Age

It is widely known that not only the gonadal estradiol (E2) but also hippocampal E2 plays an essential role in memory process. However, the role of hippocampal E2-enhanced memory mechanism during aging is largely unknown. The aim of the present study was to investigate the effect of age on E2 concentration, the expression level of its receptors, and key steroidogenic enzymes in hippocampus. We also investigated the effect of microglia activation on E2 synthesis in hippocampal neurons. The results showed that serum E2 was higher in 19-month-old (aged) rats, which exhibited spatial memory decline in the Morris water maze (MWM) test when compared to the younger rats. Hence, serum E2 may not be associated with the reduced spatial memory performance in aging. In contrast, the level of E2 and the expressions of its receptors were significantly decreased in hippocampus of aged female rat compared to younger females. Furthermore, the expressions of key hippocampal steroidogenic enzymes, steroidogenic acute regulatory protein (StAR), and cytochrome P450 (P450) also significantly decreased with age, which resulted in lower hippocampal E2 levels. In addition, we found that the microglia of aged brain highly expressed interleukin 6 (IL-6), which directly inhibited E2 synthesis in hippocampal neurons via suppression of P450 synthesis. Taken together, we summarized that the microglia-derived IL-6 inhibited hippocampal E2 synthesis in aged rats which, in turn, contributed to the deficit of spatial memory performance