nicotine-induced seizures in mice. In order to investigate the effect of alpha7 nAChRs on seizure sensitivity, we tested heterozygous mice with a threonine for leucine substitution at position 250 (L250T) within the channel domain, which is known to increase current amplitude and decreases desensitization of the channel. We show that administration of low doses of nicotine to these mutant mice increased the sensitivity to nicotine-induced seizures and the mortality rate. EEG recordings showed high amplitude rhythmic activity during tonic-clonic seizures. Pretreatment with the alpha7 nicotinic receptor antagonist methyllycaconitine inhibited the seizures induced by nicotine. These findings further suggest an important role for alpha7 nAChRs in the nicotine-induced seizures model of epilepsy.

Animals↗

Low-affinity Ca(2+) and Ba(2+) binding sites in the pore of alpha7 nicotinic acetylcholine receptors.

alpha7 nicotinic receptors are highly permeable to Ca(2+) as well as monovalent cations. We extended the characterization of the Ca(2+) permeation of non-desensitizing chick alpha7 receptors (S240T/L247T alpha7 nAChRs) expressed in Xenopus oocytes by (1) measuring the concentration dependence of conductance under conditions in which Ca(2+) or Ba(2+) were the only permeant cations in the extracellular solution, and (2) measuring the concentration dependence of Ca(2+) block of K(+) currents through the receptors. The first set of experiments yielded an apparent affinity of 0.96 mM Ca(2+) activity (2.4 mM concentration) for Ca(2+) permeation and an apparent affinity of 0.65 mM Ba(2+) activity (1.7 mM concentration) for Ba(2+) permeation. The apparent affinity of Ca(2+) inhibition of K(+) currents was 0.49 mM activity (1.5 mM concentration). The similarity of these apparent affinities in the millimolar range suggests that the pore of alpha7 receptors has one or more low-affinity Ca(2+) binding sites and no high-affinity sites.

Animals↗

Ric-3 promotes functional expression of the nicotinic acetylcholine receptor alpha7 subunit in mammalian cells.

Expression of functional, recombinant alpha7 nicotinic acetylcholine receptors in several mammalian cell types, including HEK293 cells, has been problematic. We have isolated the recently described human ric-3 cDNA and co-expressed it in Xenopus oocytes and HEK293 cells with the human nicotinic acetylcholine receptor alpha7 subunit. In addition to confirming the previously reported effect on alpha7 receptor expression in Xenopus oocytes we demonstrate that ric-3 promotes the formation of functional alpha7 receptors in mammalian cells, as determined by whole cell patch clamp recording and surface alpha-bungarotoxin binding. Upon application of 1 mm nicotine, currents were undetectable in HEK293 cells expressing only the alpha7 subunit. In contrast, co-expression of alpha7 and ric-3 cDNAs resulted in currents that averaged 42 pA/pF with kinetics similar to those observed in cells expressing endogenous alpha7 receptors. Immunoprecipitation studies demonstrate that alpha7 and ric-3 proteins co-associate. Additionally, cell surface labeling with biotin revealed the presence of alpha7 protein on the plasma membrane of cells lacking ric-3, but surface alpha-bungarotoxin staining was only observed in cells co-expressing ric-3. Thus, ric-3 appears to be necessary for proper folding and/or assembly of alpha7 receptors in HEK293 cells.

Animals↗

Fractional Ca(2+) current through human neuronal alpha7 nicotinic acetylcholine receptors.

The neuronal alpha7 nicotinic acetylcholine (ACh) receptor is believed to be a highly Ca(2+) permeable ligand-gated receptor-channel. However, the contribution of Ca(2+) to cationic current generated by ACh has not yet been directly measured to date. Simultaneous fluorescence and whole-cell current measurements using the Ca(2+) indicator dye fura-2 were made in GH4C1 pituitary cells stably expressing human alpha7 receptors and the fractional Ca(2+) current (the proportion of whole-cell current carried by Ca(2+); P(f)) was determined. We report that the P(f) value was 11.4+/-1.3%. This value was significantly larger than P(f) of human L248Talpha7 receptor mutant (P(f)=6.3+/-1.0%) and of rat alpha7 receptor (P(f)=8.8+/-1.5%) both determined in transiently transfected GH4C1 cells. In our knowledge, the findings here reported indicate the human alpha7 receptors are the most Ca(2+) conductive homomeric ligand-gated receptor-channels expressed in a heterologous cell system.

