Artificial association of memory events by optogenetic stimulation of hippocampal CA3 cell ensembles.

Previous gain-of-function studies using an optogenetic technique showed that manipulation of the hippocampal dentate gyrus or CA1 cell ensembles is important for memory reactivation and to generate synthetic or false memory. However, gain-of-function study manipulating CA3 cell ensembles has not been reported. The CA3 area of the hippocampus comprises a recurrent excitatory circuit, which is thought to be important for the generation of associations among the stored information within one brain region. We investigated whether the coincident firing of cell ensembles in one brain region, hippocampal CA3, associates distinct events. CA3 cell ensembles responding to context exploration and during contextual fear conditioning were labeled with channelrhodopsin-2 (ChR2)-mCherry. The synchronous activation of these ensembles induced freezing behavior in mice in a neutral context, in which a foot shock had never been delivered. The recall of this artificial associative fear memory was context specific. In vivo electrophysiological recordings showed that 20-Hz optical stimulation of ChR2-mCherry-expressing CA3 neurons, which is the same stimulation protocol used in behavioral experiment, induced long-term potentiation at CA3-CA3 synapses. Altogether, these results demonstrate that the synchronous activation of ensembles in one brain region, CA3 of the hippocampus, is sufficient for the association of distinct events. The results of our electrophysiology potentially suggest that this artificial association of memory events might be induced by the strengthening of synaptic efficacy between CA3 ensembles via recurrent circuit.