A randomized, double-blind, placebo-controlled phase 2 study of α4β2 agonist ABT-894 in adults with ADHD

Current nonstimulant medications for adults with attention-deficit/hyperactivity disorder

INTRODUCTION

Stimulants, including methylphenidate and amphetamines, are the first-line pharmacological treatment of ADHD in adults. However, in patients who do not respond or poorly tolerate stimulants, non-stimulant medications are usually recommended.

AREAS COVERED

The authors provide a narrative review of the literature on non-stimulant treatments for adult ADHD, including controlled and observational clinical studies conducted on adult samples. Atomoxetine has been extensively studied and showed significant efficacy in treating adult ADHD. Issues related to dosing, treatment duration, safety, and use in the case of psychiatric comorbidity are summarized. Among other compounds indicated for ADHD in adults, antidepressants sharing at least a noradrenergic or dopaminergic component, including tricyclic compounds, bupropion, and viloxazine, have shown demonstratable efficacy. Evidence is also available for antihypertensives, particularly guanfacine, as well as memantine, metadoxine, and mood stabilizers, while negative findings have emerged for galantamine, antipsychotics, and cannabinoids.

EXPERT OPINION

While according to clinical guidelines, atomoxetine may serve as the only second-line option in adults with ADHD, several other nonstimulant compounds may be effectively used in order to personalize treatment based on comorbid conditions and ADHD features. Nevertheless, further research is needed to identify and test more personalized treatment strategies for adults with ADHD.

The Role of Alpha-2 Agonists for Attention Deficit Hyperactivity Disorder in Children: A Review

INTRODUCTION

Attention Deficit Hyperactivity Disorder (ADHD) is one of the most common neurodevelopmental disorders, characterized by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), which is marked by symptoms such as inappropriate levels of inattention, hyperactivity, and impulsivity that can affect academic, social, and personal functioning in children and adolescents. This review summarizes clinical trials demonstrating the effectiveness of Alpha-2 agonists in reducing symptoms of inattention, hyperactivity, and impulsivity in children with ADHD. Studies were identified through a systematic search of PubMed and Cochrane databases. However, these medications' long-term safety and efficacy remain uncertain, with a lack of data on their effects on growth, cardiovascular function, and other adverse events. Further studies are required to determine these medications' optimal dose and treatment duration.

METHODS

Medications that target the noradrenergic system, such as Alpha-2 agonists, have been increasingly used as a treatment option for ADHD, with guanfacine and clonidine being two of the most commonly used medications. They function by selectively targeting Alpha-2 adrenergic receptors in the brain leading to improved attention and reduced hyperactivity and impulsivity symptoms in children with ADHD.

RESULTS

Clinical trials have demonstrated the effectiveness of Alpha-2 agonists in treating ADHD in children by reducing symptoms of inattention, hyperactivity, and impulsivity. However, these medications' long-term safety and efficacy still need to be completely understood. Due to a lack of information on the effects of Alpha-2 agonists on growth, cardiovascular function, and other long-term adverse events, more studies must investigate the optimal dose and treatment duration for these medications.

CONCLUSIONS

Despite these concerns, Alpha-2 agonists remain a valuable treatment option for ADHD in children, especially those unable to tolerate stimulant medications or who have coexisting conditions such as tic disorders. Future research should continue to explore the safety and efficacy of Alpha-2 agonists in the long term. In conclusion, Alpha-2 agonists show promise as a treatment for ADHD in children; however, the safety and efficacy of these drugs in the long term are not yet completely understood. Additional studies are required to investigate the optimal dose and treatment duration for these medications in their use as a treatment for this debilitating disease.

Nonstimulant Medications for Attention-Deficit/Hyperactivity Disorder (ADHD) in Adults: Systematic Review and Meta-analysis

BACKGROUND

For some adults with Attention-Deficit/Hyperactivity Disorder (ADHD), nonstimulants need to be considered either as a monotherapy or as an adjunct to stimulants.

OBJECTIVES

The objectives of this systematic review and meta-analysis were to assess the efficacy, acceptability, and tolerability of nonstimulants in adults with ADHD.

METHODS

Data sources, searches, and study selection were based on a previously published network meta-analysis of randomized clinical trials (RCTs) by Cortese at al. (Lancet Psychiatry 5(9):727-738, 2018), which we updated in March 2022. Specifically, we searched PubMed, BIOSIS Previews, CINAHL, the Cochrane Central Register of Controlled Trials, EMBASE, ERIC, MEDLINE, PsycINFO, OpenGrey, Web of Science Core Collection, ProQuest Dissertations and Theses (UK and Ireland), ProQuest Dissertations and Theses (abstracts and international), and the WHO International Trials Registry Platform, including ClinicalTrials.gov for double-blind RCTs with a placebo arm, lasting at least one week, including adults with a diagnosis of ADHD based on DSM-III, DSM-III-R, DSM-IV(TR), DSM-5 or ICD-9- or 10, and reporting data on efficacy, tolerability (drop-out due to side effects) and acceptability (drop-out due to any cause) of guanfacine, clonidine, or atomoxetine. Additionally, we searched for RCTs of viloxazine extended release (ER), approved for ADHD in 2021. Random-effects meta-analyses were conducted, and the risk of bias for individual RCTs was assessed using the Cochrane Risk of Bias tool.

RESULTS

We included 18 studies in the meta-analyses (4308 participants) plus one additional study in the narrative synthesis (374 participants). The meta-analysis showed that atomoxetine (15 RCTs) (Hedge's g = - 0.48, 95% CI [- 0.64; - 0.33]), guanfacine (two RCTs) (Hedge's g = - 0.66, 95% CI [- 0.94; - 0.38]) and viloxazine ER (one RCT) were significantly more efficacious than placebo. Atomoxetine was less well tolerated than placebo, while tolerability of guanfacine and viloxazine ER could not be meta-analysed, since only one study, for each medication, reported on it.

CONCLUSIONS

All investigated nonstimulants were more efficacious in the treatment of ADHD in adults, than placebo, while the placebo had better acceptability and tolerability.

PROTOCOL

https://osf.io/5vnmt/?view_only=2bf87ed12ba94645babedceeee4c0120 .

Strategies for Treatment of Disease-Associated Dementia Beyond Alzheimer's Disease: An Update

Memory, cognition, dementia, and neurodegeneration are complexly interlinked processes with various mechanistic pathways, leading to a range of clinical outcomes. They are strongly associated with pathological conditions like Alzheimer's disease, Parkinson's disease, schizophrenia, and stroke and are a growing concern for their timely diagnosis and management. Several cognitionenhancing interventions for management include non-pharmacological interventions like diet, exercise, and physical activity, while pharmacological interventions include medicinal agents, herbal agents, and nutritional supplements. This review critically analyzed and discussed the currently available agents under different drug development phases designed to target the molecular targets, including cholinergic receptor, glutamatergic system, GABAergic targets, glycine site, serotonergic targets, histamine receptors, etc. Understanding memory formation and pathways involved therein aids in opening the new gateways to treating cognitive disorders. However, clinical studies suggest that there is still a dearth of knowledge about the pathological mechanism involved in neurological conditions, making the dropouts of agents from the initial phases of the clinical trial. Hence, a better understanding of the disease biology, mode of drug action, and interlinked mechanistic pathways at a molecular level is required.

