Anesthetic potencies and the unitary theory of narcosis.

Anesthetic potencies of nitrous oxide, isoflurane, enflurane, halothane, cyclopropane, or chloroform were measured in male (CD-1) mice using three end points: (a) the ability to maintain an upright position (the righting-response ED50, RR ED50); (b) the response to a tail-clamp stimulus (MAC); and (c) the response to heat applied to the tail (tail-flick ED50, TF ED50). The ratio of RR ED50 to MAC ranged from mean values of 0.475 for isoflurane, 0.507 for cyclopropane, 0.524 for enflurane, and less than 0.55 for N2O, to 0.600 for halothane and 0.621 for chloroform. The ratios of RR EDS50 to MAC for halothane differed significantly from the ratios for isoflurane and enflurane. The mean values for TF ED50/MAC and RR ED50/TG ED50 ranged from 0.688 to 1.01 and 0.664 to 0.682, respectively, for isoflurane, enflurane, and halothane. The nitrous oxide RR ED50/TF ED50, however, was 1.34. The ratios of TF ED50 to MAC were significantly different for halothane and isoflurane. The different end points were measured in an attempt to determine whether the response in mice anesthetized with a particular agent was the same with varying stimuli. The differences found suggest that a unitary mechanism of anesthetic action cannot completely explain the depression defined by these three end points.