Reimagining cholinergic therapy for Alzheimer's disease.

Currently, enhancement of cholinergic neurotransmission via cholinesterase inhibitors represents the main available approach to treat cognitive and behavioural symptoms of the early as well as late stages of Alzheimer's disease. Restoring the cholinergic system has been a primary means of improving cognition in Alzheimer's disease, as four of the six approved therapies are acetylcholinesterase inhibitors. Memantine is an N-methyl-d-aspartate antagonist with a well-documented clinical effect on behavioural symptoms, which is often added to cholinesterase inhibitors to potentiate their effect and aducanumab, targeting the amyloid pathology, has recently been approved. The early, progressive and selective degeneration of the cholinergic system together and its close relation to cognitive deficits supports the use of cholinergic therapy for Alzheimer's disease. This review provides an updated view of the basal forebrain cholinergic system, its relation to cognition and its relevance for therapy of Alzheimer's disease. It deals with the three main aspects that form the basis of the cholinergic-oriented therapy of Alzheimer's disease, its origin, its mechanism of action, its clinical effects, advantages and limits of a cholinergic therapeutic approach. It includes a new and updated overview of the involvement of muscarinic receptors in Alzheimer's disease as well as the recent development of new and highly selective M1 muscarinic receptor agonists with disease-modifying potential. It also addresses the discovery of a novel nerve growth factor metabolic pathway responsible for the trophic maintenance of the basal forebrain system and its deregulation in Alzheimer's disease. It discusses new clinical studies and provides evidence for the long-term efficacy of cholinesterase inhibitor therapy suggesting a disease-modifying effect of these drugs. The classical symptomatic cholinergic therapy based on cholinesterase inhibitors is judiciously discussed for its maximal efficacy and best clinical application. The review proposes new alternatives of cholinergic therapy that should be developed to amplify its clinical effect and supplement the disease-modifying effect of new treatments to slow down or arrest disease progression.

AF710B, an M1/sigma-1 receptor agonist with long-lasting disease-modifying properties in a transgenic rat model of Alzheimer's disease.

INTRODUCTION: AF710B (aka ANAVEX 3-71) is a novel selective allosteric M1 muscarinic and sigma-1 receptor agonist. In 3×Tg-AD mice, AF710B attenuates cognitive deficits and decreases Alzheimer-like hallmarks. We now report on the long-lasting disease-modifying properties of AF710B in McGill-R-Thy1-APP transgenic (Tg) rats. METHODS: Chronic treatment with AF710B (10 µg/kg) was initiated in postplaque 13-month-old Tg rats. Drug or vehicle was administered orally daily for 4.5 months and interrupted 5 weeks before behavioral testing. RESULTS: AF710B long-term treatment reverted the cognitive deficits associated with advanced Alzheimer-like amyloid neuropathology in Tg rats. These effects were accompanied by reductions in amyloid pathology and markers of neuroinflammation and increases in amyloid cerebrospinal fluid clearance and levels of a synaptic marker. Importantly, these effects were maintained following a 5-week interruption of the treatment. DISCUSSION: With M1/sigma-1 activity and long-lasting disease-modifying properties at low dose, AF710B is a promising novel therapeutic agent for treating Alzheimer's disease.

AF710B, a Novel M1/σ1 Agonist with Therapeutic Efficacy in Animal Models of Alzheimer's Disease.

We previously developed orthosteric M1 muscarinic agonists (e.g. AF102B, AF267B and AF292), which act as cognitive enhancers and potential disease modifiers. We now report on a novel compound, AF710B, a highly potent and selective allosteric M1 muscarinic and σ1 receptor agonist. AF710B exhibits an allosteric agonistic profile on the M1 muscarinic receptor; very low concentrations of AF710B significantly potentiated the binding and efficacy of carbachol on M1 receptors and their downstream effects (p-ERK1/2, p-CREB). AF710B (1-30 µg/kg, p.o.) was a potent and safe cognitive enhancer in rats treated with the M1 antagonist trihexyphenidyl (passive avoidance impairment). These effects of AF710B involve σ1 receptor activation. In agreement with its antiamnesic properties, AF710B (at 30 nM), via activation of M1 and a possible involvement of σ1 receptors, rescued mushroom synapse loss in PS1-KI and APP-KI neuronal cultures, while AF267B (1 µM) was less potent in PS1-KI and ineffective in APP-KI models, respectively. In female 3xTg-AD mice, AF710B (10 µg/kg, i.p./daily/2 months) (i) mitigated cognitive impairments in the Morris water maze; (ii) decreased BACE1, GSK3ß activity, p25/CDK5, neuroinflammation, soluble and insoluble Aß40, Aß42, plaques and tau pathologies. AF710B differs from conventional σ1 and M1 muscarinic (orthosteric, allosteric or bitopic) agonists. These results highlight AF710B as a potential treatment for Alzheimer's disease (e.g. improving cognitive deficits, synaptic loss, amyloid and tau pathologies, and neuroinflammation) with a superior profile over a plethora of other therapeutic strategies.

