Nicotine binding in human striatum: elevation in schizophrenia and reductions in dementia with Lewy bodies, Parkinson's disease and Alzheimer's disease and in relation to neuroleptic medication

A Systematic Review on Drugs Acting as Nicotinic Acetylcholine Receptor Agonists in the Treatment of Dementia

Acetylcholine signaling is attenuated in early Alzheimer's disease (AD) and other dementias. A significant reduction in the expression of nicotinic acetylcholine receptors (nAChRs) in the brain of AD patients has also been reported in several molecular biological and in situ labeling studies. The modulation of the functional deficit of the cholinergic system as a pharmacological target could therefore have a clinical benefit, which is not to be neglected. This systematic review was conducted to identify clinical trials, which evaluated the safety and efficacy of nicotinic acetylcholine receptor agonists using Clinicaltrial (CT) and EudraCT databases. Structured searches identified 39 trials, which used 15 different drugs designed to increase the function of the nAChRs. Most of the identified clinical trials were phase II trials, with some of them classified as ongoing for several years. The systematic screening of the literature led to the selection of 14 studies out of the 8261 bibliographic records retrieved. Six trials reported detailed data on adverse events associated with the intervention, while twelve trials reported data on efficacy measures, such as attention, behavior and cognition. Overall, smost of the physical side effects of cholinergic agonists were reported to be well tolerated. Some trials also reported improvements in attention. However, the efficacy of these drugs in other cognitive and behavioral outcomes remains highly controversial.

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.

Systemic injection of nicotinic acetylcholine receptor antagonist mecamylamine affects licking, eyelid size, and locomotor and autonomic activities but not temporal prediction in male mice

Nicotinic acetylcholine receptors are thought to be associated with a wide range of phenomena, such as movement, learning, memory, attention, and addiction. However, the causal relationship between nicotinic receptor activity and behavior remains unclear. Contrary to the studies that examined the functions of muscarinic acetylcholine receptors, the role of the nicotinic acetylcholine receptors on behavior has not been examined as extensively. Here, we examined the effects of intraperitoneal injection of mecamylamine, a nicotinic acetylcholine receptor antagonist, on the performance of male mice in a head-fixed temporal conditioning task and a free-moving open-field task. The head-fixed experimental setup allowed us to record and precisely quantify the licking response while the mice performed the behavioral task with no external cues. In addition, by combining the utility of the head-fixed experimental design with computer vision analysis based on deep learning algorithms, we succeeded in quantifying the eyelid size of awake mice. In the temporal conditioning task, we delivered a 10% sucrose solution every 10 s using a blunt-tipped needle placed within the licking distance of the mice. After the training, the mice showed increased anticipatory licking toward the timing of sucrose delivery, suggesting that the mice could predict the timing of the reward. Systemic injection of mecamylamine decreased licking behavior and caused eye closure but had no effect on learned conditioned predictive behavior in the head-fixed temporal conditioning task. In addition, the injection of mecamylamine decreased spontaneous locomotor activity in a dose-dependent manner in the free-moving open-field task. The results in the open-field experiments further revealed that the effect of mecamylamine on fecal output and urination, suggesting the effects on autonomic activities. Our achievement of successful eyelid size recording has potential as a useful approach in initial screening for drug discovery. Our study paves a way forward to understanding the role of nicotinic acetylcholine receptors on learning and behavior.

Potential Roles of Glucagon-Like Peptide-1 and Its Analogues in Dementia Targeting Impaired Insulin Secretion and Neurodegeneration

Dementia is a chronic, irreversible condition marked by memory loss, cognitive decline, and mental instability. It is clinically related to various progressive neurological diseases, including Parkinson’s disease, Alzheimer’s disease, and Huntington’s. The primary cause of neurological disorders is insulin desensitization, demyelination, oxidative stress, and neuroinflammation accompanied by various aberrant proteins such as amyloid-β deposits, Lewy bodies accumulation, tau formation leading to neurofibrillary tangles. Impaired insulin signaling is directly associated with amyloid-β and α-synuclein deposition, as well as specific signaling cascades involved in neurodegenerative diseases. Insulin dysfunction may initiate various intracellular signaling cascades, including phosphoinositide 3-kinase (PI3K), c-Jun N-terminal kinases (JNK), and mitogen-activated protein kinase (MAPK). Neuronal death, inflammation, neuronal excitation, mitochondrial malfunction, and protein deposition are all influenced by insulin. Recent research has focused on GLP-1 receptor agonists as a potential therapeutic target. They increase glucose-dependent insulin secretion and are beneficial in neurodegenerative diseases by reducing oxidative stress and cytokine production. They reduce the deposition of abnormal proteins by crossing the blood-brain barrier. The purpose of this article is to discuss the role of insulin dysfunction in the pathogenesis of neurological diseases, specifically dementia. Additionally, we reviewed the therapeutic target (GLP-1) and its receptor activators as a possible treatment of dementia.