Animals↗

Rhesus monkey alpha7 nicotinic acetylcholine receptors: comparisons to human alpha7 receptors expressed in Xenopus oocytes.

An alpha7 nicotinic acetylcholine receptor sequence was cloned from Rhesus monkey (Macaca mulatta). This clone differs from the mature human alpha7 nicotinic acetylcholine receptor in only four amino acids, two of which are in the extracellular domain. The monkey alpha7 nicotinic receptor was characterized in regard to its functional responses to acetylcholine, choline, cytisine, and the experimental alpha7-selective agonists 4OH-GTS-21, TC-1698, and AR-R17779. For all of these agonists, the EC(50) for activation of monkey receptors was uniformly higher than for human receptors. In contrast, the potencies of mecamylamine and MLA for inhibiting monkey and human alpha7 were comparable. Acetylcholine and 4OH-GTS-21 were used to probe the significance of the single point differences in the extracellular domain. Mutants with the two different amino acids in the extracellular domain of the monkey receptor changed to the corresponding sequence of the human receptor had responses to these agonists that were not significantly different in EC(50) from wild-type human alpha7 nicotinic receptors. Monkey alpha7 nicotinic receptors have a serine at residue 171, while the human receptors have an asparagine at this site. Monkey S171N mutants were more like human alpha7 nicotinic receptors, while mutations at the other site (K186R) had relatively little effect. These experiments point toward the basic utility of the monkey receptor as a model for the human alpha7 nicotinic receptor, albeit with the caveat that these receptors will vary in their agonist concentration dependency. They also point to the potential importance of a newly identified sequence element for modeling the specific amino acids involved with receptor activation.

Acetylcholine↗

Sensitivity of the alpha7 nicotinic acetylcholine receptor to isoflurane may depend on receptor inactivation.

UNLABELLED: In previous studies, we demonstrated that nicotinic acetylcholine receptors (nAChRs) composed of the alpha7 subunit are unaffected by the co-application of isoflurane with agonists at concentrations up to 640 microM (two times the minimum alveolar anesthetic concentration). Modulation of alpha7-nAChR activity by isoflurane might have important behavioral ramifications because these receptors are expressed diffusely in the central and peripheral nervous systems and play pre- and postsynaptic roles in synaptic transmission. Here we have demonstrated that under some potentially physiologically relevant circumstances, the activation of alpha7 nAChRs may be inhibited by clinically relevant concentrations of isoflurane. We evaluated isoflurane inhibition of alpha7 nAChRs from chicks and humans expressed in Xenopus oocytes using two-electrode voltage clamp methodology. We determined the influence of time of preperfusion of isoflurane, agonist concentration, and membrane potential on inhibition by isoflurane. Both activation by a large concentration of agonist and isoflurane preperfusion increased inhibition. The half-maximal inhibitory concentration for isoflurane inhibition of chick alpha7 nAChR with isoflurane preperfusion and activation by 100 microM of acetylcholine was 938 +/- 26, and when activated by 1 mM of acetylcholine, it was 408 +/- 51 microM. The increase in inhibition with isoflurane preexposure and large agonist concentration raises the possibility that isoflurane interacts preferentially with a closed or closed-desensitized state of the channel. IMPLICATIONS: Nicotinic receptors expressed in the brain have been considered a possible target for the actions of isoflurane. We studied the effect of isoflurane on alpha7 type nicotinic receptors expressed in Xenopus oocytes. We find that when activated by large concentrations of acetylcholine, alpha7 nicotinic receptors are inhibited by isoflurane at concentrations near MAC.

Acetylcholine↗

Mutational analysis of the charge selectivity filter of the alpha7 nicotinic acetylcholine receptor.

