Donepezil modulates nicotinic receptors of substantia nigra dopaminergic neurones

Donepezil-induced bradycardia in a schizophrenic patient with comorbid neurocognitive disorder: a case report and review of the literature

BACKGROUND

Trials of cholinergic and glutamatergic agents have improved cognition and memory for the geriatric schizophrenic population. Donepezil is an acetylcholinesterase inhibitor that improves cognition by preventing postsynaptic degradation of hippocampal acetylcholine in patients with mild-to-moderate dementia. Donepezil has been attributed to some adverse effects, especially gastrointestinal symptoms. However, cardiovascular adverse effects are not common as there remains a dearth of literature regarding donepezil-induced bradycardia.

CASE REPORT

Hence, we present the case of a 70-year-old Hispanic female with past psychiatry history of schizophrenia who developed bradycardia and syncope following the commencement of low-dose donepezil in the inpatient unit and subsequent resolution with cessation. She had no prior cardiovascular symptoms or diagnosis.

DISCUSSION

Considering there is no baseline cardiac monitoring requirement guideline for patients on Donepezil treatment, pre-assessment electrocardiogram is advised before the commencement of acetylcholinesterase inhibitors. Finally, routine monitoring of vital signs for at least the first 72 hours following the start of donepezil might be good proactive practice for all psychiatrists. Extending this practice to inpatient and outpatient service settings will be worthwhile.

Is Target-Based Drug Discovery Efficient? Discovery and "Off-Target" Mechanisms of All Drugs

Target-based drug discovery is the dominant paradigm of drug discovery; however, a comprehensive evaluation of its real-world efficiency is lacking. Here, a manual systematic review of about 32000 articles and patents dating back to 150 years ago demonstrates its apparent inefficiency. Analyzing the origins of all approved drugs reveals that, despite several decades of dominance, only 9.4% of small-molecule drugs have been discovered through "target-based" assays. Moreover, the therapeutic effects of even this minimal share cannot be solely attributed and reduced to their purported targets, as they depend on numerous off-target mechanisms unconsciously incorporated by phenotypic observations. The data suggest that reductionist target-based drug discovery may be a cause of the productivity crisis in drug discovery. An evidence-based approach to enhance efficiency seems to be prioritizing, in selecting and optimizing molecules, higher-level phenotypic observations that are closer to the sought-after therapeutic effects using tools like artificial intelligence and machine learning.

Functional connectivity and cognitive changes after donepezil treatment in healthy participants

RATIONALE

Donepezil is a potent, noncompetitive, reversible, clinically effective acetylcholinesterase inhibitor. The effects of this drug on healthy brains have seldom been investigated.

OBJECTIVES

The primary objective of the present study was to identify possible functional connectivity markers of the effect of donepezil in healthy young adult volunteers.

METHODS

The study had a double-blind, randomized, crossover design. 30 healthy adult volunteers underwent resting-state MRI scans during 15 days of donepezil or placebo treatment, in accordance with the design.

RESULTS

Results showed significant differences in intrinsic functional connectivity between donepezil and placebo, mainly in the right executive control network (RECN). More specifically, we found a decrease in the connectivity of the right inferior parietal node with other RECN nodes. Analysis using the cingulate cortex and parahippocampal regions as seeds also revealed complex modulation of functional connectivity in the donepezil condition.

CONCLUSIONS

In conclusion, donepezil treatment for 15 days may result in reorganization of resting-state networks, compared with placebo.

Curcumin Potentiates α7 Nicotinic Acetylcholine Receptors and Alleviates Autistic-Like Social Deficits and Brain Oxidative Stress Status in Mice

