Ca2+ permeability of nicotinic acetylcholine receptors

Dendritic nicotinic receptors modulate backpropagating action potentials and long-term plasticity of interneurons

Stratum radiatum interneurons, unlike pyramidal cells, are rich in nicotinic acetylcholine receptors (nAChRs); however, the role of these receptors in plasticity has remained elusive. As opposed to previous physiological studies, we found that functional alpha7-subunit-containing nAChRs (alpha7-nAChRs) are abundant on interneuron dendrites of rats. Moreover, dendritic Ca2+ transients induced by activation of alpha7-nAChRs increase as a function of distance from soma. The activation of these extrasynaptic alpha7-nAChRs by cholinergic agonists either facilitated or depressed backpropagating action potentials, depending on the timing of alpha7-nAChR activation. We have previously shown that dendritic alpha7-nAChRs are involved in the regulation of synaptic transmission, suggesting that alpha7-nAChRs may play an important role in the regulation of the spike timing-dependent plasticity. Here we provide evidence that long-term potentiation is indeed boosted by stimulation of dendritic alpha7-nAChRs. Our results suggest a new mechanism for a cholinergic switch in memory encoding and retrieval.

Melatonin inhibits nicotinic currents in cultured rat cerebellar granule neurons

There is limited data regarding the effects of melatonin on the activity of neuronal acetylcholine receptors (nAChRs) themselves. This study analyzes the effects of low concentrations of melatonin on nicotine-evoked currents from cerebellar granule neurons (CGNs) in culture. Using electrophysiological and Ca(2+)-imaging techniques, it was found a subset of rat CGNs to which nicotine application elicited both intracellular Ca(2+) transients and inward whole-cell currents. These responses were mediated by heteromeric nAChRs, as assessed by their sensitivity to nicotine and time constant of current decay. Preincubating the cells with low melatonin concentrations (down to 1 pm) significantly reduced the current amplitude in a dose-dependent manner, without affecting the receptor's apparent affinity and voltage-dependency, nor the current's rise and decay time course. The inhibitory effect of melatonin was significantly reduced by luzindole, a competitive antagonist of both MT(1) and MT(2) melatonin receptors. In conclusion, melatonin inhibits nicotinic currents through non-alpha7 heteromeric nAChRs expressed by CGNs in culture, an effect that appears to be at least partially mediated by melatonin membrane receptors. Direct modulation of nicotinic receptors is accomplished at doses that are likely to be physiologically relevant, thus providing a mechanism through which melatonin circadian rhythmic levels could modulate cholinergic activity.

Study of nicotinic acetylcholine receptors on cultured antennal lobe neurones from adult honeybee brains

In insects, acetylcholine (ACh) is the main neurotransmitter, and nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission. In the honeybee, nAChRs are expressed in diverse structures including the primary olfactory centres of the brain, the antennal lobes (AL) and the mushroom bodies. Whole-cell, voltage-clamp recordings were used to characterize the nAChRs present on cultured AL cells from adult honeybee, Apis mellifera. In 90% of the cells, applications of ACh induced fast inward currents that desensitized slowly. The classical nicotinic agonists nicotine and imidacloprid elicited respectively 45 and 43% of the maximum ACh-induced currents. The ACh-elicited currents were blocked by nicotinic antagonists methyllycaconitine, dihydroxy-beta-erythroidine and alpha-bungarotoxin. The nAChRs on adult AL cells are cation permeable channels. Our data indicate the existence of functional nAChRs on adult AL cells that differ from nAChRs on pupal Kenyon cells from mushroom bodies by their pharmacological profile and ionic permeability, suggesting that these receptors could be implicated in different functions.

Regulation of neuronal nicotinic receptor traffic and expression

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of cation channels widely distributed in the brain, whose subunit composition and biophysical properties vary depending on the subtype and the area of the brain in which they are found. Brain nAChRs are also the target of nicotine, the most widespread drug of abuse. Chronic nicotine exposure differentially affects the number, subunit composition, stoichiometry and functional state of some nAChR subtypes, leaving others substantially unaffected. In this review, we will summarise recent data concerning the nAChR subtypes expressed in the CNS, and how they are regulated by means of chronic nicotine and/or nicotinic drugs. We will particularly focus on the possible mechanisms involved in the up-regulation of nAChRs.

Drug resistance and neurotransmitter receptors of nematodes: recent studies on the mode of action of levamisole

Here we review recent studies on the mode of action of the cholinergic anthelmintics (levamisole, pyrantel etc.). We also include material from studies on the free living nematode Caenorhabditis elegans. The initial notion that these drugs act on a single receptor population, while attractive, has proven to be an oversimplification. In both free living and parasitic nematodes there are multiple types of nicotinic acetylcholine receptor (nAChR) on the somatic musculature. Each type has different (sometimes subtly so) pharmacological properties. The implications of these findings are: (1) combinations of anthelmintic that preferentially activate a broad range of nAChR types would be predicted to be more effective; (2) in resistant isolates of parasite where a subtype has been lost, other cholinergic anthelmintics may remain effective. Not only are there multiple types of nAChR, but relatively recent research has shown these receptors can be modulated; it is possible to increase the response of a parasite to a fixed concentration of drug by altering the receptor properties (e.g. phosphorylation state). These findings offer a potential means of increasing efficacy of existing compounds as an alternative to the costly and time consuming development of new anthelmintic agents.

The Endocannabinoid Anandamide Inhibits the Function of α4β2 Nicotinic Acetylcholine Receptors

The effects of the endocannabinoid anandamide (arachidonylethanolamide, AEA) on the function of alpha4beta2 nicotinic acetylcholine receptors (nAChR) stably expressed in SH-EP1 cells were investigated using the whole-cell patch-clamp technique. In the concentration range of 200 nM to 2 microM, AEA significantly reduced the maximal amplitudes and increased the desensitization of acetylcholine (ACh)-induced currents. The effects of AEA could be neither replicated by the exogenous cannabinoid Delta(9)-tetrahydrocannabinol (1 microM) nor reversed by the selective CB1 receptor antagonist 5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide (SR-141716A) (1 microM). The actions of AEA were apparent when applied extracellularly but not during intracellular dialysis. Furthermore, the effects of AEA ACh currents were not altered by the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. The onset and washout of the AEA effects required several minutes (10-30 min), but the latter was significantly decreased in the presence of lipid-free bovine serum albumin (BSA). Moreover, BSA alone increased peak ACh current amplitudes and diminished desensitization rates in naive cells, suggesting a tonic modulation of alpha4beta2 nAChR function by an endogenous AEA-like lipid. Further analysis of AEA effects on alpha4beta2 nAChR-mediated currents, using a two-stage desensitization model, indicated that the first forward rate constant leading to desensitization, k(1), increased nearly 30-fold as a linear function of the AEA concentration. In contrast, the observation that the other three rate constants were unaltered by AEA suggested that AEA raised the energy of the activated state. These results indicate that AEA directly inhibits the function of alpha4beta2 nAChRs in a CB1 receptor-independent manner.

Cholinergic axons modulate GABAergic signaling among hippocampal interneurons via postsynaptic alpha 7 nicotinic receptors

Homopentameric alpha7 nicotinic receptors have a high affinity for acetylcholine (ACh), are permeable to Ca2+ ions, and are abundant in hippocampal interneurons. Although nicotinic agonists evoke inward currents and Ca2+ transients in stratum radiatum interneurons, the role of endogenous ACh in modulating synaptic integration by interneurons is incompletely understood. Many cholinergic axonal varicosities do not have postsynaptic specializations, but alpha7 receptors frequently occur close to synaptic GABA(A) receptors. These observations raise the possibility that alpha7 nicotinic receptors activated by ACh released from cholinergic axons modulate GABAergic transmission in interneurons. We show that agonists of alpha7 receptors profoundly depress GABAergic IPSCs recorded in stratum radiatum interneurons in the CA1 region of the hippocampus. This depression is accompanied by a small increase in GABA release. Alpha7 nicotinic receptor agonists also depress GABA- or muscimol-evoked currents in interneurons, indicating that the major effect is a postsynaptic modulation of GABA(A) receptors. The depression of GABA-evoked currents is abolished by chelating Ca2+ in the recorded interneuron and attenuated by inhibitors of PKC. We also show that stimuli designed to release endogenous ACh from cholinergic axons evoke an alpha7 receptor-dependent heterosynaptic depression of GABAergic IPSCs in interneurons. This heterosynaptic modulation is amplified by blocking cholinesterases. These results reveal a novel mechanism by which cholinergic neurons modulate information processing in the hippocampus.