Potential Role of Heterocyclic Aromatic Amines in Neurodegeneration

Heterocyclic aromatic amines (HAAs) are mainly formed in the pyrolysis process during high-temperature cooking of meat. Meat consumption is very typical of the western diet, and the amount of meat consumption in the eastern countries is growing rapidly; HAAs represents widespread exposure. HAAs are classified as possible human carcinogens; numerous epidemiological studies have demonstrated regular consumption of meat with HAAs as risk factor for cancers. Specific HAAs have received major attention. For example, 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine has been extensively studied as a genotoxicant and mutagen, with emergent literature on neurotoxicity. Harmane has been extensively studied for a role in essential tremors and potentially Parkinson's disease (PD). Harmane levels have been demonstrated to be elevated in blood and brain in essential tremor patients. Meat consumption has been implicated in the etiology of neurodegenerative diseases; however, the role of toxicants formed during meat preparation has not been studied. Epidemiological studies are currently examining the association between HAAs and risk of neurodegenerative diseases such as essential tremors and PD. Studies from our laboratory and others have provided strong evidence that HAA exposure produces PD and Alzheimer's disease-relevant neurotoxicity in cellular and animal models. In this review, we summarize and critically evaluate previous studies on HAA-induced neurotoxicity and the molecular basis of potential neurotoxic effects of HAAs. The available studies provide strong support for the premise that HAAs may impact neurological function and that addressing gaps in understanding of adverse neurological outcomes is critical to determine whether these compounds are modifiable risk factors.

Cholinergic Receptor Modulation as a Target for Preventing Dementia in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative condition characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) in the midbrain resulting in progressive impairment in cognitive and motor abilities. The physiological and molecular mechanisms triggering dopaminergic neuronal loss are not entirely defined. PD occurrence is associated with various genetic and environmental factors causing inflammation and mitochondrial dysfunction in the brain, leading to oxidative stress, proteinopathy, and reduced viability of dopaminergic neurons. Oxidative stress affects the conformation and function of ions, proteins, and lipids, provoking mitochondrial DNA (mtDNA) mutation and dysfunction. The disruption of protein homeostasis induces the aggregation of alpha-synuclein (α-SYN) and parkin and a deficit in proteasome degradation. Also, oxidative stress affects dopamine release by activating ATP-sensitive potassium channels. The cholinergic system is essential in modulating the striatal cells regulating cognitive and motor functions. Several muscarinic acetylcholine receptors (mAChR) and nicotinic acetylcholine receptors (nAChRs) are expressed in the striatum. The nAChRs signaling reduces neuroinflammation and facilitates neuronal survival, neurotransmitter release, and synaptic plasticity. Since there is a deficit in the nAChRs in PD, inhibiting nAChRs loss in the striatum may help prevent dopaminergic neurons loss in the striatum and its pathological consequences. The nAChRs can also stimulate other brain cells supporting cognitive and motor functions. This review discusses the cholinergic system as a therapeutic target of cotinine to prevent cognitive symptoms and transition to dementia in PD.

Effects of nicotinic antagonists on working memory performance in young rhesus monkeys

Acetylcholine plays a pivotal neuromodulatory role in the brain, influencing neuronal activity and cognitive function. Nicotinic receptors, particularly α7 and α4β2 receptors, modulate firing of dorsolateral prefrontal (dlPFC) excitatory networks that underlie successful working memory function. Minimal work however has been done examining working memory following systemic blockade of nicotinic receptor systems in nonhuman primates, limiting the ability to explore interactions of other neuromodulatory influences with working memory impairment caused by nicotinic antagonism. In this study, we investigated working memory performance after administering three nicotinic antagonists, mecamylamine, methyllycaconitine, and dihydro-β-erythroidine, in rhesus macaques tested in a spatial delayed response task. Surprisingly, we found that no nicotinic antagonist significantly impaired delayed response performance compared to vehicle. In contrast, the muscarinic antagonist scopolamine reliably impaired delayed response performance in all monkeys tested. These findings suggest there are some limitations on using systemic nicotinic antagonists to probe the involvement of nicotinic receptors in aspects of dlPFC-dependent working memory function, necessitating alternative strategies to understand the role of this system in cognitive deficits seen in aging and neurodegenerative disease.

The role of nicotinic cholinergic neurotransmission in delusional thinking

Delusions are a difficult-to-treat and intellectually fascinating aspect of many psychiatric illnesses. Although scientific progress on this complex topic has been challenging, some recent advances focus on dysfunction in neural circuits, specifically in those involving dopaminergic and glutamatergic neurotransmission. Here we review the role of cholinergic neurotransmission in delusions, with a focus on nicotinic receptors, which are known to play a part in some illnesses where these symptoms appear, including delirium, schizophrenia spectrum disorders, bipolar disorder, Parkinson, Huntington, and Alzheimer diseases. Beginning with what we know about the emergence of delusions in these illnesses, we advance a hypothesis of cholinergic disturbance in the dorsal striatum where nicotinic receptors are operative. Striosomes are proposed to play a central role in the formation of delusions. This hypothesis is consistent with our current knowledge about the mechanism of action of cholinergic drugs and with our abstract models of basic cognitive mechanisms at the molecular and circuit levels. We conclude by pointing out the need for further research both at the clinical and translational levels.

Visual processing speed in freezing and non-freezing Parkinson's disease patients

Introduction

Parkinson's disease patients are usually characterized by body motor dysfunction due to dopaminergic reduction in the central nervous system. Freezing of gait is a motor disorder that affects certain Parkinson's disease patients. However, it is hypothesized that non-motor functions mediated by the cholinergic system are also involved in developing freezing of gait. Visual information processing speed, or inspection time is independent of the motor response, and can be used a reliable measure of the cholinergic system integrity.

Objective

Inspection time can be used to investigate whether Parkinson's disease patients with freezing of gait symptoms have a larger impairment in cholinergic mediated functions than those patients who have no freezing of gait symptoms and healthy controls.

