Figure 6. Effect of 1 mg/kg intravenous morphine (upper panel) or 10 micro gram/kg intravenous fentanyl, on microdialysate concentrations of norepinephrine from probes inserted in ventral horn of the spinal cord. Each bar represents the mean+/-SEM of 3–6 probes in three animals. Morphine or fentanyl was injected at time 0. *P < 0.05 versus baseline (time 0).