Excitatory Responses Of Hippocampal CA1 Neurons To The Stimulus Memorized In Associative Fear Learning And The Synapse Unsilencing Mechanism

The hippocampus is critical for memory acquisition and initial storage, a process that relies on activity-dependent neuronal plasticity. To understand hippocampal memory function, a large amount of studies have shown the molecular and cellular mechanisms that account for neuronal plasticity, such as the contribution of NMDA receptors, Ca2+ channels, protein kinases C, as well as gene expression. On the other hand, learning-induced plasticity has been found in hippocampal circuits, such as the changes in synaptic connections and neuronal excitability. However, the learning-induced plasticity that directly contributes to memory encoding remains unclear.In this study, we investigated the neuronal activity evoked by a memorized stimulus in hippocampal CA1 pyramidal cells (PCs), with the use of in vivo whole-cell recording from anaesthetized and awake rodents. We found that before learning, an unfamiliar visual stimulus (a 100-ms flash) could evoke membrane-potential (MP) responses in a small fraction (-30%) of CA1 PCs. Remarkably, previously unresponsive PCs showed excitatory responsiveness to the flash stimulus shortly (within 3 days) after learning, a task during which the animal learned the association of this flash with an aversive electric foot-shock stimulus. We also found the amplitude or onset latency of memorized flash stimulus evoked responses in CA1 PCs was not changed after learning. Furthermore, we found that the synchronization of spontaneous activities between CA1 PCs were significantly increased on day 1 after learning, and this increase was disappeared on day 5.Next, we investigated the synaptic mechanisms of these learning induced excitatory responses in CA1 PCs. In unlearned animals, normally unresponsive PCs were found to be able to show N-methyl-D-aspartate (NMDA) receptor-dependent responses to the flash stimulus when the membrane potential was depolarized, indicating the existence of potentially responding silent synapses, a form of synapses that contain only NMDA receptors but not AMPA receptors. To testify whether these silent synapses were involved in the learning-induced CA1 excitatory responsiveness, we next examined whether the learning-induced CA1 excitation originated from CA1 plasticity. With the use of CA1-specific NR1 konckout mice, we found that the associative fear learning could not induce the emergence of CA1 excitatory responses to the flash stimulus, indicating this plasticity depended on CA1 NMDA receptor activity (i.e., CA1 plasticity). Further, in the observations from learned rats (day 1), we found very similar onset latencies and temporal profiles between flash evoked excitatory responses at -35 mV and -70 mV in the same cell, indicating that previously silent synapses had been unsilenced after learning. We conclude that associative fear learning can induce widespread CA1 excitation to the memorized stimulus and that potentiation at silent synapses rather than at functional synapses may be a critical mechanism that accounts for memory acquisition and initial storage in the hippocampus.