Investigating hippocampal-neocortical interactions around sharp-wave ripples

Coordinated activity in the hippocampal-neocortical network around hippocampal sharp-wave ripples (SWRs) plays an instrumental role in memory processing in the brain. SWRs occur in both sleep and awake states, though under two significantly different behavioural and chemical circumstances. Previous studies have reported different patterns of peri-SWR neocortical modulations between these states; however, their findings have been limited to one or a few discrete regions of the neocortex. To extend previous findings, we conducted wide-field optical imaging of the mouse neocortical voltage and glutamate activity combined with hippocampal electrophysiological recording. We found topographically- and temporally-organized patterns of neocortical glutamate and voltage activity around sleep and awake SWRs, though with pronounced differences. These findings highlight the state-dependency of the hippocampal-neocortical network’s computations and possibly functions. Moreover, they provide a spatiotemporal map of the neocortex around SWRs that could guide future studies on the role of hippocampal-neocortical interactions in memory consolidation

Inhibition is a prevalent mode of activity in the neocortex around awake hippocampal ripples in mice

Coordinated peri-ripple activity in the hippocampal-neocortical network is essential for mnemonic information processing in the brain. Hippocampal ripples likely serve different functions in sleep and awake states. Thus, the corresponding neocortical activity patterns may differ in important ways. We addressed this possibility by conducting voltage and glutamate wide-field imaging of the neocortex with concurrent hippocampal electrophysiology in awake mice. Contrary to our previously published sleep results, deactivation and activation were dominant in post-ripple neocortical voltage and glutamate activity, respectively, especially in the agranular retrosplenial cortex (aRSC). Additionally, the spiking activity of aRSC neurons, estimated by two-photon calcium imaging, revealed the existence of two subpopulations of excitatory neurons with opposite peri-ripple modulation patterns: one increases and the other decreases firing rate. These differences in peri-ripple spatiotemporal patterns of neocortical activity in sleep versus awake states might underlie the reported differences in the function of sleep versus awake ripples