Acetylcholine receptors: too many channels, too few functions

Homeostasis as a fundamental principle for a coherent theory of brains

'Brains' may be considered to be computation engines, with neurons and synapses analogized to electronic components wired into networks that process information, learn and evolve. Alternatively, 'brains' are cognitive systems, which contain elements of intentionality, purposefulness and creativity that do not fit comfortably into a brain-as-computer metaphor. I address the question of how we may think most constructively about brains in their various forms-solid, liquid or fluid-and whether there is a coherent theory that unites them all. In this essay, I explore cognitive systems in the context of new understanding of life's distinctive nature, in particular the core concept of homeostasis, and how this new understanding lays a sound conceptual foundation for an expansive theory of brains. This article is part of the theme issue 'Liquid brains, solid brains: How distributed cognitive architectures process information'.

An Update on GABAρ Receptors

The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.

The effective opening of nicotinic acetylcholine receptors with single agonist binding sites

We have identified a means by which agonist-evoked responses of nicotinic receptors can be conditionally eliminated. Modification of α7L119C mutants by the sulfhydryl reagent 2-aminoethyl methanethiosulfonate (MTSEA) reduces responses to acetylcholine (ACh) by more than 97%, whereas corresponding mutations in muscle-type receptors produce effects that depend on the specific subunits mutated and ACh concentration. We coexpressed α7L119C subunits with pseudo wild-type α7C116S subunits, as well as ACh-insensitive α7Y188F subunits with wild-type α7 subunits in Xenopus laevis oocytes using varying ratios of cRNA. When mutant α7 cRNA was coinjected at a 5:1 ratio with wild-type cRNA, net charge responses to 300 µM ACh were retained by α7L119C-containing mutants after MTSEA modification and by the ACh-insensitive Y188F-containing mutants, even though the expected number of ACh-sensitive wild-type binding sites would on average be fewer than two per receptor. Responses of muscle-type receptors with one MTSEA-sensitive subunit were reduced at low ACh concentrations, but much less of an effect was observed when ACh concentrations were high (1 mM), indicating that saturation of a single binding site with agonist can evoke strong activation of nicotinic ACh receptors. Single-channel patch clamp analysis revealed that the burst durations of fetal wild-type and α1β1γδL121C receptors were equivalent until the α1β1γδL121C mutants were exposed to MTSEA, after which the majority (81%) of bursts were brief (≤2 ms). The longest duration events of the receptors modified at only one binding site were similar to the long bursts of native receptors traditionally associated with the activation of receptors with two sites containing bound agonists.

Agonist-activated ion channels

This paper looks at ion channels as an example of the pharmacologist's stock in trade, the action of an agonist on a receptor to produce a response. Looked at in this way, ion channels have been helpful because they are still the only system which is simple enough for quantitative investigation of transduction mechanisms. A short history is given of attempts to elucidate what happens between the time when agonist first binds, and the time when the channel opens.

Acetylcholine

This article traces the development of knowledge about the physiology and pharmacology of acetylcholine and its receptors between 1930 and 2005, with emphasis on contributions by members of the British Pharmacological Society, and by other British pharmacologists and physiologists.

Nicotinic-acetylcholine receptors are functionally coupled to the nitric oxide/cGMP-pathway in insect neurons

In addition to their ionotropic role, neuronal nicotinic acetylcholine receptors (nAChRs) can influence second messenger levels, transmitter release and gene transcription. In this study, we show that nAChRs in an insect CNS control cGMP levels by coupling to NO production. In conditions that inhibit spiking, nicotine induced cGMP synthesis. This increase in cGMP was blocked by nicotinic antagonists, and by inhibitors of both nitric oxide synthase and soluble guanylyl cyclase. The nicotinic-evoked increase in cGMP was localized to specific NO-sensitive neurons in the CNS, several of which are identified motoneurons. Because NO production requires Ca2+, we investigated the effect of nicotinic stimulation on [Ca2+]i in cultured neurons. We found that activation of nAChRs increased [Ca2+]i, which was blocked by nAChR antagonists. Nicotinic stimulation of neurons in the isolated CNS in low-Na+, also evoked increases in [Ca2+]i independent of fast changes in voltage. In addition, approximately 10% of the nicotinic-evoked [Ca2+]i increase in cultured neurons persisted when voltage-gated Ca2+ channels were blocked by Ni2+. Under the same conditions, nicotinic stimulation of cGMP in the CNS was unaffected. These combined results suggest that nicotinic stimulation is coupled to NOS potentially by directly gating Ca2+.

