Supraspinal kappa-opioid receptors: new therapeutic strategies for pain, pruritus, and negative emotions

Abstract

Research on kappa-opioid receptor (KOR) regulation of pain and itching has focused primarily on spinal and peripheral levels. However, the role of central KOR in this process, as well as the mechanisms exacerbating negative emotional responses to pain and itching, remains unknown. Therefore, this study aimed to utilize the advantages of intracerebroventricular (i.c.v.) administration of U50488H to explore supraspinal KOR activation on pain, itching, and negative emotions. U50488H, a prototypical KOR agonist, was administered i.c.v., with physiological saline as the control. The Hargreaves test and intradermal injection of histamine and chloroquine were conducted to assess thermal pain and itch behavior, respectively. The elevated plus maze (EPM), open field test (OFT), and tail suspension test (TST) were performed to evaluate negative emotions. i.c.v. administration of U50488H increased thermal pain latencies, reduced scratching behavior, and decreased locomotor activity in the central zone of the OFT and in the open arms of the EPM, while increasing immobility in the TST. i.c.v. pretreatment with the KOR antagonist nor-Binaltorphimine dihydrochloride reversed all of the above behaviors. In conclusion, central administration of U50488H can exhibit analgesic and antipruritic effects while also inducing negative emotional responses. Our results highlight the potential of supraspinal KOR as a promising therapeutic target in the combined treatment of pain, pruritus, and negative emotions.

Keywords: Anxiety; Depression; Itch; Kappa-opioid receptor; Pain; U50488H.

Fig. 1

Experimental design. (A) Schedule of the rotarod, pain, and itch behavioral tests. After a 7-day recovery following cranial cannula placement, mice in groups 1–4 (U50488H groups, 8 mice per group) and groups 9–12 (nor-BNI + U50488H groups, 7 mice per group) sequentially underwent the rotarod, Hargreaves, histamine, and chloroquine tests on days 3, 10, 17, and 24, respectively. (B) Schedule of negative emotions test. After a 7-day recovery, mice in groups 5–8 (U50488H groups, 8 mice per group) and groups 13–16 (nor-BNI + U50488H groups, 7 mice per group) sequentially underwent the OFT, EPM, and TST experimental assessments on days 1, 8, and 15, respectively

Fig. 2

Intracerebroventricular administration of U50488H exhibits analgesic effects on radiant heat-induced pain and is reversed by nor-BNI. Hargreaves PWL of the right hind paw (A) [13 vs. 0 µg/µL P < 0.0001; 13 vs. 3.25 µg/µL P < 0.01; 6.5 vs. 0 µg/µL P < 0.05] and left hind paw (B) [13 vs. 0 µg/µL P < 0.0001; 13 vs. 3.25 µg/µL P < 0.0001; 13 vs. 6.5 µg/µL P < 0.0001; 6.5 vs. 0 µg/µL P < 0.0001; 6.5 vs. 3.25 µg/µL P < 0.0001] in mice were compared between different U50488H groups (n = 8). Data are presented as the mean ± S.E.M. and significance was assessed by the Kruskal–Wallis test in (A) and one-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc test in (B) between different U50488H groups. Comparisons between the U50488H and nor-BNI + U50488H groups were performed using an unpaired t-test (n = 7 mice per nor-BNI + U50488H group). * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001

