The subnucleus reticularis dorsalis is involved in antinociception produced by a low dose of naloxone during carrageenan-induced inflammation.

The present study was designed to investigate a role of the subnucleus reticularis dorsalis (SRD) in the analgesia produced by a low dose of naloxone during carrageenan-induced inflammation. Male Sprague-Dawley rats were divided into the following two groups: (1) rats with bilateral lesions of the SRD (n = 13) and 2) sham-operated rats (n = 24). In each group, effects of a low dose of naloxone (5 microg/kg, i.p.) on thermal nociception were examined 4 h, 7 and 28 days after the induction of unilateral inflammation. Carrageenan (6 mg in 0.15 ml saline) was injected subcutaneously into the plantar surface of the left hindpaw. The analgesic effect was assessed by prolongation of the paw withdrawal latency (PWL) to heating. Prior to carrageenan injection, a low dose of naloxone did not prolong PWLs in either group. Four hours after carrageenan, a low dose of naloxone produced a prolongation of PWLs in both sham-operated and SRD-lesioned rats. Seven days after carrageenan, naloxone failed to produce analgesia in the SRD-lesioned rats but did produce analgesia in the sham-operated rats. At 28 days, a low dose of naloxone induced hyperalgesia in the inflamed paw of both groups, whereas naloxone was ineffective in the contralateral non-inflamed paw. These results suggest that the SRD plays a role in naloxone-induced analgesia during the subacute phase of inflammation (e.g. 7 days after induction of inflammation).