Nicotine addiction: the possible role of functional upregulation

Upregulation of binding to nicotinic acetylcholine receptors (nAChRs) is observed in the brains of both smokers and animals chronically exposed to nicotine, although whether this in vivo change is accompanied by an increase in receptor function is unknown. In vitro recordings indicate that alpha4beta2- and alpha7-subtypes of nAChRs, which are the most abundant subtypes in the brain, are functionally upregulated following prolonged exposure to nicotine. The possible consequences of functional upregulation for nicotine addiction are discussed. Moreover, we propose a new paradigm that describes the unusual behavior of these neuronal nAChRs and helps to explain the effects of nicotine in the CNS and the diffuse effects of ACh.

The chaperone protein 14-3-3eta interacts with the nicotinic acetylcholine receptor alpha 4 subunit. Evidence for a dynamic role in subunit stabilization

By using the large cytoplasmic domain of the nicotinic acetylcholine receptor (AChR) alpha4 subunit as a bait in the yeast two-hybrid system, we isolated the first cytosolic protein, 14-3-3eta, known to interact directly with neuronal AChRs. 14-3-3eta is a member of a family of proteins that function as regulatory or chaperone/ scaffolding/adaptor proteins. 14-3-3eta interacted with the recombinant alpha4 subunit alone in tsA 201 cells following activation of cAMP-dependent protein kinase by forskolin. The interaction of 14-3-3eta with recombinant alpha4 subunits was abolished when serine 441 of the alpha4 subunit was mutated to alanine (alpha4(S441A)). The surface levels of recombinant wild-type alpha4beta2 AChRs were approximately 2-fold higher than those of mutant alpha4(S441A)beta2 AChRs. The interaction significantly increased the steady state levels of the alpha4 subunit and alpha4beta2 AChRs but not that of the mutant alpha4(S441A) subunit or mutant alpha4(S441A)beta2 AChRs. The EC50 values for activation by acetylcholine were not significantly different for alpha4beta2 AChRs and alpha4(S441A)beta2 AChRs coexpressed with 14-3-3eta in oocytes following treatment with forskolin. 14-3-3 coimmunopurified with native alpha4 AChRs from brain. These results support a role for 14-3-3 in dynamically regulating the expression levels of alpha4beta2 AChRs through its interaction with the alpha4 subunit.

Nicotine and its interaction with beta-amyloid protein: a short review

Two features of Alzheimer's disease (AD) are beta-amyloid protein (betaAP) deposition and a severe cholinergic deficit. beta-Amyloid protein is a 39- to 43-amino acid transmembrane fragment of a larger precursor molecule, amyloid precursor protein. It is a major constituent of senile plaque, a neuropathologic hallmark of AD, and has been shown to be neurotoxic in vivo and in vitro. The cholinergic neurotransmission system is seen as the primary target of AD. However, other systems are also found to show functional deficit. An association between cholinergic deficit and betaAP is suggested by a negative correlation between cigarette smoking and AD. Evidence hitherto suggests that betaAP causes neuronal death possibly via apoptosis by disrupting calcium homeostasis, which may involve direct activation or enhancement of ligand-gated or voltage-dependent calcium channels. Selective second messengers such as protein kinases are triggered that signal neuronal death. Nicotine or acetylcholinesterase inhibitors can partially prevent the neurotoxicity of betaAP in vivo and in vitro. However, the exact mechanism by which nicotine provides its protective effects is not fully understood, but clearly there are protective roles for nicotine. Here, some aspects of betaAP neurotoxicity and nicotinic intervention as a protective agent are discussed.

