Hypothesis

Age‑related gut dysbiosis elevates circulating tryptophan‑derived kynurenine, which activates aryl hydrocarbon receptor (AhR) signaling in microglia. This shifts microglia toward a phagocytic state that excessively tags and removes synapses via complement‑dependent mechanisms, producing the behavioral rigidity labeled as “over‑consolidation.” Blocking kynurenine synthesis or AhR activation will reduce maladaptive synaptic pruning, reinstate synaptic turnover, and improve cognitive flexibility without globally increasing baseline plasticity.

Mechanistic Rationale

• Gut microbes modulate host tryptophan metabolism; aged microbiota show increased expression of tryptophanase enzymes that generate kynurenine (1).
• Kynurenine is a ligand for AhR, a transcription factor known to upregulate microglial complement components (C1q, C3) that flag synapses for engulfment (2).
• Complement‑mediated synaptic tagging drives microglial phagocytosis, a process demonstrated to cause hippocampal synapse loss in aged mice receiving aged‑donor fecal transplants (3).
• Reducing kynurenine levels (via IDO1 inhibition) or blocking AhR signaling lowers complement deposition, preserving synapses while allowing experience‑dependent remodeling.

Novel Insight

The prevailing view treats age‑related synaptic loss as either passive neurodegeneration or excessive consolidation. We propose that the loss is an active immune‑mediated pruning program triggered by a microbial metabolite, and that the resulting circuit rigidity reflects a biased prediction engine that cannot incorporate surprise because synapses needed for new learning are prematurely eliminated. Restoring microbial balance or interrupting the kynurenine‑AhR‑complement axis re‑introduces the synaptic substrate necessary for uncertainty‑driven learning.

Testable Predictions

1. Biochemical – Aged mice will have higher plasma kynurenine and hippocampal AhR‑target gene expression than young controls; IDO1 treatment will normalize both.
2. Cellular – Microglia from aged brains will show increased C1q colocalization with synaptic markers (PSD‑95, Synaptophysin); this will be reduced by AhR antagonism.
3. Behavioral – In the attentional set‑shifting test, aged mice treated with IDO1 inhibitor or AhR antagonist will commit fewer reversal errors, performing similarly to young mice, whereas microbiota transplant alone will not achieve full rescue.
4. Synaptic Dynamics – In vivo two‑photon imaging will reveal elevated baseline spine turnover in treated aged mice, indicating restored capacity for structural change without global hyperplasticity.

Experimental Design (Falsifiable)

• Groups (n=12 per group, male & female aged 20‑month mice):
  1. Vehicle control
  2. IDO1 inhibitor (e.g., epacadostat)
  3. AhR antagonist (e.g., CH‑223191)
  4. Bifidobacterium pseudolongum supplementation (positive control from prior work)
  5. Combination IDO1 inhibitor + B. pseudolongum
• Measurements (after 4 weeks):
  • Plasma kynurenine/tryptophan ratio (LC‑MS)
  • Hippocampal AhR target gene expression (qPCR)
  • Microglial C1q immunoreactivity and synaptic colocalization (confocal)
  • Attentional set‑shifting performance (trials to criterion)
  • In vivo dendritic spine turnover (thumbnail imaging)

Potential Outcomes

• Support: Groups 2‑5 show reduced kynurenine, AhR signaling, microglial synaptic tagging, and improved set‑shifting; spine turnover increases modestly but remains within youthful range.
• Refutation: No significant differences in kynurenine/AhR readouts, microglial pruning, or behavior across treatments; or spine turnover rises excessively, indicating global hyperplasticity rather than targeted rescue.

If the hypothesis holds, it reframes cognitive aging as a metabolite‑driven immune pruning problem solvable by targeting the gut‑derived kynurenine pathway, offering a precise, mechanistic alternative to vague “over‑consolidation” narratives.