Rapid enhancement of visual and place memory by estrogens in rats

Estrogenic effects on visual (object recognition) and place (object placement) memory were investigated. Ovariectomized (OVX) rats received acute sc injections 30 min before a sample trial (viewing objects), and 4 h later a recognition/retention trial was performed. During recognition/retention trials, discrimination between sample (old) and new objects (visual memory) or between objects in sample (old) and new locations (place memory) was tested. Subjects given 17alpha- or 17beta-estradiol or diethylstilbestrol (DES) 30 min before sample trials discriminated between objects or locations during recognition/retention trials whereas vehicle-treated, OVX rats did not. Estrogens were given a postsample trial to investigate whether enhancements were due to effects on memory processes or psychological/performance parameters. Hormones were given immediately after or 2 h after sample trials (delayed injections), and recognition/retention were tested 4 h after the sample trial. Both object and place discriminations were enhanced when estrogens were given immediately after sample trials, but not when injections were delayed. These results provide evidence that estrogen rapidly enhances visual and place memory. Moreover, posttraining injections suggest effects on mnemonic processes, consolidation, or encoding, not on performance parameters. Place memory enhancements required higher estrogen doses, both pre- and postsample trial. The rapid time course, stereospecificity of responses (alpha- and beta-estradiol are effective), and efficacy of various estrogens suggest interactions at other than classic estrogen alpha- or beta-receptors in mediating the effects. Thus, these results provide the first demonstration of rapid memory enhancements by estrogen and implicate nongenomic mechanisms, possibly an extranuclear receptor(s), in mediating the response.