Synapse-Specific Modulation of Synaptic Responses by Brain States in Hippocampal Pathways

Synaptic changes play a major role in memory processes. Modulation of synaptic responses by brain states remains, however, poorly understood in hippocampal networks, even in basal conditions. We recorded evoked synaptic responses at five hippocam-pal pathways in freely moving male rats. We showed that, at the perforant path to dentate gyrus (PP-DG) synapse, responses increase during wakefulness compared with sleep. At the Schaffer collaterals to CA1 (SC-CA1) synapse, responses increase dur-ing non-REM sleep (NREM) compared with the other states. During REM sleep (REM), responses decreased at the PP-DG and SC-CA1 synapses compared with NREM, while they increased at the fornix to nucleus accumbens synapse (Fx-NAc) during REM compared with the other states. In contrast, responses at the fornix to medial PFC synapse (Fx-PFC) and at the fornix to amyg-dala synapse (Fx-Amy) were weakly modulated by vigilance states. Extended sleep periods led to synaptic changes at PP-DG and Fx-Amy synapses but not at the other synapses. Synaptic responses were also linked to local oscillations and were highly corre-lated between Fx-PFC and Fx-NAc but not between Fx-Amy and these synapses. These results reveal synapse-specific modula-tions that may contribute to memory consolidation during the sleep-wake cycle.

Automated summary

Synaptic changes in hippocampal networks during different brain states were investigated in freely moving male rats. It was found that synaptic responses at the PP-DG synapse increased during wakefulness, while responses at the SC-CA1 synapse increased during NREM sleep. REM sleep led to decreased responses at the PP-DG and SC-CA1 synapses and increased responses at the Fx-NAc synapse. The results suggest synapse-specific modulations that may contribute to memory consolidation during the sleep-wake cycle.