Effect of Nitrous Oxide on Excitatory and Inhibitory Synaptic Transmission in Hippocampal Cultures

Abstract

Nitrous oxide (N₂O; laughing gas) has been a widely used anesthetic/analgesic since the 19th century, although its cellular mechanism of action is not understood. Here we characterize the effects of N₂O on excitatory and inhibitory synaptic transmission in microcultures of rat hippocampal neurons, a preparation in which anesthetic effects on monosynaptic communication can be examined in a setting free of polysynaptic network variables. Eighty percent N₂O occludes peak NMDA receptor-mediated (NMDAR) excitatory autaptic currents (EACs) with no effect on the NMDAR EAC decay time course. N₂O also mildly depresses AMPA receptor-mediated (AMPAR) EACs. We find that N₂O inhibits both NMDA and non-NMDA receptor-mediated responses to exogenous agonist. The postsynaptic blockade of NMDA receptors exhibits slight apparent voltage dependence, whereas the blockade of AMPA receptors is not voltage dependent. Although the degree of ketamine and Mg²⁺ blockade of NMDA-induced responses is dependent on permeant ion concentration, the degree of N₂O blockade is not. We also observe a slight and variable prolongation of GABA_A receptor-mediated (GABAR) postsynaptic currents likely caused by previously reported effects of N₂O on GABA_A receptors. Despite the effects of N₂O on both NMDA and non-NMDA ionotropic receptors, glial glutamate transporter currents and metabotropic glutamate receptor-mediated synaptic depression are not affected. Paired-pulse depression, the frequency of spontaneous miniature excitatory synaptic currents, and high-voltage–activated calcium currents are not affected by N₂O. Our results suggest that the effects of N₂O on synaptic transmission are confined to postsynaptic targets.