Heterogeneity and complexity of native brain nicotinic receptors

Neuronal cholinergic nicotinic receptors (nAChRs) are a heterogeneous class of cationic channels that are widely distributed in the nervous system that have specific functional and pharmacological properties. They consist of homologous subunits encoded by a large multigene family, and their opening is physiologically controlled by the acetylcholine neurotransmitter or exogenous ligands such as nicotine. Their biophysical and pharmacological properties depend on their subunit composition, which is therefore central to understanding receptor function in the nervous system and discovering new subtype-selective drugs. We will review rodent brain subtypes by discussing their subunit composition, pharmacology and localisation and, when possible, comparing them with the same subtypes present in the brain of other mammalian species or chick. In particular, we will focus on the nAChRs present in the visual pathway (retina, superior colliculus and nucleus geniculatus lateralis), in which neurons express most, if not all, nAChR subunits. In addition to the major alpha4beta2 and alpha7 nAChR subtypes, the visual pathway selectively expresses subtypes with a complex subunit composition. By means of ligand binding and immunoprecipitation and immunopurification experiments on tissues obtained from control and lesioned rats, and wild-type and nAChR subunit knockout mice, we have qualitatively and quantitatively identified, and pharmacologically characterised, the multiple complex native subtypes containing up to four different subunits.

Dendritic Ca2+ signalling due to activation of alpha 7-containing nicotinic acetylcholine receptors in rat hippocampal neurons

Neuronal nicotinic acetylcholine receptors (nAChRs) are in the superfamily of Cys-loop ligand-gated ion channels, which are widely expressed in the brain. Among the many different subtypes of nAChRs known to be expressed in the rat brain, the alpha 7-containing nAChRs are considered to be the most permeable to Ca2+. Utilizing highly localized and rapid iontophoretic agonist delivery, combined with patch-clamp electrophysiology and fura-2 fluorescence imaging techniques, we examined the alpha 7 nAChR-mediated currents and [Ca2+]i transients in the dendrites of rat hippocampal CA1 interneurons in the slice. We found that in the dendrites, whereas the amplitudes of the current responses were smaller and the decay kinetics faster than the responses in the soma, the amplitudes of the [Ca2+]i signals were significantly larger. Cultured hippocampal neurons were studied since the dendritic field lies in the same focal plane, which allowed for a broader investigation of the spatiotemporal dynamics of [Ca2+]i signalling. In cultured neurons, the [Ca2+]i signals in the dendrites were similar to those in slices. Interestingly in cultures, even though the amplitude of the alpha 7 nAChR-mediated currents dramatically decreased with distance from the soma (from approximately 20-250 microm), the amplitude of the [Ca2+]i signals did not correlate with distance. This indicates that the relative efficacy of alpha 7 nAChR activation to increase [Ca2+]i levels in dendrites increased severalfold with distance from the soma. These results may have implications for the role that alpha 7 nAChRs have in regulating various signal transduction cascades, synaptic plasticity, and memory processes, via significant changes in [Ca(2+)]i levels.

The effect of epibatidine on spontaneous and evoked neurotransmitter release in the mouse and guinea pig isolated vas deferens

BACKGROUND AND PURPOSE

Nicotinic agonists increase sympathetic field-stimulus-evoked contraction of the rodent vas deferens, presumably by increasing evoked neurotransmitter release. This presumption was tested in two species.

EXPERIMENTAL APPROACH

The effect of the nicotinic acetylcholine receptor (nAChR) agonist epibatidine on neurotransmitter release in mouse and guinea pig isolated vas deferens was investigated using contraction studies and conventional intracellular recording techniques.

KEY RESULTS

In 12 of 14 mouse vasa deferentia, slow bath application of epibatidine (100 nM) had no significant effect on excitatory junction potential (EJP) amplitude and spontaneous EJP (SEJP) frequency. However, rapid application of epibatidine to the mouse vas deferens caused an increase in SEJP frequency (by 530%), with no effect on EJP amplitude. Despite the absence of an effect on EJPs, electrically-evoked contractions of the mouse vas deferens were significantly increased in the presence of epibatidine (by 50%). A transient contraction was reliably induced by a higher epibatidine concentration (1 microM). This contraction was significantly reduced in the presence of prazosin, tetrodotoxin, or alpha,beta-methyleneATP. Epibatidine did not induce a contraction in the presence of a combination of prazosin, alpha,beta-methyleneATP and cyclopentolate. In guinea pig vasa deferentia, bath-applied epibatidine potentiated EJP amplitude in a biphasic pattern, lasting for at least 30 minutes.

CONCLUSION AND IMPLICATIONS

The nAChR-mediated augmentation of neurogenic contraction is indeed prejunctional, but in the mouse arises from an increase in spontaneous neurotransmitter release that primes smooth muscle for subsequent contraction, while in the guinea pig there is a direct augmentation of evoked neurotransmitter (ATP) release.

Chronic nicotine-induced switch in Src-family kinase signaling for long-term potentiation induction in hippocampal CA1 pyramidal cells

Here, we show that chronic nicotine exposure induces changes in Src signaling for the modulation of N-methyl-D-aspartate receptor (NMDAR) function and LTP induction in CA1 pyramidal cells. Activation of muscarinic receptors normally potentiates NMDAR responses in pyramidal cells via a Gq/protein kinase C (PKC)/proline-rich tyrosine kinase 2/Src signaling cascade. However, muscarinic, PKC and Src stimulation had no effect on NMDAR responses after chronic nicotine treatment. The lack of effect was apparently due to enhanced tyrosine phosphorylation, and therefore further stimulation of the signaling cascade caused no effect on NMDAR responses. Interestingly, another Src-family kinase potentiated NMDAR responses after, but not before, chronic nicotine treatment. In control pyramidal cells, Src inhibitor peptides prevented tetanus-induced long-term potentiation (LTP). Conversely, in chronic nicotine-exposed cells, the inhibitor was ineffective in blocking tetanus-induced LTP. Furthermore, in control pyramidal cells, applying exogenous Src and administration of an endogenous Src-family kinase activator increased alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR)-mediated responses. This increase was blocked by Src inhibitor peptides and occluded tetanus-induced LTP, as reported previously. In contrast, in chronic nicotine-treated pyramidal cells, applying exogenous Src had no effect on AMPAR-mediated responses and a tetanus-induced LTP. Interestingly, however, administration of an endogenous Src-family kinase activator enhanced AMPAR-mediated responses, which occluded tetanus-induced LTP. This enhancement was not prevented by co-application of Src inhibitor peptides. Thus, it appears that chronic nicotine exposure recruits another member of the Src-family for the regulation of NMDAR function and LTP induction. The nicotine-induced distinct signaling cascades may be involved in long-lasting memories of nicotine misuse.

Differential coupling of α7 and non-α7 nicotinic acetylcholine receptors to calcium-induced calcium release and voltage-operated calcium channels in PC12 cells

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that can modulate various neuronal processes by altering intracellular Ca(2+) levels. Following nAChR stimulation Ca(2+) can enter cells either directly, through the intrinsic ion channel, or indirectly following voltage-operated Ca(2+) channel (VOCC) activation; Ca(2+) levels can subsequently be amplified via Ca(2+)-induced Ca(2+) release from intracellular stores. We have used subtype-selective nAChR agonists to investigate the Ca(2+) sources contributing to alpha7 and non-alpha7 nAChR-mediated increases in intracellular Ca(2+) in PC12 cells. Application of the alpha7 nAChR positive allosteric modulator PNU 120596 (10 mum), in conjunction with the alpha7 nAChR agonist, compound A [(R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide), 10 nm], produces a rapid increase in fluo-3 fluorescence that is prevented by the selective alpha7 nAChR antagonist alpha-bungarotoxin. The non-alpha7 nAChR agonist 5-Iodo-A-85380 produces alpha-bungarotoxin-insensitive increases in intracellular Ca(2+) (EC(50) = 11.2 mum). Using these selective agonists or KCl in conjunction with general and selective VOCC inhibitors, we demonstrate that the primary route of Ca(2+) entry following either non-alpha7 nAChR activation or KCl stimulation is via L-type VOCCs. In contrast, the alpha7 nAChR-mediated response is unaffected by VOCC blockers but is inhibited by modulators of intracellular Ca(2+) stores. These results indicate that alpha7 and non-alpha7 nAChRs are differentially coupled to Ca(2+)-induced Ca(2+) release and VOCCs, respectively.