Autistic spectrum disorder (ASD) refers to a group of neurodevelopmental disorders characterized by impaired social interaction and cognitive deficit, restricted repetitive behaviors, altered immune responses, and imbalanced oxidative stress status. In recent years, there has been a growing interest in studying the role of nicotinic acetylcholine receptors (nAChRs), specifically α7-nAChRs, in the CNS. Influence of agonists for α7-nAChRs on the cognitive behavior, learning, and memory formation has been demonstrated in neuro-pathological condition such as ASD and attention-deficit hyperactivity disorder (ADHD). Curcumin (CUR), the active compound of the spice turmeric, has been shown to act as a positive allosteric modulator of α7-nAChRs. Here we hypothesize that CUR, acting through α7-nAChRs, influences the neuropathology of ASD. In patch clamp studies, fast inward currents activated by choline, a selective agonist of α7-nAChRs, were significantly potentiated by CUR. Moreover, choline induced enhancement of spontaneous inhibitory postsynaptic currents was markedly increased in the presence of CUR. Furthermore, CUR (25, 50, and 100 mg/kg, i.p.) ameliorated dose-dependent social deficits without affecting locomotor activity or anxiety-like behaviors of tested male Black and Tan BRachyury (BTBR) mice. In addition, CUR (50 and 100 mg/kg, i.p.) mitigated oxidative stress status by restoring the decreased levels of superoxide dismutase (SOD) and catalase (CAT) in the hippocampus and the cerebellum of treated mice. Collectively, the observed results indicate that CUR potentiates α7-nAChRs in native central nervous system neurons, mitigates disturbed oxidative stress, and alleviates ASD-like features in BTBR mice used as an idiopathic rodent model of ASD, and may represent a promising novel pharmacological strategy for ASD treatment.

Comparative risk of cardiac arrhythmias associated with acetylcholinesterase inhibitors used in treatment of dementias - A narrative review

Donepezil, galantamine, and rivastigmine are the three acetylcholinesterase inhibitors (AChEIs), out of a total of only four medications prescribed in the treatment of Alzheimer's Disease (AD) and related dementias. These medications are known to be associated with bradycardia given their mechanism of action of increasing acetylcholine (ACh). However, in March 2015, donepezil was added to the CredibleMeds "known-risk" category, a list where medications have a documented risk for acquired long-QT syndrome (ALQTS) and torsades de pointes (TdP) - a malignant ventricular arrhythmia that is a different adverse event than bradycardia (and is not necessarily associated with ACh action). The purpose of this article is to review the three AChEIs, especially with regards to mechanistic differences that may explain why only donepezil poses this risk; several pharmacological mechanisms may explain why. However, from an empirical point-of-view, aside from some case-reports, only a limited number of studies have generated relevant information regarding AChEIs' and electrocardiogram findings; none have specifically compared donepezil against galantamine or rivastigmine for malignant arrhythmias such as TdP. Currently, the choice of one of the three AChEIs for treatment of AD symptoms is primarily dependent upon clinician and patient preference. However, clinicians should be aware of the potential increased risk associated with donepezil. There is a need to examine the comparative risk of malignant arrhythmias among AChEIs users in real-world practice; this may have important implications with regards to changes in AChEI prescribing patterns.

The acetylcholinesterase (AChE) inhibitor and anti-Alzheimer drug donepezil interacts with human erythrocytes

Donepezil is used to treat symptomatically the Alzheimer's disease (AD). This drug is a specific inhibitor of the enzyme acetylcholinesterase (AChE), whose main physiological function is to hydrolyze the neurotransmitter acetylcholine. The main objective of this work was to study the effect of donepezil on human erythrocytes as AChE is present in its membrane. For this purpose, human erythrocytes and molecular model of its membrane built-up of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) were used. The latter correspond to classes of phospholipids present in the outer and inner monolayers of the human erythrocyte membrane, respectively. Our experimental evidences obtained from X-ray diffraction and differential scanning calorimetry (DSC) analysis indicated that donepezil was capable of interacting with both phospholipids. Fluorescence spectroscopy results showed a moderate increase in the fluidity of the hydrophobic tails of DMPC and isolated unsealed human erythrocyte membranes (IUM). On the other hand, results by scanning electron microscopy (SEM) and optical defocusing microscopy (DM) showed that the drug changed the normal biconcave shape of the erythrocytes inducing the formation of stomatocytes (cup-shaped cells). This effect was explained by the incorporation of donepezil molecules into the erythrocyte membrane and interactions with AChE.

