NMDA receptor-gated visual responses in hippocampal CA1 neurons

Hippocampal processing of environmental information is critical for hippocampus-dependent brain functions that result from experience-induced hippocampal plasticity, such as memory acquisition and storage. Hippocampal responses to sensory stimulation have been extensively investigated, particularly with respect to spike activity. However, the synaptic mechanism for hippocampal processing of sensory stimulation has been much less understood. Here, we performed in vivo whole-cell recording on hippocampal CA1 pyramidal cells (PCs) from adult rodents to examine CA1 responses to a flash of visual stimulation. We first found in recordings obtained at resting potentials that approximate to 30% of CA1 PCs exhibited significant excitatory/inhibitory membrane-potential (MP) or membrane-current (MC) responses to the flash stimulus. Remarkably, in the other (approximate to 70%) CA1 PCs, although no responses could be detected at resting potentials, clear excitatory MP or MC responses to the same flash stimulus were observed at depolarizing potentials, and these responses were further found to depend on NMDA receptors. Our findings demonstrate the presence of NMDA receptor-mediated gating of visual responses in hippocampal CA1 neurons, a synaptic mechanism for hippocampal processing of sensory information that may play important roles in hippocampus-dependent functions such as learning and memory.