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

Neuronal AChE splice variants and their non-hydrolytic functions: redefining a target of AChE inhibitors?

AChE enzymatic inhibition is a core focus of pharmacological intervention in Alzheimer's disease (AD). Yet, AChE has also been ascribed non-hydrolytic functions, which seem related to its appearance in various isoforms. Neuronal AChE presents as a tailed form (AChE-T) predominantly found on the neuronal synapse, and a facultatively expressed readthough form (AChE-R), which exerts short to medium-term protective effects. Notably, this latter form is also found in the periphery. While these non-hydrolytic functions of AChE are most controversially discussed, there is evidence for them being additional targets of AChE inhibitors. This review aims to provide clarification as to the role of these AChE splice variants and their interplay with other cholinergic parameters and their being targets of AChE inhibition: AChE-R is particularly involved in the mediation of (anti-)apoptotic events in cholinergic cells, involving adaptation of various cholinergic parameters and a time-dependent link to the expression of neuroprotective factors. The AChE-T C-terminus is central to AChE activity regulation, while isolated AChE-T C-terminal fragments mediate toxic effects via the α7 nicotinic acetylcholine receptor. There is direct evidence for roles of AChE-T and AChE-R in neurodegeneration and neuroprotection, with these roles involving AChE as a key modulator of the cholinergic system: in vivo data further encourages the use of AChE inhibitors in the treatment of neurodegenerative conditions such as AD since effects on both enzymatic activity and the enzyme's non-hydrolytic functions can be postulated. It also suggests that novel AChE inhibitors should enhance protective AChE-R, while avoiding the concomitant up-regulation of AChE-T.

Effects of cholinesterase inhibitor metrifonate on naive rats and rats with a model of hypoxia-induced impaired memory

Cholinesterase inhibitors are currently used in the therapy of different kind of dementia to improve brain memory functions. The acetylcholinesterase inhibitor metrifonate was studied in naive rats and in rats with a model of sodium nitrite-induced hypoxia. One active avoidance test and in two passive avoidance tests were used. In the active avoidance test metrifonate increased the number of avoidances during the learning session only. In both passive avoidance tests, metrifonate prolonged latency differently during the learning session and in short-term or in long-term memory retention. Hypoxic rats showed lower numbers of avoidances in learning and memory retention sessions. Metrifonate increased the number of avoidances during the learning session for hypoxic rats. In the step-through passive avoidance test, metrifonate increased the latency of reactions in the learning session and in long-term memory retention tests. In the step-down passive avoidance test, the groups with hypoxia and metrifonate did not change the latency of reaction in the learning and long-term memory retention sessions, but increased the latency of reactions in the short-term memory retention test. Morphological data showed a significant impaired neuronal structure in a CA1 zone of the hippocampus in hypoxic rats and a tendency to preserving in rats treated with metrifonate. Our results suggest that metrifonate improves cognitive functions in naive and in hypoxic rats.

Choline availability and acetylcholine synthesis in the hippocampus of acetylcholinesterase-deficient mice

Mice deficient for acetylcholinesterase (AChE) have strongly increased extracellular levels of acetylcholine (ACh) in the dorsal hippocampus [Hartmann, J., Kiewert, C., Duysen, E.G., Lockridge, O., Greig, N.H., Klein, J., 2007. Excessive hippocampal acetylcholine levels in acetylcholinesterase-deficient mice are moderated by butyrylcholinesterase activity. J. Neurochem. 100, 1421-1429]. Using microdialysis, we found that increased ACh levels are accompanied by decreased levels of extracellular choline which were 1.60 microM in AChE-deficient mice and 4.36 microM in wild-type mice. Addition of choline (10 microM) to the perfusion fluid, while ineffective in wild-type animals, more than doubled extracellular ACh levels in AChE-deficient mice. High-affinity choline uptake (HACU), as measured ex vivo in corticohippocampal synaptosomes, was more than doubled in AChE-deficient mice. Inhibition of HACU by hemicholinium-3 (HC-3) in vivo reduced extracellular levels of ACh by 60% in wild-type mice but by more than 90% in AChE-deficient mice. Decreased ACh levels caused by infusion of HC-3 or tetrodotoxin (TTX) were accompanied by increased levels of free choline. Infusion of scopolamine (1 microM) caused a fivefold increase of ACh levels in wild-type animals but only a 50% increase in AChE-deficient mice. In conclusion, absence of AChE causes dynamic changes in the ratio of choline to ACh. High levels of extracellular ACh are accompanied by reduced levels of extracellular choline, and ACh release becomes strongly dependent on choline availability. Similar changes may take place in patients chronically exposed to AChE inhibitors.

