Acute Opioid Administration Undermines Recovery after SCI: Adverse Effects Are Not Restricted to Morphine

Previous studies have shown that administration of high doses of morphine in the acute phase of spinal cord injury (SCI) significantly undermines locomotor recovery and increases symptoms of chronic pain in a rat spinal contusion model. Similarly, SCI patients treated with high doses of opioid for the first 24 h postinjury have increased symptoms of chronic pain 1 year later. Whether these adverse effects are driven by morphine only or all opioids compromise recovery after SCI, however, is unknown. Based on our previous findings we hypothesized that activation of the kappa opioid receptor (KOR) is key in the morphine-induced attenuation of locomotor recovery after SCI. Thus, we posited that opioids that engage KOR-mediated signaling pathways (morphine, oxycodone) would undermine recovery, and clinically relevant opioids with less KOR activity (fentanyl and buprenorphine) would not. To test this, we compared the effects of the clinically relevant opioids on locomotor recovery and pain in a male rat spinal contusion model. Rats were given a moderate spinal contusion injury followed by 7 days of intravenous morphine, oxycodone, fentanyl, buprenorphine, or saline, and recovery was assessed for 28 days. All opioids produced analgesia on tests of thermal, mechanical, and incremented shock reactivity. However, tolerance developed rapidly with buprenorphine administration, particularly with daily administrations of 5 morphine milligram equivalent (MME) buprenorphine. Opioid-induced hyperalgesia (OIH) also developed across days following administration of higher doses (10 MME, 20 MME) of morphine and oxycodone. Fentanyl and buprenorphine did not produce OIH. Contrary to our hypothesis, however, we found that high doses of all opioids reduced recovery of locomotor function. Unlike the other opioids, the effects of buprenorphine on locomotor recovery appeared transient, but it also produced chronic pain. Morphine, oxycodone, and buprenorphine decreased reactivity thresholds on tests of mechanical and incremented shock stimulation. In sum, all opioids undermined long-term recovery in the rat model. Further interrogation of the molecular mechanisms driving the adverse effects is essential. This study provides critical insight into pain management strategies in the acute phase of SCI and potential long-term consequences of early opioid administration.