Correlation of cholinergic drug induced quenching of acetylcholinesterase bound thioflavin-T fluorescence with their inhibition activity

The development of new acetylcholinesterase inhibitors (AChEIs) and subsequent assay of their inhibition efficiency is considered to be a key step for AD treatment. The fluorescence intensity of thioflavin-T (ThT) bound in the active site of acetylcholinesterase (AChE) quenches substantially in presence of standard AChEI drugs due to the dynamic replacement of the fluorophore from the AChE active site as confirmed from steady state emission as well as time-resolved fluorescence anisotropy measurement and molecular dynamics simulation in conjunction with docking calculation. The parametrized % quenching data for individual system shows excellent correlation with enzyme inhibition activity measured independently by standard Ellman AChE assay method in a high throughput plate reader system. The results are encouraging towards design of a fluorescence intensity based AChE inhibition assay method and may provide a better toolset to rapidly evaluate as well as develop newer AChE-inhibitors for AD treatment.

Protection by a transdermal patch containing eserine and pralidoxime chloride for prophylaxis against (±)-Anatoxin A poisoning in rats

The prophylactic and neuroprotective impact of a transdermal patch containing eserine and pralidoxime chloride (2-PAM) against (±)-Anatoxin A poisoning was investigated using Wistar strain albino rats. Rats were smooth-shaved on the dorsal side, attached with a drug-in-adhesive matrix type prophylactic transdermal patch for 72 h and challenged with subcutaneous injection of three doses (1.0, 1.5 and 2.0×LD50) of (±)-Anatoxin A. The LD50 value of (±)-Anatoxin A was determined to be 1.25mg/kg, and at this particular dose (1.0×LD50) of toxin induced severe clinical symptom including extreme seizures in rats, resulting acute brain injuries in discrete brain regions, leading to 100% mortality within 5 min. The anticonvulsant effect, antiarrythmic effect, nerve conduction study, clinical observations and mortality, neuroprotective effect as well as skin histopathology of the prophylactic transdermal patch against (±)-Anatoxin A poisoning were investigated systematically. It was found that seizures, tachycardia, nerve damage, clinical symptoms, brain injuries and mortality induced by such lethal toxin were effectively prevented by the prophylactic patch treatment up to certain LD50 level. Hence, it could be a choice of potential therapeutic regimen against such lethal poisoning.

Potentially procholinergic effects of medications commonly used in older adults

BACKGROUND

Older adults are susceptible to a variety of illnesses, many of which can be treated with medications that may need to be used for the long term. Considerable attention has been paid to drugs that, in addition to their intended function, may have an anticholinergic effect that results in undesirable side effects, including impairment in cognition. Cholinesterase inhibitors are used as procholinergic drugs to improve cognitive dysfunction in Alzheimer's disease. We hypothesized that some of the drugs commonly used by older adults might, in addition to their intended function, also have procholinergic effects by virtue of inhibiting cholinesterases.

OBJECTIVE

To determine the potential procholinergic nature of some of the commonly used drugs by examining their cholinesterase inhibiting properties.

METHODS

The Ellman spectrophotometric method was used with human acetylcholinesterase and butyrylcholinesterase, in the absence and presence of increasing concentrations of each test drug. To compare inhibition potencies, from enzyme kinetic data, we determined half maximal inhibitory concentration (IC(50) values) for each cholinesterase by each drug.

RESULTS

Of the 28 drugs examined, over half (17/28) inhibited one or both of the human cholinesterases. The inhibition potencies were often within 1 to 2 orders of magnitude of reversible cholinesterase inhibitors currently used to treat Alzheimer's disease. These included trazodone, quetiapine, risperidone, indapamide, and perindopril.

CONCLUSIONS

Many drugs used by older adults for other reasons have potentially clinically relevant procholinergic effects. The effect of cumulative cholinesterase inhibition merits clinical evaluation.

Alzheimer's disease drugs: an application of the hormetic dose-response model

This article provides an evaluation of the dose-response features of drugs that are intended to improve memory, some of which have been used in the treatment of Alzheimer's disease (AD). A common feature of these drugs is that they act via an inverted U-shaped dose response, consistent with the hormetic dose response model. This article assesses historical foundations that lead to the development of AD drugs, their dose-response features and how the quantitative features of such dose responses affected drug discovery and development, and the successes and possible failures of such agents in preclinical and clinical settings. This story begins about 150 years ago with the discovery of an active agent in the Calabar bean plant called physostigmine, its unfolding medical applications, and its implications for dose-response relationships, memory enhancement, and improved drug discovery activities. The article also demonstrates the occurrence of U-shaped dose responses for memory with numerous endogenous agonists including neurosteroids, various peptides (e.g., vasopressin, CCK-8, neuropeptide Y), and other agents (e.g., epinephrine, antagonists for platelet activity factor and nicotinic receptors), supporting the generalizability of the hormetic biphasic dose response. Finally, the significance of the U-shaped dose response is critical for successful clinical application, since it defines the therapeutic window.

Preparation andin vivo evaluation of huperzine A-loaded PLGA microspheres

Huperzine A-loaded microspheres composed of poly(D,L-lactide-co-glycolide) were prepared by an ONV emulsion solvent evaporation method. The characterization of the microspheres such as drug loading, size, shape and release profile was described. The in vitro release in the initial 7 days was nearly linear with 10% released per day. Thereafter drug release rate became slow gradually and about 90% drug released at day 21. The in vitro release rate determined by dialysis bag method had a good correlation with the in vivo release rate. Huperzine A aqueous solution was intramuscularly injected (i.m.) at 0.4 mg/kg and microspheres were intramuscularly injected at 8.4 mg eq huperzine A/kg in rats. The maxium plasma concentration (Cmax ) after i.m. microspheres was only 32% of that after i.m. solution. Drug in plasma could be detected until day 14 and about 5% of administered dose was residued at the injection site at day 14. The relative bioavailability of huperzine A microspheres over a period of 14 days was 94.7%. Inhibition of acyecholinesterase activity (AchE) in rat's cortex, hippocampus and striatum could sustain for about 14 days. In conclusion, huperzine A-loaded microspheres possessed a prolonged and complete drug release with significant inhibition of AchE for 2 weeks in rats.

Long-term cholinesterase inhibitor treatment of Alzheimer's disease

The most prevalent cause of dementia--Alzheimer's disease--is characterised by an early cholinergic deficit that is in part responsible for the cognitive deficits, especially memory and attention defects, seen with this condition. Three cholinesterase inhibitors (ChEIs), namely donepezil, rivastigmine and galantamine, are widely used for the symptomatic treatment of patients with Alzheimer's disease. Placebo-controlled, randomised clinical trials have shown significant effects of these drugs on global function, cognition, activities of daily living (ADL) and behavioural symptoms in patients with this disorder. These trials have been conducted for up to 12 months and were followed by open-label extension studies. One placebo-controlled, randomised clinical trial followed patients for up to 4 years. Both retrospective and prospective follow-up studies suggest a treatment effect for ChEIs that lasts for up to 5 years. Studies have shown comparable effects for ChEIs in patients with moderate-to-severe Alzheimer's disease or mild Alzheimer's disease. Clinically relevant responses consist not only of improvement over 3-6 months but also stabilisation and possibly slower than expected decline. Lack of overt clinical improvement in one domain (e.g. global function, cognition, ADL or behaviour) does not preclude clinically relevant benefit(s) in other domains. If it is judged that the patient has experienced a treatment effect from ChEI therapy during the first 6 months, it is recommended that treatment be continued for at least 1 year before discontinuation is considered again. On average, patients will return to their pre-treatment status between 9 and 12 months of initiation of treatment. However, this return to pre-treatment level does not mean that the treatment effect has disappeared. At this point in time, the patient may still function better than he or she would have without treatment. Setting a fixed measurement, e.g. a Mini-Mental State Examination score, as a 'when to stop treatment limit' is not clinically rational. The length of treatment should depend on several individual patient factors. The earlier the diagnosis is made and the slower the rate of disease progression, the longer the treatment period will tend to be. Treatment duration must therefore be evaluated on an individual basis, and the patient's status compared with what would have been expected without treatment. If a clinical evaluation is conducted with a view to stopping or switching treatment, it is crucial that all domains are evaluated and that the patient is evaluated at more than one point in time before the decision is made.

