Hypothesis

Osteocalcin‑GPR158 signaling in the brainstem sets the gain of descending pain inhibition, and its age‑related decline directly drives reduced pain tolerance, making pain sensitivity a functional read‑out of biological age.

Mechanistic core

1. Osteocalcin crosses the blood‑brain barrier and binds GPR158 in the rostroventral medulla and locus coeruleus, where it stimulates IP3‑dependent BDNF release ([1][1]).
2. BDNF enhances serotonergic and GABAergic tone that fuels the diffuse noxious inhibitory controls (DNIC) and conditioned pain modulation (CPM) pathways ([2][2]).
3. With aging, circulating osteocalcin falls ([1][1]), diminishing GPR158‑BDNF signaling, weakening descending inhibition, and lowering pain thresholds.
4. Osteocalcin‑GPR158 also modulates mitochondrial respiration in vagal preganglionic neurons; loss of this signal increases ROS, reduces vagal tone, and further disinhibits pain circuits ([3][3]).
5. Consequently, an individual’s pain tolerance integrates osteocalcin‑dependent neurochemical and autonomic state, mirroring the cumulative damage captured by epigenetic clocks.

Testable predictions

• Pharmacological rescue: Chronic osteocalcin infusion (or a GPR158 agonist) in 18‑month‑old mice will raise hot‑plate latency and improve CPM scores to levels seen in 3‑month controls, while hippocampal BDNF rises ([1][1],[3][3]).
• Genetic ablation: Mice lacking GPR158 in the brainstem will exhibit early‑onset pain hypersensitivity despite normal osteocalcin levels, and will show accelerated epigenetic aging in blood and brain tissue.
• Human correlation: Plasma osteocalcin will predict pain tolerance (pressure pain threshold and CPM efficiency) independent of CRP, IL‑6, and chronological age in a cohort of 200 adults aged 30‑80. Adding osteocalcin to a multivariable model will improve prediction of GrimAge acceleration beyond standard covariates.
• Activity link: Voluntary wheel running will blunt the age‑related drop in osteocalcin and preserve pain modulation; blocking GPR158 during exercise will abolish this benefit ([4][4]).

Falsifiability

If osteocalcin elevation fails to alter pain thresholds or CPM in aged animals, or if plasma osteocalcin shows no association with human pain tolerance after adjusting for inflammation and epigenetic age, the hypothesis is refuted. Likewise, demonstrating that GPR158 knockout does not affect pain modulation despite altered osteocalcin would break the proposed causal chain.