Silent Synapse-based Mechanism For Sensory Representation In CA1 Hippocampus

In mammals,hippocampus plays an important role in learning and memory.As the"online" processing system for information,activity-dependent plasticity is induced on relevant hippocampal synapses to store the experience during memory acquisition and short-term storage.It is thought that as a multimodal brain region,the hippocampus processes sensory information based on "sparse coding",i.e.,only a small proportion of cells are involved in coding certain sensory information.This was found in the sparse CA1 neuronal responses to spatial as well as non-spatial information.However,the cellular and circuitry mechanisms of sparse representation of sensory information in the hippocampus remain largely unknown.To explore the underlying mechanisms and the possible physiological significance,we performed in vivo whole-cell recording on anaesthetized and awake adult rodents,and investigated visual stimuli-evoked responses of hippocampal CA1 pyramidal cells(PCs).First we found that under anaesthetized state,flash stimuli could evoke responses in only?30%CA1 PCs,which is consistent with the sparse coding scheme of dealing with sensory information in the hippocampus.By depolarizing the cells not responding at resting potentials,we observed clear excitatory responses to the same stimulus.Further studies showed that these excitatory responses were mediated by silent synapses,which contain the NMDA subtype but not AMPA subtype of glutamate receptors.When the flash stimulus and postsynaptic depolarization were paired,long-term potentiation could be induced on the relevant silent synapses.The responses observed at resting potential after pairing may be accounted by the emerging functional synapses underwent silent synapse unsilencing.Next we found that on the head-fixed awake mice,the normally unresponsive cells could also respond to the flash stimuli at depolarized membrane potential.In addition,by receiving input conveying specific sensory information,different visual stimulus evoked distinct responses on the same CA1 PCs.We therefore conclude that the largely distributed silent synapses in the hippocampal CA1 receive widespread and specific projections,and mediate potential responses with physiological significance.These sensory-associate silent synapses on the great number of CA1 PCs provide the capacity for activity-dependent plasticity accompanying memory acquisition and storage.