Acetylcholinesterase Inhibitors and Drugs Acting on Muscarinic Receptors- Potential Crosstalk of Cholinergic Mechanisms During Pharmacological Treatment

BACKGROUND

Pharmaceuticals with targets in the cholinergic transmission have been used for decades and are still fundamental treatments in many diseases and conditions today. Both the transmission and the effects of the somatomotoric and the parasympathetic nervous systems may be targeted by such treatments. Irrespective of the knowledge that the effects of neuronal signalling in the nervous systems may include a number of different receptor subtypes of both the nicotinic and the muscarinic receptors, this complexity is generally overlooked when assessing the mechanisms of action of pharmaceuticals.

METHODS

We have search of bibliographic databases for peer-reviewed research literature focused on the cholinergic system. Also, we have taken advantage of our expertise in this field to deduce the conclusions of this study.

RESULTS

Presently, the life cycle of acetylcholine, muscarinic receptors and their effects are reviewed in the major organ systems of the body. Neuronal and non-neuronal sources of acetylcholine are elucidated. Examples of pharmaceuticals, in particular cholinesterase inhibitors, affecting these systems are discussed. The review focuses on salivary glands, the respiratory tract and the lower urinary tract, since the complexity of the interplay of different muscarinic receptor subtypes is of significance for physiological, pharmacological and toxicological effects in these organs.

CONCLUSION

Most pharmaceuticals targeting muscarinic receptors are employed at such large doses that no selectivity can be expected. However, some differences in the adverse effect profile of muscarinic antagonists may still be explained by the variation of expression of muscarinic receptor subtypes in different organs. However, a complex pattern of interactions between muscarinic receptor subtypes occurs and needs to be considered when searching for selective pharmaceuticals. In the development of new entities for the treatment of for instance pesticide intoxication, the muscarinic receptor selectivity needs to be considered. Reactivators generally have a muscarinic M2 receptor acting profile. Such a blockade may engrave the situation since it may enlarge the effect of the muscarinic M3 receptor effect. This may explain why respiratory arrest is the major cause for deaths by esterase blocking.

[Pharmacotherapy and dementia]

The review considers treatment aspects of dementia in Alzheimer's disease (DAD), vascular dementia (VD) and some other forms of dementia. Pathogenetic pharmacotherapy of DAD aims to maintain the activity of cholinergic neurons with anticholinesterase drugs including donepezil, which is used most often. The main objective of VD treatment is prevention of new cerebral infarctions using cardiotropic medications, antiaggregants, anticoagulants, hypotensive drugs and statins. In Russia, patients with DAD and VD receive polymodal drugs as well. These drugs have been included in the list of antidementia medications due to their wide use in treatment of discirculatory encephalopathy and chronic brain ischemia.

Memantine and cholinesterase inhibitors: complementary mechanisms in the treatment of Alzheimer's disease

This review describes the preclinical mechanisms that may underlie the increased therapeutic benefit of combination therapy-with the N-methyl-D-aspartate receptor antagonist, memantine, and an acetylcholinesterase inhibitor (AChEI)-for the treatment of Alzheimer's disease (AD). Memantine, and the AChEIs target two different aspects of AD pathology. Both drug types have shown significant efficacy as monotherapies for the treatment of AD. Furthermore, clinical observations indicate that their complementary mechanisms offer superior benefit as combination therapy. Based on the available literature, the authors have considered the preclinical mechanisms that could underlie such a combined approach. Memantine addresses dysfunction in glutamatergic transmission, while the AChEIs serve to increase pathologically lowered levels of the neurotransmitter acetylcholine. In addition, preclinical studies have shown that memantine has neuroprotective effects, acting to prevent glutamatergic over-stimulation and the resulting neurotoxicity. Interrelations between the glutamatergic and cholinergic pathways in regions of the brain that control learning and memory mean that combination treatment has the potential for a complex influence on disease pathology. Moreover, studies in animal models have shown that the combined use of memantine and the AChEIs can produce greater improvements in measures of memory than either treatment alone. As an effective approach in the clinical setting, combination therapy with memantine and an AChEI has been a welcome advance for the treatment of patients with AD. Preclinical data have shown how these drugs act via two different, but interconnected, pathological pathways, and that their complementary activity may produce greater effects than either drug individually.

