OPRK1 and Anxiety: Mice injected with the selective KOR-agonist U50,488H (5 mg/kg, i.p.)spent significantly less time in the open arm than their matched saline-injected controls (Fig. 1c, n = 5–7, one-way ANOVA, F(2,17) = 29.87; p<0.001, Saline vs. U50; Bonferroni post-hoc), implying that KOR activation was sufficient to produce anxiety-like behaviors.