Methods

The inspection time was determined by a simple length discrimination task. Twenty-two Parkinson's disease patients with freezing of gait, 25 Parkinson's disease patients without freezing of gait, and 25 aged matched healthy controls participated in the study.

Results

Based on the log values of IT score, Parkinson's disease patients with freezing of gait symptoms had statistically significant slower inspection times (mean of 1.793 ms) than Parkinson's disease patients without freezing of gait (mean of 1.655 ms) and healthy controls (mean of 1.523 ms). Inspection times for the Parkinson's disease patients without FOG symptoms were also significantly slower than healthy controls.

Conclusion

The results of this study support the hypothesis that the cholinergic system integrity is affected more in Parkinson's disease patients with freezing of gait symptoms.

•

Non-motor symptoms are associated with Parkinson’s disease patients.

•

Inspection time can be used as a reliable tool to evaluate the integrity of the cholinergic system.

•

Patients with Parkinson’s disease especially those who showed freezing of gait symptoms had longer inspection time score which suggest they have deficit in the cholinergic system.

PhIP exposure in rodents produces neuropathology potentially relevant to Alzheimer's disease

Alzheimer's disease (AD) is a public health crisis due to debilitating cognitive symptoms and lack of curative treatments, in the context of increasing prevalence. Thus, it is critical to identify modifiable risk factors. High levels of meat consumption may increase AD risk. Many toxins are formed during meat cooking such as heterocyclic aromatic amines (HAAs). Our prior studies have shown that HAAs produce dopaminergic neurotoxicity. Given the mechanistic and pathological overlap between AD and dopaminergic disorders we investigated whether exposure to 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP), a prevalent dietary HAA formed during high-temperature meat cooking, may produce AD-relevant neurotoxicity. Here, C57BL/6 mice were treated with 100 or 200 mg/kg PhIP for 8 h or 75 mg/kg for 4 weeks and 16 weeks. PhIP exposure for 8 h produced oxidative damage, and AD-relevant alterations in hippocampal synaptic proteins, Amyloid-beta precursor protein (APP), and β-Site amyloid precursor protein cleaving enzyme 1 (BACE1). PhIP exposure for 4 weeks resulted in an increase in BACE1. PhIP exposure for 16 weeks resulted in increased hippocampal oxidative damage, APP, BACE1, Aβ aggregation, and tau phosphorylation. Quantification of intracellular nitrotyrosine revealed oxidative damage in cholinergic neurons after 8 h, 4 weeks and 16 weeks of PhIP exposure. Our study demonstrates that increase in oxidative damage, APP and BACE1 might be a possible mechanism by which PhIP promotes Aβ aggregation. Given many patients with AD or PD exhibit neuropathological overlap, our study suggests that HAA exposure should be further studied for roles in mediating pathogenic overlap.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

Identification of Natural Compounds against Neurodegenerative Diseases Using In Silico Techniques

The aim of this study was to identify new potentially active compounds for three protein targets, tropomyosin receptor kinase A (TrkA), N-methyl-d-aspartate (NMDA) receptor, and leucine-rich repeat kinase 2 (LRRK2), that are related to various neurodegenerative diseases such as Alzheimer's, Parkinson's, and neuropathic pain. We used a combination of machine learning methods including artificial neural networks and advanced multilinear techniques to develop quantitative structure⁻activity relationship (QSAR) models for all target proteins. The models were applied to screen more than 13,000 natural compounds from a public database to identify active molecules. The best candidate compounds were further confirmed by docking analysis and molecular dynamics simulations using the crystal structures of the proteins. Several compounds with novel scaffolds were predicted that could be used as the basis for development of novel drug inhibitors related to each target.

Reversal of mecamylamine-induced effects in healthy subjects by nicotine receptor agonists: Cognitive and (electro) physiological responses

AIMS

Establishing a pharmacological challenge model could yield an important tool to understand the complex role of the nicotinic cholinergic system in cognition and to develop novel compounds acting on the nicotinic acetylcholine receptor.

METHODS

This randomized, double-blind, double-dummy, placebo-controlled, four-way crossover study examined the effects of the nicotinic antagonist mecamylamine on a battery of cognitive and neurophysiological test with coadministration of a placebo, nicotine or galantamine in order to reverse the cognitive impairment caused by mecamylamine.

RESULTS

Thirty-three healthy subjects received a single oral dose of 30 mg of mecamylamine (or placebo) in combination with either 16 mg of oral galantamine or 21 mg of transdermal nicotine (or its double-dummy). Mecamylamine 30 mg induced significant disturbances of cognitive functions. Attention and execution of visual (fine) motor tasks was decreased, short- and long-term memory was impaired and the reaction velocity during the test was slower when compared to placebo. Mecamylamine 30 mg produced a decrease in posterior α and β power in the surface electroencephalogram, effects that were reversed by nicotine coadministration. Memory and motor coordination tests could be partially reversed by the coadministration of nicotine.

CONCLUSIONS

Mecamylamine administration induced slowing of the electroencephalogram and produced decrease in performance of tests evaluating motor coordination, sustained attention and short- and long-term memory. These effects could be partially reversed by the coadministration of nicotine, and to a lesser extent by galantamine.

