Inhibitory effects of donepezil hydrochloride (E2020) on cholinesterase activity in brain and peripheral tissues of young and aged rats

Effect of donepezil hydrochloride on the transgenic Drosophila expressing human Aβ-42

CONCLUSION

The results suggest that donepezil hydrochloride is potent enough to reduce the AD symptoms being mimicked in transgenic flies.

Donepezil inhibits neuromuscular junctional acetylcholinesterase and enhances synaptic transmission and function in isolated skeletal muscle

BACKGROUND AND PURPOSE

Donepezil, a piperidine inhibitor of acetylcholinesterase (AChE) prescribed for treatment of Alzheimer's disease, has adverse neuromuscular effects in humans, including requirement for higher concentrations of non-depolarising neuromuscular blockers during surgery. Here, we examined the effects of donepezil on synaptic transmission at neuromuscular junctions (NMJs) in isolated nerve-muscle preparations from mice.

EXPERIMENTAL APPROACH

We measured effects of therapeutic concentrations of donepezil (10 nM to 1 μM) on AChE enzymic activity, muscle force responses to repetitive stimulation, and spontaneous and evoked endplate potentials (EPPs) recorded intracellularly from flexor digitorum brevis muscles from CD01 or C57BlWldS mice.

KEY RESULTS

Donepezil inhibited muscle AChE with an approximate IC50 of 30 nM. Tetanic stimulation in sub-micromolar concentrations of donepezil prolonged post-tetanic muscle contractions. Preliminary Fluo4-imaging indicated an association of these contractions with an increase and slow decay of intracellular Ca2+ transients at motor endplates. Donepezil prolonged spontaneous miniature EPP (MEPP) decay time constants by about 65% and extended evoked EPP duration almost threefold. The mean frequency of spontaneous MEPPs was unaffected but the incidence of 'giant' MEPPs (gMEPPs), some exceeding 10 mV in amplitude, was increased. Neither mean MEPP amplitude (excluding gMEPPs), mean EPP amplitude, quantal content or synaptic depression during repetitive stimulation were significantly altered by concentrations of donepezil up to 1 μM.

CONCLUSION AND IMPLICATIONS

Adverse neuromuscular signs associated with donepezil therapy, including relative insensitivity to neuromuscular blockers, are probably due to inhibition of AChE at NMJs, prolonging the action of ACh on postsynaptic nicotinic acetylcholine receptors but without substantively impairing evoked ACh release.

Analgesic and preventive effects of donepezil in animal models of chemotherapy-induced peripheral neuropathy: Involvement of spinal muscarinic acetylcholine M2 receptors

BACKGROUND

Donepezil, a cholinesterase inhibitor approved in Alzheimer's disease, has demonstrated analgesic and preventive effects in animal models of oxaliplatin-induced neuropathy. To improve the clinical interest of donepezil for the management and prevention of chemotherapy-induced peripheral neuropathy (CIPN), a broader validation is required in different animal models of CIPN.

METHODS

using rat models of CIPN (bortezomib, paclitaxel, and vincristine), the analgesic and preventive efficacies of donepezil were evaluated on tactile, cold and heat hypersensitivities. The involvement of muscarinic M2 acetylcholine receptors (m2AChRs) in analgesic effects was investigated at the spinal level. The absence of interference of donepezil with the cytotoxic effect of chemotherapy has been controlled in cancer cell lines.

RESULTS

the analgesic efficacy of donepezil was demonstrated for all CIPN models, mainly on tactile hypersensitivity (maximal efficacy at 60 min, p < 0.05 vs. vehicle group). This effect was suppressed by an intrathecal injection of methoctramine (m2AChR antagonist). Regarding preventive effects, donepezil limited tactile hypersensitivity induced by paclitaxel, but not for other CIPN models. Donepezil did not modify the viability of cancer cells or the efficacy of anticancer drugs.

CONCLUSIONS

donepezil had a broad analgesic effect on animal models of CIPN and this effect involved spinal m2AChRs. This work validates the repositioning of donepezil in the management of CIPN.

Adverse Drug Reactions of Acetylcholinesterase Inhibitors in Older People Living with Dementia: A Comprehensive Literature Review

The rising of global geriatric population has contributed to increased prevalence of dementia. Dementia is a neurodegenerative disease, which is characterized by progressive deterioration of cognitive functions, such as judgment, language, memory, attention and visuospatial ability. Dementia not only has profoundly devastating physical and psychological health outcomes, but it also poses a considerable healthcare expenditure and burdens. Acetylcholinesterase inhibitors (AChEIs), or so-called anti-dementia medications, have been developed to delay the progression of neurocognitive disorders and to decrease healthcare needs. AChEIs have been widely prescribed in clinical practice for the treatment of Alzheimer's disease, which account for 70% of dementia. The rising use of AChEIs results in increased adverse drug reactions (ADRs) such as cardiovascular and gastrointestinal adverse effects, resulting from overstimulation of peripheral cholinergic activity and muscarinic receptor activation. Changes in pharmacokinetics (PK), pharmacodynamics (PD) and pharmacogenetics (PGx), and occurrence of drug interactions are said to be major risk factors of ADRs of AChEIs in this population. To date, comprehensive reviews in ADRs of AChEIs have so far been scarcely studied. Therefore, we aimed to recapitulate and update the diverse aspects of AChEIs, including the mechanisms of action, characteristics and risk factors of ADRs, and preventive strategies of their ADRs. The collation of this knowledge is essential to facilitate efforts to reduce ADRs of AChEIs.

[Donepezil Reduces Amyloid Precursor Protein Endocytosis by Resulting from Increase in the Expression of Sorting Nexin Protein 33]

Donepezil, the most widely used drug for the treatment of Alzheimer's disease (AD), is an acetylcholinesterase (AChE) inhibitor and is thought to improve cognition by stimulating cholinergic neurotransmission. However, no correlation has yet been established between the inhibitory role of AChE inhibitors and their therapeutic effects when used in AD patients. The cleavage pathway of amyloid precursor protein (APP) includes amyloidgenic (β, γ-cleavage) and non-amyloidgenic (α-cleavage) pathways. The intracellular transportation of APP is important in determining these cleavage pathways. It has been suggested that sorting nexin (SNX) family proteins regulates the intracellular transport of APP, thereby enhancing α-cleavage. In this study, we examined the effects of donepezil on SNX33 expression changes and APP processing in primary cultures of fetal rat cortical neurons. While donepezil treatment increased the levels of SNX33 expression and soluble APPα (sAPPα) in culture media, no changes were observed regarding full-length APP expression in the cell lysate. Donepezil also reduced the release of amyloid β (Aβ) into culture media in a concentration- and time-dependent manner. This reduction was not affected by acetylcholine receptor antagonists. The membrane surface expression of APP was elevated by donepezil. Furthermore, SNX knockdown by antisense morpholino oligos prevented the effects of donepezil. These results indicated that donepezil increased APP expression at the surface of the plasma membrane by decreasing APP endocytosis through upregulation of SNX33, suggesting donepezil might stimulate the non-amyloidogenic pathway. This new mechanism of action for the currently used anti-AD drug may provide a valuable basis for future drug discovery.

