Hypothesis: Long‑term NSAID use damages ileal enterocytes, reducing ASBT‑dependent bile acid reabsorption and lowering circulating bile acids that normally activate spinal TGR5 and FXR receptors to enhance GABAergic tone and suppress glial activation. This loss of bile‑acid‑driven analgesic signaling forces the nervous system to rely on sensitized nociceptors, increasing pain perception while simultaneously impairing bile‑acid‑mediated metabolic regulation (e.g., glucose homeostasis and lipid metabolism). The combined effect creates a vicious loop: heightened pain prompts further analgesic use, worsening gut injury and bile‑acid deficiency, which accelerates cellular senescence and metabolic dysfunction in aging organisms.

Testable predictions:

1. In aged mice receiving chronic low‑dose ibuprofen, ileal ASBT mRNA and protein will be significantly reduced compared with vehicle‑treated controls, accompanied by a measurable decrease in total fecal bile acid pool.
2. These mice will exhibit lowered spinal TGR5/FXR activation (measured by p‑ERK and cAMP levels in dorsal horn lysates) and elevated markers of glial activation (Iba1, GFAP) and nociceptor sensitization (p‑CREB, Nav1.8).
3. Metabolically, the NSAID group will develop insulin intolerance (higher AUC during GTT) and hepatic steatosis despite identical caloric intake.
4. Oral administration of a gut‑restricted TGR5 agonist (e.g., INT‑777) to NSAID‑treated mice will restore spinal bile‑acid signaling, attenuate glial activation and nociceptor sensitization, normalize pain thresholds, and improve glucose tolerance without altering systemic NSAID exposure.
5. Conversely, genetic overexpression of ASBT specifically in ileal enterocytes will protect against NSAID‑induced bile‑acid loss, preserving spinal analgesic tone and metabolic health even in the presence of chronic NSAID exposure.

Falsification: If chronic NSAID treatment fails to alter ileal ASBT expression or fecal bile acid levels, or if TGR5/FXR agonism does not rescue pain sensitivity and metabolic parameters despite confirmed target engagement, the hypothesis would be refuted. Likewise, if ileal‑specific ASBT overexpression does not mitigate NSAID‑induced metabolic or neuropathic phenotypes, the proposed gut‑CNS bile‑acid axis would be deemed insufficient to explain the observed phenotypes.

Mechanistic insight: NSAIDs inhibit cyclooxygenase‑1, reducing prostaglandin‑E2 synthesis that normally supports mucosal integrity and mucus secretion. This compromises the ileal epithelial barrier, leading to increased luminal bile‑acid loss via impaired ASBT recycling. Since bile acids act as endocrine signals that engage neuronal TGR5/FXR to potentiate inhibitory GABAergic transmission and dampen neuroinflammation, their depletion removes a key endogenous brake on pain signaling. The nervous system compensates by upregulating pronociceptive pathways, which in turn feed back to the gut via stress‑axis hormones, further damaging enterocytes. Thus, analgesics intended to relieve pain may inadvertently dismantle a bile‑acid‑dependent feedback loop that simultaneously curbs nociception and preserves metabolic homeostasis, hastening age‑related decline.