Neuronal nicotinic receptors: function,modulation and structure

In the present paper, we discuss the diversity of nicotinic receptors (nAChRs) expressed in hippocampal neurons and their functional properties. Three distinct types of whole-cell currents can be identified in hippocampal neurons by brief application of nicotinic agonists. These currents, which are referred to as types IA, II and III, were distinguished pharmacologically on the basis of their differential sensitivities to various nicotinic agonists and antagonists, and functionally on the basis of their rectification properties and rundown. Most of the hippocampal neurons show type IA current in response to nicotinic agonists, and the single channels that account for these currents in addition to having a very short open time and a high conductance, have a high Ca²⁺permeability and inactivate very fast. Based on the comparison of the properties of the nicotinic currents elicited in hippocampal neurons with those elicited by activation of nAChRs transiently expressed in oocytes, we have concluded that type IA currents may be subserved by α 7-bearing nAChRs, type II currents by α4β2 nAChRs, and type III currents by α3β4 nAChRs. Indeed,in-situhybridization shows that mRNAs coding for α7, α4 and β2 nAChR subunits are present in hippocampal neurons. These findings altogether have given new insights for the studies of neurophysiological processes and neuropathological conditions in which neuronal nAChRs may be implicated.     

Neuronal nicotinic receptors in the human brain

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand gated ion channels which are widely distributed in the human brain. Multiple subtypes of these receptors exist, each with individual pharmacological and functional profiles. They mediate the effects of nicotine, a widely used drug of abuse, are involved in a number of physiological and behavioural processes and are additionally implicated in a number of pathological conditions such as Alzheimer's disease, Parkinson's disease and schizophrenia. The nAChRs have a pentameric structure composed of five membrane spanning subunits, of which nine different types have thus far been identified and cloned. The multiple subunits identified provide the basis for the heterogeneity of structure and function observed in the nAChR subtypes and are responsible for the individual characteristics of each. A substantial amount of information on human nAChR structure and function has come from studies on neuroblastoma cell lines which naturally express nAChRs and from recombinant nAChRs expressed in Xenopus oocytes. In vitro brain nAChR distribution can be mapped with a number of appropriate agonist and antagonist radioligands and subunit distribution may be mapped by in situ hybridization using subunit specific mRNA probes. Receptor distribution in the living human brain can be studied with noninvasive imaging techniques such as PET and SPECT, with a significant reduction in nAChRs in the brains of Alzheimer's patients having been identified with [11C] nicotine in PET studies. Despite the significant body of knowledge now accumulated about nAChRs, much remains to be elucidated. This review will attempt to describe the current knowledge on the nAChR subtypes in the human brain, their functional roles and neuropathological involvement.     

Neuronal nicotinic alpha7 receptors modulate early neutrophil infiltration to sites of skin inflammation

Background  

A major site of initiation of inflammatory responses upon physical perturbation(s) and infection by invading organisms is the skin. Control of responses in this organ is, in part, modulated by the neuronal nicotinic acetylcholine receptor (nAChR) alpha7.     

Neuronal nicotinic acetylcholine receptors and autosomal dominant nocturnal frontal lobe epilepsy: a critical review

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is a distinct human epileptic syndrome. In some families, it is associated with mutations of the alpha4 or the beta2 subunit of the neuronal nicotinic acetylcholine receptor (nAChR). It has been suggested that these mutations are the causative factors responsible for the induction and expression of this syndrome. However, the pathogenic mechanisms leading to ADNFLE are unknown and, in this review, we discuss the following yet unresolved questions concerning the involvement of mutated nAChRs in the phenotypic development of the disorder: (1) why do seizures associated with ADNFLE arise explicitly from the frontal lobe of the neocortex? (2) why do the seizures arise mainly from sleep? (3) why does ADNFLE starts predominantly during childhood? A survey of our current knowledge on neocortical and thalamic cholinergic systems, including their ontogenetic development, leads us to the conclusion that there are, at least at the moment, no convincing answers to these questions. Furthermore, we believe that, even in those cases where mutations of the alpha4 or the beta2 subunit of the nAChR cosegregate with ADNFLE, there must be some crucial additional factors contributing to the development of the specific symptoms of ADNFLE.     