Fig. 3

Intracerebroventricular administration of U50488H inhibits itch behavior and is reversed by nor-BNI. (A) After i.c.v. administration of saline and U50488H at concentrations of 3.25, 6.5, and 13 µg/µL, followed by intradermal injections of chloroquine, scratching numbers were compared between different U50488H groups for 30 min [13 vs. 0 µg/µL P < 0.0001; 13 vs. 3.25 µg/µL P < 0.0001; 13 vs. 6.5 µg/µL P < 0.0001] (n = 8 mice per U50488H group). (B) After i.c.v. administration of saline and U50488H at concentrations of 3.25, 6.5, and 13 µg/µL, followed by intradermal injections of histamine, scratching numbers were compared between different U50488H groups for 30 min [13 vs. 0 µg/µL P < 0.0001; 13 vs. 3.25 µg/µL P < 0.0001; 13 vs. 6.5 µg/µL P < 0.01; 6.5 vs. 0 µg/µL P < 0.01] (n = 8 mice per U50488H group). Data are presented as the mean ± S.E.M. and significance was assessed by one-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc test between different U50488H groups. Comparisons between the U50488H and nor-BNI + U50488H groups were performed using an unpaired t-test (n = 7 mice per nor-BNI + U50488H group). * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001

Fig. 4

Effects of U50488H on open-field test behaviors in mice. Representative two-dimensional heat maps showing movement trajectories in the OFT after i.c.v. administration of saline (A) and U50488H at concentrations of 3.25 (B), 6.5 (C), and 13 µg/µL (D). Total distance (E), peripheral area distance (F), and central area distance (G) in the different U50488H concentration groups and saline control group (G: 13 vs. 0 µg/µL P < 0.0001; 6.25 vs. 0 µg/µL P < 0.05; 3.25 vs. 0 µg/µL P < 0.01). Durations in the peripheral zone (H) and central zone (I) were recorded (H: 13 vs. 0 µg/µL P < 0.05; I: 13 vs. 0 µg/µL P < 0.05) between the different U50488H groups. Significance was assessed by one-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc test between different U50488H groups (n = 8). Comparisons between the U50488H and nor-BNI + U50488H groups were performed using an unpaired t-test (n = 7 mice per nor-BNI + U50488H group). * p < 0.05, ** p < 0.01, **** p < 0.0001, no significance (ns)

Fig. 5

Effects of U50488H on elevated plus maze behaviors in mice. Representative heat maps showing movement trajectories in the EPM after i.c.v. administration of 0.9% saline (A) and U50488H at concentrations of 3.25 (B), 6.5 (C), and 13 µg/µL (D). Total distance (E) [13 vs. 0 µg/µL P < 0.0001; 13 vs. 3.25 µg/µL P < 0.0001; 6.5 vs. 0 µg/µL P < 0.05; 6.5 vs. 3.25 µg/µL P < 0.05], closed arm distance (F) [13 vs. 0 µg/µL P < 0.05], and open arm distance (G) [13 vs. 0 µg/µL P < 0.001; 13 vs. 3.25 µg/µL P < 0.0001; 6.5 vs. 0 µg/µL P < 0.05; 6.5 vs. 3.25 µg/µL P < 0.05] traveled in the different U50488H groups and saline control group. Closed arm duration (H) [13 vs. 0 µg/µL P < 0.01, 13 vs. 3.25 µg/µL P < 0.001] and open arm duration (I) [13 vs. 0 µg/µL P < 0.001, 13 vs. 3.25 µg/µL P < 0.001] were recorded between different U50488H groups (n = 8). Data are presented as the mean ± S.E.M. and significance was assessed by Tamhane’s test in (I) and one-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc test in (A–H) between the different U50488H groups. An unpaired t-test was used for the U50488H and nor-BNI + U50488H groups (n = 7 mice per nor-BNI + U50488H group). * p < 0.05, ** p < 0.01, *** p < 0.001, **** p < 0.0001

Fig. 6

Effects of U50488H on tail suspension test behaviors in mice. (A) Mobility time and (B) immobility time in different U50488H groups and the saline control group (A: 13 vs. 0 µg/µL P < 0.05; B: 13 vs. 0 µg/µL P < 0.01, 6.5 vs. 0 µg/µL P < 0.05). Data are presented as the mean ± S.E.M. and significance was assessed by one-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc test between the different U50488H groups (n = 8). An unpaired t-test was used for the U50488H and nor-BNI + U50488H groups (n = 7 mice per nor-BNI + U50488H group). * p < 0.05, ** p < 0.01