Exercise-induced pain and analgesia? Underlying mechanisms and clinical translation

The Effects of Exercise on Pain and Reproductive Performance in Female Pregnant Mice With Neuropathic Pain

Pain can have negative, physiological and psychological impacts on pregnancy. Pregnant women are fearful of using pain medication because of teratogenic effects. In this study, we evaluated whether exercise could lower pain sensitivity in pregnant mice with neuropathic pain and reduce the negative effects of maternal pain on newborns. We randomly assigned 32 female mice to one of four groups (eight mice/group): Sham surgery with standard environment (SE) or enriched environment (EE) or spare nerve injury (SNI) with SE or EE. Mice in EE groups had access to an exercise wheel. Mothers were evaluated for mechanical sensitivity with Von Frey filaments and for exercise performance with computerized running wheels. Mice were impregnated 2 weeks after the initiation of EE. Pups were weighed and measured for length at birth and evaluated for negative geotaxis, righting, forelimb grasping, rooting, and crawling at 3 days postpartum and for crawling at 6 days postpartum. Following euthanasia, mothers' frontal cortexes were analyzed for selected neuropeptides. After exercise exposure, only SNI-SE females remained neuropathic. Exercise levels were similar between EE groups. Some brain neuropeptides (endorphins, enkephalins, and oxytocin) from SNI females showed significant differences with exercise. Number of pups was significantly smaller in the SNI-SE group. Significantly more pups died at birth in the SNI-SE group, but pup behavior tests (except righting) were similar across groups. Exercise can reduce neuropathic pain in pregnant mice. Neuropathic pain does not impact motor neurodevelopment of mice pups but does appear to affect litter size and neonatal mortality.

Associations of physical activity or sedentary behaviour with pain sensitivity in young adults of the Raine Study

Background and aims

There is high level evidence for physical activity (PA) improving outcomes in persistent pain disorders and one of the mechanisms proposed is the effect of exercise on central nociceptive modulation. Although laboratory studies and small field intervention studies suggest associations between physical activity and pain sensitivity, the association of objectively measured, habitual PA and sedentary behaviour (SB) with pain sensitivity requires further investigation. Current evidence suggests PA typically lowers pain sensitivity in people without pain or with single-site pain, whereas PA is frequently associated with an increase in pain sensitivity for those with multisite pain. The aim of this study was to explore the relationships of PA and SB with pain sensitivity measured by pressure pain thresholds and cold pain thresholds, considering the presence of single-site and multisite pain and controlling for potential confounders.

Methods

Participants from the Western Australian Pregnancy Cohort (Raine) Study (n = 714) provided data at age 22-years. PA and SB were measured via accelerometry over a 7-day period. Pain sensitivity was measured using pressure pain threshold (4 sites) and cold pain threshold (wrist). Participants were grouped by number of pain areas into “No pain areas” (n = 438), “Single-site pain” (n = 113) and “Multisite pain” (n = 163) groups. The association of PA and SB variables with pain sensitivity was tested separately within each pain group by multivariable regression, adjusting for potential confounders.

Results

For those with “Single-site pain”, higher levels (>13 min/day) of moderate-vigorous PA in ≥10 min bouts was associated with more pressure pain sensitivity (p = 0.035). Those with “Multisite pain” displayed increased cold pain sensitivity with greater amounts of vigorous PA (p = 0.011). Those with “No pain areas” displayed increased cold pain sensitivity with decreasing breaks from sedentary time (p = 0.046).

Conclusions

This study was a comprehensive investigation of a community-based sample of young adults with “No pain areas”, “Single-site pain” and “Multisite pain” and suggests some associations of measures of PA and SB with pain sensitivity.

Implications

The findings suggest that the pattern of accumulation of PA and SB may be important to inform improved clinical management of musculoskeletal pain disorders. This study provides a baseline for follow-up studies using the Raine Study cohort. Future research should consider temporal influences of PA and SB on pain sensitivity, pain experience and consider using a broader range of pain sensitivity measures.

Moderate running exercise prevents excessive immune system activation

Benefits of exercise have been documented for many diseases with a chronic progression, including obesity, diabetes mellitus, cardiovascular diseases, neurodegenerative diseases, certain types of cancers, and overall mortality. Low-grade systemic inflammation is a key component of these pathologies and it has been demonstrated that can be prevented by performing regularly physical exercise. The aim of this study was to examine the effect of lipopolysaccharide (LPS)-induced inflammation on glucose and insulin tolerance, exercise performance, production of urinary neopterin and striatal neurotransmitters levels in adult male C57BL/6 mice. Increased blood glucose clearance and insulin sensitivity were observed after a single administration of glucose (2 g/kg, p.o.) or insulin (0.5 U/kg, i.p.). However, the repeated injection of LPS (0.33 mg/kg/day, i.p.) decreased glucose tolerance and increase urinary neopterin levels, pointing to systemic inflammation. In parallel to the urinary-increased neopterin, it was observed a significant reduction in the striatal dopamine levels and an increase in the serotonin/dopamine ratio. While a single LPS injection (0.33 mg/kg, i.p.) showed impaired performance in the incremental loading test (10 m/min, with 2 m/min increment every 3 min, at 9% grade), a moderate physical exercise protocol (treadmill for three weeks; 5 sessions/week; up to 50 min/day) prevented the exacerbation of immune system activation and preserved mitochondrial activity in skeletal muscle from mice with continuous LPS infusion (infusion pumps: 0.83 mg/kg/day, i.p.). In conclusion, the peripheral-induced inflammation elicited metabolic alterations that provoked impairment in striatal dopamine metabolism. The moderate exercise prevented the increase of urinary neopterin and preserved mitochondrial activity under LPS-induced inflammatory conditions.