Acute nicotinic blockade produces cognitive impairment in normal humans

Effect of estradiol with or without micronized progesterone on cholinergic-related cognitive performance in postmenopausal women

Introduction

Women are at a higher risk of developing Alzheimer's disease (AD), and the decline in estrogens post-menopause is thought of as a factor increasing this risk. Estradiol (E2) is important in supporting cholinergic neuronal integrity, and cholinergic functioning may be negatively impacted following the loss of E2 post-menopause. The use of exogenous E2 has been observed to enhance cholinergically mediated cognitive performance in healthy post-menopausal women, which indicates a potentially protective mechanism. However, E2 is often co-administered with progestin or progesterone to prevent endometrial proliferation. Progesterone/progestins have previously been shown to have a detrimental effect on E2-mediated biological and cognitive effects mediated by cholinergic systems in preclinical models, therefore the present study aimed to assess whether progesterone would modify the effect of E2 to influence cognition during cholinergic blockade.

Methods

Twenty participants completed 3-months of oral E2 treatment with micronized progesterone (mPRO) or with placebo (PLC) in a repeated-measures within-subjects crossover design, in which they also completed five anticholinergic challenge days per hormone treatment condition. During the challenge participants were administered low or high doses of the nicotinic cholinergic antagonist mecamylamine, the muscarinic cholinergic antagonist scopolamine, or placebo. Following drug administration participants performed cognitive tests sensitive to cholinergic tone, assessing attention, episodic memory, and working memory.

Results

Significant decrements were found on some tasks when participants were taking E2+mPRO compared to E2 alone. Specifically, under more challenging task conditions and larger anticholinergic doses, participants showed poorer performance on the Critical Flicker Fusion task and the Stroop test and responded more conservatively on the N-back working memory task. Other tasks showed no differences between treatments under cholinergic blockade.

Discussion

The findings show that mPRO when taken in concert with E2, was detrimental to effortful cognitive performance, in the presence of cholinergic blockade. These results are important for assessing the impact of combined postmenopausal hormone treatment on cognitive performance that is dependent on cholinergic functioning after menopause.

Estradiol treatment in young postmenopausal women with self-reported cognitive complaints: Effects on cholinergic-mediated cognitive performance

OBJECTIVE

Older women are at increased risk of developing Alzheimer's disease compared to men. One proposed reason is that following menopause there is a decline in estrogens. Estrogens are important for cholinergic functioning and attenuate the impact of cholinergic antagonists on cognitive performance in postmenopausal women. Self-reported or subjective cognitive complaints in middle or older age may represent a harbinger of cognitive decline and those who endorse cognitive complaints appear more likely to develop future cognitive impairment. However, the response of individuals with cognitive complaints after menopause to estrogen and the relationship to cholinergic functioning has not been investigated. This study investigated the effect of estrogen treatment using 17β-estradiol on cognitive performance following anticholinergic blockade in postmenopausal women and the relationship of this interaction with the level of self-reported (subjective) postmenopausal cognitive complaints.

METHODS

Forty postmenopausal women (aged 50-60 years) completed a 3-month treatment regimen of either 1 mg oral estradiol or placebo. Participants then completed four challenge days in which they completed cognitive and behavioral tasks after one of four cholinergic antagonist drug conditions (oral mecamylamine (MECA), intravenous scopolamine, combined MECA and scopolamine, or PLC).

RESULTS

Compared to PLC, the estradiol treated group performed worse on attention tasks under cholinergic challenge including the choice reaction time task and the critical flicker fusion task. In addition, participants who endorsed greater cognitive complaints showed reduced performance on the N-back working memory task, regardless of whether they received estradiol treatment.

CONCLUSIONS

The findings of this study indicate that estradiol treatment was unable to mitigate anticholinergic blockade in postmenopausal women with subjective cognitive complaints, and worsened performance on attention tasks. Moreover, the present study suggests that greater levels of cognitive complaints following menopause may be associated with an underlying decline in cholinergic function that may manifest as an inability to compensate during working memory tasks.

Nicotinic receptor modulation of the default mode network

RATIONALE

Previous neuroimaging studies of cognition involving nicotinic acetylcholine receptor (nAChR) agonist administration have repeatedly found enhanced task-induced deactivation of regions of the default mode network (DMN), a group of brain systems that is more active at rest and mediates task-independent thought processes. This effect may be related to pro-cognitive nAChR agonist effects OBJECTIVES: The present study sought to test whether nAChR modulation of the DMN is bi-directional, i.e., whether a nAChR antagonist would reduce task-induced deactivation.

METHODS

Eighteen healthy non-smokers underwent functional magnetic resonance imaging while performing a letter N-back task. Scans were performed after nicotine administration (7 mg/24 h, transdermally), after administration of the nAChR antagonist mecamylamine (7.5 mg, p.o.), and after double placebo, in counterbalanced sequence. Blood-oxygen-level-dependent (BOLD) signal was analyzed within ventromedial prefrontal cortex (vmPFC) and posterior cingulate cortex (PCC) regions of interest-central hubs of the DMN in which consistent nAChR agonist-induced changes had previously been identified.

RESULTS

Nicotine enhanced hit rate in both the 0-back and 2-back condition, while mecamylamine slowed reaction time in the 2-back condition. Mecamylamine reduced task-induced deactivation of vmPFC and PCC. Nicotine had no significant effects on the BOLD signal.

CONCLUSIONS

The finding that nAChR tone reduction by mecamylamine weakened task-induced DMN deactivation indicates that a constant tone of nAChR activation helps regulate DMN activity in healthy individuals. This suggests that low nAChR tone may play a causal role in DMN dysregulation seen in conditions such as mild cognitive impairment or Alzheimer's disease.

