Involvement of TrkB- and p75M^NTR -signaling pathways in two contrasting forms of long-lasting synaptic plasticity

The repetition of experience is often necessary to establish long-lasting memory. However, the cellular mechanisms underlying this repetition-dependent consolidation of memory remain unclear. We previously observed in organotypic slice cultures of the rodent hippocampus that repeated inductions of long-term potentiation (LTP) led to a slowly developing long-lasting synaptic enhancement coupled with synaptogenesis. We also reported that repeated inductions of long-term depression (LTD) produced a long-lasting synaptic suppression coupled with synapse elimination. We proposed these phenomena as useful in vitro models for analyzing repetition-dependent consolidation. Here, we hypothesized that the enhancement and suppression are mediated by the brain-derived neurotrophic factor (BDNF)-TrkB signaling pathway and the proBDNF-p75 NTR pathway, respectively. When we masked the respective pathways, reversals of the enhancement and suppression resulted. These results suggest the alternative activation of the p75 NTR pathway by BDNF under TrkB-masking conditions and of the TrkB pathway by proBDNF under p75 NTR -masking conditions, thus supporting the aforementioned hypothesis.