Repeated systemic administration of morphine promotes recovery of locomotor function after spinal cord injury in rats

Targeting activation of astrocytes during early stages of spinal cord injury (SCI) has been viewed as a potential strategy to improve neurological functions. Repeated systemic administration of morphine (RSAM) can induce activation of astrocytes in the spinal cord. Moreover, previous studies also revealed neuroprotective effects of morphine. Therefore, we investigated the effect of RSAM during an acute stage of SCI on neurological outcome in rats. RSAM significantly improved the Basso, Beattie, and Bresnahan score from 7 d after SCI and the inclined plane score from 14 d after SCI. However, the differences in thermal nociceptive thresholds and tactile sensory thresholds between the 2 groups were not statistically significant. RSAM increased expression levels of spinal IL-17, and decreased plasma levels of IL-17 at 1 d and 3 d after SCI. Furthermore, RSAM promoted activation of astrocytes around the damaged area, which prevented the spreading of damage into the surrounding tissue. At 28 d after SCI, RSAM preserved a greater area of white matter. These observations suggest that RSAM during an acute stage of SCI promotes recovery of locomotor function without promoting development of hyperalgesia and allodynia.