Abstract
Elevated levels of circulating estrogen in female rats result in increased spine and synapse density and parallel increases in NMDA receptor binding in area CA1 of the hippocampus. Estrogen also influences cholinergic neurochemistry in the basal forebrain and hippocampus. The objectives of the present study were to determine the role of acetylcholine in the estrogen-induced increase in NMDA receptor binding in CA1 of the hippocampus and to investigate the relationship between increased NMDA receptor binding in CA1 and performance on a task of working memory. In the current experiments, elevating endogenous levels of acetylcholine in ovariectomized rats by 3 d of continuous administration of physostigmine, an acetylcholinesterase inhibitor, increased NMDA receptor binding in CA1 as measured by quantitative autoradiography. This increase was comparable with the increase in NMDA receptor binding induced by injections of estradiol benzoate 72 and 48 hr before death. Additionally, the administration of 5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1-oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (BIBN 99), an M2 receptor antagonist, blocked the ability of both estrogen and physostigmine to increase NMDA receptor binding. The regimen of estradiol replacement that was demonstrated to increase NMDA receptor binding in CA1 of ovariectomized rats also improved arm-choice accuracy in a working memory task in an eight-arm radial maze. The estrogen-induced improvement in working memory performance was blocked by BIBN 99, which also blocked the increase in NMDA receptor binding. These results indicate that acetylcholine acts at M2 muscarinic receptors to mediate the estrogen-induced increase in NMDA receptor binding in CA1 of the hippocampus as well as the associated improvement in working memory.