Galantamine attenuates some of the subjective effects of intravenous nicotine and improves performance on a Go No-Go task in abstinent cigarette smokers: a preliminary report

RATIONALE

Galantamine (GAL), a reversible and competitive inhibitor of acetylcholinesterase, is used clinically in the treatment of Alzheimer's dementia. Some preclinical and clinical studies support the potential efficacy of cholinesterase inhibitors for smoking cessation, although their effects on the behavioral and physiological responses to nicotine have not been examined. The goal of this study was to characterize GAL's actions on multiple outcomes, including withdrawal severity and cognitive performance, as well as subjective and physiological responses to nicotine administered intravenously.

METHODS

A total of 12 smokers participated in a double-blind, placebo-controlled, crossover study. Smokers had two 4-day treatment periods, assigned in random sequence, to GAL (8 mg/day) or placebo treatment. On day 4 of each treatment phase, smokers had an experimental session in which they received an intravenous (IV) dose of saline or 1 mg/70 kg nicotine, 1 h apart, in a random order.

RESULTS

GAL attenuated the self-reported rating of "craving for cigarettes" and prevented decrements in performance in a Go/No-Go task. In response to IV nicotine, GAL treatment attenuated the self-report ratings of "like the drug effects," "good drug effects," "bad drug effects," and "stimulated."

CONCLUSIONS

These findings support the potential utility of GAL as a treatment for smoking cessation.

Additive protective effects of donepezil and nicotine against salsolinol-induced cytotoxicity in SH-SY5Y cells

Although the etiology of Parkinson's disease (PD) remains elusive, a number of toxins including elevated salsolinol, an endogenous metabolite of dopamine may contribute to its pathology. It was reported recently that nicotine may have protective effects against salsolinol-induced toxicity in human neuroblastoma derived SH-SY5Y cells and that these effects of nicotine are mediated by nicotinic receptors. Donepezil (Aricept) is a reversible non-competitive acetylcholinesterase inhibitor that is approved for use in mild to moderate Alzheimer's disease. The increase in acetylcholine concentrations is believed to be the major contributory factor in donepezil's therapeutic efficacy. However, cholinesterase inhibitors may also directly interact with nicotinic receptors and possess neuroprotective properties. In this study, we sought to determine whether donepezil may have protective effects against salsolinol-induced toxicity in SH-SY5Y cells and whether the combination of donepezil and nicotine may result in additive protection. Moreover, it was of interest to elucidate the role of nicotinic receptors as well as cell cycle and apoptosis in mechanism of action of these compounds. SH-SY5Y cells were exposed to 0.6 mM salsolinol with and without various drug pretreatments for 48 h. Nicotine (50 muM) resulted in approximately 54% protection and donepezil (5 muM) resulted in approximately 40% protection, and the combination of the two resulted in an additive (approximately 93%) protection against salsolinol-induced toxicity. Salsolinol caused an arrest of the cells in G(1)-phase of cell cycle and an increase in apoptotic indices that were blocked by the combination of donepezil and nicotine. Mecamylamine, a non-selective nicotinic receptor antagonist completely blocked the effects of nicotine and partially attenuated the effects of donepezil. A combination of atropine, a muscarinic receptor antagonist and mecamylamine completely blocked the effects of donepezil, indicating involvement of both nicotinic and muscarinic receptors in donepezil's actions. The findings suggest a therapeutic potential for the combination of donepezil and nicotine in PD.

