Hypothesis

Age‑related loss of excitatory cholinergic neurons in the myenteric plexus results from a preferential decline in mitochondrial complex I activity within this subpopulation, rendering them vulnerable to oxidative stress and calcium dysregulation. Preserved nitrergic neurons maintain complex I function but develop structural abnormalities due to chronic nitric‑oxide‑mediated protein modifications.

Mechanistic Rationale

Cholinergic myenteric neurons exhibit higher basal firing rates and rely heavily on oxidative phosphorylation to sustain acetylcholine synthesis and vesicular release【1】. This metabolic profile makes them uniquely sensitive to defects in complex I, the primary entry point for electrons into the electron transport chain. Age‑associated accumulation of mitochondrial DNA mutations or oxidative damage to complex I subunits would therefore impair ATP production disproportionately in cholinergic cells, triggering energetic failure, axonal degeneration, and ultimately cell death.

Nitrergic neurons, by contrast, utilize a greater proportion of glycolytic ATP and possess upregulated antioxidant enzymes (e.g., SOD2, glutathione peroxidase) that buffer reactive oxygen species generated during nitric oxide synthesis【2】. This metabolic bias spares them from outright loss but exposes them to persistent S‑nitrosylation of cytoskeletal proteins (e.g., tubulin, neurofilaments), leading to axonal swelling and structural compromise without triggering apoptosis.

The distal colon shows the steepest cholinergic decline because its myenteric plexus receives the least vagal trophic support, reducing compensatory NAD+ salvage pathways【3】. Consequently, regional differences in complex I resilience mirror the spatial pattern of motility dysfunction.

Testable Predictions

1. Biochemical: Isolated cholinergic myenteric neurons from aged mice will show ↓ complex I activity (measured by NADH‑ubiquinone reductase assay) and ↑ ROS relative to nitrergic peers; complex I activity will correlate with cholinergic neuron density across proximal‑mid‑distal colonic segments.
2. Genetic: Neuron‑specific knockdown of Ndufs4 (a core complex I subunit) in Chat‑positive cells will recapitulate the age‑related cholinergic loss and motility deficits seen in wild‑type aged animals, whereas the same knockdown in Nos1‑positive cells will not reduce neuron number but will reproduce axonal swelling.
3. Pharmacological: Chronic oral administration of the mitochondria‑targeted peptide SS‑31 (which stabilizes cardiolipin and enhances complex I efficiency) will preserve cholinergic neuron counts, reduce axonal swelling in nitrergic fibers, and improve colonic transit measured by the 3D‑Transit capsule system in aged rodents【4】. Failure of SS‑31 to rescue cholinergic loss would falsify the hypothesis.
4. Rescue Experiment: Viral‑mediated overexpression of NDUFA4L2 (a complex I stabilizer) selectively in cholinergic neurons will prevent age‑related decline in acetylcholine release and restore propagating contractile complexes without altering nitrergic morphology.

Implications

If validated, this hypothesis shifts the focus from generic oxidative stress to a specific bioenergetic vulnerability, explaining why cholinergic neurons die while nitrergic cells persist but malfunction. It also suggests that therapies targeting mitochondrial complex I—rather than broad antioxidants—could selectively protect the excitatory arm of the ENS, restoring the excitatory‑inhibitory balance essential for propulsive motility.