"Brain-specific" nutrients: a memory cure?

OBJECTIVE

We review the experimental evaluations of several widely marketed nonprescription compounds claimed to be memory enhancers and treatments for age-related memory decline. We generally limit our review to double-blind placebo-controlled studies. The compounds examined are phosphatidylserine (PS), phosphatidylcholine (PC), citicoline, piracetam, vinpocetine, acetyl-L-carnitine (ALC), and antioxidants (particularly vitamin E).

RESULTS

In animals, PS has been shown to attenuate many neuronal effects of aging, and to restore normal memory on a variety of tasks. Preliminary findings with humans, though, are limited. For older adults with probable Alzheimer's disease, a single study failed to demonstrate positive effects of PS on memory performance. For older adults with moderate cognitive impairment, PS has produced consistently modest increases in recall of word lists. Positive effects have not been as consistently reported for other memory tests. There is one report of consistent benefits across a number of memory tests for a subset of normal adults who performed more poorly than their peers at baseline. The choline compounds PC and citicoline are thought to promote synthesis and transmission of neurotransmitters important to memory. PC has not proven effective for improving memory in patients with probable Alzheimer's disease. The issue remains open for older adults without serious degenerative neural disease. Research on citicoline is practically nonexistent, but one study reported a robust improvement in story recall for a small sample of normally aging older adults who scored lower than their peers in baseline testing. Animal studies suggest that piracetam may improve neuronal efficiency, facilitate activity in neurotransmitter systems, and combat the age-related decrease in receptors on the neuronal membrane. However, for patients with probable Alzheimer's disease, as well as for adults with age-associated memory impairment, there is no clear-cut support for a mnemonic benefit of piracetam. Vinpocetine increases blood circulation and metabolism in the brain. Animal studies have shown that vinpocetine can reduce the loss of neurons due to decreased blood flow. In three studies of older adults with memory problems associated with poor brain circulation or dementia-related disease, vinpocetine produced significantly more improvement than a placebo in performance on global cognitive tests reflecting attention, concentration, and memory. Effects on episodic memory per se have been tested minimally, if at all. ALC participates in cellular energy production, a process especially important in neurons, and in removal of toxic accumulation of fatty acids. Animal studies show that ALC reverses the age-related decline in the number of neuron membrane receptors. Studies of patients with probable Alzheimer's disease have reported nominal advantages over a range of memory tests for ALC-treated patients relative to placebo groups. Significant differences have been reported rarely, however. Whether ALC would have mnemonic benefits for aging adults without brain disease is untested as far as we know. Antioxidants help neutralize tissue-damaging free radicals, which become more prevalent as organisms age. It is hypothesized that increasing antioxidant levels in the organism might retard or reverse the damaging effects of free radicals on neurons. Thus far, however, studies have found that vitamin E does not significantly slow down memory decline for Alzheimer's patients and does not produce significant memory benefits among early Parkinson's patients. Neither did a combination of vitamins E and C significantly improve college students' performance on several cognitive tasks.

CONCLUSIONS

In sum, for most of the "brain-specific" nutrients we review, some mildly suggestive effects have been found in preliminary controlled studies using standard psychometric memory assessments or more general tests designed to reveal cognitive impairment. We suggest that future evaluations of the possible memory benefits of these supplements might fruitfully focus on memory processes rather than on memory tests per se.

Aromatic amino-acid residues at the active and peripheral anionic sites control the binding of E2020 (AriceptR) to cholinesterases

E2020 (R,S)-1-benzyl-4-[(5,6-dimethoxy-1-indanon)-2-yl]methyl)piperidine hydrochloride is a piperidine-based acetylcholinesterase (AChE) inhibitor that was approved for the treatment of Alzheimer's disease in the United States. Structure-activity studies of this class of inhibitors have indicated that both the benzoyl containing functionality and the N-benzylpiperidine moiety are the key features for binding and inhibition of AChE. In the present study, the interaction of E2020 with cholinesterases (ChEs) with known sequence differences, was examined in more detail by measuring the inhibition constants with Torpedo AChE, fetal bovine serum AChE, human butyrylcholinesterase (BChE), and equine BChE. The basis for particular residues conferring selectivity was then confirmed by using site-specific mutants of the implicated residue in two template enzymes. Differences in the reactivity of E2020 toward AChE and BChE (200- to 400-fold) show that residues at the peripheral anionic site such as Asp74(72), Tyr72(70), Tyr124(121), and Trp286(279) in mammalian AChE may be important in the binding of E2020 to AChE. Site-directed mutagenesis studies using mouse AChE showed that these residues contribute to the stabilization energy for the AChE-E2020 complex. However, replacement of Ala277(Trp279) with Trp in human BChE does not affect the binding of E2020 to BChE. Molecular modeling studies suggest that E2020 interacts with the active-site and the peripheral anionic site in AChE, but in the case of BChE, as the gorge is larger, E2020 cannot simultaneously interact at both sites. The observation that the KI value for mutant AChE in which Ala replaced Trp286 is similar to that for wild-type BChE, further confirms our hypothesis.

"Brain-Specific" Nutrients: A Memory Cure?