Mechanosensitivity of nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are heteropentameric ligand-gated ion channels that mediate excitatory neurotransmission at the neuromuscular junction (NMJ) and other peripheral and central synapses. At the NMJ, acetylcholine receptors (AChRs) are constantly exposed to mechanical stress resulting from muscle contraction. It is therefore of interest to understand if their function is influenced by mechanical stimuli. In this study, patch-clamp recordings showed that AChR channel activity was enhanced upon membrane stretching in both cultured Xenopus muscle cells and C2C12 myotubes. To examine how this property is physiologically regulated, effects of membrane-intrinsic and membrane-extrinsic factors on AChRs expressed in HEK293T cells were studied. As in muscle cells, AChR single channel currents recorded under cell-attached configuration were significantly increased??without change in current amplitude??when negative pressure was applied through the patch pipette. GsMTx-4, a peptide toxin that blocks mechanically activated cation channels, inhibited this effect on AChRs. The mechanosensitivity decreased when cells were treated with M??CD, latrunculin A or cytochalasin D, but increased when exposed to lysophosphatidylcholine, indicating contributions from both membrane lipids and the cytoskeleton. Rapsyn, which binds to AChRs and mediates their cytoskeletal interaction in muscle, suppressed AChR mechanosensitivity when co-expressed in HEK293T cells, but this influence of rapsyn was impaired following the deletion of rapsyn??s AChR-binding domain or upon cytoskeletal disruption by cytochalasin D. These results suggest a mechanism for regulating AChR??s mechanosensitivity through its cytoskeletal linkage via rapsyn, which may serve to protect the receptors and sarcolemmal integrity under high mechanical stress encountered by the NMJ.     

Neuronal bungarotoxin blocks the nicotinic stimulation of endogenous dopamine release from rat striatum

Nicotinic receptors in the brain are receiving increased attention due in part to the recent cloning of receptor subunits and to postmortem studies revealing alterations in receptor density associated with Alzheimer's disease. The peptide neurotoxin neuronal bungarotoxin (NBT) has been shown to block nicotinic cholinergic responses in autonomic ganglia and in retinal ganglion cells. These findings suggest that NBT may be a useful probe for studying nicotinic receptors in the brain. Therefore, we have investigated the effects of NBT on the nicotine-mediated enhancement of endogenous dopamine release from rat striatal slices. It was found that the transient increase in dopamine release caused by 100 microM nicotine was completely blocked by 100 nM NBT, indicating that NBT is a functional nicotinic antagonist in this system.     

Neuronal nicotinic receptor antagonist reduces anxiety-like behavior in mice

Brain cholinergic neurotransmission has been implicated in the modulation of anxiety in humans and evidence suggests that drugs targeting neuronal nicotinic acetylcholine receptor (nAChR) could have potential for the treatment of anxiety. The objective of present study was to examine anxiolytic effects of lobeline (0.04 or 0.1 mg/kg), a nAChR antagonist, in C57BL/6J mice using elevated plus-maze (EPM) and marble-burying test. Lobeline (0.04 mg/kg) significantly increased open arm time on EPM and reduced number of marbles buried. Similarly, mecamylamine (0.3 mg/kg) produced anxiolytic effects, while peripherally acting hexamethonium (0.3 mg/kg) failed to produce any response. These results provide evidence that lobeline has anxiolytic potential and nAChR antagonists may represent a new class of anxiolytics in humans.     

Cholinergic receptors: functional role of nicotinic ACh receptors in brain circuits and disease

The neurotransmitter acetylcholine (ACh) can regulate neuronal excitability throughout the nervous system by acting on both the cys-loop ligand-gated nicotinic ACh receptor channels (nAChRs) and the G protein-coupled muscarinic ACh receptors (mAChRs). The hippocampus is an important area in the brain for learning and memory, where both nAChRs and mAChRs are expressed. The primary cholinergic input to the hippocampus arises from the medial septum and diagonal band of Broca, the activation of which can activate both nAChRs and mAChRs in the hippocampus and regulate synaptic communication and induce oscillations that are thought to be important for cognitive function. Dysfunction in the hippocampal cholinergic system has been linked with cognitive deficits and a variety of neurological disorders and diseases, including Alzheimer’s disease and schizophrenia. My lab has focused on the role of the nAChRs in regulating hippocampal function, from understanding the expression and functional properties of the various subtypes of nAChRs, and what role these receptors may be playing in regulating synaptic plasticity. Here, I will briefly review this work, and where we are going in our attempts to further understand the role of these receptors in learning and memory, as well as in disease and neuroprotection.     

Nonsynaptic chemical transmission through nicotinic acetylcholine receptors

This review attempts to organize the different aspects of nicotinic transmission in the context of nonsynaptic interactions. Nicotinic acetylcholine receptors (nAChRs) dominantly operate in the nonsynaptic mode in the central nervous system despite their ligand-gated ion-channel nature, which would otherwise be better suited for fast synaptic transmission. This fast form of nonsynaptic transmission, most likely unique to nAChRs, represents a new avenue in the communication platforms of the brain. Cholinergic messages received by nAChRs, arriving at a later phase following synaptic activation, can interfere with dendritic signal integration. Nicotinic transmission plays a role in both neural plasticity and cellular learning processes, as well as in long-term changes in basic activity through fast activation, desensitization of receptors, and fluctuations of the steady-state levels of ACh. ACh release can contribute to plastic changes via activation of nAChRs in neurons and therefore plays a role in learning and memory in different brain regions. Assuming that nAChRs in human subjects are ready to receive long-lasting messages from the extracellular space because of their predominantly nonsynaptic distribution, they offer an ideal target for drug therapy at low, nontoxic drug levels.     

