Stability of hippocampal place cell activity across the rat estrous cycle

Findings from both in vitro and in vivo studies have shown that estrogen exerts pronounced effects on hippocampal morphology and physiology. The degree to which these molecular findings influence hippocampal processing in freely behaving animals is unclear. The present study assessed the effect of the estrous cycle on hippocampal place cells in naturally cycling rats during two behavioral states. Female Sprague-Dawley rats were trained to alternate on a U-shaped runway for food reinforcement. Single-unit recordings of hippocampal CA1 cells were conducted under two conditions: (1) at rest on a holder, and (2) running on the maze. Spatial firing characteristics of the cells were examined at different stages of the estrous cycle (i.e., diestrus, proestrus, and estrus). Specifically, information was collected on (1) mean firing rates; (2) basic place field parameters; and (3) changes in the firing dynamics of these cells (e.g., burst properties). The findings showed a decrease in mean firing rate on the maze during proestrus. However, other basic measures of spatial tuning and burst properties were unchanged. The current study suggests that there is relative stability of hippocampal place cells across the estrous cycle during a well-trained task.