We review the experimental evaluations of several widely marketed nonprescription compounds claimed to be memory enhancers and treatments for age-related memory decline. We generally limit our review to double-blind placebo-controlled studies. The compounds examined are phos-phatidylserine (PS), phosphatidylcholine (PC), citicoline, piracetam, vinpocetine, acetyl-L-carnitine (ALC), and antiox-idants (particularly vitamin E). In animals, PS has been shown to attenuate many neuronal effects of aging, and to restore normal memory on a variety of tasks. Preliminary findings with humans, though, are limited. For older adults with probable Alzheimer's disease, a single study failed to demonstrate positive effects of PS on memory performance. For older adults with moderate cognitive impairment, PS has produced consistently modest increases in recall of word lists. Positive effects have not been as consistently reported for other memory tests. There is one report of consistent benefits across a number of memory tests for a subset of normal adults who performed more poorly than their peers at baseline. The choline compounds PC and citicoline are thought to promote synthesis and transmission of neurotransmitters important to memory. PC has not proven effective for improving memory in patients with probable Alzheimer's disease. The issue remains open for older adults without serious degenerative neural disease. Research on citicoline is practically nonexistent, but one study reported a robust improvement in story recall for a small sample of normally aging older adults who scored lower than their peers in baseline testing. Animal studies suggest that piracetam may improve neuronal efficiency, facilitate activity in neurotransmitter systems, and combat the age-related decrease in receptors on the neuronal membrane. However, for patients with probable Alzheimer's disease, as well as for adults with age-associated memory impairment, there is no clear-cut support for a mnemonic benefit of piracetam. Vinpocetine increases blood circulation and metabolism in the brain. Animal studies have shown that vinpocetine can reduce the loss of neurons due to decreased blood flow. In three studies of older adults with memory problems associated with poor brain circulation or dementia-related disease, vinpocetine produced significantly more improvement than a placebo in performance on global cognitive tests reflecting attention, concentration, and memory. Effects on episodic memory per se have been tested minimally, if at all. ALC participates in cellular energy production, a process especially important in neurons, and in removal of toxic accumulation of fatty acids. Animal studies show that ALC reverses the age-related decline in the number of neuron membrane receptors. Studies of patients with probable Alzheimer's disease have reported nominal advantages over a range of memory tests for ALC-treated patients relative to placebo groups. Significant differences have been reported rarely, however. Whether ALC would have mnemonic benefits for aging adults without brain disease is untested as far as we know. Antioxidants help neutralize tissue-damaging free radicals, which become more prevalent as organisms age. It is hypothesized that increasing antioxidant levels in the organism might retard or reverse the damaging effects of free radicals on neurons. Thus far, however, studies have found that vitamin E does not significantly slow down memory decline for Alzheimer's patients and does not produce significant memory benefits among early Parkinson's patients. Neither did a combination of vitamins E and C significantly improve college students' performance on several cognitive tasks. In sum, for most of the "brain-specific" nutrients we review, some mildly suggestive effects have been found in preliminary controlled studies using standard psychometric memory assessments or more general tests designed to reveal cognitive impairment. We suggest that future evaluations of the possible memory benefits of these supplements might fruitfully focus on memory processes rather than on memory tests per se.

Do cholinesterase inhibitors have disease-modifying effects in Alzheimer's disease?

During the last decade, a systematic effort to develop a pharmacological treatment for Alzheimer disease (AD) has resulted in drugs being registered for the first time in the US and Europe for this specific indication. The 3 agents registered are cholinesterase inhibitors (ChEIs). The major therapeutic effect of ChEIs in patients with AD is the maintenance of cognitive function, as compared with placebo, during a 6-month to 1-year period of treatment. Additional drug effects that may occur are the slowing of cognitive deterioration and improvement of behaviour and daily living activities. Comparison of clinical effects of 6 ChEIs demonstrates a rather similar magnitude of improvement in cognitive outcome measures. For some drugs, this level may represent an upper limit, while for others it may be possible to increase the benefit further. In order to maximise and prolong positive drug effects it is important to start treatment early and adjust the dosage during treatment. Recent studies that used this administration strategy have shown that in many patients, the stabilisation effect produced by ChEIs can be prolonged for as long as 36 months. This long-lasting effect suggests mechanisms of action other than symptomatic ones. In this article, the effects of ChEIs on beta-amyloid metabolism are postulated to explain the stabilising (i.e. disease-modifying) effects of the drugs. Evidence for such a mechanism is available at the experimental but not yet at the clinical level.

Is anti-cholinesterase therapy of Alzheimer's disease delaying progression?

During the last decade, a systematic effort to develop a pharmacological treatment for Alzheimer's disease (AD) resulted in three drugs being registered for the first time in the US and Europe. All three compounds are cholinesterase inhibitors (ChEI). The major therapeutic effect of ChEI on AD patients is to maintain cognitive function at a stable level during a 6-month to 1-year period of treatment, as compared to placebo. Additional drug effects are to slow down cognitive deterioration and improve behavioral and daily living activity. Recent studies show that in many patients the cognitive stabilization effect can be prolonged up to 24 months. This long-lasting effect suggests a mechanism of action other than symptomatic, and directly cholinergic. In vitro and in vivo studies have consistently demonstrated a link between cholinergic activation and amyloid precursor protein (APP) metabolism. Lesions of cholinergic nuclei cause a rapid increase in cortical APP and cholinergic synaptic function; the effect of such lesions can be reversed by ChEI treatment. A reduction in cholinergic neurotransmission, experimental or pathological, leads to amyloidogenic metabolism and contributes to the development of neuropathology and cognitive dysfunction. To explain the long-term effect of ChEI, for which evidence is available on an experimental as well as clinical level, a mechanism based on beta-amyloid metabolism is postulated. The question whether cholinergic stabilization implies simply slowing down progression of disability or also involves delay of disease progression is discussed.

The experimental Alzheimer drug phenserine: preclinical pharmacokinetics and pharmacodynamics

Phenserine, a phenylcarbamate of physostigmine, is a new potent and highly selective acetylcholinesterase (AChE) inhibitor, with a > 50-fold activity versus butyrylcholinesterase (BChE), in clinical trials for the treatment of Alzheimer's disease (AD). Compared to physostigmine and tacrine, it is less toxic and robustly enhances cognition in animal models. To determine the time-dependent effects of phenserine on cholinergic function, AChE activity, brain and plasma drug levels and brain extracellular acetylcholine (ACh) concentrations were measured in rats before and after phenserine administration. Additionally, its maximum tolerated dose, compared to physostigmine and tacrine, was determined. Following i.v. dosing, brain drug levels were 10-fold higher than those achieved in plasma, peaked within 5 min and rapidly declined with half-lives of 8.5 and 12.6 min, respectively. In contrast, a high (> 70%) and long-lasting inhibition of AChE was achieved (half-life > 8.25 h). A comparison between the time-dependent plasma AChE inhibition achieved after similar oral and i.v. doses provided an estimate of oral bioavailability of 100%. Striatal, in vivo microdialysis in conscious, freely-moving phenserine-treated rats demonstrated > 3-fold rise in brain ACh levels. Phenserine thus is rapidly absorbed and cleared from the body, but produces a long-lasting stimulation of brain cholinergic function at well tolerated doses and hence has superior properties as a drug candidate for AD. It selectively inhibits AChE, minimizing potential BChE side effects. Its long duration of action, coupled with its short pharmacokinetic half-life, reduces dosing frequency, decreases body drug exposure and minimizes the dependence of drug action on the individual variations of drug metabolism commonly found in the elderly.

Effects of ZK 93426 on muscarinic and nicotinic antagonist or nucleus basalis lesioning-induced electrocortical slowing

The present study investigated the effects of a benzodiazepine receptor antagonist, beta-carboline ZK 93426 (1, 3 and 10 mg/kg, IP), on scopolamine and nucleus basalis (NB) quisqualic acid lesion-induced neocortical electrocortical activity slowing in rats. Scopolamine induced a dose dependent increase in EEG spectral values and slow delta waves (0.3 < 0.9 = 2.7 mg/kg IP). ZK 93426 partially reversed EEG slowing induced by the smallest scopolamine dose (0.3 mg/kg), but had no effect on the EEG changes induced by higher doses. A combination of scopolamine at 0.3 mg/kg and mecamylamine (a centrally active nicotinic antagonist) at 10 mg/kg induced an EEG slowing that was not reversed by ZK 93426. NB lesions markedly decreased cortical choline acetyltransferase (ChAT) activity (-77%) and increased EEG slow waves. ZK 93426 had no effect on the NB lesion-induced slow wave activity increase. The present results support the idea that beta-carboline ZK 93426 may increase cortical cholinergic activity by disinhibiting the NB cholinergic neurons. However, if the activity of "NB to cortex" cholinergic system is greatly decreased by either a marked reduction in NB cell number (in NB-lesioned rats), a near complete cortical post-synaptic muscarinic receptor blockade (large scopolamine dose) or by a combination of nicotinic (decrease acetylcholine release) and muscarinic receptor blockade, the effects of beta-carboline ZK 93426 on EEG slowing may be negligible.

