Expression of nicotinic acetylcholine receptors in Alzheimer's disease: postmortem investigations and experimental approaches

Adiponectin-Receptor Agonistic Dipeptide Tyr-Pro Stimulates the Acetylcholine Nervous System in NE-4C Cells

The dipeptide Tyr-Pro has physiological potential for intact transportability into the brain parenchyma, prevention of cognitive impairment, and an adiponectin receptor 1 (AdipoR1) agonistic effect. The present study aimed to understand the effect of Tyr-Pro on the acetylcholine (ACh) nervous system and its underlying mechanism in NE-4C nerve cells. Concentration-dependent ACh production was induced by stimulation with Tyr-Pro and AdipoRon (an AdipoR1 agonist), along with the expression of AdipoR1 and choline acetyltransferase (ChAT) in NE-4C cells. By knocking down AdipoR1 in the cells, Tyr-Pro promoted ChAT expression, along with the activations of AMPK and ERK 1/2. Tyr-Pro did not alter acetylcholinesterase or ACh receptors, indicating that the dipeptide might operate as an ACh accelerator in nerve cells. This study provides the first evidence that the AdipoR1 agonistic Tyr-Pro is a promising dipeptide responsible for the stimulation of the ACh nervous system by AdipoR1-induced ChAT activation.

Nicotinic acetylcholine receptors in neurological and psychiatric diseases

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that are widely distributed both pre- and post-synaptically in the mammalian brain. By modulating cation flux across cell membranes, neuronal nAChRs regulate neuronal excitability and the release of a variety of neurotransmitters to influence multiple physiologic and behavioral processes including synaptic plasticity, motor function, attention, learning and memory. Abnormalities of neuronal nAChRs have been implicated in the pathophysiology of neurologic disorders including Alzheimer's disease, Parkinson's disease, epilepsy, and Tourette´s syndrome, as well as psychiatric disorders including schizophrenia, depression, and anxiety. The potential role of nAChRs in a particular illness may be indicated by alterations in the expression of nAChRs in relevant brain regions, genetic variability in the genes encoding for nAChR subunit proteins, and/or clinical or preclinical observations where specific ligands showed a therapeutic effect. Over the past 25 years, extensive preclinical and some early clinical evidence suggested that ligands at nAChRs might have therapeutic potential for neurologic and psychiatric disorders. However, to date the only approved indications for nAChR ligands are smoking cessation and the treatment of dry eye disease. It has been argued that progress in nAChR drug discovery has been limited by translational gaps between the preclinical models and the human disease as well as unresolved questions regarding the pharmacological goal (i.e., agonism, antagonism or receptor desensitization) depending on the disease.

Disruption of Cholinergic Circuits as an Area for Targeted Drug Treatment of Alzheimer's Disease: In Vivo Assessment of Short-Term Plasticity in Rat Brain

The search for new therapeutics for the treatment of Alzheimer’s disease (AD) is still in progress. Aberrant pathways of synaptic transmission in basal forebrain cholinergic neural circuits are thought to be associated with the progression of AD. However, the effect of amyloid-beta (Aβ) on short-term plasticity (STP) of cholinergic circuits in the nucleus basalis magnocellularis (NBM) is largely unknown. STP assessment in rat brain cholinergic circuitry may indicate a new target for AD cholinergic therapeutics. Thus, we aimed to study in vivo electrophysiological patterns of synaptic activity in NBM-hippocampus and NBM-basolateral amygdala circuits associated with AD-like neurodegeneration. The extracellular single-unit recordings of responses from the hippocampal and basolateral amygdala neurons to high-frequency stimulation (HFS) of the NBM were performed after intracerebroventricular injection of Aβ 25–35. We found that after Aβ 25–35 exposure the number of hippocampal neurons exhibiting inhibitory responses to HFS of NBM is decreased. The reverse tendency was seen in the basolateral amygdala inhibitory neural populations, whereas the number of amygdala neurons with excitatory responses decreased. The low intensity of inhibitory and excitatory responses during HFS and post-stimulus period is probably due to the anomalous basal synaptic transmission and excitability of hippocampal and amygdala neurons. These functional changes were accompanied by structural alteration of hippocampal, amygdala, and NBM neurons. We have thus demonstrated that Aβ 25–35 induces STP disruption in NBM-hippocampus and NBM-basolateral amygdala circuits as manifested by unbalanced excitatory/inhibitory responses and their frequency. The results of this study may contribute to a better understanding of synaptic integrity. We believe that advancing our understanding of in vivo mechanisms of synaptic plasticity disruption in specific neural circuits could lead to effective drug searches for AD treatment.

Allosterism of Nicotinic Acetylcholine Receptors: Therapeutic Potential for Neuroinflammation Underlying Brain Trauma and Degenerative Disorders

Inflammation is a key physiological phenomenon that can be pervasive when dysregulated. Persistent chronic inflammation precedes several pathophysiological conditions forming one of the critical cellular homeostatic checkpoints. With a steady global surge in inflammatory diseases, it is imperative to delineate underlying mechanisms and design suitable drug molecules targeting the cellular partners that mediate and regulate inflammation. Nicotinic acetylcholine receptors have a confirmed role in influencing inflammatory pathways and have been a subject of scientific scrutiny underlying drug development in recent years. Drugs designed to target allosteric sites on the nicotinic acetylcholine receptors present a unique opportunity to unravel the role of the cholinergic system in regulating and restoring inflammatory homeostasis. Such a therapeutic approach holds promise in treating several inflammatory conditions and diseases with inflammation as an underlying pathology. Here, we briefly describe the potential of cholinergic allosterism and some allosteric modulators as a promising therapeutic option for the treatment of neuroinflammation.

Pharmacological Characterization of the Novel and Selective α7 Nicotinic Acetylcholine Receptor-Positive Allosteric Modulator BNC375

Treatments for cognitive deficits associated with central nervous system (CNS) disorders such as Alzheimer disease and schizophrenia remain significant unmet medical needs that incur substantial pressure on the health care system. The α7 nicotinic acetylcholine receptor (nAChR) has garnered substantial attention as a target for cognitive deficits based on receptor localization, robust preclinical effects, genetics implicating its involvement in cognitive disorders, and encouraging, albeit mixed, clinical data with α7 nAChR orthosteric agonists. Importantly, previous orthosteric agonists at this receptor suffered from off-target activity, receptor desensitization, and an inverted U-shaped dose-effect curve in preclinical assays that limit their clinical utility. To overcome the challenges with orthosteric agonists, we have identified a novel selective α7 positive allosteric modulator (PAM), BNC375. This compound is selective over related receptors and potentiates acetylcholine-evoked α7 currents with only marginal effect on the receptor desensitization kinetics. In addition, BNC375 enhances long-term potentiation of electrically evoked synaptic responses in rat hippocampal slices and in vivo. Systemic administration of BNC375 reverses scopolamine-induced cognitive deficits in rat novel object recognition and rhesus monkey object retrieval detour (ORD) task over a wide range of exposures, showing no evidence of an inverted U-shaped dose-effect curve. The compound also improves performance in the ORD task in aged African green monkeys. Moreover, ex vivo ¹³C-NMR analysis indicates that BNC375 treatment can enhance neurotransmitter release in rat medial prefrontal cortex. These findings suggest that α7 nAChR PAMs have multiple advantages over orthosteric α7 nAChR agonists for the treatment of cognitive dysfunction associated with CNS diseases. SIGNIFICANCE STATEMENT: BNC375 is a novel and selective α7 nicotinic acetylcholine receptor (nAChR) positive allosteric modulator (PAM) that potentiates acetylcholine-evoked α7 currents in in vitro assays with little to no effect on the desensitization kinetics. In vivo, BNC375 demonstrated robust procognitive effects in multiple preclinical models across a wide exposure range. These results suggest that α7 nAChR PAMs have therapeutic potential in central nervous system diseases with cognitive impairments.

