Alpha and beta nicotinic acetylcholine receptors subunits and synaptophysin in putamen from Parkinson's disease

The role of astrocytic α7 nicotinic acetylcholine receptors in Alzheimer disease

The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.

Nanodelivery of Histamine H3/H4 Receptor Modulators BF-2649 and Clobenpropit with Antibodies to Amyloid Beta Peptide in Combination with Alpha Synuclein Reduces Brain Pathology in Parkinson's Disease

Parkinson's disease (PD) in military personnel engaged in combat operations is likely to develop in their later lives. In order to enhance the quality of lives of PD patients, exploration of novel therapy based on new research strategies is highly warranted. The hallmarks of PD include increased alpha synuclein (ASNC) and phosphorylated tau (p-tau) in the cerebrospinal fluid (CSF) leading to brain pathology. In addition, there are evidences showing increased histaminergic nerve fibers in substantia niagra pars compacta (SNpc), striatum (STr), and caudate putamen (CP) associated with upregulation of histamine H3 receptors and downregulation of H4 receptors in human brain. Previous studies from our group showed that modulation of potent histaminergic H3 receptor inverse agonist BF-2549 or clobenpropit (CLBPT) partial histamine H4 agonist with H3 receptor antagonist induces neuroprotection in PD brain pathology. Recent studies show that PD also enhances amyloid beta peptide (AβP) depositions in brain. Keeping these views in consideration in this review, nanowired delivery of monoclonal antibodies to AβP together with ASNC and H3/H4 modulator drugs on PD brain pathology is discussed based on our own observations. Our investigation shows that TiO₂ nanowired BF-2649 (1 mg/kg, i.p.) or CLBPT (1 mg/kg, i.p.) once daily for 1 week together with nanowired delivery of monoclonal antibodies (mAb) to AβP and ASNC induced superior neuroprotection in PD-induced brain pathology. These observations are the first to show the modulation of histaminergic receptors together with antibodies to AβP and ASNC induces superior neuroprotection in PD. These observations open new avenues for the development of novel drug therapies for clinical strategies in PD.

The α3 and α4 nicotinic acetylcholine receptor (nAChR) subunits in the brainstem medulla of sudden infant death syndrome (SIDS)

SIDS occurs in early infancy and predominantly during a sleep period. Abnormalities in nicotine receptor binding and in the expression of the nicotinic acetylcholine receptor (nAChR) subunits α7 and β2 have been reported in the brainstem of SIDS infants. This study focuses on the α3 and α4 nAChR subunits as α3 is important for early postnatal survival while α4 is crucial for nicotine-elicited antinociception and sleep-wake cycle regulation. Tissue from the rostral medulla of infants who died with a known cause of death (eSUDI, n = 7), and from SIDS classified as SIDS I (n = 8) and SIDS II (n = 27), was immunohistochemically stained for the α3 and α4 nAChR subunits and quantified in 9 nuclei comparing amongst these groups. The association with risk factors of sex, cigarette smoke exposure, upper respiratory tract infection (URTI), prone sleeping and bedsharing was also evaluated. Results showed that only α4 changes (increase) were evident in SIDS, occurring in the hypoglossal and cuneate nuclei of SIDS II infants and the nucleus of the spinal trigeminal tract of SIDS I infants. Amongst the SIDS infants, cigarette smoke exposure was only associated with decreased α4 in cribriform fibre tracts, while sex and bedsharing were associated with increases in α3 in the dorsal motor nucleus of the vagus and solitary nucleus, respectively. Combined, these findings suggest that abnormalities in endogenous acetylcholine synthesis and regulation may underlie the altered α3 and α4 nAChR subunit expressions in the SIDS brainstem medulla since the changes were not related to cigarette smoke exposure.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Human α4β2 nicotinic acetylcholine receptor as a novel target of oligomeric α-synuclein

Cigarette smoking is associated with a decreased incidence of Parkinson disease (PD) through unknown mechanisms. Interestingly, a decrease in the numbers of α4β2 nicotinic acetylcholine receptors (α4β2-nAChRs) in PD patients suggests an α4β2-nAChR-mediated cholinergic deficit in PD. Although oligomeric forms of α-synuclein have been recognized to be toxic and involved in the pathogenesis of PD, their direct effects on nAChR-mediated cholinergic signaling remains undefined. Here, we report for the first time that oligomeric α-synuclein selectively inhibits human α4β2-nAChR-mediated currents in a dose-dependent, non-competitive and use-independent manner. We show that pre-loading cells with guanyl-5'-yl thiophosphate fails to prevent this inhibition, suggesting that the α-synuclein-induced inhibition of α4β2-nAChR function is not mediated by nAChR internalization. By using a pharmacological approach and cultures expressing transfected human nAChRs, we have shown a clear effect of oligomeric α-synuclein on α4β2-nAChRs, but not on α4β4- or α7-nAChRs, suggesting nAChR subunit selectivity of oligomeric α-synuclein-induced inhibition. In addition, by combining the size exclusion chromatography and atomic force microscopy (AFM) analyses, we find that only large (>4 nm) oligomeric α-synuclein aggregates (but not monomeric, small oligomeric or fibrillar α-synuclein aggregates) exhibit the inhibitory effect on human α4β2-nAChRs. Collectively, we have provided direct evidence that α4β2-nAChR is a sensitive target to mediate oligomeric α-synuclein-induced modulation of cholinergic signaling, and our data imply that therapeutic strategies targeted toward α4β2-nAChRs may have potential for developing new treatments for PD.