Acute Effects of Muscarinic M1 Receptor Modulation on AßPP Metabolism and Amyloid-ß Levels in vivo: A Microdialysis Study.

Indirect modulation of cholinergic activity by cholinesterase inhibition is currently a widely established symptomatic treatment for Alzheimer's disease (AD). Selective activation of certain muscarinic receptor subtypes has emerged as an alternative cholinergic-based amyloid-lowering strategy for AD, as selective muscarinic M1 receptor agonists can reduce amyloid-ß (Aß) production by shifting endoproteolytic amyloid-ß protein precursor (AßPP) processing toward non-amyloidogenic pathways. In this study, we addressed the hypothesis that acute stimulation of muscarinic M1 receptors can inhibit Aß production in awake and freely moving AßPP transgenic mice. By combining intracerebral microdialysis with retrodialysis, we determined hippocampal Aß concentrations during simultaneous pharmacological modulation of brain M1 receptor function. Infusion with a M1 receptor agonist AF102B resulted in a rapid reduction of interstitial fluid (ISF) Aß levels while treatment with the M1 antagonist dicyclomine increased ISF Aß levels reaching significance within 120 minutes of treatment. The reduction in Aß levels was associated with PKCα and ERK activation resulting in increased levels of the α-secretase ADAM17 and a shift in AßPP processing toward the non-amyloidogenic processing pathway. In contrast, treatment with the M1 receptor antagonist dicyclomine caused a decrease in levels of phosphorylated ERK that was independent of PKCα, and led to an elevation of ß-secretase levels associated with increased amyloidogenic AßPP processing. The results of this study demonstrate rapid effects of in vivo M1 receptor modulation on the ISF pool of Aß and suggest that intracerebral microdialysis with retrodialysis is a useful technical approach for monitoring acute treatment effects of muscarinic receptor modulators on AßPP/Aß metabolism.

[11C]AF150(S), an agonist PET ligand for M1 muscarinic acetylcholine receptors.

BACKGROUND: The M1 muscarinic acetylcholine receptor (M1ACh-R) is a G protein-coupled receptor that can occur in interconvertible coupled and uncoupled states. It is enriched in the basal ganglia, hippocampus, olfactory bulb, and cortical areas, and plays a role in motor and cognitive functions. Muscarinic M1 agonists are potential therapeutic agents for cognitive disorders. The aim of this study was to evaluate [11C]AF150(S) as a putative M1ACh-R agonist PET ligand, which, owing to its agonist properties, could provide a tool to explore the active G protein-coupled receptor. METHODS: Regional kinetics of [11C]AF150(S) in rat brain were measured using a high-resolution research tomograph, both under baseline conditions and following pre-treatment with various compounds or co-administration of non-radioactive AF150(S). Data were analysed by calculating standard uptake values and by applying the simplified reference tissue model (SRTM). RESULTS: [11C]AF150(S) was rapidly taken up in the brain, followed by a rapid clearance from all brain regions. Analysis of PET data using SRTM revealed a binding potential (BPND) of 0.25 for the striatum, 0.20 for the hippocampus, 0.16 for the frontal cortical area and 0.15 for the posterior cortical area, all regions rich in M1ACh-R. BPND values were significantly reduced following pre-treatment with M1ACh-R antagonists. BPND values were not affected by pre-treatment with a M3ACh-R antagonist. Moreover, BPND was significantly reduced after pre-treatment with haloperidol, a dopamine D2 receptor blocker that causes an increase in extracellular acetylcholine (ACh). The latter may compete with [11C]AF150(S) for binding to the M1ACh-R; further pharmacological agents were applied to investigate this possibility. Upon injection of the highest dose (49.1 nmol kg-1) of [11C]AF150(S) diluted with non-radioactive AF150(S), brain concentration of AF150(S) reached 100 nmol L-1 at peak level. At this concentration, no sign of saturation in binding to M1ACh-R was observed. CONCLUSIONS: The agonist PET ligand [11C]AF150(S) was rapidly taken up in the brain and showed an apparent specific M1ACh-R-related signal in brain areas that are rich in M1ACh-R. Moreover, binding of the agonist PET ligand [11C]AF150(S) appears to be sensitive to changes in extracellular ACh levels. Further studies are needed to evaluate the full potential of [11C]AF150(S) for imaging the active pool of M1ACh-R in vivo.