Cholinesterase inhibitor therapy stabilizes symptoms of Alzheimer disease

Cholinesterase inhibitors tested in clinical trials in Europe, the United States, and Japan include fewer than 10 drugs; however, most of these compounds have advanced to clinical phase III trials. Based on results related to a population of more than 8,000 patients, we conclude that several of these compounds have shown significant clinical efficacy and safety in the treatment of Alzheimer disease. There are, however, differences with regard to side effects. The major clinical effect is stabilization of cognitive function during a 6- to 12-month period with an improvement of behavioral symptoms. The long-term effect of cholinesterase inhibitors extending to a 2-year period was reported. Future applications of these drugs include treatment of other types of dementias such as Lewy bodies dementia, vascular dementia, and Down syndrome dementia. The combination of cholinesterase inhibitors with estrogens, antioxidants, and anti-inflammatories may represent a further improvement of the therapy. From an economical point of view, treatment with cholinesterase inhibitors is not cost neutral.

RBC cholinesterase inhibition: a useful surrogate marker for cholinesterase inhibitor activity in Alzheimer disease therapy?

Red blood cell (RBC) acetylcholinesterase (AChE) inhibition has been used as a peripheral surrogate marker for the activity of centrally acting AChE inhibitors (AChEIs) in the treatment of Alzheimer disease. As a valid peripheral surrogate marker, RBC AChE inhibition should reflect the central pharmacodynamic activity of the compound and should demonstrate a relation with cognitive or global improvement in patients with Alzheimer disease. As a useful clinical tool, RBC AChE inhibition should also provide an advantage in dose optimization. However, the application of surrogate markers in research and clinical use is controversial (Prentice, 1989; Gotzsche, 1996; Colburn, 1997; De Gruttola et al., 1997). For instance, surrogate markers that have been identified or applied inappropriately can lead to erroneous conclusions, slowing the drug development process (Colburn, 1997). Also, the validation of surrogate markers for the pharmacodynamic activity of central nervous system drugs is not always possible because samples of brain tissue cannot be analyzed in humans. Finally, although validation of peripheral markers for central nervous system drugs has been approached via analysis of cerebrospinal fluid (Cutler et al., 1998a), few markers have been subjected to such rigorous evaluation in clinical studies. The extent to which measures of peripheral AChE inhibition accurately model central drug activity and therapeutic effectiveness of AChEIs, both as individual agents and as a drug class, is the focus of this review. AChEIs comprise a group of structurally diverse compounds with a wide range of relative specificities for the various molecular species of cholinesterase found in plasma, RBCs, and the brain. Studies of RBC AChE inhibition after administration of AChEIs in animals are of limited utility because of the differential sensitivity of AChEIs for human versus animal forms of AChE, the poor correlation between effective doses in animals and humans, and the lack of standardized measurements of effectiveness. Although clinical studies of donepezil, metrifonate, and eptastigmine have suggested the potential use of RBC AChE inhibition as a predictor of clinical response, the degree of inhibition yielding maximum cognitive improvements was highly variable from compound to compound (30-80%). Further, investigators did not prove a relation between central and peripheral pharmacodynamics or demonstrate an advantage over dose in the ability of RBC AChE inhibition to predict clinical response. A study of rivastigmine in patients with Alzheimer disease revealed that cerebrospinal fluid AChE inhibition correlated well with cognitive performance, whereas peripheral inhibition did not. Therefore, RBC cholinesterase inhibition is not a reliable surrogate marker for the activity of AChEIs as a class of drugs, and its usefulness as a dose optimization tool for individual agents has yet to be demonstrated clearly.

Effects of cholinesterase inhibitors on a two-component chained schedule performance in rats

The acetylcholinesterase (AChE) inhibitors physostigmine (PHY), tacrine (THA), and heptylphysostigmine (HEP) have been evaluated as potential therapeutics for treatment of Alzheimer's disease (AD) and as prophylactics against organophosphate (OP) poisoning. The above medical applications are based upon the neurochemical principles of elevation of transient levels of acetylcholine (ACh) in brain and reversible inhibition of AChE in blood and brain, respectively. The present study was undertaken to evaluate the effects of these drugs on performance of a two-component chained schedule of differential-reinforcement-of-high-rate (DRH) reward/differential-reinforcement-of-low-rate (DRL) nonreward contingencies, for water reinforcement in 2-h experimental sessions in rats. Both PHY (0.031-0.25 mg/kg, SC) and HEP (0.625-10.0 mg/kg, SC) decreased overall reinforcement rate and nonreinforced response in a dose-related and parallel manner, whereas THA (0.625-5.0 mg/kg) decreased the overall reinforcement rate in a dose-related manner, but did not significantly affect nonreinforced response. The least significant doses of PHY (0.625 mg/kg), THA (1.25 mg/kg), and HEP (1. 25 mg/kg) on the behavioral performance were associated with oral movements and/or muscle fasciculation. Moderate to high doses of these drugs (i.e., PHY >/= 0.624, THA >/= 1.25, and HEP >/= 1.25 mg/kg) produced behavioral suppression, which resulted mostly from the cessation of responding in the presence of cholinergic adverse events. The ED(50) values of the behavioral disruption (as documented by overall reinforcer loss) for PHY, THA, and HEP were 0. 081, 3.87, and 2.89 mg/kg, respectively, and the behavioral-deficit-free (BDF) doses were 0.031, 0.625, and 0.625 mg/kg, respectively. Preclinical data revealed that the BDF doses of both PHY and HEP have moderate inhibition of AChE, which leads to an efficacious elevation of ACh level in the brain, whereas the BDF dose of THA shows no significant AChE inhibition or elevation of brain ACh level; however, they all have cognition enhancing effects at their respective safety doses. The above data suggest that the BDF doses of PHY and HEP may have prophylactic efficacy against OP poisoning, whereas THA may not. The BDF doses of these three drugs are comparable to the maximum tolerated doses in clinical practice, suggesting that the present rat model may have potential value in predicting the clinical safety of AChE inhibitors developed for therapy of AD and prophylaxis against OP poisoning as well.

Present and future of Alzheimer therapy

Three major lines of drugs have been developed or are under development for the treatment of Alzheimer Disease (AD): cholinergic drugs (mainly cholinesterase inhibitors), anti-beta-amyloid drugs, estrogens and anti-inflammatories. Cholinesterase inhibitors are the only drugs presently approved in USA and Europe for the indication of AD. Cholinesterase inhibitors tested in clinical trials in Europe, USA and Japan include less than ten drugs, however most of these compounds have advanced to clinical trials III. Based on results related to a population of over 8,000 patients we conclude that several of these compounds have shown significant clinical efficacy and safety in the treatment of Alzheimer disease. There are, however, differences with regard to side effects. The major clinical effect is stabilization of cognitive function during a six- to 12-months period with a parallel improvement of behavioral symptoms. Long-term effect of cholinesterase inhibitors extending to a two year-period has been reported. Future applications of these drugs are treatment of other types dementias such as Lewy body dementia, vascular dementia and Down Syndrome dementia. Combination of cholinesterase inhibitors with estrogens, anti-oxidants and anti-inflammatories may represent a further improvement of the therapy. From the economical point of view, treatment with cholinesterase inhibitors is not cost neutral.