Neuronal nicotinic acetylcholine receptors: from the gene to the disease

The neuronal nicotinic acetylcholine receptors are excitatory ligand-gated channels. Widely expressed throughout the peripheral and central nervous system, their properties depend upon their subunit composition. Furthermore, genetic studies have revealed a high degree of variation at the genomic level and alternative splicing of the mRNAs coding for these integral membrane proteins. In particular, genes coding for alpha4 and alpha7 subunits harbour a high degree of polymorphisms. Although well characterised at their molecular and functional level, the role of these receptors in the central nervous system remains obscure. Despite accumulating evidence for the participation of nicotinic receptors in disorders of the central nervous system including nicotinic addiction, Parkinson's disease, Alzheimer's disease and Tourette's syndrome, the exact role of these receptors is still speculative. Because most of these phenotypes are complex and genetically heterogeneous, the investigation is difficult. However, in the past few years, significant progress has been made in understanding the contribution of nicotinic acetylcholine receptors to the origin of epilepsies and schizophrenia. By concentrating on the latest results gained for these diseases, we discuss in this review the possible relationships between neuronal nicotinic receptors and neurological and psychiatric disorders.

Modulatory role of presynaptic nicotinic receptors in synaptic and non-synaptic chemical communication in the central nervous system

Neuronal nicotinic acetylcholine receptors (nAChRs) belong to a family of ligand-gated channels closely related to but distinct from the muscle nAChRs. Recent progress in neurochemical and pharmacological methods supports the hypothesis of presynaptically located nAChRs on axon terminals and indicates that the major effect of nAChR is the modulation rather than processing of fast synaptic transmission. Strong neurochemical evidence indicate that the most important function of presynaptic nAChRs in either synaptic or non-synaptic localization is to increase transmitter release initiated by axonal firing, or directly induce Na(+) and Ca(2+) influx followed by a depolarization sufficient to activate local voltage-sensitive Ca(2+) channels, as a result transmitter of vesicular origin will be released. Therefore, it is somewhat expected that nicotine-induced transmitter release of different monoamines including norepinephrine (NE), dopamine (DA), serotonin (5-HT) can be tetrodotoxin (TTX)- and [Ca(2+)](o)-sensitive. However, some of the nAChR agonists at higher concentrations (1, 1-dimethyl-4-phenylpiperazinium (DMPP) and lobeline), besides their effects on presynaptic nAChRs, are able to inhibit the uptake of NE and 5-HT into nerve terminals, thereby their transmitter releasing effects are extended in time and space. The effect on the uptake process is different from classical nicotinic actions, not being sensitive to nAChR antagonism, but can be prevented by selective uptake blockers or reduced temperature. Considering neurochemical, pharmacological and electrophysiological evidence it seems likely that presynaptic nAChRs on monoaminergic fibers are composed of alpha3 or alpha4 subunits in combination with the beta2 subunit. This is supported by the observation that nicotinic agonists have no presynaptic effect on transmitter release in knockout mice lacking the beta2 nAChR subunit gene. The essential brain function lies not only in impulse transmission within a hard-wired neuronal circuitry but also within synaptic and non-synaptic communication subjected to presynaptic modulation. Since the varicose noradrenergic, dopaminergic, serotonergic, glutamatergic and cholinergic axon terminals mainly do not make synaptic contact, but their varicosities are equipped with nAChRs and these non-synaptically localized receptors are of high affinity, it is suggested that nicotine inhaled during smoking might exert its behavioral, psychological, neurological and neuroendocrinological effects via these receptors.