Proteomic Investigation of Murine Neuronal α7-Nicotinic Acetylcholine Receptor Interacting Proteins

The α7-nicotinic acetylcholine receptor (α7-nAChR) is a ligand-gated ion channel that is expressed widely in vertebrates and is the principal high-affinity α-bungarotoxin (α-bgtx) binding protein in the mammalian CNS. α7-nAChRs associate with proteins that can modulate its properties. The α7-nAChR interactome is the summation of proteins interacting or associating with α7-nAChRs in a protein complex. To identify an α7-nAChR interactome in neural tissue, we isolated α-bgtx-affinity protein complexes from wild-type and α7-nAChR knockout (α7 KO) mouse whole brain tissue homogenates using α-bgtx-affinity beads. Affinity precipitated proteins were trypsinized and analyzed with an Orbitrap Fusion mass spectrometer. Proteins isolated with the α7-nAChR specific ligand, α-bgtx, were determined to be α7-nAChR associated proteins. The α7-nAChR subunit and 120 additional proteins were identified. Additionally, 369 proteins were identified as binding to α-bgtx in the absence of α7-nAChR expression, thereby identifying nonspecific proteins for α7-nAChR investigations using α-bgtx enrichment. These results expand on our previous investigations of α7-nAChR interacting proteins using α-bgtx-affinity bead isolation by controlling for differences between α7-nAChR and α-bgtx-specific proteins, developing an improved protein isolation methodology, and incorporating the latest technology in mass spectrometry. The α7-nAChR interactome identified in this study includes proteins associated with the expression, localization, function, or modulation of α7-nAChRs, and it provides a foundation for future studies to elucidate how these interactions contribute to human disease.

18F-XTRA PET for Enhanced Imaging of the Extrathalamic α4β2 Nicotinic Acetylcholine Receptor

Reduced density of the α4β2 nicotinic acetylcholine receptor (α4β2-nAChR) in the cortex and hippocampus of the human brain has been reported in aging and patients with neurodegenerative disease. This study assessed the pharmacokinetic behavior of ¹⁸F-(-)-JHU86428 (¹⁸F-XTRA), a new radiotracer for in vivo PET imaging of the α4β2-nAChR, particularly in extrathalamic regions of interest in which the α4β2-nAChR is less densely expressed than in thalamus. ¹⁸F-XTRA was also used to evaluate the α4β2-nAChR in the hippocampus in human aging. Methods: Seventeen healthy nonsmoker adults (11 men, 6 women; age, 30-82 y) underwent PET neuroimaging over 90 or 180 min in a high-resolution research tomograph after bolus injection of ¹⁸F-XTRA. Methods to quantify binding of ¹⁸F-XTRA to the α4β2-nAChR in the human brain were compared, and the relationship between age and binding in the hippocampus was tested. Results: ¹⁸F-XTRA rapidly entered the brain, and time-activity curves peaked within 10 min after injection for extrathalamic regions and at approximately 70 min in the thalamus. The 2-tissue-compartment model (2TCM) predicted the regional time-activity curves better than the 1-tissue-compartment model, and total distribution volume (V_T) was well identified by the 2TCM in all ROIs. V_T values estimated using Logan analysis with metabolite-corrected arterial input were highly correlated with those from the 2TCM in all regions, and values from 90-min scan duration were on average within 5% of those values from 180 min of data. Parametric images of V_T were consistent with the known distribution of the α4β2-nAChR across the brain. Finally, an inverse correlation between V_T in the hippocampus and age was observed. Conclusion: Our results extend support for use of ¹⁸F-XTRA with 90 min of emission scanning in quantitative human neuroimaging of the extrathalamic α4β2-nAChR, including in studies of aging.

Evaluating Commercially Available Antibodies for Rat α7 Nicotinic Acetylcholine Receptors

Alpha7 nicotinic acetylcholine receptors (α7 nAChRs) are important drug targets in neurological disorders and inflammation, making their detection and localization by validated antibodies highly desirable. However, tests in knockout animals raised questions about specificity of antibodies to mouse α7 nAChRs. To date, methods for validating antibodies for rat or human α7 nAChR have not been reported. We developed a gel-shift assay for western blots using GH4C1 cells expressing either native rat receptors or α7 nAChR-green fluorescent protein (GFP) chimeras to evaluate seven commercially available α7 nAChR antibodies. Blots with anti-GFP antibody detected GFP or α7 nAChR-GFP expressed in GH4C1 cells, and ¹²⁵I-α-bungarotoxin binding and RNA analysis demonstrated α7 nAChR expression. Validated samples were used to evaluate α7 nAChR antibodies by western blot and immunofluorescence studies. These methods confirmed that two of seven α7 nAChR antibodies identify gel-shifts for α7 nAChR/nAChR-GFP but only one antibody demonstrated low background and significant immunofluorescence differences between wild-type and α7 nAChR expressing GH4C1 cells. However, that polyclonal antibody displayed lot-to-lot variability. Our findings suggest that careful validation methods are required for all α7 nAChR receptor species and antibody lots and that the gel-shift assay may allow for relatively rapid antibody screening.

Gene networks in neurodegenerative disorders

Three neurodegenerative diseases [Amyotrophic Lateral Sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD)] have many characteristics like pathological mechanisms and genes. In this sense some researchers postulate that these diseases share the same alterations and that one alteration in a specific protein triggers one of these diseases. Analyses of gene expression may shed more light on how to discover pathways, pathologic mechanisms associated with the disease, biomarkers and potential therapeutic targets. In this review, we analyze four microarrays related to three neurodegenerative diseases. We will systematically examine seven genes (CHN1, MDH1, PCP4, RTN1, SLC14A1, SNAP25 and VSNL1) that are altered in the three neurodegenerative diseases. A network was built and used to identify pathways, miRNA and drugs associated with ALS, AD and PD using Cytoscape software an interaction network based on the protein interactions of these genes. The most important affected pathway is PI3K-Akt signalling. Thirteen microRNAs (miRNA-19B1, miRNA-107, miRNA-124-1, miRNA-124-2, miRNA-9-2, miRNA-29A, miRNA-9-3, miRNA-328, miRNA-19B2, miRNA-29B2, miRNA-124-3, miRNA-15A and miRNA-9-1) and four drugs (Estradiol, Acetaminophen, Resveratrol and Progesterone) for new possible treatments were identified.

