Background and Objectives To investigate whether endogenous opioids might be involved in the mechanisms that underlie hemorrhagic shock-induced analgesia, formalin tests were performed after hemorrhage and reinfusion in naloxone pretreated and untreated rats.

Methods Twenty-four adult male Sprague-Dawley rats were divided into control (n = 6), saline (n = 6), naloxone 10 mg/kg (n = 6), and naloxone 100 mg/kg (n = 6) groups. The mean blood pressure (mBP) was kept at 50 to 60 mm Hg for 30 minutes by draining arterial blood in the saline group and the naloxone groups. After 15 minutes of returning mBP to normal levels by reinfusion of the drained shed blood, 10% formalin (3.7% formaldehyde solution, 0.1 mL) was injected into the left rear paw. Nociceptive behaviors were observed for 1 hour after the formalin injection.

Results Nociceptive behaviors of the posthemorrhagic shock groups were significantly lower than those of the control group. No significant difference was seen in nociceptive behaviors among the saline and naloxone groups.

Conclusion Naloxone did not reverse the hemorrhagic shock-induced analgesia, which suggests that endogenous opioids might not be a major factor that governs stress-induced analgesia (SIA) after hemorrhagic shock.