Ca2+ Dyshomeostasis Disrupts Neuronal and Synaptic Function in Alzheimer's Disease

Ca²⁺ homeostasis is essential for multiple neuronal functions and thus, Ca²⁺ dyshomeostasis can lead to widespread impairment of cellular and synaptic signaling, subsequently contributing to dementia and Alzheimer's disease (AD). While numerous studies implicate Ca²⁺ mishandling in AD, the cellular basis for loss of cognitive function remains under investigation. The process of synaptic degradation and degeneration in AD is slow, and constitutes a series of maladaptive processes each contributing to a further destabilization of the Ca²⁺ homeostatic machinery. Ca²⁺ homeostasis involves precise maintenance of cytosolic Ca²⁺ levels, despite extracellular influx via multiple synaptic Ca²⁺ channels, and intracellular release via organelles such as the endoplasmic reticulum (ER) via ryanodine receptor (RyRs) and IP₃R, lysosomes via transient receptor potential mucolipin channel (TRPML) and two pore channel (TPC), and mitochondria via the permeability transition pore (PTP). Furthermore, functioning of these organelles relies upon regulated inter-organelle Ca²⁺ handling, with aberrant signaling resulting in synaptic dysfunction, protein mishandling, oxidative stress and defective bioenergetics, among other consequences consistent with AD. With few effective treatments currently available to mitigate AD, the past few years have seen a significant increase in the study of synaptic and cellular mechanisms as drivers of AD, including Ca²⁺ dyshomeostasis. Here, we detail some key findings and discuss implications for future AD treatments.

Muscarinic and Nicotinic Modulation of Memory but not Verbal Problem-solving

Aspects of cognitive flexibility are modulated by the noradrenergic system, which is important in arousal and attention. Acetylcholine also modulates arousal and attention, as well as working memory. Effects of muscarinic and nicotinic antagonism on memory are well established. Our purpose was to test whether muscarinic and nicotinic antagonism affect aspects of cognitive flexibility, specifically verbal problem-solving, as well as memory, given acetylcholine's role in attention and arousal. Eighteen participants attended three testing sessions. Two hours before testing, participants received either 0.6 mg scopolamine, 10 mg mecamylamine, or placebo. Then, participants were tested on three memory tasks (Buschke Selective Reminding Test [BSRT], California Verbal Learning Test [CVLT], Rey Complex Figure Test), two verbal problem-solving/cognitive flexibility tasks (Compound Remote Associates Test, a timed anagram test), and a spatial inductive reasoning task (Raven's Progressive Matrices). Task order and drug order were counterbalanced. Memory impairment was seen on one BSRT measure and multiple CVLT measures with scopolamine, and with one BSRT measure with mecamylamine. There were no effects of either drug on any of the tasks involving cognitive flexibility, including verbal problem-solving. Specific memory impairments were detected using muscarinic, and to a marginal extent, nicotinic antagonists, as expected, but no effect was seen on cognitive flexibility. Therefore, although both the noradrenergic and cholinergic systems play important roles in arousal and cortical signal-to-noise processing, the cholinergic system does not appear to have the same effect as the noradrenergic system on cognitive flexibility, including verbal problem-solving.

Intermittent stimulation in the nucleus basalis of meynert improves sustained attention in rhesus monkeys

Sustained attention is essential in important behaviors in daily life. Many neuropsychiatric disorders are characterized by a compromised ability to sustain attention, making this cognitive domain an important therapeutic target. In this study, we tested a novel method of improving sustained attention. Monkeys were engaged in a continuous performance task (CPT) while the nucleus basalis of Meynert (NB), the main source of cholinergic innervation of the neocortex, was stimulated. Intermittent NB stimulation improved the animals' performance by increasing the hit rate and decreasing the false alarm rate. Administration of the cholinesterase inhibitor donepezil or the muscarinic antagonist scopolamine alone impaired performance, whereas the nicotinic antagonist mecamylamine alone improved performance. Applying NB stimulation while mecamylamine or donepezil were administered impaired CPT performance. Methylphenidate, a monoaminergic psychostimulant, was applied in conjunction with intermittent stimulation as a negative control, as it does not directly modulate cholinergic output. Methylphenidate also improved performance, and it produced further improvement when combined with NB stimulation. The additive effect of the combination suggested NB stimulation altered behavior independently from methylphenidate effects. We conclude that basal forebrain projections contribute to sustained attention, and that intermittent NB stimulation is an effective way of improving performance.

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.

An anti-nicotinic cognitive challenge model using mecamylamine in comparison with the anti-muscarinic cognitive challenge using scopolamine

AIMS

The muscarinic acetylcholine receptor antagonist scopolamine is often used for proof-of-pharmacology studies with pro-cognitive compounds. From a pharmacological point of view, it would seem more rational to use a nicotinic rather than a muscarinic anticholinergic challenge to prove pharmacology of a nicotinic acetylcholine receptor agonist. This study aims to characterize a nicotinic anticholinergic challenge model using mecamylamine and to compare it to the scopolamine model.

METHODS

In this double-blind, placebo-controlled, four-way cross-over trial, 12 healthy male subjects received oral mecamylamine 10 and 20 mg, intravenous scopolamine 0.5 mg and placebo. Pharmacokinetics were analysed using non-compartmental analysis. Pharmacodynamic effects were measured with a multidimensional test battery that includes neurophysiological, subjective, (visuo)motor and cognitive measurements.

RESULTS

All treatments were safe and well tolerated. Mecamylamine had a tmax of 2.5 h and a Cmax of 64.5 ng ml-1 for the 20 mg dose. Mecamylamine had a dose-dependent effect decreasing the adaptive tracking performance and VAS alertness, and increasing the finger tapping and visual verbal learning task performance time and errors. Scopolamine significantly affected almost all pharmacodynamic tests.

CONCLUSION

This study demonstrated that mecamylamine causes nicotinic receptor specific temporary decline in cognitive functioning. Compared with the scopolamine model, pharmacodynamic effects were less pronounced at the dose levels tested; however, mecamylamine caused less sedation. The cognitive effects of scopolamine might at least partly be caused by sedation. Whether the mecamylamine model can be used for proof-of-pharmacology of nicotinic acetylcholine receptor agonists remains to be established.

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 test of the cognitive-enhancing potential of low-dose mecamylamine in healthy non-smokers

RATIONALE

The beneficial effects of nicotinic acetylcholine receptor (nAChR) agonists on cognitive performance have been widely shown. Paradoxically, recent preclinical studies employing extremely low doses of nAChR antagonists have also found cognitive enhancement, perhaps pointing to a novel treatment mechanism for cognitive deficits.

OBJECTIVES

The aim was to test whether low doses of the nAChR antagonist mecamylamine would benefit performance in human volunteers.