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.

Cellular prion protein co-localizes with nAChR beta4 subunit in brain and gastrointestinal tract

PrP(C), the cellular isoform of prion protein, is widely expressed in most tissues, including brain, muscle and gastrointestinal tract. Despite its involvement in several bioprocesses, PrP has still no apparent physiological role. During propagation of transmissible spongiform encephalopathies (TSE), prion protein is converted to the pathological isoform, PrP(Sc), in a process believed to be mediated by unknown host factors. The identification of proteins associated with PrP may provide information about both the biology of prions and the pathogenesis of TSE. Thus far, PrP(C) has been shown to interact with synaptic proteins, components of the cytoskeleton and intracellular proteins involved in signalling pathways. Here, we describe the association of PrP with the beta4 subunit of nicotinic acetylcholine receptor (nAChR), as indicated by co-immunoprecipitation assays and double-label immunofluorescence. The interaction between prion protein and native beta4 subunit was further studied by affinity chromatography, using immobilized and refolded recombinant PrP as a bait and brain homogenates from normal individuals. Additionally, the participation of beta4 subunit in the pathogenesis of TSE was studied by in vivo assays. beta4(-/-) and wild-type mice were challenged with the RML (Rocky Mountain Laboratories) infectious agent. Transgenic animals displayed altered incubation times but the deletion of beta4 subunit did not result in a significant change of the incubation period of the disease. Our results suggest that PrP(C) is a member of a multiprotein membrane complex participating in the formation and function of alpha3beta4 nAChR.

Presynaptic nicotinic receptors: a dynamic and diverse cholinergic filter of striatal dopamine neurotransmission

The effects of nicotine on dopamine transmission from mesostriatal dopamine neurons are central to its reinforcing properties. Only recently however, has the influence of presynaptic nicotinic receptors (nAChRs) on dopaminergic axon terminals within striatum begun to be understood. Here, rather than simply enhancing (or inhibiting) dopamine release, nAChRs perform the role of a presynaptic filter, whose influence on dopamine release probability depends on presynaptic activity in dopaminergic as well as cholinergic neurons. Both mesostriatal dopaminergic neurons and striatal cholinergic interneurons play key roles in motivational and sensorimotor processing by the basal ganglia. Moreover, it appears that the striatal influence of dopamine and ACh cannot be fully appreciated without an understanding of their reciprocal interactions. We will review the powerful filtering by nAChRs of striatal dopamine release and discuss its dependence on activity in dopaminergic and cholinergic neurons. We will also review how nicotine, acting via nAChR desensitization, promotes the sensitivity of dopamine synapses to activity. This filtering action might provide a mechanism through which nicotine promotes how burst activity in dopamine neurons facilitates goal-directed behaviour and reinforcement processing. More generally, it indicates that we should not restrict our view of presynaptic nAChRs to simply enhancing neurotransmitter release. We will also summarize current understanding of the forms and functions of the diverse nAChRs purported to exist on dopaminergic axons. A greater understanding of nAChR form and function is imperative to guide the design of ligands with subtype-selective efficacy for improved therapeutic interventions in nicotine addiction as well as Parkinson's disease.

Differential nicotinic regulation of the nigrostriatal and mesolimbic dopaminergic pathways: implications for drug development

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate dopaminergic function. Discovery of their multiplicity has lead to the search for subtype-selective nAChR agonists that might be therapeutically beneficial in diseases linked to brain dopaminergic pathways. The regulation and responses of the nigrostriatal and mesolimbic dopaminergic pathways are often similar, but some differences do exist. The cerebral distribution and characteristics of various nAChR subtypes differ between nigrostriatal and mesolimbic dopaminergic pathways. Comparison of nicotine and epibatidine, two nAChR agonists whose relative affinities for various nAChR subtypes differ, revealed differences in the nAChR-mediated regulation of dopaminergic activation between these dopamine systems. Nicotine preferentially stimulates the mesolimbic pathway, whereas epibatidine's stimulatory effect falls on the nigrostriatal pathway. Thus, it may be possible to stimulate the nigrostriatal pathway with selective nAChR agonists that do not significantly affect the mesolimbic pathway, and thus lack addictive properties. Furthermore, dopamine uptake inhibition revealed a novel inhibitory effect of epibatidine on accumbal dopamine release, which could form a basis for novel antipsychotics that could alleviate the elevated accumbal dopaminergic tone found in schizophrenia during the active psychotic state. Different regulation of nigrostriatal and mesolimbic dopaminergic pathways by nAChRs could be an important basis for developing novel drugs for treatment of Parkinson's disease and schizophrenia.