Applications of Isosteres of Piperazine in the Design of Biologically Active Compounds: Part 2

Applications of piperazine and homopiperazine in drug design are well-established, and these heterocycles have found use as both scaffolding and terminal elements and also as a means of introducing a water-solubilizing element into a molecule. In the accompanying review (10.1021/acs.jafc.2c00726), we summarized applications of piperazine and homopiperazine and their fused ring homologues in bioactive compound design along with illustrations of the use of 4-substituted piperidines and a sulfoximine-based mimetic. In this review, we discuss applications of pyrrolidine- and fused-pyrrolidine-based mimetics of piperazine and homopiperazine and illustrate derivatives of azetidine that include stretched and spirocyclic motifs, along with applications of a series of diaminocycloalkanes.

Prospects for new drugs to treat binge-eating disorder: Insights from psychopathology and neuropharmacology

BACKGROUND

Binge-eating disorder (BED) is a common psychiatric condition with adverse psychological and metabolic consequences. Lisdexamfetamine (LDX) is the only approved BED drug treatment. New drugs to treat BED are urgently needed.

METHODS

A comprehensive review of published psychopathological, pharmacological and clinical findings.

RESULTS

The evidence supports the hypothesis that BED is an impulse control disorder with similarities to ADHD, including responsiveness to catecholaminergic drugs, for example LDX and dasotraline. The target product profile (TPP) of the ideal BED drug combines treating the psychopathological drivers of the disorder with an independent weight-loss effect. Drugs with proven efficacy in BED have a common pharmacology; they potentiate central noradrenergic and dopaminergic neurotransmission. Because of the overlap between pharmacotherapy in attention deficit hyperactivity disorder (ADHD) and BED, drug-candidates from diverse pharmacological classes, which have already failed in ADHD would also be predicted to fail if tested in BED. The failure in BED trials of drugs with diverse pharmacological mechanisms indicates many possible avenues for drug discovery can probably be discounted.

CONCLUSIONS

(1) The efficacy of drugs for BED is dependent on reducing its core psychopathologies of impulsivity, compulsivity and perseveration and by increasing cognitive control of eating. (2) The analysis revealed a large number of pharmacological mechanisms are unlikely to be productive in the search for effective new BED drugs. (3) The most promising areas for new treatments for BED are drugs, which augment noradrenergic and dopaminergic neurotransmission and/or those which are effective in ADHD.

Atomoxetine in comorbid ADHD/PTSD: A randomized, placebo controlled, pilot, and feasibility study

BACKGROUND

PTSD and ADHD often occur comorbidly. Research indicates that the cognitive deficits in PTSD may be related to the same disturbance of fronto-temporal systems as observed in ADHD, and ADHD has been shown to impact PTSD treatment outcomes. The presented study evaluated the safety and efficacy of atomoxetine in Veterans with comorbid ADHD/PTSD.

METHODS

A double blind, randomized, placebo controlled, cross-over pilot and feasibility study was conducted. Atomoxetine was examined as an adjunctive treatment over this 10 weeks, two phase, crossover study which compared treatment with atomoxetine 80 mg daily to placebo daily. The primary outcome was improvement in ADHD symptoms as measured by the Conners' Adult ADHD Rating Scales-Self-Report: Short Version (CAARS-S:S), the Barkley Adult ADHD Rating Scale-IV (BAARS-IV), and the Adult ADHD Quality of Life-29 (AAQoL-29). Secondary outcomes included the Clinician Administered PTSD Scale (CAPS), Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36), and the response inhibition task Go/NoGo (GNG).

RESULTS

Atomoxetine treated patients had greater reductions in ADHD symptoms as defined by total scores on the CAARS-S:S (F_{(1, 29)}  = 6.37, p = .017); both the BAARS-IV (F_{(1, 26)}  = 3.16, p = .087); and GNG overall errors test (F_{(1, 29)}  = 3.88, p = .06), reached a trend level of significance. No significant differences were noted in quality of life assessments, GNG latency periods, or CAPS scores. Atomoxetine was well-tolerated with no serious adverse events observed.

CONCLUSIONS

In Veterans with ADHD comorbid with PTSD, atomoxetine demonstrated modest efficacy for ADHD symptoms; quality of life measures and PTSD symptoms were not affected.

New Drugs to Treat ADHD: Opportunities and Challenges in Research and Development

Since the landmark MTA (Multimodal Treatment of ADHD) trial unequivocally demonstrated the efficacy of methylphenidate, catecholaminergic drugs, especially stimulants, have been the therapeutic mainstay in treatment of Attention-Deficit Hyperactivity Disorder (ADHD). We review the new drugs which have entered the ADHD formulary. The lessons learned from drug-candidates that have succeeded in clinical trials together with those that have not have also been considered. What emerges confirms and consolidates the hypothesis that clinically effective ADHD drugs indirectly or directly increase catecholaminergic neurotransmission in the prefrontal cortex (PFC). Attempts to enhance catecholaminergic signalling through modulatory neurotransmitter systems or cognitive-enhancing drugs have all failed. New drugs approved for ADHD are catecholaminergic reuptake inhibitors and releasing agents, or selective noradrenaline reuptake inhibitors. Triple reuptake inhibitors with preferential effects on dopamine have not been successful. The substantial number of failures probably accounts for a continued focus on developing novel catecholaminergic and noradrenergic drugs, and a dearth of drug-candidates with novel mechanisms entering clinical development. However, substantial improvements in ADHD pharmacotherapy have been achieved by the almost exclusive use of once-daily medications and prodrugs, e.g. lisdexamfetamine and Azstarys^®, which improve compliance, deliver greater efficacy and reduce risks for diversion and abuse.

Galantamine-Memantine Combination and Kynurenine Pathway Enzyme Inhibitors in the Treatment of Neuropsychiatric Disorders

The kynurenine pathway (KP) is a major route for L-tryptophan (L-TRP) metabolism, yielding a variety of bioactive compounds including kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN), and picolinic acid (PIC). These tryptophan catabolites are involved in the pathogenesis of many neuropsychiatric disorders, particularly when the KP becomes dysregulated. Accordingly, the enzymes that regulate the KP such as indoleamine 2,3-dioxygenase (IDO)/tryptophan 2,3-dioxygenase, kynurenine aminotransferases (KATs), and kynurenine 3-monooxygenase (KMO) represent potential drug targets as enzymatic inhibition can favorably rebalance KP metabolite concentrations. In addition, the galantamine-memantine combination, through its modulatory effects at the alpha7 nicotinic acetylcholine receptors and N-methyl-D-aspartate receptors, may counteract the effects of KYNA. The aim of this review is to highlight the effectiveness of IDO-1, KAT II, and KMO inhibitors, as well as the galantamine-memantine combination in the modulation of different KP metabolites. KAT II inhibitors are capable of decreasing the KYNA levels in the rat brain by a maximum of 80%. KMO inhibitors effectively reduce the central nervous system (CNS) levels of 3-HK, while markedly boosting the brain concentration of KYNA. Emerging data suggest that the galantamine-memantine combination also lowers L-TRP, kynurenine, KYNA, and PIC levels in humans. Presently, there are only 2 pathophysiological mechanisms (cholinergic and glutamatergic) that are FDA approved for the treatment of cognitive dysfunction for which purpose the galantamine-memantine combination has been designed for clinical use against Alzheimer's disease. The alpha7 nicotinic-NMDA hypothesis targeted by the galantamine-memantine combination has been implicated in the pathophysiology of various CNS diseases. Similarly, KYNA is well capable of modulating the neuropathophysiology of these disorders. This is known as the KYNA-centric hypothesis, which may be implicated in the management of certain neuropsychiatric conditions. In line with this hypothesis, KYNA may be considered as the "conductor of the orchestra" for the major pathophysiological mechanisms underlying CNS disorders. Therefore, there is great opportunity to further explore and compare the biological effects of these therapeutic modalities in animal models with a special focus on their effects on KP metabolites in the CNS and with the ultimate goal of progressing to clinical trials for many neuropsychiatric diseases.