Neurobiological and Neurophysiological Mechanisms Underlying Nicotine Seeking and Smoking Relapse

Tobacco-related morbidity and mortality continue to be a significant public health concern. Unfortunately, current FDA-approved smoking cessation pharmacotherapies have limited efficacy and are associated with high rates of relapse. Therefore, a better understanding of the neurobiological and neurophysiological mechanisms that promote smoking relapse is needed to develop novel smoking cessation medications. Here, we review preclinical studies focused on identifying the neurotransmitter and neuromodulator systems that mediate nicotine relapse, often modeled in laboratory animals using the reinstatement paradigm, as well as the plasticity-dependent neurophysiological mechanisms that facilitate nicotine reinstatement. Particular emphasis is placed on how these neuroadaptations relate to smoking relapse in humans. We also highlight a number of important gaps in our understanding of the neural mechanisms underlying nicotine reinstatement and critical future directions, which may lead toward the development of novel, target pharmacotherapies for smoking cessation.

Acute caffeine ingestion affects surround suppression of perceived contrast

Caffeine is a widely used psychostimulant that is associated with increased acetylcholine levels in mammalian brain and acetycholinesterase antagonism. Acetylcholine, a neuromodulator, plays an important role in the processing of visual information. One key example in human vision, thought to at least partly involve cholinergic neuromodulation, is perceptual surround suppression of contrast, whereby the perceived contrast of a pattern is altered by the presence of a neighbouring pattern. Perceptual surround suppression is weaker with pharmacological administration of donepezil (a centrally-acting acetylcholine enzyme inhibitor) in healthy human observers. Here, we test whether temporarily manipulating caffeine levels (from complete washout to a controlled dose of caffeine) has a similar effect on perceptual surround suppression in 21 healthy young adults (aged 20-24 years, 11 females). Neither ingestion of a caffeine pill nor placebo altered contrast judgments when the target pattern was presented on a uniform grey background ( p=0.54). With caffeine ingestion, perceptual surround suppression strength was reduced relative to baseline (prior to pill ingestion, p=0.003) and placebo ( p=0.029), irrespective of whether the surround was oriented parallel or orthogonal to the central target. While daily habitual caffeine consumption of low-to-moderate doses (<400 mg/day, estimated from a written questionnaire) is not predictive of performance, our study indicates that acute consumption of caffeine on the day of testing influences perceptual surround suppression strength. Perceptual surround suppression is predominantly attributed to inhibitory processes involving the major cortical inhibitory neurotransmitter, gamma-aminobutyric acid. Our results point to the involvement of other neuromodulators, possibly cholinergic, in perceptual surround suppression.

Inhibitory effect of donepezil on bradykinin-induced increase in the intracellular calcium concentration in cultured cortical astrocytes

Donepezil is a potent and selective acetylcholinesterase inhibitor developed for the treatment of Alzheimer's disease. In the present study, we investigated the responses of astrocytes to bradykinin, an inflammatory mediator, and the effect of donepezil on these responses using cultured cortical astrocytes. Bradykinin induced a transient increase of intracellular calcium concentration ([Ca²⁺]_i) in cultured astrocytes. Bradykinin-induced [Ca²⁺]_i increase was inhibited by the exposure to thapsigargin, which depletes Ca²⁺ stores on endoplasmic reticulum, but not by the exclusion of extracellular Ca²⁺. Twenty four hours pretreatment of donepezil reduced the bradykinin-induced [Ca²⁺]_i increase. This reduction was inhibited not only by mecamylamine, a nAChR antagonist, but also by PI3K and Akt inhibitors. In addition, donepezil inhibited bradykinin-induced increase of the intracellular reactive oxygen species level in astrocytes. These results suggest that donepezil inhibits the inflammatory response induced by bradykinin via nAChR and PI3K-Akt pathway in astrocytes.

Donepezil regulates energy metabolism and favors bone mass accrual

The autonomous nervous system regulates bone mass through the sympathetic and parasympathetic arms. The sympathetic nervous system (SNS) favors bone loss whereas the parasympathetic nervous system (PNS) promotes bone mass accrual. Donepezil, a central-acting cholinergic agonist, has been shown to down-regulate SNS and up-regulate PNS signaling tones. Accordingly, we hypothesize that the use of donepezil could have beneficial effects in regulating bone mass. To test our hypothesis, two groups of healthy female mice were treated either with donepezil or saline. Differences in body metabolism and bone mass of the treated groups were compared. Body and visceral fat weights as well as serum leptin level were increased in donepezil-treated mice compared to control, suggesting that donepezil effects on SNS influenced metabolic activity. Donepezil-treated mice had better bone quality than controls due to a decrease in osteoclasts number. These results indicate that donepezil is able to affect whole body energy metabolism and favors bone mass in young female WT mice.

Repeated administration of an acetylcholinesterase inhibitor attenuates nicotine taking in rats and smoking behavior in human smokers

Tobacco smoking remains the leading cause of preventable death worldwide and current smoking cessation medications have limited efficacy. Thus, there is a clear need for translational research focused on identifying novel pharmacotherapies for nicotine addiction. Our previous studies demonstrated that acute administration of an acetylcholinesterase inhibitor (AChEI) attenuates nicotine taking and seeking in rats and suggest that AChEIs could be repurposed for smoking cessation. Here, we expand upon these findings with experiments designed to determine the effects of repeated AChEI administration on voluntary nicotine taking in rats as well as smoking behavior in human smokers. Rats were trained to self-administer intravenous infusions of nicotine (0.03 mg kg(-1) per 0.59 ml) on a fixed-ratio-5 schedule of reinforcement. Once rats maintained stable nicotine taking, galantamine or donepezil was administered before 10 consecutive daily nicotine self-administration sessions. Repeated administration of 5.0 mg kg(-1) galantamine and 3.0 mg kg(-1) donepezil attenuated nicotine self-administration in rats. These effects were reinforcer-specific and not due to adverse malaise-like effects of drug treatment as repeated galantamine and donepezil administration had no effects on sucrose self-administration, ad libitum food intake and pica. The effects of repeated galantamine (versus placebo) on cigarette smoking were also tested in human treatment-seeking smokers. Two weeks of daily galantamine treatment (8.0 mg (week 1) and 16.0 mg (week 2)) significantly reduced smoking rate as well as smoking satisfaction and reward compared with placebo. This translational study indicates that repeated AChEI administration reduces nicotine reinforcement in rats and smoking behavior in humans at doses not associated with tolerance and/or adverse effects.

Dose-dependent sigma-1 receptor occupancy by donepezil in rat brain can be assessed with (11)C-SA4503 and microPET

RATIONALE

Sigma-1 receptor agonists are under investigation as potential disease-modifying agents for several CNS disorders. Donepezil, an acetylcholinesterase inhibitor used for the symptomatic treatment of Alzheimer's disease, is also a high-affinity sigma-1 agonist.

