Ibec Seminar: Dr Pascale Quilichini

The consolidation of episodic memory during sleep relies on coordinated interactions between the hippocampus (HPC) and the prefrontal cortex (PFC), although these regions lack direct reciprocal connections. The thalamic nucleus reuniens (NR), which is bidirectionally connected to both structures, is therefore a strong candidate for mediating this dialogue. Using simultaneous silicon-probes recordings from the HPC, NR, and PFC in freely sleeping rats, we identified a previously undescribed infra-slow rhythm (ISR) in NR neuronal activity predominantly during non-rapid eye movement (NREM) sleep.

HPC and PFC neurons were differentially entrained to distinct phases of this rhythm. Consistently, hippocampal sharp-wave ripples (SWRs) and prefrontal spindles occurred also preferentially during slightly different ISR phases. Coupled SWR–spindle events peaked at the onset of the ON phase.

Furthermore, this coupling was stronger during NREM epochs dominated by ISR compared to those without ISR, and, most importantly, was significantly enhanced during ISR-dominated NREM sleep following spatial learning.

Together, these findings reveal a novel infra-slow population dynamic in the NR that may temporally coordinate hippocampo–prefrontal interactions during sleep, positioning this nucleus as a key hub in this network supporting memory consolidation.