Nicotine withdrawal suppresses nicotinic modulation of long-term potentiation induction in the hippocampal CA1 region

We have previously reported that acute and chronic nicotine exposure lower the threshold for long-term potentiation (LTP) induction in the rat hippocampal CA1 region, and acute application of nicotine in the chronic-nicotine-treated hippocampus further reduces the threshold. However, it is unknown how withdrawal from chronic nicotine exposure affects the induction of LTP. Here, we show that, following nicotine withdrawal, the threshold for LTP induction fluctuates before returning to the basal level and acute nicotine is no longer effective in lowering the threshold at 4 days after withdrawal. Chronic nicotine-induced enhancement of N-methyl-d-aspartate receptor responses slowly diminishes and returns to the control level by 8 days of withdrawal. In 4-day-withdrawn hippocampi, there is functional up-regulation of postsynaptic alpha7 nicotinic acetylcholine receptors (nAChRs) on interneurons in the stratum radiatum, whereas the release of gamma-aminobutyric acid from their terminals is reduced. In both control and chronic nicotine-exposed hippocampi, acute nicotine depresses monosynaptic inhibitory postsynaptic currents recorded in pyramidal cells but has almost no effect at 4 days of withdrawal. The lack of effect is due, at least in part, to the loss of a presynaptic nicotine effect. These withdrawal-induced changes are accompanied by decreases in normal nicotine-induced enhancement of N-methyl-d-aspartate receptor responses, which may be responsible for the lack of acute nicotine-mediated facilitation of LTP induction in 4-day-withdrawn hippocampi. These withdrawal-induced changes may contribute to the cellular basis of unpleasant withdrawal symptoms and, thus, nicotine dependence.

Role of membrane potential in calcium signaling during rhythmic bursting in tritonia swim interneurons

Rhythmic bursting in neurons is accompanied by dynamic changes in intracellular Ca(2+) concentration. These Ca(2+) signals may be caused by membrane potential changes during bursting and/or by synaptic inputs. We determined that membrane potential is responsible for most, if not all, of the cytoplasmic Ca(2+) signal recorded during rhythmic bursting in two neurons of the escape swim central pattern generator (CPG) of the mollusk, Tritonia diomedea: ventral swim interneuron B (VSI) and cerebral neuron 2 (C2). Ca(2+) signals were imaged with a confocal laser scanning microscope while the membrane potential was recorded at the soma. During the swim motor pattern (SMP), Ca(2+) signals in both neurons transiently increased during each burst of action potentials with a more rapid decay in secondary than in primary neurites. VSI and C2 were then voltage-clamped at the soma, and each neuron's own membrane potential waveform recorded during the SMP was played back as the voltage command. In all regions of VSI, this completely reproduced the amplitude and time course of Ca(2+) signals observed during the SMP, but in C2, the amplitude was lower in the playback experiments than during the SMP, possibly due to space clamp problems. Therefore in VSI, the cytoplasmic Ca(2+) signal during the SMP can be accounted for by its membrane potential excursions, whereas in C2 the membrane potential excursions can account for most of the SMP Ca(2+) signal.

Role of acetylcholine in neurotransmission of the carotid body

Acetylcholine (ACh) has been considered an important excitatory neurotransmitter in the carotid body (CB). Its physiological and pharmacological effects, metabolism, release, and receptors have been well documented in several species. Various nicotinic and muscarinic ACh receptors are present in both afferent nerve endings and glomus cells. Therefore, ACh can depolarize or hyperpolarize the cell membrane depending on the available receptor type in the vicinity. Binding of ACh to its receptor can create a wide variety of cellular responses including opening cation channels (nicotinic ACh receptor activation), releasing Ca(2+) from intracellular storage sites (via muscarinic ACh receptors), and modulating activities of K(+) and Ca(2+) channels. Interactions between ACh and other neurotransmitters (dopamine, adenosine, nitric oxide) have been known, and they may induce complicated responses. Cholinergic biology in the CB differs among species and even within the same species due to different genetic composition. Development and environment influence cholinergic biology. We discuss these issues in light of current knowledge of neuroscience.

Inhibitory synaptic transmission in area postrema neurons of the rat showing robust presynaptic facilitation mediated by nicotinic ACh receptors

Inhibitory synaptic transmission and its modulation in neurons of the area postrema (AP), one of autonomic nuclei in the medulla, were studied using whole-cell patch-electrodes applied to slices from rats on postnatal days 10-24. When glycine (100 microM) or GABA (10 microM) was applied to AP neurons from a "Y tube", large outward currents that showed reversal potential of - 67 mV (approximate Cl- equilibrium potential estimated) were induced. At a holding potential of - 10 mV, application of high K+ to the AP neurons evoked massive inhibitory postsynaptic currents (IPSCs) in the neurons. Most of the evoked synaptic currents were blocked by bicuculline, while the remaining currents were sensitive to strychnine, indicating that the major inhibitory transmission in the area postrema was GABAergic. When nicotine (5-100 microM) was applied to AP neurons, robust IPSCs having GABAergic identity were evoked. Even in the presence of tetrodotoxin, nicotine could induce GABAergic IPSCs, most of which, however, disappeared in the presence of 5 mM Mg2+. Presynaptic facilitation was also induced by other nicotinic agonists, including cytisine, 1,1-dimethyl-4-phenyl-piperazinium iodide, ACh and choline. The nicotine-induced presynaptic facilitation was inhibited by mecamylamine and slightly inhibited by dihydro-beta-erythroidine or alpha-Bungarotoxin. These results indicate that nicotinic receptors are expressed at GABAergic presynaptic terminals in the area postrema and induce Ca2+ influx to trigger vesicular release. The major nicotinic receptors involved are thought to be heteromeric subtypes such as alpha3beta4 receptors, which may regulate inhibitory transmission potently responding to endogenous or exogenous nicotinic agents appeared in this area.

Ca2+Permeability of the (α4)3(β2)2Stoichiometry Greatly Exceeds That of (α4)2(β2)3Human Acetylcholine Receptors

Human alpha4beta2 nicotinic acetylcholine receptors (AChRs) expressed in Xenopus laevis oocytes or transfected cell lines are present as a mixture of two stoichiometries, (alpha4)2(beta2)3 and (alpha4)3(beta2)2, which differ depending on whether a beta2 or alpha4 subunit occupies the accessory subunit position corresponding to beta1 subunits of muscle AChRs. Pure populations of each stoichiometry can be expressed in oocytes by combining a linked pair of alpha4 and beta2 with free beta2 to produce the (alpha4)2(beta2)3 stoichiometry or with free alpha4 to produce the (alpha4)3(beta2)2 stoichiometry. We show that the (alpha4)3(beta2)2 stoichiometry and the (alpha4)2(beta2)2beta3 and (alpha4)2(beta2)2alpha5 subtypes in which beta3 or alpha5occupy the accessory positions have much higher permeability to Ca2+ than does (alpha4)2(beta2)3 and suggest that this could be physiologically significant in triggering signaling cascades if this stoichiometry or these subtypes were found in vivo. We show that Ca2+ permeability is determined by charged amino acids at the extracellular end of the M2 transmembrane domain, which could form a ring of amino acids at the outer end of the cation channel. Alpha4, alpha5, and beta3 subunits all have a homologous glutamate in M2 that contributes to high Ca2+ permeability, whereas beta2 has a lysine at this position. Subunit combinations or single amino acids changes at this ring that have all negative charges or a mixture of positive and negative charged amino acids are permeable to Ca2+. All positive charges in the ring prevent Ca2+ permeability. Increasing the proportion of negative charges is associated with increasing permeability to Ca2+.

Failure of nicotine-dependent enhancement of synaptic efficacy at Schaffer-collateral CA1 synapses of AD11 anti-nerve growth factor transgenic mice

Alzheimer's disease is a neurodegenerative disorder characterized by neuronal loss associated with a progressive impairment of cognitive functions. Early consequences of Alzheimer's disease include deficit of cholinergic signalling in particular regions controlling memory processes, such as the cortex and hippocampus, and accumulation of beta-amyloid (Abeta) peptide in neuritic plaques. The cholinergic system depends for its integrity and function on nerve growth factor. Chronic nerve growth factor deprivation in transgenic mice (AD11) engineered to produce recombinant neutralizing anti-nerve growth factor antibodies leads to progressive age-dependent Alzheimer's-like neurodegenerative pathology similar to that found in patients with Alzheimer's disease, associated with a selective loss of cholinergic neurones in the basal forebrain. Here we show that in the hippocampus of 6-month-old AD11 mice, Abeta aggregates started appearing in the CA1 region. The accumulation of Abeta was associated with a loss of cholinergic function at CA3-CA1 synapses. Whereas in wild-type mice nicotine induced a persistent increase of synaptic efficacy via alpha7 nicotine acetylcholine receptors, in AD11 mice this alkaloid failed to modify synaptic strength. Moreover, nicotine failed to transiently enhance the frequency of spontaneous miniature glutamatergic currents (miniature excitatory postsynaptic currents) recorded from CA1 but not from CA3 pyramidal neurones of AD11 mice. However, in CA3 principal cells of AD11 mice, the potentiating effect of nicotine on miniature excitatory postsynaptic currents was prevented when Abeta peptide 1-42 was added to the extracellular solution. These data suggest that in AD11 mice, Abeta interferes with nicotine acetylcholine receptors at the level of presynaptic glutamatergic terminals, inhibiting their function possibly through calcium signalling via presynaptic alpha7 nicotine acetylcholine receptors.