Effect of varenicline on behavioral deficits in a rat model of Parkinson's disease induced by unilateral 6-hydroxydopamine lesion of substantia nigra

Nicotinic acetylcholine receptors (nAChRs) are implicated in the pathogenesis of Parkinson's disease (PD). Varenicline tartrate is a partial agonist at α4β2 and full agonist at α7 neuronal nAChR subunits. A unilateral lesion of the substantia nigra (SN) has been used as a reliable model of PD. This study aimed to investigate the effect of varenicline on locomotor and nonlocomotor behavioral deficits induced by a unilateral lesion of the SN induced by 6-hydroxydopamine (6-OHDA) (8 µg/4 µl). Varenicline (1 mg/kg) was administered to the lesioned rats daily for 2 weeks, which commenced 3 weeks after 6-OHDA administration. The results showed that varenicline improved motor deficits induced by 6-OHDA. It improved locomotor and nonlocomotor activities such as forelimb use, rotarod performance, and forelimb asymmetry. Varenicline did not change rearing or vibrissae-elicited forelimb placing but did increase apomorphine-induced rotation. In conclusion, the present results suggest that drugs with specific partial/full agonistic activity on nAChR subunits could be of value in the treatment of neurodegenerative disorders such as PD.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

Brain cell apoptosis inhibition by butylphthalide in Alzheimer's disease model in rats

The present study was designed to test the hypothesis that butylphthalide protects the brain of Alzheimer's disease (AD) model rats by inhibiting apoptosis. Ninety Sprague-Dawley rats were randomly divided into drug, control and blank groups of 30 rats in each. The rats in the drug and control groups were treated to induce AD. Then, the rats in the drug group were administered with butylphthalide daily, the rats in the AD control group were given normal saline, and the rats in the healthy group were fed routinely. All rats were sacrificed after 30 days; the brain tissues were used for testing for apoptosis by the terminal deoxynucleotidyl-transferase-mediated dUTP nick end labelling (TUNEL) staining method, for determining mitogen-activated protein kinase (MAPK), ERK and P21 protein by western blot analysis, and their cognate mRNA levels by RT-PCR. The results of the TUNEL staining indicated that apoptosis of the brain tissues of rats in the drug group was significantly less than that in the control group and blank group. The protein expression levels of MAPK in the drug group were significantly lower than that in the control group, but higher than that in the normal healthy group (P<0.05). The mRNA expression levels of MAPK in the drug group were significantly lower than those in the control group, but higher than those in the normal healthy group (P<0.05). Based on these results, butylphthalide showed a protective apoptosis-inhibition effect on the brain tissues of the AD rats and this seems to be a consequence of its inhibition of the expressions of MAPK mRNA and MAPK protein in the brain of the rat.

Pharmacokinetics and pharmacodynamics of oral mecamylamine - development of a nicotinic acetylcholine receptor antagonist cognitive challenge test using modelling and simulation

A pharmacologic challenge model with a nicotinic antagonist could be an important tool not only to understand the complex role of the nicotinic cholinergic system in cognition, but also to develop novel compounds acting on the nicotinic acetylcholine receptor. The objective was to develop a pharmacokinetic-pharmacodynamic (PKPD) model using nonlinear mixed effects (NLME) methods to quantitate the pharmacokinetics of three oral mecamylamine doses (10, 20 and 30 mg) and correlate the plasma concentrations to the pharmacodynamic effects on a cognitive and neurophysiologic battery of tests in healthy subjects. A one-compartment linear kinetic model best described the plasma concentrations of mecamylamine. Mecamylamine's estimated clearance was 0.28 ± 0.015 L min^-1. The peripheral volume of distribution (291 ± 5.15 L) was directly related to total body weight. Mecamylamine impaired the accuracy and increased the reaction time in tests evaluating short term working memory with a steep increase in the concentration-effect relationship at plasma concentrations below 100 μg L^-1. On the other hand, mecamylamine induced a decrease in performance of tests evaluating visual and fine motor coordination at higher plasma concentrations (EC₅₀ 97 μg L^-1). Systolic and diastolic blood pressure decreased exponentially after a plasma mecamylamine concentration of 80 μg L^-1, a known effect previously poorly studied in healthy subjects. The developed mecamylamine PKPD model was used to quantify the effects of nicotinic blockade in a set of neurophysiological tests in humans with the goal to provide insight into the physiology and pharmacology of the nicotinic system in humans and the possibility to optimize future trials that use mecamylamine as a pharmacological challenge.

A Controlled Evaluation of a Training Course for Staff Who Work with People with Dementia

This controlled intervention was a pilot study that examined the impact of a brief training programme on N= 40 care-workers. The six-week course (one hour per week) produced significant changes in the staff’s attitude and knowledge base; no change was evident for their coping style. In accounting for the results, this article presents a qualitative analysis of the process features underpinning the improvements. The training employed an integrative approach, combining cognitive behavioural and experiential learning methods.