In the alpha7 nicotinic acetylcholine receptors, we analyze the contribution of mutations E237A and V251T, together with the proline insertion P236', in the conversion of the charge selectivity from cationic to anionic. We show that the triple mutant exhibits spontaneous openings displaying anionic selectivity. Furthermore, at position 251, hydrophilic or even negatively charged residues are compatible with an anionic channel. In contrast, the additional proline yields an anionic channel only when inserted between positions 234 and 237; insertion before 234 yields a cationic channel and after 238 alters the receptor surface expression. The coiled 234-238 loop thus directly contributes to the charge selectivity filter of the alpha7 channel.

Amino Acid Sequence↗

Characterization of a rat neuronal nicotinic acetylcholine receptor alpha7 promoter.

Neuronal nicotinic acetylcholine receptors (nAChRs) containing the alpha7 subunit are expressed in the central nervous system, autonomic nervous system, retina, adrenal medulla, and PC12 cells. alpha7 nAChRs have been implicated in several important biological activities apart from synaptic transmission such as mediating neurite growth and presynaptic control of neurotransmitter release. A 178-base pair promoter was sufficient to drive high level expression of the alpha7 gene in PC12 cells. The alpha7 promoter was also cell-specific, expressing in PC12 cells but not in L6 rat muscle cells. Within our minimal rat alpha7 nAChR promoter we identified two sequences important for basal level expression. Mutation of a GC-rich sequence at -172 relative to the translational start site led to an increase in activity of the promoter, indicating the presence of a negative regulatory element. Upstream stimulatory factor-1 acted to regulate alpha7 expression positively by binding to an E-box at -116. A site directly adjacent to the upstream stimulatory factor-1 binding site was shown to bind Egr-1. Sp1 and Sp3 binding also occurred downstream from or overlapping the Egr-1 binding site in the rat alpha7 promoter. Several transcription factors interact in close proximity to control expression of the rat alpha7 nicotinic receptor gene.

Adrenal Gland Neoplasms↗

Chronic exposure to typical or atypical antipsychotics in rodents: temporal effects on central alpha7 nicotinic acetylcholine receptors.

A decrease in alpha7 nicotinic acetylcholine receptors in the hippocampus has been hypothesized to contribute to alterations in auditory gating and other behavioral impairments in schizophrenia. However, while both typical and atypical neuroleptics are routinely used in the therapeutics of schizophrenia, little is known about their effects on auditory gating or alpha7 nicotinic acetylcholine receptor expression particularly when they are administered for extended periods of time (which is common in the clinical setting). In the present study in normal rats, the residual effects of prior chronic treatment (90 or 180 days) with representative typical and atypical neuroleptics (oral haloperidol, 2.0 mg/kg/day; chlorpromazine, 10.0 mg/kg/day, risperidone, 2.5 mg/kg/day; or olanzapine, 10.0 mg/kg/day) on prepulse inhibition of the auditory gating response were investigated. The densities of alpha7 nicotinic acetylcholine receptors were subsequently measured using [125I]-alpha-bungarotoxin autoradiography. The results indicated that none of the compounds significantly altered the startle amplitude or prepulse inhibition response either during drug treatment (day 60) or after 90 or 180 days of treatment (i.e. during a drug free washout). However, prior exposure to chlorpromazine, risperidone and olanzapine for 90 days resulted in modest but significant (P<0.01) decreases in [125I]-alpha-bungarotoxin binding sites in some brain regions (e.g. posterior cortical amygdala). After 180 days of treatment, decreases in [(125I]-alpha-bungarotoxin binding ranging from approximately 12% (lateral dentate gyrus) up to 24% (e.g. CA1 hippocampal region) were evident in the risperidone group in 13 of the 36 regions analyzed while decreases associated with the other neuroleptics agents were still present, but not statistically significant. These data indicate that the commonly used atypical neuroleptic, risperidone is associated with time dependent and persistent negative effects on an important biological substrate of memory (i.e. the alpha7 nicotinic receptor), but that the magnitude of the deficits was not sufficient to impair auditory gating.

Acoustic Stimulation↗

Microglial alpha7 nicotinic acetylcholine receptors drive a phospholipase C/IP3 pathway and modulate the cell activation toward a neuroprotective role.

