Figure 3. Intraspinal ketamine (KET) given in a dose of 12 micro gram (50 nmol) prevents the rightward shift in the intraspinal morphine (MOR) dose-response curve on day 5. Tolerance was produced by administering MOR three times a day at 10 micro gram on day 1, at 20 micro gram on day 2, and at 40 micro gram on day 3. KET or saline (SAL) was administered 10 min before each MOR dose. On day 1, cumulative MOR dose-response assessment preceded each treatment. On day 5, the SAL + MOR curve shifted 46 times, whereas curves for day 5 KET + MOR, KET + SAL, and SAL + SAL groups were not significantly different from the curve for day 1 before treatment (see Table 3for ED⁵⁰values).