Pharmacologic treatment of attention deficit hyperactivity disorder in adults: A systematic review and network meta-analysis

Background

Attention deficit hyperactivity disorder (ADHD) affects approximately 3% of adults globally. Many pharmacologic treatments options exist, yet the comparative benefits and harms of individual treatments are largely unknown. We performed a systematic review and network meta-analysis to assess the relative effects of individual pharmacologic treatments for adults with ADHD.

Methods

We searched English-language published and grey literature sources for randomized clinical trials (RCTs) involving pharmacologic treatment of ADHD in adults (December 2018). The primary outcome was clinical response; secondary outcomes were quality of life, executive function, driving behaviour, withdrawals due to adverse events, treatment discontinuation, serious adverse events, hospitalization, cardiovascular adverse events, and emergency department visits. Data were pooled via pair-wise meta-analyses and Bayesian network meta-analyses. Risk of bias was assessed by use of Cochrane’s Risk of Bias tool, and the certainty of the evidence was assessed by use of the GRADE framework.

Results

Eighty-one unique trials that reported at least one outcome of interest were included, most of which were at high or unclear risk of at least one important source of bias. Notably, only 5 RCTs were deemed at overall low risk of bias. Included pharmacotherapies were methylphenidate, atomoxetine, dexamfetamine, lisdexamfetamine, guanfacine, bupropion, mixed amphetamine salts, and modafinil. As a class, ADHD pharmacotherapy improved patient- and clinician-reported clinical response compared with placebo (range: 4 to 15 RCTs per outcome); however, these findings were not conserved when the analyses were restricted to studies at low risk of bias, and the certainty of the finding is very low. There were few differences among individual medications, although atomoxetine was associated with improved patient-reported clinical response and quality of life compared with placebo. There was no significant difference in the risk of serious adverse events or treatment discontinuation between ADHD pharmacotherapies and placebo; however, the proportion of participants who withdrew due to adverse events was significantly higher among participants who received any ADHD pharmacotherapy. Few RCTs reported on the occurrence of adverse events over a long treatment duration.

Conclusions

Overall, despite a class effect of improving clinical response relative to placebo, there were few differences among the individual ADHD pharmacotherapies, and most studies were at risk of at least one important source of bias. Furthermore, the certainty of the evidence was very low to low for all outcomes, and there was limited reporting of long-term adverse events. As such, the choice between ADHD pharmacotherapies may depend on individual patient considerations, and future studies should assess the long-term effects of individual pharmacotherapies on patient-important outcomes, including quality of life, in robust blinded RCTs.

Registration

PROSPERO no. CRD 42015026049

Emerging drugs for the treatment of attention-deficit hyperactivity disorder (ADHD)

INTRODUCTION

Attention-deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder affecting up to 5.3% of children and 2.5% of adults depending on the country considered. Current pharmacological treatments for ADHD are based on stimulant or non-stimulant medications, targeting dopaminergic and noradrenergic systems in the frontal cortex and dopaminergic system in the basal ganglia. These drugs are effective and safe for the majority of patients, whereas about 20% of treated patients do not tolerate current therapies or experience insufficient efficacy. The adequate treatment of ADHD is necessary to allow a proper social placement and prevent the acquisition of additional, more severe, comorbidities.

AREAS COVERED

We conducted a review of the scientific literature and of unpublished/ongoing clinical trials to summarize the advances made in the last 10 years (2010-2020) for the pharmacological treatment of ADHD. We found many pharmacological mechanisms beyond dopaminergic and noradrenergic ones have been investigated in patients.

EXPERT OPINION

Some emerging drugs for ADHD may be promising as add-on treatment especially in children, amantadine to enhance cognitive functions and tipepidine for hyperactivity/impulsivity. Stand-alone emerging treatments for ADHD include viloxazine and dasotraline, which will soon have more clinical data available to support market access requests.

Characterization of Amphetamine, Methylphenidate, Nicotine, and Atomoxetine on Measures of Attention, Impulsive Action, and Motivation in the Rat: Implications for Translational Research

Amphetamine (AMP), methylphenidate (MPH), and atomoxetine (ATX) are approved treatments for ADHD, and together with nicotine (NIC), represent pharmacological agents widely studied on cognitive domains including attention and impulsive action in humans. These agents thus represent opportunities for clinical observation to be reinvestigated in the preclinical setting, i.e., reverse translation. The present study investigated each drug in male, Long Evans rats trained to perform either (1) the five-choice serial reaction time task (5-CSRTT), (2) Go/NoGo task, or (3) a progressive ratio (PR) task, for the purpose of studying each drug on attention, impulsive action and motivation. Specific challenges were adopted in the 5-CSRTT designed to tax attention and impulsivity, i.e., high frequency of stimulus presentation (sITI), variable reduction in stimulus duration (sSD), and extended delay to stimulus presentation (10-s ITI). Initially, performance of a large (> 80) cohort of rats in each task variant was conducted to examine performance stability over repeated challenge sessions, and to identify subgroups of "high" and "low" attentive rats (sITI and sSD schedules), and "high" and "low" impulsives (10-s ITI). Using an adaptive sequential study design, the effects of AMP, MPH, ATX, and NIC were examined and contrasting profiles noted across the tests. Both AMP (0.03-0.3 mg/kg) and MPH (1-6 mg/kg) improved attentional performance in the sITI but not sSD or 10-s ITI condition, NIC (0.05-0.2 mg/kg) improved accuracy across all conditions. ATX (0.1-1 mg/kg) detrimentally affected performance in the sITI and sSD condition, notably in "high" performers. In tests of impulsive action, ATX reduced premature responses notably in the 10-s ITI condition, and also reduced false alarms in Go/NoGo. Both AMP and NIC increased premature responses in all task variants, although AMP reduced false alarms highlighting differences between these two measures of impulsive action. The effect of MPH was mixed and appeared baseline dependent. ATX reduced break point for food reinforcement suggesting a detrimental effect on motivation for primary reward. Taken together these studies highlight differences between AMP, MPH, and ATX which may translate to their clinical profiles. NIC had the most reliable effect on attentional accuracy, whereas ATX was reliably effective against all tests of impulsive action.

