Figure 4.

Effects of the mu-opioid agonist oxycodone alone or in combination with the noncompetitive NMDA antagonist MK-801 on capsaicin-induced thermal allodynia (left panels) and rates of schedule-controlled responding (right panels). Upper panels show dose-effect functions for oxycodone alone or in combination with MK-801 and lower panels show isobolograms at the ED₅₀ effect level for oxycodone or MK-801 alone or as part of a mixture. Upper horizontal axes: unit oxycodone dose (left panel) or cumulative oxycodone dose (right panel) in mg/kg/injection. Upper vertical axes: percent control rate of responding. Lower horizontal axes: ED₅₀ values for oxycodone alone or in a mixture in mg/kg (linear scale). Lower vertical axes: ED₅₀ values for MK-801 alone or in a mixture mg/kg (linear scale). Each point represents mean ± SEM of 3–4 monkeys. Asterisk indicates that the mixture produced a sub-additive effect in the assay of capsaicin-induced thermal allodynia as determined by dose-addition analysis (see Table 5).