Arrestins contribute to amyloid beta-induced cell death via modulation of autophagy and the α7nAch receptor in SH-SY5Y cells

Amyloid β-protein (Aβ) is believed to contribute to the development of Alzheimer’s disease (AD). Here we showed that Aβ_25-35 rapidly caused activation of autophagy, subsequently leading to reduction of autophagy associated with cellular apoptosis. Further investigation revealed that the accumulation of β-arrestin 1 (ARRB1) caused by Aβ_25-35 contributed to the induction of autophagic flux. The depletion of ARRB1 led to decreases in the expression of LC3B, Atg7, and Beclin-1, which are essential for the initiation of autophagy. ARRB1 depletion also reduced downstream ERK activity and promoted Aβ_25-35-induced cell death. As with ARRB1, transient upregulation of ARRB2 by Aβ_25-35 was observed after short treatment durations, whereas genetic reduction of ARRB2 caused a marked increase in the expression of the α7nAch receptor at the cell surface, which resulted in partial reversal of Aβ_25-35-induced cell death. Although expression of both ARRB1 and ARRB2 was reduced in serum from patients with AD, the levels of ARRB1 were much lower than those of ARRB2 in AD. Thus, our findings indicate that ARRB1/2 play different roles in Aβ_25-35 cytotoxicity, which may provide additional support for exploring the underlying molecular mechanism of AD.

Characterization of the binding mode of the PET tracer [18F]ASEM to a chimera structure of the α7 nicotinic acetylcholine receptor

The binding free energy profile of the radio-ligand [¹⁸F]ASEM with the α7 nicotinic acetylcholine receptor was revealed by metadynamic simulations.

Neuronal Nicotinic Acetylcholine Receptors: Involvement in Alzheimer’s Disease and Schizophrenia

Nicotinic acetylcholine receptors (nAChRs) play a role in a variety of diseases of the central nervous system including Alzheimer's disease (AD) and schizophrenia. There is great interest in evaluating disease-related nAChR changes, and pharmacological treatment of nAChR deficits is a promising therapy. In AD, 7 nAChRs remain relatively stable, contrasting to 4 2 nAChRs that are lost in substantial numbers. -amyloid, a major neuropathology in AD, blocks 4 2 and 7 nAChRs. Agonists selective to 7 nAChRs are neuroprotective against amyloid. Paradoxically, 7 nAChRs may function as receptors for -amyloid. These results indicate 7 nAChR antagonists may be appropriate therapy in AD. In schizophrenia, 7 nAChRs are significantly reduced in hippocampus and neocortex. The exceptionally high rate of smoking in schizophrenics is likely a form of self-medication. Therapy with 7 nAChR agonists relieves some schizophrenic symptoms. Despite disparities in etiology and symptomatology, AD and schizophrenia share a target for therapeutic intervention— 7 nAChRs.

Predicting the neural effect of switching from donepezil to galantamine based on single-photon emission computed tomography findings in patients with Alzheimer's disease

BACKGROUND

A number of neuroimaging studies have addressed the specific effect of treatment with cholinesterase inhibitors on the frontal lobe in patients with Alzheimer's disease (AD). However, the neural effects of cholinesterase inhibitors on both apathy and executive dysfunction remain unclear. We examined whether baseline regional cerebral blood flow, as determined by using single-photon emission computed tomography, is capable of predicting changes in apathy and executive dysfunction in response to AD patients switching from donepezil to galantamine therapy.

METHODS

We conducted a 24-week, prospective, open-label study of AD patients treated with galantamine who did not respond to previous treatment with donepezil. Single-photon emission computed tomography was performed at baseline, and behaviour and cognitive assessments including the Mini-Mental State Examination, the Japanese version of the Alzheimer's Disease Assessment Scale-cognitive subscale, the Frontal Assessment Battery, the Neuropsychiatry Inventory Brief Questionnaire Form, and the Dysexecutive Questionnaire were conducted at three time points (baseline and after 12 and 24 weeks of galantamine therapy).

RESULTS

After galantamine therapy, the Neuropsychiatry Inventory Brief Questionnaire Form scores (apathy, irritability, and aberrant motor symptoms) and the Dysexecutive Questionnaire score improved significantly. The single-photon emission computed tomography findings showed that lower baseline regional cerebral blood flow values in several frontal areas, including the dorsolateral and ventrolateral prefrontal cortex, the anterior cingulate, and the orbitofrontal cortex, predicted greater reductions in the score for apathy (distress) on the Neuropsychiatry Inventory Brief Questionnaire Form and the Dysexecutive Questionnaire score after patients switched from donepezil to galantamine therapy.

CONCLUSIONS

Our study suggests that galantamine therapy, unlike donepezil, is characterized by a dual mechanism of action that may increase acetylcholine and the nicotinic receptor-modulation effect within the frontal lobe, both of which are associated with apathy and executive dysfunction in AD patients.

Resistance to Inhibitors of Cholinesterase 3 (Ric-3) Expression Promotes Selective Protein Associations with the Human α7-Nicotinic Acetylcholine Receptor Interactome

The α7-nicotinic acetylcholine receptor (α7-nAChR) is a ligand-gated ion channel widely expressed in vertebrates and is associated with numerous physiological functions. As transmembrane ion channels, α7-nAChRs need to be expressed on the surface of the plasma membrane to function. The receptor has been reported to associate with proteins involved with receptor biogenesis, modulation of receptor properties, as well as intracellular signaling cascades and some of these associated proteins may affect surface expression of α7-nAChRs. The putative chaperone resistance to inhibitors of cholinesterase 3 (Ric-3) has been reported to interact with, and enhance the surface expression of, α7-nAChRs. In this study, we identified proteins that associate with α7-nAChRs when Ric-3 is expressed. Using α-bungarotoxin (α-bgtx), we isolated and compared α7-nAChR-associated proteins from two stably transfected, human tumor-derived cell lines: SH-EP1-hα7 expressing human α7-nAChRs and the same cell line further transfected to express Ric-3, SH-EP1-hα7-Ric-3. Mass spectrometric analysis of peptides identified thirty-nine proteins that are associated with α7-nAChRs only when Ric-3 was expressed. Significantly, and consistent with reports of Ric-3 function in the literature, several of the identified proteins are involved in biological processes that may affect nAChR surface expression such as post-translational processing of proteins, protein trafficking, and protein transport. Additionally, proteins affecting the cell cycle, the cytoskeleton, stress responses, as well as cyclic AMP- and inositol triphosphate-dependent signaling cascades were identified. These results illuminate how α-bgtx may be used to isolate and identify α7-nAChRs as well as how the expression of chaperones such as Ric-3 can influence proteins associating with α7-nAChRs. These associating proteins may alter activities of α7-nAChRs to expand their functionally-relevant repertoire as well as to affect biogenesis and membrane trafficking of α7-nAChRs.

The nicotinic acetylcholine receptor alpha 4 subunit contains a functionally relevant SNP Haplotype

BACKGROUND

Non-coding single nucleotide polymorphisms within the nicotinic acetylcholine receptor alpha 4 subunit gene (CHRNA4) are robustly associated with various neurological and behavioral phenotypes including schizophrenia, cognition and smoking. The most commonly associated polymorphisms are located in exon 5 and segregate as part of a haplotype. So far it is unknown if this haplotype is indeed functional, or if the observed associations are an indirect effect caused by linkage disequilibrium with not yet identified adjacent functional variants. We therefore analyzed the functional relevance of the exon 5 haplotype alleles.

RESULTS

Using voltage clamp experiments we were able to show that the CHRNA4 haplotype alleles differ with respect to their functional effects on receptor sensitivity including reversal of receptor sensitivity between low and high acetylcholine concentrations. The results indicate that underlying mechanisms might include differences in codon usage bias and changes in mRNA stability.

CONCLUSIONS

Our data demonstrate that the complementary alleles of the CHRNA4 exon 5 haplotype are functionally relevant, and might therefore be causative for the above mentioned associations.