[Neurobehavioral Comorbidities in Pentylenetetrazol-kindled Mice and Role of the α4β2 Nicotinic Acetylcholine Receptor]

Patients with epilepsy are often affected by not only seizures but also a variety of cognitive and psychiatric comorbidities that further impair their quality of life. However, it is unclear whether epilepsy is associated with psychic function. The aim of the present study was to clarify the effects of kindling-induced epileptic seizures on psychic functioning, using behavioral pharmacological tests. Pentylenetetrazol (PTZ)-kindled mice displayed impaired motor coordination (in the rotarod test), and social approach impairment (in the three-chamber social test) compared with vehicle mice. Intraperitoneal ABT-418 treatment (0.05 mg/kg) alleviated these behavioral abnormalities in PTZ-kindled mice. Immunolabeling of tissue sections demonstrated that expression of the α4 subunit of the α4β2 nicotinic acetylcholine receptor in the piriform cortex was significantly decreased in PTZ-kindled mice. In contrast, expression of the synaptic adhesion molecule neuroligin 3 (NLG3) was significantly higher in the piriform cortex of PTZ-kindled mice compared with vehicle mice. Collectively, our findings suggest that attention deficit/hyperactivity disorder (ADHD)-like or autistic-like behavioral abnormalities associated with epilepsy are closely linked to downregulation of the α4 subunit of the α4β2 receptor and upregulation of NLG3 in the mouse piriform cortex. In summary, this study indicates that ABT-418 is a good candidate for the treatment of patients with epilepsy complicated by psychiatric symptoms such as autism and ADHD.

Cholinergic double duty: cue detection and attentional control

Cholinergic signaling in the cortex involves fast or transient signaling as well as a relatively slower neuromodulatory component. These two components of cholinergic activity mediate separate yet interacting aspects of cue detection and attentional control. The transient component appears to support the activation of cue-associated task or response sets, whereas the slower modulatory component stabilizes task-set and context representations, therefore potentially facilitating top-down control. Evidence from humans expressing genetic variants of the choline transporter as well as from patients with degenerating cholinergic systems supports the hypothesis that attentional control capacities depend on levels of cholinergic neuromodulation. Deficits in cholinergic-attentional control impact diverse cognitive functions, including timing, working memory, and complex movement control.

Profiling attention and cognition enhancing drugs in a rat touchscreen-based continuous performance test

RATIONALE

A novel rodent continuous performance test (CPT) was developed as one of the goals of the NEWMEDS (Novel Methods leading to New Medications in Depression and Schizophrenia) consortium to improve its translatability to the CPT test used in human subjects.

OBJECTIVES

The objective of the study is to investigate the effects of attention and cognition enhancing drugs in rodent CPT.

METHODS

A single cohort of rats were trained to asymptotic performance in the test. Pharmacological test sessions were then performed twice per week in a full crossover design with the following drugs tested: methylphenidate (0.3, 1, and 3 mg/kg), the α4β2 nicotinic agonist ABT-594 (0.0023, 0.007 and 0.023 mg/kg), modafinil (8, 16, and 32 mg/kg), atomoxetine (0.3, 1, and 3 mg/kg), donepezil (0.1, 0.3, and 1 mg/kg), and memantine (1.25, 2.5, and 5 mg/kg).

RESULTS

The stimulant-like drugs methylphenidate, ABT-594, and modafinil were found to increase measures of impulsivity and overall responding with generally no positive effects on d', a putative measure of attention, with the exception of ABT-594 which improved d' at the highest dose tested. Atomoxetine and the memory-enhancing drugs donepezil and memantine, on the other hand, were found to reduce measures of impulsivity and responding and had either negligible or worsening effects on d'.

CONCLUSIONS

Our results suggest rodent CPT can detect changes in impulsivity resulting from drugs known to improve attention in rodents and humans. However, additional work is needed to assess the sensitivity and validity of this assay for assessing effects on attention.

Designing selective modulators for the nicotinic receptor subtypes: challenges and opportunities

Nicotinic receptors are membrane proteins involved in several physiological processes. They are considered suitable drug targets for various CNS disorders or conditions, as shown by the large number of compounds which have entered clinical trials. In recent years, nonconventional agonists have been discovered: positive allosteric modulators, allosteric agonists, site-specific agonists and silent desensitizers are compounds able to modulate the receptor interacting at sites different from the orthodox one, or to desensitize the receptor without prior opening. While these new findings can further complicate the pharmacology of these proteins and the design and optimization of ligands, they undoubtedly offer new opportunities to find drugs for the many therapeutic indications involving nicotinic receptors.

Nicotinic α4β2 Cholinergic Receptor Influences on Dorsolateral Prefrontal Cortical Neuronal Firing during a Working Memory Task

The primate dorsolateral prefrontal cortex (dlPFC) subserves top-down regulation of attention and working memory abilities. Depletion studies show that the neuromodulator acetylcholine (ACh) is essential to dlPFC working memory functions, but the receptor and cellular bases for cholinergic actions are just beginning to be understood. The current study found that nicotinic receptors comprised of α4 and β2 subunits (α4β2-nAChR) enhance the task-related firing of delay and fixation cells in the dlPFC of monkeys performing a working memory task. Iontophoresis of α4β2-nAChR agonists increased the neuronal firing and enhanced the spatial tuning of delay cells, neurons that represent visual space in the absence of sensory stimulation. These enhancing effects were reversed by coapplication of a α4β2-nAChR antagonist, consistent with actions at α4β2-nAChR. Delay cell firing was reduced when distractors were presented during the delay epoch, whereas stimulation of α4β2-nAChR protected delay cells from these deleterious effects. Iontophoresis of α4β2-nAChR agonists also enhanced the firing of fixation cells, neurons that increase firing when the monkey initiates a trial, and maintain firing until the trial is completed. These neurons are thought to contribute to sustained attention and top-down motor control and have never before been the subject of pharmacological inquiry. These findings begin to build a picture of the cellular actions underlying the beneficial effects of ACh on attention and working memory. The data may also help to explain why genetic insults to α4 subunits are associated with working memory and attentional deficits and why α4β2-nAChR agonists may have therapeutic potential.SIGNIFICANCE STATEMENT The acetylcholine (ACh) arousal system in the brain is needed for robust attention and working memory functions, but the receptor and cellular bases for its beneficial effects are poorly understood in the newly evolved primate brain. The current study found that ACh stimulation of nicotinic receptors comprised of α4 and β2 subunits (α4β2-nAChR) enhanced the firing of neurons in the primate prefrontal cortex that subserve top-down attentional control and working memory. α4β2-nAChR stimulation also protected neuronal responding from the detrimental effects of distracters presented during the delay epoch, when information is held in working memory. These results illuminate how ACh strengthens higher cognition and help to explain why genetic insults to the α4 subunit weaken cognitive and attentional abilities.

Ligand Binding at the 4-4 Agonist-Binding Site of the 42 nAChR Triggers Receptor Activation through a Pre-Activated Conformational State

The α4β2 nicotinic acetylcholine receptor (nAChR) is the most abundant subtype in the brain and exists in two functional stoichiometries: (α4)₃(β2)₂ and (α4)₂(β2)₃. A distinct feature of the (α4)₃(β2)₂ receptor is the biphasic activation response to the endogenous agonist acetylcholine, where it is activated with high potency and low efficacy when two α4-β2 binding sites are occupied and with low potency/high efficacy when a third α4-α4 binding site is occupied. Further, exogenous ligands can bind to the third α4-α4 binding site and potentiate the activation of the receptor by ACh that is bound at the two α4-β2 sites. We propose that perturbations of the recently described pre-activation step when a third binding site is occupied are a key driver of these distinct activation properties. To investigate this, we used a combination of simple linear kinetic models and voltage clamp electrophysiology to determine whether transitions into the pre-activated state were increased when three binding sites were occupied. We separated the binding at the two different sites with ligands selective for the α4-β2 site (Sazetidine-A and TC-2559) and the α4-α4 site (NS9283) and identified that when a third binding site was occupied, changes in the concentration-response curves were best explained by an increase in transitions into a pre-activated state. We propose that perturbations of transitions into a pre-activated state are essential to explain the activation properties of the (α4)₃(β2)₂ receptor by acetylcholine and other ligands. Considering the widespread clinical use of benzodiazepines, this discovery of a conserved mechanism that benzodiazepines and ACh potentiate receptor activation via a third binding site can be exploited to develop therapeutics with similar properties at other cys-loop receptors.

