Volatile anaesthetics as central nervous system excitants.

Volatile anaesthetics (VAs) act essentially as central nervous system (CNS) depressants. However, many evidences obviously demonstrate that VAs are also CNS excitants. In this review, we adduce our previous studies which are relevant to the behavioural evidences of the CNS excitation actions of VAs. Anaesthetic-induced opisthotonus (OP) found in mice is thought to be a sign of CNS stimulation and the order of the incidence inversely coincides with that of blood/gas partition coefficient, that is, the smaller the coefficient, the higher the incidence of OP as far as the halogenated ether anaesthetics are concerned. In other words, the shorter the induction time to anaesthesia, the greater the incidence. A striking observation is that the incidence due to isoflurane is much higher than that to enflurane, while in humans, enflurane often produces convulsion whereas isoflurane when administered alone has not been found to cause electroencephalographic or clinical seizures in anaesthetized patients. We also found that enflurane-induced OP is possibly mediated by N-methyl-D-aspartate- and glycine-receptor but not GABAA-receptor and that the intensity of immunoreactivity of methionine-enkephalin in hippocampal mossy fibre is well correlated with the incidence among three strains of mice. We demonstrated that the open-field activity in ddN mice significantly increased on and after emergence from anaesthesia with enflurane and isoflurane. The effect of very low concentrations of halothane, enflurane and isoflurane on acquired active avoidance training was assessed.(ABSTRACT TRUNCATED AT 250 WORDS)