Effects of tramadol on minimum alveolar concentration (MAC) of isoflurane in rats

It has been suggested previously that tramadol increases central nervoussystem activity and 'lightens' anaesthesia with volatile agents. Weassessed the effects of tramadol on the minimum alveolar concentration(MAC) of isoflurane in 56 Wistar rats, instrumented chronically with anarterial and central venous catheter. The MAC of isoflurane was determinedusing the tail clamp method under three conditions: (1) after injection ofsaline (control); (2) after administration of tramadol 10 mg kg-1 i.v.; and(3) after administration of morphine 1 mg kg-1 i.v. The studies wererepeated after treatment with the antagonists naloxone or yohimbine.Tramadol and morphine both reduced the MAC of isoflurane from mean 1.38(SEM 0.05)% to 1.22 (0.06)% and 1.17 (0.06)%, respectively (P < 0.05).Concomitant administration of yohimbine did not abolish this reduction inMAC. In contrast, after pretreatment with naloxone, tramadol (1.47 (0.04)%)or morphine (1.38 (0.07)%) did not cause a reduction in the MAC ofisoflurane compared with controls (1.39 (0.06)%). We conclude that tramadoland morphine reduced the MAC of isoflurane to a small but significantextent. For both drugs, this effect was related to their action at opioidreceptors.