Microglia perform both neuroprotective and neurotoxic functions in the brain, with this depending on their state of activation and their release of mediators. Upon P2X(7) receptor stimulation, for example, microglia release small amounts of TNF, which protect neurons, whereas LPS causes massive TNF release leading to neuroinflammation. Here we report that, in rat primary cultured microglia, nicotine enhances P2X(7) receptor-mediated TNF release, whilst suppressing LPS-induced TNF release but without affecting TNF mRNA expression via activation of alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs). In microglia, nicotine elicited a transient increase in intracellular Ca(2+) levels, which was abolished by specific blockers of alpha7 nAChRs. However, this response was independent of extracellular Ca(2+) and blocked by U73122, an inhibitor of phospholipase C (PLC), and xestospongin C, a blocker of the IP(3) receptor. Repeated experiments showed that currents were not detected in nicotine-stimulated microglia. Moreover, nicotine modulation of LPS-induced TNF release was also blocked by xestospongin C. Upon LPS stimulation, inhibition of TNF release by nicotine was associated with the suppression of JNK and p38 MAP kinase activation, which regulate the post-transcriptional steps of TNF synthesis. In contrast, nicotine did not alter any MAP kinase activation, but enhanced Ca(2+) response in P2X(7) receptor-activated microglia. In conclusion, microglial alpha7 nAChRs might drive a signaling process involving the activation of PLC and Ca(2+) release from intracellular Ca(2+) stores, rather than function as conventional ion channels. This novel alpha7 nAChR signal may be involved in the nicotine modification of microglia activation towards a neuroprotective role by suppressing the inflammatory state and strengthening the protective function.

Animals↗

Single amino acid residue in the extracellular portion of transmembrane segment 2 in the nicotinic alpha7 acetylcholine receptor modulates sensitivity to ketamine.

BACKGROUND: Ketamine inhibits the activation of both heteromeric and homomeric nicotinic acetylcholine receptors. The site of molecular interaction is unknown. METHODS: The inhibition of alpha7 nicotinic acetylcholine receptors by ketamine was compared to that of 5-hydroxytryptamine-3A (5HT3A) receptors that are resistant to ketamine inhibition in Xenopus laevis oocytes. To determine whether the region of transmembrane segments 2 and 3 is relevant for ketamine inhibition of nicotinic receptors, the authors identified single amino acid residues that differ in the sequence alignment of the two proteins. They created 22 mutant alpha7 nicotinic receptors that contain the single homologous amino acid residue in the 5HT3A sequence. RESULTS: Of the 22 mutant alpha7 nicotinic receptors tested, only one (alpha7 A258S) was significantly resistant to 20 microM ketamine. The ketamine concentration response relationship for the alpha7 A258S mutant was shifted to the right with the IC50 for ketamine increased from 17 +/- 2 for wild type to 30 +/- 3 microM in the mutant (P < 0.001). Agonist activation was unchanged by the mutation. The homologous amino acid residue in the 5HT3A receptor was mutated to the alanine that occurs in the wild-type nicotinic receptor. This mutation made the previously insensitive 5HT3A receptor sensitive to ketamine (P < 0.001). CONCLUSIONS: Conservative mutation of a single amino acid in the extracellular transmembrane segment 2 domain induces resistance to ketamine inhibition in the alpha7 nicotinic receptor and sensitivity to inhibition in the 5HT3A receptor. This region may represent a ketamine binding site in the alpha7 nicotinic receptor, or it may be an important transduction site for ketamine action.

Amino Acid Substitution↗

Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation.

Alpha7 nicotinic acetylcholine receptors (nAChRs) modulate network activity in the CNS. Thus, functional regulation of alpha7 nAChRs could influence the flow of information through various brain nuclei. It is hypothesized here that these receptors are amenable to modulation by tyrosine phosphorylation. In both Xenopus oocytes and rat hippocampal interneurons, brief exposure to a broad-spectrum protein tyrosine kinase inhibitor, genistein, specifically and reversibly potentiated alpha7 nAChR-mediated responses, whereas a protein tyrosine phosphatase inhibitor, pervanadate, caused depression. Potentiation was associated with an increased expression of surface alpha7 subunits and was not accompanied by detectable changes in receptor open probability, implying that the increased function results from an increased number of alpha7 nAChRs. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated exocytosis was shown to be a plausible mechanism for the rapid delivery of additional alpha7 nAChRs to the plasma membrane. Direct phosphorylation/dephosphorylation of alpha7 subunits was unlikely because mutation of all three cytoplasmic tyrosine residues did not prevent the genistein-mediated facilitation. Overall, these data are consistent with the hypothesis that the number of functional cell surface alpha7 nAChRs is controlled indirectly via processes involving tyrosine phosphorylation.