OBJECTIVES

The objectives of the present study were to investigate if the sigma-1 agonist tracer (11)C-SA4503 and microPET can be used to determine sigma-1 receptor occupancy (RO) of donepezil in the rat brain; to establish RO of donepezil at doses commonly used in rodent behavioural studies; and to determine the effective plasma concentration of donepezil required for 50 % of max-min occupancy (EC50).

METHODS

Male Wistar rats were pre-treated with donepezil (0.1 to 10 mg/kg) for about 1 h before microPET scans using (11)C-SA4503. The total distribution volume (V T) of the tracer was determined by Logan graphical analysis using time activity curves from arterial plasma and regions of interest drawn around the entire brain and individual brain regions. RO by donepezil was calculated from a modified Lassen plot, and ED50 was estimated from the sigmoidal dose-response curves obtained when the RO was plotted against log donepezil dose.

RESULTS

A dose-dependent reduction was observed for V T in the whole brain as well as individual brain regions. RO increased dose-dependently and was 93 % at 10 mg/kg. ED50 was 1.29 mg/kg.

CONCLUSIONS

Donepezil, in the common dose range, was found to dose-dependently occupy a significant fraction of the sigma-1 receptor population. The data indicate that it is possible to determine sigma-1 RO by an agonist drug in rat brain, using (11)C-SA4503 and microPET.

Distribution of intravenously administered acetylcholinesterase inhibitor and acetylcholinesterase activity in the adrenal gland: 11C-donepezil PET study in the normal rat

Purpose

Acetylcholinesterase (AChE) inhibitors have been used for patients with Alzheimer's disease. However, its pharmacokinetics in non-target organs other than the brain has not been clarified yet. The purpose of this study was to evaluate the relationship between the whole-body distribution of intravenously administered ¹¹C-Donepezil (DNP) and the AChE activity in the normal rat, with special focus on the adrenal glands.

Methods

The distribution of ¹¹C-DNP was investigated by PET/CT in 6 normal male Wistar rats (8 weeks old, body weight  = 220±8.9 g). A 30-min dynamic scan was started simultaneously with an intravenous bolus injection of ¹¹C-DNP (45.0±10.7 MBq). The whole-body distribution of the ¹¹C-DNP PET was evaluated based on the Vt (total distribution volume) by Logan-plot analysis. A fluorometric assay was performed to quantify the AChE activity in homogenized tissue solutions of the major organs.

Results

The PET analysis using Vt showed that the adrenal glands had the 2nd highest level of ¹¹C-DNP in the body (following the liver) (13.33±1.08 and 19.43±1.29 ml/cm³, respectively), indicating that the distribution of ¹¹C-DNP was the highest in the adrenal glands, except for that in the excretory organs. The AChE activity was the third highest in the adrenal glands (following the small intestine and the stomach) (24.9±1.6, 83.1±3.0, and 38.5±8.1 mU/mg, respectively), indicating high activity of AChE in the adrenal glands.

Conclusions

We demonstrated the whole-body distribution of ¹¹C-DNP by PET and the AChE activity in the major organs by fluorometric assay in the normal rat. High accumulation of ¹¹C-DNP was observed in the adrenal glands, which suggested the risk of enhanced cholinergic synaptic transmission by the use of AChE inhibitors.

Insights into direct nose to brain delivery: current status and future perspective

Now a day's intranasal (i.n) drug delivery is emerging as a reliable method to bypass the blood-brain barrier (BBB) and deliver a wide range of therapeutic agents including both small and large molecules, growth factors, viral vectors and even stem cells to the brain and has shown therapeutic effects in both animals and humans. This route involves the olfactory or trigeminal nerve systems which initiate in the brain and terminate in the nasal cavity at the olfactory neuroepithelium or respiratory epithelium. They are the only externally exposed portions of the central nervous system (CNS) and therefore represent the most direct method of noninvasive entry into the brain. This approach has been primarily used to explore therapeutic avenues for neurological diseases. The potential for treatment possibilities with olfactory transfer of drugs will increase as more effective formulations and delivery devices are developed. Recently, the apomorphine hydrochloride dry powders have been developed for i.n. delivery (Apomorphine nasal, Lyonase technology, Britannia Pharmaceuticals, Surrey, UK). The results of clinical trial Phase III suggested that the prepared formulation had clinical effect equivalent to subcutaneously administered apomorphine. In coming years, intranasal delivery of drugs will demand more complex and automated delivery devices to ensure accurate and repeatable dosing. Thus, new efforts are needed to make this noninvasive route of delivery more efficient and popular, and it is also predicted that in future a range of intranasal products will be used in diagnosis as well as treatment of CNS diseases. This review will embark the existing evidence of nose-to-brain transport. It also provides insights into the most relevant pre-clinical studies of direct nose-brain delivery and delivery devices which will provide relative success of intranasal delivery system. We have, herein, outlined the relevant aspects of CNS drugs given intranasally to direct the brain in treating CNS disorders like Alzheimer's disease, depression, migraine, schizophrenia, etc.

Development and evaluation of rivastigmine loaded chitosan nanoparticles for brain targeting

The rivastigmine (RHT) loaded chitosan nanoparticles (CS-RHT NPs) were prepared by ionic gelation method to improve the bioavailability and enhance the uptake of RHT to the brain via intranasal (i.n.) delivery. CS-RHT NPs were characterized for particles size, particle size distribution (PDI), encapsulation efficiency, zeta potential and in vitro release study. Nose-to-brain delivery of placebo nanoparticles (CS-NPs) was investigated by confocal laser scanning microscopy technique using rhodamine-123 as a marker. The brain/blood ratio of RHT for different formulations were 0.235, 0.790 and 1.712 of RHT (i.v.), RHT (i.n.), and CS-RHT NPs (i.n.) respectively at 30 min are indicative of direct nose to brain transport bypassing the BBB. The brain concentration achieved from i.n. administration of CS-NPs (966 ± 20.66 ng ml(-1); t(max) 60 min) was significantly higher than those achieved after i.v. administration of RHT sol (387 ± 29.51 ngml(-1); t(max) 30 min), and i.n. administration of RHT solution (508.66 ± 22.50 ng ml(-1); t(max) 60 min). The higher drug transport efficiency (355 ± 13.52%) and direct transport percentage (71.80 ± 6.71%) were found with CS-RHT NPs as compared to other formulation. These results suggest that CS-RHT NPs have better brain targeting efficiency and are a promising approach for i.n. delivery of RHT for the treatment and prevention of Alzheimer's disease (AD).

Investigation of the in vitro and ex vivo acetylcholinesterase and antioxidant activities of traditionally used Lycopodium species from South America on alkaloid extracts

ETHNOPHARMACOLOGICAL RELEVANCE

The study was aimed at evaluating medicinal and therapeutic potentials of two Lycopodiaceae species, Lycopodium clavatum (L.) and Lycopodium thyoides (Humb. & Bonpl. ex Willd), both used in South American folk medicine for central nervous system conditions. Alkaloid extracts were evaluated for chemical characterization, acetylcholinesterase and antioxidant activities.

