FIGURE 1.

Estradiol regulates spine synapses on hippocampal pyramidal neurons, which are sites of excitatory neurotransmission important for learning and memory. A: in the diestrus phase, which is the beginning of the estrus cycle when estradiol levels are low, spine densities are also low. During proestrus when ovulation occurs, estrogen levels peak and spine densities increase in parallel. In the estrus phase, the day after proestrus, the system begins to reset itself for the next cycle and spine densities return to baseline. [Image adapted from McEwen and Schmeck (134a) with permission.] B: axospinous synapse density is higher in the proestrus phase compared with the estrus phase. C and D: in ovariectomized (OVX) rats, estradiol (E) treatment increases dendritic spine density (C) as well as axospinous synapse density (D). V, vehicle-treated. Histograms (B–D) are plotted from data originally presented in Gould et al. (58) and Woolley and McEwen (236).