The α4β2 nicotinic acetylcholine receptor modulates autism-like behavioral and motor abnormalities in pentylenetetrazol-kindled mice

Epilepsy is associated with several psychiatric disorders, including cognitive impairment, autism and attention deficit/hyperactivity disorder (ADHD). However, the psychopathology of epilepsy is frequently unrecognized and untreated in patients. In the present study, we investigated the effects of ABT-418, a neuronal nicotinic acetylcholine receptor agonist, on pentylenetetrazol (PTZ)-kindled mice with behavioral and motor abnormalities. PTZ-kindled mice displayed impaired motor coordination (in the rotarod test), anxiety (in the elevated plus maze test) and social approach impairment (in the three-chamber social test) compared with control mice. ABT-418 treatment (0.05 mg/kg, intraperitoneally) alleviated these behavioral abnormalities in PTZ-kindled mice. Immunolabeling of tissue sections demonstrated that expression of the α4 nicotinic acetylcholine receptor subunit in the medial habenula was similar in control and PTZ-kindled mice. However, expression was significantly decreased in the piriform cortex in PTZ-kindled mice. In addition, we examined the expression of the synaptic adhesion molecule neuroligin 3 (NLG3). NLG3 expression in the piriform cortex was significantly higher in PTZ-kindled mice compared with control mice. Collectively, our findings suggest that ADHD-like or autistic-like behavioral abnormalities associated with epilepsy are closely related to the downregulation of the α4 nicotinic receptor and the upregulation of NLG3 in the piriform cortex. In summary, this study indicates that ABT-418 might have therapeutic potential for attentional impairment in epileptic patients with psychiatric disorders such as autism and ADHD.

Ion channels gated by acetylcholine and serotonin: structures, biology, and drug discovery

The nicotinic acetylcholine receptors (nAChRs) and the 5-HT3 receptors (5-HT3Rs) are cation-selective members of the pentameric ligand-gated ion channels (pLGICs), which are oligomeric protein assemblies that convert a chemical signal into an ion flux through postsynaptic membrane. They are critical components for synaptic transmission in the nervous system, and their dysfunction contributes to many neurological disorders. The diverse subunit compositions of pLGICs give rise to complex mechanisms of ligand recognition, channel gating, and ion-selective permeability, which have been demonstrated in numerous electrophysiological and molecular biological studies, and unraveled by progress in studying the structural biology of this protein family. In this review, we discuss recent insights into the structural and functional basis of two cation-selective pLGICs, the nAChR and the 5-HT3R, including their subunit compositions, ligand binding, and channel gating mechanisms. We also discuss their relevant pharmacology and drug discovery for treating various neurological disorders. Finally, we review a model of two alternative ion conducting pathways based on the latest 5-HT3A crystal structure.

Nicotinic ligands as multifunctional agents for the treatment of neuropsychiatric disorders

The challenges associated with developing more effective treatments for neurologic and psychiatric illness such as Alzheimer's disease and schizophrenia are considerable. Both the symptoms and the pathophysiology of these conditions are complex and poorly understood and the clinical presentations across different patients can be very heterogeneous. Moreover, it has become apparent that the reductionist approach to drug discovery for these illnesses that has dominated the field for decades (i.e., the development of highly selective compounds or other treatment modalities focused on a very specific pathophysiologic target) has not been widely successful. Accordingly, a variety of new strategies have emerged including the development of "multitarget-directed ligands" (MTDLs), the development and/or identification of compounds that exhibit "multifunctional" activity (e.g., pro-cognitive plus neuroprotective, pro-cognitive plus antipsychotic activity), "repurposing" strategies for existing compounds that have other clinical indications, and novel "adjunctive" treatment strategies that might enhance the efficacy of the currently available treatments. Interestingly, a variety of ligands at nicotinic acetylcholine receptors (nAChRs) appear to have the potential to fulfill one or more of these desirable properties (i.e., multifunctional, repurposing, or adjunctive treatment potential). The purpose of this review (while not all-inclusive) is to provide an overview of a variety of nAChR ligands that demonstrate potential in these categories, particularly, "multifunctional" properties. Due to their densities in the mammalian brain and the amount of literature available, the review will focus on ligands of the high affinity α4β2 nAChR and the low affinity α7 nAChR.

Behavioral-Cognitive Targets for Cholinergic Enhancement

Cholinergic mechanisms have long been considered a promising target for enhancing cognitive functions. Two distinct yet interacting components of cholinergic activity have been proposed to mediate specific cognitive functions. Transient spikes in cholinergic activity mediate the detection of cues in situations involving attentional mode shifts. More slowly changing cholinergic neuromodulation of cortical circuitry regulates task compliance specifically in response to performance challenges. Increases in cholinergic neuromodulation enhances the generation of cholinergic transients via stimulation of α4β2* nicotinic acetylcholine receptors. Stimulation of these receptors stabilizes attentional performance and increases cue detection rates. Adjunctive treatment with agonists or modulators at these receptors is predicted to benefit unstable attentional performance and low cue detection rates that are common to several brain disorders.

Engineered α4β2 nicotinic acetylcholine receptors as models for measuring agonist binding and effect at the orthosteric low-affinity α4-α4 interface

The nicotinic acetylcholine receptor α4β2 is important for normal mammalian brain function and is known to express in two different stoichiometries, (α4)2(β2)3 and (α4)3(β2)2. While these are similar in many aspects, the (α4)3(β2)2 stoichiometry differs by harboring a third orthosteric acetylcholine binding site located at the α4-α4 interface. Interestingly, the third binding site has, so far, only been documented using electrophysiological assays, actual binding affinities of nicotinic receptor ligands to this site are not known. The present study was therefore aimed at determining binding affinities of nicotinic ligands to the α4-α4 interface. Given that epibatidine shows large functional potency differences at α4-β2 vs. α4-α4 interfaces, biphasic binding properties would be expected at (α4)3(β2)2 receptors. However, standard saturation binding experiments with [(3)H]epibatidine did not reveal biphasic binding under the conditions utilized. Therefore, an engineered β2 construct (β2(HQT)), which converts the β(-) face to resemble that of an α4(-) face, was utilized to create (α4)3(β2(HQT))2 receptors harboring three α4-α4 interfaces. With this receptor, low affinity binding of epibatidine with a Kd of ∼5 nM was observed in sharp contrast to a Kd value of ∼10 pM observed for wild-type receptors. A strong correlation between binding affinities at the (α4)3(β2(HQT))2 receptor and functional potencies at the wild-type receptor of a range of nicotinic ligands highlighted the validity of using the mutational approach. Finally, large differences in activities at α4-β2 vs. α4-α4 interfaces were observed for structurally related agonists underscoring the need for establishing all binding parameters of compounds at α4β2 receptors.