MATERIALS AND METHODS

The alkaloid extracts obtained by alkaline extraction were determined for each species by GC/MS examination. The evaluation of the anticholinesterase and the antioxidant activities of the extracts were tested by determining in vitro and ex vivo models. Effects on acetylcholinesterase (AChE) were tested in vitro using rat brain homogenates and ex vivo after a single administration (25, 10 and 1mg/kg i.p.) of the alkaloid extracts in mice. The in vitro antioxidant effects were tested for the 2-deoxyribose degradation, nitric oxide (NO) interaction, 2,2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activity and total reactive antioxidant potential (TRAP). After an acute administration (25 and 10mg/kg i.p.) of the extracts in middle-aged (12 months) mice, the antioxidant effects were estimated through the thiobarbituric acid reactive substances test (TBARS), and the antioxidant enzymes activities for catalase (CAT) and superoxide dismutase (SOD) were measured.

RESULTS

AChE activity was inhibited in vitro by the alkaloid-enriched extracts of both Lycopodium species in a dose and time-dependent manner in rat cortex, striatum and hippocampus. A significant inhibition was also observed in areas of the brain after acute administration of extracts, as well as decreased lipid peroxidation and increased CAT activity in the cortex, hippocampus and cerebellum. A moderate antioxidant activity was observed in vitro for the extracts. Chemically, the main alkaloids found for the two species were lycopodine and acetyldihidrolycopodine.

CONCLUSION

This study showed that the biological properties of the folk medicinal plants Lycopodium clavatum and Lycopodium thyoides include AChE inhibitory activity and antioxidant effects, two possible mechanisms of action in Alzheimer's related processes.

ICV STZ induced impairment in memory and neuronal mitochondrial function: A protective role of nicotinic receptor

The present study was planned to evaluate the cholinergic influence on mitochondrial activity and neurodegeneration associated with impaired memory in intracerebroventricular (ICV) streptozotocin (STZ) treated rats. STZ (3mg/kg), administered ICV twice with an interval of 48h between the two doses, showed significant impairment in spatial memory tested by water maze test 14 days after first dose without altering blood glucose level and locomotor activity. Animals were sacrificed on 21st day of ICV administration. STZ significantly increased malondialdehyde (MDA), reactive oxygen species (ROS), Ca(2+) ion influx, caspase-3 activity and decreased glutathione (GSH) level. Acetylcholinesterase inhibitors tacrine and donepezil (5mg/kg, PO) pretreatment significantly prevented STZ induced memory deficit, oxidative stress, Ca(2+) influx and caspase-3 activity. Carbachol, a muscarinic cholinergic agonist (0.01mg/kg, SC) did not show any significant effect on ROS generation, Ca(2+) ion influx and caspase-3 activity. While nicotinic cholinergic agonist, nicotine, significantly attenuated ICV STZ induced mitochondrial dysfunction and caspase-3 activity. The results indicate that instead of muscarinic receptors nicotinic receptors may be involved in neuroprotection by maintaining mitochondrial functions.

Cholinergic enhancement increases the effects of voluntary attention but does not affect involuntary attention

Voluntary visual spatial attention can be allocated in a goal-oriented manner to locations containing behaviorally relevant information. In contrast, involuntary attention is automatically captured by salient events. Allocation of attention is known to be modulated by release of the neurotransmitter acetylcholine (ACh) in cerebral cortex. We used an anti-predictive spatial cueing task to assess the effects of pharmacological enhancement of cholinergic transmission on behavioral measures of voluntary and involuntary attention in healthy human participants. Each trial began with the presentation of a cue in a peripheral location. In 80% of the trials, a target then appeared in a location opposite the cue. In the remaining 20% of trials, the target appeared in the cue location. For trials with short stimulus onset asynchrony (SOA) between cue and target, involuntary capture of attention resulted in shorter reaction times (RTs) to targets presented at the cue location. For long SOA trials, allocation of voluntary attention resulted in the opposite pattern: RTs were shorter when the target appeared in the expected (opposite) location. Each subject participated in two sessions: one in which the cholinesterase inhibitor donepezil was administered to increase synaptic ACh levels and one in which placebo was administered. Donepezil selectively improved performance (reduced RT) for long SOA trials in which targets appeared in the expected location. Thus, cholinergic enhancement augments the benefits of voluntary attention but does not affect involuntary attention, suggesting that they rely on different neurochemical mechanisms.

Acetylcholinesterase inhibition in cognition-relevant brain areas of mice treated with a nootropic Amazonian herbal (Marapuama)

The goal of acetylcholinesterase inhibitors (AChEIs) used to treat Alzheimer's patients is an improvement in cholinergic transmission. While currently available AChEIs have limited success, a huge impediment to the development of newer ones is access to the relevant brain areas. Promnesic, anti-amnesic and AChEI properties were identified in a standardized ethanol extract from Ptychopetalum olacoides (POEE), a medicinal plant favored by the elderly in Amazon communities. The purpose of this study was to provide conclusive evidence that orally given POEE induces AChE inhibition in brain areas relevant to cognition. Histochemistry experiments confirmed that the anticholinesterase compound(s) present in POEE are orally bioavailable, inducing meaningful AChE inhibition in the hippocampus CA1 (∼33%) and CA3 (∼20%), and striatum (∼17%). Ellman's colorimetric analysis revealed that G1 and G4 AChE isoforms activities were markedly inhibited (66 and 72%, respectively) in hippocampus and frontal cortex (50 and 63%, respectively), while G4 appeared to be selectively inhibited (72%) in the striatum. Western blotting showed that POEE did not induce significant changes in the AChE immunocontent suggesting that its synthesis is not extensively modified. This study provides definitive proof of meaningful anticholinesterase activity compatible with the observed promnesic and anti-amnesic effects of POEE in mice, reaffirming the potential of this extract for treating neurodegenerative conditions where a hypofunctioning cholinergic neurotransmission is prominent. Adequate assessment of the safety and efficacy of this extract and/or its isolated active compound(s) are warranted.

Cholinergic enhancement augments magnitude and specificity of visual perceptual learning in healthy humans

Learning through experience underlies the ability to adapt to novel tasks and unfamiliar environments. However, learning must be regulated so that relevant aspects of the environment are selectively encoded. Acetylcholine (ACh) has been suggested to regulate learning by enhancing the responses of sensory cortical neurons to behaviorally relevant stimuli. In this study, we increased synaptic levels of ACh in the brains of healthy human subjects with the cholinesterase inhibitor donepezil (trade name: Aricept) and measured the effects of this cholinergic enhancement on visual perceptual learning. Each subject completed two 5 day courses of training on a motion direction discrimination task, once while ingesting 5 mg of donepezil before every training session and once while placebo was administered. We found that cholinergic enhancement augmented perceptual learning for stimuli having the same direction of motion and visual field location used during training. In addition, perceptual learning with donepezil was more selective to the trained direction of motion and visual field location. These results, combined with previous studies demonstrating an increase in neuronal selectivity following cholinergic enhancement, suggest a possible mechanism by which ACh augments neural plasticity by directing activity to populations of neurons that encode behaviorally relevant stimulus features.

