Working and Reference Memory tasks trigger opposed long-term synaptic changes in the rat dentate gyrus

Long-term storage of information into memory is supposed to rely on long-term synaptic plasticity processes. Detection of such synaptic changes after training in long-term or reference memory (RM) tasks has yet been scarce, variable and only studied on a short time scale. On the other hand, short-term or working memory (WM) is largely known to depend on persistent neuronal activity or short-term plasticity processes. However, processing information into WM could also involve long-term synaptic changes that could be responsible for the erasure/forgetting of items previously stored in WM playing the role of proactive interference. In order to study long-term synaptic changes associated with RM or WM, we trained chronically implanted rats in three different radial maze tasks: a classical RM task and two WM tasks involving different levels of proactive interference. Synaptic responses at the perforant path to dentate gyrus synapse were recorded on a long-time scale (24h) in freely-moving rats after training in one of these three tasks. We found that consolidation of long-term information leads to a delayed synaptic potentiation, occurring 9 hours after RM training and predicting good behavioral performance on the following day. In contrast, optimal information processing into WM triggers a synaptic depression immediately observed after training and lasting 3 hours, that could act as a mechanism for interference erasure/forgetting.