Estradiol modulates long-term synaptic depression in female rat hippocampus

Fluctuating estradiol levels in the adult, female rat modify the anatomical and functional organization of the hippocampal CA1 region. When systemic levels of estradiol are low, e.g., on estrus or in ovariectomized (OVX) rats, long-term synaptic potentiation is difficult to induce in vivo. However, little is known about the role of this ovarian hormone in long-term synaptic depression. Using multiple conditioning paradigms, we assess the magnitude of long-term depression (LTD) at CA3-CA1 synapses in vitro from adult, ovariectomized rats as a function of systemic estradiol replacement. In hippocampal slices from control OVX rats with low levels of estradiol, a low-frequency (2 Hz), asynchronous conditioning stimulation protocol does not produce LTD at 1 h postconditioning. However, this same protocol induces robust LTD in slices from estradiol-treated OVX rats. When the conditioning frequency is increased to 4 Hz, slices from both groups of rats show robust LTD in vitro. At an even higher conditioning frequency (10 Hz), the 2-Hz-based observations are reversed; no consistent changes in synaptic transmission are observed in slices from estradiol-treated OVX rats, but those from control rats (OVX + oil) show robust LTD. Thus estradiol reduces the frequency threshold for LTD induction at the CA3-CA1 synapses. Further, regardless of the conditioning frequency employed, where robust LTD is seen, its induction depends on normally functioning N-methyl-D-aspartate (NMDA) receptors during conditioning. The shift in conditioning frequency needed to elicit LTD is consistent with a decrease in NMDA receptor activation with decreasing estradiol levels.