Chronic morphine administration enhances nociceptive sensitivity and local cytokine production after incision

Abstract

Background: The chronic use of opioids prior to surgery leads to lowered pain thresholds and exaggerated pain levels after these procedures. Several mechanisms have been proposed to explain this heightened sensitivity commonly termed opioid-induced hyperalgesia (OIH). Most of these proposed mechanisms involve plastic events in the central or peripheral nervous systems. Alterations in the abundance of peripheral mediators of nociception have not previously been explored.

Results: In these experiments mice were treated with saline (control) or ascending daily doses of morphine to generate a state of OIH followed by hind paw incision. In other experiments morphine treatment was initiated at the time of incision. Both mechanical allodynia and peri-incisional skin cytokine levels were measured. Myeloperoxidase (MPO) assays were used to determine neutrophil activity near the wounds. The cytokine production inhibitor pentoxifylline was used to determine the functional significance of the excess cytokines in previously morphine treated animals. Mice treated chronically treated with morphine prior to incision were found to have enhanced skin levels of IL-1beta, IL-6, G-CSF, KC and TNFalpha after incision at one or more time points compared to saline pretreated controls. The time courses of individual cytokines followed different patterns. There was no discernable effect of chronic morphine treatment on wound area neutrophil infiltration. Pentoxifylline reduced cytokine levels and reversed the excess mechanical sensitization caused by chronic morphine administration prior to incision. Morphine treatment initiated at the time of incision did not lead to a generalized enhancement of cytokine production or nociceptive sensitization in excess of the levels observed after incision alone.

Conclusion: The enhanced level of nociceptive sensitization seen after incision in animals chronically exposed to morphine is associated with elevated levels of several cytokines previously reported to be relevant to this incisional pain model. The cytokines may be functional in supporting nociceptive sensitization because pentoxifylline reverses both peri-incisional skin cytokine levels and OIH. Opioid administration beginning at the time of incision does not seem to have the same cytokine enhancing effect. Approaches to postoperative pain control involving a reduction of cytokines may be an effective way to control excessive pain in patients chronically using opioids prior to surgical procedures.

Figures

Figure 1

Mechanical nociceptive thresholds after drug pretreatment and hindpaw incision. Panel A displays the mechanical thresholds of mice in four different treatment groups: saline pretreatment/no incision, saline pretreatment/incision, morphine pretreatment/no incision, morphine pretreatment/incision. The values labeled pre-incision represent the nociceptive thresholds measured after 4 days of saline or morphine treatment but prior to hind paw incision. In panel B data are presented representing mechanical nociceptive thresholds in mice undergoing saline or morphine treatment beginning at the time of incision. Nociceptive thresholds were measured immediately before that day's dose of morphine. The statistical analysis presented reflects the results of two-way ANOVA comparing saline/incision values to morphine/incision ones. *p < 0.05, **p < 0.01, N = 6/group.

Figure 2

Peri-incisional skin cytokine levels after saline or morphine pretreatment. The figure displays the mechanical thresholds of mice in four different treatment groups: saline pretreatment/no incision, saline pretreatment/incision, morphine pretreatment/no incision, morphine pretreatment/incision. The cytokine levels were assessed at time points out to 72 hours after incision based on the results of the experiments presented in Figure 1. The statistical analysis presented reflects the results of two-way ANOVA comparing saline/incision values to morphine/incision ones over the 2 to 72 hour time period. *p < 0.05, **p < 0.01, N = 8/group.

Figure 3

Peri-incisional skin cytokine levels for mice treated with saline or morphine beginning at the time of incision. The figure displays cytokine measurements from mice in four different treatment groups: saline, incision/saline, no incision/morphine, incision/morphine. The cytokine levels were assessed at 120 hours after the incisions were made (18 hours after the last dose of morphine). Statistical analysis was performed to detect differences between each set of conditions. N = 8/group.

Figure 4

Myeloperoxidase (MPO) activity after hind paw incisions. For these experiments MPO activity was measured in peri-incisional skin as an index of infiltrating neutrophil activity. Skin was harvested from separate groups of mice at the indicated time points and processed for MPO assays as described in Methods. The time course for analysis was the same as that used for the behavioral and cytokine assays. Statistical analysis failed to detect between group differences at any of these time points. N = 8/group.

Figure 5

Mechanical allodynia after incision as affected by morphine and pentoxifylline. In these experiments mice were subjected to nociceptive testing of the hind paw to establish the baseline threshold. Mice were then pre-treated with either saline or morphine for 4 days prior to hind paw incisions being made. After incision, groups of mice were treated daily with daily intraperitoneal saline or pentoxifylline as described in Methods. Nociceptive testing 72 hours after hind paw incision (2 hours after the last dose of pentoxifylline). **p < 0.01, N = 8/group.