A phase II study in patients with Alzheimer's disease to assess the preliminary efficacy and maximum tolerated dose of rivastigmine (Exelon)

Rivastigmine is a carbamate acetylcholinesterase (AChE) inhibitor with central selectivity. Early studies showed that daily doses up to 6 mg/day have some efficacy in patients with dementia of the Alzheimer type (DAT). The present study was designed to assess the safety, tolerability and efficacy of rivastigmine at doses up to 12 mg/day. A total of 114 patients with mild-moderate DAT were randomly assigned to either rivastigmine (b.i.d. (twice daily) or t.i.d. (three times daily)) or placebo in a double-blind fashion titrated to their maximum tolerated dose over 10 weeks followed by an eight-week maintenance phase. The mean maximum tolerated dose was approximately 10 mg/day (b.i.d. or t.i.d.). Gastrointestinal complaints, the majority of which were mild to moderate, were the most frequently reported adverse events. No clinically relevant changes in vital signs, haematology or organ function were detected. Significantly more patients taking rivastigmine b.i.d. were considered improved according to the Clinicians' Interview-Based Impression of Change-Plus (CIBIC-Plus) vs. placebo (57% vs. 16%, respectively; P = 0.027). The Nurses' Observation Scale for Geriatric Patients (NOSGER) (memory component) and the Alzheimer's Disease Assessment Scale-cognitive subscale (ADAS-cog) also improved in the rivastigmine b.i.d. group vs. placebo (mean change from baseline on NOSGER = -0.7 vs. +1.3, respectively; P = 0.037: mean change from baseline on ADAS-cog = -2.7 vs. +0.2, respectively; P = 0.054). Despite the relatively small size and limited duration of the study, the finding that rivastigmine induced changes in the same (positive) direction in all three dimensions measured suggests that rivastigmine at doses of up to 12 mg/day has useful efficacy in patients with mild-moderate DAT. Reports from larger phase III studies confirm this finding. The results of this study also suggest that b.i.d. is the more efficacious regimen and has comparable tolerability to the t.i.d. regimen.

Phenylmethylsulfonyl fluoride inhibitory effects on acetylcholinesterase of brain and muscle

Differential inhibition of brain versus peripheral acetylcholinesterase (AChE) by phenylmethylsulfonyl fluoride (PMSF) suggested that PMSF might preferentially inhibit different AChE molecular forms. AChE inhibition was examined after systemic and in vitro PMSF treatment. Systemic administration resulted in no overt behavioral changes but produced a 71% reduction in brain AChE; hemidiaphragm, extensor digitorum longus and soleus muscles showed 65, 50 and 41% reductions. Muscle asymmetric AChE was reduced to the greatest extent (50-80%). The tetrameric form was inhibited in brain and hemidiaphragm (60-76%) but spared in other muscles (18-22%). Monomeric AChE was spared in all tissues. When PMSF was added to a muscle homogenate all forms were inhibited equally. Purified monomer and tetramer forms were inhibited equally in vitro. These results suggest that PMSF inhibition of AChE is a consequence of a selective inhibition of membrane-associated forms and that the apparent brain selectivity is related to the greater fraction of membrane-associated AChE in brain.

Cholinesterase inhibitors: a therapeutic strategy for Alzheimer disease

OBJECTIVE

To provide a review of acetylcholinesterase inhibitors (AChEIs) tested as therapeutic agents for Alzheimer disease (AD).

DATA SOURCES

MEDLINE searches (January 1986-July 1998) identified pertinent literature. Selected references from these articles, as well as abstracts from recent meetings and package insert literature from approved compounds, were also used as source material.

DATA EXTRACTION

AChEIs were reviewed with regard to chemical structure, mechanism of inhibition, substrate specificity, pharmacokinetics/pharmacodynamics, safety/tolerability, and efficacy.

DATA SYNTHESIS

Cholinergic deficits, leading to cognitive impairment, are a significant aspect of neurodegeneration in AD. AChEIs reduce the degradation of acetylcholine, thus enhancing cholinergic transmission. In addition to the two agents approved by the Food and Drug Administration, tacrine and donepezil, six other compounds of diverse chemical structure and mechanism of inhibition including physostigmine, metrifonate, rivastigmine, and galantamine are under investigation as potential therapy for AD. These compounds are structurally diverse, possess unique patterns of specificities for the various forms of cholinesterase enzymes, use distinct mechanisms of enzyme inhibition, present unique adverse event profiles, and offer relatively similar mean gains in cognitive abilities to patients with AD in controlled clinical trials.

CONCLUSIONS

Relative to placebo, new AChEIs in development provide modest improvements in cognition for patients with mild to moderate AD, with improved tolerability profiles and more convenient dosing relative to tacrine. The availability of a wide array of AChEIs soon to be accessible to patients with AD will provide additional options to those who cannot tolerate or do not respond to drugs currently used for AD.

Counteractive effects of a partial (sabcomeline) and a full (RS86) muscarinic receptor agonist on deficits in radial maze performance induced by S-AMPA lesions of the basal forebrain and medial septal area

After S-AMPA (8.0 mM) lesions to the nucleus basalis and medial septal regions, at the source of the cortical and hippocampal branches of the forebrain cholinergic projection system, rats displayed long-lasting and relatively stable impairment in long-term reference and short-term working memory in both spatial (place) and associative (cue) radial maze tasks. Treatment with four doses of the partial agonist at the M1 cholinergic muscarinic receptor, sabcomeline (formerly known as SB 202026: 0.01-0.156 mg/kg), substantially reduced working and reference memory errors in both tasks in lesioned rats, in a mainly dose-related manner. These effects were more consistent than those found with the direct muscarinic agonist RS86 (0.05-0.781 mg/kg). The performance of non-lesioned controls was largely unaffected by either treatment. These findings are consistent with previous evidence for cholinergic participation in the radial maze deficits induced by excitotoxic lesions to the forebrain cholinergic projection system. They show that with a relatively selective lesion, which respectively, reduced choline acetyltransferase activity to 36.5 and 22.5% of control level in frontal and dorsolateral cortex, and to 61.8 and 69.2% of control level in dorsal and ventral hippocampus, lesioned rats were responsive to pharmacological treatments aimed to enhance cholinergic function by full or partial agonist activity at M1 receptors. Findings that nicotine (0.1 mg/kg) also reduced radial maze errors in the lesioned animals supports the suggestion that lesion-induced deficits in radial maze performance were amenable to improvement by cholinergic receptor manipulation. However, given the potential adverse side effects of full receptor agonists, which nonselectively target cholinergic receptors throughout the organism, the functional efficacy of sabcomeline, which shows regional selectivity for the central M1 receptor subtypes, suggests that deleterious effects of cholinergic depletion on cognition can be counteracted without incurring the risk of unwanted side effects.