Impaired hippocampal acetylcholine release parallels spatial memory deficits in Tg2576 mice subjected to basal forebrain cholinergic degeneration

The Alzheimer's disease (AD) mouse model Tg2576 overexpresses an AD associated mutant variant of human APP and accumulates amyloid beta (Aβ) in an age-dependent manner. Using the selective cholinergic immunotoxin mu p75-saporin (SAP), we induced a partial basal forebrain cholinergic degeneration (BFCD) in 3 months old male Tg2576 mice to co-express cholinergic degeneration with Aβ overexpression as these characteristics constitutes key hallmarks of AD. At 9 months, SAP lesioned Tg2576 mice were cognitively impaired in two spatial paradigms addressing working memory and mid to long-term memory. Conversely, there was no deterioration of cognitive functioning in sham lesioned Tg2576 mice or wild type littermates (wt) receiving the immunotoxin. At 10 months of age, release of acetylcholine (ACh) was addressed by microdialysis in conscious mice. Scopolamine-induced increases in hippocampal ACh efflux was significantly reduced in SAP lesioned Tg2576 mice compared to sham lesioned Tg2576 mice. Intriguingly, there was no significant difference in ACh efflux between wt treatment groups. Following SAP treatment, choline acetyltransferase activity was reduced in the hippocampus and frontal cortex and the reduction was comparable between groups. Our results suggest that partial BFCD acts collectively with increased levels of Aβ to induce cognitive decline and to compromise cholinergic release. Tg2576 mice with BFCD may constitute a new and suitable AD mouse model to study the interrelations between cholinergic deficits and amsyloid deposition.

Radiosynthesis and first evaluation in mice of [(18)F]NS14490 for molecular imaging of α7 nicotinic acetylcholine receptors

[(18)F]NS14490, a new potential radiotracer for neuroimaging of α7 nicotinic acetylcholine receptors (α7 nAChRs), was synthesized and evaluated in vitro and in vivo. Radioligand binding studies using [(3)H]methyllycaconitine and NS14490 as competitor showed a good target affinity (K(i,α7) = 2.5 nM) and a high selectivity towards other nAChRs. Radiosynthesis of [(18)F]NS14490 was performed by two different labelling procedures: a two-step synthesis using a prosthetic group, which led to 7% labelling yield, and the convenient direct nucleophilic substitution of the corresponding tosylate precursor, which resulted in 70% labelling yield. After optimisation of the isolation, purification and formulation process, biodistribution studies were performed in CD-1 mice. The brain uptake of [(18)F]NS14490 was comparably low (0.16% ID g(-1) wet weight at 5 min p.i.). The radiotracer showed a high metabolic stability in plasma and brain. Also, the target specificity was proven by pre-administration of a highly affine α7 ligand providing a rationale basis for further in vivo evaluation.

Brain imaging of nicotinic receptors in Alzheimer's disease

Neuronal nicotinic acetylcholine receptors (nAChRs) are a family of ligand-gated ion channels which are widely distributed in the human brain. Several lines of evidence suggest that two major subtypes (α4β2 and α7) of nAChRs play an important role in the pathophysiology of Alzheimer's disease (AD). Postmortem studies demonstrated alterations in the density of these subtypes of nAChRs in the brain of patients with AD. Currently, nAChRs are one of the most attractive therapeutic targets for AD. Therefore, several researchers have made an effort to develop novel radioligands that can be used to study quantitatively the distribution of these two subtypes in the human brain with positron emission tomography (PET) and single-photon emission computed tomography (SPECT). In this paper, we discuss the current topics on in vivo imaging of two subtypes of nAChRs in the brain of patients with AD.

Amyloid precursor protein gene mutated at Swedish 670/671 sites in vitro induces changed expression of nicotinic acetylcholine receptors and neurotoxicity

In order to investigate the influence of amyloid precursor protein (APP) over-expression on the levels of nicotinic acetylcholine receptors (nAChRs), the pCDNA 3.0 carrying the Swedish 670/671 APP double mutation (APP(SWE)) gene was transfected into human neuroblastoma (SH-SY5Y) cells and primary culture of rat hippocampal neurons. The mRNA level of APP, and nAChR α3, α4 and α7 subunits were detected by real-time PCR, and their corresponding proteins as well as α-secreted APP (αAPPs) by Western blotting. [3H]Epibatidine binding sites were measured by the receptor binding assay. The results showed that significantly concomitant increases in mRNA and protein levels of SH-SY5Y cells and primary cultured neurons transfected with APP(SWE) were observed. Interestingly, a decreased αAPPs level was detected in both cells treated with APP(SWE) transfection. In addition, decreases in mRNA and protein levels of α3 nAChR subunit in SH-SY5Y cells or α4 subunit in primary cultured neurons with APP(SWE) transfection were observed. For α7 nAChR, the increased protein and mRNA levels were found in SH-SY5Y cells and primary cultured neurons with APP(SWE) transfection. The number of cholinergic receptor binding site of [3H]epibatidine was decreased in the SH-SY5Y cells transfected with APP(SWE). Elevations in the activities of AChE and BuChE and in the level of lipid peroxidation were detected in both types of cultured cells transfected with APP(SWE). These results indicated that the over-expression of APP(SWE) gene can influence the expression of nAChRs and resulted in neurotoxicity, in which this process might play an important role in the pathogenesis of Alzheimer's disease.

Alzheimer's disease amyloid beta-protein and synaptic function

Alzheimer's disease (AD) is characterized neuropathologically by the deposition of different forms of amyloid beta-protein (A beta) including variable amounts of soluble species that correlate with severity of dementia. The extent of synaptic loss in the brain provides the best morphological correlate of cognitive impairment in clinical AD. Animal research on the pathophysiology of AD has therefore focussed on how soluble A beta disrupts synaptic mechanisms in vulnerable brain regions such as the hippocampus. Synaptic plasticity in the form of persistent activity-dependent increases or decreases in synaptic strength provide a neurophysiological substrate for hippocampal-dependent learning and memory. Acute treatment with human-derived or chemically prepared soluble A beta that contains certain oligomeric assemblies, potently and selectively disrupts synaptic plasticity causing inhibition of long-term potentiation (LTP) and enhancement of long-term depression (LTD) of glutamatergic transmission. Over time these and related actions of A beta have been implicated in reducing synaptic integrity. This review addresses the involvement of neurotransmitter intercellular signaling in mediating or modulating the synaptic plasticity disrupting actions of soluble A beta, with particular emphasis on the different roles of glutamatergic and cholinergic mechanisms. There is growing evidence to support the view that NMDA and possibly nicotinic receptors are critically involved in mediating the disruptive effect of A beta and that targeting muscarinic receptors can indirectly modulate A beta's actions. Such studies should help inform ongoing and future clinical trials of drugs acting through the glutamatergic and cholinergic systems.