Imaging single-channel calcium microdomains

The Ca(2+) microdomains generated around the mouth of open ion channels represent the basic building blocks from which cytosolic Ca(2+) signals are constructed. Recent improvements in optical imaging techniques now allow these microdomains to be visualized as single channel calcium fluorescence transients (SCCaFTs), providing information about channel properties that was previously accessible only by electrophysiological patch-clamp recordings. We review recent advances in single channel Ca(2+) imaging methodologies, with emphasis on total internal reflection fluorescence microscopy (TIRFM) as the technique of choice for recording SCCaFTs from voltage- and ligand-gated plasmalemmal ion channels. This technique of 'optical patch-clamp recording' is massively parallel, permitting simultaneous imaging of hundreds of channels; provides millisecond resolution of gating kinetics together with sub-micron spatial resolution of channel locations; and is applicable to diverse families of membrane channels that display partial permeability to Ca(2+) ions.

Nicotinic modulation of gene expression in SH-SY5Y neuroblastoma cells

Exposure to nicotine has a broad range of physiological and psychological effects that can be long lasting and contribute to nicotine dependence. On a time course longer than that needed to activate nicotinic acetylcholine receptor (nAChR) function, nicotine exposure induces functional inactivation of nAChR, upregulation of nAChR radioligand binding sites, and other alterations of cellular functions. To identify possible mechanisms underlying nicotine-induced changes in nAChR numbers and function, we defined changes in gene expression in neuron-like, SH-SY5Y human neuroblastoma cells following 24 h of continuous exposure to 1 mM nicotine. This treatment condition produces both functional inactivation and upregulation of nAChR. Repeat and cross-controlled microarray ( approximately 5000 genes queried) analyses revealed 163 genes whose expression was consistently altered at the p<0.01 level following nicotine treatment. Quantitative, real-time, reverse transcription-polymerase chain reaction analyses confirmed altered expression of thirteen out of fourteen of these genes chosen for further study, including contactin 1, myozenin 2, and ubiquitin-conjugating enzymes E2C and E2S. Inhibition or reversal of these effects by the general nAChR antagonist, d-tubocurarine, indicated that gene expression changes are dependent on nAChR activation. Studies using other nAChR subtype-selective antagonists identified gene expression changes that required activation of both alpha7- and alpha3*-nAChR, alpha7-nAChR alone, or either alpha7- or alpha3beta4*-nAChR, suggesting some convergent and some divergent pathways of gene activation coupled to these nAChR subtypes. These results suggest that longer-term physiological and psychological effects of nicotine exposure and changes in nAChR expression may be due in part to effects on gene expression initiated by interactions with nAChR.

The Prototoxin lynx1 Acts on Nicotinic Acetylcholine Receptors to Balance Neuronal Activity and Survival In Vivo

Nicotinic acetylcholine receptors (nAChRs) affect a wide array of biological processes, including learning and memory, attention, and addiction. lynx1, the founding member of a family of mammalian prototoxins, modulates nAChR function in vitro by altering agonist sensitivity and desensitization kinetics. Here we demonstrate, through the generation of lynx1 null mutant mice, that lynx1 modulates nAChR signaling in vivo. Its loss decreases the EC(50) for nicotine by approximately 10-fold, decreases receptor desensitization, elevates intracellular calcium levels in response to nicotine, and enhances synaptic efficacy. lynx1 null mutant mice exhibit enhanced performance in specific tests of learning and memory. Consistent with reports that mutations resulting in hyperactivation of nAChRs can lead to neurodegeneration, aging lynx1 null mutant mice exhibit a vacuolating degeneration that is exacerbated by nicotine and ameliorated by null mutations in nAChRs. We conclude that lynx1 functions as an allosteric modulator of nAChR function in vivo, balancing neuronal activity and survival in the CNS.

The Magnitude of α7 Nicotinic Receptor Currents in Rat Hippocampal Neurons Is Dependent upon GABAergic Activity and Depolarization

Hippocampal alpha7(*) nicotinic acetylcholine receptors modulate the release of GABA and glutamate. The control of functional receptor pools by cell firing or synaptic activity could therefore allow for a local adjustment of the sensitivity to cholinergic input upon changes in neuronal activity. We first investigated whether tonic depolarization or cell firing affected the function of alpha7(*). The amplitude of alpha7(*)-gated whole-cell currents in cultured rat hippocampal neurons exposed to high-extracellular K(+) (40 mM KCl) for 24 to 48 h increased 1.3 to 5.5 times. The proportion of alpha7(*)-responsive neurons (99%), the potency of acetylcholine, and the sensitivity to nicotinic antagonists were all unaffected. In contrast, block of spontaneous cell firing with tetrodotoxin for 24 h led to a 37% reduction in mean current amplitude. Reduced alpha7(*) responses were seen after a 24-h blockade of N-type calcium channels but not of L-type calcium channels, N-methyl-d-aspartate (NMDA), or non-NMDA receptor channels, protein kinase C, or calcium-calmodulin kinases II and IV. The N-type or L-type calcium channel antagonists omega-conotoxin GVIA and nifedipine did not prevent the current-potentiating effect of KCl. The GABA(A) antagonist picrotoxin led to a 44% reduction of the currents, despite increasing action potential firing, and also reversed the potentiating effect of KCl. Treatment with GABA, midazolam, or a GABA uptake blocker led to increased currents. These data indicate that alpha7(*)-gated currents in hippocampal neurons are regulated by GABAergic activity and suggest that depolarization-induced GABA release may underlie the effect of increased extracellular KCl.

C3-halogenation of cytisine generates potent and efficacious nicotinic receptor agonists

Neuronal nicotinic acetylcholine receptors subserve predominantly modulatory roles in the brain, making them attractive therapeutic targets. Natural products provide key leads in the quest for nicotinic receptor subtype-selective compounds. Cytisine, found in Leguminosae spp., binds with high affinity to alpha4beta2* nicotinic receptors. We have compared the effect of C3 and C5 halogenation of cytisine and methylcytisine (MCy) on their interaction with native rat nicotinic receptors. 3-Bromocytisine (3-BrCy) and 3-iodocytisine (3-ICy) exhibited increased binding affinity (especially at alpha7 nicotinic receptors; Ki approximately 0.1 microM) and functional potency, whereas C5-halogenation was detrimental. 3-BrCy and 3-ICy were more potent than cytisine at evoking [3H]dopamine release from striatal slices (EC50 approximately 11 nM), [3H]noradrenaline release from hippocampal slices (EC50 approximately 250 nM), increases in intracellular Ca2+ in PC12 cells and inward currents in Xenopus oocytes expressing human alpha3beta4 nicotinic receptor (EC50 approximately 2 microM). These compounds were also more efficacious than cytisine. C3-halogenation of cytisine is proposed to stabilize the open conformation of the nicotinic receptor but does not enhance subtype selectivity.

The pathophysiology of traumatic brain injury in α7 nicotinic cholinergic receptor knockout mice

The alpha7 nicotinic cholinergic receptor is a ligand-gated ion channel with calcium permeability similar to that of ionotrophic glutamate receptors. Previous studies from our laboratory have implicated changes in expression alpha7 nicotinic cholinergic receptors in the pathophysiology of traumatic brain injury (TBI). In rats, TBI causes a time-dependent and significant decrease in cortical and hippocampal alpha-[(125)I]-bungarotoxin (BTX) binding. We have postulated that deficits in alpha7 expression may contribute to TBI-induced cognitive impairment and that nicotinic receptor agonists can reverse alpha7 binding deficits and result in significant cognitive improvement compared to saline-treated controls. Thus, alpha7 nAChRs could be involved in a form of cholinergically mediated excitotoxicity following brain injury. In the current study, wild-type, heterozygous and null mutant mice were employed to test the hypothesis that genotypic depletion of the alpha7 receptor would render animals less sensitive to tissue loss and brain inflammation following experimental brain injury. Mice were anesthetized and subjected to a 0.5-mm cortical contusion injury of the somatosensory cortex. Brain inflammation, changes in nicotinic receptor expression and cortical tissue sparing were evaluated in wild-type, heterozygous and homozygous mice 1 week following TBI. In wild-type mice, brain injury caused a significant decrease in BTX binding in several hippocampal regions, consistent with what we have measured in rat brain following TBI. However, there were no genotypic differences in cortical tissue sparing or brain inflammation in this experiment. Although the results of this study were largely negative, it is still plausible that changes in the activity/expression of native alpha7 receptors contribute to pathophysiology following TBI. However, when null mutant mice develop in the absence of central alpha7 expression, it is possible that compensatory changes occur that confound the results obtained.