Selective excitation of subtypes of neocortical interneurons by nicotinic receptors

The cellular mechanisms by which neuronal nicotinic cholinergic receptors influence many aspects of physiology and pathology in the neocortex remain primarily unknown. Whole-cell recordings and single-cell reverse transcription (RT)-PCR were combined to analyze the effect of nicotinic receptor agonists on different types of neurons in acute slices of rat neocortex. Nicotinic receptor agonists had no effect on pyramidal neurons and on most types of interneurons, including parvalbumin-expressing fast spiking interneurons and somatostatin-expressing interneurons, but selectively excited a subpopulation of interneurons coexpressing the neuropeptides vasoactive intestinal peptide (VIP) and cholecystokinin. This excitation persisted in the presence of glutamate, GABA, and muscarinic receptor antagonists and in the presence of tetrodotoxin and low extracellular calcium, suggesting that the depolarization was mediated through the direct activation of postsynaptic nicotinic receptors. The responses were blocked by the nicotinic receptor antagonists dihydro-beta-erythroidine and mecamylamine and persisted in the presence of the alpha7 selective nicotinic receptor antagonist methyllycaconitine, suggesting that the involved nicotinic receptors lacked the alpha7 subunit. Single-cell RT-PCR analysis indicated that the majority of the interneurons that responded to nicotinic stimulation coexpressed the alpha4, alpha5, and beta2 nicotinic receptor subunits. Therefore, these results provide a role for non-alpha7 nicotinic receptors in the selective excitation of a subpopulation of neocortical interneurons. Because the neocortical interneurons expressing VIP have been proposed previously to regulate regional cortical blood flow and metabolism, these results also provide a cellular basis for the neuronal regulation of cortical blood flow mediated by acetylcholine.

Single channel properties of human alpha3 AChRs: impact of beta2, beta4 and alpha5 subunits

1. We performed single channel analysis on human alpha3 acetylcholine receptors (AChRs) in Xenopus oocytes and native AChRs from the human neuroblastoma cell line IMR-32. alpha3 AChRs exhibit channel properties that reflect subunit composition. 2. alpha3beta2 AChR open times were 0.71 +/- 0.14 and 3.5 +/- 0.4 ms with a predominant conductance of 26 pS. alpha3beta4 AChRs had open times of 1.4 +/- 0.2 and 6.5 +/- 0.8 ms and a predominant conductance of 31 pS. Burst times were 0.82 +/- 0.12 and 5.3 +/- 0.7 ms for alpha3beta2 and 1.7 +/- 0.1 and 16 +/- 1 ms for alpha3beta4. Desensitization was faster for AChRs with the beta2 subunit than for those with the beta4 subunit. 3. One open time for alpha3alpha5beta2 AChRs (5.5 +/- 0.3 ms) was different from those of alpha3beta2 AChRs. For alpha3alpha5beta4 AChRs, an additional conductance, open time and burst time (36 pS, 22 +/- 3 ms and 43 +/- 4 ms, respectively) were different from those for alpha3beta4 AChRs. 4. alpha3 AChRs were inhibited by hexamethonium or mecamylamine. The rate constants for block of alpha3beta4 by hexamethonium and of alpha3beta2 by mecamylamine were 1.2 x 107 and 4.6 x 107 M-1 s-1, respectively. 5. AChRs from IMR-32 cells had a predominant conductance of 32 pS and open times of 1.5 +/- 0.3 and 9.6 +/- 1.2 ms. These properties were most similar to those of alpha3beta4 AChRs expressed in oocytes. Antibodies revealed that 5 +/- 2 % of IMR-32 alpha3 AChRs contained alpha5 subunits and 6 +/- 2 % contained beta2 subunits. IMR-32 alpha3 AChRs are primarily alpha3beta4 AChRs.