Molecular changes in the postmortem parkinsonian brain

Parkinson disease (PD) is the second most common neurodegenerative disease after Alzheimer disease. Although PD has a relatively narrow clinical phenotype, it has become clear that its etiological basis is broad. Post-mortem brain analysis, despite its limitations, has provided invaluable insights into relevant pathogenic pathways including mitochondrial dysfunction, oxidative stress and protein homeostasis dysregulation. Identification of the genetic causes of PD followed the discovery of these abnormalities, and reinforced the importance of the biochemical defects identified post-mortem. Recent genetic studies have highlighted the mitochondrial and lysosomal areas of cell function as particularly significant in mediating the neurodegeneration of PD. Thus the careful analysis of post-mortem PD brain biochemistry remains a crucial component of research, and one that offers considerable opportunity to pursue etiological factors either by 'reverse biochemistry' i.e. from defective pathway to mutant gene, or by the complex interplay between pathways e.g. mitochondrial turnover by lysosomes. In this review we have documented the spectrum of biochemical defects identified in PD post-mortem brain and explored their relevance to metabolic pathways involved in neurodegeneration. We have highlighted the complex interactions between these pathways and the gene mutations causing or increasing risk for PD. These pathways are becoming a focus for the development of disease modifying therapies for PD. Parkinson's is accompanied by multiple changes in the brain that are responsible for the progression of the disease. We describe here the molecular alterations occurring in postmortem brains and classify them as: Neurotransmitters and neurotrophic factors; Lewy bodies and Parkinson's-linked genes; Transition metals, calcium and calcium-binding proteins; Inflammation; Mitochondrial abnormalities and oxidative stress; Abnormal protein removal and degradation; Apoptosis and transduction pathways. This article is part of a special issue on Parkinson disease.

Cholinergic imaging in dementia spectrum disorders

The multifaceted nature of the pathology of dementia spectrum disorders has complicated their management and the development of effective treatments. This is despite the fact that they are far from uncommon, with Alzheimer's disease (AD) alone affecting 35 million people worldwide. The cholinergic system has been found to be crucially involved in cognitive function, with cholinergic dysfunction playing a pivotal role in the pathophysiology of dementia. The use of molecular imaging such as SPECT and PET for tagging targets within the cholinergic system has shown promise for elucidating key aspects of underlying pathology in dementia spectrum disorders, including AD or parkinsonian dementias. SPECT and PET studies using selective radioligands for cholinergic markers, such as [(11)C]MP4A and [(11)C]PMP PET for acetylcholinesterase (AChE), [(123)I]5IA SPECT for the α4β2 nicotinic acetylcholine receptor and [(123)I]IBVM SPECT for the vesicular acetylcholine transporter, have been developed in an attempt to clarify those aspects of the diseases that remain unclear. This has led to a variety of findings, such as cortical AChE being significantly reduced in Parkinson's disease (PD), PD with dementia (PDD) and AD, as well as correlating with certain aspects of cognitive function such as attention and working memory. Thalamic AChE is significantly reduced in progressive supranuclear palsy (PSP) and multiple system atrophy, whilst it is not affected in PD. Some of these findings have brought about suggestions for the improvement of clinical practice, such as the use of a thalamic/cortical AChE ratio to differentiate between PD and PSP, two diseases that could overlap in terms of initial clinical presentation. Here, we review the findings from molecular imaging studies that have investigated the role of the cholinergic system in dementia spectrum disorders.

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.

The nicotinic cholinergic system function in the human brain

Research on the nicotinic cholinergic system function in the brain was previously mainly derived from animal studies, yet, research in humans is growing. Up to date, findings allow significant advances on the understanding of nicotinic cholinergic effects on human cognition, emotion and behavior using a range of functional brain imaging approaches such as pharmacological functional magnetic resonance imaging or positron emission tomography. Studies provided insights across various mechanistic psychological domains using different tasks as well as at rest in both healthy individuals and patient populations, with so far partly mixed results reporting both enhancements and decrements of neural activity related to the nicotinic cholinergic system. Moreover, studies on the relation between brain structure and the nicotinic cholinergic system add important information in this context. The present review summarizes the current status of human brain imaging studies and presents the findings within a theoretical and clinical perspective as they may be useful not only for an advancement of the understanding of basic nicotinic cholinergic-related mechanisms, but also for the development and integration of psychological and pharmacological treatment approaches. Patterns of functional neuroanatomy and neural circuitry across various cognitive and emotional domains may be used as neuropsychological markers of mental disorders such as addiction, Alzheimer's disease, Parkinson disease or schizophrenia, where nicotinic cholinergic system changes are characteristic. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Nicotinic acetylcholine receptor density in cognitively intact subjects at an early stage of Parkinson's disease

We investigated in vivo brain nicotinic acetylcholine receptor (nAChR) distribution in cognitively intact subjects with Parkinson’s disease (PD) at an early stage of the disease. Fourteen patients and 13 healthy subjects were imaged with single photon emission computed tomography and the radiotracer 5-[¹²³I]iodo-3-[2(S)-2-azetidinylmethoxy]pyridine ([¹²³I]5IA). Patients were selected according to several criteria, including short duration of motor signs (<7 years) and normal scores at an extensive neuropsychological evaluation. In PD patients, nAChR density was significantly higher in the putamen, the insular cortex and the supplementary motor area and lower in the caudate nucleus, the orbitofrontal cortex, and the middle temporal gyrus. Disease duration positively correlated with nAChR density in the putamen ipsilateral (ρ = 0.56, p < 0.05) but not contralateral (ρ = 0.49, p = 0.07) to the clinically most affected hemibody. We observed, for the first time in vivo, higher nAChR density in brain regions of the motor and limbic basal ganglia circuits of subjects with PD. Our findings support the notion of an up-regulated cholinergic activity at the striatal and possibly cortical level in cognitively intact PD patients at an early stage of disease.