Decreased nicotinic receptors and cognitive deficit in rats intracerebroventricularly injected with beta-amyloid peptide(1-42) and fed a high-cholesterol diet

To investigate whether the changes in nicotinic receptors (nAChRs) and in learning and memory associated with Alzheimer's disease (AD) are influenced by both beta-amyloid peptide (Abeta) and cholesterol in vivo, we examined the effects of intracerebroventricular injection of Abeta(1-42) and/or a high-cholesterol diet on brain levels of nAChRs and learning and memory in rats. The levels of nAChR subunit proteins and the corresponding mRNA were measured by Western blotting and RT-PCR, respectively; and learning and memory were evaluated with the Morris Water Maze examination. Injection of Abeta(1-42) resulted in deposition of this peptide, activation of astrocytes, decreased levels of the alpha7 and alpha4 protein subunits of the nAChR, and elevated expression of alpha7 mRNA, as well as impaired learning and spatial memory. A high-cholesterol diet activated astrocytes and, more importantly, potentiated the toxic effects of Abeta on nAChR subunit levels and on learning and memory. These findings may be highly relevant to the mechanisms underlying the cognitive deficits associated with AD.

Site-specific regulation of gene expression by estrogen in the hypothalamus of adult female rats

Estrogen plays critical roles in the neuroendocrine system of adult female rats through separate actions, respectively, in the preoptic area (POA) and the ventromedial nucleus of the hypothalamus (VMH). Seven-week-old rats were treated with/without estrogen after they were ovariectomized, and four estrogen-responsive, neuronal system-related genes, encoding alpha4 neuronal nicotinic acetylcholine receptor (Chrna4), GABA(A) receptor delta (Gabrd), serotonin receptor 6 (Htr6), and GABA transporter 2 (Slc6a13), were investigated by real-time RT-PCR and Western blot analyses to examine their differential regulation by estrogen between the anterior part containing POA and the posterior part containing VMH. We further examined Bax, Bcl2, and Prkce, the former two genes to be involved in the gene expression network of Chrna4 and the latter gene, that of Gabrd. The regulation of Bax and Bcl2 by estrogen differed between the anterior and posterior parts. The results demonstrated differential regulation of these neuronal system-related genes by estrogen between the anterior and posterior parts of the hypothalamus and suggested the roles of gene expression networks for the respective genes in the neuroendocrine system of adult female rats.

Alpha7 nicotinic acetylcholine receptor: a link between inflammation and neurodegeneration

Alzheimer's disease (AD) is the leading cause of dementia affecting over 25 million people worldwide. Classical studies focused on the description and characterization of the pathological hallmarks found in AD patients including the neurofibrillary tangles and the amyloid plaques. Current strategies focus on the etiology of these hallmarks and the different mechanisms contributing to neurodegeneration. Among them, recent studies reveal the close interplay between the immunological and the neurodegenerative processes. This article examines the implications of the alpha7 nicotinic acetylcholine receptor (alpha7nAChR) as a critical link between inflammation and neurodegeneration in AD. Alpha7nAChRs are not only expressed in neurons but also in Glia cells where they can modulate the immunological responses contributing to AD. Successful therapeutic strategies against AD should consider the connections between inflammation and neurodegeneration. Among them, alpha7nAChR may represent a pharmacological target to control these two mechanisms during the pathogenesis of neurodegenerative and behavioral disorders.

Abeta peptides can enter the brain through a defective blood-brain barrier and bind selectively to neurons

We have investigated the possibility that soluble, blood-borne amyloid beta (Abeta) peptides can cross a defective blood-brain barrier (BBB) and interact with neurons in the brain. Immunohistochemical analyses revealed extravasated plasma components, including Abeta42 in 19 of 21 AD brains, but in only 3 of 13 age-matched control brains, suggesting that a defective BBB is common in AD. To more directly test whether blood-borne Abeta peptides can cross a defective BBB, we tracked the fate of fluorescein isothiocyanate (FITC)-labeled Abeta42 and Abeta40 introduced via tail vein injection into mice with a BBB rendered permeable by treatment with pertussis toxin. Both Abeta40 and Abeta42 readily crossed the permeabilized BBB and bound selectively to certain neuronal subtypes, but not glial cells. By 48 h post-injection, Abeta42-positive neurons were widespread in the brain. In the cerebral cortex, small fluorescent, Abeta42-positive granules were found in the perinuclear cytoplasm of pyramidal neurons, suggesting that these cells can internalize exogenous Abeta42. An intact BBB (saline-injected controls) blocked entry of blood-borne Abeta peptides into the brain. The neuronal subtype selectivity of Abeta42 and Abeta40 was most evident in mouse brains subjected to direct intracranial stereotaxic injection into the hippocampal region, thereby bypassing the BBB. Abeta40 was found to preferentially bind to a distinct subset of neurons positioned at the inner face of the dentate gyrus, whereas Abeta42 bound selectively to the population of large neurons in the hilus region of the dentate gyrus. Our results suggest that the blood may serve as a major, chronic source of soluble, exogenous Abeta peptides that can bind selectively to certain subtypes of neurons and accumulate within these cells.

Blockade of nicotinic acetylcholine receptors suppresses hippocampal long-term potentiation in wild-type but not ApoE4 targeted replacement mice

Both impaired nicotinic neurotransmission and the inheritance of apoE4 are associated with increased risk for Alzheimer disease (AD) as well as other deficiencies in memory-related behavior. Long-term potentiation (LTP), a cellular model of memory, is known to be altered by nicotinic agents. Recent studies also support an emergent role for apoE in LTP. We compared the effects of mecamylamine, a nonspecific antagonist of nicotinic acetylcholine receptors (nAChRs), on basal synaptic transmission and LTP in hippocampal slices from wild-type (wt) mice and targeted replacement mice expressing human apoE4 (apoE4-TR). Field excitatory postsynaptic potentials (EPSPs) were recorded in the dentate gyrus (DG) in response to medial perforant path activation, and theta burst stimulation was used to induce LTP. Bath application of mecamylamine (3 microM) did not alter input-output relationships or paired pulse depression in either mouse strain. Under control conditions, apoE4-TR mice showed significantly less LTP than wt mice (17.5% +/- 3.2%, n = 9, vs. 30.1% +/- 3.9%, n = 11, P < 0.02). Mecamylamine reduced LTP in wt mice to a level that was similar to control levels for apoE4-TR mice (15.7% +/- 3.4%, n = 9), whereas apoE4-TR showed no further reduction of LTP (16.6% +/- 3.7%, n = 8) by mecamylamine. Thus mice expressing human apoE4 differ from wt mice both in their capacity for LTP and in the effect on LTP of nicotinic cholinergic blockade. It is possible that nicotinic neurotransmission is already compromised in apoE4-TR mice and, hence, that interference with the integrity of this cholinergic system represents a mechanism by which inheritance of the apoE4 allele contributes to cognitive risk.

Chronic nicotine exposure during adolescence differentially influences calcium-binding proteins in rat anterior cingulate cortex

We have recently shown that chronic amphetamine exposure selectively up-regulates parvalbumin (PV) calcium-binding proteins in the anterior cingulate cortex (ACC). In this study, we evaluated the effects of chronic nicotine (NIC) exposure on PV, calbindin D28k (CB) and calretinin (CR) calcium-binding protein immunostaining in ACC GABAergic interneurons. Chronic NIC exposure for 3 weeks in adolescent rats, either via drinking water (the oral group) or by twice daily subcutaneous injections (the injection group), resulted in the expression of high levels of CR proteins in the ACC but not in the parietal cortex. Larger increases in the density of CR-immunoreactive (ir) neurons were noted in the NIC-injected rats at 0-day withdrawal (45% increase) compared with the oral group (26% increase). The larger increases in CR-ir neuron density in the NIC-injected rats were also reflected by prominent CR-ir processes across cortical layers. The density of PV-ir neurons was also increased (37%) at 0-day withdrawal but only in the oral NIC group and no changes in CB-ir neuron density were observed in either NIC group. Combined dual-immunofluorescence and confocal microscopy revealed that somatodendritic alpha4 nicotinic acetylcholine receptors colocalized with cortical neurons stained positively for CR, PV or CB. These results suggest that CR- and/or PV-ir-containing GABA interneurons may be involved in channeling the effects of NIC in the ACC, which is closely associated with the ventral basal ganglia circuit that is linked to brain reward function.