Use of a novel radiometric method to assess the inhibitory effect of donepezil on acetylcholinesterase activity in minimally diluted tissue samples

BACKGROUND AND PURPOSE

Cholinesterase inhibitors have been widely used for the treatment of patients with dementia. Monitoring of the cholinesterase activity in the blood is used as an indicator of the effect of the cholinesterase inhibitors in the brain. The selective measurement of cholinesterase with low tissue dilution is preferred for accurate monitoring; however, the methods have not been established. Here, we investigated the effect of tissue dilution on the action of cholinesterase inhibitors using a novel radiometric method with selective substrates, N-[(14)C]methylpiperidin-4-yl acetate ([(14)C]MP4A) and (R)-N- [(14)C]methylpiperidin-3-yl butyrate ([(14)C]MP3B_R), for AChE and butyrylcholinesterase (BChE) respectively.

EXPERIMENTAL APPROACH

We investigated the kinetics of hydrolysis of [(14)C]-MP4A and [(14)C]-MP3B_R by cholinesterases, and evaluated the selectivity of [(14)C]MP4A and [(14)C]MP3B_R for human AChE and BChE, respectively, compared with traditional substrates. Then, IC(50) values of cholinesterase inhibitors in minimally diluted and highly diluted tissues were measured with [(14)C]MP4A and [(14)C]MP3B_R.

KEY RESULTS

AChE and BChE activities were selectively measured as the first-order hydrolysis rates of [(14)C]-MP4A and [(14)C]MP3B_R respectively. The AChE selectivity of [(14)C]MP4A was an order of magnitude higher than traditional substrates used for the AChE assay. The IC(50) values of specific AChE and BChE inhibitors, donepezil and ethopropazine, in 1.2-fold diluted human whole blood were much higher than those in 120-fold diluted blood. In addition, the IC(50) values of donepezil in monkey brain were dramatically decreased as the tissue was diluted.

CONCLUSIONS AND IMPLICATIONS

This method would effectively monitor the activity of cholinesterase inhibitors used for therapeutics, pesticides and chemical warfare agents.

The effect of donepezil treatment on cardiovascular mortality

The acetylcholinesterase inhibitor donepezil hydrochloride improves cognitive function in patients with Alzheimer's disease and vascular dementia. Given acetylcholine's important actions on the heart, we undertook a retrospective cohort investigation to assess whether donepezil usage affects cardiovascular mortality. In patients treated with donepezil, hazard ratios for total and cardiovascular mortality were 0.68 (P = 0.045, 95% confidence interval 0.46-0.99) and 0.54 (P = 0.042, 95% confidence interval 0.30-0.98), respectively. The apparent survival benefit in donepezil-treated patients should not be overinterpreted. Prospective clinical trials are warranted.

Ladostigil prevents age-related glial activation and spatial memory deficits in rats

Oxidative stress and glial activation occur in the aging brain. Ladostigil is a new monoamine oxidase (MAO) and acetylcholinesterase (AChE) inhibitor designed for the treatment of Alzheimer's disease. It has neuroprotective and antioxidant activities in cellular models at much lower concentrations than those inhibiting MAO or AChE. When ladostigil (1mg/kg/day) was given for 6 months to 16-month-old rats it prevented the age-related increase in activated astrocytes and microglia in several hippocampal and white matter regions and increased proNGF immunoreactivity in the hippocampus towards the levels in young rats. Ladostigil also prevented the age-related reduction in cortical AChE activity and the increase in butyrylcholinesterase activity in the hippocampus, in association with the reduction in gliosis. The immunological and enzymatic changes in aged rats were associated with improved spatial memory. Ladostigil treatment had no effect on memory, glial or proNGF immunoreactivity in young rats. Early treatment with ladostigil could slow disease progression in conditions like Alzheimer's disease in which oxidative stress and inflammatory processes are present.

Effect of donepezil and tacrine on oxidative stress in intracerebral streptozotocin-induced model of dementia in mice

Oxidative stress is a major factor implicated in the degeneration of cholinergic neurons in Alzheimer's disease. Presently, cholinesterase inhibitors are the mainstay of therapy for Alzheimer's disease. However, the potential of cholinesterase inhibitors as antioxidants, an important aspect for neuroprotection, has not been properly investigated. Therefore, the present study was designed to investigate the influence of antidementia drugs, tacrine and donepezil, on biochemical markers of oxidative stress, glutathione (GSH) and malondialdehyde (MDA), and acetylcholinesterase activity in the brain in a streptozotocin-induced experimental model of dementia in mice. Intracerebral (i.c.) injection of streptozotocin at a dose of 0.5 mg/kg on 1st and 3rd days caused significant deficits in memory function, as evaluated in a passive avoidance test and Morris Water Maze (spatial memory) test 14 days after the 1st dose. Mice were treated with tacrine and donepezil at a dose of 5 mg/kg orally in separate groups. Both tacrine- and donepezil-treated mice showed a significant improvement of the streptozotocin (i.c.)-induced memory impairment. Streptozotocin (i.c.) administration caused a significant decrease in GSH and increase in MDA as compared to control, indicating a state of oxidative stress in the brain of streptozotocin (i.c.) amnesic mice. Treatment of streptozotocin (i.c.) amnesic mice with tacrine or donepezil did not cause significant changes in GSH and MDA levels in the brain as compared to control. Streptozotocin amnesic mice had raised acetylcholinesterase activity in the brain while there was a significant decrease in brain acetylcholinesterase activity in tacrine- and donepezil-treated streptozotocin (i.c.) mice. Thus, results indicate that tacrine and donepezil, beside inhibition of acetylcholinesterase, may also suppress oxidative stress.

Combination of rimonabant and donepezil prolongs spatial memory duration

The observations that the cannabinoid(1)(CB(1)) receptor antagonist/inverse agonist, rimonabant, and the selective noncompetitive inhibitor of acetylcholinesterase (AChE), donepezil, improve performance in a variety of animal memory models, suggest that these neurochemical systems play integral roles in cognition. The present study tested whether each of these agents administered alone or in combination will prolong the duration of spatial memory. Rats were trained in a two-phase radial-arm maze procedure, consisting of acquisition and retrieval tests, which were separated by an 18 h delay. Each drug was administered 30 min before the acquisition phase, immediately after the acquisition phase, or 30 min before the retrieval test to assess acquisition/consolidation, consolidation, and retrieval mnemonic processes, respectively. Rimonabant or donepezil administered before the acquisition phase, but not immediately after acquisition or before retrieval, led to a significant decrease in the number of errors committed during the retrieval test. Combined administration of subthreshold doses of rimonabant and donepezil that had no discernable effects on performance when given alone, enhanced memory. These results taken together demonstrate that the delay radial-arm maze task is sufficiently sensitive to detect memory enhancing effects of these drugs. Moreover, these findings suggest that combined administration of subthreshold doses of rimonabant and donepezil can improve memory and may represent a novel approach to treat cognitive deficits associated with neurodegenerative disorders.

