Electroacupuncture analgesia, stress responses, and variations in sensitivity in rats anesthetized with different sub-MAC anesthetics

The use of anesthetics to stabilize animals for the purpose of electroacupuncture (EA) analgesic studies can be problematic because of the interference of differential physiological responses to EA and pain. In this study, EA-induced physiological profiles were surveyed under a sub-minimal alveolar concentration (sub-MAC) of two different anesthetics in a previously proposed minimal stress model. First, to select an adequate concentration, compliance with EA and tail-flick stimulation was evaluated under various concentrations of halothane and isoflurane. Second, using the chosen concentrations, low- (4-Hz) and high-frequency (100-Hz) EA were conducted on the right hind limb. The EA effects of the two gases were compared by tail-flick latency (TFL), hemodynamic variables, and individual variations in analgesic sensitivity. The optimal concentrations for halothane and isoflurane were 0.5% and 0.75%, respectively. TFLs were stable under these anesthetic levels, but rats under 0.75% isoflurane had better compliance than those under 0.5% halothane. EA inhibited TFLs with distinct analgesic patterns when comparing high- and low-frequency EA, but TFL suppression did not differ between the two gases. Heart rate and blood pressure showed temporal and differential responses to low- vs. high-frequency EA, but were comparable between groups under the two anesthetics. The ratios of EA non-responders in the isoflurane and halothane groups were 32.4% and 26.7%, respectively, without statistical difference. We concluded that sub-MAC halothane and isoflurane provide optimal conditions for the study of EA-induced analgesia in rats. In this model, 0.75% isoflurane appears to be a better choice than 0.5% halothane in terms of EA compliance.