Molecular analysis of sex difference in the hippocampus, a center for learning and memory

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The goal of the current study is elucidation of sex difference in the hippocampus, a center for learning and memory. (1) Concentration of sex hormones in the hippocampus exhibited clear sex difference. Hippocampal concentration of estradiol (E2), the most potent estrogen, was higher in male than that in female. Hippocampal E2 fluctuated across the estrous cycle. The concentration of hippocampal testosterone (T) was higher in male than female, which did not fluctuate. (2) Surprisingly, the expression level of the steroidogenic enzymes and receptors in the hippocampus exhibited neither sex difference nor fluctuation across the estrous cycle.(3) The density of dendritic spines in female hippocampus fluctuated across the estrous cycle. The total spine density showed the maximum at Pro and D1, and the minimum at Est and D2 whereas the total spine density in male was as high as that in female at Pro or D1. The fluctuating pattern of spine density corresponded to that of hippocampal concentration of sex steroids. Taken together, there is clear sex difference in the hippocampus-synthesized sex hormones which play a crucial role in regulating the density of dendritic spines