Differential effects of halothane and isoflurane on lumbar dorsal horn neuronal windup and excitability

Background. Windup of spinal nociceptive neurones may underlietemporal summation of pain, influencing the minimum alveolarconcentration (MAC) of anaesthetics required to prevent movementto supramaximal stimuli. We hypothesized that halothane andisoflurane would differentially affect windup of dorsal hornneurones. Methods. We recorded 18 nociceptive dorsal horn neurones exhibitingwindup to 1 Hz electrical hindpaw stimuli in rats. Effects of0.8 and 1.2 MAC isoflurane and halothane were recorded in thesame neurones (counterbalanced, crossover design). Windup wascalculated as the total number of C-fibre (100–400 mslatency) plus afterdischarge (400–1000 ms latency) spikes/20stimuli (area under curve, AUC) or absolute windup (C-fibreplus afterdischarge–20xinitial response). Results. Increasing isoflurane from 0.8 to1.2 MAC did not affectAUC, but increased absolute windup from 429 (62) to 618 (84)impulses/20 stimuli (P<0.05) and depressed the initial C-fibreresponse from 14 (3) to 8 (2) impulses (P<0.05). Increasinghalothane from 0.8 to1.2 MAC depressed AUC from 690 (79) to537 (65) impulses/20 stimuli (P<0.05) and the initial responsefrom 18 (2) to 13 (2) impulses (P<0.05), but absolute windupwas not affected. Absolute windup was 117% greater during 1.2MAC isoflurane compared with 1.2 MAC halothane. Conclusions. Windup was significantly greater under isofluranethan halothane anaesthesia at 1.2 MAC, whereas the initial C-fibreresponse was suppressed more by isoflurane. These findings suggestthat these two anaesthetics have mechanistically distinct effectson neuronal windup and excitability.