Attention and the Cholinergic System: Relevance to Schizophrenia

Traditional methods of drug discovery often rely on a unidirectional, "bottom-up" approach: A search for molecular compounds that target a particular neurobiological substrate (e.g., a receptor type), the refinement of those compounds, testing in animal models using high-throughput behavioral screening methods, and then human testing for safety and effectiveness. Many attempts have found the "effectiveness" criterion to be a major stumbling block, and we and others have suggested that success may be improved by an alternative approach that considers the neural circuits mediating the effects of genetic and molecular manipulations on behavior and cognition. We describe our efforts to understand the cholinergic system's role in attention using parallel approaches to test main hypotheses in both rodents and humans as well as generating converging evidence using methods and levels of analysis tailored to each species. The close back-and-forth between these methods has enhanced our understanding of the cholinergic system's role in attention both "bottom-up" and "top-down"-that is, the basic neuroscience identifies potential neuronal circuit-based mechanisms of clinical symptoms, and the patient and genetic populations serve as natural experiments to test and refine hypotheses about its contribution to specific processes. Together, these studies have identified (at least) two major and potentially independent contributions of the cholinergic system to attention: a neuromodulatory component that influences cognitive control in response to challenges from distractors that either make detection more difficult or draw attention away from the distractor, and a phasic or transient cholinergic signal that instigates a shift from ongoing behavior and the activation of cue-associated response. Right prefrontal cortex appears to play a particularly important role in the neuromodulatory component integrating motivational and cognitive influences for top-down control across populations, whereas the transient cholinergic signal involves orbitofrontal regions associated with shifts between internal and external attention. Understanding how these two modes of cholinergic function interact and are perturbed in schizophrenia will be an important prerequisite for developing effective treatments.

Targeting the nicotinic cholinergic system to treat attention-deficit/hyperactivity disorder: rationale and progress to date

Attention-deficit/hyperactivity disorder (ADHD) is a common chronic neurobehavioral disorder related to clinically significant levels of inattention, hyperactivity, and/or impulsivity. ADHD begins in childhood and symptoms persist into adulthood for the majority of those with the disorder. Associated features of ADHD include emotion dysregulation and cognitive impairments, which contribute to the considerable functional impairments in this disorder. Current approved treatments are reasonably effective; however, a significant need remains for new pharmacotherapies, both for individuals who do not achieve a full therapeutic response and for symptoms that are under-treated including cognition and emotion regulation. The striking relationship between ADHD and cigarette smoking and the known effects of nicotine on cognition has spurred research into the therapeutic potential of nicotinic agents for ADHD. Although there are no approved medications for ADHD that target nicotinic acetylcholine receptor (nAChR) function, results from many trials of nicotinic drugs are available and reviewed in this article. ADHD symptoms were reduced in the majority of published studies of nicotine and novel α4β2 nicotinic agonists in adult ADHD. The drugs were generally well tolerated, with mild to moderate side effects reported, which were largely consistent with cholinergic stimulation and included nausea, dizziness, and gastrointestinal distress. Within-subject crossover study designs were used in the majority of positive studies. This design may be particularly useful in ADHD trials because it minimizes variability in this notoriously heterogeneous diagnostic group. In addition, many studies found evidence for a beneficial effect of nicotinic stimulation on cognitive and emotional domains. Thus, targeting nAChRs in ADHD appears to have a modest clinical benefit in adult ADHD. Continued refinement of nAChR agonists with greater specificity and fewer side effects may lead to even more effective nAChR agonists for ADHD. Future clinical trials in ADHD should include direct measures of neuropsychological performance and emotion regulation.

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.

Efficacy of atomoxetine in adults with attention deficit hyperactivity disorder: an integrated analysis of the complete database of multicenter placebo-controlled trials

Persistence of attention deficit hyperactivity disorder (ADHD) into adulthood can be disabling or lead to substantial impairment. Several clinical trials of atomoxetine (ATX) in adults with ADHD have been reported following the National Institute for Health and Clinical Excellence (NICE) guidelines issued in 2008. We performed an integrated analysis of all Eli Lilly-sponsored, randomized, double-blind, placebo-controlled studies of ATX in adults with ADHD completed as of May 2012. Individual patient data were pooled from six short-term (10-16 week) studies (1961 patients) and three longer-term (six-month) studies (1413 patients). In the short-term analysis, ATX patients achieved a significantly greater mean reduction in ADHD symptoms than placebo patients (-12.2 vs -8.1; Conners' Adult ADHD Rating Scale-Investigator-Rated: Screening Version (CAARS-Inv: SV); p<0.001). In the longer-term analysis, respective improvements after six months were -13.2 vs -9.7 (p<0.001). Response rates at study endpoints for ATX vs placebo, based on CAARS-Inv: SV improvement ≥ 30% and Clinical Global Impressions of ADHD-Severity (CGI-ADHD-S) ≤ 3 were 34.8% vs 22.3% in the short-term and 43.4% vs 28.0% after six months, and CAARS-Inv: SV improvements ≥ 40% were 41.3% vs 25.3% in the short-term and 44.0% vs 31.4% after six months (all p<0.001). Overall, ATX had a clinically significant effect in adults with ADHD, with reductions in core symptoms and clinically meaningful responder rates.

The α7 nicotinic receptor agonist ABT-107 decreases L-Dopa-induced dyskinesias in parkinsonian monkeys

Previous studies in Parkinsonian rats and monkeys have shown that β2-selective nicotinic acetylcholine receptor (nAChR) agonists reduce l-Dopa-induced dyskinesias (LIDs), a serious complication of l-Dopa therapy for Parkinson's disease. Since rodent studies also suggested an involvement of α7 nAChRs in LIDs, we tested the effect of the potent, selective α7 agonist ABT-107 [5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole]. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-lesioned monkeys were gavaged with l-Dopa/carbidopa (10 and 2.5 mg/kg, respectively) twice daily, which resulted in stable LIDs. A dose-response study (0.03-1.0 mg/kg) showed that oral ABT-107 decreased LIDs by 40-60%. LIDs returned to control levels only after a 6-week ABT-107 washout, suggesting that long-term molecular changes were involved. Subsequent readministration of ABT-107 decreased LIDs by 50-60%, indicating that tolerance did not develop. ABT-107 had no effect on Parkinsonism or cognitive performance. We next tested ABT-107 together with the β2 agonist ABT-894 [(3-(5,6-dichloro-pyridin-3-yl)-1(S),5 (S)-3,6-diazabicyclo[3.2.0]heptane], previously shown to reduce LIDs in Parkinsonian monkeys. In one study, the monkeys were first given oral ABT-894 (0.01 mg/kg), which maximally decreased LIDs by 50-60%; they were then also treated with 0.1 mg/kg ABT-107, a dose that maximally reduced LIDs. The effect of combined treatment on LIDs was similar to that with either drug alone. Comparable results were observed in a group of monkeys first treated with ABT-107 and then also given ABT-894. Thus, α7 and β2 nAChR-selective drugs may function via a final common mechanism to reduce LIDs. The present results suggest that drugs targeting either α7 or β2 nAChRs may be useful as antidyskinetic agents in Parkinson's disease.

