Fig. 1. Effects of 30% nitrous oxide on excitatory synaptic responses. a) Nitrous oxide significantly increased the ratio of second population spike/first population spike (  P < 0.05 by paired Student  t test, n = 6). b) Traces show representative excitatory postsynaptic potentials (  upper ) and population spikes (  lower ) depicted before (  solid lines ) and 20 min after (  dotted lines ) nitrous oxide administration. Nitrous oxide facilitated the second population spike with no effect on first population spike amplitude or first and second excitatory postsynaptic potential slope. c) Input-output curves for second population spike amplitude recorded before (  diamonds ) and 20 min after (  squares ) nitrous oxide administration in a single slice. The increased amplitude of the second population spike was greater than the expected value calculated from the control input-output curves at the level of 50% to 60% of the maximal second population spike amplitude.