Regional acetylcholinesterase activity and its correlation with behavioral performances in 15-month old transgenic mice expressing the human C99 fragment of APP

In addition to Abeta plaques and neurofibrillary tangles, Alzheimer's disease (AD) is characterized by increased brain levels of APP C-terminal fragments. In the present investigation, the cholinergic innervation in forebrain regions of transgenic mice (Tg13592) expressing the human betaAPP C99 fragment was compared to that of non-transgenic controls by measuring the activity of the non-specific catabolic enzyme, acetylcholinesterase (AChE). The AchE activity of Tg13592 mice was altered in several regions implicated in the functional loop of regulation between septum and hippocampus, vulnerable in Alzheimer pathology and critically involved in cognitive functions. In particular, AChE activity was upregulated in three basal forebrain regions containing cholinergic cell bodies, prelimbic cortex, anterior subiculum, and paraventricular thalamus, but downregulated in lateral septum and reticular thalamus. The increased activity in medial septum and anterior subiculum was linearly correlated with poor performances in a spatial learning task, possibly due to cell stress mechanisms. Because of some similarities in terms of neurochemistry and behavior, this mouse model may be of use for studying prodromal AD.

Expression of nicotinic receptors on primary cultures of rat astrocytes and up-regulation of the α7, α4 and β2 subunits in response to nanomolar concentrations of the β-amyloid peptide1–42

Neuronal nicotinic acetylcholine receptors (nAChRs) are thought to be involved in the pathogenesis of Alzheimer's disease (AD). Interestingly, in the brains of patients with this disease, losses of several subtypes of nAChRs on neurons have been reported, while an increase in alpha7 nAChRs was recently detected in the astrocytes. However, little is presently known about the expressions of individual subunits of nAChR on rat astrocytes in primary culture or the possible influence of exposure to beta-amyloid peptide (Abeta), a neuropathological hallmark of AD, on this expression. Thus, in the present investigation the levels of individual nAChR subunits on primary rat astrocytes and the possible direct influence of Abetas on the receptors were examined by RT-PCR, Western blotting, monitoring intracellular free calcium and immunohistochemistry. The alpha4, alpha7, beta2 and beta3 subunits and related calcium channel responses were found in these cells, whereas neither alpha2 nor alpha3 could be detected. Elevation in the levels of alpha7, alpha4 and beta2 mRNAs and proteins were observed in astrocytes exposed to 0.1-100nM Abeta(1-42). In contrast, incubation with 1muM Abeta(1-42) or Abeta(35-25) did not affect these levels. We propose that the enhanced expression of alpha7, alpha4 and beta2 nAChRs by astrocytes stimulated directly by nanomolar concentrations of Abeta(1-42) might be related to ongoing defensive or compensative mechanisms.

Neuronal nicotinic acetylcholine receptor subunits in autism: an immunohistochemical investigation in the thalamus

The cholinergic system has been implicated in the development of autism on the basis of neuronal nicotinic acetylcholine receptor (nAChR) losses in cerebral and cerebellar cortex. In the present study, the first to explore nAChRs in the thalamus in autism, alpha4, alpha7 and beta2 nAChR subunit expression in thalamic nuclei of adult individuals with autism (n=3) and age-matched control cases (n=3) was investigated using immunochemical methods. Loss of alpha7- and beta2- (but not alpha4-) immunoreactive neurons occurred in the paraventricular nucleus (PV) and nucleus reuniens in autism. Preliminary results indicated glutamic acid decarboxylase immunoreactivity occurred at a low level in PV, co-expressed with alpha7 in normal and autistic cases and was not reduced in autism. This suggested loss of neuronal alpha7 in autism is not caused by loss of GABAergic neurons. These findings indicate nicotinic abnormalities that occur in the thalamus in autism which may contribute to sensory or attentional deficits.

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.

High selective expression of alpha7 nicotinic receptors on astrocytes in the brains of patients with sporadic Alzheimer's disease and patients carrying Swedish APP 670/671 mutation: a possible association with neuritic plaques

In the present study, we have investigated the expression of nicotinic acetylcholine receptors (nAChRs) on astrocytes and neurons in the hippocampus and temporal cortex of subjects carrying the Swedish amyloid precursor protein (APP) 670/671 mutation (APPswe), patients with sporadic Alzheimer's disease (AD), and age-matched control subjects. Significant increases in the total numbers of astrocytes and of astrocytes expressing the alpha7 nAChR subunit, along with significant decreases in the levels of alpha7 and alpha4 nAChR subunits on neurons, were observed in the hippocampus and temporal cortex of both APPswe and sporadic AD brains. Both of these phenomena were more pronounced in APPswe than sporadic AD cases. Furthermore, the number of [(125)I]alpha-BTX binding sites (alpha7 nAChR) in the temporal cortex of the APPswe brain was significant lower than in the younger control group, reflecting the lower neuronal level of alpha7 nAChR. The increase in the level of expression of alpha7 nAChR on astrocytes was positively correlated with the extent of neuropathological alternations, especially the number of neuritic plaques, in the AD brain. The elevated expression of alpha7 nAChR on astrocytes might participate in Abeta cascade and formation of neuritic plaques, thereby playing an important role in the pathogenesis of AD.

Chronic nicotine administration exacerbates tau pathology in a transgenic model of Alzheimer's disease

The association between nicotinic acetylcholine receptor (nAChR) dysfunction and cognitive decline in Alzheimer's disease (AD) has been widely exploited for its therapeutic potential. The effects of chronic nicotine exposure on Abeta accumulation have been studied in both humans and animal models, but its therapeutic efficacy for AD neuropathology is still unresolved. To date, no in vivo studies have addressed the consequences of activating nAChRs on tau pathology. To determine the effects of chronic nicotine administration on Abeta and tau pathology, we chronically administrated nicotine to a transgenic model of AD (3xTg-AD) in their drinking water. Here, we show that chronic nicotine intake causes an up-regulation of nicotinic receptors, which correlated with a marked increase in the aggregation and phosphorylation state of tau. These data show that nicotine exacerbates tau pathology in vivo. The increase in tau phosphorylation appears to be due to the activation of p38-mitogen-activated protein kinase, which is known to phosphorylate tau in vivo and in vitro. We also show that the 3xTg-AD mice have an age-dependent reduction of alpha7nAChRs compared with age-matched nontransgenic mice in specific brain regions. The reduction of alpha7nAChRs is first apparent at 6 months of age and is restricted to brain regions that show intraneuronal Abeta(42) accumulation. Finally, this study highlights the importance of testing compounds designed to ameliorate AD pathology in a model with both neuropathological lesions because of the differential effects it can have on either Abeta or tau.

