Effects of nitrous oxide on glycinergic transmission in rat spinal neurons

We investigated the effects of nitrous oxide (N2O) on glycinergic inhibitory whole-cell and synaptic responses using a "synapse bouton preparation," dissociated mechanically from rat spinal sacral dorsal commissural nucleus (SDCN) neurons. This technique can evaluate pure single- or multi-synaptic responses from native functional nerve endings and enable us to accurately quantify how N2O influences pre- and postsynaptic transmission. We found that 70 % N2O enhanced exogenous glycine-induced whole-cell currents (I-Gly) at glycine concentrations lower than 3 x 10(-5) M, but did not affect IGly at glycine concentrations higher than 10(-4) M. N2O did not affect the amplitude and 1/e decay-time of both spontaneous and miniature glycinergic inhibitory postsynaptic currents recorded in the absence and presence of tetrodotoxin (sIPSCs and mIPSCs, respectively). The decrease in frequency induced by N2O was observed in sIPSCs but not in mIPSCs, which was recorded in the presence of both tetrodotoxin and Cd2+, which block voltage-gated Na+ and Ca2+ channels, respectively. N2O also decreased the amplitude and increased the failure rate and paired-pulse ratio of action potential-evoked glycinergic inhibitory postsynaptic currents. N2O slightly decreased the Ba2+ currents mediated by voltage-gated Ca2+ channels in SDCN neurons. We found that (NO)-O-2 suppresses glycinergic responses at synaptic levels with presynaptic effect having much more predominant role. The difference between glycinergic whole-cell and synaptic responses suggests that extrasynaptic responses seriously modulate whole-cell currents. Our results strongly suggest that these responses may thus in part explain analgesic effects of N2O via marked glutamatergic inhibition by glycinergic responses in the spinal cord.