Functional Modifications In Hippocampal CA1 Pyramidal Cells Accompanying Fear Memory

Hippocampal formation is crucial for the multimodal information associations and memory storage. Previous studies have intensively demonstrated the molecular and cellular modifications induced in hippocampal neurons and synapses after the implementation of task-learning. However, the functional plasticity, accompanying the molecular modifications, remains to be elucidated for the understanding of the hippocampus-dependent information storage and memory retrieval. Here we develop a new technique of in vivo whole-cell recording in hippocampus of anaesthetized rats, and investigate the functional plasticity of CA1 pyramidal cell (PCs) after fear condition - a straightforward form of associative learning which has been demonstrated to be hippocampus dependent.In control rats without the experience of the given context fear, brief flashing of visual stimuli (conditional stimulation, CS)was found to elicit significant responses in-30% PCs, Out of all CA1 PCs examined (N = 98), membrane potential (MP) responses were either depolarization (15 of 32 neurons,46.9%) or hyperpolarization (11 of 32 neurons,34.4%), or both (6 of 32 neurons,18.9%).Interestingly, in the rats previously conditioned with context fear - the same CS (flash stimulation) associated with a delayed US (electrical shock) - 1 day in advance, all the CA1 PCs were found to be responsive to the conditioned CS. The excitatory and inhibitory responses were observed in 60% and 40%, respectively, in these PCs. The data recorded at different days (0-7 day) after the training showed that the increase in the proportion of the responsive CA1 PCs disappeared 5 days after fear conditioning.Thus, fear conditioning results in dramatic functional modifications in the entire hippocampal CA 1 network, by increasing the neuronal sensitivity to CS in a widespread population of CA1 neurons, which may in turn serve as a neuronal basis of hippocampal information storage.