Figure 4. Effect of 1 mg/kg intravenous morphine on microdialysate concentrations of norepinephrine (upper panel) or acetylcholine (lower panel) from probes inserted in dorsal horn of the spinal cord. Each bar represents the mean+/-SEM of 5–8 probes in three animals. Morphine was injected at time 0. *P < 0.05 versus baseline (time 0).