Hypothesis

We propose that sleep-dependent glymphatic flux actively clears corticotropin-releasing factor (CRF) and associated inflammatory peptides from the amygdala extracellular space. When this nocturnal clearance is impaired—due to aging, sleep fragmentation, or glymphatic dysfunction—CRF accumulates, sustains basolateral amygdala (BLA) hyperexcitability via enhanced CRF‑R1 signaling, and blocks fear extinction independent of memory consolidation (see [1][2]).

Mechanistic Basis

During wakefulness, CRF released from central amygdala (CeA) neurons diffuses into the BLA parenchyma, where it binds CRF‑R1 on glutamatergic principal cells, increasing intracellular calcium and promoting a hyperexcitable state. Concurrently, inflammatory mediators such as IL‑1β and TNF‑α are co‑released, amplifying neuronal excitability through NF‑κB–dependent up‑regulation of NMDA receptor subunits. The glymphatic system, driven by arterial pulsatility and aquaporin‑4 (AQP4)‑mediated convective flow, preferentially clears soluble peptides and cytokines from the interstitial fluid during NREM sleep. Evidence shows that AQP4 polarization declines with age, reducing convective clearance. If CRF and inflammatory peptides are not efficiently removed, their extracellular concentration builds up, leading to persistent CRF‑R1 activation, sustained ERK phosphorylation, and a shift in the excitation/inhibition balance toward excitation. This molecular milieu impairs the synaptic plasticity mechanisms underlying extinction learning, such as long‑term depression (LTD) at BLA‑prefrontal synapses, without necessarily affecting the storage of the original fear memory (see [3][4][5]).

Novel Insight

We extend the “autopsy” metaphor by positing that the brain does not merely discard waste; it performs a selective triage where high‑affinity CRF‑binding extracellular vesicles are earmarked for glymphatic removal. Failure to clear these vesicles results in their uptake by microglia, triggering NLRP3 inflammasome activation and a secondary wave of IL‑1β release that further exacerbates CRF signaling—a positive feedback loop that locks the amygdala in a hyperreactive state.

Predictions

1. Aged mice with fragmented sleep will show higher extracellular CRF levels in the BLA measured by microdialysis during the light phase compared with young, well‑sleeping controls.
2. Pharmacological enhancement of glymphatic flow (e.g., via intranasal administration of dilute hypertonic saline to increase arterial pulsatility) will reduce BLA CRF concentration and rescue extinction learning in aged, sleep‑disrupted mice, even when total sleep time remains unchanged.
3. Conditional knockout of AQP4 in astrocytes of the BLA will mimic the effects of sleep fragmentation: increased CRF accumulation, heightened CRF‑R1‑dependent ERK activation, and impaired extinction, without altering baseline fear memory.
4. Inhibiting microglial NLRP3 (with MCC950) will break the CRF‑IL‑1β feedback loop and partially restore extinction in aged mice with impaired glymphatic clearance, indicating that the maladaptive inflammatory response is downstream of clearance failure.

Experimental Approach

• Subjects: Young (3 mo) and aged (18‑20 mo) C57BL/6J mice; subgroups: ad libitum sleep, sleep fragmentation (gentle handling every 2 min during NREM), and glymphatic enhancement (intranasal hypertonic saline 0.5 % twice daily).
• Procedures: Fear conditioning (tone‑shock), extinction training 24 h later, extinction recall 24 h after training. Microdialysis probes in BLA to sample extracellular CRF and IL‑1β across sleep‑wake cycles. Western blot for p‑ERK, CRF‑R1, NLRP3, and ASC in microdissected BLA. Immunohistochemistry for AQP4 polarization and microglial Iba1/NLRP3 colocalization.
• Analysis: Two‑way ANOVA (age × treatment) for CRF levels, extinction scores, and signaling markers. Correlation analysis between extracellular CRF concentration and extinction performance. Significance set at p < 0.05.

If the data confirm that restoring glymphatic clearance lowers amygdalar CRF and rescues extinction despite persistent sleep fragmentation, the hypothesis would support the view that sleep’s role in anxiety regulation is primarily a molecular housekeeping function rather than a memory‑consolidation one. Conversely, if clearance manipulation fails to affect CRF or extinction, the hypothesis would be falsified, directing focus toward alternative sleep‑dependent mechanisms.