Fig. 2.

L5 nerve transection enhances the development of morphine tolerance to analgesic and antiallodynic actions. Chronic morphine (10 mg/kg, s.c., twice daily for 5 d) treatment was initiated on postoperative day 6 to L5 nerve-transected (L5Tx) and sham-operated rats. Antinociceptive and antiallodynic activity of morphine in these rats was accessed on days 1, 3, and 5 of the treatment (i.e., postoperative days 6, 8, and 10). Behavior recorded before the beginning of morphine treatment represents baseline responses. Analgesic activity of morphine against noxious thermal and mechanical stimuli was expressed as a percentage of MPE, whereas antiallodynic activity was expressed as average numbers of paw withdrawals to 30 stimuli of 12 gm of von Frey filament. A significant decrease in the antinociceptive action of morphine (both in the tail-flick and in the paw-pressure tests) in L5 nerve-transected rats (L5Tx-morphine) compared with sham rats (Sham-morphine) on day 3 of the treatment indicates early development of morphine tolerance in neuropathic conditions. Values are mean ± SEM (n = 8–10). *p < 0.05 versus antinociceptive or antiallodynic activity of morphine observed on the first day of its treatment;⁺p < 0.05 versus morphine-treated, sham-operated (Sham-morphine) rats (Bonferroni test). Note that sham-operated rats did not develop tactile allodynia, preventing evaluation of the antiallodynic activity of morphine in these animals.