Figure 1. Time course of visceromotor threshold change after intrathecal clonidine at four doses. Each point and bar represent the mean value and SEM in 10-13 rats. Control threshold for each dose was approximately 22 mmHg. Peak effect time was between 10 and 20 min. Clonidine increased the thresholds in a dose-dependent manner with a significant change (*P < 0.05 and P < 0.01 by analysis of variance) at the indicated times. Intrathecal yohimbine (20 micro gram, n = 7), when administered at 18 min after 10 micro gram clonidine, decreased the thresholds significantly at 30 and 45 min (P < 0.05).