Donepezil in Alzheimer's disease: From conventional trials to pharmacogenetics

Donepezil is the leading compound for the treatment of Alzheimer's disease (AD) in more than 50 countries. As compared with other conventional acetylcholinesterase inhibitors (AChEIs), donepezil is a highly selective and reversible piperidine derivative with AChEI activity that exhibits the best pharmacological profile in terms of cognitive improvement, responders rate (40%-58%), dropout cases (5%-13%), and side-effects (6%-13%) in AD. Although donepezil represents a non cost-effective treatment, most studies convey that this drug can provide a modest benefit on cognition, behavior, and activities of the daily living in both moderate and severe AD, contributing to slow down disease progression and, to a lesser exetnt, to delay institutionalization. Patients with vascular dementia might also benefit from donepezil in a similar fashion to AD patients. Some potential effects of donepezil on the AD brain, leading to reduced cortico-hippocampal atrophy, include the following: AChE inhibition, enhancement of cholinergic neurotransmission and putative modulation of other neurotransmitter systems, protection against glutamate-induced excitotoxicity, activation of neurotrophic mechanisms, promotion of non-amyloidodgenic pathways for APP processing, and indirect effects on cerebrovascular function improving brain perfusion. Recent studies demonstrate that the therapeutic response in AD is genotype-specific. Donepezil is metabolized via CYP-related enzymes, especially CYP2D6, CYP3A4, and CYP1A2. Approximately, 15%-20% of the AD population may exhibit an abnormal metabolism of AChEIs; about 50% of this population cluster would show an ultrarapid metabolism, requiring higher doses of AChEIs to reach a therapeutic threshold, whereas the other 50% of the cluster would exhibit a poor metabolism, displaying potential adverse events at low doses. In AD patients treated with a multifactorial therapy, including donepezil, the best responders are the CYP2D6-related extensive (EM)(*1/*1, *1/*10) (57.47%) and intermediate metabolizers (IM)(*1/*3, *1/*5, *1/*6, *7/*10) (25.29%), and the worst responders are the poor (PM) (*4/*4)(9.20%) and ultra-rapid metabolizers (UM) (*1xN/*1) (8.04%). Pharmacogenetic and pharmacogenomic factors may account for 75%-85% of the therapeutic response in AD patients treated with donepezil and other AChEIs metabolized via enzymes of the CYP family. The implementation of pharmacogenetic protocols can optimize AD therapeutics.

Galantamine enhances dopaminergic neurotransmission in vivo via allosteric potentiation of nicotinic acetylcholine receptors

Clinical studies suggest that adjunct galantamine may improve negative and cognitive symptoms in schizophrenia. These symptoms may be related to impaired dopaminergic function in the prefrontal cortex. Indeed, galantamine has been shown to increase dopamine release in vitro. Galantamine is an allosteric modulator of nicotinic acetylcholine receptors (nAChRs) and, at higher doses, an acetylcholine esterase (AChE) inhibitor. We have previously shown that nicotine, through stimulation of nAChRs in the ventral tegmental area (VTA), activates midbrain dopamine neurons and, hence, potentiation of these receptors could be an additional mechanism by which galantamine can activate dopaminergic pathways. Therefore, the effects of galantamine (0.01-1.0 mg/kg s.c.) on dopamine cell firing were tested in anaesthetized rats. Already at a low dose, unlikely to result in significant AchE inhibition, galantamine increased firing activity of dopaminergic cells in the VTA. The effect of galantamine was prevented by the nAChR antagonist mecamylamine (1.0 mg/kg s.c.), but not the muscarinic receptor antagonist scopolamine (0.1 mg/kg s.c.), and it was not mimicked by the selective AChE inhibitor donepezil (1.0 mg/kg s.c.). Our data thus indicate that galantamine increases dopaminergic activity through allosteric potentiation of nAChRs. Galantamine's effect was also prevented by the alpha7 nAChR antagonist methyllycaconitine (6.0 mg/kg i.p.) as well as the N-methyl-D-aspartate antagonist CGP39551 (2.5 mg/kg s.c.), indicating a mechanism involving presynaptic facilitation of glutamate release. In parallel microdialysis experiments, galantamine was found to increase extracellular levels of dopamine in the medial prefrontal cortex. These results may have bearing on the enhancement of negative and cognitive symptoms in schizophrenia.

Acetylcholinesterase inhibitors used in treatment of Alzheimer's disease prevent glutamate neurotoxicity via nicotinic acetylcholine receptors and phosphatidylinositol 3-kinase cascade

