Figure 1

(A) Response thresholds of ipsilateral whisker pad to usually innocuous tactile (von Frey Hair) stimuli in WT sham, WT pIONL, KOR-/- sham, and KOR-/- pIONL mice during 25 days after partial infraorbital nerve ligation. After pIONL, both WT and KOR-/- mice developed allodynic response as evident from the decreased thresholds to tactile stimulation compared with WT and KOR-/- mice that received sham-ligation surgeries. One week after pIONL, WT mice with pIONL began to show the increase of the mechanical threshold, whereas KOR-/- pIONL group showed much slower recovery and significantly greater allodynic response than WT pIONL group. (B)On the 25^th day after surgery, we injected the kappa opioid receptor agonist U50,488 intraperitoneally at 10 mg/kg in each group of mice. The von Frey hair force returned to baseline in WT pIONL group, but not in KOR-/- pIONL group. U50,488 injection did not affect the mechanical threshold in sham operated groups. Asterisks indicate significant decreases in the threshold to stimuli by the von Frey hair applied to ipsilateral side of whisker pad in KOR-/- pIONL group compared to WT pIONL group (p < 0.05, ANOVA followed by Student-Newman test). Data are presented as the mean ± SEM von Frey hair threshold in grams; n = 8 per time point).