Selective changes in nicotinic acetylcholine receptor subtypes related to tobacco smoking: an immunohistochemical study

Increases in neuronal nicotinic receptors (nAChRs) in response to nicotine exposure have been reported in cell cultures, rodent brains, and in the brains of human smokers. The present study examines alterations in alpha4 and alpha7 nAChR subunit cellular expression in human hippocampus and entorhinal cortex from normal elderly individuals with known smoking history. There were significant increases in the intensity of alpha4 immunoreactive neuropil, but not the number of cell bodies, in many regions of hippocampus and entorhinal cortex in smokers compared to age-matched non-smokers and ex-smokers. There was also an increase in alpha7 immunoreactive perikarya in the granular cell layer of dentate gyrus in smokers but not other regions examined. There was, in contrast, a significant reduction in alpha7 immunoreactive astrocytes in smokers and ex-smokers compared to non-smokers. These findings suggest exposure to tobacco smoke acutely up-regulates alpha4 receptors in axon terminals and dendrites but not perikarya, whereas tobacco smoking induced down-regulation of alpha7 expression on astrocytes is a long-term effect. As the alpha4 subunit decreases with ageing and degenerative diseases such as Alzheimer's disease, whereas alpha7 increases in astrocytes in Alzheimer's disease, the findings further indicate the therapeutic relevance of nicotinic agonists such as nicotine.

5-substituted derivatives of 6-halogeno-3-((2-(S)-azetidinyl)methoxy)pyridine and 6-halogeno-3-((2-(S)-pyrrolidinyl)methoxy)pyridine with low picomolar affinity for alpha4beta2 nicotinic acetylcholine receptor and wide range of lipophilicity: potential probes for imaging with positron emission tomography

Potential positron emission tomography (PET) ligands with low picomolar affinity at the nicotinic acetylcholine receptor (nAChR) and with lipophilicity (log D) ranging from -1.6 to +1.5 have been synthesized. Most members of the series, which are derivatives of 5-substituted-6-halogeno-A-85380, exhibited a higher binding affinity at alpha4beta2-nAChRs than epibatidine. An analysis, by molecular modeling, revealed an important role of the orientation of the additional heterocyclic ring on the binding affinity of the ligands with nAChRs. The existing nicotinic pharmacophore models do not accommodate this finding. Two compounds of the series, 6-[(18)F]fluoro-5-(pyridin-3-yl)-A-85380 ([(18)F]31) and 6-chloro-3-((2-(S)-azetidinyl)methoxy)-5-(2-[(18)F]fluoropyridin-5-yl)pyridine) ([(18)F]35), were radiolabeled with (18)F. Comparison of PET data for [(18)F]31 and 2-[(18)F]FA shows the influence of lipophilicity on the binding potential. Our recent PET studies with [(18)F]35 demonstrated that its binding potential values in Rhesus monkey brain were ca. 2.5 times those of 2-[(18)F]FA. Therefore, [(18)F]35 and several other members of the series, when radiolabeled, will be suitable for quantitative imaging of extrathalamic nAChRs.

Bioactivity of a peptide derived from acetylcholinesterase: involvement of an ivermectin-sensitive site on the alpha 7 nicotinic receptor

A peptide fragment of 14 amino acids, derived from the C-terminus of acetylcholinesterase (AChE), might underlie the now well-established noncholinergic effects of the enzyme. This peptide is bioactive in a variety of systems including acute (brain slices) and chronic (organotypic culture) preparations of hippocampus, a pivotal area in Alzheimer's disease (AD); invariably, the action of the peptide is mediated specifically via an as yet unknown receptor. In this study, the allosteric alpha 7 agent, ivermectin (IVM), had a modest inhibitory effect, whilst that of the peptide was significantly more marked. However, ivermectin rendered ineffective the toxicity of high doses of the peptide, that is, when the two were co-applied, only the smaller effects of ivermectin were seen. Ivermectin, therefore, is presumably acting at a site that is identical to, or at least strongly interactive with, the normal binding site for AChE-peptide. This observation could have important implications for eventual therapeutic targeting of the action of AChE-peptide, in neurodegeneration.

The nicotinic acetylcholine receptor gene family of the pufferfish, Fugu rubripes☆

Nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission at nerve-muscle junctions and in the brain. However, the complete gene family of nAChRs has not so far been reported for any vertebrate organism. We have identified the complete nAChR gene family from the reference genome of the pufferfish, Fugu rubripes. It consists of 16 alpha and 12 non-alpha candidate subunits, making it the largest vertebrate nAChR gene family known to date. The gene family includes an unusual set of muscle-like nAChR subunits comprising two alpha1s, two beta1s, one delta, one epsilon, and one gamma. One of the beta1 subunits possesses an aspartate residue and N-glycosylation sites hitherto shown to be necessary for delta-subunit function. Potential Fugu orthologs of neuronal nAChR subunits alpha2-4, alpha6, and beta2-4 have been identified. Interestingly, the Fugu alpha5 counterpart appears to be a non-alpha subunit. Fugu possesses an expanded set of alpha7-9-like subunits and no alpha10 ortholog has been found. Two new candidate beta subtypes, designated beta5 and beta6, may represent subunits yet to be found in the human genome. The Fugu nAChR gene structures are considerably more diverse than those of higher vertebrates, with evidence of "intron gain" in many cases. We show, using RT-PCR, that the Fugu nAChR subunits are expressed in a variety of tissues.

Molecular biology and electrophysiology of neuronal nicotinic receptors of rat chromaffin cells

Neuronal nicotinic acetylcholine receptors of chromaffin cells in the adrenal medulla are physiologically activated by acetylcholine to mediate catecholamine release into the bloodstream. The present study examined the subunit composition and functional properties of rat chromaffin cell neuronal nicotinic acetylcholine receptors using molecular biology, immunocytochemistry and whole-cell patch-clamp. Reverse transcription-polymerase chain reaction analysis indicated the presence of alpha2, alpha3, alpha4, alpha5, alpha7, beta2 and beta4 transcripts (alpha6 and beta3 could not be detected). Immunocytochemistry revealed most cells positive for alpha3, beta2, beta4 and alpha5 proteins. Few cells were immunoreactive for alpha2 and alpha4, while none was for alpha7. At single-cell level, colocalization could be demonstrated for alpha3alpha5 and alpha4beta2. Western blot analysis confirmed antibody specificity for alpha3, alpha4, alpha5, beta2 and beta4 subunits. Inward currents elicited by nicotine pulses were insensitive to alpha-bungarotoxin and low doses of methyllycaconitine, demonstrating lack of functional alpha7 receptors. Partial block of nicotine currents was observed with either AuIB alpha-conotoxin (selective against alpha3beta4 receptors) or MII alpha-conotoxin (selective against alpha3beta2 receptors). With high concentrations of co-applied toxins, antagonism occlusion developed, suggesting loss of subunit selectivity. Antagonism by dihydro-beta-erythroidine summated nonlinearly with AuIB and MII inhibition, confirming heterogeneity of neuronal nicotinic acetylcholine receptor block. The present results suggest that the most frequently encountered receptors of rat chromaffin cells should comprise alpha3beta4, alpha3beta2 with the addition of alpha5 subunits. Because of the prevailing subunit composition, rat chromaffin cell neuronal nicotinic acetylcholine receptors are suitable models, particularly for the alpha3beta4 subclasses of mammalian brain receptors recently demonstrated in discrete cerebral areas.

