GABA acts as a ligand chaperone in the early secretory pathway to promote cell surface expression of GABAA receptors

GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in brain. The fast inhibitory effect of GABA is mediated through the GABA(A) receptor, a postsynaptic ligand-gated chloride channel. We propose that GABA can act as a ligand chaperone in the early secretory pathway to facilitate GABA(A) receptor cell surface expression. Forty-two hours of GABA treatment increased the surface expression of recombinant receptors expressed in HEK 293 cells, an effect accompanied by an increase in GABA-gated chloride currents. In time-course experiments, a 1h GABA exposure, followed by a 5h incubation in GABA-free medium, was sufficient to increase receptor surface expression. A shorter GABA exposure could be used in HEK 293 cells stably transfected with the GABA transporter GAT-1. In rGAT-1HEK 293 cells, the GABA effect was blocked by the GAT-1 inhibitor NO-711, indicating that GABA was acting intracellularly. The effect of GABA was prevented by brefeldin A (BFA), an inhibitor of early secretory pathway trafficking. Coexpression of GABA(A) receptors with the GABA synthetic enzyme glutamic acid decarboxylase 67 (GAD67) also resulted in an increase in receptor surface levels. GABA treatment failed to promote the surface expression of GABA binding site mutant receptors, which themselves were poorly expressed at the surface. Consistent with an intracellular action of GABA, we show that GABA does not act by stabilizing surface receptors. Furthermore, GABA treatment rescued the surface expression of a receptor construct that was retained within the secretory pathway. Lastly, the lipophilic competitive antagonist (+)bicuculline promoted receptor surface expression, including the rescue of a secretory pathway-retained receptor. Our results indicate that a neurotransmitter can act as a ligand chaperone in the early secretory pathway to regulate the surface expression of its receptor. This effect appears to rely on binding site occupancy, rather than agonist-induced structural changes, since chaperoning is observed with both an agonist and a competitive antagonist.

Prefrontal beta2 subunit-containing and alpha7 nicotinic acetylcholine receptors differentially control glutamatergic and cholinergic signaling

One-second-long increases in prefrontal cholinergic activity ("transients") were demonstrated previously to be necessary for the incorporation of cues into ongoing cognitive processes ("cue detection"). Nicotine and, more robustly, selective agonists at alpha4beta2* nicotinic acetylcholine receptors (nAChRs) enhance cue detection and attentional performance by augmenting prefrontal cholinergic activity. The present experiments determined the role of beta2-containing and alpha7 nAChRs in the generation of prefrontal cholinergic and glutamatergic transients in vivo. Transients were evoked by nicotine, the alpha4beta2* nAChR agonist ABT-089 [2-methyl-3-(2-(S)-pyrrolindinylmethoxy) pyridine dihydrochloride], or the alpha7 nAChR agonist A-582941 [2-methyl-5-(6-phenyl-pyridazin-3-yl)-octahydro-pyrrolo[3,4-c]pyrrole]. Transients were recorded in mice lacking beta2 or alpha7 nAChRs and in rats after removal of thalamic glutamatergic or midbrain dopaminergic inputs to prefrontal cortex. The main results indicate that stimulation of alpha4beta2* nAChRs evokes glutamate release and that the presence of thalamic afferents is necessary for the generation of cholinergic transients. ABT-089-evoked transients were completely abolished in mice lacking beta2* nAChRs. The amplitude, but not the decay rate, of nicotine-evoked transients was reduced by beta2* knock-out. Conversely, in mice lacking the alpha7 nAChR, the decay rate, but not the amplitude, of nicotine-evoked cholinergic and glutamatergic transients was attenuated. Substantiating the role of alpha7 nAChR in controlling the duration of release events, stimulation of alpha7 nAChR produced cholinergic transients that lasted 10- to 15-fold longer than those evoked by nicotine. alpha7 nAChR-evoked cholinergic transients are mediated in part by dopaminergic activity. Prefrontal alpha4beta2* nAChRs play a key role in evoking and facilitating the transient glutamatergic-cholinergic interactions that are necessary for cue detection and attentional performance.