Relevance of Donepezil in Enhancing Learning and Memory in Special Populations: A Review of the Literature

This review discusses the laboratory and clinical research supporting the rationale for the efficacy of donepezil (Aricept USA) in enhancing cognition in autism, Alzheimer disease, Down syndrome, traumatic brain injury, Attention Deficit Hyperactivity Disorder (ADHD), and schizophrenia. While preliminary animal models have shown effective, human studies exclusive of Alzheimer disease are sparse. Although attention and memory are unlikely a sole operation of the cholinergic system, evidence indicates a promising direction for further examination of this hypothesis in autism. Studies that examine changes in operationally defined behaviors and reliable and valid measure of changes in attention and memory are needed.

Effects on Cholinergic Markers in Rat Brain and Blood after Short and Prolonged Administration of Donepezil

Donepezil is a selective inhibitor of acetylcholinesterase (AChE) clinically used for treating Alzheimer's disease. Cholinergic effects after short-term exposure of donepezil (up to 12 h) have been extensively studied in rats, but few have addressed the potential long-term effects. After 14 days administration (1x3 mg/kg, decapitation 4 h after the last injection) the cerebral acetylcholine level was increased by 35% and the AChE activity was decreased by 66% and 32% in brain and blood, respectively. No change was detected in choline acetyltransferase activity, or the levels of vesicular acetylcholine transporter, choline transporter, or muscarinic receptors. Expression of various cholinergic genes was unaffected. Preliminary results of AChE activity in human blood showed 60-97% and 43-89% of pre-exposed level after one and three days of donepezil administration (5 mg daily), respectively. In conclusion, donepezil exposure in rats at doses that do not inhibit brain AChE continuously during the day, will not lead to tolerance development.

(10) Ex vivo anticholinesterase activity of benzoic acid derivatives

p-Aminobenzoic acid (p-ABA) derivatives were evaluated as acetylcholinesterase inhibitors (AChEIs). Finding a correlation between AChE activity with the partition coefficient (log P) and the highest occupied molecular orbital (HOMO) energy of the compounds.

Effect of donepezil and lercanidipine on memory impairment induced by intracerebroventricular streptozotocin in rats

Intracerebroventricular (ICV) injection of streptozotocin (STZ) causes cognitive impairment in rats. ICV STZ is known to impair cholinergic neurotransmission by decreasing choline acetyltransferase (ChAT) levels, glucose and energy metabolism in brain and synthesis of acetyl CoA. However, no reports are available regarding the cholinesterase inhibitors in this model. In aging brain, reduced energy metabolism increases glutamate release, which is blocked by L-type calcium channel blockers. These calcium channel blockers have shown beneficial effects on learning and memory in various models of cognitive impairment. The present study was designed to investigate the influence of chronic administration of donepezil (cholinesterase inhibitor, 1 and 3 mg/kg) and lercanidipine (L-type calcium channel blocker, 0.3 and 1 mg/kg) on cognitive impairment in male Sprague-Dawley rats injected twice with ICV STZ (3 mg/kg) bilaterally on days 1 and 3. ICV STZ injected rats developed a severe deficit in learning and memory indicated by deficits in passive avoidance paradigm and elevated plus maze as compared to control rats. Cholinesterase activity in brain was significantly increased in ICV STZ injected rats. Donepezil dose-dependently inhibited cholinesterase activity and improved performance in memory tests at both the doses. Lercanidipine (0.3 mg/kg) showed significant improvement in memory. When administered together, the effect of combination of these two drugs on memory and cholinesterase activity was higher than that obtained with either of the drugs when used alone.

Brain levels and acetylcholinesterase inhibition with galantamine and donepezil in rats, mice, and rabbits

Galantamine is a rather weak acetylcholinesterase (AChE) inhibitor, currently approved for the symptomatic treatment of Alzheimer's disease, with possible additional allosteric potentiating effects at the nicotinic ACh receptor (nAChR). Earlier data from in vitro biochemical tests suggest that donepezil is 40- to 500-fold more potent than galantamine in inhibiting AChE. In this study, both brain levels and Ki values for AChE inhibition for donepezil and galantamine in rat, mouse, and rabbit after subcutaneous application were determined. Clearance of galantamine from the brain is in general faster that donepezil and is faster in rabbits compared to rats and mice. The brain-to-plasma ratio for galantamine and donepezil, respectively, ranges from 1.2 to 1.5 in the rabbit, 3.3 to 5.2 in the mouse, and 6.6 to 13 in the rat. Galantamine doses between 1.5 and 5 mg/kg are appropriate to reach brain levels within the documented optimal allosteric potentiating ligand dose-response. Ki values of brain AChE inhibition for galantamine and donepezil, respectively, are 7.1 and 2.3 microg/g in rats, 8.3 and 0.65 microg/g for mice, and 19.1 and 1.3 microg/g in rabbits. The data also suggest that for a similar degree of brain AChE inhibition, 3-15 times higher galantamine than donepezil doses are needed.

Comparison of the effect of TAK-147 (zanapezil) and E-2020 (donepezil) on extracellular acetylcholine level and blood flow in the ventral hippocampus of freely moving rats

The effects of zanapezil (TAK-147) and donepezil (E2020) on extracellular acetylcholine (ACh) levels were investigated by HPLC-microdialysis of ventral hippocampus (VH) in freely moving intact rats. The results showed that the basal ACh release rate in the VH is 116.7 +/- 12.4 to 158.4 +/- 22.86 fmol/20 microl. At 2, 5 and 10 mg/kg, single p.o., each drug increased ACh level by 9.4%, 106.5%, 50.8% (TAK-147) and 14.8%, 76.1%, 120.94% (E2020), respectively. The ED50 values were 4.52 mg/kg (1.43 - 14.29; R=0.52) and 4.07 mg/kg (1.77 - 9.37; R=0.985) for TAK-147 and E2020, respectively. Analysis of data revealed that the relative TAK-147/E2020 potency ratio is 0.773, but the effect of E2020 was accompanied by more prominent skeletal muscle fasciculation, gnawing, increased defecation and to lesser extent salivation. Moreover, the significant effect of TAK-147 was observed earlier (20 min) than E2020 (60 min). In this study, we also investigated the effect of both drugs at dose of 5 mg/kg p.o. on blood flow in the VH using Laser Doppler Flowmetry. The results showed that the average blood flow rate in the VH is 6.5 +/- 0.9 ml/min/100 g. TAK-147 did not change blood flow, but E2020 increased blood flow in a biphasic manner. The first increment was obtained between 5 and 40 min (11.5 +/- 2.2 to 12.7 +/- 2.2 ml/min/100 g), and the second one 80-105 min (10.7 +/- 1.6 to 13.4 +/- 3.6 ml/min/100 g). In conclusion, the present results indicate that both TAK-147 and E2020 increase ACh level in the VH. E2020 showed greater potency than TAK-147, but it induced more fasciculation and other side effects than TAK-147. Moreover, the blood flow increasing properties of E2020 could be beneficial in some patients with Alzheimer' disease especially those with chronic vascular dementia, but at the same time, it could also indicate less specific ACh increasing activity than TAK-147 and higher risk of cerebral hemorrhage. On the other hand, the fast and specific effect of TAK-147 may be useful for cure of early stages of Alzheimer's disease (AD).