Nicotinic acetylcholine receptors in attention circuitry: the role of layer VI neurons of prefrontal cortex

Cholinergic modulation of prefrontal cortex is essential for attention. In essence, it focuses the mind on relevant, transient stimuli in support of goal-directed behavior. The excitation of prefrontal layer VI neurons through nicotinic acetylcholine receptors optimizes local and top-down control of attention. Layer VI of prefrontal cortex is the origin of a dense feedback projection to the thalamus and is one of only a handful of brain regions that express the α5 nicotinic receptor subunit, encoded by the gene chrna5. This accessory nicotinic receptor subunit alters the properties of high-affinity nicotinic receptors in layer VI pyramidal neurons in both development and adulthood. Studies investigating the consequences of genetic deletion of α5, as well as other disruptions to nicotinic receptors, find attention deficits together with altered cholinergic excitation of layer VI neurons and aberrant neuronal morphology. Nicotinic receptors in prefrontal layer VI neurons play an essential role in focusing attention under challenging circumstances. In this regard, they do not act in isolation, but rather in concert with cholinergic receptors in other parts of prefrontal circuitry. This review urges an intensification of focus on the cellular mechanisms and plasticity of prefrontal attention circuitry. Disruptions in attention are one of the greatest contributing factors to disease burden in psychiatric and neurological disorders, and enhancing attention may require different approaches in the normal and disordered prefrontal cortex.

Stimulant treatment of ADHD and cigarette smoking: a meta-analysis

BACKGROUND AND OBJECTIVE

Individuals with attention-deficit/hyperactivity disorder (ADHD) have a significantly higher risk of cigarette smoking. The nature of the relationship between smoking and psychostimulant medications commonly used to treat ADHD is controversial. Our objective was to examine the relationship between stimulant treatment of ADHD and cigarette smoking by using meta-analysis, and to identify study and sample characteristics that moderate this relationship.

METHODS

Literature searches on PubMed and PsycInfo databases identified published studies for inclusion. Included studies compared cigarette smoking outcomes for stimulant-treated and untreated ADHD individuals. Seventeen studies met inclusion criteria, and 14 (total n = 2360) contained sufficient statistical information for inclusion in the meta-analysis. Two authors extracted odds ratios or frequencies of smokers in the treatment or nontreatment groups, and coded study characteristics including sample source, percentage of male participants, follow-up length, treatment consistency, type of smoking measure, prospective study, and controlling for comorbidities.

RESULTS

Meta-analysis revealed a significant association between stimulant treatment and lower smoking rates. Meta-regression indicated that effect sizes were larger for studies that used clinical samples, included more women, measured smoking in adolescence rather than adulthood, conceptualized stimulant treatment as consistent over time, and accounted for comorbid conduct disorder.

CONCLUSIONS

Nearly all studies were naturalistic, precluding causal inferences. Available data were insufficient to examine additional influences of patient demographics, treatment effectiveness, or other comorbidities. Consistent stimulant treatment of ADHD may reduce smoking risk; the effect was larger in samples with more severe psychopathology. Implications for further research, treatment of ADHD, and smoking prevention are discussed.

Role for the nicotinic cholinergic system in movement disorders; therapeutic implications

A large body of evidence using experimental animal models shows that the nicotinic cholinergic system is involved in the control of movement under physiological conditions. This work raised the question whether dysregulation of this system may contribute to motor dysfunction and whether drugs targeting nicotinic acetylcholine receptors (nAChRs) may be of therapeutic benefit in movement disorders. Accumulating preclinical studies now show that drugs acting at nAChRs improve drug-induced dyskinesias. The general nAChR agonist nicotine, as well as several nAChR agonists (varenicline, ABT-089 and ABT-894), reduces l-dopa-induced abnormal involuntary movements or dyskinesias up to 60% in parkinsonian nonhuman primates and rodents. These dyskinesias are potentially debilitating abnormal involuntary movements that arise as a complication of l-dopa therapy for Parkinson's disease. In addition, nicotine and varenicline decrease antipsychotic-induced abnormal involuntary movements in rodent models of tardive dyskinesia. Antipsychotic-induced dyskinesias frequently arise as a side effect of chronic drug treatment for schizophrenia, psychosis and other psychiatric disorders. Preclinical and clinical studies also show that the nAChR agonist varenicline improves balance and coordination in various ataxias. Lastly, nicotine has been reported to attenuate the dyskinetic symptoms of Tourette's disorder. Several nAChR subtypes appear to be involved in these beneficial effects of nicotine and nAChR drugs including α4β2*, α6β2* and α7 nAChRs (the asterisk indicates the possible presence of other subunits in the receptor). Overall, the above findings, coupled with nicotine's neuroprotective effects, suggest that nAChR drugs have potential for future drug development for movement disorders.

Where attention falls: Increased risk of falls from the converging impact of cortical cholinergic and midbrain dopamine loss on striatal function

Falls are a major source of hospitalization, long-term institutionalization, and death in older adults and patients with Parkinson's disease (PD). Limited attentional resources are a major risk factor for falls. In this review, we specify cognitive-behavioral mechanisms that produce falls and map these mechanisms onto a model of multi-system degeneration. Results from PET studies in PD fallers and findings from a recently developed animal model support the hypothesis that falls result from interactions between loss of basal forebrain cholinergic projections to the cortex and striatal dopamine loss. Striatal dopamine loss produces inefficient, low-vigor gait, posture control, and movement. Cortical cholinergic deafferentation impairs a wide range of attentional processes, including monitoring of gait, posture and complex movements. Cholinergic cell loss reveals the full impact of striatal dopamine loss on motor performance, reflecting loss of compensatory attentional supervision of movement. Dysregulation of dorsomedial striatal circuitry is an essential, albeit not exclusive, mediator of falls in this dual-system model. Because cholinergic neuromodulatory activity influences cortical circuitry primarily via stimulation of α4β2* nicotinic acetylcholine receptors, and because agonists at these receptors are known to benefit attentional processes in animals and humans, treating PD fallers with such agonists, as an adjunct to dopaminergic treatment, is predicted to reduce falls. Falls are an informative behavioral endpoint to study attentional-motor integration by striatal circuitry.

ABT-089 and ABT-894 reduce levodopa-induced dyskinesias in a monkey model of Parkinson's disease

Levodopa-induced dyskinesias (LIDs) are a serious complication of levodopa therapy for Parkinson's disease for which there is little treatment. Accumulating evidence shows that nicotinic acetylcholine receptor (nAChR) drugs decrease LIDs in parkinsonian animals. Here, we examined the effect of two β2 nAChR agonists, ABT-089 and ABT-894, that previously were approved for phase 2 clinical trials for other indications. Two sets of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys were administered levodopa/carbidopa (10 mg/kg and 2.5 mg/kg, respectively) twice daily 5 days a week until they were stably dyskinetic. Each set had a vehicle-treated group, an nAChR agonist-treated group, and a nicotine-treated group as a positive control. Set A monkeys had previously received other nAChR drugs (nAChR drug-primed), whereas Set B monkeys were initially nAChR drug-naive. Both sets were administered the partial agonist ABT-089 (range, 0.01-1.0 mg/kg) orally 5 days a week twice daily 30 minutes before levodopa with each dose given for 1 to 5 weeks. ABT-089 decreased LIDs by 30% to 50% compared with vehicle-treated monkeys. Nicotine reduced LIDs by 70% in a parallel group. After 4 weeks of washout, the effect of the full agonist ABT-894 (range, 0.0001-0.10 mg/kg) was assessed on LIDs in Set A and Set B. ABT-894 reduced LIDs by 70%, similar to nicotine. Both drugs acted equally well at α4β2* and α6β2* nAChRs; however, ABT-089 was 30 to 60 times less potent than ABT-894. Tolerance did not develop for the time periods tested (range, 3-4 months). The nAChR drugs did not worsen parkinsonism or cognitive ability. Emesis, a common problem with nAChR drugs, was not observed. ABT-894 and ABT-089 appear to be good candidate nAChR drugs for the management of LIDs in Parkinson's disease.