METHODS

The study employed a double-blind within-subject design. Over four separate days, healthy adult non-smokers (n = 23) were tested with placebo and three trace doses of mecamylamine (0.25-1 mg, p.o.), adjusted for body weight. Participants performed three computerized tasks: a task of spatial selective attention and stimulus detection, the rapid visual information processing task (RVIPT) taxing sustained attention and working memory, and a change detection short-term memory task. Subjective state and vital signs were assessed repeatedly.

RESULTS

Mecamylamine did not improve performance in any of the tasks. Any trends that were observed instead pointed toward performance impairment. Mecamylamine also had no effects on subjective state or vital signs.

CONCLUSIONS

The present results do not support the hypothesized cognitive-enhancing potential of low doses of mecamylamine. Contrary to preclinical reports, these findings speak against low-dose nAChR antagonism as a novel avenue for treating cognitive deficits.

[Pharmacological properties of vortioxetine and its pre-clinical consequences]

Selective Serotonin Reuptake Inhibitors (SSRIs) are extensively used for the treatment of major depressive disorder (MDD). SSRIs are defined as indirect receptor agonists since the activation of postsynaptic receptors is a consequence of an increase in extracellular concentrations of serotonin (5-HT) mediated by the blockade of serotonin transporter. The activation of some serotoninergic receptors (5-HT1A, post-synaptic, 5-HT1B post-synaptic, 5-HT2B, and 5-HT4), but not all (5-HT1A, pre-synaptic, 5-HT1B pre-synaptic, 5-HT2A, 5-HT2C, 5-HT3, and probably 5-HT6), induces anxiolytic/antidepressive - like effects. Targetting specifically some of them could potentially improve the onset of action and/or efficacy and/or prevent MD relapse. Vortioxetine (Brintellix, 1- [2-(2,4-dimethylphenyl-sulfanyl)-phenyl]-piperazine) is a novel multi-target antidepressant drug approved by the Food and Drug Administration (FDA) and by European Medicines Agency. Its properties are markedly different from the extensively prescribed SSRIs. Compared to the SSRIs, vortioxetine is defined as a multimodal antidepressant drug since it is not only a serotonin reuptake inhibitor, but also a 5-HT1D, 5-HT3, 5-HT7 receptor antagonist, 5-HT1B receptor partial agonist and 5-HT1A receptor agonist. This specific pharmacological profile enables vortioxetine to affect not only the serotoninergic and noradrenergic systems, but also the histaminergic, cholinergic, gamma-butyric acid (GABA) ergic and glutamatergic ones. Thus, vortioxetine not only induces antidepressant-like or anxiolytic-like activity but also improves cognitive parameters in several animal models. Indeed, vortioxetine was shown to improve working memory, episodic memory, cognitive flexibility and spatial memory in young adult rodents and also in old animal models. These specific effects of the vortioxetine are of interest considering that cognitive dysfunction is a common comorbidity to MDD. Altogether, even though this molecule still needs to be investigated further, especially in the insufficient-response to antidepressant drugs, vortioxetine is already an innovative therapeutic option for the treatment of major depression.

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'.

Cognition as a therapeutic target in late-life depression: potential for nicotinic therapeutics

Depression is associated with impairments to cognition and brain function at any age, but such impairments in the elderly are particularly problematic because of the additional burden of normal cognitive aging and in some cases, structural brain pathology. Individuals with late-life depression exhibit impairments in cognition and brain structural integrity, alongside mood dysfunction. Antidepressant treatment improves symptoms in some but not all patients, and those who benefit may not return to the cognitive and functional level of nondepressed elderly. Thus, for comprehensive treatment of late-life depression, it may be necessary to address both the affective and cognitive deficits. In this review, we propose a model for the treatment of late-life depression in which nicotinic stimulation is used to improve cognitive performance and improve the efficacy of an antidepressant treatment of the syndrome of late-life depression. The cholinergic system is well-established as important to cognition. Although muscarinic stimulation may exacerbate depressive symptoms, nicotinic stimulation may improve cognition and neural functioning without a detriment to mood. While some studies of nicotinic subtype specific receptor agonists have shown promise in improving cognitive performance, less is known regarding how nicotinic receptor stimulation affects cognition in depressed elderly patients. Late-life depression thus represents a new therapeutic target for the development of nicotinic agonist drugs. Parallel treatment of cognitive dysfunction along with medical and psychological approaches to treating mood dysfunction may be necessary to ensure full resolution of depressive illness in aging.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

Reward learning as a potential target for pharmacological augmentation of cognitive remediation for schizophrenia: a roadmap for preclinical development

Rationale: Impaired cognitive abilities are a key characteristic of schizophrenia. Although currently approved pharmacological treatments have demonstrated efficacy for positive symptoms, to date no pharmacological treatments successfully reverse cognitive dysfunction in these patients. Cognitively-based interventions such as cognitive remediation (CR) and other psychosocial interventions however, may improve some of the cognitive and functional deficits of schizophrenia. Given that these treatments are time-consuming and labor-intensive, maximizing their effectiveness is a priority. Augmenting psychosocial interventions with pharmacological treatments may be a viable strategy for reducing the impact of cognitive deficits in patients with schizophrenia.

Objective: We propose a strategy to develop pharmacological treatments that can enhance the reward-related learning processes underlying successful skill-learning in psychosocial interventions. Specifically, we review clinical and preclinical evidence and paradigms that can be utilized to develop these pharmacological augmentation strategies. Prototypes for this approach include dopamine D1 receptor and α7 nicotinic acetylcholine receptor agonists as attractive targets to specifically enhance reward-related learning during CR.

Conclusion: The approach outlined here could be used broadly to develop pharmacological augmentation strategies across a number of cognitive domains underlying successful psychosocial treatment.

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.