Output signals of the SCN

The suprachiasmatic nucleus (SCN) of the hypothalamus controls circadian rhythmicity in mammals (for reviews, see Refs. 33 and 59). Responses modulated by the SCN are numerous and include rhythms in sleep/wake cycles, locomotor, gnawing and general activity, temperature, ingestive behavior, and rhythms of hormonal and peptide secretions. Though a great deal is known about the neuroanatomical organization of the SCN, many elements of the structure-function relationships remain to be discovered. For example, it is not known which cellular components of the SCN function as driving pacemakers or which output signal(s) of these pacemakers are important for each of its functions. While some signals from pacemaker cells reach target regions by neural efferents, there is also evidence that rhythmic responses can be controlled by diffusible signals. This article reviews output signals from the SCN. The data available suggest that neural efferents are not necessary for the control of locomotor activity rhythms. Evidence that a diffusible signal is sufficient to restore activity rhythms in SCN-lesioned animals is described. Finally, possible physiological mechanisms for diffusible signals are suggested.

A reporter mutation approach shows incorporation of the "orphan" subunit beta3 into a functional nicotinic receptor

We have investigated whether the neuronal nicotinic subunit beta3 can participate in the assembly of functional recombinant receptors. Although beta3 is expressed in several areas of the central nervous system, it does not form functional receptors when expressed heterologously together with an alpha or another beta nicotinic subunit. We inserted into the human beta3 subunit a reporter mutation (V273T), which, if incorporated into a functional receptor, would be expected to increase its agonist sensitivity and maximum response to partial agonists. Expressing the mutant beta3(V273T) in Xenopus oocytes together with both the alpha3 and the beta4 subunits resulted in the predicted changes in the properties of the resulting nicotinic receptor when compared with those of alpha3 beta4 receptors. This indicated that some of the receptors incorporated the mutant beta3 subunit, as part of a "triplet" alpha3 beta4 beta3 receptor. The proportion of triplet receptors was dependent on the ratios of the alpha3:beta4:beta3 cRNA injected. We conclude that, like the related alpha5 subunit, the beta3 subunit can form functional receptors only if expressed together with both alpha and beta subunits.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

N-methyl-D-aspartate receptor antagonism in the ventral tegmental area diminishes the systemic nicotine-induced dopamine release in the nucleus accumbens

Systemic nicotine enhances burst firing of dopamine neurons in the ventral tegmental area and dopamine release in the nucleus accumbens, mainly via stimulation of nicotinic acetylcholine receptors in the ventral tegmental area. Given that both the neuronal activity of mesolimbic dopamine neurons and terminal dopamine release are regulated by excitatory amino acid inputs to the ventral tegmental area and that nicotine facilitates glutamatergic transmission in brain, we investigated the putative role of ionotropic glutamate receptors within the ventral tegmental area for the effects of nicotine on dopamine release in the nucleus accumbens using microdialysis, with one probe implanted in the ventral tegmental area for drug application and another in the ipsilateral nucleus accumbens for measuring dopamine, in awake rats. Systemic nicotine (0.5 mg/kg, s.c.) and infusion of nicotine (1.0 mM) into the ventral tegmental area increased dopamine output in the nucleus accumbens. Intrategmental infusion of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (0.1 mM) or N-methyl-D-aspartate (0.3 mM) increased accumbal dopamine release; these effects were antagonized by concomitant infusion of a selective antagonist at N-methyl-D-aspartate receptors, 2-amino-5-phosphonopentanoic acid (0.3 mM), and non-N-methyl-D-aspartate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (0.3 mM), respectively. Infusion of either antagonist (0.3 or 1.0 mM) into the ventral tegmental area did not affect basal dopamine levels, whereas infusion of 2-amino-5-phosphonopentanoic acid, but not 6-cyano-7-nitroquinoxaline-2,3-dione, starting 40 min before nicotine injection dose-dependently attenuated the nicotine-induced increase in accumbal dopamine release. Concurrent intrategmental infusion of 2-amino-5-phosphonopentanoic acid and nicotine decreased nicotine-induced dopamine release in the nucleus accumbens. These results indicate that the stimulatory action of nicotine on the mesolimbic dopamine system is to a considerable extent mediated via stimulation of N-methyl-D-aspartate receptors within the ventral tegmental area.