Cholinergic modulation of the medial prefrontal cortex: the role of nicotinic receptors in attention and regulation of neuronal activity

Acetylcholine (ACh) release in the medial prefrontal cortex (mPFC) is crucial for normal cognitive performance. Despite the fact that many have studied how ACh affects neuronal processing in the mPFC and thereby influences attention behavior, there is still a lot unknown about how this occurs. Here we will review the evidence that cholinergic modulation of the mPFC plays a role in attention and we will summarize the current knowledge about the role between ACh receptors (AChRs) and behavior and how ACh receptor activation changes processing in the cortical microcircuitry. Recent evidence implicates fast phasic release of ACh in cue detection and attention. This review will focus mainly on the fast ionotropic nicotinic receptors and less on the metabotropic muscarinic receptors. Finally, we will review limitations of the existing studies and address how innovative technologies might push the field forward in order to gain understanding into the relation between ACh, neuronal activity and behavior.

Developments and challenges in the diagnosis and treatment of ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a prevalent neurodevelopmental disorder, often associated with other psychiatric comorbidities, functional impairments, and poor long-term outcomes. The objective of this selected review is to describe current advances and challenges in the diagnosis and treatment of ADHD. The disorder is associated with neurobiological underpinnings and is highly heterogeneous in various aspects, such as symptom profiles, cognitive impairments, and neurobiological and genetic features. The efficacy and safety of short-term pharmacological treatments across the life cycle is well studied, but further research investigating long-term treatment, impact of treatment in preschoolers, and non-pharmacological interventions is needed. Future research is also needed to better characterize the neurodevelopmental pathways of the disorder, linking clinical and neurobiological information, less investigated populations, and new interventions.

Selective potentiation of (α4)3(β2)2 nicotinic acetylcholine receptors augments amplitudes of prefrontal acetylcholine- and nicotine-evoked glutamatergic transients in rats

Prefrontal glutamate release evoked through activation of α4β2* nicotinic acetylcholine receptors (nAChRs) situated on thalamic glutamatergic afferents mediates cue detection processes and thus contributes to attentional performance. However, little is known about the respective contributions of the high sensitivity and low sensitivity (LS) stoichiometries of the α4β2 nAChR, (α4)2(β2)3 and (α4)3(β2)2, to these processes. In the present study we employed glutamate-sensitive microelectrodes and the (α4)3(β2)2-selective positive allosteric modulator (PAM) NS9283 to investigate the importance of the LS α4β2 nAChR for glutamate release in the rat medial prefrontal cortex (mPFC). Firstly, the signaling evoked by physiologically relevant ACh concentrations through the (α4)3(β2)2 nAChR in HEK293 cells was potentiated by NS9283, consistent with the classification of NS9283 as a PAM. In urethane-anesthetized rats, intra-prefrontal pressure ejections of NS9283 evoked glutamatergic transients. Importantly, this glutamate release was attenuated by removal of cholinergic projections to the recording area. This finding indicates that the effects of NS9283 depend on endogenous ACh, again consistent with effects of a PAM. We then conducted microdialysis to demonstrate the presence of extracellular ACh in urethane-anesthetized control rats. While detectable, those levels were significantly lower than in awake rats. Finally, the amplitudes of glutamatergic transients evoked by local pressure ejections of a low concentration of nicotine were significantly augmented following systemic administration of NS9283 (3.0mg/kg). In conclusion, our results indicate that a LS α4β2 nAChR PAM such as NS9283 may enhance the cholinergic modulation of glutamatergic neurotransmission in the cortex, thereby perhaps alleviating the attentional impairments common to a range of brain disorders.

Cholinergic control over attention in rats prone to attribute incentive salience to reward cues

Some rats [sign-trackers (STs)] are especially prone to attribute incentive salience to reward cues, relative to others [goal-trackers (GTs)]. Thus, reward cues are more likely to promote maladaptive reward-seeking behavior in STs than GTs. Here, we asked whether STs and GTs differ on another trait that can contribute to poor restraint over behavior evoked by reward cues. We report that, relative to GTs, STs have poor control over attentional performance, due in part to insufficient cholinergic stimulation of cortical circuitry. We found that, relative to GTs, STs showed poor performance on a sustained attention task (SAT). Furthermore, their performance fluctuated rapidly between periods of good to near-chance performance. This finding was reproduced using a separate cohort of rats. As demonstrated earlier, performance on the SAT was associated with increases in extracellular levels of cortical acetylcholine (ACh); however, SAT performance-associated increases in ACh levels were significantly attenuated in STs relative to GTs. Consistent with the view that the modulatory effects of ACh involve stimulation of α4β2* nicotinic ACh receptors (nAChRs), systemic administration of the partial nAChR agonist ABT-089 improved SAT performance in STs and abolished the difference between SAT-associated ACh levels in STs and GTs. Neither the nonselective nAChR agonist nicotine nor the psychostimulant amphetamine improved SAT performance. These findings suggest that individuals who have a propensity to attribute high-incentive salience to reward cues also exhibit relatively poor attentional control. A combination of these traits may render individuals especially vulnerable to disorders, such as obesity and addiction.

α4β2* neuronal nicotinic receptor ligands (agonist, partial agonist and positive allosteric modulators) as therapeutic prospects for pain

α4β2* neuronal nicotinic acetylcholine receptor are ligand-gated ion channels and widely expressed throughout the central and peripheral nervous system. α4β2* neuronal nicotinic acetylcholine receptor play crucial role in pain signaling via modulation of multiple neurotransmitters like acetylcholine, dopamine, γ-amino butyric acid (GABA) and norepinephrine. Both spinal and supraspinal pathways are involved in the mechanisms by which α4β2* neuronal nicotinic acetylcholine receptor ligands modulate the neuropathic and inflammatory pain. Selective α4β2* neuronal nicotinic acetylcholine receptor ligands are being developed for the treatment of neuropathic and inflammatory pain as they show considerable efficacy in a wide range of preclinical pain models. Agonists/partial agonists of α4β2* neuronal nicotinic acetylcholine receptor show efficacy in animal models of pain and their anti-nociceptive properties are blocked by nicotinic antagonists. Positive allosteric modulators are being developed with the aim to increase the potency or therapeutic window of agonists/partial agonists. Accumulating evidences suggest that anti-nociceptive effects of nicotinic acetylcholine receptor ligands may not be mediated solely by α4β2* neuronal nicotinic acetylcholine receptor. We have also reviewed the stage of clinical development of various α4β2* neuronal nicotinic acetylcholine receptor ligands.