Activation of heteroliganded mouse muscle nicotinic receptors

The activation of the mouse muscle-type nicotinic acetylcholine receptor was studied in the presence of carbachol, and in the simultaneous presence of carbachol and choline. The channel currents were recorded under steady-state conditions using cell-attached single-channel patch clamp, and during transient exposures to the agonists using a piezo-driven fast application system. The presence of choline resulted in inhibition of currents elicited by carbachol. The inhibitory effect of choline manifested as a reduction in the effective opening rate (increase in the mean intracluster closed time duration) in single-channel recordings. In the fast application experiments, the peak current amplitude was reduced and the current rise time increased when choline was co-applied with carbachol. The data were analysed according to a model in which receptor interactions with carbachol and choline resulted in three types of ligation: receptors occupied by two carbachol molecules, receptors occupied by two choline molecules, and receptors in which one agonist binding site was occupied by carbachol and the other by choline, i.e. heteroliganded receptors. All three agonist-bound receptor populations could open albeit with different efficacies. The affinity of the resting receptor to choline was estimated to be 1–2 m m , and heteroliganded receptors opened with an opening rate constant of ∼3000 s⁻¹. The results of the analysis suggest that the presence of choline in the neuromuscular junction in vivo has little effect on the time course of synaptic currents. Nevertheless, the contribution of heteroliganded receptors should be taken into consideration when the receptor is exposed simultaneously to two or more agonists.     

Receptor protection studies comparing recombinant and native nicotinic receptors: Evidence for a subpopulation of mecamylamine-sensitive native alpha3beta4* nicotinic receptors

Studies involving receptor protection have been used to define the functional involvement of specific receptor subtypes in tissues expressing multiple receptor subtypes. Previous functional studies from our laboratory demonstrate the feasibility of this approach when applied to neuronal tissues expressing multiple nicotinic acetylcholine receptors (nAChRs). In the current studies, the ability of a variety of nAChR agonists and antagonists to protect native and recombinant alpha3beta4 nAChRs from alkylation were investigated using nAChR binding techniques. Alkylation of native alpha3beta4* nAChRs from membrane preparations of bovine adrenal chromaffin cells resulted in a complete loss of specific [(3)H]epibatidine binding. This loss of binding to native nAChRs was preventable by pretreatment with the agonists, carbachol or nicotine. The partial agonist, cytisine, produced partial protection. Several nAChR antagonists were also tested for their ability to protect. Hexamethonium and decamethonium were without protective activity while mecamylamine and tubocurarine were partially effective. Addition protection studies were performed on recombinant alpha3beta4 nAChRs. As with native alpha3beta4* nAChRs, alkylation produced a complete loss of specific [(3)H]epibatidine binding to recombinant alpha3beta4 nAChRs which was preventable by pretreatment with nicotine. However, unlike native alpha3beta4* nAChRs, cytisine and mecamylamine, provide no protection for alkylation. These results highlight the differences between native alpha3beta4* nAChRs and recombinant alpha3beta4 nAChRs and support the use of protection assays to characterize native nAChR subpopulations.     

Prion diseases: from protein to cell pathology

Prion diseases or transmissible spongiform encephalopathies are fatal neurodegenerative conditions in humans and animals that originate spontaneously, genetically or by infection. Conformational change of the normal (cellular) form of prion protein (PrP c) to a pathological, disease-associated form (PrP TSE) is considered central to pathogenesis and formation of the infectious agent or prion. Neuronal damage is central to clinical manifestation of prion diseases but poorly understood. In this review, we analyze the major pathogenetic pathways that lead to tissue pathology in different forms of disease. Neuropathogenesis of prion diseases evolves in complex ways on several front lines, most but not all of which exist also in other neurodegenerative as well as infectious diseases. Whereas intracellular accumulation of PrP forms might significantly impair cell function and lead to cytopathology, mere extracellular deposition of PrP TSE is questionable as a direct cytotoxic factor. Tissue damage may result from several parallel, interacting, or subsequent pathways. Future studies should clarify the trigger(s) and sequence of these processes and whether, and which, one is dominating or decisive.     

Chemokines in autoimmunity: from pathology to therapeutics

Leukocyte recruitment, accumulation, and activation have been both a unifying and enigmatic feature of a variety of autoimmune pathologies. While these processes were not well-known for decades, recent scientific discoveries have underscored the importance of specific chemokines in the evolution of autoimmunity. This has been supported by in vivo data from clinical studies and animal model experiments. Although chemokines are an attractive target for drug development, there are hurdles that need to be cleared. Nonetheless, the quest to understand chemokine biology and develop effective therapies continues to capture the imagination of scientists in academia and pharma alike.     