Reproducible Analysis of Rat Brain PET Studies Using an Additional [(18)F]NaF Scan and an MR-Based ROI Template.

Background. An important step in the analysis of positron emission tomography (PET) studies of the brain is the definition of regions of interest (ROI). Image coregistration, ROI analysis, and quantification of brain PET data in small animals can be observer dependent. The purpose of this study was to investigate the feasibility of ROI analysis based on a standard MR template and an additional [(18)F]NaF scan. Methods. [(18)F]NaF scans of 10 Wistar rats were coregistered with a standard MR template by 3 observers and derived transformation matrices were applied to corresponding [(11)C]AF150(S) images. Uptake measures were derived for several brain regions delineated using the MR template. Overall agreement between the 3 observers was assessed by interclass correlation coefficients (ICC) of uptake data. In addition, [(11)C]AF150(S) ROI data were compared with ex vivo biodistribution data. Results. For all brain regions, ICC analysis showed excellent agreement between observers. Reproducibility, estimated by calculation of standard deviation of the between-observer differences, was demonstrated by an average of 17% expressed as coefficient of variation. Uptake of [(11)C]AF150(S) derived from ROI analysis closely matched ex vivo biodistribution data. Conclusions. The proposed method provides a reproducible and tracer-independent method for ROI analysis of rat brain PET data.

Cholinergic modulation of amyloid precursor protein processing with emphasis on M1 muscarinic receptor: perspectives and challenges in treatment of Alzheimer's disease.

The prescribed drugs for treatment of cognitive deficits in Alzheimer's disease (AD) patients are regarded as symptomatic drugs. Effective disease modifying therapies are not yet prescribed in AD patients. Three major hallmarks of AD (e.g. cholinergic hypofunction, Aß and tau neuropathologies) are closely linked raising the expectation that restoring the cholinergic hypofunction to normal, in particular via selective activation of M1 muscarinic receptors, may alter the onset or progression of AD dementia. This review is focused mainly on modulation of amyloid precursor processing and Aß levels in the brain via cholinergic treatment strategies based on M1 muscarinic agonists versus other cholinergic treatments (e.g. cholinesterase inhibitors prescribed for treatment of AD, M2 antagonists and nicotinic agonists). Advantages and potential drawbacks of these treatment modalities are reviewed versus the notion that due to an elusive etiology of AD, future disease modifiers should address comprehensively most of these AD hallmarks (e.g. Aß pathology, tau and tangle pathologies, as well as the cholinergic hypofunction and cognitive impairments). This major requirement may be fulfilled with M1-selective muscarinic agonists and less with other reviewed cholinergic treatments.

ERK1-independent α-secretase cut of ß-amyloid precursor protein via M1 muscarinic receptors and PKCα/ε.

