Hypothesis

During NREM sleep rising extracellular adenosine activates astrocytic A1 receptors, which increases AQP4 polarization and drives glymphatic flow while simultaneously tagging low‑activity synapses for complement‑mediated removal. Sleep deprivation blunts this adenosine signal, reducing both waste clearance and synaptic triage, leading to the accumulation of dysfunctional connections and contributing to cognitive decline.

Mechanistic Basis

• Adenosine levels rise with prolonged wakefulness and peak during slow‑wave sleep, activating A1 receptors on astrocytes (see adenosine and sleep article).
• A1 receptor activation triggers intracellular cAMP reduction, promoting AQP4 redistribution to perivascular endfeet, a state required for efficient glymphatic influx (see AQP4 polarization study).
• The same adenosine‑A1 pathway upregulates astrocytic release of cytokines that stimulate microglial C1q expression, initiating the complement cascade that tags synapses with low recent activity for removal (see complement synaptic pruning paper).
• Glymphatic influx then carries away complement‑tagged synaptic debris and extracellular metabolites, a process visualized by increased CSF tracer clearance during sleep (see glymphatic MRI study).

Testable Predictions

1. Pharmacological enhancement of adenosine signaling during wakefulness (using an A1 agonist such as CPA) will increase glymphatic influx measured by intrathecal CSF tracer kinetics and elevate CSF levels of complement‑tagged synaptic proteins (e.g., C3b‑synaptophysin complexes).
2. Genetic astrocytic knockout of A1 receptors will attenuate both AQP4 polarization and complement deposition on synapses after sleep, resulting in higher cortical synaptic density measured by SV2A PET despite normal sleep duration.
3. In humans, a night of selective slow‑wave sleep deprivation will reduce CSF adenosine concentration, decrease AQP4 perivascular localization inferred from diffusion‑tensor MRI perivascular space enlargement, and lower CSF C3 levels relative to a control sleep night.
4. Chronic caffeine consumption (an adenosine receptor antagonist) will correlate with reduced glymphatic clearance rates (via intrathecal Gd‑DOTA clearance) and higher burden of complement‑positive synapses in post‑mortem tissue from aged donors.

Falsifiability

If augmenting adenosine signaling fails to raise glymphatic flow or complement‑mediated synaptic tagging, or if blocking A1 receptors does not impair these processes, the hypothesis would be refuted. Conversely, demonstrating that adenosine manipulation independently alters both clearance kinetics and synaptic complement deposition would support the proposed mechanism.

Implications

This framework re‑positions sleep not merely as a passive cleanup shift but as an active decision‑making window where neuromodulatory state (adenosine) couples fluid dynamics to immune‑like synaptic selection. Disrupting this coupling could explain why sleep loss accelerates neurodegeneration beyond simple toxin buildup, offering a dual‑target therapeutic avenue: enhance adenosine signaling to boost both glymphatic flow and synaptic triage.