Estradiol treatment altered anticholinergic-related brain activation during working memory in postmenopausal women

Estradiol has been shown to affect cholinergic modulation of cognition in human and nonhuman animal models. This study examined the brain-based interaction of estradiol treatment and anticholinergic challenge in postmenopausal women during the performance of a working memory task and functional MRI. Twenty-four postmenopausal women were randomly and blindly placed on 1mg oral 17-β estradiol or matching placebo pills for three months after which they participated in three anticholinergic challenge sessions. During the challenge sessions, subjects were administered the antimuscarinic drug scopolamine, the antinicotinic drug mecamylamine, or placebo. After drug administration, subjects completed an fMRI session during which time they performed a visual verbal N-back test of working memory. Results showed that scopolamine increased activation in the left medial frontal gyrus (BA 10) and mecamylamine increased activation in the left inferior frontal gyrus (BA 46). Estradiol treatment compared to placebo treatment significantly reduced the activation in this left medial frontal region during scopolamine challenge. Estradiol treatment also increased activation in the precuneus (BA 31) during mecamylamine challenge. These data are the first to show that estradiol modulated antimuscarinic- and anitnicotinic-induced brain activity and suggest that estradiol affected cholinergic system regulation of cognition-related brain activation in humans.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Nicotine: specific role in angiogenesis, proliferation and apoptosis

Nowadays, tobacco smoking is the cause of ~5-6 million deaths per year, counting 31% and 6% of all cancer deaths (affecting 18 different organs) in middle-aged men and women, respectively. Nicotine is the addictive component of tobacco acting on neuronal nicotinic receptors (nAChR). Functional nAChR, are also present on endothelial, haematological and epithelial cells. Although nicotine itself is regularly not referred to as a carcinogen, there is an ongoing debate whether nicotine functions as a 'tumour promoter'. Nicotine, with its specific binding to nAChR, deregulates essential biological processes like regulation of cell proliferation, apoptosis, migration, invasion, angiogenesis, inflammation and cell-mediated immunity in a wide variety of cells including foetal (regulation of development), embryonic and adult stem cells, adult tissues as well as cancer cells. Nicotine seems involved in fundamental aspects of the biology of malignant diseases, as well as of neurodegeneration. Investigating the biological effects of nicotine may provide new tools for therapeutic interventions and for the understanding of neurodegenerative diseases and tumour biology.

Differences in neonatal neurotoxicity of brominated flame retardants, PBDE 99 and TBBPA, in mice

Flame retardants such as polybrominated diphenyl ethers (PBDE) and tetrabromobisphenol A are used as flame retardants and detected in the environmental, wildlife species and human tissues. Exposure to PBDEs during the neonatal development of the brain has been shown to affect behavior and learning and memory in adult mice, while neonatal exposure to TBBPA (another brominated flame retardant) did not affect behavioral variables in the adult. In this study, we hypothesized that the effects of these compounds could be reflected by changes in biochemical substrates and cholinergic receptors and have examined the levels of four proteins involved in maturation of the brain, neuronal growth and synaptogenesis and the densities of both muscarinic and nicotinic cholinergic receptors. We measured the levels of radioactivity in the brain after administration of (14)C-labelled TBBPA at different time points and saw that levels of TBBA peaked earlier and decreased faster than the earlier reported levels of PBDE 99. The protein analysis in the neonatal brain showed changes in the levels of calcium/calmodulin-dependent protein kinase II (CaMKII), growth associated protein-43 (GAP-43) and synaptophysin following neonatal exposure to PBDE 99 (21 μmol/kg body weight), but not following exposure TBBPA. Furthermore, neonatal exposure to PBDE 99 and TBBPA caused a decrease in binding sites of the nicotinic ligand cytisine in frontal cortex. These results confirm earlier reported data that PBDE 99 can act as a developmental neurotoxicant, possibly due to its different uptake and retention in the brain compared to TBBPA. In addition, the changes in protein levels are interesting leads in the search for mechanisms behind the developmental neonatal neurotoxicity of PBDEs in general and PBDE 99 in particular, since also other compounds inducing similar adult behavioral disturbances as PBDE 99, affect these proteins during the period of rapid brain development.

Functional brain imaging of nicotinic effects on higher cognitive processes

Significant advances in human functional brain imaging offer new opportunities for direct observation of the effects of nicotine, novel nicotinic agonists and nicotinic antagonists on human cognitive and behavioral performance. Careful research over the last decade has enabled investigators to explore the role of nicotinic systems on the functional neuroanatomy and neural circuitry of cognitive tasks in domains such as selective attention, working memory, episodic memory, cognitive control, and emotional processing. In addition, recent progress in understanding functional connectivity between brain regions utilized during cognitive and emotional processes offers new opportunities for examining drug effects on network-related activity. This review will critically summarize available nicotinic functional brain imaging studies focusing on the specific cognitive domains of attention, memory, behavioral control, and emotional processing. Generally speaking, nicotine appears to increase task-related activity in non-smokers and deprived smokers, but not active smokers. By contrast, nicotine or nicotinic stimulation decreases the activity of structures associated with the default mode network. These particular patterns of activation and/or deactivation may be useful for early drug development and may be an efficient and cost-effective method of screening potential nicotinic agents. Further studies will have to be done to clarify whether such activity changes correlate with cognitive or affective outcomes that are clinically relevant. The use of functional brain imaging will be a key tool for probing pathologic changes related to brain illness and for nicotinic drug development.

The cholinergic hypothesis of cognitive aging revisited again: cholinergic functional compensation

It is now possible to reevaluate the cholinergic hypothesis of age-related cognitive dysfunction based on a synthesis of new evidence from cholinergic stimulation studies and cognitive models. We propose that a change of functional circuitry that can be observed through a combination of pharmacologic challenge and functional neuroimaging is associated with age-related changes in cholinergic system functioning. Psychopharmacological manipulations using cholinergic agonists and antagonists have been consistent in replicating patterns of aging seen in functional imaging studies. In addition, studies of anticholinesterase drugs in patients with Alzheimer's disease and mild cognitive impairment show support for the proposal that cholinergic compensation causes alterations in task-related brain activity. Thus, the cholinergic hypothesis of age-related cognitive dysfunction deserves further consideration as new methodologies for evaluating its validity are increasingly being used. Future directions for testing hypotheses generated from this model are presented.

