Abscissae: Drug treatment. Ordinates: Number of acid-stimulated stretches. (a) The left panel shows effects of SNC80 alone in a group of 6 rats (mean±SEM baseline number of stretches after vehicle treatment =18.2±1.9). One-way ANOVA revealed a significant effect of SNC80 dose [F(3,15)=22.1, p<0.0001], and asterisks (*) indicate a significant difference from vehicle treatment (Dunnett post hoc test; p<0.05). (b) The right panel shows effects of 30 min pretreatment with the delta antagonist naltrindole (NTI, 1.0 mg/kg) or 24hr pretreatment with SNC80 (0.1-10 mg/kg) on antinociception produced by 10 mg/kg SNC80 in a different group of 6 rats (control number of stretches ± SEM=24.8±2.2). One-way ANOVA revealed a significant effect of treatment [F(5,25)=28.8, p<0.0001]. The antinociceptive effect of SNC80 was blocked by naltrindole pretreatment and dose-dependently attenuated by SNC80 pretreatment. Asterisks (*) indicate significantly different from vehicle, and crosses (†) indicate significantly different from 10 SNC80 (Newman-Keuls post hoc test; p<0.05). All bars show mean±SEM.