Effect of subchronic metrifonate treatment on cerebral glucose metabolism in young and aged rats

The effects of subchronic administration of metrifonate, a long-lasting cholinesterase inhibitor, on local cerebral glucose utilization were assessed in 3- and 27-month old Sprague-Dawley rats, using the autoradiographic [14C]2-deoxyglucose technique. Rats were treated twice daily with metrifonate (80 or 120 mg/kg) for 3 weeks. The [14C]2-deoxyglucose experiment was performed 18 h after the last metrifonate administration. In 3-month old rats, metrifonate 80 mg/kg increased the average hemispheric cerebral glucose utilization by 12% (P > 0.001). Significant effects were observed in 19 of the 54 regions studied, including cortical and limbic regions. The higher dose induced a larger effect (average increase 17%, 24 of the 54 regions affected). In 27-month old rats, very similar effects were obtained. These results show that repeated administration of metrifonate leads to a sustained metabolic activation in rat brain, at a level comparable to the activation observed previously after a single administration of the drug.

N-[11C]methylpiperidine esters as acetylcholinesterase substrates: an in vivo structure-reactivity study

A series of simple esters incorporating the N-[11C]methylpiperidine structure were examined as in vivo substrates for acetylcholinesterase in mouse brain. 4-N-[11C]Methylpiperidinyl esters, including the acetate, propionate and isobutyrate esters, are good in vivo substrates for mammalian cholinesterases. Introduction of a methyl group at the 4-position of the 4-piperidinol esters, to form the ester of a teritary alcohol, effectively blocks enzymatic action. Methylation of 4-N-[11C]methylpiperidinyl propionate at the 3-position gives a derivative with increased in vivo reactivity toward acetylcholinesterase. Esters of piperidinecarboxylic acids (nipecotic, isonipecotic and pipecolinic acid ethyl esters) are not hydrolyzed by acetylcholinesterase in vivo, nor do they act as in vivo inhibitors of the enzyme. This study has identified simple methods to both increase and decrease the in vivo reactivity of piperidinyl esters toward acetylcholinesterase.

Effect of subchronic treatment with metrifonate and tacrine on brain cholinergic function in aged F344 rats

The effects of 21-day treatment with the acetylcholinesterase inhibitors metrifonate (80 mg kg(-1) per os (p.o.)) and tacrine (3 mg kg(-1) p.o.), twice daily, on cortical and hippocampal cholinergic systems were investigated in aged rats (24-26 months). Extracellular acetylcholine levels were measured by transversal microdialysis in vivo; choline acetyltransferase and acetylcholinesterase activities were measured ex vivo by means of radiometric methods. Basal cortical and hippocampal extracellular acetylcholine levels, measured 18 h after the last metrifonate treatment, were about 15 and two folds higher, respectively, than in control and tacrine-treated rats. A challenge with metrifonate further increased cortical and hippocampal acetylcholine levels by about three and four times, respectively. Basal extracellular acetylcholine levels, measured 18 h after the last treatment with tacrine were not statistically different from those of the control rats. A challenge with tacrine increased cortical and hippocampal extracellular acetylcholine levels by about four and two times. A 75% inhibition of cholinesterase activity was found 18 h after the last metrifonate administration, while only a 15% inhibition was detectable 18 h after the last tacrine administration. The challenge with metrifonate or tacrine resulted in 90 and 80% cholinesterase inhibition, respectively. These results demonstrate that in aging rats a subchronic treatment with metrifonate results in a long-lasting, cholinesterase inhibition, and a persistent increase in acetylcholine extracellular levels which compensate for the age-associated cholinergic hypofunction. Metrifonate is therefore a potentially useful agent for the cholinergic deficit accompanying Alzheimer's disease.

Kinetics of human erythrocyte acetylcholinesterase inhibition by a novel derivative of physostigmine: phenserine

The effect of phenserine, a novel cholinesterase inhibitor, was assessed for the first time on kinetic parameters of human erythrocyte acetylcholinesterase (AChE). Phenserine (0.025-0.40 microM) inhibited the activity of human erythrocyte AChE in a concentration-dependent fashion, the IC50 was 0.0453 microM. The Michaelis-Menten constant (K(m)) for the hydrolysis of acetylthiocholine iodide was found to be 0.124 mM and the Vmax was 0.980 mumol/min/mg protein. Dixon as well as Lineweaver-Burk plots and their secondary replots indicated that the nature of the inhibition was of the noncompetitive type. The value of Ki was estimated as 0.048 microM by the primary and secondary replots of the Dixon as well as secondary replots of the Lineweaver-Burk plot. A novel relationship between Ki and substrate concentration was also identified which permits more precise prediction of the specific type of noncompetitive inhibition of various enzymes by a wide variety of drugs, chemicals and, in some circumstances, by their own substrates.

Cholinergic foundations of Alzheimer's disease therapy

Cholinesterase inhibitors (ChEI) represent the drug of choice for Alzheimer's disease (AD) treatment. They produce significant improvement on cognitive as well as non-cognitive function for a period up to 1 year during the first 3 years following clinical diagnosis. The magnitude of cognitive improvements is similar for different ChEI, however, differences are seen with regard to incidence and severity of side effects, optimal ChE inhibition, pharmacokinetic properties and mode of administration.

Effect of tetrahydroaminoacridine, a cholinesterase inhibitor, on cognitive performance following experimental brain injury

An emerging literature exists in support of deficits in cholinergic neurotransmission days to weeks following experimental traumatic brain injury (TBI). In addition, novel cholinomimetic therapeutics have been demonstrated to improve cognitive outcome following TBI in rats. We examined the effects of repeated postinjury administration of a cholinesterase inhibitor, tetrahydroaminoacridine (THA), on cognitive performance following experimental TBI. Rats were either injured at a moderate level of central fluid percussion TBI (2.1+/-0.1 atm) or were surgically prepared but not delivered a fluid pulse (sham injury). Beginning 24 h after TBI or sham injury, rats were injected (IP) daily for 15 days with an equal volume (1.0 ml/kg) of either 0.0, 1.0, 3.0, or 9.0 mg/kg THA (TBI: n = 8, 8, 10, and 7, respectively, and Sham: n = 5, 7, 8, 7, respectively). Cognitive performance was assessed on Days 11-15 after injury in a Morris water maze (MWM). Analysis of maze latencies over days indicated that chronic administration of THA produced a dose-related impairment in MWM performance in both the injured and sham groups, with the 9.0 mg/kg dose producing the largest deficit. The 1.0 and 3.0 mg/kg doses of THA impaired MWM performance without affecting swimming speeds. Thus, the results of this investigation do not support the use of THA as a cholinomimetic therapeutic for the treatment of cognitive deficits following TBI.

Cholinergic modulation, visual function and Alzheimer's dementia

Electrophysiological evidence at a cellular level and in vivo macroelectrode recordings converge in indicating a degree of specificity of acetylcholine action in vision. Acetylcholine (ACh) function is also thought to play a significant role in memory, learning and other cognitive processes. In this respect, ACh action is suggested to serve in both sensory and cognitive processes. The pharmacological blocking of brain muscarinic transmission has been proposed as a model of geriatric memory impairment and Alzheimer's dementia. Visual electrophysiological testing is deemed of diagnostic specificity for this disease. ACh brain neurotransmission, however, mostly contributes to the modulation of nonspecific aspects of cognition, such as arousal or attention. Alzheimer's dementia results from complex neuron alterations [which also affect muscarinic receptors among other (sub)cellular structures] rather than simply reflecting ACh impoverishment. A substantial loss of retinal ganglion cells is documented in patients with Alzheimer's disease and is consistent with electrophysiological observations. However, it is unclear to what extent the dysfunction of the visual system observable in Alzheimer's dementia is qualitatively different from that occurring spontaneously during aging. The dissimilarities between the effect of acute muscarinic blocking (e.g. by scopolamine) and dementia outnumber the similarities. Accordingly, the conventional ACh agonist-antagonist model of dementia now appears questionable, and replacement treatment with compounds enhancing ACh function proved disappointing. It is suggested that (nonspecific) ACh action becomes function-specific, as determined by the architecture of local brain circuits in which it is involved.