The amyloid precursor protein (ßAPP) undergoes several proteolytic cleavages. While ß- and γ-secretases are responsible for the production of the 40-43 amino-acid long amyloid ß peptide (Aß), the α-secretase cut performed by the disintegrins ADAM10 and ADAM17, occurs in the middle of the Aß sequence, thereby preventing its formation and leading to the secretion of the large sAPPα neuroprotective fragment. Here we showed that a series of M1 muscarinic receptor agonists dose-dependently stimulated sAPPα secretion without interfering with ßAPP subcellular distribution. Carbachol- and PDBu-induced sAPPα secretions were blocked by the general PKC inhibitor GF109203X. We established that HEK293 and rhabdhomyosarcoma cells overexpressing constitutively active (CA) PKCα or PKCε secrete increased amounts of sAPPα while those expressing PKCδ were unable to modify sAPPα recovery. Conversely, the overexpression of PKCα or PKCε dominant negative (DN) constructs abolished PDBU-stimulated sAPPα secretion, whereas DN-PKCδ remained inert. In agreement, PKCα knockout lowered sAPPα recovery in primary cultured fibroblasts. We also demonstrated that the regulated α-secretase processing of ßAPP is not controlled by the Extracellular-Regulated Kinase-1/MAP-ERK Kinase (ERK1/MEK) cascade and likely does not require ADAM17 phosphorylation on its threonine735 residue. Because the muscarinic-dependent α-secretase-like processing of PrP(c) is fully dependent on ADAM17 phosphorylation on its threonine735 residue by ERK1, these results indicate that a single extracellular signal triggers ADAM17-dependent regulated cleavages of ßAPP and PrP(c) through distinct signalling cascades. This opens new potential therapeutic strategies aimed, in the context of Alzheimer's disease, at selectively activating ADAM17 towards ßAPP without affecting the cleavages of its numerous other substrates.

M1 agonists as a potential disease-modifying therapy for Alzheimer's disease.

Cholinergic deficit is a cardinal feature of Alzheimer's disease, and cholinesterase inhibitors represent one of the most prominent means of mitigating this dysfunction. Cholinesterase inhibitors provide mild symptomatic relief, although they lose their efficacy over time most likely because they are not disease-modifying agents. An alternative strategy for restoring cholinergic function and attenuating the cognitive decline involves acting on the receptors on which acetylcholine acts. Stimulation of muscarinic acetylcholine receptors and in particular the M1 subtype has been shown to have a beneficial effect in restoring cognition in patients with Alzheimer's disease and in attenuating Abeta and tau pathology in different animal models. In this review, we discuss the role of M1 agonists as a potential disease-modifying therapy for Alzheimer's disease.

Isoform-specific contribution of protein kinase C to prion processing.

The cellular prion protein (PrP(c)) undergoes a physiological cleavage between amino acids 111 and 112, thereby leading to the secretion of an amino-terminal fragment referred to as N1. This proteolytic event is either constitutive or regulated by protein kinase C (PKC) and is operated by the disintegrins ADAM9/ADAM10 or ADAM17 respectively. We recently showed that the stimulation of the M1/M3 muscarinic receptors potentiates this cleavage via the phosphorylation and activation of ADAM17. We have examined the contribution of various PKC isoforms in the regulated processing of PrP(c). First we show that the PDBu- and carbachol-stimulated N1 secretions are blocked by the general PKC inhibitor GF109203X. We establish that HEK293 and human-derived rhabdhomyosarcoma cells over-expressing constitutively active PKCalpha, PKCdelta or PKCepsilon, but not PKCzeta, produce increased amounts of N1 and harbor enhanced ability to hydrolyze the fluorimetric substrate of ADAM17, JMV2770. Conversely, over-expression of the corresponding dominant negative proteins abolishes PDBU-stimulated N1 secretion and restores N1 to levels comparable to constitutive production. Moreover, deletion of PKCalpha lowers N1 recovery in primary cultured fibroblasts. Importantly, mutation of threonine 735 of ADAM17 significantly lowers the PDBu-induced N1 formation while transient over-expression of constitutively active PKCalpha, PKCdelta or PKCepsilon, but not PKCzeta, induced both the phosphorylation of ADAM17 on its threonine residues and N1 secretion. As a corollary, T735A mutation concomitantly reversed PKCalpha-, PKCdelta- and PKCepsilon-induced ADAM17 phosphorylation and N1 recovery. Finally, we established that PKCepsilon-dependent N1 production is fully prevented by ADAM17 deficiency. Altogether, the present results provide strong evidence that the activation of PKCalpha, delta and epsilon, but not zeta, isoforms leads to increased N1 secretion via the phosphorylation and activation of ADAM17, a process that likely accounts for M1/M3 muscarinic receptors-mediated control of N1 production.

Cholinergic treatments with emphasis on m1 muscarinic agonists as potential disease-modifying agents for Alzheimer's disease.

