Comparison of Galantamine and Donepezil for Effects on Nerve Growth Factor, Cholinergic Markers, and Memory Performance in Aged Rats

Donepezil- and scopolamine-induced rCMRglu changes assessed by PET in conscious rhesus monkeys

OBJECTIVE

[(18)F]Fluoro-2-deoxyglucose positron emission tomography (FDG-PET) is a useful tool for measuring the regional cerebral metabolic rate of glucose (rCMRglu), which is an index of neuronal activity. Donepezil, an acetylcholine esterase inhibitor (AChEI), has been recommended as a treatment option for patients with Alzheimer's disease (AD). We aimed to characterize the effects of donepezil on rCMRglu using FDG-PET in non-human primates.

METHODS

We investigated the effects of administration of donepezil (500 microg/kg, i.m.), the non-selective muscarinic ACh receptor antagonist scopolamine (30 microg/kg, i.m.), and the coadministration of both drugs on the rCMRglu of conscious young rhesus monkeys.

RESULTS

Donepezil increased the rCMRglu in all regions of interest except in the thalamus. Scopolamine treatment also increased the rCMRglu in all regions of interest except the cerebellum and thalamus. However, these effects disappeared with coadministration of the drugs.

CONCLUSIONS

This PET study showed that administration of donepezil or scopolamine alone increased the rCMRglu in conscious rhesus monkeys. We also found that the donepezil-induced increase was abolished by simultaneous administration of scopolamine, suggesting that muscarinic ACh receptor function plays an important role in the effect of donepezil.

Donepezil treatment restores the ability of estradiol to enhance cognitive performance in aged rats: evidence for the cholinergic basis of the critical period hypothesis

Recent studies suggest that the ability of estradiol to enhance cognitive performance diminishes with age and/or time following loss of ovarian function. We hypothesize that this is due, in part, to a decrease in basal forebrain cholinergic function. This study tested whether donepezil, a cholinesterase inhibitor, could restore estradiol effects on cognitive performance in aged rats that had been ovariectomized as young adults. Rats were ovariectomized at 3 months of age, and then trained on a delayed matching to position (DMP) T-maze task, followed by a configural association (CA) operant condition task, beginning at 12-17 or 22-27 months of age. Three weeks prior to testing, rats started to receive either donepezil or vehicle. After one week, half of each group also began receiving estradiol. Acclimation and testing began seven days later and treatment continued throughout testing. Estradiol alone significantly enhanced DMP acquisition in middle-aged rats, but not in aged rats. Donepezil alone had no effect on DMP acquisition in either age group; however, donepezil treatment restored the ability of estradiol to enhance DMP acquisition in aged rats. This effect was due largely to a reduction in the predisposition to adopt a persistent turn strategy during acquisition. These same treatments did not affect acquisition of the CA task in middle-aged rats, but did have small but significant effects on response time in aged rats. The data are consistent with the idea that estrogen effects on cognitive performance are task specific, and that deficits in basal forebrain cholinergic function are responsible for the loss of estradiol effect on DMP acquisition in aged ovariectomized rats. In addition, the data suggest that enhancing cholinergic function pharmacologically can restore the ability of estradiol to enhance acquisition of the DMP task in very old rats following long periods of hormone deprivation. Whether donepezil has similar restorative effects on other estrogen-sensitive tasks needs to be explored.

Galantamine-induced improvements in cognitive function are not related to alterations in alpha(4)beta (2) nicotinic receptors in early Alzheimer's disease as measured in vivo by 2-[18F]fluoro-A-85380 PET

INTRODUCTION

The nicotinic acetylcholine receptor (nAChR) system plays a regulatory role in a number of cognitive processes. Cholinesterase inhibitors (i.e., galantamine) that potentiate cholinergic neurotransmission improve cognitive function in Alzheimer's disease (AD); however, the relationship between these effects and associated changes in nAChRs are yet to be established in vivo.

MATERIALS AND METHODS

2-[18F]Fluoro-A-85380 (2-FA) binds to nAChRs and with positron emission tomography (PET) imaging provides a composite measure of receptor density and ligand affinity. This study aimed to: (1) quantify nAChRs in vivo in 15 drug-naïve patients with mild AD before and after chronic treatment with galantamine, using 2-FA and PET, and (2) examine the relationship between treatment-induced changes in nAChRs and improvements in cognitive function. Participants were nonsmokers and underwent extensive cognitive testing and a PET scan after injection of approximately 200 MBq of 2-FA on two occasions (before and after 12 weeks, galantamine treatment). A 3-day washout period preceded the second scan. Brain regional 2-FA binding was assessed through a simplified estimation of distribution volume (DV(S)).