Proline-induced potentiation of glutamate transmission

The amino acid proline has long been suspected to serve as a modulator of synaptic transmission in the mammalian brain, but no such function has been identified. The selective expression of high affinity proline transport by a subset of glutamate pathways suggested that proline might play a role in synaptic transmission at these sites. This idea was tested with use of one such pathway, the Schaffer collateral-commissural projection to CA1 pyramidal cells of the rat hippocampus. Proline enhanced the initial slope of the field EPSP without affecting axonal excitability or the magnitude of paired-pulse facilitation. Proline-induced potentiation far outlasted the period of proline application and required the activation of NMDA receptors. Proline enhanced Schaffer collateral-commissural synaptic transmission even when the connections between areas CA1 and CA3 had been interrupted. Potentiation was observed with a proline concentration normally present in human CSF (3 microM). A concentration typical of CSF in persons with the genetic disorder hyperprolinemia type II (30 microM) produced a somewhat greater effect. Occlusion experiments suggested that proline-induced potentiation and tetanus-induced long-term potentiation utilize largely distinct transduction mechanisms. Proline-induced potentiation could be blocked by a prior high frequency stimulus, whether or not the stimulus evoked long-term potentiation. These results suggest that endogenous extracellular proline regulates the basal function of some glutamate synapses by maintaining them in a partially potentiated state. They may also facilitate understanding of the seizures and/or mental retardation associated with genetic disorders of proline metabolism.

Nicotinic receptor distribution in the human thalamus: autoradiographical localization of [3H]nicotine and [125I] alpha-bungarotoxin binding

The thalamus plays a major role in relaying and transforming information that is relayed to the cortex and in turn modulates cortical outputs. The reticular nucleus projects to the other thalamic nuclei, modulating and integrating their activity. The distribution of high affinity nicotine and alpha-bungarotoxin (alpha BTX) receptors in the human thalamus has been investigated by radioligand autoradiography in post mortem human tissue. [3H]nicotine binding in the human thalamus was high in most thalamic nuclei, especially in the lateral dorsal, the medial geniculate, lateral geniculate and anterior nuclei. The distribution of [125I] alpha BTX binding was quite distinct from [3H]nicotine binding. [125I] alpha BTX binding was generally lower (< 0.26-11.62 fmol/mg protein compared with 6.68-36.17 fmol/mg protein for nicotine binding) and concentrated in the reticular nucleus, with discrete groups of cells displaying higher binding in the latter. These results indicate differences between the distribution of nicotinic receptors in humans and those previously reported in mice and monkeys. Changes in high affinity nicotine and alpha BTX receptors in the thalamus may contribute to symptoms observed in neuropathological conditions associated with disorders of perception and movement such as Dementia with Lewy Bodies, Alzheimer's Disease and Schizophrenia.

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

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

Presynaptic nicotinic ACh receptors

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

Nicotine for the treatment of Tourette's syndrome

Recent evidence has demonstrated that nicotine may obtund the symptoms of Tourette's syndrome (TS). TS is a neuropsychiatric disorder characterized by motor and vocal tics, obsessions and compulsions, and frequently with impulsivity, distractibility, and visual-motor deficits. While neuroleptics, such as haloperidol, are most effective for treatment of the motor and vocal tics of TS, these medications have many side effects. In this article, we review the evidence, consistent with findings in animals, that administration of nicotine (either 2 mg nicotine gum or 7 mg transdermal nicotine patch) potentiates the therapeutic properties of neuroleptics in treating TS patients and that a single patch may be effective for a variable number of days. These findings suggest that transdermal nicotine could serve as an effective adjunct to neuroleptic therapy for TS.

