SYSTEMIC OPIOIDS DO NOT SUPPRESS SPINAL SENSITIZATION AFTER SUBCUTANEOUS FORMALIN IN RATS

Background: Preemptive treatment with a combination of inhalation anes thesia plus intrathecal morphine has been shown to inhibit development of hyperalgesia that follows subcutaneous (SC) formalin injection in rats. Using a similar paradigm, this study sought to determine whether moderate doses of opioids, administered systemically, could inhibit d evelopment of a hyperalgesic state. Methods: Flinches per minute were observed 1 and 5 min after formalin injection (phase 1) and at 5-min i ntervals for the remainder of 1 h (phase 2) for five groups of rats. A ll animals received isoflurane 1% during and for 6 min after formalin injection. Groups 1 and 2 received SC alfentanil 200 mu g/ kg or norma l saline, respectively, before formalin and 0.5 mg/ kg naloxone SC 6 m in after formalin. Groups 3 and 4 received SC morphine 20 mg/kg or SC normal saline, respectively, before formalin and SC naloxone 0.5 mg/kg plus naltrexone 0.5 mg/kg after formalin. Group 5 received normal sal ine at both injection times. Results: Phase 2 activity was nearly iden tical for the three control groups. Total phase 2 activity for group 1 (alfentanil) was 16% less than control (not significant, P>0.05). Tot al phase 2 activity for group 3 (morphine) was almost identical to con trol. Conclusions: Administration of 1% isoflurane plus systemic opioi ds, administered in doses that produce profound analgesia in standard analgesic testing paradigms, do not produce the significant suppressio n of subsequent hyperalgesia that has been reported with inhalation an esthesia plus intrathecal opioids.