Pii: S0306-4522(00)00180-9

ÐThe 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 af®nity, 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 signi®cantly 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 ®rst demonstration of cholinergic modulation of synaptic plasticity over the time-course of days. Furthermore, the durability of long-term potentiation was signi®cantly, positively correlated with nicotinic receptor binding in the hippocampus. Chronic treatment with donepezil or galantamine had no signi®cant 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. q 2000 IBRO. Published by Elsevier Science Ltd. All rights reserved.