Synaptic engram underlies memory

Memory encoding cells, or engram cells, are sparsely distributed across the brain. Recent findings have shown that memory formation coincides with the strengthening of interregional synaptic connectivity between engram ensembles. However, whether this learning-induced synaptic reorganization causally supports memory, independently or in concert with heterosynaptic and intrinsic plasticity, remains elusive. Here we developed optically implemented spike-timing-dependent plasticity (oSTDP), which induces synaptic depression selectively at target synapses by controlling the relative timing of pre- and postsynaptic activity. In vivo application of oSTDP to the synapses between entorhinal cortex (EC) and dentate gyrus (DG) engram cells caused a significant synaptic depression and the resultant memory impairment. Our results demonstrate that plasticity of functional connectivity between memory-encoding ensembles causally underlies memory.