Figure 4. Histogram of visceromotor thresholds 15 min after the administration of intrathecal morphine (M) (2.5 micro gram) or [D-Pen2, D-Pen5] enkephalin (D) (10 micro gram) with or without naloxone (NX) (2.5 micro gram) or naltrindole (NTI) (5 micro gram). Numbers of animals are shown under the x-axis. Each bar represents the SEM. By one-way analysis of variance, it was determined that the effect of morphine was antagonized by NX but not by NTI and that the effect of D was antagonized by NTI.