A role for hippocampal actin rearrangement in object placement memory in female rats

Actin rearrangement, the polymerization of globular actin (G-actin) to filamentous actin, causes morphological changes in dendritic spines and is hypothesized to be a substrate of learning and memory. The ovarian hormone estradiol promotes hippocampal actin rearrangement and enhances performance on hippocampus-dependent tasks, including object placement memory. The goals of the current study were to determine a role for actin rearrangement and its regulatory pathway in object placement memory in female rats and to determine if estradiol impacts actin rearrangement in ovariectomized rats during the performance of the task. In an initial experiment, young adult Long-Evans rats were ovariectomized and implanted with capsules containing either cholesterol vehicle or estradiol. Bilateral intrahippocampal infusions of aCSF vehicle or the actin rearrangement inhibitor, latrunculin A, were administered 15 min prior to initiation of the object placement task. Latrunculin A dose-dependently impaired object placement memory. Estradiol had no impact on the ability of latrunculin A to affect performance. In a second experiment, rats were ovariectomized and received implants containing cholesterol or estradiol. Half of each hormone treatment group was exposed to the object placement memory task and half underwent control procedures. Immediately following completion of behavior, rats were euthanized and hippocampi removed. Western blotting was used to measure hippocampal levels of phosphorylated and total levels of a regulator of actin polymerization, the actin depolymerization factor cofilin. Exposure to the object placement memory task resulted in significant increases in phosphorylated levels of cofilin. Estradiol treatment had no impact on protein levels. These data support a role for hippocampal actin rearrangement and its regulatory proteins in object placement memory in female rats and suggest that chronic estradiol treatment does not impact hippocampal actin arrangement.