Nicotine exposurein vivoinduces long-lasting enhancement of NMDA receptor-mediated currents in the hippocampus

The use of nicotine via cigarette smoking forms long-lasting memories that are recalled in response to environmental cues associated with previous nicotine use. However, the changes in brain memory systems that underlie these long-lasting memories are not well understood. The N-methyl-D-aspartate receptor (NMDAR) is critical for long-lasting modifications of synapses. Here we show that in vivo nicotine exposure induces the enhancement of NR2B-containing NMDAR-mediated currents in the hippocampus, a brain region associated with the formation of memories. This nicotine effect is maintained during continued nicotine exposure and is accompanied by increased tyrosine phosphorylation of NR2B. Furthermore, long-term potentiation (LTP), which is considered to be a cellular substrate of learning and memory, induced in nicotine-exposed hippocampi contains a protein synthesis-independent long-lasting component. An NR2B-selective antagonist blocks a long-lasting component of LTP, but not LTP. These results suggest that exposure to nicotine provides conditions that promote the induction of long-lasting modifications of synapses, which may be involved in the formation of memories involving nicotine use.

alpha3alpha5beta2-Nicotinic acetylcholine receptor contributes to the wound repair of the respiratory epithelium by modulating intracellular calcium in migrating cells

Nicotinic acetylcholine receptors (nAChRs), present in human bronchial epithelial cells (HBECs), have been shown in vitro to modulate cell shape. Because cell spreading and migration are important mechanisms involved in the repair of the bronchial epithelium, we investigated the potential role of nAChRs in the wound repair of the bronchial epithelium. In vivo and in vitro, alpha3alpha5beta2-nAChRs accumulated in migrating HBECs involved in repairing a wound, whereas alpha7-nAChRs were predominantly observed in stationary confluent cells. Wound repair was improved in the presence of nAChR agonists, nicotine, and acetylcholine, and delayed in the presence of alpha3beta2 neuronal nAChR antagonists, mecamylamine, alpha-conotoxin MII, and kappa-bungarotoxin; alpha-bungarotoxin, an antagonist of alpha7-nAChR, had no effect. Addition of nicotine to a repairing wound resulted in a dose-dependent transient increase of intracellular calcium in migrating cells that line the wound edge. Mecamylamine and kappa-bungarotoxin inhibited both the cell-migration speed and the nicotine-induced intracellular calcium increase in wound-repairing migrating cells in vitro. On the contrary alpha-bungarotoxin had no significant effect on migrating cells. These results suggest that alpha3alpha5beta2-nAChRs actively contribute to the wound repair process of the respiratory epithelium by modulating intracellular calcium in wound-repairing migrating cells.

The human adult subtype ACh receptor channel has high Ca2+ permeability and predisposes to endplate Ca2+ overloading

Slow-channel congenital myasthenic syndrome, caused by mutations in subunits of the endplate ACh receptor (AChR), results in prolonged synaptic currents and excitotoxic injury of the postsynaptic region by Ca2+ overloading. The Ca2+ overloading could be due entirely to the prolonged openings of the AChR channel or could be abetted by enhanced Ca2+ permeability of the mutant channels. We therefore measured the fractional Ca2+ current, defined as the percentage of the total ACh-evoked current carried by Ca2+ ions (Pf), for AChRs harbouring the alphaG153S or the alphaV249F slow-channel mutation, and for wild-type human AChRs in which Pf has not yet been determined. Experiments were performed in transiently transfected GH4C1 cells and human myotubes with simultaneous recording of ACh-evoked whole-cell currents and fura-2 fluorescence signals. We found that the Pf of the wild-type human endplate AChR was unexpectedly high (Pf approximately 7%), but neither the alphaV249F nor the alphaG153S mutation altered Pf. Fetal human AChRs containing either the wild-type or the mutated alpha subunit had a much lower Pf (2-3%). We conclude that the Ca2+ permeability of human endplate AChRs is higher than that reported for any other human nicotinic AChR, with the exception of alpha7-containing AChRs (Pf > 10%); and that neither the alphaG153S nor the alphaV249F mutations affect the Pf of fetal or adult endplate AChRs. However, the intrinsically high Ca2+ permeability of human AChRs probably predisposes to development of the endplate myopathy when opening events of the AChR channel are prolonged by altered AChR-channel kinetics.

Nicotinic modulation of neuronal networks: from receptors to cognition

RATIONALE

Nicotine affects many aspects of human cognition, including attention and memory. Activation of nicotinic acetylcholine receptors (nAChRs) in neuronal networks modulates activity and information processing during cognitive tasks, which can be observed in electroencephalograms (EEGs) and functional magnetic resonance imaging studies.

OBJECTIVES

In this review, we will address aspects of nAChR functioning as well as synaptic and cellular modulation important for nicotinic impact on neuronal networks that ultimately underlie its effects on cognition. Although we will focus on general mechanisms, an emphasis will be put on attention behavior and nicotinic modulation of prefrontal cortex. In addition, we will discuss how nicotinic effects at the neuronal level could be related to its effects on the cognitive level through the study of electrical oscillations as observed in EEGs and brain slices.

RESULTS/CONCLUSIONS

Very little is known about mechanisms of how nAChR activation leads to a modification of electrical oscillation frequencies in EEGs. The results of studies using pharmacological interventions and transgenic animals implicate some nAChR types in aspects of cognition, but neuronal mechanisms are only poorly understood. We are only beginning to understand how nAChR distribution in neuronal networks impacts network functioning. Unveiling receptor and neuronal mechanisms important for nicotinic modulation of cognition will be instrumental for treatments of human disorders in which cholinergic signaling have been implicated, such as schizophrenia, attention deficit/hyperactivity disorder, and addiction.

Nicotine reverses consolidated long-term potentiation in the hippocampal CA1 region

Long-term potentiation (LTP) has a memory-like consolidation period during which it becomes progressively stabilized. However, it is unknown how the consolidation is achieved. The present study demonstrates that nicotine reverses stabilized LTP in the hippocampal CA1 region, providing the first evidence that consolidated LTP can be reversed. The nicotine-induced reversal appeared to work by reversing cellular processes involved in stabilizing LTP, as LTP was readily induced again after reversal. The effect of nicotine was mediated, in large part, via desensitization of alpha7 nicotinic acetylcholine receptors (nAChRs), as an alpha7 nAChR-selective antagonist mimicked the nicotine effect. A non-selective N-methyl-d-aspartate receptor (NMDAR) antagonist completely abolished the nicotine-induced reversal, whereas an NR2B-containing NMDAR-selective antagonist had no effect. Furthermore, both the protein phosphatase 1/protein phosphatase 2A inhibitor okadaic acid and the protein phosphatase 2B (calcineurin) inhibitor cyclosporin A blocked the nicotine-induced reversal. Taken together, our results suggest that the reversal of stabilized LTP depends on the activation of NR2A-containing NMDARs and dephosphorylation. Thus, the consolidation of LTP appears to be the interruption of signaling leading to NR2A-containing NMDAR-dependent activation of protein phosphatases, which can be circumvented by nicotine-induced signaling. LTP induced in chronic nicotine-treated hippocampi contained a component that is immune to reversal, and thus acute nicotine was no longer effective to reverse consolidated LTP. These results demonstrate the differential effects of acute and chronic nicotine exposure on the cellular processes that are potentially involved in learning and memory.