Mecamylamine interactions with galantamine and donepezil: effects on learning, acetylcholinesterase, and nicotinic acetylcholine receptors

Patch-clamp recordings of single ion channel activity demonstrated that donepezil, but not galantamine, could be blocked by the nicotinic cholinergic antagonist mecamylamine, suggesting that galantamine acted at a separate (allosteric) site. The aim of this experiment was to demonstrate at a whole organism, behavioral level that galantamine, but not donepezil, could reverse mecamylamine-induced learning impairment. Forty-four young female rabbits received 15 sessions in the 750-ms delay eyeblink classical conditioning procedure, after one of five drug treatments: 0.5 mg/kg mecamylamine, 3.0 mg/kg donepezil, 0.5 mg/kg mecamylamine plus 3.0 mg/kg galantamine, 0.5 mg/kg mecamylamine plus 3.0 mg/kg donepezil, or sterile saline vehicle. An additional 24 young female rabbits were tested in the explicitly unpaired condition after treatment with the same mecamylamine plus galantamine or donepezil combinations or with vehicle. In a previous study we demonstrated that 3.0 mg/kg galantamine facilitated learning in young rabbits. Donepezil (3.0 mg/kg) did not facilitate learning in this experiment. However, both galantamine and donepezil reversed the deleterious effects of mecamylamine on learning. Significant differences in plasma and brain acetylcholinesterase levels were detected among the drug treatment groups. Fifteen daily injections did not produce statistically significant changes in nicotinic receptor binding in any of the five treatment groups. One possible interpretation of these results is that donepezil affected nicotinic acetylcholine receptors by raising the synaptic level of acetylcholine and hence, the probability of receptor activation, whereas galantamine bound to distinct allosteric sites not blocked by mecamylamine.

Loss of nicotinic receptors induced by beta-amyloid peptides in PC12 cells: possible mechanism involving lipid peroxidation

The mechanisms involved in the loss of nicotinic acetylcholine receptors (nAChRs), seen in brains of patients with Alzheimer's disease (AD) and in cultured cells treated by beta-amyloid peptides (A betas), remain elusive. We give results to show that lipid peroxidation induced directly by A beta might be involved in the deficits of nAChRs. In the study, PC12 cells were treated by addition of 5 microM of A beta(25-35) and A beta(1-40), respectively, with or without a antioxidant, vitamin E. Besides significantly decreased MTT (3-(4,5-dimethylthiazol-2-yl)-2,5,diphenyltetrazolium bromide) reduction, an increased lipid peroxidation was detected in the cells, but no protein oxidation. Significant reductions in [(3)H]epibatidine and [(125)I]alpha-bungarotoxin binding sites and in the protein levels of the alpha 3 and alpha 7 nAChR subunits were observed in the cells treated with A betas. Furthermore, A beta(25-35) decreased the level of ubiquinone-9 in PC12 cells, but did not change the amount of cholesterol, providing further evidence for lipid peroxidation. Interestingly, when PC12 cells were pretreated by antioxidant before the addition of A betas, the lipid peroxidation and the decreased ubiquinone resulted from A betas were prohibited. The decreases of nAChR binding sites and subunit proteins resulted from A betas were mostly prevented by the pretreatment with antioxidant. These findings suggest that lipid peroxidation stimulated by A betas might be a mechanism for the loss of nAChRs associated with the pathogenesis of 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.

How mutations in the nAChRs can cause ADNFLE epilepsy

PURPOSE

The linkage between autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) and neuronal nicotinic acetylcholine receptor has been strongly reinforced by the report of five distinct mutations in the two genes coding for the major brain alpha4beta2 nicotinic acetylcholine (ACh) receptors. As a first step toward understanding the basic mechanisms underlying this genetically transmissible neurologic disorder, we examined the similarities and differences of the functional properties displayed by naturally occurring mutant forms of this ligand-gated channel.

METHODS

Functional studies of neuronal nicotinic ACh receptors reconstituted in Xenopus oocytes were designed to analyze the common traits displayed by the different mutations associated with ADNFLE.

RESULTS

Coexpression of the control and mutated alleles harboring the alpha4S248F mutation obtained from patient DNAs yielded ACh-evoked currents of amplitude comparable to the control responses but with a higher sensitivity and desensitization to the natural agonist. Alternatively, the other mutants (alpha4L776ins3, alpha4S252L, and beta2V287M) displayed an increased ACh sensitivity without pronounced desensitization. In addition, whereas a reduction of calcium permeability was observed for the mutants (alpha4S248F and alpha4L776ins3), no significant modification of ionic selectivity could be detected in the alpha4S252L mutation. Hence increase in ACh sensitivity is the only common characteristic so far observed between the four naturally occurring mutant receptors investigated.

CONCLUSIONS

Analyses of functional properties of four nAChR mutants associated with ADNFLE indicate that a gain of function of these mutant receptors may be at the origin of the neuronal network dysfunction that causes the epileptic seizures. These data are discussed in the context of our latest knowledge of the pyramidal cell function.

Intracellular accumulation of beta-amyloid(1-42) in neurons is facilitated by the alpha 7 nicotinic acetylcholine receptor in Alzheimer's disease

Amyloid beta(1-42), a major component of amyloid plaques, binds with exceptionally high affinity to the alpha 7 nicotinic acetylcholine receptor and accumulates intracellularly in neurons of Alzheimer's disease brains. In this study, we investigated the possibility that this binding plays a key role in facilitating intraneuronal accumulation of amyloid beta(1-42). Consecutive section immunohistochemistry and digital imaging were used to reveal the spatial relationship between amyloid beta(1-42) and the alpha 7 receptor in affected neurons of Alzheimer's disease brains. Results showed that neurons containing substantial intracellular accumulations of amyloid beta(1-42) invariably express relatively high levels of the alpha 7 receptor. Furthermore, this receptor is highly co-localized with amyloid beta(1-42) within neurons of Alzheimer's disease brains. To experimentally test the possibility that the binding interaction between exogenous amyloid beta(1-42) and the alpha 7 receptor facilitates internalization and intracellular accumulation of amyloid beta(1-42) in Alzheimer's disease brains, we studied the fate of exogenous amyloid beta(1-42) and its interaction with the alpha 7 receptor in vitro using cultured, transfected neuroblastoma cells that express elevated levels of this receptor. Transfected cells exhibited rapid binding, internalization and accumulation of exogenous amyloid beta(1-42), but not amyloid beta(1-40). Furthermore, the rate and extent of amyloid beta(1-42) internalization was related directly to the alpha 7 receptor protein level, since (1) the rate of amyloid beta(1-42) accumulation was much lower in untransfected cells that express much lower levels of this receptor and (2) internalization was effectively blocked by alpha-bungarotoxin, an alpha 7 receptor antagonist. As in neurons of Alzheimer's disease brains, the alpha 7 receptor in transfected cells was precisely co-localized with amyloid beta(1-42) in prominent intracellular aggregates. Internalization of amyloid beta(1-42) in transfected cells was blocked by phenylarsine oxide, an inhibitor of endocytosis. We suggest that the intraneuronal accumulation of amyloid beta(1-42) in Alzheimer's disease brains occurs predominantly in neurons that express the alpha 7 receptor. In addition, internalization of amyloid beta(1-42) may be facilitated by the high-affinity binding of amyloid beta(1-42) to the alpha 7 receptor on neuronal cell surfaces, followed by endocytosis of the resulting complex. This provides a plausible explanation for the selective vulnerability of neurons expressing the alpha 7 receptor in Alzheimer's disease brains and for the fact that amyloid beta(1-42) is the dominant amyloid beta peptide species in intracellular accumulations and amyloid plaques.