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

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

Cerebellar choline acetyltransferase positive mossy fibres and their granule and unipolar brush cell targets: a model for central cholinergic nicotinic neurotransmission

A subset of cerebellar mossy fibres is rich in choline acetyltransferase, the rate-limiting enzyme for the synthesis of acetylcholine. These choline acetyltransferase-positive mossy fibres are concentrated in the vestibulocerebellum and originate predominantly from the medial vestibular nucleus. The granular layer of the vestibulocerebellum is also enriched in unipolar brush cells, an unusual type of small neuron that form giant synapses with mossy fibres. In this immunocytochemical light and electron microscopic study, we explored whether choline acetyltransferase-positive mossy fibres innervate unipolar brush cells of the rat cerebellum. We utilized monoclonal antibodies to rat choline acetyltransferase of proven specificity, and immunoperoxidase procedures with 3,3'-diaminobenzidine tetrahydrochloride as the chromogen. A high density of choline acetyltransferase-positive fibres occurred in the nodulus and ventral uvula, where they showed an uneven, zonal distribution. Immunostained mossy fibre rosettes contained high densities of round synaptic vesicles and mitochondria. They formed asymmetric synaptic junctions with dendritic profiles of both granule cells and unipolar brush cells. The synaptic contacts between choline acetyltransferase-immunoreactive mossy fibres and unipolar brush cells were very extensive, and did not differ from synapses of choline acetyltransferase-negative mossy fibres with unipolar brush cells. Analysis of a total area of 1.25 mm2 of the nodulus from three rats revealed that 14.2% of choline acetyltransferase-immunoreactive mossy fibre rosettes formed synapses with unipolar brush cells profiles. Choline acetyltransferase-positive rosettes accounted for 21.7% of the rosettes forming synapses with unipolar brush cells. Thus, the present data demonstrate that unipolar brush cells are innervated by a heterogeneous population of mossy fibres, and that some unipolar brush cells receive cholinergic synaptic input from the medial vestibular nucleus. The ultrastructure of these synapses is compatible with the possibility that choline acetyltransferase-positive mossy fibres co-release acetylcholine and glutamate. As the granular layer of the vestibulocerebellum contains nicotinic binding sites, the choline acetyltransferase-positive mossy fibres may be a model for studying nicotinic neurotransmission in the CNS.

The pore domain of the nicotinic acetylcholine receptor: molecular modeling, pore dimensions, and electrostatics

The pore domain of the nicotinic acetylcholine receptor has been modeled as a bundle of five kinked M2 helices. Models were generated via molecular dynamics simulations incorporating restraints derived from 9-A resolution cryoelectron microscopy data (Unwin, 1993; 1995), and from mutagenesis data that identify channel-lining side chains. Thus, these models conform to current experimental data but will require revision as higher resolution data become available. Models of the open and closed states of a homopentameric alpha 7 pore are compared. The minimum radius of the closed-state model is less than 2 A; the minimum radius of the open-state models is approximately 6 A. It is suggested that the presence of "bound" water molecules within the pore may reduce the effective minimum radii below these values by up to approximately 3 A. Poisson-Boltzmann calculations are used to obtain a first approximation to the potential energy of a monovalent cation as it moves along the pore axis. The differences in electrostatic potential energy profiles between the open-state models of alpha 7 and of a mutant of alpha 7 are consistent with the experimentally observed change in ion selectivity from cationic to anionic. Models of the open state of the heteropentameric Torpedo nicotinic acetylcholine receptor pore domain are also described. Relatively small differences in pore radius and electrostatic potential energy profiles are seen when the Torpedo and alpha 7 models are compared.

