EFFECTS OF VOLATILE ANESTHETICS ON SPONTANEOUS ACTION-POTENTIAL FIRING OF CEREBELLAR PURKINJE-CELLS IN-VITRO DO NOT FOLLOW THE MEYER-OVERTON RULE

We have investigated in rat brain slices the effects of the volatile a naesthetics enflurane, isoflurane and halothane on spontaneous dischar ge patterns and mean firing rates of cerebellar Purkinje cells. In the absence of these anaesthetics, Purkinje cells fired bursts of action potentials separated by quiescent periods lasting less than 2 s. Mean discharge rates were 10.8 (SEM 0.4) Hz at 23+/-1 degrees C and 25.6 (1 .2) Hz at 35+/-1 degrees C. The agents exhibited qualitatively differe nt effects when applied at concentrations corresponding to 1-3 MAC. En flurane markedly lengthened burst and inter-burst durations. Isofluran e acted in a similar manner, but effects were less pronounced. In cont rast with isoflurane and enflurane, halothane shortened burst duration s. At concentrations corresponding to 1-1.5 MAC, halothane, isoflurane and enflurane significantly depressed action potential firing by 15-3 0% (P<0.05). Enflurane 1.2 mmol litre(-1) (2.0 MAC), isoflurane 0.9 mm ol litre(-1) (2.8 MAC) and halothane 0.9 mmol litre(-1) (3.8 MAC) depr essed spontaneous spike rates by 50%. The changes in discharge pattern s and the concentration-dependent decrease in the firing rates were si milar at 23+/-1 degrees C and 35+/-1 degrees C. In summary, we observe d that neither the anaesthetic-induced alterations in spontaneous disc harge patterns nor the EC50 values of the concentration-dependent depr ession of the mean firing rates were in accordance with the Meyer-Over ton rule. However, at clinically relevant concentrations, depression o f average spike rates did not differ significantly between the anaesth etics and thus followed the rule. Our results suggest that anaesthetic actions, which are in accordance with the rule, are frequently masked by several side effects.