Fig. 6. Rightward shifts of intrathecal morphine dose–response curves in scratching by a single dose (32 μg/kg, administered subcutaneously) of nalmefene pretreatment. Abscissae (all panels): intrathecal doses (micrograms) of morphine. Ordinates (all panels): total scratches accumulated in six test sessions. Open circles represent total scratching responses after intrathecal morphine. Solid circles represent total scratching responses after intrathecal morphine in the presence of nalmefene pretreatment. See figure 2, table 1, and Materials and Methods for other details.