Fig. 6.  (A ) Effects of ruthenium red on the prevention of fentanyl-induced hyperalgesia by gabapentin. Four fentanyl injections (60 μg/kg) were performed subcutaneously at 15-min intervals. Intrathecal gabapentin, 300 μg, with intrathecal ruthenium red (20 ng) (n = 8) or saline (n = 8) was administered 30 min before the first fentanyl injection. The basal nociceptive threshold was evaluated after treatment for 5 h on D⁰and then daily for 5 days (D⁺¹–D⁺⁵). Mean nociceptive thresholds (±SD) are expressed in grams. * Dunnett test,  P < 0.05 for comparison between saline-injected  versus gabapentin-injected groups. # Dunnett test,  P < 0.05 for comparison between ruthenium red-injected  versus saline-injected groups in gabapentin-administered rats.  (B ) Algesic indices of the effects of ruthenium red on the prevention of fentanyl-induced hyperalgesia by gabapentin. Plus symbol (+): pharmacologic treatments (fentanyl, gabapentin, ruthenium red); minus symbol (−): saline injections. * Mann–Whitney test,  P < 0.05 compared with the reference algesic index: long-lasting hyperalgesia induced by fentanyl injections in saline-treated rats. # Mann–Whitney test,  P < 0.05 for comparison between ruthenium red-injected  versus saline-injected groups in gabapentin-administered rats. IT = intrathecal; SC = subcutaneous.