Metrifonate for Alzheimer's disease

BACKGROUND

Metrifonate is a long-acting irreversible cholinesterase inhibitor, originally used to treat schistosomiasis. Its potential to enhance central nervous system cholinergic neurotransmission led to clinical trials for the treatment of people with Alzheimer's disease (AD). Although low incidence of serious side effects occurred during short-term use as an antihelmintic, in studies of the treatment of AD extending over 6 months, 20 patients experienced respiratory paralysis and problems with neuromuscular transmission. These findings have led to a halt to trials of metrifonate for AD and Bayer, the pharmaceutical company, has withdrawn its FDA application.

OBJECTIVES

1) To establish the efficacy of metrifonate for patients with Alzheimer's disease, in terms of cognition, global impression, functional activity, non cognitive symptoms, rate of institutionalization and mortality.2) Assess the safety and tolerability of metrifonate.

SEARCH STRATEGY

The Cochrane Dementia and Cognitive Improvement Group's Specialized Register was searched on 5 December 2005 using the term metrifonat*. This Register is regularly updated with records from all major health care databases (MEDLINE, EMBASE, CINAHL, PsycINFO) and many trials databases. One of the authors (LS), as member of the Metrifonate Study Group has had the opportunity to contact other metrifonate trialists to obtain data from potentially non published data of metrifonate clinical trials.

SELECTION CRITERIA

All unconfounded, randomized double-blind clinical controlled trials comparing metrifonate to placebo in people with AD.

DATA COLLECTION AND ANALYSIS

Data were extracted by the two reviewers, cross-checked, and pooled when appropriate and possible.

MAIN RESULTS

Most studies assessed changes in cognitive function, global function, activities of daily living, behavioural problems, severity of disease and adverse events. Occasionally the results were not reported in sufficient detail to allow extraction of data for the meta-analyses. The treatment regimens were varied: loading doses were used in some trials. The range of maintenance doses and studies were not pooled unless the treatment regimens were considered comparable. The lengths of treatment varied from 6 to 26 weeks and studies were not pooled unless the treatment duration was similar. The results are derived from the ITT populations. Metrifonate at various doses, fixed and loading doses, was associated with significant cognitive improvement compared to placebo, except for weekly doses where there was no difference from placebo: MMSE (metrifonate 60-80 mg/day with initial loading at 26 weeks; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD 1.85, 95% CI 1.06 to 2.64, p<0.00001); ADAS-Cog (metrifonate 60-80 mg/day with initial loading at 26 weeks MD -3.24, 95% CI -4.40 to -2.08, p<0.00001)In most trials, there was improvement in clinical global impression: CIBIC-Plus (metrifonate 30-55 mg/day, approximately 0.65 mg/kg body weight, with initial loading at 26 weeks MD -0.25, 95% CI -0.41 to -0.09 p=0.002; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD -0.20, 95% CI -0.39 to -0.01, p=0.04). There were generally-significant drug-placebo differences in activities of daily living but this often depended on sample size and the characteristics of the instrument used: DAD (metrifonate 30-55 mg/day, 0.65 mg/kg body weight, with initial loading at 26 weeks MD 2.72, 95% CI 0.66 to 4.77, p=0.01; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD 4.07, 95% CI 0.29 to 7.85, p=0.03)Also there were differences associated with metrifonate compared with placebo for different doses of metrifonate in scores on a behavioural symptom scale, caregiver burden scale, and severity of disease scale. Adverse events occurring more often with metrifonate included abdominal pain, bloating, bradycardia, diarrhoea, leg cramps, nausea and rhinitis and were described as mostly mild and transient, but occasionally moderately severe, and infrequently severe and serious. Analysis of the number of patients suffering at least one mild, moderate, severe or serious adverse event before the end of treatment showed that there was usually no difference between placebo and metrifonate.

