Fig. 1. Putative neuronal pathways involved in the antinociceptive effect of N2O in rats. Closed triangles indicate excitatory synapses, and open triangles indicate inhibitory synapses. Small closed circles indicate the nuclei of cells activated by N2O exposure or nociception, and small open circles indicate the nuclei of cells inactivated by N2O exposure. In the present study, muscimol, a GABAAreceptor agonist, was administered intracerebroventricularlly (supraspinally), gabazine, a GABAAreceptor antagonist, was administered intrathecally (spinally), and midazolam, a benzodiazepine receptor agonist with GABA-mimetic properties, was administered intraperitoneally (i.e. , systemically).