Chronic nicotine selectively enhances alpha4beta2* nicotinic acetylcholine receptors in the nigrostriatal dopamine pathway

These electrophysiological experiments, in slices and intact animals, study the effects of in vivo chronic exposure to nicotine on functional alpha4beta2* nAChRs in the nigrostriatal dopaminergic (DA) pathway. Recordings were made in wild-type and alpha4 nicotinic acetylcholine receptor (nAChR) subunit knock-out mice. Chronic nicotine enhanced methyllycaconitine citrate hydrate-resistant, dihydro-beta-erythroidine hydrobromide-sensitive nicotinic currents elicited by 3-1000 mum ACh in GABAergic neurons of the substantia nigra pars reticulata (SNr), but not in DA neurons of the substantia nigra pars compacta (SNc). This enhancement leads to higher firing rates of SNr GABAergic neurons and consequently to increased GABAergic inhibition of the SNc DA neurons. In the dorsal striatum, functional alpha4* nAChRs were not found on the neuronal somata; however, nicotine acts via alpha4beta2* nAChRs in the DA terminals to modulate glutamate release onto the medium spiny neurons. Chronic nicotine also increased the number and/or function of these alpha4beta2* nAChRs. These data suggest that in nigrostriatal DA pathway, chronic nicotine enhancement of alpha4beta2* nAChRs displays selectivity in cell type and in nAChR subtype as well as in cellular compartment. These selective events augment inhibition of SNc DA neurons by SNr GABAergic neurons and also temper the release of glutamate in the dorsal striatum. The effects may reduce the risk of excitotoxicity in SNc DA neurons and may also counteract the increased effectiveness of corticostriatal glutamatergic inputs during degeneration of the DA system. These processes may contribute to the inverse correlation between tobacco use and Parkinson's disease.

Alterations in cholinergic sensitivity of respiratory neurons induced by pre-natal nicotine: a mechanism for respiratory dysfunction in neonatal mice

Nicotine may link cigarette smoking during pregnancy with sudden infant death syndrome (SIDS). Pre-natal nicotine leads to diminished ventilatory responses to hypercarbia and reduced central chemoreception in mice at post-natal days 0-3. We studied how pre-natal nicotine exposure changes the cholinergic contribution to central respiratory chemoreception in neonatal isolated brainstem-spinal cord and slice preparations. Osmotic minipumps, implanted subcutaneously into 5-7 days pregnant mice, delivered saline or nicotine ditartrate 60 mg kg(-1) d(-1) for up to 28 days. In control preparations, acidification of the superfusion medium from pH 7.4 to 7.3 increased the frequency and reduced the amplitude of fictive respiration. In nicotine-exposed neonatal mice, the reduction in amplitude induced by acidification was reduced. In control preparations, atropine suppressed respiratory responses to acidification, while hexamethonium did not. By contrast, in nicotine-exposed preparations, hexamethonium blocked chemosensory responses but atropine did not. Our results indicate that pre-natal nicotine exposure switches cholinergic mechanisms of central chemosensory responses from muscarinic receptors to nicotinic receptors. Modification of the cholinergic contribution to central chemoreception may produce respiratory dysfunctions, as suggested by receptor-binding studies in victims of SIDS.

Differential contribution of genetic variation in multiple brain nicotinic cholinergic receptors to nicotine dependence: recent progress and emerging open questions

