Acute increases in intramuscular inflammatory cytokines are necessary for the development of mechanical hypersensitivity in a mouse model of musculoskeletal sensitization.

Musculoskeletal pain is a widespread health problem in the United States. Back pain, neck pain, and facial pain are three of the most prevalent types of chronic pain, and each is characterized as musculoskeletal in origin. Despite its prevalence, preclinical research investigating musculoskeletal pain is limited. Musculoskeletal sensitization is a preclinical model of muscle pain that produces mechanical hypersensitivity. In a rodent model of musculoskeletal sensitization, mechanical hypersensitivity develops at the hind paws after injection of acidified saline (pH 4.0) into the gastrocnemius muscle. Inflammatory cytokines contribute to pain during a variety of pathologies, and in this study we investigate the role of local, intramuscular cytokines in the development of mechanical hypersensitivity after musculoskeletal sensitization in mice. Local intramuscular concentrations of interleukin-1ß (IL-1), IL-6 and tumor necrosis factor-α (TNF) were quantified following injection of normal (pH 7.2) or acidified saline into the gastrocnemius muscle. A cell-permeable inhibitor was used to determine the impact on mechanical hypersensitivity of inhibiting nuclear translocation of the transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) prior to musculoskeletal sensitization. The role of individual cytokines in mechanical hypersensitivity following musculoskeletal sensitization was assessed using knockout mice lacking components of the IL-1, IL-6 or TNF systems. Collectively, our data demonstrate that acidified saline injection increases intramuscular IL-1 and IL-6, but not TNF; that intramuscular pre-treatment with an NF-κB inhibitor blocks mechanical hypersensitivity; and that genetic manipulation of the IL-1 and IL-6, but not TNF systems, prevents mechanical hypersensitivity following musculoskeletal sensitization. These data establish that actions of IL-1 and IL-6 in local muscle tissue play an acute regulatory role in the development of mechanical hypersensitivity following musculoskeletal sensitization.

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

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.

Sleep fragmentation exacerbates mechanical hypersensitivity and alters subsequent sleep-wake behavior in a mouse model of musculoskeletal sensitization.

STUDY OBJECTIVES: Sleep deprivation, or sleep disruption, enhances pain in human subjects. Chronic musculoskeletal pain is prevalent in our society, and constitutes a tremendous public health burden. Although preclinical models of neuropathic and inflammatory pain demonstrate effects on sleep, few studies focus on musculoskeletal pain. We reported elsewhere in this issue of SLEEP that musculoskeletal sensitization alters sleep of mice. In this study we hypothesize that sleep fragmentation during the development of musculoskeletal sensitization will exacerbate subsequent pain responses and alter sleep-wake behavior of mice. DESIGN: This is a preclinical study using C57BL/6J mice to determine the effect on behavioral outcomes of sleep fragmentation combined with musculoskeletal sensitization. METHODS: Musculoskeletal sensitization, a model of chronic muscle pain, was induced using two unilateral injections of acidified saline (pH 4.0) into the gastrocnemius muscle, spaced 5 days apart. Musculoskeletal sensitization manifests as mechanical hypersensitivity determined by von Frey filament testing at the hindpaws. Sleep fragmentation took place during the consecutive 12-h light periods of the 5 days between intramuscular injections. Electroencephalogram (EEG) and body temperature were recorded from some mice at baseline and for 3 weeks after musculoskeletal sensitization. Mechanical hypersensitivity was determined at preinjection baseline and on days 1, 3, 7, 14, and 21 after sensitization. Two additional experiments were conducted to determine the independent effects of sleep fragmentation or musculoskeletal sensitization on mechanical hypersensitivity. RESULTS: Five days of sleep fragmentation alone did not induce mechanical hypersensitivity, whereas sleep fragmentation combined with musculoskeletal sensitization resulted in prolonged and exacerbated mechanical hypersensitivity. Sleep fragmentation combined with musculoskeletal sensitization had an effect on subsequent sleep of mice as demonstrated by increased numbers of sleep-wake state transitions during the light and dark periods; changes in nonrapid eye movement (NREM) sleep, rapid eye movement sleep, and wakefulness; and altered delta power during NREM sleep. These effects persisted for at least 3 weeks postsensitization. CONCLUSIONS: Our data demonstrate that sleep fragmentation combined with musculoskeletal sensitization exacerbates the physiological and behavioral responses of mice to musculoskeletal sensitization, including mechanical hypersensitivity and sleep-wake behavior. These data contribute to increasing literature demonstrating bidirectional relationships between sleep and pain. The prevalence and incidence of insufficient sleep and pathologies characterized by chronic musculoskeletal pain are increasing in the United States. These demographic data underscore the need for research focused on insufficient sleep and chronic pain so that the quality of life for the millions of individuals with these conditions may be improved.