Ca2+ permeability through rat cloned alpha9-containing nicotinic acetylcholine receptors

We investigated the functional properties of rat alpha9 and alpha9alpha10 nicotinic acetylcholine receptors (nAChRs) expressed by transient transfection in the rat GH4C1 cell line, using both Ca(2+) imaging and whole-cell recording. Acute applications of ACh generated short-delay fast-rising and quick-decaying Ca(2+) transients, suppressed in Ca(2+)-free medium and invariably accompanied by the activation of whole-cell inward currents. The mean amplitude of ACh-induced currents was as small as -16 pA in alpha9 subunit cDNA-transfected GH4C1 cells (alpha9-GH4C1), while they were much larger (range: -150 to -300 pA) in alpha9alpha10 subunit cDNAs-transfected GH4C1 cells (alpha9alpha10-GH4C1). Currents were not activated by nicotine, were blocked by methyllycaconitine and were ACh concentration-dependent. Because the Ca(2+) permeability of alpha9-containing nAChRs has been estimated in immortalized cochlear UB/OC-2 mouse cells, we also characterized the ACh-induced responses in these cells. Unlike alpha9- and alpha9alpha10-GH4C1 cells, UB/OC-2 cells responded to ACh with both long-delay methyllycaconitine-insensitive whole-cell currents and long-lasting Ca(2+) transients, the latter being detected in the absence of Ca(2+) in the extracellular medium and being suppressed by the Ca(2+)-ATPase inhibitor thapsigargin, known to deplete IP(3)-sensitive stores. These results indicated the involvement of muscarinic nAChRs and the lack of functional ACh-gated receptor channels in UB/OC-2 cells. Thus, we measured the fractional Ca(2+) current (P(f), i.e. the percentage of total current carried by Ca(2+) ions) in alpha9alpha10-GH4C1, obtaining a P(f) value of 22 +/- 4%; this is the largest value estimated to date for a ligand-gated receptor channel. The physiological role played by Ca(2+) entry through alpha9-containing nAChRs gated by ACh is discussed.

Acetylcholine causes an increase of intracellular calcium in human sperm

Sperm nicotinic acetylcholine receptors (nAChRs) can influence motility and the initiation of acrosome reaction (AR). We report that AR initiation by acetylcholine (ACh) in capacitated human sperm requires both Na+ and Ca2+ in the external medium. Pre-incubation with 50 microM 3-quinuclidinyl benzilate (QNB) or 50 nM strychnine failed to inhibit the ACh-initiated AR, demonstrating that muscarinic AChRs and nAChRs containing alpha9 subunits do not mediate this event. Choline (2.5, 5 and 10 mM), a highly specific but low potency agonist of the alpha7 nAChR initiated AR, with its effect blocked by the nAChR antagonist methyllycaconitine (MLA). ACh (50-400 microM) stimulated a small transient rise in the intracellular Ca2+ in sperm populations loaded with FURA-2, with 200 microM ACh being maximal (146 nM +/- 23 SEM). The nAChR antagonists, alpha-bungarotoxin (alpha-BTX) and MLA, reduced the ACh-initiated Ca2+ rise by 75 and 78%, respectively, demonstrating the majority of the rise is mediated through nAChRs containing alpha7 or alpha9 subunits. Single cell imaging studies using FLUO-3 resolved two patterns of ACh-stimulated Ca2+ increase in the sperm head: 94% of responding sperm displayed a rise (59.6% +/- 5.7 SEM increase from resting fluorescence intensity), returning to resting levels over a period of 2-3 min. The remaining sperm (6%) displayed a sharp spike of Ca2+ ( approximately 1 min; 86% +/- 4.3 SEM change in fluorescence intensity), followed by abrupt loss of fluorescence, a pattern suggestive of AR. A Ca2+ influx in the sperm midpiece appeared to accompany the Ca2+ influx seen in the head. These observations confirm an ionotropic role for nAChRs in sperm function.

Nicotinic cholinergic intercellular communication: implications for the developing auditory system

In this paper, research on the temporal and spatial distribution of cholinergic-related molecules in the lower auditory brainstem, with an emphasis on nicotinic acetylcholine receptors (nAChRs), is reviewed. The possible functions of acetylcholine (ACh) in driving selective auditory neurons before the onset of hearing, inducing glutamate receptor gene expression, synaptogenesis, differentiation, and cell survival are discussed. Experiments conducted in other neuronal and non-neuronal systems are drawn on extensively to discuss putative functions of ACh and nAChRs. Data from other systems may provide insight into the functions of ACh and nAChRs in auditory processing. The mismatch of presynaptic and postsynaptic markers and novel endogenous agonists of nAChRs are discussed in the context of non-classical interneuronal communication. The molecular mechanism that may underlie the many functions of ACh and its agonists is the regulation of intracellular calcium through nAChRs. The possible reorganization that may take place in the auditory system by the exposure to nicotine during critical developmental periods is also briefly considered.

From ligand design to therapeutic efficacy: the challenge for nicotinic receptor research

S-Nicotine, the principal psychoactive constituent of Nicotiana tabacum, underpins addiction to tobacco smoking. Although tobacco consumption is a leading cause of death worldwide, nicotine itself is also proposed to have potential therapeutic benefits for a diverse range of conditions. Nicotine interacts with its cognate receptors in the central nervous system to exert a predominantly modulatory influence, making neuronal nicotinic receptors attractive therapeutic targets. Here, we focus on three natural products as lead compounds for drug discovery programs, nicotine, epibatidine and cytisine, and consider the aims and limitations that shape these drug discovery endeavors.

The alpha7 nicotinic acetylcholine receptor: molecular modelling, electrostatics, and energetics

The structure of a homopentameric alpha7 nicotinic acetylcholine receptor is modelled by combining structural information from two sources: the X-ray structure of a water soluble acetylcholine binding protein from Lymnea stagnalis, and the electron microscopy derived structure of the transmembrane domain of the Torpedo nicotinic receptor. The alpha7 nicotinic receptor model is generated by simultaneously optimising: (i) chain connectivity, (ii) avoidance of stereochemically unfavourable contacts, and (iii) contact between the beta1-beta2 and M2-M3 loops that have been suggested to be involved in transmission of conformational change between the extracellular and transmembrane domains. A Gaussian network model was used to predict patterns of residue mobility in the alpha7 model. The results of these calculations suggested a flexibility gradient along the transmembrane domain, with the extracellular end of the domain more flexible that the intracellular end. Poisson-Boltzmann (PB) energy calculations and atomistic (molecular dynamics) simulations were used to estimate the free energy profile of a Na+ ion as a function of position along the axis of the pore-lining M2 helix bundle of the transmembrane domain. Both types of calculation suggested a significant energy barrier to exist in the centre of the (closed) pore, consistent with a "hydrophobic gating" model. Estimations of the PB energy profile as a function of ionic strength suggest a role of the extracellular domain in determining the cation selectivity of the alpha7 nicotinic receptor. These studies illustrate how molecular models of members of the nicotinic receptor superfamily of channels may be used to study structure-function relationships.

Nicotine-induced switch in the nicotinic cholinergic mechanisms of facilitation of long-term potentiation induction

Nicotine facilitates the induction of long-term potentiation (LTP) in the hippocampal CA1 region. The present study reveals the potential mechanisms underlying this effect of nicotine. Timed ACh-mediated activation of alpha7 nicotinic acetylcholine receptors (nAChRs) on pyramidal cells is known to promote LTP induction. Nicotine could suppress this timing-dependent mechanism by desensitizing nAChRs. Timed ACh-mediated activation of alpha7 nAChRs on feedforward interneurons can prevent LTP induction by inhibiting pyramidal cells. Nicotine diminished this ACh-mediated inhibition by desensitizing alpha7 nAChRs, thereby reducing the inhibitory influence on pyramidal cells. In addition to these desensitizing effects, nicotine activated presynaptic non-alpha7 nAChRs on feedforward interneurons to decrease the evoked release of gamma-aminobutyric acid (GABA) onto pyramidal cells. Furthermore, nicotine increased the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in pyramidal cells, and concomitantly caused a reduction in the size of responses to focal GABA application onto the dendrites of pyramidal cells, suggesting that the nicotine-induced increase in interneuronal activity leads ultimately to a use-dependent depression of evoked IPSCs in pyramidal cells. These nicotine-induced suppressions of inhibition of pyramidal cells were accompanied by enhanced N-methyl-D-aspartate (NMDA) responses in pyramidal cells. Thus, our results suggest that nicotine promotes the induction of LTP by diminishing inhibitory influences on NMDA responses while suppressing the ACh-mediated mechanisms. These ACh-independent mechanisms probably contribute to the nicotine-induced cognitive enhancement observed in the presence of cholinergic deficits, such as those in Alzheimer's disease patients.

