A hypothalamo-septo-hippocampal circuit for REM sleep-dependent consolidation of social memory

Reviewer #1 (Public review):

In this manuscript, the authors aim to define how rapid eye movement sleep supports memory consolidation by identifying the brain circuits that are selectively engaged during this sleep state. They focus on a pathway linking a hypothalamic region involved in sleep regulation to the medial septum and onward to a hippocampal subregion that is critical for social memory. By combining recordings of neural activity with sleep-state-specific circuit manipulations, the study seeks to explain how information is routed during sleep to support distinct types of memory.

A major strength of the work is the use of state-of-the-art circuit-based approaches to link sleep dynamics to defined long-range connections and behavioral outcomes. The authors show that neurons in the lateral supramammillary region projecting to the medial septum are selectively active during rapid eye movement sleep, and that silencing this pathway during this sleep state disrupts consolidation of both social and contextual fear memories. Further dissection of downstream circuitry reveals that inhibition of the medial septum-to-hippocampal CA2 pathway during rapid eye movement sleep selectively impairs social memory. These results provide support for functional specialization within parallel pathways and suggest that this circuit acts as a hub for routing memory-related information during sleep.

While the evidence supporting a role for this circuit in sleep-dependent memory consolidation is compelling, several important mechanistic details remain unresolved. The chemical signaling used by the neurons connecting the hypothalamus to the medial septum is not clearly defined, leaving open whether these cells release excitatory signals, inhibitory signals, or a combination of both. In addition, the medial septum contains multiple neuronal populations with distinct downstream targets, and the specific cell types receiving input from this pathway are not clearly identified. Similarly, the nature of the signals delivered from the medial septum to the hippocampus remains unclear, making it difficult to link circuit anatomy to the observed behavioral specificity. Finally, because different circuit segments are manipulated independently, the causal relationship between upstream and downstream pathways remains suggestive rather than definitive and should be discussed explicitly as a limitation or addressed experimentally.

Overall, the authors largely achieve their aims by identifying a rapid eye movement sleep-specialized circuit that contributes to memory consolidation in a modality-specific manner. The findings are likely to have a meaningful impact on the field by advancing understanding of how sleep organizes memory through parallel neural pathways and by providing a useful framework for future studies of sleep-dependent brain state regulation. With additional clarification of circuit mechanisms or a clearer discussion of current limitations, the study would offer even greater value to the neuroscience community.

Reviewer #2 (Public review):

Summary:

This study systematically characterizes the activity patterns of a lateral supramammillary nucleus (SuM)-medial septum (MS)-hippocampus circuit across sleep-wake cycles and its role in memory consolidation. The authors demonstrate that the lateral SuM-MS projection is specifically active during REM sleep, and that REM-selective inhibition of this circuit, and of its downstream MS-CA2 pathway, impairs the consolidation of social memory. The work is well-designed, and the data are robust in supporting clear and significant conclusions. It provides important new insights into how distinct memory modalities could be processed by parallel, sleep-active subcortical-hippocampal circuits. The manuscript is of high quality overall, with some points to address as detailed below.

Strengths:

(1) Novel finding:
The identification of a REM-specialized subpopulation within the lateral SuM-MS pathway and its specific role in social memory consolidation via the lateral SuM-MS-CA2 projection is a significant advance. It effectively complements the previously described direct SuM-CA2 pathway and supports a model of the SuM as a "REM-hub" routing information through dedicated downstream targets.

(2) Technical rigor:
The combination of retrograde tracing, in vivo calcium imaging, single-unit identification via optrode recording, and temporally precise (REM-sleep-specific) optogenetic manipulation provides strong correlative and causal evidence.

(3) Appropriate controls:
Behavioral experiments include crucial controls for optogenetic inhibition (GtACR1 group, NREM/Wake inhibition control, mCherry control), effectively ruling out nonspecific effects of light or timing.

Weaknesses:

(1) Figure titles/descriptions:
For clarity, the authors should consider specifying the recording method in the figure titles or legends. For instance, Figure 2: "Bulk Ca2+ activity (fiber photometry) of lateral SuM-MS projecting neurons..." and Figure 3: "Single-unit activity patterns (optrode recordings) of lateral SuM-MS projecting neurons...".

(2) Statistical details:
The use of "LSD post-hoc comparison" following ANOVA is noted. LSD is sensitive but can increase Type I error risk with multiple comparisons. Please justify its use or consider employing a more conservative post-hoc test (e.g., Tukey's or Bonferroni) for key comparisons like the social preference index in Figure 4h to bolster robustness.

(3) Data presentation:
When reporting statistical results in figure legends (e.g., Figures 2d, 3i-k), please provide the specific test statistic values (e.g., F, χ²) and exact P values where possible, rather than only significance asterisks.

(4) Deepening mechanistic insight:
The study excellently demonstrates "what" the circuit does. The discussion could be strengthened by further exploring "how" it might work. The finding that SuM-MS inhibition does not affect CA1 theta power is interesting and distinguishes it from other MS/hippocampal pathways. The suggestion of a theta-independent mechanism is plausible. Could the authors hypothesize more specifically? For example, might this circuit modulate reactivation events in the local CA2 network, neurochemical milieu (e.g., acetylcholine), or synapse-specific plasticity during REM sleep to facilitate social memory consolidation?

(5) Implications of regional heterogeneity:
The functional divergence between lateral (90% REM-active) and medial SuM-MS neurons is intriguing. A brief discussion on the potential anatomical basis (differential inputs/outputs) and functional significance (e.g., integration of specific affective or arousal signals) of this subdivision would be valuable.