Galantamine facilitates acquisition of hippocampus-dependent trace eyeblink conditioning in aged rabbits

Cholinergic systems are critical to the neural mechanisms mediating learning. Reduced nicotinic cholinergic receptor (nAChR) binding is a hallmark of normal aging. These reductions are markedly more severe in some dementias, such as Alzheimer's disease. Pharmacological central nervous system therapies are a means to ameliorate the cognitive deficits associated with normal aging and aging-related dementias. Trace eyeblink conditioning (EBC), a hippocampus- and forebrain-dependent learning paradigm, is impaired in both aged rabbits and aged humans, attributable in part to cholinergic dysfunction. In the present study, we examined the effects of galantamine (3 mg/kg), a cholinesterase inhibitor and nAChR allosteric potentiating ligand, on the acquisition of trace EBC in aged (30-33 months) and young (2-3 months) female rabbits. Trace EBC involves the association of a conditioned stimulus (CS) with an unconditioned stimulus (US), separated by a stimulus-free trace interval. Repeated CS-US pairings results in the development of the conditioned eyeblink response (CR) prior to US onset. Aged rabbits receiving daily injections of galantamine (Aged/Gal) exhibited significant improvements compared with age-matched controls in trials to eight CRs in 10 trial block criterion (P = 0.0402) as well as performance across 20 d of training [F(1,21) = 5.114, P = 0.0345]. Mean onset and peak latency of CRs exhibited by Aged/Gal rabbits also differed significantly [F(1,21) = 6.120/6.582, P = 0.0220/0.0180, respectively] compared with age-matched controls, resembling more closely CR timing of young drug and control rabbits. Galantamine did not improve acquisition rates in young rabbits compared with age-matched controls. These data indicate that by enhancing nicotinic and muscarinic transmission, galantamine is effective in offsetting the learning deficits associated with decreased cholinergic transmission in the aging brain.

Evaluation of the binding characteristics of [5-(11)C-methoxy]Donepezil in the rat brain for in vivo visualization of acetylcholinesterase

Donepezil, an acetylcholinesterase (AChE) inhibitor, has not been evaluated for its binding characteristics using a radioactive tracer, although its inhibitory action on AChE has been studied. The aim of this research is to examine whether AChE can be visualized in vivo and in vitro with [(11)C]donepezil. [5-(11)C-methoxy]Donepezil was synthesized by O-methylation using [(11)C]methyl triflate. The binding of [(11)C]donepezil to brain homogenates was higher in the brain stem and striatum, and it was lowest in the cerebellum. The in vitro autoradiographic study successfully demonstrated the specific binding of [(11)C]donepezil to AChE in the rat brain. The IC(50) value of binding was approximately 10 nM, which is comparable to the reported value for inhibiting enzyme activity (6 nM). Saturation experiments revealed that the B(max) and K(d) of [(11)C]donepezil binding in vitro are 65 fmol/mg tissue and 39.8 nM, respectively. In accordance with the in vitro bindings, the in vivo distribution of [(11)C]donepezil was heterogeneous in the rat brain. In the blocking experiments, the heterogeneous distribution disappeared in the presence of a large amount of unlabeled donepezil. These data suggest that [5-(11)C-methoxy]donepezil can be potentially useful to image AChE non-invasively in the human brain by positron emission tomography.

Combined treatment with galanthaminium bromide, a new cholinesterase inhibitor, and RS 67333, a partial agonist of 5-HT4 receptors, enhances place and object recognition in young adult and old rats

The present study was designed to investigate whether a combination of a new acetylcholinesterase inhibitor we have synthesized, galanthaminium bromide, and an agonist of 5-hydroxytryptamine(4) receptors, RS 67333, at doses ineffective alone, improves performance in tasks involving place and object recognition memory. Dose responses of each compound were determined in order to select doses without effect alone. Accordingly, young adult rats were injected intraperitoneally with galanthaminium bromide (0.3 mg/kg)+RS 67333 (0.01 mg/kg), and old rats with galanthaminium bromide (0.1 mg/kg for place and 0.3 mg/kg for object recognition)+RS 67333 (1 mg/kg). Drugs were injected before the acquisition phase, immediately after it, or before the retrieval phase to determine the stage of information processing affected by treatments. Doses of galanthaminium bromide and RS 67333, without effect on their own, jointly improved both place and object recognition in young adult rats via an enhancement of acquisition and consolidation information processing. In old rats, the combined treatment enhanced performance by acting on the acquisition processes of place recognition and on the acquisition and consolidation processes of object recognition. These results indicate that combining agents that act on different neuronal targets may be more powerful than either treatment alone, enabling use of lower doses of each compound, thereby attenuating the adverse effects of the individual drugs. A bitherapeutic strategy of this kind might thus be of interest in the treatment of the cognitive deficits related to "normal" or pathological aging.

Research and development of donepezil hydrochloride, a new type of acetylcholinesterase inhibitor

A wide range of evidence shows that cholinesterase (ChE) inhibitors can interfere with the progression of Alzheimer's disease (AD). The earliest known ChE inhibitors, namely, physostigmine and tacrine, showed modest improvement in the cognitive function of AD patients. However, clinical studies show that physostigmine has poor oral activity, brain penetration and pharmacokinetic parameters, while tacrine has hepatotoxic liability. Studies were then focused on finding a new type of acetylcholinesterase (AChE) inhibitor that would overcome the disadvantages of these two compounds. During the study, by chance we found a seed compound. We then conducted a structure-activity relationship study of this compound. After four years of exploratory research, we found donepezil hydrochloride (donepezil). Donepezil showed several positive characteristics including the following: 1) It has a novel structure compared to other conventional ChE inhibitors; 2) It shows strong anti-AChE activity and has long lasting efficacy; 3) The inhibitory characteristic of donepezil shows that it is highly selective for AChE as compared to butyrylcholinesterase (BuChE) and showed reversibility; 4) The results of clinical studies on donepezil show a very high significant difference on ADAS cog and CIBIC plus scores of AD patients. Donepezil is currently marketed in 56 countries all over the world.

