Estrogen replacement in ovariectomized rats affects strategy selection in the Morris water maze

While estrogen enhances performance on some tasks of learning and memory, it has impairing or no effects on others. It has been proposed that estrogen differentially affects performance on various tasks of learning and memory by influencing the strategy used to solve a task. The goal of the present study was to determine if estrogen would influence strategy selection in the Morris water maze. Long-Evans rats were ovariectomized and implanted with Silastic capsules containing 25% estradiol diluted in cholesterol or 100% cholesterol. Rats were trained in a water maze task in which multiple strategies were available for use to locate a hidden escape platform that was moved to a new location for each set of four daily trials. During 10 days of acquisition trials, a visible floating landmark was always located in a static position relative to the hidden escape platform. Additionally, fixed extramaze cues visible to the animals surrounded the maze. Following acquisition, 2 days of probe trials were conducted in which the static landmark was removed. Estrogen replacement in ovariectomized rats resulted in impaired performance across 10 days of acquisition. Additionally, while removal of the visible landmark during the probe trials had no effect on the performance of the females receiving estrogen, it significantly disrupted performance of females receiving cholesterol treatment. These results indicate that estrogen replacement in ovariectomized rats biases an animal against using a landmark or static cue to aid in the location of a hidden escape platform in the water maze.