Figure 7. Acute injection of morphine (3 mg/kg) on day 5 in animals that received saline over 4 days (vehicle group) did not increase dorsal horn tissue levels of 8-OHdG and did not activate PARP when compared with animals that received an equivalent volume of its vehicle (naive group).

On the other hand, acute administration of morphine on day 5 after repeated administration of morphine (morphine group) led to significant increase in dorsal horn tissue levels of 8-OHdG (A) and substantially activated PARP (B). These biochemical changes were significantly attenuated by coadministration of morphine over 4 days with l-NAME (10 mg/kg/d), MnTBAP^3– (10 mg/kg/d), or FeTM-4-PyP⁵⁺ (30 mg/kg/d). When compared with the naive groups, the acute dose of morphine did not increase levels of these cytokines. Results are expressed as mean ± SEM for 12 animals. *P < 0.001 for morphine alone versus vehicle; ^†P < 0.001 for morphine plus drug versus morphine alone.