Figure 1.

Effect of benzamide on morphine tolerance. Tolerance to the antinociceptive effect of morphine developed in rats treated with 20 μg of morphine for 7 days. Coadministration of 20 μg of morphine with 200 or 400 nmol (not 100 nmol) benzamide for 7 days reliably attenuated the development of tolerance. Neither baseline tail-flick latency nor the response to a single injection of 20 μg of morphine changed after repeated saline treatment for seven days. ∗∗, P < 0.01, as compared with that of day 1 in each corresponding group. Neither repeated benzamide (400 nmol) treatment alone nor a single injection of 20 of μg morphine on day 8 (the 20*/0 group) affected the degree of tolerance as compared with the saline group. MPAE%, percent of maximal possible antinociceptive effect.