Acetylcholine↗

alpha7 Nicotinic acetylcholine receptors and temporal memory: synergistic effects of combining prenatal choline and nicotine on reinforcement-induced resetting of an interval clock.

We previously showed that prenatal choline supplementation could increase the precision of timing and temporal memory and facilitate simultaneous temporal processing in mature and aged rats. In the present study, we investigated the ability of adult rats to selectively control the reinforcement-induced resetting of an internal clock as a function of prenatal drug treatments designed to affect the alpha7 nicotinic acetylcholine receptor (alpha7 nAChR). Male Sprague-Dawley rats were exposed to prenatal choline (CHO), nicotine (NIC), methyllycaconitine (MLA), choline + nicotine (CHO + NIC), choline + nicotine + methyllycaconitine (CHO + NIC + MLA), or a control treatment (CON). Beginning at 4-mo-of-age, rats were trained on a peak-interval timing procedure in which food was available at 10-, 30-, and 90-sec criterion durations. At steady-state performance there were no differences in timing accuracy, precision, or resetting among the CON, MLA, and CHO + NIC + MLA treatments. It was observed that the CHO and NIC treatments produced a small, but significant increase in timing precision, but no change in accuracy or resetting. In contrast, the CHO + NIC prenatal treatment produced a dramatic increase in timing precision and selective control of the resetting mechanism with no change in overall timing accuracy. The synergistic effect of combining prenatal CHO and NIC treatments suggests an organizational change in alpha7 nAChR function that is dependent upon a combination of selective and nonselective nAChR stimulation during early development.

Aconitine↗

alpha7 Nicotinic acetylcholine receptor stimulation inhibits lipopolysaccharide-induced interleukin-18 and -12 production in monocytes.

Nicotine inhibited interleukin (IL)-18 and -12 production in lipopolysaccharide (LPS)-stimulated monocytes, and the action of nicotine was antagonized by a non-selective and a selective alpha7 nicotinic acetylcholine receptor (alpha7-nAChR) antagonist, suggesting that the stimulation of alpha7-nAChR may be involved in the action of nicotine. Nicotine is reported to induce prostaglandin E(2) (PGE(2)) production in monocytes through the up-regulation of cyclooxygenase (COX)-2 expression. PGE(2) is known to increase cAMP levels and to activate protein kinase A (PKA). COX-2 and PKA inhibitors prevented the action of nicotine, indicating that the mechanism of action of nicotine may be via endogenous PGE(2) production.

Bungarotoxins↗

Stimulation of alpha7 nicotinic acetylcholine receptor inhibits CD14 and the toll-like receptor 4 expression in human monocytes.

The lipopolysaccharide (LPS)-receptor complex, CD14/toll-like receptor 4, is known to play a role in the immune responses during sepsis. Excessive inflammation and tumor necrosis factor (TNF)-alpha synthesis have been reported to cause morbidity and mortality in endotoxemia and sepsis. Cell-to-cell interaction through the engagement between intercellular adhesion molecule 1, B7.1, and CD40 on monocytes and their ligands on T cells has been suggested to play a role in the inflammatory response such as TNF-alpha and interleukin 10 production. Nicotine, with the stimulation of the nicotinic acetylcholine receptor alpha7 subunit (alpha7-nAChR), has now become the focus of attention because of its anti-inflammatory effects. However, little is known about the mechanism of the inhibitory effects induced by nicotine on the LPS-induced immune responses. In the present study, we found that nicotine suppressed the expression of CD14, toll-like receptor 4, intercellular adhesion molecule 1, B7.1, and CD40 on monocytes and the production of TNF-alpha, but not interleukin 10, in human peripheral blood mononuclear cells in the presence of LPS. The actions of nicotine were reversed by a nonselective and a selective alpha7-nAChR antagonist, mecamylamine and alpha-bungarotoxin, respectively. Therefore, nicotine might inhibit the LPS receptor complex expression via alpha7-nAChR, thus leading to a decrease in the adhesion molecule expression and TNF-alpha production. Moreover, we demonstrated that a nuclear factor-kappaB and a p38 mitogen-activated protein kinase inhibitor mimicked the actions of nicotine in the presence of LPS. These results suggested that the nuclear factor-kappaB and p38 mitogen-activated protein kinase might be involved in the actions of nicotine.