Cognitive performance and cholinergic transmission: influence of muscarinic and nicotinic receptor blockade

The cholinergic system is essential in mediating cognitive processes. Although there has been extensive research regarding cholinergic receptor subsystems, the specific contribution of the muscarinic and nicotinic receptor system to cognitive processes still has not been sufficiently explored. In the present study, we examined the selective contribution of muscarinic and nicotinic antagonism to cognitive performance in healthy human subjects. A single-blind, double-dummy, time-elapsed, repeated measures cross-over design was used on 15 healthy males. Subjects completed a neuropsychological test battery assessing a wide range of cognitive domains after 0.4 mg scopolamine (intravenous), 0.2 mg/kg mecamylamine (max. 15 mg; oral) or placebo. Subjects were tested under three conditions: placebo/placebo (PP), scopolamine/placebo (SP) and mecamylamine/placebo (MP). Results show that scopolamine significantly impaired the free recall and recognition performance in the verbal learning test. No other cognitive domain was affected, neither by scopolamine nor by mecamylamine. In line with the existing literature, antagonism of muscarinic receptors resulted in specific cognitive impairments, predominantly memory performance.

Impact of the principal stroke risk factors on human movements

In the context of the occidental population aging, new preventive approaches must be developed to reduce the disability related to brain stroke in the elderly. The stroke susceptibility assessment based on the analysis of human movements is one of the potential avenues needing investigation. As a first step in this direction, this paper reports results on the relationship linking the most important stroke risk factors to some characteristics of human movement. Various features were extracted using the Sigma-Lognormal model on 1440 stereotypical triangular movements performed by 120 subjects having different health conditions. These features were combined through a linear modeling to maximize the predictability of presence of stroke risk factors in the studied cohort. The receiver operating characteristic (ROC) curve and the area under this curve (AUC) were used to evaluate the clinical significance of this relationship. Using only the information derived from the movements, the six tested risk factors (cardiac problems, diabetes mellitus, hypercholesterolemia, hypertension, obesity, and cigarette smoking) can be predicted with an AUC ranging from .68 to .82.

Mecamylamine - a nicotinic acetylcholine receptor antagonist with potential for the treatment of neuropsychiatric disorders

Mecamylamine (Inversine), the first orally available antihypertensive agent launched in the 1950s, is rarely used today for hypertension because of its widespread ganglionic side effects at antihypertensive doses (25 - 90 mg/day). However, more recent clinical studies suggest that mecamylamine is effective at much lower doses for blocking the central and peripheral effects of nicotine. Pharmacologically, mecamylamine has been well characterized as a nonselective and noncompetitive antagonist of nicotinic acetylcholine receptors (nAChRs). Because mecamylamine easily crosses the blood - brain barrier at relatively low doses (2.5 - 10 mg), it has been used by several research groups over the past two decades investigating the role of central nAChRs in the etiology and treatment of various neuropsychiatric disorders, including addiction disorders, Tourette's syndrome, schizophrenia and various cognitive and mood disorders. Two independent Phase II clinical trials recently confirmed mecamylamine's hypothesized antidepressant activity and suggest that it may be effective as an augmentation pharmacotherapy for SSRI treatment resistant major depression. These areas of investigation for mecamylamine are reviewed and recommendations for future research directions are proposed.

Nicotinic antagonist effects on functional attention networks

Cholinergic neurotransmission has been implicated in memory and attention. We investigated the effect of the non-competitive nicotinic antagonist mecamylamine on three components of attention processes (i.e. alerting, orienting and executive control) in 12 healthy male subjects whilst performing the Attention Network Task (ANT) in a magnetic resonance imaging (MRI) scanner. Participants received 15 mg mecamylamine in a single blind and placebo- controlled randomized procedure 90 min prior to obtaining functional MRI data. Our results confirm previous reports of beneficial effects of cueing (alerting and orienting) and detrimental effects of conflict (executive control) on reaction times when performing the ANT. The functional MRI data confirmed distinct neural networks associated with each of the three attention components. Alerting was associated with increased left temporal lobe activation while orienting increased bilateral prefrontal, right precuneus and left caudate activation. Executive control activated anterior cingulate and precuneus. Mecamylamine slowed overall response time and down-regulated brain activation associated with orienting and to some extent brain activation associated with executive control when compared to placebo. These findings are consistent with nicotinic modulation of orienting attention by cueing and executive control when responding to conflicting information. The latter nicotine antagonist effect may be mediated via cholinergic modulation of dopamine neurotransmission in mesolimbic pathways.

Nicotinic receptors: allosteric transitions and therapeutic targets in the nervous system

Nicotinic receptors - a family of ligand-gated ion channels that mediate the effects of the neurotransmitter acetylcholine - are among the most well understood allosteric membrane proteins from a structural and functional perspective. There is also considerable interest in modulating nicotinic receptors to treat nervous-system disorders such as Alzheimer's disease, schizophrenia, depression, attention deficit hyperactivity disorder and tobacco addiction. This article describes both recent advances in our understanding of the assembly, activity and conformational transitions of nicotinic receptors, as well as developments in the therapeutic application of nicotinic receptor ligands, with the aim of aiding novel drug discovery by bridging the gap between these two rapidly developing fields.

Cortical alpha7 nicotinic acetylcholine receptor and beta-amyloid levels in early Alzheimer disease

OBJECTIVE

To examine alpha7 nicotinic acetylcholine receptor (nAChR) binding and beta-amyloid (Abeta) peptide load in superior frontal cortex (SFC) across clinical and neuropathological stages of Alzheimer disease (AD).

DESIGN

Quantitative measures of alpha7 nAChR by [(3)H]methyllycaconitine binding and Abeta concentration by enzyme-linked immunosorbent assay in SFC were compared across subjects with antemortem clinical classification of no cognitive impairment, mild cognitive impairment, or mild to moderate AD, and with postmortem neuropathological diagnoses.

SETTING

Academic medical center. Subjects Twenty-nine elderly retired clergy.

MAIN OUTCOME MEASURES

Quantitative measures of alpha7 nAChR binding and Abeta peptide concentration in SFC.

RESULTS

Higher concentrations of total Abeta peptide in SFC were associated with clinical diagnosis of mild to moderate AD (P = .02), lower Mini-Mental State Examination scores (P = .003), presence of cortical Abeta plaques (P = .02), and likelihood of AD diagnosis by the National Institute on Aging-Reagan criteria (P = .002). Increased alpha7 nAChR binding was associated with National Institute on Aging-Reagan diagnosis (P = .02) and, albeit weakly, the presence of cortical Abeta plaques (P = .08). There was no correlation between the 2 biochemical measures.