Nicotinic receptors in human brain: topography and pathology

Brain nicotinic acetylcholine receptors (nAChR) are a class of ligand-gated channels composed of and β 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 4β2, in the case of autism, and 7 possibly in addition to 4β2 nAChRs in the case of schizophrenia. In ageing and age-related neurodegenerative disorders nAChR deficits are predominantly associated with 4-containing receptors, although some studies also indicate the involvement of 3 and 7 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.     

烟碱型乙酰胆碱受体在认知功能中的作用

烟碱型乙酰胆碱受体（nAChR）是化学（配体）门控的离子通道蛋白，属于半胱氨酸环受体家族，广泛分布于中枢和周围神经系统中。中枢nAChR参与许多复杂的功能，如：注意、学习、觉醒和认知等。越来越多的研究表明，中枢nAChR在认知功能活动中发挥了重要的作用。临床资料提示，nAChR与一些认知功能障碍性疾病的发病机制有关，如：阿尔茨海默病（AD）、帕金森氏病（PD）以及癫痫等。     

Physiology and pathology of collagen receptors

Just before the transition from pre-genomic to the post-genomic era, the two latest members of the mammalian integrin family were identified. These integrins, which were named alpha10beta1 and alpha11beta1, are both collagen receptors and are related. Rather than being twins, they can be regarded as close cousins. They both belong to the subfamily of integrins that contain an I-domain in the alpha subunit. This domain is also the part that endows these integrins with the capacity to bind the GFOGER sequence in collagens. In the current review, we summarize and update the current knowledge about the in vitro and in vivo functions of these integrins.     

Neuronal localisation of benzodiazepine receptors in cerebellum.

The cellular localisation of benzodiazepine receptors was studied. Kainic acid induced neuronal lesions (2 x 0.25-2 x 2 micrograms; 2-26 days in rat cerebellum decreased specific binding of [3H]flunitrazepam down to 35% of controls. Specific binding of [3H]flunitrazepam was also decreased (to 80% of controls) in the cerebllum of mutant nervous mouse (nr/nr) where Purkinje cells are degenerated but in the mutant weaver mouse where granule cells are degenerated. These results show that benzodiazepine receptors are located mainly on neurons; both on Purkinje cells and other neurons, but not to a great extent on granule cells.     

Understanding human thiol dioxygenase enzymes: structure to function,and biology to pathology

Amino acid metabolism is a significant metabolic activity in humans, especially of sulphur‐containing amino acids, methionine and cysteine (Cys). Cys is cytotoxic and neurotoxic in nature; hence, mammalian cells maintain a constant intracellular level of Cys. Metabolism of Cys is mainly regulated by two thiol dioxygenases: cysteine dioxygenase (CDO) and 2‐aminoethanethiol dioxygenase (ADO). CDO and ADO are the only human thiol dioxygenases reported with a role in Cys metabolism and localized to mitochondria. This metabolic pathway is important in various human disorders, as it is responsible for the synthesis of antioxidant glutathione and is also for the synthesis of hypotaurine and taurine. CDO is the most extensively studied protein, whose high‐resolution crystallographic structures have been solved. As compared to CDO, ADO is less studied, even though it has a key role in cysteamine metabolism. To further understand ADO's structure and function, the three‐dimensional structures have been predicted from I‐TASSER and SWISS‐MODEL servers and validated with PROCHECK software. Structural superimposition approach using iPBA web server further confirmed near‐identical structures (including active sites) for the predicted protein models of ADO as compared to CDO. In addition, protein–protein interaction and their association in patho‐physiology are crucial in understanding protein functions. Both ADO and CDO interacting partner profiles have been presented using STRING database. In this study, we have predicted a 3D model structure for ADO and summarized the biological roles and the pathological consequences which are associated with the altered expression and functioning of ADO and CDO in case of cancer, neurodegenerative disorders and other human diseases.     

Striatum-hippocampus balance: from physiological behavior to interneuronal pathology

In neurological disorders in which the cross-talk between striatal and hippocampal memory systems is affected, such as epilepsy, Down syndrome and Huntington's disease, cell-type specific alterations in synaptic plasticity lead to distinctive patterns causing functional imbalance between the two memory systems. Despite the complex network in which their neuronal activity is likely to be engaged, a common property of striatal and hippocampal neurons is to undergo bidirectional synaptic plasticity that relies on activity of interneurons and correlates with specific learning skills. As interneuronal dysfunction plays a primary role in the pathogenesis of these disorders, interneurons can be viewed as critical elements in neurophysiological substrates of such flexible relationships between these two memory systems.