Complex relationships of nicotinic receptor actions and cognitive functions

Nicotine has been shown in a variety of studies to improve cognitive function including learning, memory and attention. Nicotine both stimulates and desensitizes nicotinic receptors, thus acting both as an agonist and a net antagonist. The relative roles of these two actions for nicotine-induced cognitive improvement have not yet been fully determined. We and others have found that acute nicotinic antagonist treatment can improve learning and attention. Nicotine acts on a variety of nicotinic receptor subtypes. The relative role and interactions of neuronal nicotinic receptor subtypes for cognition also needs to be better characterized. Nicotine acts on nicotinic receptors in a wide variety of brain areas. The role of some of these areas such as the hippocampus has been relatively well studied but other areas like the thalamus, which has the densest nicotinic receptor concentration are still only partially characterized. In a series of studies we characterized nicotinic receptor actions, anatomic localization and circuit interactions, which are critical to nicotine effects on the cognitive functions of learning, memory and attention. The relative role of increases and decreases in nicotinic receptor activation by nicotine were determined in regionally specific studies of the hippocampus, the amygdala, the frontal cortex and the mediodorsal thalamic nucleus with local infusions of antagonists of nicotinic receptor subtypes (α7 and α4β2). The understanding of the functional neural bases of cognitive function is fundamental to the more effective development of nicotinic drugs for treating cognitive dysfunction.

α6β2* and α4β2* nicotinic acetylcholine receptors as drug targets for Parkinson's disease

Parkinson's disease is a debilitating movement disorder characterized by a generalized dysfunction of the nervous system, with a particularly prominent decline in the nigrostriatal dopaminergic pathway. Although there is currently no cure, drugs targeting the dopaminergic system provide major symptomatic relief. As well, agents directed to other neurotransmitter systems are of therapeutic benefit. Such drugs may act by directly improving functional deficits in these other systems, or they may restore aberrant motor activity that arises as a result of a dopaminergic imbalance. Recent research attention has focused on a role for drugs targeting the nicotinic cholinergic systems. The rationale for such work stems from basic research findings that there is an extensive overlap in the organization and function of the nicotinic cholinergic and dopaminergic systems in the basal ganglia. In addition, nicotinic acetylcholine receptor (nAChR) drugs could have clinical potential for Parkinson's disease. Evidence for this proposition stems from studies with experimental animal models showing that nicotine protects against neurotoxin-induced nigrostriatal damage and improves motor complications associated with l-DOPA, the "gold standard" for Parkinson's disease treatment. Nicotine interacts with multiple central nervous system receptors to generate therapeutic responses but also produces side effects. It is important therefore to identify the nAChR subtypes most beneficial for treating Parkinson's disease. Here we review nAChRs with particular emphasis on the subtypes that contribute to basal ganglia function. Accumulating evidence suggests that drugs targeting α6β2* and α4β2* nAChR may prove useful in the management of Parkinson's disease.

Effects of moderate-dose treatment with varenicline on neurobiological and cognitive biomarkers in smokers and nonsmokers with schizophrenia or schizoaffective disorder

CONTEXT

The administration of nicotine transiently improves many neurobiological and cognitive functions in schizophrenia and schizoaffective disorder. It is not yet clear which nicotinic acetylcholine receptor (nAChR) subtype or subtypes are responsible for these seemingly pervasive nicotinic effects in schizophrenia and schizoaffective disorder.

OBJECTIVE

Because α4β2 is a key nAChR subtype for nicotinic actions, we investigated the effect of varenicline tartrate, a relatively specific α4β2 partial agonist and antagonist, on key biomarkers that are associated with schizophrenia and are previously shown to be responsive to nicotinic challenge in humans.

DESIGN

A double-blind, parallel, randomized, placebo-controlled trial of patients with schizophrenia or schizoaffective disorder to examine the effects of varenicline on biomarkers at 2 weeks (short-term treatment) and 8 weeks (long-term treatment), using a slow titration and moderate dosing strategy for retaining α4β2-specific effects while minimizing adverse effects.

SETTING

Outpatient clinics.

PARTICIPANTS

A total of 69 smoking and nonsmoking patients; 64 patients completed week 2, and 59 patients completed week 8. Intervention Varenicline.

MAIN OUTCOME MEASURES

Prepulse inhibition, sensory gating, antisaccade, spatial working memory, eye tracking, processing speed, and sustained attention.

RESULTS

A moderate dose of varenicline (1) significantly reduced the P50 sensory gating deficit in nonsmokers after long-term treatment (P = .006), (2) reduced startle reactivity (P = .02) regardless of baseline smoking status, and (3) improved executive function by reducing the antisaccadic error rate (P = .03) regardless of smoking status. A moderate dose of varenicline had no significant effect on spatial working memory, predictive and maintenance pursuit measures, processing speed, or sustained attention by Conners' Continuous Performance Test. Clinically, there was no evidence of exacerbation of psychiatric symptoms, psychosis, depression, or suicidality using a gradual titration (1-mg daily dose).

CONCLUSIONS

Moderate-dose treatment with varenicline has a unique treatment profile on core schizophrenia-related biomarkers. Further development is warranted for specific nAChR compounds and dosing and duration strategies to target subgroups of schizophrenic patients with specific biological deficits.

[Brain SPECT in Lewy body dementia]

Dementia of Lewy bodies (DLB) is the second cause of degenerative dementia. There is many clinical presentation of the disease. Brain single photon computed tomography (SPECT) is a simple way to investigate routinely the cerebral blood flow. On cerebral perfusion SPECT, DLB is accompanied by diffuse cortical hypoperfusion predominantly at the posterior cortex and may affect the associative and primary visual areas in relation to neuronal loss or dysfunction. DLB patients have striatal hypofixation on cerebral neurotranmission SPECT-DaTSCAN(®), related with dopaminergic loss. Brain SPECT is useful in the differential diagnosis between DLB and other dementia.