Cells, Cultured↗

Alpha7 nicotinic acetylcholine receptor expression in Alzheimer's disease: receptor densities in brain regions of the APP(SWE) mouse model and in human peripheral blood lymphocytes.

The brains of people with Alzheimer's disease (AD) display several characteristic pathological features, including deposits (plaques) of beta-amyloid 1-42 (Abeta1-42), intraneuronal accumulations (tangles) of hyperphosphorylated tau, degeneration of the basal forebrain cholinergic pathway, and gliosis. Abeta1-42 plaques develop in specific brain regions, including hippocampus and cortex, as well as in the vasculature. Abeta1-42 might promote neurodegeneration through the induction of free radicals and disruption of Ca2+ homeostasis, giving rise to the symptoms of AD. Abeta1-42 interacts with the alpha7 subtype of the nicotinic acetylcholine receptor (alpha7 nAChR), which is widely expressed throughout the central and peripheral nervous systems, as well as in several nonneuronal loci, such as epithelial cells, lymphoid tissues, and peripheral blood lymphocytes. Western blot and autoradiographic analyses indicate that the alpha7 nAChR subunit protein is up-regulated in human brain samples from Alzheimer patients, as well as in animal models of AD (Dineley et al., 2001; Bednar et al., 2002), and might be involved in nicotine-mediated reduction of Abeta1-42 deposition (Hellstrom et al., 2004), although the nature of this relationship remains ill-defined. We have undertaken a semiquantitative histological evaluation of alpha7 nAChR expression in a mouse model of AD pathology, as well as a comparison of alpha7 nAChR levels in lymphocytes from AD patients and control subjects.

Alzheimer Disease↗

Inhibition of alpha7-nicotinic acetylcholine receptor expression by arsenite in the vascular endothelial cells.

The alpha7-nicotinic acetylcholine receptor (alpha7-nAChR), expressed in the neuronal and non-neuronal cells, has been shown to regulate cell proliferation. However, the expression and function of the alpha7-nAChR in the proliferation of the vascular endothelial cells remain unclear. In this study, we investigated the expression of the alpha7-nAChR in the arsenite-exposed vascular endothelial cells. The vascular endothelial cells SVEC4-10 and porcine aorta endothelial cells (PAEC) expressed the alpha7-nAChR proteins. Moreover, the location of the alpha7-nAChR proteins on cell membrane of the vascular endothelial cells was identified by the alpha7-nAChR binding to a tetramethylrhodamine-labeled alpha-bungarotoxin (alpha-BTX). Arsenite (20 microM, 24 h) significantly induced the cytotoxicity, cell growth inhibition, and apoptosis in the vascular endothelial cells. The level of alpha7-nAChR proteins was concentration dependently decreased in the arsenite-treated endothelial cells. Furthermore, a specific alpha7-nAChR antagonist, alpha-BTX, inhibited the cell viability in the vascular endothelial cells. Nevertheless, alpha-BTX, and a alpha7-nAChR agonist, nicotine, did not significantly alter the cytotoxicity in the arsenite-treated endothelial cells. In addition, arsenite decreased the level of endothelial nitric oxide synthase proteins but did not alter choline acetyltransferase proteins in the SVEC4-10 endothelial cells. Together, our results indicate that arsenite can inhibit the alpha7-nAChR protein expression and cause the cell injury in the vascular endothelial cells.

Animals↗