Effect of donepezil hydrochloride (E2020) on basal concentration of extracellular acetylcholine in the hippocampus of rats

Abstract

The effects of oral administration of the centrally acting acetylcholinesterase (AChE) inhibitors, donepezil hydrochloride (donepezil: E2020: (±)-2-[(1-benzylpiperidin-4-yl)methyl]-5,6-dimethoxy-indan-1-one monohydrochloride), tacrine (9-amino-1,2,3,4-tetrahydroacridine hydrochloride) and ENA-713 (rivastigmine: (S)-N-ethyl-3-[(1-dimethyl-amino)ethyl]-N-methyl-phenylcarbamate hydrogentartrate), which have been developed for the treatment of Alzheimer's disease, on the extracellular acetylcholine concentration in the hippocampus of rats were evaluated by using a microdialysis technique without adding cholinesterase inhibitor to the perfusion solution. We also compared the inhibition of brain AChE and the brain concentrations of these drugs. Donepezil at 2.5 mg/kg and tacrine at 5 mg/kg showed significant effects for more than 6 h. At these doses, the maximum increases were observed at about 1.5 h after administration of donepezil, and at about 2 h with tacrine, and were 499% and 422% of the pre-level, respectively. ENA-713 produced significant effects at doses of 0.625, 1.25 and 2.5 mg/kg, which lasted for about 1, 2 and 4 h, respectively. The maximum increases produced by these doses at about 0.5 h after administration were 190, 346 and 458% of the pre-level, respectively. The time courses of brain AChE inhibition with donepezil at 2.5 mg/kg, tacrine at 10 mg/kg and ENA-713 at 2.5 mg/kg were mirror images of the extracellular acetylcholine-increasing action at the same doses. The time courses of the brain concentrations of drugs after oral administration of donepezil at 2.5 mg/kg and tacrine at 10 mg/kg were consistent with those of brain AChE inhibition at the same doses, and there was a linear relation between these parameters. Brain concentration of ENA-713 at 2.5 mg/kg was below the limit of quantification at all time points measured. These results suggest that oral administration of donepezil, tacrine and ENA-713 increases acetylcholine concentration in the synaptic cleft of the hippocampus mostly through AChE inhibition, and that donepezil has a more potent activity than tacrine and a longer-lasting effect than ENA-713 on the central cholinergic system.

Introduction

Alzheimer's disease is a progressive neurodegenerative disease characterized by deficits in memory and cognitive function. One of the most pronounced changes in the brain of Alzheimer's disease patients occurs in the cholinergic systems. Cholinesterase inhibitors are the only class of drugs currently approved for the treatment of Alzheimer's disease.

We have developed a novel, piperidine-based, acetylcholinesterase (AChE) inhibitor, donepezil hydrochloride (donepezil: E2020: (±)-2-[(1-benzylpiperidin-4-yl)methyl]-5,6-dimethoxy-indan-1-one monohydrochloride), for the treatment of Alzheimer's disease (Sugimoto et al., 1995). Donepezil was approved by the U.S. Food and Drug Administration (FDA) in 1996, and is now being prescribed world-wide. A large-scale multicenter, double-blind clinical study has demonstrated that donepezil is a well-tolerated drug that improves cognitive performance and global function in patients with mild to moderate Alzheimer's disease (Rogers and Friedhoff, 1998, Rogers et al., 1996, Rogers et al., 1998a, Rogers et al., 1998b). Pharmacological studies in vitro have revealed that donepezil is a reversible and non-competitive cholinesterase inhibitor, and is a far more selective inhibitor of AChE, relative to butyrylcholinesterase, than is tacrine or physostigmine. It produces marked and long-lasting inhibition of brain AChE and increases the brain content of acetylcholine in vivo. Moreover, donepezil significantly ameliorates performance deficits in several learning and memory tasks including eight-arm radial maze impairments after scopolamine, and passive avoidance deficits in nucleus basalis magnocellularis-lesioned rats Ogura et al., 1988, Yamanishi et al., 1988, Rogers et al., 1991. Other cholinesterase inhibitors available for the treatment of Alzheimer's disease are tacrine (9-amino-1,2,3,4-tetrahydroacridine hydrochloride) and ENA-713 (rivastigmine: (S)-N-ethyl-3-[(1-dimethyl-amino)ethyl]-N-methyl-phenylcarbamate hydrogentartrate). Tacrine is the first drug approved by the U.S. FDA for the treatment of Alzheimer's disease, although it has adverse effects related to its actions on the peripheral nervous system and to hepatic toxicity (Beermann, 1993). ENA-713 is a pseudo-irreversible, carbamate-based cholinesterase inhibitor Weinstock et al., 1994, Anand and Gharabawi, 1996 and is currently available in some European countries.

Since the extracellular acetylcholine concentration measured by the intracerebral microdialysis technique reflects the acetylcholine concentration in the synaptic cleft, it is a useful parameter with which to compare the potential efficacy of various cholinesterase inhibitors in the central cholinergic system. The effects of donepezil, tacrine and ENA-713 on extracellular acetylcholine concentration have been studied by microdialysis. However, in those studies, the drug effects were evaluated after intraperitoneal or subcutaneous injection Kawashima et al., 1994, Giacobini et al., 1996, or in the presence of a second cholinesterase inhibitor, in the perfusion solution in order to increase extracellular acetylcholine to a detectable level (Ohara et al., 1997). In the present study, we compared the effects of orally administered donepezil, tacrine and ENA-713 on the basal concentration of extracellular acetylcholine in the hippocampus of rats under the same experimental conditions. Moreover, in order to validate the microdialysis data, we measured the inhibition of brain AChE and the brain concentrations of these drugs.