Mice Deficient in CHRNA7, a Subunit of the Nicotinic Acetylcholine Receptor, Produce Sperm with Impaired Motility1

In this study we investigate the role of the CHRNA7 subunit (also known as the alpha7 subunit) of the nicotinic acetylcholine receptor in mouse sperm function. We confirm by reverse-transcription-polymerase chain reaction the expression in adult mouse testis of Chrna7 mRNA and demonstrate the subunit's presence in mouse sperm by immunoblot. Alpha-bungarotoxin binds a range of nicotinic acetylcholine receptor subunits, including the CHRNA7 subunit. Localization studies using a fluorescent alpha-bungarotoxin-tetramethyl-rhodamine conjugate revealed specific binding sites on the midpiece of mouse sperm with fainter alpha-bungarotoxin binding on the remainder of the flagellum. Mice engineered with a double-null disruption of the Chrna7 gene displayed only faint fluorescence on the midpiece, suggesting that the CHRNA7 contributed the majority of the observed alpha-bungarotoxin binding sites. The location of alpha-bungarotoxin binding suggested that nicotinic acetylcholine receptors may play an ionotropic role in sperm motility. Sperm from Chrna7(-/-) mice display no difference in number, morphology, viability or spontaneous acrosome reaction rate compared with Chrna7(+/+) sperm. Studies using computer-assisted sperm analysis indicate the motility of Chrna7(-/-) sperm is significantly impaired. This impairment is characterized by significantly reduced swimming velocities, failure to maintain vigorous swimming, and lower levels of hyperactivated swimming patterns in Chrna7(-/-) sperm compared with Chrna7(+/+) sperm. This is the first genetic evidence that sperm nicotinic acetylcholine receptors are important for maintenance of normal sperm motility.

"Optical patch-clamping": single-channel recording by imaging Ca2+ flux through individual muscle acetylcholine receptor channels

We describe an optical technique using total internal reflection fluorescence (TIRF) microscopy to obtain simultaneous and independent recordings from numerous ion channels via imaging of single-channel Ca²⁺ flux. Muscle nicotinic acetylcholine (ACh) receptors made up of αβγδ subunits were expressed in Xenopus oocytes, and single channel Ca²⁺ fluorescence transients (SCCaFTs) were imaged using a fast (500 fps) electron-multiplied c.c.d. camera with fluo-4 as the indicator. Consistent with their arising through openings of individual nicotinic channels, SCCaFTs were seen only when a nicotinic agonist was present in the bathing solution, were blocked by curare, and increased in frequency as roughly the second power of [ACh]. Their fluorescence amplitudes varied linearly with membrane potential and extrapolated to zero at about +60 mV. The rise and fall times of fluorescence were as fast as 2 ms, providing a kinetic resolution adequate to characterize channel gating kinetics; which showed mean open times of 7.9 and 15.8 ms when activated, respectively, by ACh or suberyldicholine. Simultaneous records were obtained from >400 channels in the imaging field, and we devised a novel “channel chip” representation to depict the resultant large dataset as a single image. The positions of SCCaFTs remained fixed (<100 nm displacement) over tens of seconds, indicating that the nicotinic receptor/channels are anchored in the oocyte membrane; and the spatial distribution of channels appeared random without evidence of clustering. Our results extend single-channel TIRFM imaging to ligand-gated channels that display only partial permeability to Ca²⁺, and demonstrate an order-of-magnitude improvement in kinetic resolution. We believe that functional single-channel imaging opens a new approach to ion channel study, having particular advantages over patch-clamp recording in that it is massively parallel, and provides high-resolution spatial information that is inaccessible by electrophysiological techniques.

Precise localization of alpha7 nicotinic acetylcholine receptors on glutamatergic axon terminals in the rat ventral tegmental area

Alpha7 neuronal nicotinic acetylcholine receptors (nAChRs) constitute one of the predominant nAChR subtypes in the mammalian brain. Within the ventral tegmental area (VTA), nicotine application, paired with postsynaptic stimulation, contributes to a form of long-term potentiation, an effect attributed to presynaptic alpha7 nAChRs on glutamatergic afferents (Mansvelder and McGehee, 2000). The aim of this study was to examine the precise subcellular distribution of alpha7 nAChRs in the adult rat VTA to establish whether these receptors are indeed present on glutamatergic axon terminals and to determine their relationship with cholinergic afferents. The spatial relationship between alpha7 nAChRs, labeled using the alpha7 nAChR-specific antagonist alpha-bungarotoxin, and the local neurochemical environment was investigated by the application of multiple labeling strategies with antibodies against tyrosine hydroxylase, vesicular glutamate transporters (VGluTs), vesicular acetylcholine transporter, and glial fibrillary acidic protein. alpha7 nAChRs were localized at both somatodendritic and presynaptic loci within the VTA: on subpopulations of dopaminergic and nondopaminergic neurons and glutamatergic and nonglutamatergic terminals. There was no detectable alpha7 nAChR expression within astrocytes in the VTA. Most alpha7 nAChRs were cytoplasmic (82%), and the remainder were associated with the plasma membrane. Most presynaptic receptors (75%) were on glutamatergic axon terminals, with similar levels of alpha-bungarotoxin binding present on both VGluT1- and VGluT2-immunoreactive boutons. Both preembedding and postembedding electron microscopy revealed that presynaptic alpha7 nAChRs are often located at extrasynaptic (27%) and perisynaptic (61%) loci. alpha7 nAChRs were not associated with cholinergic synapses, consistent with their activation by a paracrine mode of acetylcholine or choline delivery.

Ca2+ permeability of nicotinic acetylcholine receptors in rat hippocampal CA1 interneurones

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely expressed in the brain where they are involved in a variety of physiological processes, including cognition and development. The nAChRs are ligand-gated cationic channels, and different subtypes are known to be differentially permeable to Ca2+; the alpha7-containing nAChRs are generally considered to be the most permeable. Ca2+ can activate and regulate a variety of signal transduction cascades, and the influx of Ca2+ through these receptors may have implications for synaptic plasticity. To determine the Ca2+ permeability of the nAChRs in rat hippocampal interneurones in the slice, which contain diverse subtypes of alpha7- and non-alpha7-containing nAChRs, we combined patch-clamp electrophysiology recordings with conventional fura-2 fluorescence imaging techniques. We estimated the relative Ca2+ permeability of the channels by determining the ratio of the increase in [Ca2+]i level (Delta[Ca2+]i) in the soma to the integrated transmembrane current (charge, Q) induced by the activation of the nAChRs, and compared this ratio to the highly Ca2+ permeable NMDA subtype of glutamate receptor channel. In all cells tested, the Delta[Ca2+]i/Q ratio was significantly larger (i.e. more than twice as big) for responses activated by NMDA than for alpha7-containing nAChRs in interneurones; the activation of the non-alpha7 nAChRs did not produce any significant increase in [Ca2+]i. Interestingly, the Ca2+ permeability of native alpha7 nAChRs in PC12 cells was significantly larger than in hippocampal interneurones, and not significantly different from NMDA receptors. Therefore, the alpha7-containing nAChRs in rat hippocampal interneurones are significantly less permeable to Ca2+ than not only NMDA receptors but also alpha7 nAChRs in PC12 cells.

Ca2+ permeability of nicotinic acetylcholine receptors from rat dorsal root ganglion neurones

Ca2+ entry through neuronal nicotinic ACh receptors (nAChRs) modulates many biological processes in nervous tissue. In order to study the functional role of nAChRs in peripheral sensory signalling, we measured their Ca2+ permeability in rat dorsal root ganglion (DRG) neurones, and analysed the effects of nAChR-mediated Ca2+ influx on the function of the vanilloid receptor TRPV1. The fractional Ca2+ current (P(f), i.e. the percentage of current carried by Ca2+ ions) flowing through nAChR channels was measured by Ca2+ imaging fluorescence microscopy in combination with the patch-clamp technique. Functional nAChRs were expressed in a subset of adult DRG neurones (about 24% of the cells), typically with small to medium size as measured by their capacitance (40 +/- 3 pF). In most cells, ACh evoked slowly desensitizing currents, insensitive to methyllycaconitine (MLA, 10 nm), a potent antagonist of homomeric nAChRs. Fast decaying currents, probably mediated by alpha7*-nAChRs (i.e. native alpha7-containing nAChRs), were observed in 15% of ACh-responsive cells, in which slowly decaying currents, mediated by heteromeric nAChRs, were simultaneously present. The nAChRs of adult DRG neurones exhibited a P(f) value of 2.2 +/- 0.6% in the presence of MLA and 1.9 +/- 0.6% (P > 0.1) in the absence of MLA, indicating that homomeric MLA-sensitive nAChRs do not contribute to Ca2+ entry into adult DRG neurones. Conversely, 10% of neonatal DRG neurones showed ACh-evoked currents completely blocked by MLA. In these neurones, nAChRs showed a larger P(f) value (9.5 +/- 1.5%), indicating the expression of bona fide alpha7*-nAChRs. Finally, we report that Ca2+ influx through nAChRs in adult DRG neurones negatively modulated the TRPV1-mediated responses, representing a possible mechanism underlying the analgesic properties of nicotinic agonists on sensory neurones.

