Chronic DHEAS administration facilitates hippocampal long-term potentiation via an amplification of Src-dependent NMDA receptor signaling.

Dehydroepiandrosterone sulfate (DHEAS) has well characterized effects on memory and cognitive performances. Recently we have reported that repetitive administration of DHEAS lowers the threshold pulse number in inducing activity-dependent long-term potentiation (LTP) in rat hippocampal Schaffer collateral-CA1 synapses, in which a sub-threshold high frequency stimulation (HFS, 30 pulses at 100 Hz) for normal rats could induce robust LTP in DHEAS-treated rats (Chen et al., 2006). Here we report that the sub-threshold HFS could trigger the phosphorylation of Src and ERK2 in the DHEAS-treated rats, but not in control rats. We found in slices obtained from the DHEAS-treated rats that NMDA-induced intracellular Ca2+([Ca2+]i) transients in CA1 pyramidal neurons were significantly potentiated, which was essential for the Src and ERK2 phosphorylations. The activation of ERK2, a downstream factor of Src family kinase, was required for the DHEAS-facilitated LTP. The Src family kinase inhibitor PP2, but not its inactive homologue PP3, attenuated the NMDA-induced [Ca2+]i increase and abolished the DHEAS-facilitated LTP. These findings suggest that the chronic administration of DHEAS brings the NMDA receptor (NMDAr) to a potentiated state that causes an enough level of [Ca2+]i increase for LTP induction even by the sub-threshold HFS. The potentiated [Ca2+]i transient by the sub-threshold HFS may trigger the Src phosphorylation that will further potentiate NMDAr followed by an activation of ERK2 and LTP induction. This novel postsynaptic NMDAr/Src-mediated signal amplification through "NMDAr-Ca2+-->Src-->NMDAr-Ca2+" cycle may play a pivotal role in the DHEAS-facilitated LTP induction.