Effects of inhalational general anaesthetics on native glycine receptors in rat medullary neurones and recombinant glycine receptors in Xenopus oocytes

1. Glycine responses were studied under voltage clamp in Xenopus oocytes injected with cDNA encoding mammalian glycine receptor subunits and in rat medullary neurones. Bath application of glycine gave strychnine-sensitive currents which reversed close to the expected equilibrium potentials for chloride ions. The peak currents for the receptors expressed in oocytes fitted a Hill equation with EC50 = 215 +/- 5 microM and Hill coefficient nH = 1.70 +/- 0.05 (means +/- s.e. means). The peak currents from the receptors in medullary neurones fitted a Hill equation with EC50 = 30 +/- 1 microM and Hill coefficient nH = 1.76 +/- 0.08. The current-voltage relationship for the receptors expressed in oocytes showed strong outward rectification (with Vrev = -21 +/- 2 mV), while that for the glycine responses from the medullary neurones in symmetrical Cl- was linear (with Vrev = 3.2 +/- 0.6 mV). 2. Inhalational general anaesthetics, at concentrations close to their human minimum alveolar concentrations (MACs), potentiated responses to low concentrations of glycine. The potentiation observed with the recombinant receptors (between 60-22%) was approximately twice that found with the medullary neurones (between 40-80%). For both the recombinant receptors and the receptors in medullary neurones, the degree of potentiation increased in the order of methoxyflurane approximately sevoflurane < halothane approximately isoflurane approximately enflurane. There was no significant difference between the potentiations observed for the two optical isomers of isoflurane. 3. For both the recombinant and native receptors, isoflurane potentiated the currents in a dose-dependent manner at low concentrations of glycine, although at high glycine concentrations the anaesthetic had no significant effect on the glycine-activated responses. The major effect of isoflurane was to cause a parallel leftward shift in the glycine concentration-response curves. The glycine EC50 concentration for the recombinant receptors decreased from a control value of 215 +/- 5 microM to 84 +/- 7 microM glycine at 610 microM isoflurane, while that for the medullary neurones decreased from a control value of 30 +/- 1 microM to 18 +/- 2 microM glycine at the same concentration of isoflurane. The potentiation was independent of membrane potential. 4. Isoflurane also potentiated responses to taurine, a partial agonist at the glycine receptor. This was observed for receptors expressed in oocytes at both low and saturating concentrations of taurine. The EC50 concentration decreased from a control value of 1.6 +/- 0.2 to 0.9 +/- 0.1 mM taurine in the presence of 305 microM isoflurane, while the maximum response to taurine increased from 47 +/- 2 to 59 +/- 2% of the maximum response to glycine. 5. Glycine receptors, like other members of the fast ligand-gated receptor superfamily, are sensitive to clinically relevant concentrations of inhalational general anaesthetics. Effects at these receptors may, therefore, play some role in the maintenance of the anaesthetic state.

Nicotine blocks angiotensin II inhibition of LTP in the dentate gyrus

Angiotensin (ANG)-containing axons, terminals, and receptors have been found in the hippocampus. When angiotensin II (ANG II) is administered to the dentate gyrus, long-term potentiation (LTP) induction, in response to medial perforant path stimulation, is inhibited and it can be blocked by losartan, an ANG II AT1 receptor antagonist. ANG II has been shown to mediate impairment of the retention of an inhibitory shock avoidance response and to be involved in ethanol and diazepam inhibition of dentate gyrus LTP, all of which can be blocked by losartan. Nicotine acetylcholine receptors are found in the hippocampus and nicotine is involved in the enhancement of complex and important psychological functions that are mediated by the hippocampus; therefore, the possibility that nicotine prevents the ANG II inhibition of dentate granule cell LTP was examined. Nicotine pretreatment reduced ANG II inhibition of LTP induction in a dose-dependent manner. Mecamylamine blocked the nicotine antagonism of ANG II-induced LTP inhibition and normal LTP occurred, whereas hexamethonium was ineffective in blocking these central effects of nicotine. Nicotine by itself did not affect normal LTP under these conditions. Nicotinic blocking of the ANG II inhibition of a frequency dependent type of synaptic plasticity provides a function for central nicotinic receptors and a possible mechanism of action a) to explain the enhancement of learning and memory by nicotine, b) an explanation for tobacco smoking while drinking alcohol, and c) a possible basis for the excessive use of tobacco in depression and schizophrenia that supports a possible therapeutic use of nicotine in some mental disorders.