Potentiation and depression following stimulus interruption in young rat hippocampi.

We examined the effect of stimulus interruption on dual component field EPSPs in the hippocampal CA1 region. Resuming test stimulation at 0.1 Hz after 10-60 min silent periods led to an increase of the response followed by a decline, involving AMPA and NMDA components to a similar extent. Similar changes were seen when stimulation was initially applied to a naive pathway or the stimulus strength was increased during an experiment. The potentiation of the AMPA response was largely blocked by prior application of the NMDA antagonist AP5 while application of this drug immediately after the initial potentiation prevented the following decline. The results demonstrate that NMDA-dependent potentiation and depression, possibly equivalent to LTP and LTD, can both be induced by the same, very low, test stimulus frequency. Furthermore, the depression appeared to have a longer time window for its induction than the potentiation.