Figure 4.

Effects of intrathecal (a) atipamezole (0.8 ng), (b) yohimbine (0.02 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) on the induction of acute tolerance to morphine (Mor) in the paw pressure test. Tolerance was induced by intrathecal injection of morphine (15 μg) at 0, 90 and 180 min. Morphine was administered with atipamezole, yohimbine, mirtazapine and idazoxan as a singe injection. Nociceptive testing was performed every 30 min post-injection. The data are presented as mean±s.e.mean; n=five to six animals per treatment group. Significant difference from the action of morphine ^**P< 0.01, ^***P< 0.001.