The only prescribed drugs for treatment of Alzheimer's disease (AD) are acetylcholinesterase inhibitors (e.g., donepezil, rivastigmine, galantamine, and tacrine) and memantine, an NMDA antagonist. These drugs ameliorate mainly the symptoms of AD, such as cognitive impairments, rather than halting or preventing the causal neuropathology. There is currently no cure for AD and there is no way to stop its progression, yet there are numerous therapeutic approaches directed against various pathological hallmarks of AD that are extensively being pursued. In this context, the three major hallmark characteristics of AD (i.e., the CNS cholinergic hypofunction, formation of beta-amyloid plaques, and tangles containing hyperphosphorylated tau proteins) are apparently linked. Such linkages may have therapeutic implications, and this review is an attempt to analyze these versus the advantages and drawbacks of some cholinergic compounds, such as acetylcholinesterase inhibitors, M1 muscarinic agonists, M2 antagonists, and nicotinic agonists. Among the reviewed treatments, M1 selective agonists emerge, in particular, as potential disease modifiers.

M1 muscarinic agonists target major hallmarks of Alzheimer's disease--the pivotal role of brain M1 receptors.

The M1 muscarinic receptor (M1 mAChR) is a therapeutic target in Alzheimer's disease (AD) and the M1-selective muscarinic agonists AF102B, AF150(S) and AF267B are cognitive enhancers and potential disease modifiers. Notably, AF267B decreased cerebrospinal fluid beta-amyloid (Abeta(40) and Abeta(42)) in rabbits, decreased brain Abeta levels in hypercholesterolemic rabbits and vascular Abeta(42) deposition from the cortex in cholinotoxin-treated rabbits. In triple transgenic AD mice, AF267B reduced cognitive deficits and decreased Abeta(42) and tau pathologies in the cortex and hippocampus (not amygdala), via M1 mAChR activation of protein kinase C and a disintegrin and metalloproteinase domain 17 (ADAM17 or TACE) and decreased beta-site amyloid precursor protein-cleaving enzyme 1 and glycogen synthase kinase 3beta, respectively. AF267B is the first reported low-molecular-weight therapy that targets the major AD hallmarks.

M1 and M3 muscarinic receptors control physiological processing of cellular prion by modulating ADAM17 phosphorylation and activity.

The cellular prion protein (PrP(c)) undergoes a physiological processing yielding the N-terminal fragment referred to as N1, the production of which can be constitutive or protein kinase C regulated. We show that activation of endogenous muscarinic receptors by carbachol and by the M1-selective agonist AF267B increases N1 recovery in an atropine-sensitive manner, in mouse embryonic primary neurons. To identify the muscarinic receptor subtype involved, we used human embryonic kidney HEK293 (HEK) cells stably overexpressing M1, M2, M3, or M4 receptor subtype. Carbachol and the selective M1 agonist AF267B dose dependently increased N1 release by HEK-M3 and HEK-M1 cells, respectively, whereas carbachol did not modify N1 production by HEK-M2 or HEK-M4 cells. We demonstrate that the increase of N1 was not attributable to modified trafficking to the membrane of either PrP(c) or the disintegrin metalloproteases ADAM10 or ADAM17. Furthermore, we establish that carbachol affects the overall phosphorylation of ADAM17 on its threonine and tyrosine but not serine residues, whereas levels of phosphorylated ADAM9 were not affected. Interestingly, carbachol also increases the hydrolysis of the fluorimetric substrate JMV2770, which mimicked the sequence encompassing the N1 site cleavage and was shown previously to behave as an ADAM protease substrate. Mutations of threonine 735 but not of tyrosine 702 of the ADAM17 cytoplasmic tail abolishes the carbachol-induced increase of N1, ADAM17 phosphorylation, and JMV2770-hydrolyzing activity in M1- and M3-expressing HEK293 cells. Thus, our data provide strong evidence that muscarinic receptor activation increases the physiological processing of PrP(c) by upregulating the phosphorylation state and activity of ADAM17 protease.

M1 muscarinic agonists target major hallmarks of Alzheimer's disease--an update.