Partial agonists as therapeutic agents at neuronal nicotinic acetylcholine receptors

Improved understanding of how brain function is altered in neurodegenerative disease states, pain and conditions, such as schizophrenia and attention deficit disorder, has highlighted the role of nicotinic acetylcholine receptors (nAChRs) in these conditions and identified them as promising therapeutic targets. nAChRs are widely expressed throughout the peripheral and central nervous system, and this widespread nature underlines the need for new ligands with different selectivities and pharmacological profiles if we are to avoid the adverse side effects associated with many of the nAChR modulators currently identified. Partial agonists have the unique property of being able to act both as agonists or antagonists depending on the concentration of endogenous neurotransmitter. Moreover, the agonist action of partial agonists has a 'ceiling' effect, giving them a large safety margin and making them an attractive proposition for therapeutic molecules. Partial agonists of nAChRs are currently being developed as a nicotine replacement therapy for smoking cessation and for the treatment of a number of neurological diseases associated with a loss of cholinergic function. This commentary will discuss the pharmacological properties of partial agonists and review recent research developments in the field of partial agonists acting at nicotinic receptors.

Selective nicotinic acetylcholine receptor subunit deficits identified in Alzheimer's disease, Parkinson's disease and dementia with Lewy bodies by immunoprecipitation

Antibodies raised against human alpha2-6 and beta2-4 nicotinic receptor subunits were utilized to fractionate (3)H-epibatidine binding in human temporal cortex and striatum. The predominant receptor subtypes in both regions contained alpha4 and beta2 subunits. In normal cortex, 10% of binding was also associated with alpha2 subunits, whereas in the striatum, contributions by alpha6 (17%) and beta3 (23%) were observed. Minimal binding (< or =5%) was associated with alpha3. In Alzheimer's disease and dementia with Lewy bodies, cortical loss of binding was associated with reductions in alpha4 (50%, P < 0.01) and beta2 (30-38%, P < 0.05). In Parkinson's disease and dementia with Lewy bodies, striatal deficits in alpha6 (91 and 59% respectively, P < 0.01) and beta3 (72 and 75%, P < 0.05) tended to be greater than for alpha4 and beta2 (50-58%, P < 0.05). This study demonstrates distinct combinations of subunits contributing to heteromeric nicotinic receptor binding in the human brain that are area/pathway specific and differentially affected by neurodegeneration.

In vitro evaluation of nicotinic acetylcholine receptors with 2-[18F]F-A85380 in Parkinson's disease

Nicotinic acetylcholine receptors (nAChR) are involved in many physiological functions and appear to be affected in neurodegenerative diseases like Alzheimer's disease and Parkinson's disease (PD). Here, we describe the in vitro evaluation of nAChRs in PD with 2-[18F]F-A85380, a ligand with high affinity to the beta2 nAChR subunit. Autoradiography with 2-[18F]F-A85380 in untreated rat brain corresponded to the known distribution of alpha4beta2 nAChRs with high uptake in the thalamus, moderate uptake in the striatum and cortex and low uptake in the cerebellum (47%, 43% and 19% of the thalamus, respectively). The localization of alpha4beta2 nAChRs in the striatum was investigated in rodents with unilateral lesion of the substantia nigra. 2-[18F]F-A85380 binding was significantly reduced in the striatum ipsilateral to the lesion side (to 64% of the contralateral side), indicating that a fraction of alpha4beta2 nAChRs is located on dopaminergic terminals, whereas another fraction resides on striatal interneurons or cortical afferents. Similarly, in human brain sections of PD patients, 2-[18F]F-A85380 uptake was significantly reduced not only in the caudate and putamen but also in the thalamus (approximately 30% of the binding of control brain in all three regions); within the striatum, nAChRs in the putamen were significantly more severely affected as in the caudate. The observed pattern of alpha4beta2* nAChR loss demonstrates the potential of 2-[18F]F-A85380 for further investigations of this positron emission tomography ligand for in vivo studies of alpha4beta2* nAChRs in PD.

