Suvorexant and mirtazapine improve chronic pain-related changes in parameters of sleep and voluntary physical performance in mice with sciatic nerve ligation

Abstract

Both chronic pain and sleep disorders are associated with a reduction in the quality of life. They can be both a cause and a consequence of each other, and should therefore be simultaneously treated. However, optimal treatments for chronic pain-related sleep disorders are not well established. Here, we aimed to investigate the effects of suvorexant, a novel sleep drug, and mirtazapine, a noradrenergic and specific serotonergic antidepressant, on pain-related changes in sleep parameters in a preclinical chronic pain mice model, by partial sciatic nerve ligation. We evaluated the quantity, duration, and depth of sleep by analyzing the electroencephalogram and voluntary activity by counting the number of wheel rotations to determine various symptoms of sleep disorders, including reduced total sleep time, fragmentation, low quality, and impaired activity in the daytime. Suvorexant and mirtazapine normalized the reduction in sleep time and fragmented sleep, further regaining the sleep depth at sleep onset in the chronic pain state in nerve-ligated mice. Mirtazapine also increased the percentage of rapid eye movement sleep in mice. Suvorexant decreased voluntary activity, which was prolonged after administration; however, mirtazapine did not decrease it. Although the effects of suvorexant and mirtazapine on sleep and activity are different, both suvorexant and mirtazapine could be potential therapeutic agents for chronic pain-related sleep disorders.

Fig 1. Experimental timeline.

Mice were subjected to von Frey and Hargreaves tests at certain test points in Experiment 1 (a). They were treated with suvorexant (30 mg/kg, p.o.), mirtazapine (1 mg/kg, i.p.), or vehicles once daily at ZT 0 for 7 days after nerve ligation. Mice treated with suvorexant or mirtazapine were subjected to EEG for 24 h in Experiment 2 (b). The number of wheel rotations was recorded in naïve mice 1 day before PSNL surgery and at 7, 14, and 21 days after surgery in Experiment 3 (c). The number of wheel rotations was recorded in the mice treated with suvorexant or mirtazapine at the test points in Experiment 4 (d). Abbreviations: ZT, Zeitgeber time; PSNL, partial sciatic nerve ligation; EEG, electroencephalogram.

Fig 2. Suvorexant and mirtazapine improved mechanical allodynia and thermal hyperalgesia in the neuropathic pain model.

They were assessed for mechanical allodynia (a, c) and thermal hyperalgesia (b, d) with four test points, with the automated von Frey and Hargreaves tests. Data are expressed as means ± SEMs (PSNL-vehicle, n = 8–9; PSNL-suvorexant, n = 8; PSNL-mirtazapine, n = 9). *P < 0.05, ****P < 0.0001, compared by two-way ANOVA with Bonferroni post-test. Abbreviations: ANOVA, analysis of variance; PSNL, partial sciatic nerve ligation; SEM, standard error of the mean; ZT, Zeitgeber time.

Fig 3. Suvorexant and mirtazapine improved the amount of sleep time in PSNL mice.

The time changes of the sleep percentage are shown for every 2 h (a, d) and every 12 h (b, e). The proportions of REM sleep per total sleep were shown (c, f). Data are expressed as means ± SEMs (sham-vehicle, n = 5–6; PSNL-vehicle, n = 5; PSNL-suvorexant, n = 6; PSNL-mirtazapine, n = 5). *P < 0.05, **P < 0.01, and ***P < 0.001 compared between each sham-vehicle and PSNL-vehicle group, and ⁺P < 0.05, ⁺⁺P < 0.01, ⁺⁺⁺P < 0.001, and ⁺⁺⁺⁺P < 0.0001 compared between each PSNL-vehicle and PSNL-drug (suvorexant or mirtazapine) group by two-way ANOVA with Bonferroni post-test. Abbreviations: ANOVA, analysis of variance; EEG, electroencephalogram; PSNL, partial sciatic nerve ligation; REM, rapid eye movement; SEM, standard error of the mean; ZT, Zeitgeber time.

Fig 4. Influence of daily administration of suvorexant or mirtazapine on the duration of arousal and sleep in PSNL.

The mean durations of wakefulness, non-REM sleep, and REM sleep in both suvorexant (a) and mirtazapine (b) experiments, were calculated every 6 h. Data are expressed as means ± SEMs (sham-vehicle, n = 5–6; PSNL-vehicle, n = 5; PSNL-suvorexant, n = 6; PSNL-mirtazapine, n = 5). *P < 0.05, **P < 0.01, ***P < 0.001, and ****P < 0.0001 compared between each sham-vehicle and PSNL-vehicle group, and ⁺P < 0.05, ⁺⁺P < 0.01, ⁺⁺⁺P < 0.001, and ⁺⁺⁺⁺P < 0.0001 compared between each PSNL-vehicle and PSNL-drug (suvorexant or mirtazapine) group by two-way ANOVA with Bonferroni post-test. Abbreviations: ANOVA, analysis of variance; PSNL, partial sciatic nerve ligation; REM, rapid eye movement; SEM, standard error of the mean; ZT, Zeitgeber time.

Fig 5. Influence of suvorexant or mirtazapine on the changes in power density of δ wave in PSNL mice.

The change in the normalized power density of the δ wave in non-REM sleep was calculated every 3 h. Data are expressed as means ± SEMs (sham-vehicle, n = 5; PSNL-vehicle, n = 4–5; PSNL-suvorexant, n = 5; PSNL-mirtazapine, n = 5). *P < 0.05, compared between each sham-vehicle and PSNL-vehicle group, and ⁺⁺P < 0.01, compared between each PSNL-vehicle and PSNL-drug (suvorexant or mirtazapine) groups by two-way ANOVA with Bonferroni post-test. Abbreviations: ANOVA, analysis of variance; PSNL, partial sciatic nerve ligation; REM, rapid eye movement; SEM, standard error of the mean; ZT, Zeitgeber time.

Fig 6. Changes in voluntary activity upon suvorexant and mirtazapine administration following PSNL surgery.

The number of rotations in naïve mice was evaluated before and 3 weeks after PSNL surgery and compared with the sham group (a). **P < 0.01, two-way ANOVA with Bonferroni post-test (n = 6). We compared wheel rotations at six different time points with the number in mice treated with each drug at day -1 defined as the baseline (b, c). *P < 0.05, **P < 0.01, compared with baseline by one-way ANOVA with Bonferroni post-test in suvorexant (b) and mirtazapine (d) experiments (n = 8). Data are expressed as means ± SEMs. Abbreviations: ANOVA, analysis of variance; PSNL, partial sciatic nerve ligation; SEM, standard error of the mean; ZT, Zeitgeber time.