Nicotine dependence (ND), a major public health challenge, is a complex, multifactorial behavior, in which both genetic and environmental factors have a role. Brain nicotinic acetylcholine receptor (nAChR)-encoding genes are among the most prominent candidate genes studied in the context of ND, because of their biological relevance as binding sites for nicotine. Until recently, most research on the role of nAChRs in ND has focused on two of these genes (encoding the alpha4- and beta2-subunits) and not much attention has been paid to the possible contribution of the other nine brain nAChR subunit genes (alpha2-alpha3, alpha5-alpha7, alpha9-alpha10, beta3-beta4) to the pathophysiology and genetics of ND. This situation has changed dramatically in the last 2 years during which intensive research had addressed the issue, mainly from the genetics perspective, and has shown the importance of the CHRNA5-CHRNA3-CHRNB4 and CHRNA6-CHRNB3 loci in ND-related phenotypes. In this review, we highlight recent findings regarding the contribution of non-alpha4/beta2-subunit containing nAChRs to ND, based on several lines of evidence: (1) human genetics studies (including linkage analysis, candidate-gene association studies and whole-genome association studies) of several ND-related phenotypes; (2) differential pharmacological and biochemical properties of receptors containing these subunits; (3) evidence from genetically manipulated mice; and (4) the contribution of nAChR genes to ND-related personality traits and neurocognitive profiles. Combining neurobiological genetic and behavioral perspectives, we suggest that genetic susceptibility to ND is not linked to one or two specific nAChR subtype genes but to several. In particular, the alpha3, alpha5-6 and beta3-4 nAChR subunit-encoding genes may play a much more pivotal role in the neurobiology and genetics of ND than was appreciated earlier. At the functional level, variants in these subunit genes (most likely regulatory) may have independent as well as interactive contributions to the ND phenotype spectrum. We address methodological challenges in the field, highlight open questions and suggest possible pathways for future research.

The expression of CYP2D22, an ortholog of human CYP2D6, in mouse striatum and its modulation in 1-methyl 4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson's disease phenotype and nicotine-mediated neuroprotection

The main contributory factors of Parkinson's disease (PD) are aging, genetic factors, and environmental exposure to pesticides and heavy metals. CYP2D22, a mouse ortholog of human CYP2D6, is expected to participate in a chemically induced PD phenotype due to its structural resemblance with CYP2D6. Despite its expected participation in PD, its expression in the nigrostriatal tissues and modulation by the chemicals that induce PD or offer neuroprotection have not yet been investigated. The present study was undertaken to investigate CYP2D22 expression in mouse striatum and to assess its involvement in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD phenotype and nicotine-mediated neuroprotection. The animals were treated intraperitoneally daily with nicotine (1 mg/kg) for 8 weeks, followed by MPTP (20 mg/kg) + nicotine (1 mg/kg) for 2-4 weeks and vice versa, along with respective controls. In some sets of experiments, the animals were treated with ketoconazole (300 mg/kg), a CYP3AH/CYP2D22 inhibitor, along with nicotine and/or MPTP. Tyrosine hydroxylase (TH)-immunoreactivity in the substantia nigra, the expression of nicotinic acetylcholine receptors (nAChRs) alpha6 and alpha4, dopamine content, and 1-methyl-4-phenylpyridinium ion (MPP(+)) level in the striatum were measured to confirm the MPTP-induced PD phenotype and nicotine-mediated neuroprotection. CYP2D22 and nAChRs expressions were measured in the striatum by RT-PCR/western blotting and dopamine level; CYP2D22 catalytic activity and MPP(+) content were determined by high-performance liquid chromatography (HPLC). MPTP increased dopaminergic neuronal degeneration and the striatal MPP(+) level and reduced striatal dopamine content; it attenuated expression/activity of CYP2D22 and nAChRs that were significantly restored in nicotine-treated animals. Ketoconazole reduced the nicotine-mediated increase in CYP2D22 expression and activity, dopamine content, and TH-immunoreactivity. The results indicate the expression of CYP2D22 in mouse striatum and its possible role in the MPTP-induced PD phenotype and nicotine-mediated neuroprotection.

Smoking, nicotine and neuropsychiatric disorders

Tobacco smoking is an extremely addictive and harmful form of nicotine (NIC) consumption, but unfortunately also the most prevalent. Although disproportionately high frequencies of smoking and its health consequences among psychiatric patients are widely known, the neurobiological background of this epidemiological association is still obscure. The diverse neuroactive effects of NIC and some other major tobacco smoke constituents in the central nervous system may underlie this association. This present paper summarizes the pharmacology of NIC and its receptors (nAChR) based on a systematic review of the literature. The role of the brain's reward system(s) in NIC addiction and the results of functional and structural neuroimaging studies on smoking-related states and behaviors (i.e. dependence, craving, withdrawal) are also discussed. In addition, the epidemiological, neurobiological, and genetic aspects of smoking in several specific neuropsychiatric disorders are reviewed and the clinical relevance of smoking in these disease states addressed.

