Passive Oscillatory Body Motion Attenuates Pain-Related Behaviors and Increases β-Endorphin Immunoreactivity in a Murine Fracture Model: A Preliminary Study.

OBJECTIVE

This preliminary study aimed to investigate whether passive oscillatory body motion (PBM), a mechanical stimulus that reproduces vertical accelerations comparable to those generated during treadmill running, could activate the β-endorphin (β-END)-related hypothalamus-periaqueductal gray (PAG) pathway and alleviate fracture-induced pain.

METHODS

A murine fracture model was established in C57BL/6J mice by inducing a left tibial fracture followed by 3 weeks of immobilization. Mice were randomly divided into a PBM group and a control group, and PBM was initiated the day after fracture (±5 mm amplitude, 2 Hz, 30 min/day, 5 days/week, for 3 weeks). To measure pain thresholds, paw withdrawal thresholds were measured in an unblinded manner using the von Frey filament test. β-END immunoreactivity and opioid receptor mu 1 (Oprm1) mRNA in the PAG, as well as pro-opiomelanocortin (Pomc) mRNA expression in the hypothalamus, were analyzed by immunofluorescence and qPCR, respectively.

RESULTS

PBM significantly increased the paw withdrawal threshold ratio (p < 0.001) and β-END immunoreactivity (p = 0.02) in the PAG. Moreover, as an exploratory observation, Pomc mRNA expression tended to increase in the PBM group (p = 0.07), whereas Oprm1 mRNA expression did not differ between the two groups (p = 0.69).

CONCLUSION

This preliminary study provides initial evidence that PBM may attenuate fracture-induced pain-related behaviors and increase β-END immunoreactivity in the PAG, possibly via the hypothalamus-PAG pathway. PBM may represent a potential exercise-mimetic approach for pain relief, although further studies are needed to confirm its efficacy and underlying mechanisms.