Altered expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in Parkinson disease

Glutamatergic pathways play a key role in the functional organization of neuronal circuits involved in Parkinson disease (PD). Recently, three vesicular glutamate transporters (VGLUT1-3) were identified. VGLUT1 and VGLUT2 are responsible for the uploading of glutamate into synaptic vesicles and are the first specific markers of glutamatergic neurons available. Here, we analyzed the expression of VGLUT1 and VGLUT2 in autopsy tissues of PD patients and matched controls using Western blot and immunoautoradiography. VGLUT1 and VGLUT2 expression was increased in the Parkinsonian putamen by 24% and 29%, respectively (p<0.01). In contrast, only VGLUT1 was dramatically decreased in the prefrontal and temporal cortex of PD patients (approximately 50%, p<0.01 and p<0.001, respectively). These findings demonstrate the existence of profound alterations of glutamatergic transmission in PD, which are likely to contribute to the motor and cognitive impairments associated with the disease, and should thus be taken into account in the treatment of PD.

Pathophysiology of synuclein aggregation in Lewy body disease

We provide an overview of synaptic pathology in dementia with Lewy bodies (DLB) and related neurodegenerative disorders that are characterised by intraneuronal accumulation of alpha-synuclein aggregates. The review addresses the clinico-neuropathological correlates of synaptic pathology in Lewy body disease, and concentrates on: altered alpha-synuclein metabolism, mechanisms leading to alpha-synuclein fibril formation (self-polymerisation, alpha-synuclein mutations and post-translational modifications) and how these influence the axonal transport and synaptic network in ageing and disease process. Understanding the mechanisms leading to intraneuronal alpha-synuclein accumulation are crucial for the development of novel therapies for treatment of Lewy body disease.

Neuronal nicotinic receptors: from structure to pathology

Neuronal nicotinic receptors (NAChRs) form a heterogeneous family of ion channels that are differently expressed in many regions of the central nervous system (CNS) and peripheral nervous system. These different receptor subtypes, which have characteristic pharmacological and biophysical properties, have a pentameric structure consisting of the homomeric or heteromeric combination of 12 different subunits (alpha2-alpha10, beta2-beta4). By responding to the endogenous neurotransmitter acetylcholine, NAChRs contribute to a wide range of brain activities and influence a number of physiological functions. Furthermore, it is becoming evident that the perturbation of cholinergic nicotinic neurotransmission can lead to various diseases involving nAChR dysfunction during development, adulthood and ageing. In recent years, it has been discovered that NAChRs are present in a number of non-neuronal cells where they play a significant functional role and are the pathogenetic targets in several diseases. NAChRs are also the target of natural ligands and toxins including nicotine (Nic), the most widespread drug of abuse. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining their regional and cellular localisation and the molecular basis of their functional diversity mainly in pharmacological and biochemical terms. The recent availability of mice with the genetic ablation of single or double nicotinic subunits or point mutations have shed light on the role of nAChRs in major physiological functions, and we will here discuss recent data relating to their behavioural phenotypes. Finally, the role of NAChRs in disease will be considered in some details.

Subunit Composition of Nicotinic Receptors in Monkey Striatum: Effect of Treatments with 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine or l-DOPA

Nicotinic acetylcholine receptors (nAChRs) represent an important modulator of striatal function both under normal conditions and in pathological states such as Parkinson's disease. Because different nAChR subtypes may have unique functions, immunoprecipitation and ligand binding studies were done to identify their subunit composition. As in the rodent, alpha2, alpha4, alpha6, beta2, and beta3 nAChR subunit immunoreactivity was identified in monkey striatum. However, distinct from the rodent, the present results also revealed the novel presence of alpha3 nAChR subunit-immunoreactivity in this same region, but not that for alpha5 and beta4. Relatively high levels of alpha2 and alpha3 subunits were also identified in monkey cortex, in addition to alpha4 and beta2. Experiments were next done to determine whether striatal subunit expression was changed with nigrostriatal damage. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment decreased alpha6 and beta3 subunit immunoreactivity by approximately 80% in parallel with the dopamine transporter, suggesting that they are predominantly expressed on nigrostriatal dopaminergic projections. In contrast, alpha3, alpha4, and beta2 subunit immunoreactivity was decreased approximately 50%, whereas alpha2 was not changed. These data, together with those from dual immunoprecipitation and radioligand binding studies ([(3)H]cytisine, (125)I-alpha-bungarotoxin, and (125)I-alpha-conotoxin MII) suggest the following: that alpha6beta2beta3, alpha6alpha4beta2beta3, and alpha3beta2* nAChR subtypes are present on dopaminergic terminals and that the alpha4beta2 subtype is localized on both dopaminergic and nondopaminergic neurons, whereas alpha2beta2* and alpha7 receptors are localized on nondopaminergic cells in monkey striatum. Overall, these results suggest that drugs targeting non-alpha7 nicotinic receptors may be useful in the treatment of disorders characterized by nigrostriatal dopaminergic damage, such as Parkinson's disease.

