Anaesthetic suppression of transmitter actions in neocortex

1. The effects of general anaesthetics were investigated on neuronal sensitivities to transmitter substances, which were determined by iontophoretic applications of acetylcholine, glutamate, N-methyl-D-aspartate (NMDA) and gamma-aminobutyrate (GABA) during intracellular recording in in vitro slice preparations of neocortex (guinea-pig). 2. In most of the 65 neurones studied, perfusion of isoflurane (0.5-2.5 minimum alveolar concentration (MAC)) or Althesin (25-200 microM) and, in some cases, halothane (0.5-2 MAC), markedly reduced the depolarizing responses and associated membrane conductance changes evoked by dendritic applications of acetylcholine, glutamate, NMDA and GABA. 3. The order of depression was acetylcholine greater than glutamate or NMDA much greater than GABA. This selectivity could also be assessed from the EC50 for the isoflurane-induced depression of the just-maximal responses to acetylcholine, which was 0.9 MAC compared with an EC50 = 1.9 MAC for the suppression of glutamate responses. The selectivity was less pronounced in the case of the actions of Althesin, where the EC50s were 75 microM for the depression of acetylcholine responses and 90 microM for the depression of glutamate responses. 4. The hyperpolarizing responses observed when GABA was applied near the perikaryon in 7 neurones, were slightly reduced (approximately 15%) in 4, and unchanged in 3 neurones during anaesthetic application. 5. The pronounced depression of the responsiveness to the putative arousal transmitters and an observed blockade of acetylcholine-induced potentiation of glutamate actions suggest that anaesthetics produce unconsciousness, at least in part, by interfering with subsynaptic mechanisms of neocortical activation.