CONCLUSIONS

These observations suggest that during the clinical progression from normal cognition to neurodegenerative disease state, total Abeta peptide concentration increases while alpha7 nAChRs remain relatively stable in SFC. Regardless of subjects' clinical status, however, elevated alpha7 nAChR binding is associated with increased Abeta plaque pathology, supporting the hypothesis that cellular expression of these receptors may be upregulated selectively in Abeta plaque-burdened brain areas.

Exposure to polybrominated diphenyl ethers 203 and 206 during the neonatal brain growth spurt affects proteins important for normal neurodevelopment in mice

The period of rapid brain growth and development (BGS) is postnatal in mice and rats, spanning the first 3-4 weeks of life, reaching its peak around postnatal day 10, whereas in humans, the BGS is perinatal. CaMKII, GAP-43, synaptophysin, and tau play important roles during the BGS. One class of flame retardants, polybrominated diphenyl ethers (PBDEs), is present and increasing in the environment and in human milk. The only congener still in use, decabrominated diphenyl ether (PBDE 209), is thought to be debrominated into lower brominated congeners. In the present study, nona- and octabrominated PBDEs were examined. Neonatal mice were exposed to 21 mumol PBDE 203 or 206/kg bodyweight on postnatal day 10, and different brain regions were analyzed for CaMKII, GAP-43, synaptophysin, and tau, 24 h after exposure. The protein analysis showed that CaMKII and synaptophysin increased significantly in the hippocampus, but not in the cerebral cortex, after neonatal exposure to PBDE 203 or 206. Furthermore, there were no significant changes in the levels of GAP-43 and tau in the cerebral cortex or hippocampus after neonatal exposure to PBDE 203 or 206. This shows that PBDE 203 and 206 affect important proteins involved in normal maturation of the brain and strengthens our findings that highly brominated PBDEs cause developmental neurotoxicity. In addition, the increases in CaMKII and synaptophysin are the same changes seen after neonatal PBDE 209 exposure; supporting the suggestion that PBDE 209 must be metabolized, likely debrominated into lower brominated PBDEs, to exert its neurotoxic effects.

Differential involvement of glutamatergic mechanisms in the cognitive and subjective effects of smoking

There is growing preclinical evidence for the involvement of glutamate in the behavioral actions of nicotine. The aim of this study, was to investigate the role of N-methyl-D-aspartate (NMDA) receptors in the cognitive and subjective effects of smoking in humans. Sixty regular smokers took part in this double-blind placebo controlled study, that investigated the effect of the NMDA-antagonist memantine (40 mg) and the nicotinic-receptor antagonist mecamylamine (10 mg) on smoking-induced improvement in performance of a task of sustained attention and on smoking-induced changes in subjective effects and craving. Increases in subjective ratings of 'buzzed' following smoking were reversed by memantine, but not by mecamylamine. In contrast, improvement on a Rapid Visual Information Processing task by smoking was opposed by mecamylamine, but not by memantine. Smoking reduced craving for cigarettes, but neither drug altered this effect. Our results suggest that glutamatergic mechanisms may have differential involvement in the subjective and cognitive actions of smoking. Further investigations using different ligands are warranted to fully characterize the role of glutamate underlying the consequences of smoking behavior.

Nicotinic versus muscarinic blockade alters verbal working memory-related brain activity in older women

OBJECTIVES

An important aspect of furthering our understanding of the central nervous system function after menopause is to examine the cerebral circuitry that appears to be influenced by cholinergic antagonist drugs in the presence and absence of estrogen. This pilot study investigated the effects of two anticholinergic drugs on brain activation and working memory performance in postmenopausal women not taking estrogen. This approach simulates the effects of age- or disease-related neuroreceptor or neuronal loss by temporarily blocking pre- and postsynaptic muscarinic and nicotinic cholinergic receptors.

DESIGN

Six healthy postmenopausal women took part in three drug challenges using the antinicotinic drug mecamylamine (MECA, 20 mg, oral), the antimuscarinic drug scopolamine (SCOP, 2.5 microg/kg, i.v.), and placebo during functional magnetic resonance imaging. The cognitive measure was a visually presented verbal N-back test of working memory.

RESULTS

Neither MECA nor SCOP significantly impaired performance on the verbal N-back. Functional magnetic resonance imaging results showed greater increases in frontal lobe activation in the placebo condition relative to each drug condition with different specific regional activation for MECA and SCOP.

CONCLUSIONS

These preliminary results suggest that brain activation patterns are sensitive to cholinergic modulation in postmenopausal women and that differential effects may be observed following nicotinic versus muscarinic blockade. This approach offers a potentially valuable method for modeling age-related changes in brain function, and the findings may have implications for cholinergic contributions to normal and pathologic aging.

Neonatal exposure to decabrominated diphenyl ether (PBDE 209) results in changes in BDNF, CaMKII and GAP-43, biochemical substrates of neuronal survival, growth, and synaptogenesis

Mammals have a marked period of rapid brain growth and development (BGS), which is postnatal in mice and rats, spanning the first 3-4 weeks of life and reaching its peak around postnatal day 10. CaMKII, GAP-43 and BDNF play important roles during the BGS in mammals. One class of flame retardants, polybrominated diphenyl ethers (PBDEs), are present and increasing in the environment and in human milk, which is also true for the only congener still in use, decabrominated diphenyl ether (PBDE 209). In the present study, the brains from 1, 3, 7, 10, 14 and 28 days old mice, were analysed for CaMKII and GAP-43. The level of CaMKII increases continuously during the neonatal period, while GAP-43 has a bell-shaped ontogeny curve, which peaks around postnatal day 10, in mouse brain. Furthermore, the effects of PBDE 209 on the developmental expression of CaMKII, GAP-43 and BDNF were examined in mice. Neonatal NMRI-male mice were orally exposed on days 3-20.1mgPBDE 209/kg body weight. The animals were euthanized 7 days after exposure to PBDE 209 and levels of CaMKII, GAP-43 and BDNF were analysed in different brain regions. The protein analysis showed that CaMKII increased significantly in hippocampus, but not in cortex, in animals 7 days after exposure to PBDE 209. GAP-43 showed a significant increase in hippocampus and a significant decrease in cortex of animals 7 days after exposure to PBDE 209. BDNF decreased significantly in hippocampus, but not in cortex, in mice 7 days after exposure to PBDE 209. This shows that PBDE 209 affects important proteins involved in normal maturation of the brain and further strengthen our findings concerning PBDE 209 as a developmental neurotoxicological agent.