Molecular analysis of nicotinic receptor expression in autism

Autism is a developmental disorder of unknown aetiopathology and lacking any specific pharmacological therapeutic intervention. Neurotransmitters such as serotonin, gamma-aminobutyric acid (GABA) and acetylcholine have been implicated. Abnormalities in nicotinic acetylcholine receptors have been identified including cortical loss of binding to the alpha4/beta2 subtype and increase in cerebellar alpha7 binding. Receptor expression (mRNA) has not so far been systematically examined. This study aims to further explore the role of nicotinic receptors in autism by analysing nicotinic receptor subunit mRNA in conjunction with protein levels and receptor binding in different brain areas. Quantitative RT-PCR for alpha4, alpha7 and beta2 subunit mRNA expression levels; alpha3, alpha4, alpha7 and beta2 subunit protein expression immunochemistry and specific radioligand receptor binding were performed in adult autism and control brain samples from cerebral cortex and cerebellum. Alpha4 and beta2 protein expression and receptor binding density as well as alpha4 mRNA levels were lower in parietal cortex in autism, while alpha7 did not change for any of these parameters. In cerebellum, alpha4 mRNA expression was increased, whereas subunit protein and receptor levels were decreased. Alpha7 receptor binding in cerebellum was increased alongside non-significant elevations in mRNA and protein expression levels. No significant changes were found for beta2 in cerebellum. The data obtained, using complementary measures of receptor expression, indicate that reduced gene expression of the alpha4beta2 nicotinic receptor in the cerebral cortex is a major feature of the neurochemical pathology of autism, whilst post-transcriptional abnormalities of both this and the alpha7 subtype are apparent in the cerebellum. The findings point to dendritic and/or synaptic nicotinic receptor abnormalities that may relate to disruptions in cerebral circuitry development.

Loss of alpha-conotoxinMII- and A85380-sensitive nicotinic receptors in Parkinson's disease striatum

Multiple nicotinic receptors are present in rodent and monkey striatum, with a selective localization of alpha-conotoxinMII-sensitive sites in the striatum and preferential declines in their numbers after nigrostriatal damage. Here we report the presence of 125I-alpha-conotoxinMII and alpha-conotoxinMII-sensitive 125I-epibatidine nicotinic receptors in human control and Parkinson's disease striatum. 125I-alpha-ConotoxinMII bound to control striatum with the characteristics of a nicotinic receptor ligand although the number of sites was approximately fivefold lower than in rodent and monkey. Competition analyses of alpha-conotoxinMII with 125I-epibatidine showed that toxin-sensitive sites comprised approximately 15% of nicotinic receptors in human striatum. In Parkinson's disease caudate, there was a approximately 50% decline in 125I-alpha-conotoxinMII sites with a similar decline in the dopamine transporter. In putamen, there were substantially greater losses of the dopamine transporter (80-90%) but only 50-60% decreases in 125I-alpha-conotoxinMII sites with corresponding declines in alpha-conotoxinMII-sensitive 125I-epibatidine sites, 125I-epibatidine (multiple) sites and 125I-A85380 (beta2-containing) nicotinic receptors. The greater loss of the transporter compared with nicotinic sites suggests that only a subpopulation of nicotinic receptors is located pre-synaptically on striatal dopaminergic neurons in man. Correlation analyses between changes in nicotinic receptors and the dopamine transporter in Parkinson's disease striatum suggest that alpha-conotoxinMII-sensitive 125I-epibatidine sites (low-affinity sites), 125I-A85380 and 125I-epibatidine sites are localized in part to dopaminergic terminals. In summary, these results show that alpha-conotoxinMII-sensitive sites are present in human striatum and that there are high- and low-affinity subtypes which are both decreased in Parkinson's disease.

Assembly of alpha4beta2 nicotinic acetylcholine receptors assessed with functional fluorescently labeled subunits: effects of localization, trafficking, and nicotine-induced upregulation in clonal mammalian cells and in cultured midbrain neurons

