Fig. 3. Effects of butorphanol pretreatment on systemic morphine dose–response curves for scratching and antinociception. Butorphanol (mg/kg) was administered subcutaneously 15 min before the injection of the first dose of subcutaneous morphine.  A and  B : Effects of butorphanol on morphine-induced scratching responses.  C and  D : Effects of butorphanol on morphine-induced antinociception against 50°C water.  E and  F : Effects of butorphanol combined with morphine on the sedation. Each value represents mean ± SEM (n = 6).  Symbols represent different experimental conditions for the same monkeys. * Significant difference from the vehicle pretreatment condition, subcutaneous morphine alone (  P < 0.05). See  figures 1 and 2 for other details.