Nicotine-induced upregulation of nicotinic receptors: underlying mechanisms and relevance to nicotine addiction

A major hurdle in defining the molecular biology of nicotine addiction has been characterizing the different nicotinic acetylcholine receptor (nAChR) subtypes in the brain and how nicotine alters their function. Mounting evidence suggests that the addictive effects of nicotine, like other drugs of abuse, occur through interactions with its receptors in the mesolimbic dopamine system, particularly ventral tegmental area (VTA) neurons, where nicotinic receptors act to modulate the release of dopamine. The molecular identity of the nicotinic receptors responsible for drug seeking behavior, their cellular and subcellular location and the mechanisms by which these receptors initiate and maintain addiction are poorly defined. In this commentary, we review how nicotinic acetylcholine receptors (nAChRs) are upregulated by nicotine exposure, the potential posttranslational events that appear to cause it and how upregulation is linked to nicotine addiction.

Does smoking reduce the risk of Parkinson's disease through stimulation of the ubiquitin-proteasome system?

Parkinson's disease is a neurodegenerative condition characterized by tremor, rigidity, bradykinesia, and postural instability. Smoking is an inverse risk factor for Parkinson's disease, although the mechanism for this apparent neuroprotection is not definitively established. Smoking consistently upregulates nicotinic acetylcholine receptor levels in various brain regions known to be involved in Parkinson's disease. The ubiquitin-proteasome system--the system that tags and removes unwanted, misfolded, or damaged proteins from cells--regulates nicotinic receptor levels. The ubiquitin-proteasome system has also been implicated in Parkinson's disease, with aberrant activity identified in both sporadic and familial forms of the disease. The involvement of the ubiquitin-proteasome system in nicotinic receptor regulation and Parkinson's disease pathology suggests a link between the two, which forms the basis of the present hypothesis. Specifically, this paper considers the hypothesis that smoking reduces the risk of Parkinson's disease through the upregulation of nicotinic cholinergic receptors in key brain regions involved in Parkinson's disease. This receptor upregulation is hypothesized to increase activity of the ubiquitin-proteasome system, which is believed to prevent neurodegeneration caused by the accumulation of misfolded or damaged proteins or other consequences of inadequate protein sequestration and/or degradation. This hypothesis is supported by evidence documenting the upregulation of nicotinic receptors in the brains of smokers, neuroprotective effects of nicotine, reduced activity of the ubiquitin-proteasome in Parkinson's disease, and increased activity of the ubiquitin-proteasome system in animals exposed to chronic nicotine. Additional research is needed to test several predictions of the hypothesis, including increased activity of the ubiquitin-proteasome system in key brain regions of smokers.

Structural and functional diversity of native brain neuronal nicotinic receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand-gated ion channels present in the central and peripheral nervous systems, that are permeable to mono- and divalent cations. They share a common basic structure but their pharmacological and functional properties arise from the wide range of different subunit combinations making up distinctive subtypes. nAChRs are involved in many physiological functions in the central and peripheral nervous systems, and are the targets of the widely used drug of abuse nicotine. In addition to tobacco dependence, changes in their number and/or function are associated with neuropsychiatric disorders, ranging from epilepsy to dementia. Although some of the neural circuits involved in the acute and chronic effects of nicotine have been identified, much less is known about which native nAChR subtypes are involved in specific physiological functions and pathophysiological conditions. We briefly review some recent findings concerning the structure and function of native nAChRs, focusing on the subtypes identified in the mesostriatal and habenulo-interpeduncular pathways, two systems involved in nicotine reinforcement and withdrawal. We also discuss recent findings concerning the effect of chronic nicotine on the expression of native subtypes.

Molecular and cellular mechanisms of action of nicotine in the CNS

Nicotine achieves its psychopharmacological effects by interacting with nicotinic acetylcholine receptors (nAChRs) in the brain. There are numerous subtypes of nAChR that differ in their properties, including their sensitivity to nicotine, permeability to calcium and propensity to desensitise. The nAChRs are differentially localised to different brain regions and are found on presynaptic terminals as well as in somatodendritic regions of neurones. Through their permeability to cations, these ion channel proteins can influence both neuronal excitability and cell signalling mechanisms, and these various responses can contribute to the development or maintenance of dependence. However, many questions and uncertainties remain in our understanding of these events and their relevance to tobacco addiction. In this chapter, we briefly overview the fundamental characteristics of nAChRs that are germane to nicotine's effects and then consider the cellular responses to acute and chronic nicotine, with particular emphasis on dopamine systems because they have been the most widely studied in the context of nicotine dependence. Where appropriate, methodological aspects are critically reviewed.