Donepezil dose-dependently inhibits acetylcholinesterase activity in various areas and in the presynaptic cholinergic and the postsynaptic cholinoceptive enzyme-positive structures in the human and rat brain

In the symptomatic treatment of mild to moderately severe dementia associated with Alzheimer's disease, donepezil (E2020) has been introduced for the inhibition of acetylcholinesterase activity in the human brain. However, there is no morphological evidence as to how this chemical agent affects the acetylcholinesterase-positive structures in the various areas of the human and the rat CNS. This study demonstrates by histochemical means that donepezil exerts a dose-dependent inhibitory effect in vitro on acetylcholinesterase activity. The most sensitive areas were the cortex and the hippocampal formation. Within the different layers of the cortex, the cholinoceptive acetylcholinesterase-positive postsynaptic pyramidal cell bodies were more sensitive than the presynaptic cholinergic axonal processes. In the cortex, the cell body staining was already abolished by even 2 x 10(-8)M donepezil, whereas the axonal staining could be eliminated only by at least 5 x 10(-8)M donepezil. In the hippocampus, the axonal acetylcholinesterase reaction end-product was eliminated by 5 x 10(-7)M donepezil. The most resistant region was the putamen, where the staining intensity was moderately reduced by 1 x 10(-6)M donepezil. In the rat brain, the postsynaptic cholinoceptive and presynaptic cholinergic structures were inhibited by nearly the same dose of donepezil as in the human brain. These histochemical results provide the first morphological evidence that, under in vitro circumstances, donepezil is not a general acetylcholinesterase inhibitor in the CNS, but rather selectively affects the different brain areas and, within these, the cholinoceptive and cholinergic structures. The acetylcholinesterase staining in the nerve fibers (innervating the intracerebral blood vessels of the human brain and the extracerebral blood vessels of the rat brain) and at the neuromuscular junction in the diaphragm and gastrocnemius muscle of rat, was also inhibited dose dependently by donepezil. It is concluded that donepezil may be a valuable tool with which to influence both the pre- and the postsynaptic acetylcholinesterase-positive structures in the human and rat central and peripheral nervous systems.

Galantamine: effect on nicotinic receptor binding, acetylcholinesterase inhibition, and learning

Classical eyeblink conditioning is a well-characterized model paradigm that engages the septohippocampal cholinergic system. This form of associative learning is impaired in normal aging and severely disrupted in Alzheimer's disease (AD). Some nicotinic cholinergic receptor subtypes are lost in AD, making the use of nicotinic allosterically potentiating ligands a promising therapeutic strategy. The allosterically potentiating ligand galantamine (Gal) modulates nicotinic cholinergic receptors to increase acetylcholine release as well as acting as an acetylcholinesterase (AChE) inhibitor. Gal was tested in two preclinical experiments. In Experiment 1 with 16 young and 16 older rabbits, Gal (3.0 mg/kg) was administered for 15 days during conditioning, and the drug significantly improved learning, reduced AChE levels, and increased nicotinic receptor binding. In Experiment 2, 53 retired breeder rabbits were tested over a 15-wk period in four conditions. Groups of rabbits received 0.0 (vehicle), 1.0, or 3.0 mg/kg Gal for the entire 15-wk period or 3.0 mg/kg Gal for 15 days and vehicle for the remainder of the experiment. Fifteen daily conditioning sessions and subsequent retention and relearning assessments were spaced at 1-month intervals. The dose of 3.0 mg/kg Gal ameliorated learning deficits significantly during acquisition and retention in the group receiving 3.0 mg/kg Gal continuously. Nicotinic receptor binding was significantly increased in rabbits treated for 15 days with 3.0 mg/kg Gal, and all Gal-treated rabbits had lower levels of brain AChE. The efficacy of Gal in a learning paradigm severely impaired in AD is consistent with outcomes in clinical studies.

Galantamine: Effect on nicotinic receptor binding, acetylcholinesterase inhibition, and learning

Classical eyeblink conditioning is a well-characterized model paradigm that engages the septohippocampal cholinergic system. This form of associative learning is impaired in normal aging and severely disrupted in Alzheimer's disease (AD). Some nicotinic cholinergic receptor subtypes are lost in AD, making the use of nicotinic allosterically potentiating ligands a promising therapeutic strategy. The allosterically potentiating ligand galantamine (Gal) modulates nicotinic cholinergic receptors to increase acetylcholine release as well as acting as an acetylcholinesterase (AChE) inhibitor. Gal was tested in two preclinical experiments. In Experiment 1 with 16 young and 16 older rabbits, Gal (3.0 mg/kg) was administered for 15 days during conditioning, and the drug significantly improved learning, reduced AChE levels, and increased nicotinic receptor binding. In Experiment 2, 53 retired breeder rabbits were tested over a 15-wk period in four conditions. Groups of rabbits received 0.0 (vehicle), 1.0, or 3.0 mg/kg Gal for the entire 15-wk period or 3.0 mg/kg Gal for 15 days and vehicle for the remainder of the experiment. Fifteen daily conditioning sessions and subsequent retention and relearning assessments were spaced at 1-month intervals. The dose of 3.0 mg/kg Gal ameliorated learning deficits significantly during acquisition and retention in the group receiving 3.0 mg/kg Gal continuously. Nicotinic receptor binding was significantly increased in rabbits treated for 15 days with 3.0 mg/kg Gal, and all Gal-treated rabbits had lower levels of brain AChE. The efficacy of Gal in a learning paradigm severely impaired in AD is consistent with outcomes in clinical studies.

Chronic treatment of old rats with donepezil or galantamine: effects on memory, hippocampal plasticity and nicotinic receptors

The function of the cholinergic system is known to change during normal aging and in pathological conditions such as Alzheimer's disease. The present study was designed to assess, within the same group of old animals, the behavioral, electrophysiological and neurochemical effects of chronic treatment with agents that increase the function of the cholinergic system through both muscarinic and nicotinic mechanisms. Doses were determined that produced 60% cholinesterase inhibition by donepezil and galantamine for the old rats. This was chosen to be analogous to therapeutic levels achieved for treatment of human Alzheimer's disease patients with these agents. Because of the well-known age-related changes in spatial memory and hippocampal synaptic plasticity, spatial working memory in the radial eight-arm maze and hippocampal long-term potentiation induction and decay, as well as nicotinic receptor density and affinity, were measured in old rats implanted with minipumps that delivered donepezil, galantamine or saline. There was no effect of drug treatment on baseline synaptic transmission or on the threshold or magnitude of long-term potentiation induction. Both drug treatment groups, however, showed significantly extended long-term potentiation decay times at the perforant path-granule cell synapse over the saline control animals, as measured during the week following induction. Both drugs also elevated the number of nicotinic receptors within the hippocampus and neocortex. This is the first demonstration of cholinergic modulation of synaptic plasticity over the time-course of days. Furthermore, the durability of long-term potentiation was significantly, positively correlated with nicotinic receptor binding in the hippocampus. Chronic treatment with donepezil or galantamine had no significant effect on a well-learned spatial working memory task on the radial maze. These data suggest that the therapeutic doses of cholinesterase inhibitors used to treat patients with Alzheimer's disease may have effects on neurophysiology and neurochemistry that are close to the threshold for producing detectable behavioral improvements.