Fura-2 recording of Ca2+ influx was used to show that incubation in 1 microm nicotine (2-6 d) upregulates several pharmacological components of acetylcholine (ACh) responses in ventral midbrain cultures, including a MLA-resistant, DHbetaE-sensitive component that presumably corresponds to alpha4beta2 receptors. To study changes in alpha4beta2 receptor levels and assembly during this upregulation, we incorporated yellow and cyan fluorescent proteins (YFPs and CFPs) into the alpha4 or beta2 M3-M4 intracellular loops, and these subunits were coexpressed in human embryonic kidney (HEK) 293T cells and cultured ventral midbrain neurons. The fluorescent receptors resembled wild-type receptors in maximal responses to ACh, dose-response relations, ACh-induced Ca2+ influx, and somatic and dendritic distribution. Transfected midbrain neurons that were exposed to nicotine (1 d) displayed greater levels of fluorescent alpha4 and beta2 nicotinic ACh receptor (nAChR) subunits. As expected from the hetero-multimeric nature of alpha4beta2 receptors, coexpression of the alpha4-YFP and beta2-CFP subunits resulted in robust fluorescence resonance energy transfer (FRET), with a FRET efficiency of 22%. In midbrain neurons, dendritic alpha4beta2 nAChRs displayed greater FRET than receptors inside the soma, and in HEK293T cells, a similar increase was noted for receptors that were translocated to the surface during PKC stimulation. When cultured transfected midbrain neurons were incubated in 1 microm nicotine, there was increased FRET in the cell body, denoting increased assembly of alpha4beta2 receptors. Thus, changes in alpha4beta2 receptor assembly play a role in the regulation of alpha4beta2 levels and responses in both clonal cell lines and midbrain neurons, and the regulation may result from Ca2+-stimulated pathways.

Dementia rating and nicotinic receptor expression in the prefrontal cortex in schizophrenia

BACKGROUND

The etiology of dementia that occurs in patients with schizophrenia is not well understood. Nicotinic acetylcholine receptors have been implicated in cognitive function, and deficits in these receptors have been reported in schizophrenia.

METHODS

The present study investigates possible associations of nicotinic receptor subunit expression in the dorsal lateral prefrontal cortex, an area known to be affected in schizophrenia, and dementia rating.

RESULTS

alpha7 immunoreactivity was reduced by 20% to 28% and [(3)H]epibatidine binding was increased twofold in groups of patients with schizophrenia compared to normal control subjects matched for age, postmortem delay, and low levels of brain nicotine and cotinine. In contrast, no significant differences in alpha4, alpha3, or beta2 immunoreactivity or alpha7 messenger RNA expression were observed in schizophrenia patients compared with control subject values. Clinical dementia ratings in patients with schizophrenia were correlated with neither [(3)H]epibatidine binding nor nicotinic receptor subunit expression.

CONCLUSIONS

These data indicate no relationship between the trend for reduced neocortical alpha7 subunit protein expression in schizophrenia and dementia. Further investigations are required to establish whether the reduction in alpha7 protein in the dorsal lateral prefrontal cortex is associated with clinical features other than dementia in schizophrenia.

Differential nicotinic acetylcholine receptor subunit expression in the human hippocampus

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels composed of alpha and beta subunits with specific structural, functional and pharmacological properties. In this study the distribution of alpha3, alpha4, alpha7, beta2 and beta4 nAChR subunits in the human hippocampus was investigated using immunohistochemistry. Most pyramidal neurons, pre-alpha cells of the entorhinal cortex and dentate granule cells were immunoreactive for all subunits. Small islands of alpha7 immunoreactive cells were present in the outer presubiculum. alpha4 and beta2, and alpha3, alpha4 and beta2 immunoreactive fibre tracts were present in the stratum radiatum and subiculum, respectively, suggesting nAChRs may play a role in modulating inputs to the hippocampus via Schaffer collaterals and along the perforant pathway. Some astrocytes were immunoreactive for alpha3, alpha7 and beta4 subunits. Immunoreactivity to all subunits was noted in association with blood vessels. These results indicate the involvement of multiple nAChR subtypes in the modulation of both neuronal and non-neuronal functions in the human hippocampus.

Nicotinic receptors in the putamen of patients with dementia with Lewy bodies and Parkinson's disease: relation to changes in alpha-synuclein expression

A reduction in nicotinic receptor (nAChR) binding has previously been observed in putamen in Parkinson's disease (PD) and dementia with Lewy bodies (DLB). The present study demonstrates no concommitant reduction in the expression of alpha2-alpha7, beta2 and beta3 nAChR subunit proteins. Alphasynuclein, which can interfere with membrane protein function and is a key constituent of PD and DLB pathology, was increased (insoluble fraction) in both disorders, although nAChR binding loss did not correlate with alpha-synuclein expression within patient groups. The results point to a possible abnormality of striatal nicotinic receptor assembly in PD and DLB.