Cellular events in nicotine addiction

In the 25 years since the observation that chronic exposure to nicotine could regulate the number and function of high affinity nicotine binding sites in the brain there has been a major effort to link alterations in nicotinic acetylcholine receptors (nAChRs) to nicotine-induced behaviors that drive the addiction to tobacco products. Here we review the proposed roles of various nAChR subtypes in the addiction process, with emphasis on how they are regulated by nicotine and the implications for understanding the cellular neurobiology of addiction to this drug.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

The subunit arrangement and assembly of ionotropic receptors

Ionotropic receptors mediate rapid communication between neurons. These receptors are oligomers and are usually assembled from multiple subunit types. Receptors built from different subunit combinations have distinct functional properties, such as single-channel conductances, rates of desensitization and sensitivities to activators and inactivators; they can also have different intracellular locations. Methods are now available for determining not only the subunit stoichiometry but also the subunit arrangement within ionotropic receptors. This information will inform experiments designed to understand the molecular basis of receptor assembly and function. It will also permit the modelling of potential ligand-binding sites at the interfaces between the subunits and should lead to a more rational approach to drug development.

Assembly and trafficking of nicotinic acetylcholine receptors (Review)

Nicotinic acetylcholine receptors (nAChRs) are members of an extensive super-family of neurotransmitter-gated ion channels. In humans, nAChRs are expressed within the nervous system and at the neuromuscular junction and are important targets for pharmaceutical drug discovery. They are also the site of action for neuroactive pesticides in insects and other invertebrates. Nicotinic receptors are complex pentameric transmembrane proteins which are assembled from a large family of subunits; seventeen nAChR subunits (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) have been identified in vertebrate species. This review will discuss nAChR subunit diversity and factors influencing receptor assembly and trafficking.

Long-term nicotine treatment differentially regulates striatal alpha6alpha4beta2* and alpha6(nonalpha4)beta2* nAChR expression and function

Nicotine treatment has long been associated with alterations in alpha4beta2(*) nicotinic acetylcholine receptor (nAChR) expression that modify dopaminergic function. However, the influence of long-term nicotine treatment on the alpha6beta2(*) nAChR, a subtype specifically localized on dopaminergic neurons, is less clear. Here we used voltammetry, as well as receptor binding studies, to identify the effects of nicotine on striatal alpha6beta2(*) nAChR function and expression. Long-term nicotine treatment via drinking water enhanced nonburst and burst endogenous dopamine release from rat striatal slices. In control animals, alpha6beta2(*) nAChR blockade with alpha-conotoxin MII (alpha-CtxMII) decreased release with nonburst stimulation but not with burst firing. These data in control animals suggest that varying stimulus frequencies differentially regulate alpha6beta2(*) nAChR-evoked dopamine release. In contrast, in nicotine-treated rats, alpha6beta2(*) nAChR blockade elicited a similar pattern of dopamine release with nonburst and burst firing. To elucidate the alpha6beta2(*) nAChR subtypes altered with long-term nicotine treatment, we used the novel alpha-CtxMII analog E11A in combination with alpha4 nAChR knockout mice. (125)I-alpha-CtxMII competition studies in striatum of knockout mice showed that nicotine treatment decreased the alpha6alpha4beta2(*) subtype but increased the alpha6(nonalpha4)beta2(*) nAChR population. These data indicate that alpha6beta2(*) nAChR-evoked dopamine release in nicotine-treated rats is mediated by the alpha6(nonalpha4)beta2(*) nAChR subtype and suggest that the alpha6alpha4beta2(*) nAChR and/or alpha4beta2(*) nAChR contribute to the differential effect of higher frequency stimulation on dopamine release under control conditions. Thus, alpha6beta2(*) nAChR subtypes may represent important targets for smoking cessation therapies and neurological disorders involving these receptors such as Parkinson's disease.