RESULTS

Performance on global measures of cognition significantly improved following galantamine treatment (p < 0.05). This improvement extended to specific cognitive measures of language and verbal learning. No significant differences in nAChR DV(S) before and after galantamine treatment were found. The treatment-induced improvement in cognition was not correlated with regional or global nAChR DV(S), suggesting that changes in nAChRs may not be responsible for the improvements in cognition following galantamine in patients with mild AD.

Nicotinic acetylcholine receptors and modulation of learning in 4- and 27-month-old rabbits

Using drugs acting on nicotinic acetylcholine receptors (nAChRs), we examined temporal-parietal and frontal cortex, hippocampus, and cerebellum to identify sites of cognition enhancement in 4- and 27-month rabbits. First, we compared radioligand receptor binding for neuronal alphabeta heteromeric nAChRs ([3H]epibatidine) and alpha7 homomeric nAChRs ([3H]methyllycaconitine) in rabbits and rats. In cerebellum, nAChR levels of both species are low, about at the detection limit of the radioligand binding assays. Next, we compared nAChRs in 4- and 27-month vehicle-treated rabbits trained in delay eyeblink conditioning. Older rabbits conditioned more poorly and had lower alphabeta heteromeric nAChR binding in hippocampus than young rabbits. For cognition enhancement, galantamine (mild cholinesterase inhibitor and allosteric modulator of nAChRs) or MEM-3389 (alpha7nAChR agonist formerly identified as AR-R 17779) was injected before conditioning. Drugs improved learning in both age groups. In 27-month rabbits, drugs increased expression of frontal and temporal-parietal alphabeta heteromeric nAChRs and hippocampal alphabeta and alpha7nAChRs. In 4-month rabbits, drugs increased expression of alpha7 homomeric nAChRs in frontal and temporal-parietal cortex and hippocampus, but increased expression of alphabeta heteromeric nAChRs only occurred in temporal-parietal cortex. Increased expression of alphabeta nAChRs was more extensive in older drug-treated rabbits, whereas increased expression of alpha7nAChRs was more prevalent in younger drug-treated rabbits, suggesting different substrates for amelioration (27-month rabbits) vs facilitation (4-month rabbits) of learning. Results provide evidence for cortical as well as hippocampal nAChR modulation of delay eyeblink conditioning and demonstrate that more sensitive binding assays are required to assess nAChR effects in cerebellum.

Expression of GDNF transgene in astrocytes improves cognitive deficits in aged rats

Glial cell line-derived neurotrophic factor (GDNF) was assayed for its neurotrophic effects against the neuronal atrophy that causes cognitive deficits in old age. Aged Fisher 344 rats with impairment in the Morris water maze received intrahippocampal injections at the dorsal CA1 area of either a lentiviral vector encoding human GDNF or the same vector encoding human green fluorescent protein as a control. Recombinant lentiviral vectors constructed with human cytomegalovirus promotor and pseudotyped with lyssavirus Mokola glycoprotein specifically transduced the astrocytes in vivo. Astrocyte-secreted GDNF enhanced neuron function as shown by local increases in synthesis of the neurotransmitters acetylcholine, dopamine and serotonin. This neurotrophic effect led to cognitive improvement of the rats as early as 2 weeks after gene transduction. Spatial learning and memory testing showed a significant gain in cognitive abilities due to GDNF exposure, whereas control-transduced rats kept their performance at the chance level. These results confirm the broad spectrum of the neurotrophic action of GDNF and open new gene therapy possibilities for reducing age-related neurodegeneration.

Cognitive performance of healthy young rats following chronic donepezil administration

RATIONALE

Experimental studies have investigated the effects of chronic donepezil treatment on the behavioral deficits elicited by reduced activity or the loss of cholinergic neurons that occurs in aging or in models of dementia. However, few studies have analyzed the effects of chronic donepezil treatment on the cognitive functions of intact animals.

OBJECTIVES

The cognitive functions of healthy young rats treated chronically with the acetylcholinesterase inhibitor donepezil were evaluated using a wide behavioral test battery.

RESULTS

Chronic treatment with donepezil ameliorated memory functions and explorative strategies, speeded up the acquisition of localizing knowledge, augmented responsiveness to the context, and reduced anxiety levels. However, it did not affect spatial span, modify motivational levels, or influence associative learning.

CONCLUSIONS

The present findings show the specific profile of donepezil action on cognitive functions in the presence of unaltered cholinergic neurotransmission systems.