We show here that donepezil, galanathamine and tacrine, therapeutic acetylcholinesterase inhibitors currently being used for treatment of Alzheimer's disease, protect neuronal cells in a time- and concentration-dependent manner from glutamate neurotoxicity that involves apoptosis. The neuroprotective effects were antagonized by mecamylamine, an inhibitor of nicotinic acetylcholine receptors (nAChRs). Dihydro-beta-erythroidine and methyllycaconitine, antagonists for alpha4-nAChR and alpha7-nAChR, respectively, antagonized the protective effect of donepezil and galanthamine, but not that of tacrine. Previous reports suggest the involvement of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway in the nicotine-induced neuroprotection. Inhibitors for a non-receptor type tyrosine kinase, Fyn, and janus-activated kinase 2, suppressed the neuroprotective effect of donepezil and galanthamine, but not that of tacrine. Furthermore, LY294002, a PI3K inhibitor, also suppressed the neuroprotective effect of donepezil and galanthamine, but not that of tacrine. The phosphorylation of Akt, an effector of PI3K, and the expression level of Bcl-2, an anti-apoptotic protein, increased with donepezil and galanthamine treatment, but not with tacrine treatment. These results suggest that donepezil and galanthamine prevent glutamate neurotoxicity through alpha4- and alpha7-nAChRs, followed by the PI3K-Akt pathway, and that tacrine protects neuronal cells through a different pathway.

Protective effect of donepezil on retinal ganglion cells in vitro and in vivo

The neuroprotective effect of donepezil, an acetylcholinesterase inhibitor on retinal ganglion cells (RGCs), was examined. The survival of purified RGCs after exposure to glutamate with or without donepezil (10(- 7) M to 10(- 5) M) was measured after 3 days in culture. In vivo, the neuroprotective effect of donepezil was examined by quantifying the number of viable RGCs 7 days after axotomy in adult rats. In vitro, donepezil dose-dependently reduced RGC death caused by glutamate toxicity. Oral administration of donepezil (10 mg kg(-1) day(-1)) significantly reduced RGC death after axotomy. In conclusion, donepezil exerts a protective effect on RGCs both in vitro and in vivo.

Changes in cerebral neurotransmitters and metabolites induced by acute donepezil and memantine administrations: A microdialysis study

Cholinesterase inhibitors including donepezil, rivastigmine, and galantamine and the N-methyl-D-aspartate (NMDA) antagonist, memantine are the medications currently approved for the treatment of Alzheimer's disease (AD). In addition to their beneficial effects on cognitive and functional domains typically disrupted in AD, these agents have also been shown to slow down the emergence of behavioral and psychotic symptoms associated with this disease. However, the underlying mechanisms for these therapeutic effects remain poorly understood and could involve effects of these medications on non-cholinergic or non-glutamatergic neurotransmitter systems respectively. These considerations prompted us to initiate a series of investigations to examine the acute and chronic effects of donepezil (Aricept (+/-)-2,3-dihydro-5,6-dimethoxy-2-[[1-(phenylmethyl)-4-piperidinyl]methyl]-1H-inden-1-1 hydrochloride and memantine (1-amino-3,5-dimethyladamantane hydrochloride C12H21N.HCl)). The present study focuses on the acute effects of donepezil and memantine on brain extracellular levels of acetylcholine, dopamine, serotonin, norepinephrine and their metabolites. We assayed changes in the ventral and dorsal hippocampus and the prefrontal and medial temporal cortex by microdialysis. Memantine resulted in significant increases in extracellular dopamine (DA), norepinephrine (NE), and their metabolites, in the cortical regions, and in a reduction of DA in the hippocampus. Donepezil produced an increase in extracellular DA in the cortex and in the dorsal hippocampus. Norepinephrine increased in the cortex; with donepezil it increased in the dorsal hippocampus and the medial temporal cortex, and decreased in the ventral hippocampus. Interestingly both compounds decreased extracellular serotonin (5HT) levels. The metabolites of the neurotransmitters were increased in most areas. We also found an increase in extracellular acetylcholine (ACh) by memantine in the nucleus accumbens and the ventral tegmental area. Our results suggest both region and drug specific neurotransmitter effects of these agents as well as some similarities. We conclude that drugs influencing cognitive mechanisms induce changes in a number of neurotransmitters with the changes being both region and drug specific. Release and metabolism are altered and extracellular neurotransmitter levels can be increased or decreased by the drugs. Other studies are in progress to determine the pharmacological effects associated with chronic treatment with these compounds, which may be more pertinent to the clinical situation in which patients take these medications for months or years.

