Aaryan Amin

Background: Spinal Cord Injury (SCI) is a traumatic event that leads to disturbances in sensory, motor, or autonomic function, with an estimated 14.5 million individuals living with the condition globally.¹ Management of acute pain and anxiety is critical during the primary injury phase, and opioids are the current frontline treatment. A 2020 study found that 100% of SCI patients received opioids within 24 hours of their injury.² However, this standard poses a paradox: high opioid doses are associated with increased chronic pain and an eight-fold increase in depression risk one-year post-injury.² Current treatments force patients to trade long-term recovery for temporary pain relief. This review explores the destructive cellular mechanisms triggered by opioids and identifies next-generation therapies that can decouple the necessary pain relief from pathological tissue damage.

Objectives: To review the harmful effects of acute opioid administration on SCI recovery, elucidate the underlying mechanisms behind neuroinflammation associated with the Kappa Opioid Receptor (KOR), and explore the potential of biased KOR agonists as a safer alternative analgesic.

Search Methods: A literature search was conducted on PubMed. Search terms included keywords and MeSH terms such as “Spinal Cord Injury”, “Opioids”, and “Kappa Opioid Receptor” for the timeframe 2018-2026.

Results: Research indicates that the detrimental effects of opioids are not restricted to morphine; intravenous administration of fentanyl, oxycodone, and buprenorphine also significantly decreased motor recovery compared to a saline control.³ Mechanistically, opioids act as catalysts, amplifying the immune response, and increasing resident microglia and macrophages at the lesion site.⁴ These cells produce dynorphin, which binds to the KOR and NMDA receptors, driving neural excitotoxicity.⁵ Specifically, KOR through the b-Arrestin pathway is implicated as a driver of morphine-induced spinal tissue loss.⁵ Blocking these pathways with microglial inhibitors or KOR antagonists reduces tissue loss while preserving analgesia.^4,5 Further, novel bio-engineered biased KOR agonist peptides have been developed. These are able to selectively activate the GPCR pathway that drives analgesia without triggering the destructive b-Arrestin pathway, providing analgesia without inducing neurotoxicity.⁶

Conclusion: The current opioid-heavy standard of care for acute SCI actively undermines long-term recovery through KOR-mediated neuroinflammation and tissue destruction. To improve patient outcomes, future care must shift towards integrated pain management strategies across therapeutic phases that do not undermine the prognosis for long-term recovery. Biased KOR agonists may be an effective alternative for pain management after SCI, offering the necessary analgesia while avoiding the pathological side effects of current therapies. Future research and clinical trials should continue to investigate the efficacy and safety of GPCR-biased KOR agonists in human SCI populations.

Works Cited:

1. Kim M, Jeong W, Jang S, Park JH, Bae Y, Lee SW. Spinal Cord Injury Epidemiology and Causes: A Worldwide Analysis with 2050 Projections. Healthcare (Basel). 2025;13(20):2552. Published 2025 Oct 10. doi:10.3390/healthcare13202552
2. Stampas A, Pedroza C, Bush JN, Ferguson AR, Kramer JLK, Hook M. The first 24 h: opioid administration in people with spinal cord injury and neurologic recovery. Spinal Cord. 2020;58:1080-1089. doi:10.1038/s41393-020-0483-x.
3. Rau J, Joseph R, Weise L, et al. Acute opioid administration undermines recovery after SCI: Adverse effects are not restricted to morphine. J Neurotrauma. 2025;42(23-24):2265-2281. doi:10.1089/neu.2024.0375.
4. Aceves M, Terminel MN, Okoreeh A, et al. Morphine increases macrophages at the lesion site following spinal cord injury: Protective effects of Minocycline. Brain Behav Immun. 2019;79:125-138. doi:10.1016/j.bbi.2019.01.023.
5. Rau J, Hemphill A, Araguz K, et al. Adverse effects of repeated, intravenous morphine on recovery after spinal cord injury in young, male rats are blocked by a Kappa opioid receptor antagonist. J Neurotrauma. 2022;39(23-24):1741-1755. doi:10.1089/neu.2022.0208
6. Lohman RJ, Reddy Tupally K, Kandale A, Cabot PJ, Parekh HS. Design and development of novel, short, stable dynorphin-based opioid agonists for safer analgesic therapy. Front Pharmacol. 2023 Mar 3;14:1150313. doi: 10.3389/fphar.2023.1150313. PMID: 36937883; PMCID: PMC10020352.