Hemodynamic consequences of deformed microvessels in the brain in Alzheimer's disease

The cause of sporadic Alzheimer's disease (AD) remains a mystery. Mounting clinical and experimental data, however, suggest that a cerebral hemodynamic role may affect neuronoglial metabolism. Light and electron microscopy have consistently revealed that the microvasculature in AD brains contains structurally deformed capillaries which create a distorted intraluminal conduit for blood flow. The cerebral capillary distortions can create "disturbed" rather than "laminar" blood flow. Chronically disturbed capillary blood flow will impair normal delivery of essential nutrients to brain neurons as well as impede catabolic outflow of CNS waste products. This condition will negatively affect cerebral metabolism, primarily because of impaired glucose delivery to neurons. Impaired glucose delivery to AD brain results in a patho-chemical cascade that will impair the Na+, K(+)-ATPase ion pump and affect the syntheses of ATP, acetylcholine, and other neurotransmitters. The outcome of this metabolic dysfunction can promote neurofibrillary tangle and senile plaque formation in AD brain.

Behavioral characterization of metrifonate-improved acquisition of spatial information in medial septum-lesioned rats

We investigated the effects of acute oral pretraining treatment with an indirect acetylcholinesterase inhibitor, metrifonate, on water maze spatial navigation in medial septum-lesioned rats. We observed that metrifonate (30 mg/kg, orally) (1) does not alter the pattern of exploration of lesioned rats at the water maze pool or retrieval of spatial memory, (2) effectively reverses the acquisition defect, (3) enhances reversal learning, and (4) improves acquisition of water maze navigation by facilitating the encoding of the spatial representation of a specific environment. These results indicate that metrifonate does not improve escape performance to the hidden platform by modulating exploration strategy, but that metrifonate enhances the speed and accuracy of development and durability of spatial memory engrams, and facilitates learning capacity that depends on activity of the septo-hippocampal projection.

Effects of metrifonate on memory impairment and cholinergic dysfunction in rats

Metrifonate is an organophosphorous compound that has been used in the treatment of schistosomiasis. In this study, we investigated the effects of metrifonate on the impairment of learning and on central cholinergic dysfunction in scopolamine-treated and basal forebrain-lesioned rats. Oral administration of metrifonate (5.0-15.0 mg/kg) ameliorated the scopolamine- and basal forebrain. lesion-induced learning impairment in the water maze and passive avoidance tasks. Metrifonate (50 and 100 mg/kg) also significantly increased extracellular acetylcholine levels but decreased choline levels in the cerebral cortex of the basal forebrain-lesioned rats. The basal forebrain lesion decreased the cholinesterase activity in the cerebral cortex, and metrifonate (100 mg/kg) further reduced the cholinesterase activity. However, cholinesterase inhibition was not observed at the dose that ameliorated learning impairments. These results indicated that metrifonate ameliorated the impairment of learning in both scopolamine-treated and basal forebrain-lesioned rats by not only increasing extracellular acetylcholine levels by inhibiting cholinesterase, but also by undefined other mechanism(s). This finding suggests the usefulness of metrifonate for the therapy of Alzheimer's disease.

Effects of combined chronic nimodipine and acute metrifonate treatment on spatial and avoidance behavior

The present experiment was designed to elucidate whether chronic dietary treatment with nimodipine (3 months, 1000 ppm) enhances water maze spatial navigation, passive avoidance behavior and locomotor activity, and whether such a treatment with nimodipine would interact with the therapeutic effect of acute metrifonate treatment. In young medial septum-lesioned rats, nimodipine had no effect by its own on cognitive or motor behavior, and did not enhance the water maze and passive avoidance behavior improving action of metrifonate (3 and 10 mg/kg. p.o.). Nimodipine treatment of aged rats did not markedly affect the deficit in motor performance. Single and combined nimodipine and metrifonate (3 and 10 mg/kg, p.o.) treatment of aged rats resulted in shorter escape distance values to the hidden water maze escape platform compared to those of control aged rats. The passive avoidance performance of aged rats was more effectively facilitated by a combined nimodipine and metrifonate treatment than by either of the drugs on their own. Following a washout period of 2.5 months the rats that were treated previously with nimodipine no longer performed better than aged controls in the water maze test. Furthermore, after the washout period metrifonate 10 mg/kg was no longer effective in improving the water maze behavior of the now 26-month-old rats irrespective of their chronic pretreatment. Taken together, these findings indicate that chronic nimodipine and acute metrifonate treatment may more effectively stimulate cognitive functioning than either of the treatments on their own.

Effects of metrifonate, a cholinesterase inhibitor, on local cerebral glucose utilization in young and aged rats

The effects of the centrally acting anti-cholinesterase metrifonate (MFT) and its metabolite dichlorvos (2,2-dichlorovinyl dimethyl phosphate; DDVP) on local cerebral glucose utilization (LCGU) have been studied in 3- and 27-month-old rats, using the autoradiographic [14C]deoxyglucose technique. In 3-month-old rats, MFT (80 mg/kg i.p.) increased LCGU significantly in 17 of the 54 regions studied, including insular, cingulate, and temporal cortices, ventral hippocampus, thalamus, lateral habenula, substantia nigra, and superior colliculus. In these regions, the average MFT-induced increase in LCGU was 23% above control. The average hemispheric LCGU increased by 10% (p < 0.01). DDVP (5 mg/kg) increased LCGU in 19 regions (average increase 26%). The average hemispheric LCGU increased by 9% (p < 0.01). Regional distributions of MFT- and DDVP-induced increases in LCGU were similar and overlapped the distribution of the acetylcholinesterase activity. In 27-month-old rats, MFT was active in 18 regions (average increase 25%). The whole-brain mean LCGU increased by 10% (p < 0.01). MFT compensated for the age-related hypometabolism in some brain areas including insular, temporal, and retrosplenial cortices, substantia nigra, and superior colliculus. The effects of MFT on LCGU were preserved in old rats, at variance with other anticholinesterases (tacrine, physostigmine). Which are less active in the aged rat brain.

