Role of hippocampal M2 muscarinic receptors in the estrogen-induced enhancement of working memory

We have previously demonstrated that acetylcholine, acting at M2 muscarinic receptors, mediates the estradiol-induced increase in hippocampal N-methyl-D-aspartate receptor binding and the associated enhancement in working memory.. The goal of present experiment was to investigate the role of hippocampal M2 receptors in the behavioral aspects of these effects. Ovariectomized rats were trained to locate a hidden escape platform on a matching-to-place version of the water maze in which the platform was moved to a new location for each session of four daily trials. Following 18 days of training, rats were randomly assigned to receive one of the following treatments: 1) injections of oil vehicle delivered 72 and 48 h before testing and continuous delivery of vehicle into the dorsal hippocampus via bilateral cannulae implants connected to osmotic minipumps; 2) injections of estradiol benzoate (EB) delivered 72 and 48 h before testing and continuous delivery of vehicle into the hippocampus; 3) injections of EB delivered 72 and 48 h before testing and continuous delivery of the M2 muscarinic receptor antagonist, AFDX 116, into the hippocampus; and 4) injections of EB delivered 72 and 48 h before testing and continuous delivery of AFDX 116 into a control site in the cortex. Chronic administration of AFDX 116 into the hippocampus, but not the cortex, significantly attenuated an estrogen-induced enhancement in performance on a working memory task in the water maze as indicated by increased latency and increased path length to locate an escape platform during a test trial when a 90 min delay was imposed between the first: and second trials. These results indicate that acetylcholine acts at M2 muscarinic receptors located in the hippocampus to mediate the positive effects exerted by estrogen on working memory. (c) 2005 Published by Elsevier Ltd on behalf of IBRO.