The divergent actions of volatile anaesthetics on background neuronal activity and reactive capability in the central nervous system in cats.

The effects of halothane, isoflurane, and enflurane on background neuronal activity and reactive capability in the central nervous system were studied in cats. The background neuronal activity was assessed by midbrain reticular cell firing, which was measured by the method of multi-unit activity, and the EEG in the cortex, amygdala, and hippocampus. The reactive capability was assessed by evoked responses in the visual neuronal pathway. All anaesthetics studied suppressed reticular cell firing in a dose-dependent manner, and the suppression by halothane (43.8 +/- 10.3% of control, mean +/- SD) was less than isoflurane (66.5 +/- 5.8%, P < 0.01) and enflurane (73.1 +/- 8.8%, P < 0.05) at 1 MAC. Spontaneous EEG spikes developed at 4.8% isoflurane and 3.6% enflurane anaesthesia. Phasic activation of reticular cell firing was associated with EEG spikes during isoflurane and enflurane anaesthesia, and the activation during enflurane anaesthesia was greater than during isoflurane anaesthesia (P < 0.01). Photic stimulation provoked EEG spikes and repetitive stimulation induced seizure activity only at 3.6% enflurane anaesthesia. Halothane and isoflurane suppressed stimulation induced responses in the visual neuronal pathway. The amplitudes of N1 in visual cortical evoked responses induced by photic stimulation were suppressed to 70.1 +/- 24.5% of control at 2.4% halothane and 39.3 +/- 27.3% at 4.8% isoflurane. Enflurane, at 3.6%, augmented the evoked response induced by photic stimulation (398.4 +/- 83.0% of control in the amplitude of N1). These results indicate that all the agents studied had suppressive actions on background neuronal activity in the order halothane < isoflurane = enflurane. The effects on reactive capability were divergent among agents, e.g., enflurane enhanced, halothane suppressed, and the actions of isoflurane were intermediate. We conclude that the anaesthetic effects on background activity and on reactive capability are divergent and that suppression of reactive capability is a factor in determining the ease of clinical application of the anaesthetics.