Anti-dementia drugs and hippocampal-dependent memory in rodents

Abnormalities in hippocampal structure and function are characteristics of early Alzheimer's disease (AD). Behavioral tests measuring hippocampal-dependent memory in rodents are often used to evaluate novel treatments for AD and other dementias. In this study, we review the effects of drugs marketed for the treatment of AD, such as the acetylcholinesterase inhibitors, donepezil, rivastigmine, galantamine and the N-methyl-D-aspartic acid antagonist, memantine, in rodent models of memory impairment. We also briefly describe the effects of novel treatments for cognitive impairment in rodent models of memory impairment, and discuss issues concerning the selection of the animal model and behavioral tests. Suggestions for future research are offered.

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.

Galantamine effects on memory, spatial cue utilization, and neurotrophic factors in aged female rats

Galantamine is an acetylcholine esterase inhibitor that has been approved for use in Alzheimer's disease. However, even though clinical studies indicate efficacy in attenuating some of the symptoms associated with the disease, there are a paucity of studies evaluating the effects of galantamine administration on cognitive performance and brain parameters in aged rats. Further, because all previous animal studies using galantamine have been performed in male rats, there is no information on how females respond to galantamine treatment. Therefore, we studied the effects of 0.3, 0.6, and 1.2 mg/kg/day galantamine in 20-month-old female rats in terms of performance on the working and reference memory water radial arm maze task. Galantamine did not influence maze performance. Furthermore, a probe trial procedure to determine extra-maze cue utilization while solving the water radial arm maze established that aged female rats utilized extramaze cues, and that they did not rely on a nonspatial chaining strategy to locate hidden platforms. Galantamine treatment had no effect on use of extramaze cues or chaining. In addition, there were no significant changes in neurotrophin levels in the frontal cortex, entorhinal cortex, hippocampus, or basal forebrain after galantamine administration. Therefore, the data reported here suggest that aged animals do utilize spatial strategies for solving a working memory task, but galantamine has no appreciable effects on this task, at least not at the doses tested.

Donepezil induces a cholinergic sprouting in basocortical degeneration

One of the few currently approved therapies for Alzheimer's disease (AD) consists in the administration of acetylcholinesterase inhibitors, which enhances the lifetime of the neurotransmitter acetylcholine. Despite numerous studies on the symptomatic effect of acetylcholinesterase inhibitors, there is as yet no direct morphological evidence to indicate that they have a neurorestorative action. We investigated the effect of the acetylcholinesterase inhibitor donepezil administered subcutaneously in a rat model of partial unilateral cortical devascularization that induces a loss of the cortical cholinergic terminal network and a retrograde degeneration of the cholinergic projections that originate in the nucleus basalis. For 6 weeks, lesioned and sham-operated rats received a subcutaneous infusion of donepezil (2 mg/kg/day) or vehicle, delivered by osmotic minipumps implanted 2 weeks before the cortical devascularization. In lesioned rats, donepezil treatment increased the number and the size of vesicular acetylcholine transporter immunoreactive boutons in comparison to vehicle treatment. Donepezil had no observable effect on any of these parameters in sham-operated animals. These results show that donepezil mitigates cholinergic neuronal degeneration in vivo. This suggests a neuroplastic activity of this drug and provides evidence for a potential use of donepezil as a disease modifier in neurodegenerative diseases such as AD.

Galantamine and donepezil attenuate pharmacologically induced deficits in prepulse inhibition in rats

Acetylcholinesterase inhibitors (AChEIs) are currently being evaluated as adjunctive therapy for the cognitive dysfunction of schizophrenia. This core symptom of schizophrenia has often been attributed to impaired attention and abnormal sensory motor gating, features that are also found in Huntington's Disease, autism, and several other psychiatric and neurological disorders. The ability to improve prepulse inhibition (PPI) of the acoustic startle response may predict the efficacy of compounds as cognitive enhancers. In this study, PPI was disrupted in Wistar rats in three pharmacologic models: dopamine receptor agonism by apomorphine, NMDA receptor antagonism by MK801, or muscarinic acetylcholine receptor antagonism by scopolamine. We then evaluated the commonly used AChEIs, donepezil (0.5, 1.0, or 2.0mg/kg) and galantamine (0.3, 1.0, or 3.0mg/kg) for the capacity to improve PPI in each model. Under vehicle conditions, the prepulse stimuli (75, 80 and 85dB) inhibited the startle response to a 120dB auditory stimulus in a graded fashion. Galantamine (depending on dose) improved PPI deficits in all three PPI disruption models, whereas donepezil ameliorated PPI deficits induced by scopolamine and apomorphine, but was not effective in the MK801 model. These results indicate that some AChEIs may have the potential to improve cognition in schizophrenia by improving auditory sensory gating.