Fig. 1.

Decreased antinociceptive activity of acute morphine in L5 nerve-transected rats. Behavioral response to noxious thermal (tail-flick test) and mechanical (paw-pressure test) stimuli was recorded on postoperative days 6 or 11. Antinociceptive activity of acute morphine (M; 1–10 mg/kg) was recorded 45 min after its intravenous administration and expressed as a percentage of MPE. A rightward shift in the dose–response curve of morphine in L5 nerve-transected rats (L5Tx) compared with sham-operated rats indicates decreased antinociceptive activity of morphine in neuropathic conditions. Values are mean ± SEM (n = 6). *p < 0.05 versus basal latency (recorded before morphine administration);⁺p < 0.05 versus sham-operated rats (Bonferroni test).