Musculoskeletal sensitization and sleep: chronic muscle pain fragments sleep of mice without altering its duration

Abstract

Study objectives: Musculoskeletal pain in humans is often associated with poor sleep quality. We used a model in which mechanical hypersensitivity was induced by injection of acidified saline into muscle to study the impact of musculoskeletal sensitization on sleep of mice.

Design: A one month pre-clinical study was designed to determine the impact of musculoskeletal sensitization on sleep of C57BL/6J mice.

Methods: We instrumented mice with telemeters to record the electroencephalogram (EEG) and body temperature. We used an established model of musculoskeletal sensitization in which mechanical hypersensitivity was induced using two unilateral injections of acidified saline (pH 4.0). The injections were given into the gastrocnemius muscle and spaced five days apart. EEG and body temperature recordings started prior to injections (baseline) and continued for three weeks after musculoskeletal sensitization was induced by the second injection. Mechanical hypersensitivity was assessed using von Frey filaments at baseline (before any injections) and on days 1, 3, 7, 14, and 21 after the second injection.

Results: Mice injected with acidified saline developed bilateral mechanical hypersensitivity at the hind paws as measured by von Frey testing and as compared to control mice and baseline data. Sleep during the light period was fragmented in experimental mice injected with acidified saline, and EEG spectra altered. Musculoskeletal sensitization did not alter the duration of time spent in wakefulness, non-rapid eye movement sleep, or rapid eye movement sleep.

Conclusions: Musculoskeletal sensitization in this model results in a distinct sleep phenotype in which sleep is fragmented during the light period, but the overall duration of sleep is not changed. This study suggests the consequences of musculoskeletal pain include sleep disruption, an observation that has been made in the clinical literature but has yet to be studied using preclinical models.

Keywords: Musculoskeletal sensitization; mechanical hypersensitivity; sleep fragmentation.

Figure 1

Musculoskeletal sensitization enhances bilateral responses to von Frey testing. Mice injected with acidified saline (n = 16) exhibit mechanical hypersensitivity for at least 21 days, whereas mechanical hypersensitivity does not develop in mice injected with normal pH saline (n = 14). Responsiveness to von Frey filaments are plotted as mean ± SEM total response incidence percent ([total responses / total filament presentations] × 100) per paw. *P ≤ 0.05 vs. normal pH saline injection.

Figure 2

Sleep is fragmented after musculoskeletal sensitization with acidified saline. (A) The total number of transitions/h is plotted across the 24-h light/ dark period only for animals injected with acidified saline (n = 8). Symbols are the mean ± SEM for pre-injection baseline and for 20 days after mechanical hypersensitivity is induced. Acidified saline injections fragment of sleep during the light period. (B) The average number of transitions/h during the 12-h light or dark period is plotted for pre-injection baseline (BL), and for days 2, 8, 15, and 20 after mechanical hypersensitivity is induced. Values are the mean ± SEM for n = 8 mice. (C) Representative hypnograms from one mouse obtained during pre-injection baseline, and at days 2, 8, 15, and 20 after induction of mechanical hypersensitivity. Hypnograms are from a 1-h recording 10 h after light onset during each of the days depicted. W, Wakefulness; N, NREM; R, REM sleep. #P ≤ 0.05 vs. pre-injection baseline. *P ≤ 0.05 vs. normal pH saline injection.

Figure 3

State-specific electroencephalogram (EEG) power spectra are altered during musculoskeletal hypersensitivity. State-specific EEG power spectra were obtained from mice injected with either normal pH saline (n = 6; gray lines) or acidified pH saline (n = 8; black lines). Data presented were obtained 20 days following musculoskeletal sensitization (or control injections). Spectra were normalized as a percentage of total power within each frequency band during the 12-h light or dark period and are plotted as mean ± SEM for each frequency bin. Statistical analyses were performed on bins comprising the delta frequency band (0.5-4.5 Hz) and the theta frequency band (6.0-9.0 Hz) for NREMS and REMS, respectively. A significant change was detected between the peak theta frequency during the dark period for NREM sleep between injection groups on day 20. *P ≤ 0.05 vs. normal pH saline injection.