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 agi ng and in pathological conditions such as Alzheimer's disease. The present study was designed to assess, within the same group of old animals, the beh avioral, electrophysiological and neurochemical effects of chronic treatmen t with agents that increase the function of the cholinergic system through both muscarinic and nicotinic mechanisms. Doses were determined that produc ed 60% cholinesterase inhibition by donepezil and galantamine for the old r ats. This was chosen to be analogous to therapeutic levels achieved for tre atment of human Alzheimer's disease patients with these agents. Because of the well-known age-related changes in spatial memory and hippocampal synapt ic plasticity, spatial working memory in the radial eight-arm maze and hipp ocampal long-term potentiation induction and decay as well as nicotinic rec eptor density and affinity, were measured in old rats implanted with minipu mps that delivered donepezil, galantamine or saline. There was no effect of drug treatment on baseline synaptic transmission or on the threshold or ma gnitude of long-term potentiation induction. Both drug treatment groups, ho wever, 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 i s the first demonstration of cholinergic modulation of synaptic plasticity over the time-course of days. Furthermore, the durability of long-term pote ntiation was significantly, positively correlated with nicotinic receptor b inding 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 neuroph ysiology and neurochemistry that are close to the threshold for producing d etectable behavioral improvements. (C) 2000 IBRO, Published by Elsevier Sci ence Ltd. All rights reserved.