AUTHORS' CONCLUSIONS

Metrifonate given once per day appears to be related to clinical response in cognition, global improvement, and activities of daily living in patients with mild to moderate Alzheimer's disease. Tolerability is good with adverse events as expected from a cholinesterase inhibitor, but with a low incidence of neuromuscular dysfunction and respiratory failure, too low to be detected in this review. It has been withdrawn from further development.

Trafficking of the muscarinic m2 autoreceptor in cholinergic basalocortical neurons in vivo: differential regulation of plasma membrane receptor availability and intraneuronal localization in acetylcholinesterase-deficient and -inhibited mice

In vivo, the abundance of receptors at the neuronal plasma membrane may be critical in the mediation of pre- and postsynaptic responses. Thus, we have studied the membrane availability and intraneuronal distribution of the m2 muscarinic autoreceptor (m2R) in cholinergic neurons of the nucleus basalis magnocellularis (NBM) projecting to the frontal cortex (FC). We have studied the subcellular compartmentalization of m2R at somatodendritic postsynaptic and axonal presynaptic sites in control animals (AChE +/+) and in two animal models: mice displaying acute acetylcholinesterase (AChE) inhibition by treatment with metrifonate, and AChE-deficient mice (AChE -/-). In control animals, m2R was mainly located at the plasma membrane in the somatodendritic field of NBM and in cortical varicosities. Acute AChE inhibition and chronic AChE deficiency induced a dramatic decrease of cell surface m2R in the somatodendritic compartment. This finding was associated with two different intracytoplasmic events: (1). internalization of m2R in endosomes after acute AChE inhibition, (2). exaggerated storage of m2R in the endoplasmic reticulum and Golgi complex in AChE -/- mice. In contrast, the m2R density was higher at the membrane of cortical varicosities in AChE -/- mice but unchanged in acutely AChE-inhibited mice. Our data demonstrate that acute and chronic stimulation provoke, in vivo, depletion of the membrane store of somatodendritic m2R through different intracellular mechanisms: endocytosis of receptors from the plasma membrane to the cytoplasm (acute) or regulation of their delivery from intracytoplasmic stores to the plasma membrane (chronic). The increase of m2R at the membrane of axonal varicosities after chronic stimulation suggest modulation of presynaptic cholinergic activity, including neurotransmitter release.

Acute and chronic acetylcholinesterase inhibition regulates in vivo the localization and abundance of muscarinic receptors m2 and m4 at the cell surface and in the cytoplasm of striatal neurons

Acetylcholinesterase inhibitors (AChE-I) of various pharmacological classes have been used to provoke acute and chronic hypercholinergy in brain. Each condition induces a dramatic decrease of the abundance of muscarinic receptors at the membrane of neurons with simultaneous increase of these receptors in the cytoplasm in association with different subcellular organelles with characteristics depending on the duration of the treatment (short-term versus long term treatment). Each condition also induces a dramatic increase of cytoplasmic receptors associated with endosomes and multivesicular bodies. Chronic treatment with MTF induces a general decrease of m4R in the striatum without modification of the mRNA level but with an exaggerated abundance of muscarinic receptors in the cytoplasm at the sites of synthesis and maturation, i.e., endoplasmic reticulum, nuclear membrane and Golgi apparatus. These results suggest that the membrane abundance and intraneuronal distribution of neurotransmitter receptors are modified following drug treatment with specificity depending on the nature and the duration of treatment.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

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.