Relevance of Donepezil in Enhancing Learning and Memory in Special Populations: A Review of the Literature

This review discusses the laboratory and clinical research supporting the rationale for the efficacy of donepezil (Aricept USA) in enhancing cognition in autism, Alzheimer disease, Down syndrome, traumatic brain injury, Attention Deficit Hyperactivity Disorder (ADHD), and schizophrenia. While preliminary animal models have shown effective, human studies exclusive of Alzheimer disease are sparse. Although attention and memory are unlikely a sole operation of the cholinergic system, evidence indicates a promising direction for further examination of this hypothesis in autism. Studies that examine changes in operationally defined behaviors and reliable and valid measure of changes in attention and memory are needed.

Estrogen treatment effects on anticholinergic-induced cognitive dysfunction in normal postmenopausal women

Estrogen has been shown to interact with the cholinergic system and influence cognition in animal models. This study investigated the interaction of estrogen and cholinergic system functioning and the effects of this interaction on cognitive task performance in healthy older women. Fifteen post-menopausal women were randomly and blindly placed on 1 mg of 17-beta estradiol or placebo for 3 months after which they participated in five anticholinergic challenge sessions, where they were administered one of two doses of the antimuscarinic drug scopolamine (SCOP) or the antinicotinic drug mecamylamine (MECA) or placebo. After the first challenge phase, they were crossed over to the other hormone treatment for another 3 months and repeated the challenges. Performance in multiple domains of cognition was assessed during anticholinergic drug challenge, including attention and verbal and nonverbal learning and memory. Results showed that estrogen pretreatment attenuated the anticholinergic drug-induced impairments on tests of attention and tasks with speed components. This study is the first to demonstrate the interaction of estrogen and the cholinergic system and the effects on cognitive performance in humans. The results suggest that estrogen status may affect cholinergic system tone and may be important for cholinergic system integrity.

Evidence for a neurophysiologic auditory deficit in children with benign epilepsy with centro-temporal spikes

Benign focal epilepsy in childhood with centro-temporal spikes (BECTS) is one of the most common forms of epilepsy. Recent studies have questioned the benign nature of BECTS, as they have revealed neuropsychological deficits in many domains including language. The aim of this study was to investigate whether the epileptic discharges during the night have long-term effects on auditory processing, as reflected on electrophysiological measures, during the day, which could underline the language deficits. In order to address these questions we recorded base line electroencephalograms (EEG), sleep EEG and auditory event related potentials in 12 children with BECTS and in age- and gender-matched controls. In the children with BECTS, 5 had unilateral and 3 had bilateral spikes. In the 5 patients with unilateral spikes present during sleep, an asymmetry of the auditory event related component (P85-120) was observed contralateral to the side of epileptiform activity compared to the normal symmetrical vertex distribution that was noted in all controls and in 3 the children with bilateral spikes. In all patients the peak to peak amplitude of this event related potential component was statistically greater compared to the controls. Analysis of subtraction waveforms (deviant - standard) revealed no evidence of a mismatch negativity component in any of the children with BECTS. We propose that the abnormality of P85-120 and the absence of mismatch negativity during wake recordings in this group may arise in response to the long-term effects of spikes occurring during sleep, resulting in disruption of the evolution and maintenance of echoic memory traces. These results may indicate that patients with BECTS have abnormal processing of auditory information at a sensory level ipsilateral to the hemisphere evoking spikes during sleep.

Expanding treatment of tobacco dependence

Nicotine dependence is the leading preventable cause of adult morbidity and mortality in the world. New research on the treatment of this disorder ranges from studies evaluating access to treatment to studies elucidating the molecular mechanisms of nicotine addiction. As our understanding of the neurobiology of tobacco addiction grows, the number of potential therapeutic targets by which we can intervene in this pernicious disorder also increases. This paper presents an overview of recent research trends in the treatment of tobacco dependence. We review several novel mechanisms of action that may serve as therapeutic targets for the pharmacologic treatment of tobacco dependence, including drugs that affect monamine oxidase, selective nicotinic receptors, glutamate and gamma-aminobutyric acid receptors, and the endocannabinoid system. For each of these therapeutic targets, we discuss medications in development that affect these pathophysiologic mechanisms.

Acute effect of nicotine on auditory gating in smokers and non-smokers

This paper investigates the role of cholinergic mechanisms in auditory gating by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on behavioral and electrophysiological measures of consonant-vowel (CV) discrimination in quiet and in broadband noise (BBN). In a single-blind procedure, categorical boundaries and mismatch negativity (MMN) in two conditions (quiet, BBN) were obtained from 10 non-smokers and 4 smokers with normal hearing under two drug conditions (nicotine, placebo). After the nicotine sessions, plasma tests revealed a subject's nicotine concentration and subjects reported any symptoms. Larger MMN areas and steeper slopes at the boundary were interpreted as reflecting better electrophysiological and behavioral CV discrimination, respectively. Results indicate that, in non-smokers, the effects of nicotine on electrophysiological CV discrimination in quiet increase with an increase in severity of symptoms. Specifically, asymptomatic non-smokers (N = 5) demonstrate little improvement (and sometimes decrements) in performance while symptomatic non-smokers (N = 5) exhibit nicotine-enhanced discrimination, as do smokers. In noise, all subjects demonstrate nicotine-enhanced behavioral and electrophysiological discrimination. Additionally, in noise, smokers exhibit a larger number of measurable categorical boundaries as well as larger MMN areas than non-smokers in both placebo and nicotine sessions. Results are consistent with the hypothesis that nicotinic cholinergic mechanisms play a role in the gating of auditory stimuli.

