Fig. 1. Average time course of intrathecal morphine-induced scratching responses in monkeys (n = 8). Each value represents the mean ± SEM (two to three replications). Abscissae (both panels): time (hours) after intrathecal injection. Ordinates (both panels): scratches per 15 min during each session. Solid symbols represent different doses (micrograms) of intrathecal morphine, and open squares represent the intrathecal vehicle (saline) injection. See figure 2for individual dose–response curves of intrathecal morphine and Materials and Methods for other details.