Synchronization of the bilateral hippocampi through CA3

Title: Lateralization of CA1 assemblies in the absence of CA3 input

Journal: Nat Commun (2021) in press

Link: https://10.1038/s41467-021-26389-3

Comments:

Majority of the brain structure is paired and distributed across left and right cerebral hemispheres. Axonal projections to the contralateral hemisphere contributes to the bilateral synchrony in the cerebrum. Surprisingly, the basis underlying it remains elusive.

Study from RIKEN address this by generation of transgenic mice lacking the projection from a part of hippocampus, CA3. To this end, exogenous neurotoxin were expressed only in the CA3 region via a combination of multiple transgenic lines. This is intriguing in that 1. the first line limited transgene expression to the CA3, 2. the second line limited the timing of transgene expression via pharmacogenetic Tet-OFF system, 3. the third line allowed the expression of tetanus neurotoxin, which blocked the neurotransmission by cleaving the synaptic protein essential for fusion between synaptic vesicle and plasma membrane.

The group applied in vivo electrophysiology to record the activity of the left and right hippocampi. The results showed that the synchrony in transient field potential called sharp wave / ripple complex were impaired in the mutant as compared to the control. This suggests that a part of the local circuit of the hippocampus, i.e., CA3 played a critical role in synchronization between the left and right hippocampi.

Human imaging study repetitively reported that the unilateral hippoocampus was activated upon memory retrieval. Degree of such lateralization may be under the influence of synchronicity which the current study examined this time. Conversely, impairment of the CA3 function is likely to induce asymmetric function of the hippocampus.

We might begin to understand how the left and right hemispheres act in a similar or differential manner for asymmetry.