Nicotinic receptor mechanisms and cognition in normal states and neuropsychiatric disorders

Cigarette smoking rates in the American population are approximately 23%, whereas rates of smoking in clinical and population studies of individuals with neuropsychiatric disorders are typically two- to four-fold higher. Studies conducted in a variety of neuropsychiatric populations [e.g. attention-deficit hyperactivity disorder (ADHD), Alzheimer's disease, schizophrenia] have collectively suggested that nicotine may be efficacious in remediating selected cognitive deficits associated with these disorders, thus providing a framework for understanding the specific vulnerability of these patients to smoking initiation and maintenance. However, the specific gain in cognitive performance produced by nicotine administration in healthy subjects with normal cognitive function is less clear. This article reviews our current understanding of central nicotinic acetylcholine receptor (nAChRs) systems in normal and neuropsychiatric disease states and, specifically, their role with respect to cognitive dysfunction and clinical symptoms in several specific neuropsychiatric populations, including ADHD, Alzheimer's disease, Parkinson's disease, Tourette's Disorder, schizophrenia and affective disorders. The potential benefits of nicotinic agents for therapeutic use in neuropsychiatric disorders is discussed, as well as directions for further research in this area.

Nicotinic receptor mechanisms and cognition in normal states and neuropsychiatric disorders

Cigarette smoking rates in the American population are approximately 23%, whereas rates of smoking in clinical and population studies of individuals with neuropsychiatric disorders are typically two- to four-fold higher. Studies conducted in a variety of neuropsychiatric populations [e.g. attention-deficit hyperactivity disorder (ADHD), Alzheimer's disease, schizophrenia] have collectively suggested that nicotine may be efficacious in remediating selected cognitive deficits associated with these disorders, thus providing a framework for understanding the specific vulnerability of these patients to smoking initiation and maintenance. However, the specific gain in cognitive performance produced by nicotine administration in healthy subjects with normal cognitive function is less clear. This article reviews our current understanding of central nicotinic acetylcholine receptor (nAChRs) systems in normal and neuropsychiatric disease states and, specifically, their role with respect to cognitive dysfunction and clinical symptoms in several specific neuropsychiatric populations, including ADHD, Alzheimer's disease, Parkinson's disease, Tourette's Disorder, schizophrenia and affective disorders. The potential benefits of nicotinic agents for therapeutic use in neuropsychiatric disorders is discussed, as well as directions for further research in this area.

Effects of acute nicotine administration on behavioral inhibition in adolescents with attention-deficit/hyperactivity disorder

RATIONALE

Adolescents with attention-deficit/hyperactivity disorder (ADHD) become cigarette smokers at twice the rate of non-ADHD adolescents, and this finding continues into adulthood. Abnormal cognitive/behavioral inhibition is one core cognitive symptom of ADHD, leading to impulsive behavior in people with this disorder. Nicotine, contained in tobacco smoke, is known to improve attention, vigilance, and short-term memory. However, little is known about how nicotine might effect cognitive/behavioral inhibition.

OBJECTIVE

This study tested the hypothesis that acute nicotine administration would improve cognitive/behavioral inhibition in non-smoking adolescents with ADHD.

METHODS

This single-dose, acute, repeated-measures, double blind study in adolescents (13-17 years) with DSM-IV confirmed ADHD assessed the effects of transdermal nicotine, oral methylphenidate, and placebo on inhibition in non-smoking adolescents with ADHD. Dependent measures included tests of cognitive/behavioral inhibition (the stop signal task), cognitive interference control (the Stroop task), and a measure of verbal learning and recognition (the hi-low imagery task).

RESULTS

Results from five subjects indicated that stop signal reaction time (SSRT), an estimate of the speed of inhibiting a response, was significantly (P<0.01) improved following both nicotine and methylphenidate treatment as compared to placebo treatment. Neither "go" reaction time nor accuracy showed any effect of drug. In the Stroop task, another task of cognitive inhibition, nicotine but not methylphenidate significantly (P<0.05) decreased the Stroop effect compared to placebo.

CONCLUSIONS

These results indicate that nicotine administration has measurable positive effects on cognitive/behavioral inhibition in adolescents with ADHD. The size of the effect is at least comparable to methylphenidate. Positive effects of nicotine on inhibitional performance may contribute to higher rates of cigarette use in adolescents with ADHD.

Effects of nicotine and mecamylamine on cognition in rhesus monkeys

RATIONALE

Nicotine and other agonists of nicotinic cholinergic receptors (nAChR) have been shown to improve performance in specific memory domains in rodents and monkeys. Such beneficial effects are observed in preclinical models of age-related cognitive decline, stimulating interest in nAChR ligands as possible therapeutics. Prior work has typically focused on assays of spatial working memory in rodent studies and visual recognition memory in monkey studies.

OBJECTIVE

The current study was conducted to determine the role played by nAChRs in multiple types of memory in monkeys.

METHODS

Rhesus monkeys (n=6) were trained to perform a battery of six behavioral tasks and then serially challenged with acute doses of nicotine (3.2-56 microg/kg, i.m.) and the nAChR antagonist mecamylamine (0.32-1.78 mg/kg, i.m.).

RESULTS

Nicotine improved performance on tests designed to assay visual recognition memory, spatial working memory and visuo-spatial associative memory, while mecamylamine impaired visuo-spatial associative memory. Ballistic and fine motor performance was not significantly improved by nicotine but fine motor performance was impaired by mecamylamine.

CONCLUSIONS

Although nicotine may improve performance in multiple domains, effects on visuo-spatial associative memory is the most specifically attributable to nAChR signaling.

Norleucine1-Angiotensin IV alleviates mecamylamine-induced spatial memory deficits

The brain angiotensin AT4 receptor subtype has been implicated in cognitive processing. We initially established that intracerebroventricular administration of the nAChR-antagonist mecamylamine (mec) interfered with spatial memory performance in male Sprague-Dawley rats. Next we demonstrated that mec-induced deficits in spatial memory were overcome by the AT4 receptor-agonist Norleucine1-Angiotensin IV (Nle1-Ang IV). Nle1-Ang IV could not, however, compensate for spatial learning impairments precipitated by both mec and the mAChR-antagonist scopolamine. These findings support the importance of the AT4 receptor in cognitive processing and suggest that the ability of Nle1-Ang IV to improve spatial memory deficiencies may be dependant upon the brain cholinergic system.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.