Figure 3. Time course of visceromoter threshold change after intrathecal [D-Pen², D-Pen⁵] enkephalin (DPDPE). Each point and bar represent the mean value and SEM in six to nine animals. Control thresholds for each dose were approximately 22 mmHg. DPDPE increased the thresholds in a dose-dependent manner with a significant change (*P < 0.5 and **P < 0.01 by analysis of variance) compared with those in vehicle-treated animals at the corresponding time point (data for vehicle are shown in Table 1). Peak effect time was at approximately 15 min after administration. C = control.