Prolonged sleep fragmentation of mice exacerbates febrile responses to lipopolysaccharide.

BACKGROUND: Sleep disruption is a frequent occurrence in modern society. Whereas many studies have focused on the consequences of total sleep deprivation, few have investigated the condition of sleep disruption. NEW METHOD: We disrupted sleep of mice during the light period for 9 consecutive days using an intermittently rotating disc. RESULTS: Electroencephalogram (EEG) data demonstrated that non-rapid eye movement (NREM) sleep was severely fragmented and REM sleep was essentially abolished during the 12h light period. During the dark period, when sleep was not disrupted, neither NREM sleep nor REM sleep times differed from control values. Analysis of the EEG revealed a trend for increased power in the peak frequency of the NREM EEG spectra during the dark period. The fragmentation protocol was not overly stressful as body weights and water consumption remained unchanged, and plasma corticosterone did not differ between mice subjected to 3 or 9 days of sleep disruption and home cage controls. However, mice subjected to 9 days of sleep disruption by this method responded to lipopolysaccharide with an exacerbated febrile response. COMPARISON WITH EXISTING METHODS: Existing methods to disrupt sleep of laboratory rodents often subject the animal to excessive locomotion, vibration, or sudden movements. This method does not suffer from any of these confounds. CONCLUSIONS: This study demonstrates that prolonged sleep disruption of mice exacerbates febrile responses to lipopolysaccharide. This device provides a method to determine mechanisms by which chronic insufficient sleep contributes to the etiology of many pathologies, particularly those with an inflammatory component.

Ischemic stroke selectively inhibits REM sleep of rats.

Sleep disorders are important risk factors for stroke; conversely, stroke patients suffer from sleep disturbances including disruptions of non-rapid eye movement (NREM) and rapid eye movement (REM) sleep and a decrease in total sleep. This study was performed to characterize the effect of stroke on sleep architecture of rats using continuous electroencephalography (EEG) and activity monitoring. Rats were implanted with transmitters which enabled continuous real time recording of EEG, electromyography (EMG), and locomotor activity. Baseline recordings were performed prior to induction of either transient middle cerebral artery (MCA) occlusion or sham surgery. Sleep recordings were obtained for 60 h after surgery to identify periods of wakefulness, NREM, and REM sleep before and after stroke. Spectral analysis was performed to assess the effects of stroke on state-dependent EEG. Finally, we quantified the time in wake, NREM, and REM sleep before and after stroke. Delta power, a measure of NREM sleep depth, was increased the day following stroke. At the same time, there was a significant shift in theta rhythms to a lower frequency during REM and wake periods. The awake EEG slowed after stroke over both hemispheres. The EEG of the ischemic hemisphere demonstrated diminished theta power specific to REM in excess of the slowing seen over the contralateral hemisphere. In contrast to rats exposed to sham surgery which had slightly increased total sleep, rats undergoing stroke experienced decreased total sleep. The decrease in total sleep after stroke was the result of dramatic reduction in the amount of REM sleep after ischemia. The suppression of REM after stroke was due to a decrease in the number of REM bouts; the length of the average REM bout did not change. We conclude that after stroke in this experimental model, REM sleep of rats is specifically and profoundly suppressed. Further experiments using this experimental model should be performed to investigate the mechanisms and consequences of REM suppression after stroke.

Interactions between cognition and circadian rhythms: attentional demands modify circadian entrainment.

Animals and humans are able to predict and synchronize their daily activity to signals present in their environments. Environmental cues are most often associated with signaling the beginning or the end of a daily activity cycle, but they can also be used to time the presentation or availability of scarce resources. If the signal occurs consistently, animals can begin to anticipate its arrival and ultimately become entrained to its presence. While many stimuli can produce anticipation for a daily event, these events rarely lead to changes in activity patterns during the rest of the circadian cycle. Here the authors demonstrate that performance of a task requiring sustained attention not only produces entrainment, but produces a robust modification in the animals' activity throughout the entire circadian cycle. In particular, normally nocturnal rats, when trained during the light phase (ZT 4) adopted a significant and reversible diurnal activity pattern. Of importance, control experiments demonstrated that this entrainment could not be attributed to the noncognitive components of task performance, such as handling, water deprivation, access to water used as a reward, or animal activity associated with operant training. These findings additionally indicate that levels of cognitive performance are modulated by the circadian cycle and that such activity can act as a highly effective entrainment signal. These results form the basis for future research on the role of neuronal systems mediating interactions between cognitive activity and circadian rhythms.