Metrifonate improves spatial navigation and avoidance behavior in scopolamine-treated, medial septum-lesioned and aged rats

We investigated the effects of acute p.o. pretraining treatment with an indirect acetylcholinesterase inhibitor, metrifonate, on water maze spatial navigation and passive avoidance behavior. Metrifonate (10-100 mg/kg, orally, p.o.) did not improve the water maze or passive avoidance performance of young intact rats. However, in young rats metrifonate over a broad dosage range (10-100 mg/kg, p.o.) was able to alleviate the adverse effects of scopolamine (a muscarinic acetylcholine receptor antagonist; 0.4 and 2.0 mg/kg in water maze and passive avoidance study, respectively) and medial septum-lesioning on spatial reference and working memory and passive avoidance performance. In old (23-month-old) rats, a defect of water maze and passive avoidance behavior was observed. In old rats, metrifonate improved spatial reference memory function in the water maze and also passive avoidance at 10-30 mg/kg, but the 3 mg/kg dose was ineffective. Very old (27-month-old) rats had a more severe impairment of water maze performance than old rats, and metrifonate 3-30 mg/kg did not improve their spatial navigation. These results show that metrifonate may over a wide range of doses stimulate cognitive functioning, but during advanced aging neurobiological defects develop that may mask some of the therapeutic effects of metrifonate in rats.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Determination of acetylcholine by on-line microdialysis coupled with pre- and post-microbore column enzyme reactors with electrochemical detection

A sensitive procedure consisting of a pre- and post-microbore column reactor sequence of a LC-electrochemical detection system coupled with on-line microdialysis system is described in the present study to measure endogenous acetylcholine concentration in freely moving rats. The pre-column packed, with immobilized choline oxidase and catalase, was used to remove choline, whereas the post-column, packed with immobilized acetylcholine oxidase and choline oxidase, was used to measure acetylcholine selectively. The detection limit of acetylcholine oxidase and choline oxidase, was used to measure acetylcholine selectively. The detection limit of acetylcholine was found to be 5 fmol/microliter (50 fmol/10 microliters). The usefulness of the described methodology was evaluated by examining the change in the striatal acetylcholine concentration of freely moving rats after physostigmine (0.5 mg/kg, s.c.) administration.

Acute and chronic arecoline: effects on a scopolamine-induced deficit in complex maze learning

These studies tested the effect of arecoline, a nonselective muscarinic agonist, administered either acutely or by chronic peripheral infusion via osmotic minipumps, on a scopolamine-induced deficit in a Stone (14 unit) T-maze task in rats. Scopolamine alone (0.125-1.0 mg/kg, IP) dose-dependently impaired maze acquisition, increasing maze run-times and to a lesser extent, the number of errors committed. Neither acute administration of arecoline (5.0 and 10.0 mg/kg, IP), when tested against a deficit induced by scopolamine (0.25 mg/kg, IP), nor chronic arecoline administration (30 and 50 mg/kg per 24 h), when tested against a deficit induced by scopolamine (0.5 mg/kg), were able to ameliorate the decrements in maze performance. In fact, the higher dose of arecoline (50 mg/kg per 24 h) infused over 10 days potentiated the scopolamine-induced deficit, with respect to latency. These data indicate that dose selection is of great importance when employing arecoline in tests of learning and memory and that the influence of the method of administration of arecoline on the behavioural outcome warrants further study.

Metrifonate induces cholinesterase inhibition exclusively via slow release of dichlorvos

Metrifonate, a long-acting cholinesterase (ChE) inhibitor with very low toxicity in warm-blooded animals, inhibits rat brain and serum cholinesterase (ChE) in vitro through its hydrolytic degradation product, dichlorvos. This conclusion is based on the finding that metrifonate-induced ChE inhibition showed the same pH dependence as its reported dehydrochlorination to dichlorvos. The ChE inhibition induced by dichlorvos was not pH dependent. It was mediated by a competitive drug interaction with the catalytic site of the enzyme, which led to irreversible inhibition within several minutes of incubation. After this time, addition of further substrate to the inhibited enzyme was not able to promote drug dissociation and hence enzyme reactivation. Similar characteristics of inhibition, i.e. interaction with the substrate binding site and time-dependent switch to non-competitive inhibition were observed with the reference compound, physostigmine. However, the physostigmine-induced inhibition of ChE could be readily reversed by further substrate addition. Another reference compound, tetrahydroaminoacridine (THA), also induced a reversible inhibition of rat brain and serum cholinesterase, but with a mechanism of action different from that of both dichlorvos and physostigmine in that enzyme inhibition occurred rapidly upon drug addition at an allosteric site on the enzyme surface. It is suggested that the unique slow release plus the slow inhibition of ChE by dichlorvos is responsible for the lower toxicity of metrifonate compared to that of directly acting ChE inhibitors.

Metrifonate and dichlorvos: effects of a single oral administration on cholinesterase activity in rat brain and blood

Cholinesterase activities in rat forebrain, erythrocytes, and plasma were assessed after a single oral administration of metrifonate or dichlorvos. In 3-month-old rats, the dichlorvos (10 mg/kg p.o.)induced inhibition of cholinesterase reached its peak in brain after l5-45 min and after 10-30 min in erythrocytes and plasma. Cholinesterase activity recovered rapidly after the peak of inhibition, but did not reach control values in brain and erythrocytes within 24 h after drug administration. The recovery of plasma cholinesterase activity, in contrast, was already complete 12 h after dichlorvos treatment. Metrifonate (100 mg/kg p.o.) had qualitatively similar inhibition kinetics as dichlorvos, albeit with a slightly delayed onset. Peak values were attained 45-60 min (brain) and 20-45 min (blood), after drug administration. Apparently complete recovery of cholinesterase activity was noted in both tissues 24 h after treatment. The dose-dependence of drug-induced inhibition of cholinesterase in rat blood and brain was determined at the time of maximal inhibition, i.e., 30 min after dichlorvos treatment and 45 min after metrifonate treatment. The oral ED(50) values obtained for dichlorvos were 8 mg/kg for brain and 6 mg/kg for both erythrocyte and plasma cholinesterase. The corresponding oral ED(50) values for metrifonate were 10 to 15 times higher, i.e., 90 mg/kg in brain and 80 mg/kg in erythrocytes and plasma. In rats deprived of food for 18 h before drug treatment, the corresponding ED(50) values for metrifonate were 60 and 45 mg/kg, respectively, indicating an about two-fold higher sensitivity of fasted rats to metrifonate-induced cholinesterase inhibition compared to non-fasted rats. Compared to 3-month-old rats, 19-month-old rats showed a higher sensitivity towards metrifonate and dichlorvos. At the time of maximal inhibition, there was a strong correlation between the degree of cholinesterase inhibition in brain and blood. These results demonstrate that single oral administration of metrifonate and dichlorvos induces an inhibition of blood and brain cholinesterase in the conscious rat in a dose-dependent and apparently fully reversible manner. While the efficiency of a given dose of inhibitor may vary with the satiety status or age of the animal, the extent of brain ChE inhibition can be estimated from the level of blood ChE activity.

Synthesis and enantioselectivity of the enantiomers of PG9 and SM21, new potent analgesic and cognition-enhancing drugs

The enantiomers of two alpha-tropanyl esters, SM21 (1) and PG9 (2), derived from (+)-R-hyoscyamine, that act by increasing the central cholinergic tone, were obtained by esterification after resolution of the corresponding racemic acids [(-)-S-1, (-)-R-2 and (+)-S-2] and by stereospecific synthesis [(+)-R-1]. Their analgesic and cognition-enhancing activities were tested in mice and their ACh-releasing properties determined on rat parietal cortex. These compounds show enantioselectivity in analgesic and cognition-enhancing tests on mice, the eutomers being the isomers which possess the same spatial arrangement of the groups on the chiral atom as (+)-R hyoscyamine [(+)-R-SM21, (+)-S-PG9]. The ACh-releasing effect of the enantiomers of SM21 in rats is in agreement with the results in mice, while PG9 enantiomers do not show any appreciable enantioselectivity in this test. On the basis of the different effects of the 5-HT4 antagonist SDZ 205557 on analgesia induced by the enantiomers of 1 and 2 and by (+)-R-hyoscyamine and the alpha-tropanyl ester of 2-phenylpropionic acid 3, a mechanism of action is proposed for this class of compounds.