Estradiol enhances the induction of homosynaptic long-term depression in the CA1 region of the adult, ovariectomized rat

An ovarian steroid-dependent cycle of synaptogenesis and synapse shedding o ccurs naturally in the hippocampus of the adult female rat. The newly forme d axospinous synapses in CA1 may differ functionally from extant axospinous synapses, e.g., in terms of their modifiability. Here we assess whether es tradiol alters the induction of homosynaptic long-term depression of the Sc haffer collateral-CA1 synapses in vitro. Sprague-Dawley rats were bilateral ly ovariectomized and, beginning 6-8 days later, received a series of injec tions of either 17 beta-estradiol or sesame oil sc. Field potentials were r ecorded in hippocampal slices. In estradiol-treated animals, asynchronous, low-frequency stimulation led to significant longterm depression of the act ivated synapses in CA1 s. radiatum and no change of the inactive synapses i n s. oriens. In contrast, this conditioning stimulation did not significant ly alter any CA1 responses in oil-treated control animals. Subsequent high- frequency conditioning stimulation significantly potentiated the activated s. radiatum synapses in both estradiol- and oil-treated animals. Thus, give n the stimulation conditions used here, estradiol enables the induction of homosynaptic longterm depression at the CA3-CA1 synapses in adult females. (C) 2000 Academic Press.