Receptor-mediated tobacco toxicity: regulation of gene expression through alpha3beta2 nicotinic receptor in oral epithelial cells

Tobacco is a known cause of oral disease but the mechanism remains elusive. Nicotine (Nic) is a likely culprit of pathobiological effects because it displaces the local cytotransmitter acetylcholine from the nicotinic receptors (nAChRs) expressed by oral keratinocytes (KCs). To gain a mechanistic insight into tobacco-induced morbidity in the oral cavity, we studied effects of exposures to environmental tobacco smoke (ETS) versus equivalent concentration of pure Nic on human and murine KCs. Both ETS and Nic up-regulated expression of cell cycle and apoptosis regulators, differentiation marker filaggrin, and signal transduction factors at both the mRNA and protein levels. These changes could be abolished in cultured human oral KCs transfected with anti-alpha3 small interfering RNA or treated with the alpha3beta2-preferring antagonist alpha-conotoxin MII. Functional inactivation of alpha3-mediated signaling in alpha3-/- mutant KCs prevented most of the ETS/Nic-dependent changes in gene expression. To determine relevance of the in vitro findings to the in vivo situation, we studied gene expression in oral mucosa of neonatal alpha3+/+ and alpha3-/- littermates delivered by heterozygous mice soon after their exposures to ETS or equivalent concentration of pure Nic in drinking water. In addition to reverse transcriptase-polymerase chain reaction and Western blot, the ETS/Nic-dependent alterations in gene expression were also detected by semiquantitative immunofluorescence assay directly in KCs comprising murine oral mucosa. Only wild-type mice consistently developed significant (P < 0.05) changes in the gene expression. These results identified alpha3beta2 nAChR as a major receptor mediating effects of tobacco products on KC gene expression. Real-time polymerase chain reaction demonstrated that in all three model systems the common genes targeted by alpha3beta2-mediated ETS/Nic toxicity were p21, Bcl-2, NF-kappaB, and STAT-1. The expression of the nAChR subunits alpha5 and beta2 and the muscarinic receptor subtypes M(2) and M(3) was also altered. This novel mechanism offers innovative solutions to ameliorate the tobacco-related cell damage and intercede in disease pathways, and may shed light on general mechanisms regulating and driving tobacco-related morbidity in human cells.

Differential regulation of keratinocyte chemokinesis and chemotaxis through distinct nicotinic receptor subtypes

Nicotinergic agents can act as both chemokines and chemoattractants for cell migration. Epidermal keratinocytes both synthesize acetylcholine and use it as a paracrine and autocrine regulator of cell motility. To gain a mechanistic insight into nicotinergic control of keratinocyte motility, we determined types of nicotinic acetylcholine receptors and signaling pathways regulating keratinocyte chemokinesis and chemotaxis, using respective modifications of the agarose gel keratinocyte outgrowth assay. Random migration of keratinocytes was significantly (P<0.05) inhibited by hemicholinum-3, a metabolic inhibitor of acetylcholine synthesis, as well as by the alpha-conotoxins MII and AuIB, preferentially blocking alpha3-containing nicotinic acetylcholine receptors. The use of antisense oligonucleotides specific for nicotinic-acetylcholine-receptor subunits and knockout mice demonstrated pivotal role for the alpha3beta2 channel in mediating acetylcholine-dependent chemokinesis. Signaling pathways downstream of alpha3beta2 included activation of the protein-kinase-C isoform delta and RhoA-dependent events. The nicotinergic chemotaxis of keratinocytes was most pronounced towards the concentration gradient of choline, a potent agonist of alpha7 nicotinic acetylcholine receptor. The alpha7-preferring antagonist alpha-bungarotoxin significantly (P<0.05) diminished keratinocyte chemotaxis, further suggesting a central role for the alpha7 nicotinic acetylcholine receptor. This hypothesis was confirmed in experiments with anti-alpha7 antisense oligonucleotides and alpha7-knockout mice. The signaling pathway mediating alpha7-dependent keratinocyte chemotaxis included intracellular calcium, activation of calcium/calmodulin-dependent protein-kinase II, conventional isoforms of protein-kinase C, phosphatidylinositol-3-kinase and engagement of Rac/Cdc42. Redistribution of alpha7 immunoreactivity to the leading edge of keratinocytes upon exposure to a chemoattractant preceded crescent shape formation and directional migration. Application of high-resolution deconvolution microscopy demonstrated that, on the cell membrane of keratinocytes, the nicotinic acetylcholine receptor subunits localize with the integrin beta1. The obtained results demonstrate for the first time that alpha3 and alpha7 nicotinic acetylcholine receptors regulate keratinocyte chemokinesis and chemotaxis, respectively, and identify signaling pathways mediating these functions, which has clinical implications for wound healing and control of cancer metastases.

Persistent decrease in synaptic efficacy induced by nicotine at Schaffer collateral-CA1 synapses in the immature rat hippocampus

Neuronal nicotinic acetylcholine receptors (nAChRs) are widely distributed within the brain where they contribute to the regulation of higher cognitive functions. The loss of the cholinergic function in Alzheimer's disease patients, along with the well-known memory enhancing effect of nicotine, emphasizes the role of cholinergic signalling in memory functions. The hippocampus, a key structure in learning and memory, is endowed with nAChRs localized at pre- and postsynaptic levels. In previous work on the immature hippocampus we have shown that, at low probability (P) synapses, activation of alpha7 nAChRs by nicotine or by endogenously released acetylcholine persistently enhanced glutamate release and converted 'presynaptically silent' synapses into functional ones. Here we show that in the same preparation, at high P synapses, nicotine induces long-term depression of AMPA- and NMDA-mediated synaptic currents. This effect was mediated by presynaptic alpha7- and beta2-containing receptors and was associated with an increase in the paired pulse ratio and in the coefficient of variation. High P synapses could be converted into low P and vice versa by changing the extracellular Ca2+/Mg2+ ratio. In these conditions nicotine was able to persistently potentiate or depress synaptic responses depending on the initial P-values. A bi-directional control of synaptic plasticity by nicotine would considerably enhance the computational properties of the network during a critical period of postnatal development thus contributing to sculpt the neuronal circuit.

Nicotinic acetylcholine receptors and the regulation of neuronal signalling

Neuronal nicotinic acetylcholine (nACh) receptors in the brain are more commonly associated with modulatory events than mediation of synaptic transmission. nACh receptors have a high permeability for Ca(2+), and Ca(2+) signals are pivotal in shaping nACh receptor-mediated neuromodulatory effects. In this review, we consider the mechanisms through which nACh receptors convert rapid ionic signals into sustained, wide-ranging phenomena. The complex Ca(2+) responses that are generated after activation of nACh receptors can transmit information beyond the initial domain and facilitate the interface with many intracellular processes. These mechanisms underlie the diverse repertoire of neuronal activities of nicotine in the brain, from the enhancement of learning and memory, to addiction and neuroprotection.

Nicotinic cholinergic stimulation promotes survival and reduces motility of cultured rat cerebellar granule cells

Despite many studies on the functional expression of neuronal nicotinic acetylcholine receptors (nAChRs), an exhaustive description of the long-term effects of nicotine (Nic) stimulation in cerebellar granules is still far to be completed. For this reason, we addressed the experiments stimulating cultured cerebellar granule neurons (CGN) with Nic, focusing on the effects on cell motility and survival. Using electrophysiological and Ca(2+)-fluorescence techniques, we found a subset of rat CGN that responded to Nic by inward whole cell currents and by short-delay Ca(2+) transients. These responses were mediated through both homomeric and heteromeric nAChRs, as assessed by their sensitivity to alpha-bungarotoxin (alpha-BTX), dihydro-beta-erythroidine (DHbetaE), methyllicaconitine (MLA) and 5-hydroxyindole (5OH-indole). Once established the expression of alpha-BTX-sensitive and insensitive nAChRs and their ability to trigger Ca(2+) responses in CGN, we aimed at investigating their possible role on cell survival and motility. We demonstrate that Nic stimulation significantly increases the survival of CGN exposed to the apoptosis-promoting low K(+) medium. This anti-apoptotic effect is likely mediated through alpha7* nAChRs since we found that it was mimicked by choline, was insensitive to DHbetaE and was fully inhibited by alpha-BTX. Furthermore, we report that Nic negatively modulates CGN motility, reducing the basal cell movement through a pored membrane by the activation of alpha-BTX-insensitive nAChRs. We conclude that CGN express various types of nAChRs, which are differently involved in regulating Nic-mediated modulation of cell survival and migration, and we suggest potential regulatory roles for cholinergic receptors during cerebellar development.