Spreading depression-induced preconditioning in the mouse cortex: differential changes in the protein expression of ionotropic nicotinic acetylcholine and glutamate receptors

Preconditioning of the cerebral cortex was induced in mice by repeated cortical spreading depression (CSD), and the major ionotropic glutamate (GluRs) and nicotinic acetylcholine receptor (nAChRs) subunits were compared by quantitative immunoblotting between sham- and preconditioned cortex, 24 h after treatment. A 30% reduction in alpha-amino-3-hydroxy-5-methyl-4-iso- xazolepropionate (AMPA) GluR1 and 2 subunit immunoreactivities was observed in the preconditioned cortex (p < 0.03), but there was no significant change in the NMDA receptor subunits, NR1, NR2A and NR2B. A 12-15-fold increase in alpha7 nAChR subunit expression following in vivo CSD (p < 0.001) was by far the most remarkable change associated with preconditioning. In contrast, the alpha4 nAChR subunit was not altered. These data point to the alpha7 nAChR as a potential new target for neuroprotection because preconditioning increases consistently the tolerance of the brain to acute insults such as ischaemia. These data complement recent studies implicating alpha7 nAChR overexpression in the amelioration of chronic neuropathologies, notably Alzheimer's disease (AD).

Selective changes in the levels of nicotinic acetylcholine receptor protein and of corresponding mRNA species in the brains of patients with Parkinson's disease

Reductions in the number of neuronal nicotinic acetylcholine receptors (nAChRs) have been shown to occur in connection with Parkinson's disease (PD), but it is still unclear which subtype of this receptor is affected. In the present study we examined various nAChR subtypes employing ligand binding, as well as levels of subunit protein and mRNA in the brains of PD patients and age-matched controls. Binding of [3H]epibatidine and levels of alpha3 mRNA in the caudate nucleus and temporal cortex, but not in the hippocampus were significantly decreased in the PD brain. The level of the alpha3 protein subunit was significantly reduced in all these brain regions but there was no change in the level of alpha4. The level of the beta2 protein subunit in the temporal cortex and hippocampus and the beta2 mRNA in the temporal cortex was lowered. Both the levels of the alpha7 subunit protein and [125I]alpha-bungarotoxin binding were significantly increased in the temporal cortex of PD patients whereas the alpha7 mRNA level was unchanged. These findings reveal selective losses of the alpha3- and beta2-containing nAChRs and an increase in the alpha7 nAChRs that might be related to the pathogenesis of PD.

Immunohistochemical localisation of nicotinic acetylcholine receptor subunits in human cerebellum

Neuronal nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily composed of alpha and beta subunits with specific structural, functional and pharmacological properties. In this study we have used immunohistochemistry to investigate the presence of nicotinic acetylcholine receptor subunits in human cerebellum. Tissue was obtained at autopsy from eight adult individuals (aged 36-56 years). Histological sections were prepared from formalin-fixed paraffin-embedded material. alpha 3, alpha 4, alpha 6, alpha 7, beta 2, and beta 4 subunits were present in this brain area associated with both neuronal and non-neuronal cell types. Most Purkinje cells were immunoreactive for all the above subunits, but most strongly for alpha 4 and alpha 7. A proportion of granule cell somata were immunoreactive for all subunits except alpha 3. Punctate immunoreactivity in Purkinje cell and granule cell layers was evident with antibodies against alpha 3, alpha 4, alpha 6, and alpha 7 in parallel with synaptophysin immunoreactivity, suggesting the presence of these subunits on nerve terminals in the human cerebellum. All subunits were present in the dentate nucleus associated with neurones and cell processes. Strong immunoreactivity of neuropil in both the molecular and granule cell layers and within the dentate nucleus was noted with alpha 4, alpha 7 and beta 4 subunits. Astrocytes and astrocytic cell processes appeared to be immunoreactive for alpha 7 and cell processes observed in white matter, also possibly astrocytic, were immunoreactive for beta2. Immunoreactivity to all subunits was noted in association with blood vessels. We suggest that nicotinic acetylcholine receptor subunits may be involved in the modulation of cerebellar activity. Further investigations are warranted to evaluate the participation of nicotinic acetylcholine receptors in cerebellar pathology associated with both developmental and age-related disorders.

Nicotine and nicotinic receptors; relevance to Parkinson's disease

The development of nicotinic agonists for therapy in neurodegenerative disorders such as Parkinson's disease is an area currently receiving considerable attention. The rationale for such work stems from findings that reveal a loss of nicotinic receptors in Parkinson's disease brains. These results, coupled with reports that nicotine treatment relieves some of the symptoms of this disorder, provides support for the contention that nicotine and/or nicotinic agonists may be beneficial for acute symptomatic treatment. Moreover, the observation that there is a decreased incidence of Parkinson's disease with tobacco use, possibly due to the nicotine in tobacco products, may imply that such drugs are useful for long-term neuroprotection. However, there are multiple nicotinic receptor populations in the brain with different functional properties. Identification of the subtypes involved in nigrostriatal dopaminergic activity is therefore critical for the rational use of selective therapeutic agents for symptomatic treatment and/or neuroprotection. Accumulating evidence, both in rodents and nonhuman primates now indicate that alpha6* nicotinic receptors are present on nigrostriatal dopaminergic neurons, and furthermore, that receptors containing this subunit may be most vulnerable to nigrostriatal damage, at least in nonhuman primates. These data suggest that nicotinic receptor ligands directed to alpha6* nicotinic receptors might be particularly relevant for Parkinson's disease therapeutics.