Decrease of nicotinic receptors in the nigrostriatal system in Parkinson's disease

Smoking is associated with a lower incidence of Parkinson's disease (PD), which might be related to a neuroprotective action of nicotine. Postmortem studies have shown a decrease of cerebral nicotinic acetylcholine receptors (nAChRs) in PD. In this study, we evaluated the decrease of nAChRs in PD in vivo using positron emission tomography (PET), and we explored the relationship between nAChRs density and PD severity using both clinical scores and the measurement of striatal dopaminergic function. Thirteen nondemented patients with PD underwent two PET scans, one with 6-[(18)F]fluoro-3,4-dihydroxy-L-phenylalanine (6-[(18)F]fluoro-L-DOPA) to measure the dopaminergic function and another with 2-[(18)F]fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine (2-[(18)F]fluoro-A-85380), a radiotracer with high affinity for the nAChRs. Distribution volumes (DVs) of 2-[(18)F]fluoro-A-85380 measured in the PD group were compared with those obtained from six nonsmoking healthy controls, with regions-of-interest and voxel-based approaches. Both analyses showed a significant (P <0.05) decrease of 2-[(18)F]fluoro-A-85380 DV in the striatum (-10%) and substantia nigra (-14.9%) in PD patients. Despite the wide range of PD stages, no correlation was found between DV and the clinical and PET markers of PD severity.

UTILIZAÇÃO DE DROSOPHILA MELANOGASTER COMO BIOINDICADOR NA AVALIAÇÃO DA LETALIDADE DE EXTRATO DE NICOTIANA TABACUM

RESUMO Nicotiana tabacumLinnaeus é uma planta de importância econômica, explorada comercialmente em todo o mundo. Quimicamente, destaca-se pela presença de alcalóides pirimidínicos, como a anabasina e a nicotina, além de esteroides, cumarinas e terpenos. O objetivo do estudo foi verificar a concentração do extrato aquoso do tabaco que mata aproximadamente 50% da população do bioindicador Drosophila melanogaster Morgan em 24h (CL50). Foram utilizados o total de 689 indivíduos da linhagem selvagem de D. melanogaster que foram expostos às concentrações 0, 5, 10 e 15% do extrato aquoso de tabaco. A mortalidade dos indivíduos foi contabilizada em 24h e 48h após aplicação do extrato. Os resultados obtidos mostraram que o extrato aquoso de N. tabacum promove morte dos indivíduos em todas as concentrações testadas. Em 24h, o extrato aquoso na concentração a 10% matou 50% das moscas. Houve uma correlação entre o aumento da concentração de fumo com aumento da mortalidade. Estes resultados reforçam a relevância das investigações sobre o tabaco, devido a suas propriedades tóxicas e por ser consumido por milhões de pessoas no mundo.

Smoking, nicotine and neuropsychiatric disorders

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

Approaches to measuring the effects of wake-promoting drugs: a focus on cognitive function

OBJECTIVES

In clinical drug development, wakefulness and wake-promotion may be assessed by a large number of scales and questionnaires. Objective assessment of wakefulness is most commonly made using sleep latency/maintenance of wakefulness tests, polysomnography and/or behavioral measures. The purpose of the present review is to highlight the degree of overlap in the assessment of wakefulness and cognition, with consideration of assessment techniques and the underlying neurobiology of both concepts.

DESIGN

Reviews of four key areas were conducted: commonly used techniques in the assessment of wakefulness; neurobiology of sleep/wake and cognition; targets of wake promoting and/or cognition enhancing drugs; and ongoing clinical trials investigating wake promoting effects.

RESULTS

There is clear overlap between the assessment of wakefulness and cognition. There are common techniques which may be used to assess both concepts; aspects of the neurobiology of both concepts may be closely related; and wake-promoting drugs may have nootropic properties (and vice versa). Clinical trials of wake-promoting drugs often, though not routinely, assess aspects of cognition.

CONCLUSIONS

Routine and broad assessment of cognition in the development of wake-promoting drugs may reveal important nootropic effects, which are not secondary to alertness/wakefulness, whilst existing cognitive enhancers may have underexplored or unknown wake promoting properties.

Nicotine potentiates vascular endothelial growth factor expression in balloon-injured rabbit aortas

