Ivan Bozic

Sleep involves a relative disconnection from the environment, yet sensory stimuli can still trigger awakenings. The mechanism underlying sensory vigilance and stimulus discrimination during sleep remains unclear. Here, we showed that neutral auditory stimuli evoked responses across parallel auditory and non-auditory pathways, including the auditory cortex and thalamus, the hippocampus and centro-medial thalamus (CMT). Using a convolutional neural network, we identified CMT activity as the most discriminant hub for auditory-evoked sleep-to-wake transitions among all recorded structures. Furthermore, we found that prior associative learning of auditory cues with danger (conditioned stimulus, CS+) resulted in increased awakening upon CS+ exposure during NREM, but not REM, sleep. These sleep-to-wake transitions were blocked by optogenetic silencing of CMT neurons during CS+ exposure in sleeping mice. Altogether, these results suggest a central role of the CMT neurons in the residual processing of behaviorally-relevant information in the sleeping brain functioning as one of the major hubs for awakening in response to danger.

I comment on the article JN-2022-11-29991.R1 by quickly summarizing the findings and going further by putting it in the context of current knowledge and elaborate on open questions that this article raises

The interplay between the medial prefrontal cortex and hippocampus during non-rapid eye movement (NREM) sleep contributes to the consolidation of contextual memories. To assess the role of the thalamic nucleus reuniens (Nre) in this interaction, we investigated the coupling of neuro-oscillatory activities among prelimbic cortex, Nre, and hippocampus across sleep states and their role in the consolidation of contextual memories using multi-site electrophysiological recordings and optogenetic manipulations. We showed that ripples are time-locked to the Up state of cortical slow waves, the transition from UP to DOWN state in thalamic slow waves, the troughs of cortical spindles, and the peaks of thalamic spindles during spontaneous sleep, rebound sleep and sleep following a fear conditioning task. In addition, spiking activity in Nre increased before hippocampal ripples, and the phase-locking of hippocampal ripples and thalamic spindles during NREM sleep was stronger after acquisition of a fear memory. We showed that optogenetic inhibition of Nre neurons reduced phase-locking of ripples to cortical slow waves in the ventral hippocampus whilst their activation altered the preferred phase of ripples to slow waves in ventral and dorsal hippocampi. However, none of these optogenetic manipulations of Nre during sleep after acquisition of fear conditioning did alter sleep-dependent memory consolidation. Collectively, these results showed that Nre is central in modulating hippocampus and cortical rhythms during NREM sleep.

Significance: The hypocretin (Hcrt, also known as orexin) neuropeptides regulate sleep and wake stability, and disturbances of Hcrt can lead to sleep disorders. MicroRNAs (miRNAs) are short noncoding RNAs that fine-tune protein expression levels, and miRNA-based therapeutics are emerging. We report a functional interaction between miRNA (miR-137) and Hcrt . We demonstrate that intracellular miR-137 levels in Hcrt neurons regulate Hcrt expression with downstream effects on wakefulness. Specifically, lowering of miR-137 levels increased wakefulness in mice. We further show that the miR-137:Hcrt interaction is conserved across mice and humans, that miR-137 also regulates sleep–wake balance in zebrafish, and that the MIR137 locus is genetically associated with sleep duration in humans. Together, our findings reveal an evolutionarily conserved sleep–wake regulatory role of miR-137.