Stability of hippocampal place cell activity across the rat estrous cycle
A large amount of research has focused on how the hippocampus encodes spatial information in an environment. Both in vitro and in vivo studies have demonstrated that estrogen exerts pronounced effects on hippocampal morphology and physiology. Examining the degree to which these molecular changes influence hippocampal processing in freely behaving animals was the goal of this research.
The estrogen effects on hippocampal morphology and plasticity are reviewed. This is followed by an examination of the impact of the estrous cycle on hippocampus related behaviors. A study is presented demonstrating sex and estrous cycle differences in activity and exploration of a novel environment.
The hippocampus (via the entorhinal cortex) integrates information from a wide range of brain regions. The pyramidal neurons in the CA1 region of the hippocampus transmit the outcome of the hippocampal processing back to the neocortex (via the subiculum and entorhinal cortex). The effect of the estrous cycle on these pyramidal neurons was examined in naturally cycling rats. Single unit recordings were conducted in rats both at rest on a holder and while running on the maze. A decrease in mean firing rate on the maze was found during proestrus. However, other basic measures of spatial tuning and burst properties remained stable across the cycle. The results suggest that molecular changes occurring over the estrous cycle are not directly reflected at the single unit level and may be dependent upon task demands or novelty of the environment.