The mammalian hippocampal dentate gyrus is a unique memory circuit in which a subset of neurons is continuously generated throughout the lifespan. Previous studies have shown that the dentate gyrus neuronal population can hold fear memory traces (i.e., engrams) and that adult-born neurons (ABNs) support this process. However, it is unclear whether ABNs themselves hold fear memory traces. Therefore, we analyzed ABN activity at a population level across a fear conditioning paradigm. We found that fear learning did not recruit a distinct ABN population. In sharp contrast, a completely different ABN population was recruited during fear memory retrieval. We further provide evidence that ABN population activity remaps over time during the consolidation period. These results suggest that ABNs support the establishment of a fear memory trace in a different manner to directly holding the memory. Moreover, this activity remapping process in ABNs may support the segregation of memories formed at different times. These results provide new insight into the role of adult neurogenesis in the mammalian memory system.

Rapid eye movement (REM) sleep is primarily regulated by the brainstem pons. In particular, the sublaterodorsal tegmentum (SubLDT) in the dorsal pons contains neurons whose activity is selective to REM sleep. Elucidation of the precise identities of these neurons and their roles in REM sleep regulation is challenging, however, due to the functional and molecular heterogeneity of the SubLDT. A recent study revealed that corticotropin-releasing hormone–binding protein (Crhbp)-positive neurons in the SubLDT projecting to the medulla play a crucial role in REM sleep regulation and that loss of these Crhbp-positive neurons underlies sleep deficits observed in Parkinson's disease. The firing patterns of these neurons during sleep/wake, however, remained unknown. Here, we used an opto-tagging method and conducted cell-type-specific recordings from Crhbp-positive neurons using a glass pipette microelectrode in unanesthetized male mice. We recorded 58 Crhbp-positive neurons and found that many of these neurons are REM sleep-active neurons (41.4%) and that the remaining neurons are mostly either wake-active, wake/REM sleep-active, or non-REM sleep-active. In addition, projection-specific recordings revealed that the medulla-projecting Crhbp-positive neurons are mostly REM sleep-active neurons (75.0%). Based on clustering analysis and spike waveform analysis, REM sleep-active Crhbp-positive neurons can be further divided into different subtypes according to their electrophysiological properties, suggesting that Crhbp-positive neurons play diverse roles in REM sleep regulation.