Routing of non-mnemonic hippocampal ripples to subcortical structures

Summary

The mnemonic functions of hippocampal sharp wave ripples (SPW-Rs) have been studied extensively. Because hippocampal outputs affect not only downsteam cortical structures but also subcortical targets, we examined the impact of SPW-Rs on the firing patterns of lateral septal (LS) neurons in behaving rats. A large fraction of SPW-Rs were temporally locked to high frequency oscillations (HFOs; 120-180 Hz) in LS. SPW-R to LS HFOs coupling was strongest during NREM sleep, followed by waking immobility. However, coherence and spike-LFP coupling between the two events were low, suggesting that HFOs are generated locally within the LS GABAergic population. This hypothesis was supported by optogenetic induction of HFOs in LS. Spiking of LS neurons was independent from the sequential order of spiking in SPW-Rs but, instead, correlated strongly with the magnitude of excitatory synchrony of the hippocampal output. Thus, LS is strongly activated by SPW-Rs and may convey hippocampal population events to its motivation and action-inducing targets in the hypothalamus and brainstem.