An IL-1 receptor antagonist blocks a morphine-induced attenuation of locomotor recovery after spinal cord injury

Abstract

Morphine is one of the most commonly prescribed medications for the treatment of chronic pain after a spinal cord injury (SCI). Despite widespread use, however, little is known about the secondary consequences of morphine use after SCI. Unfortunately, our previous studies show that administration of a single dose of morphine, in the acute phase of a moderate spinal contusion injury, significantly attenuates locomotor function, reduces weight gain, and produces symptoms of paradoxical pain (Hook et al., 2009). The current study focused on the cellular mechanisms that mediate these effects. Based on data from other models, we hypothesized that pro-inflammatory cytokines might play a role in the morphine-induced attenuation of function. Experiment 1 confirmed that systemic morphine (20 mg/kg) administered one day after a contusion injury significantly increased expression levels of spinal IL-1β 24 h later. Experiment 2 extended these findings, demonstrating that a single dose of morphine (90 μg, i.t.) applied directly onto the spinal cord increased expression levels of spinal IL-1β at both 30 min and 24 h after administration. Experiment 3 showed that administration of an interleukin-1 receptor antagonist (IL-1ra, i.t.) prior to intrathecal morphine (90 μg), blocked the adverse effects of morphine on locomotor recovery. Further, pre-treatment with 3 μg IL-1ra prevented the increased expression of at-level neuropathic pain symptoms that was observed 28 days later in the group treated with morphine-alone. However, the IL-1ra also had adverse effects that were independent of morphine. Treatment with the IL-1ra alone undermined recovery of locomotor function, potentiated weight loss and significantly increased tissue loss at the injury site. Overall, these data suggest that morphine disrupts a critical balance in concentrations of pro-inflammatory cytokines in the spinal cord, and this undermines recovery of function.

Figure 1

A single dose of systemic morphine (20 mg/kg, i.p.) significantly increased expression levels of IL-1β in spinal tissue from the contusion injury site (A). The tissue was assayed 24 hrs after morphine administration, and 48 hrs after injury. Morphine did not significantly affect IL-6 levels (B). n = 8 for both groups, and ** p < 0.05.

Figure 2

As found for systemic morphine, there was a significant main effect of intrathecal morphine on expression levels of IL-1β at the contusion injury site (A). Increased expression levels were observed at both 30 min and 24 hrs following morphine administration. However, expression levels determined at 30 min were also significantly higher than those observed at 24 hrs. Similarly, there was a significant main effect of time on IL-6 expression levels (B). Although there was a strong tendency for morphine to increases levels of this pro-inflammatory cytokine also (p = 0.08), this effect was not significant. n = 8 for all groups** p < 0.05.

Figure 3

Intrathecal morphine (90 µg, i.t.) administered 24 hrs after a moderate contusion significantly attenuated recovery of locomotor function, relative to vehicle-treated controls (0 µg IL-1ra). Pretreatment with an IL-1ra (1 and 3 µg) blocked the adverse effects of morphine on recovery. Further analyses of the average BBB scores when performances stabilized (days 15–21post-injury) also revealed a significant linear trend for decreased recovery as the dose of IL-1ra was increased for vehicle-treated (no morphine) controls. n = 10 for all groups, ** p < 0.05.

Figure 4

The number of vocal responses made to the application of an innocuous von Frey filament in the girdle region is depicted. Pre-treatment with 3 µg of the IL-1ra prior to morphine, and alone, significantly decreased vocal reactivity compared with vehicle-treated subjects. For the morphine-treated groups, there was also a significant linear trend for a dose-dependent decrease in vocal reactivity to von Frey stimulation as the dose of IL-1ra increased.

Figure 5

The mean weight loss for each group (± SEM) over the 21 days recovery period is depicted as a change from baseline (Day 1) score. In the 0 µg IL-1ra groups, there was a tendency for morphine alone to increase weight loss, compared with vehicle-treated subjects, but this effect was not significant (p= 0.056). There was, however, a significant effect of the dose of IL-1ra on weight loss. Independent of morphine treatment, there was a concomitant increase in weight loss as the IL-1ra dose increased. Analyses of the average weight loss from Days 15–21 confirmed that vehicle controls (no morphine) treated with 3 µg IL-1ra lost significantly more weight than vehicle control subjects treated with 1 µg of IL-1ra or vehicle only. ** p < 0.05.

Figure 6

Morphine treatment appeared to protect the contused cord against effects of the high dose IL-1ra (3 µg) treatment on lesion size and the amount of gray and white tissue sparing. Subjects treated with morphine displayed less lesion caudal to the injury center (where the morphine and IL-1ra was applied), with less missing tissue and increased areas of gray matter sparing, compared with vehicle-treated controls (A, main effect of morphine treatment, p < 0.05). The increased tissue loss observed in the vehicle-treated subjects was driven primarily by treatment with the 3 µg of IL-1ra. Pre-treatment with 3 µg of IL-1ra significantly increased relative lesion size rostral to the injury site and decreased white matter sparing in the vehicle treated controls, but the same dose did not effect these indices in morphine-treated subjects (B, interaction between morphine and IL-1ra dose, p < 0.05). There was also a significant effect (**p <0.05) of the 3 µg IL-1ra dose on gray matter sparing at the center of the lesion, compared with the 1 µg IL-1ra dose.