Unequal Neuroprotection Afforded by the Acetylcholinesterase Inhibitors Galantamine, Donepezil, and Rivastigmine in SH-SY5Y Neuroblastoma Cells: Role of Nicotinic Receptors

Donepezil, rivastigmine, and galantamine are three drugs with acetylcholinesterase (AChE)-inhibiting activity that are currently being used to treat patients suffering from Alzheimer's disease. We have studied the neuroprotective effects of these drugs, in comparison with nicotine, on cell death caused by beta-amyloid (Abeta) and okadaic acid, two models that are relevant to Alzheimer's pathology, in the human neuroblastoma cell line SH-SY5Y. Galantamine and donepezil showed a U-shaped neuroprotective curve against okadaic acid toxicity; maximum protection was achieved at 0.3 microM galantamine and at 1 microM donepezil; at higher concentrations, protection was diminished. Rivastigmine showed a concentration-dependent effect; maximum protection was achieved at 3 microM. When apoptosis was induced by Abeta25-35, galantamine, donepezil, and rivastigmine showed maximum protection at the same concentrations: 0.3, 1, and 3 microM, respectively. Nicotine also afforded protection against Abeta- and okadaic acid-induced toxicity. The neuroprotective effects of galantamine, donepezil, and nicotine were reversed by the alpha7 nicotinic antagonist methyllycaconitine but not by the alpha4beta2 nicotinic antagonist dihydro-beta-erythroidine. The phosphoinositide 3-kinase (PI3K)-Akt blocker 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002) reversed the protective effects of galantamine, donepezil, and nicotine but not that of rivastigmine. In contrast, the bcl-2 antagonist ethyl[2-amino-6-bromo-4-(1-cyano-2-ethoxy-2-oxoethyl)]-4H-chromene-3-carboxylate (HA 14-1) reversed the protective effects of the three AChE inhibitors and that of nicotine. Our results show that galantamine, donepezil, and rivastigmine afford neuroprotection through a mechanism that is likely unrelated to AChE inhibition. Such neuroprotection seemed to be linked to alpha7 nicotinic receptors and the PI3K-Akt pathway in the case of galantamine and donepezil but not for rivastigmine.

A preclinical view of cholinesterase inhibitors in neuroprotection: do they provide more than symptomatic benefits in Alzheimer's disease?

The prevalence of Alzheimer's disease (AD), a neurodegenerative condition whose greatest risk factor is old age, is expected to rise dramatically during the next five decades, along with the trend for increased longevity. Early diagnosis and intervention with therapies that halt or slow disease progress are likely to represent an important component of effective treatment. Although much progress has been made in this area, there are currently no clinically approved interventions for AD that are classed as disease modifying or neuroprotective. Cholinesterase inhibitors are a drug class used for the symptomatic treatment of AD. Recent evidence from preclinical studies indicates that these agents can attenuate neuronal damage and death from cytotoxic insults, and therefore might affect AD pathogenesis. The mechanisms by which these actions are mediated might or might not be directly related to their primary mode of action.

Methamidophos transiently inhibits neuronal nicotinic receptors of rat substantia nigra dopaminergic neurons via open channel block

The use of acetylcholinesterase (AChE) inhibitors is the primary therapeutic strategy in the treatment of Alzheimer's disease. However, these drugs have been reported to have effects beyond the simple stimulation of neuronal acetylcholine receptors (AChRs) by elevated acetylcholine (ACh), interfering directly with the nAChR. Therefore, a pure pharmacological blockade of AChE is not usually obtained. In this study, the patch-clamp technique was utilized to determine the effects of methamidophos, a pesticide that is considered a selective AChE inhibitor, on nAChRs of substantia nigra dopaminergic neurons. In spite of the fact that methamidophos has been reported to be devoid of direct nicotinic actions, our main observation was that it selectively and reversibly blocked nAChR responses, without directly affecting the holding current. Methamidophos produced a downward shift in the dose response curve for nicotine; the mechanism accounting for this non-competitive antagonism was open channel block, in view of its voltage dependence. Pre-treatment with vesamicol did not prevent the reduction of nicotine-induced currents, indicating that the effect on nAChRs was independent from the activity of methamidophos as a cholinesterase inhibitor. Our results conclude that methamidophos has a complex blocking action on neuronal nAChRs that is unlinked to the inhibition of AChE. Therefore, it should not be considered a selective AChE inhibitor and part of its toxic effects could reside in an interference with the nicotinic neurotransmission.