UNLABELLED: The M1 muscarinic receptor (M1 mAChR), preserved in Alzheimer's disease (AD), is a pivotal target that links major hallmarks of AD, e.g. cholinergic deficiency, cognitive dysfunctions, beta-amyloid (Abeta) and tau pathologies. Some muscarinic agonists, while effective in AD, had limited clinical value due to adverse effects and lack of M1 selectivity. The M1 selective muscarinic agonists AF102B [Cevimeline], AF150(S) and AF267B - i) elevated alphaAPPs, decreased Abeta levels and tau hyperphosphorylation, and blocked Abeta-induced neurotoxicity, in vitro, via M1 mAChR-modulation of kinases (e.g. PKC, MAPK and GSK3beta); ii) restored cognitive deficits, cholinergic markers, and decreased tau hyperphosphorylation in relevant models with a wide safety margin. AF267B decreased brain Abeta levels in hypercholesterolemic rabbits and decreased CSF Abeta42 in rabbits and removed vascular Abeta42 deposition from cortex in cholinotoxin-treated rabbits. In 3x transgenic-AD mice that recapitulate the major pathologies and cognitive deficits of AD, chronic AF267B treatment rescued cognitive deficits and decreased Abeta42 and tau pathologies in the cortex and hippocampus (not amygdala), via M1 mAChR-activation of ADAM17/TACE and decreased BACE1 steady state levels and inhibition of GSK3beta, extending findings from above. CONCLUSIONS: A comprehensive therapy should target all AD hallmarks, regardless of the culprit(s) responsible for the disease. In this context, AF267B is the 1(st) reported low MW CNS-penetrable mono-therapy that meets this challenge. Clinical trials will determine if AF267B may become an important therapy in AD.

M1 receptors play a central role in modulating AD-like pathology in transgenic mice.

We investigated the therapeutic efficacy of the selective M1 muscarinic agonist AF267B in the 3xTg-AD model of Alzheimer disease. AF267B administration rescued the cognitive deficits in a spatial task but not contextual fear conditioning. The effect of AF267B on cognition predicted the neuropathological outcome, as both the Abeta and tau pathologies were reduced in the hippocampus and cortex, but not in the amygdala. The mechanism underlying the effect on the Abeta pathology was caused by the selective activation of ADAM17, thereby shifting APP processing toward the nonamyloidogenic pathway, whereas the reduction in tau pathology is mediated by decreased GSK3beta activity. We further demonstrate that administration of dicyclomine, an M1 antagonist, exacerbates the Abeta and tau pathologies. In conclusion, AF267B represents a peripherally administered low molecular weight compound to attenuate the major hallmarks of AD and to reverse deficits in cognition. Therefore, selective M1 agonists may be efficacious for the treatment of AD.

M1 muscarinic receptor activation protects neurons from beta-amyloid toxicity. A role for Wnt signaling pathway.

Amyloid-beta-peptide (Abeta) deposits are one of the hallmark features of Alzheimer's disease. Signal transduction alterations are implicate in the neuronal responses to Abeta, which include neurotransmitter systems and pathways involved in the maintenance of the nervous system. In this context, we have recently found that Abeta-neurotoxicity triggers a loss of Wnt signaling. We report here that M1-acetylcholine-muscarinic-receptor (mAChR) activation protects neurons from Abeta-toxicity. Concomitant with this effect, a modulation of the Wnt signaling was observed. M1 mAChR activation inhibits glycogen-synthase-kinase-3beta (GSK-3beta) activity, stabilizes cytoplasmic and nuclear beta-catenin, and induces the expression of the Wnt target genes engrailed and cyclin-D1, reverting the switch off of the Wnt pathway caused by Abeta-toxicity. Neurons from mice that overexpress GSK-3beta allow us to establish that M1 mAChR stimulation leads to GSK-3beta inactivation. We conclude that the cross-talk between the muscarinic signaling and Wnt components underlie the neuroprotective effect of the M1 mAChR activation.

Reduction in distractibility with AF102B and THA in the macaque.

Distractibility in primates may be influenced by central cholinergic systems. Two cholinomimetics, the m-1 muscarinic agonist (+/-)-cis-2-methyl-spiro(1,3-oxathiolane-5,3')quinuclidine (AF102B, civemeline) and the cholinesterase inhibitor tetrahydroaminoacridine (THA, tacrine), were compared to vehicle controls for effects on distractibility in an automated visuospatial attention task. The task required visual pursuit of a moving target amongst distractor stimuli that acted to impair performance and was executed by seven healthy adult bonnet macaque monkeys. Task accuracy and reaction time were measured 1.5 h after systemic administration of each substance. For the seven-subject group at individually titrated best doses, accuracy increased significantly relative to vehicle for both drugs. Reaction time at best dose decreased for both drugs, but not significantly. Muscarinic agonists and cholinesterase inhibitors may reduce distractibility in primates.