Both nicotine and vascular endothelial growth factor (VEGF) have been proposed to play an important role in the development and progression of atherosclerosis. In vitro and ex vivo studies have demonstrated that nicotine significantly stimulates VEGF expression in several cell types. This study examined the effects and the mechanisms of nicotine on the expression of VEGF in a rabbit model of balloon-injured aortas. Forty-eight male New Zealand white rabbits were randomly divided into sham, control, nicotine, and nicotine plus hexamethonium (nicotine-hex) groups. Balloon catheter denuding injury iliac artery was performed in control, nicotine, and nicotine-hex animals fed with a high-cholesterol diet beginning 2 weeks before operation. Twenty-four hours after surgery, nicotine (0.05 microg/kg) or nicotine (0.05 microg/kg) and hexamethonium (6 mg/kg) was administered daily by intramuscular injection for 3 weeks in nicotine and nicotine-hex groups, respectively. Sham and control rabbits received an identical volume of phosphate-buffered saline injection, but without nicotine or hexamethonium. VEGF protein expression and intimal cell proliferation in balloon-injured aortas were determined by enzyme-link immunosorbent assay, immunohistochemistry, and Western blot analysis. Six rabbits died during the experiment. The remaining 42 rabbits were included in the study. VEGF protein expression in nicotine group was significantly higher than that in control group (P < 0.01). VEGF positive staining was seen in vascular endothelial cells, vascular smooth muscle cells, and infiltrative inflammatory cells. The number of the proliferative cells in intima was also significantly higher in nicotine group than in control group (P < 0.01). Hexamethonium, a nonselective antagonist of nicotinic acetylcholine receptors (nAChRs), significantly inhibited nicotine-induced VEGF protein expression (P < 0.01). The present study shows that intramuscular administration of nicotine markedly potentiates the expression of VEGF protein in balloon-injured rabbit aortas, which appears to be mediated through nAChRs.

Chronic nicotine exposure has dissociable behavioural effects on control and beta2-/- mice

Nicotine exerts beneficial effects on various neurological and psychiatric pathologies, yet its effects on cognitive performance remain unclear. Mice lacking the beta2 subunit of the nicotinic receptor (beta2-/-) show characteristic deficits in executive functions and are suggested as reliable animal models for some specific endophenotypes of human pathologies, notably ADHD. We use beta2-/- and their controls to investigate the consequences of chronic nicotine exposure on cognitive behaviour. We show that in control mice, this treatment elicits somewhat slight effects, particularly affecting nocturnal activity and self-grooming. By contrast, in beta2-/- mice, chronic nicotine treatment had restorative effects on exploratory behaviour in the open-field and affected rearing, but did not modify motor functions. We confirmed that beta2-/- mice exhibit impaired exploratory and social behaviour, and further demonstrated their nocturnal hyperactivity. These data support the proposal that beta2-/- mice represent a relevant model for cognitive disorders in humans and that nicotine administered chronically at low dose may relieve some of these.

NIDA522131, a new radioligand for imaging extrathalamic nicotinic acetylcholine receptors: in vitro and in vivo evaluation

A novel radioligand, 6-chloro-3-((2-(S)-azetidinyl)methoxy)-5-(2-fluoropyridin-4-yl)pyridine (NIDA522131), for imaging extrathalamic nicotinic acetylcholine receptors (nAChRs) was characterized in vitro and in vivo using positron emission tomography. The K(d) and T(1/2) of dissociation of NIDA522131 binding measured at 37 degrees C in vitro were 4.9 +/- 0.4 pmol/L and 81 +/- 5 min, respectively. The patterns of radioactivity distribution in monkey brain in vivo was similar to that of 2-[(18)F]fluoro-3-(2(S)-azetidinylmethoxy)pyridine (2FA), a radioligand that has been successfully used in humans, and matched the alpha(4)beta(2)* nAChRs distribution. Comparison between [(18)F]NIDA522131 and 2FA demonstrated better in vivo binding properties of the new radioligand and substantially greater radioactivity accumulation in brain. Consistent with [(18)F]NIDA522131 elevated affinity for nAChRs and its increased lipophilicity, both, the total and non-displaceable distribution volumes were substantially higher than those of 2FA. Estimated binding potential values in different brain regions, characterizing the specificity of receptor binding, were 3-4 fold higher for [(18)F]NIDA522131 than those of 2FA. Pharmacological evaluation in mice demonstrated a toxicity that was comparable to 2FA and is in agreement with a 2300 fold higher affinity at alpha(4)beta(2)* versus alpha(3)beta(4)* nAChRs. These results suggest that [(18)F]NIDA522131 is a promising positron emission tomography radioligand for studying extrathalamic nAChR in humans.

Dementia with Lewy bodies

The advent of new immunostains have improved the ability to detect limbic and cortical Lewy bodies, and it is evident that dementia with Lewy bodies (DLB) is the second most common neurodegenerative dementia, after Alzheimer's disease (AD). Distinguishing DLB from AD has important implications for treatment, in terms of substances that may worsen symptoms and those that may improve them. Neurocognitive patterns, psychiatric features, extrapyramidal signs, and sleep disturbance are helpful in differentiating DLB from AD early in the disease course. Differences in the severity of cholinergic depletion and type/distribution of neuropathology contribute to these clinical differences.

Central Nicotinic Receptors: Structure, Function, Ligands, and Therapeutic Potential

The growing interest in nicotinic receptors, because of their wide expression in neuronal and non-neuronal tissues and their involvement in several important CNS pathologies, has stimulated the synthesis of a high number of ligands able to modulate their function. These membrane proteins appear to be highly heterogeneous, and still only incomplete information is available on their structure, subunit composition, and stoichiometry. This is due to the lack of selective ligands to study the role of nAChR under physiological or pathological conditions; so far, only compounds showing selectivity between alpha4beta2 and alpha7 receptors have been obtained. The nicotinic receptor ligands have been designed starting from lead compounds from natural sources such as nicotine, cytisine, or epibatidine, and, more recently, through the high-throughput screening of chemical libraries. This review focuses on the structure of the new agonists, antagonists, and allosteric ligands of nicotinic receptors, it highlights the current knowledge on the binding site models as a molecular modeling approach to design new compounds, and it discusses the nAChR modulators which have entered clinical trials.