Perineural antinociceptive effect of opioids in a rat model

Background: The research on conductive analgesia induced by perineural opio ids generated a large body of conflicting data. In this study we reassessed the antinociceptive response to perineural administration of morphine, fen tanyl or meperidine in a rat model. Methods: Analgesia was assessed using the hind paw withdrawal latency (HPWL ) response to radiant heat. The opioid dose producing 20% of maximal possib le effect (20%MPE) for systemic analgesia was calculated for each drug. The n sciatic blockade was performed with the dose corresponding to 20%NME. The injected hind paw was used to measure direct perineural effect and the con tralateral hind paw was used as an indicator of systemic effect. Results: The response latency produced by morphine or fentanyl was not sign ificantly different for ipsilateral (perineural effect) or contralateral (s ystemic effect) paw (27 +/- 11 vs 28 +/- 16 and 31 +/- 16 vs 23 +/- 16 s, r espectively). However, the meperidine group showed significantly higher %MP E for the ipsilateral paw (79 +/- 32 s) than for the contralateral paw (27 +/- 99 s). Conclusions: The results indicate that perineural fentanyl or morphine do n ot produce analgesia. Perineural block produced by meperidine was attribute d to local anesthetic-like effect, rather than to drug interaction with opi oid receptor.