Genomic organization and promoter analysis of the human nicotinic acetylcholine receptor alpha6 subunit (CHNRA6) gene: Alu and other elements direct transcriptional repression

Nicotinic acetylcholine receptors (nAChRs) form ligand-gated ion channels involved in fast synaptic transmission. Recently mutations in nAChR genes have been reported in nocturnal frontal lobe epilepsy. We performed molecular analysis on the human neuronal nAChR alpha6 subunit (CHRNA6) gene, a member of nAChR gene family, to understand its role in disease. Genomic analysis revealed that the gene consisted of six exons with an estimated size of 16 kb. We mapped the CHRNA6 gene to chromosome 8p11.21-11.22 by fluorescence in situ hybridization, the same putative region responsible for adolescent-onset idiopathic generalized epilepsy. Examination of the 5'-regulatory region failed to identify either a TATA box or GC-rich sequences, but did highlight tandem Alu sequences, located between 910 and 370 bp upstream from a potential cap site. Analyses of transcriptional activity, performed using nested deletions of the 5'-upstream region, showed that the downstream Alu repeat and another element(s) in the promoter region, function as negative regulators. Further analyses of the tandem Alu repeats, examined by fusing them to a different ion channel gene promoter, confirmed their role as transcriptional repressors regardless of their orientation and copy number. These data may explain the limited expression of the CHRNA6 gene in the brain.

Loss of nicotinic receptors in monkey striatum after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment is due to a decline in alpha-conotoxin MII sites

Nicotinic acetylcholine receptors (nAChRs) in the basal ganglia are a potential target for new therapeutics for Parkinson's disease. As an approach to detect expression of nAChRs in monkeys, we used 125I-epibatidine, an agonist at nAChRs containing alpha2 to alpha6 subunits. 125I-Epibatidine binding sites are expressed throughout the control monkey brain, including the basal ganglia. The alpha3/alpha6-selective antagonist alpha-conotoxin MII maximally inhibited 50% of binding in the caudate-putamen and had no effect on 125I-epibatidine binding in the frontal cortex or thalamus. In contrast, inhibition experiments with nicotine, cytisine, and 3-(2(S)-azetidinylmethoxy)pyridine-2HCl (A85380) showed a complete block of 125I-epibatidine binding in all regions investigated and did not discriminate between the alpha-conotoxin MII-sensitive and -insensitive populations in the striatum. To assess the effects of nigrostriatal damage, monkeys were rendered parkinsonian with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Animals with moderate striatal damage (dopamine transporter levels approximately 30% of control) had a 40 to 50% decrease in 125I-epibatidine binding. Inhibition studies showed that the decrease in epibatidine binding was due to loss of alpha-conotoxin MII-sensitive nAChRs. Monkeys with severe nigrostriatal damage (dopamine transporter levels < or = 5% of control) exhibited a 55 to 60% decrease in 125I-epibatidine binding, which seemed to be due to a complete loss of alpha-conotoxin MII nAChRs and a partial loss of other nAChR subtypes. These results show that nAChRs expressed in the primate striatum have similar affinities for nicotine, cytisine, and A85380, that alpha-conotoxin MII discriminates between nAChR populations in the caudate and putamen, and that alpha-conotoxin MII-sensitive nAChRs are selectively decreased after MPTP-induced nigrostriatal damage.

Nicotinic receptors in human brain: topography and pathology

Brain nicotinic acetylcholine receptors (nAChR) are a class of ligand-gated channels composed of alpha and beta subunits with specific structural, functional and pharmacological properties. They participate in the physiological and behavioural effects of acetylcholine and mediate responses to nicotine. They are associated with numerous transmitter systems and their expression is altered during development and ageing as well as in diseases such as autism, schizophrenia, Alzheimer's disease, Parkinson's disease and Lewy body dementia. Nicotinic receptors containing a number of different subunits are highly expressed during early human development. Disorders believed to be associated with abnormal brain maturation involve deficits in both alpha4beta2, in the case of autism, and alpha7 possibly in addition to alpha4beta2 nAChRs in the case of schizophrenia. In ageing and age-related neurodegenerative disorders nAChR deficits are predominantly associated with alpha4-containing receptors, although some studies also indicate the involvement of alpha3 and alpha7 subunits. Whilst ageing appears to be associated with reductions in subunit mRNA as well as protein expression, in Alzheimer's disease only protein loss is apparent. Nicotinic therapy may be of benefit in a number of neurological conditions, however studies evaluating further both the distribution of specific subunit involvement and the correlation of nAChR deficits with clinical symptoms are required to inform therapeutic strategy.