Increase in serum nerve growth factor but not intervertebral disc degeneration following whole-body vibration in rats

Abstract

Background: Low back pain is a leading cause of disability and is frequently associated with whole-body vibration exposure in industrial workers and military personnel. While the pathophysiological mechanisms by which whole-body vibration causes low back pain have been studied in vivo, there is little data to inform low back pain diagnosis. Using a rat model of repetitive whole-body vibration followed by recovery, our objective was to determine the effects of vibration frequency on hind paw withdrawal threshold, circulating nerve growth factor concentration, and intervertebral disc degeneration.

Methods: Male Sprague-Dawley rats were vibrated for 30 min at an 8 Hz or 11 Hz frequency every other day for two weeks and then recovered (no vibration) for one week. Von Frey was used to determine hind paw mechanical sensitivity every two days. Serum nerve growth factor concentration was determined every four days. At the three-week endpoint, intervertebral discs were graded histologically for degeneration.

Findings: The nerve growth factor concentration increased threefold in the 8 Hz group and twofold in the 11 Hz group. The nerve growth factor concentration did not return to baseline by the end of the one-week recovery period for the 8 Hz group. Nerve growth factor serum concentration did not coincide with intervertebral disc degeneration, as no differences in degeneration were observed among groups. Mechanical sensitivity generally decreased over time for all groups, suggesting a habituation (desensitization) effect.

Interpretation: This study demonstrates the potential of nerve growth factor as a diagnostic biomarker for low back pain due to whole-body vibration.

Keywords: Intervertebral disc; Nerve growth factor; Pain; Spine; Whole-body vibration.

Fig. 1.

Front and side views of the vibration device and rat enclosure. A: Sectioned polycarbonate chamber for rats. B: Motor for generating vibrations. C: Eccentric mass on the motor. The curved arrow indicates the rotational direction of the motor.

Fig. 2.

Hind paw withdrawal threshold for Control, 8 Hz, and 11 Hz groups over the vibration and recovery periods. Error bars indicate 95% confidience interval. Statistical analysis was performed using a linear mixed-effects model. (n = 12; *P<0.05, compared to baseline; ^#P<0.05, compared to Control)

Fig. 3.

Change in nerve growth factor (NGF) serum concentration (pg/mL) for Control, 8 Hz, and 11 Hz groups over the vibration and recovery periods. Error bars indicate 95% confidence interval. Statistical analysis was performed using a linear mixed-effects model. (n = 8–12; *P<0.05, compared to baseline; ^#P<0.05, compared to Control; ⁼P<0.05, compared to 8 Hz)

Fig. 4.

Representative histological images of intervertebral discs (IVDs). The extent of IVD degeneration observed using the modified histological scoring system was similar among the three groups. Arrows indicate NP clefts (second row, left and middle), serpentine lamellae of AF (third row, middle and right), and discontinuity of AF-NP boundary (third row, left). H&E, bar = 200 μm. NP = nucleus pulposus; AF = annulus fibrosus.

Fig. 5.

Boxplots of the IVD histological scoring parameters. (a) Summation of histological scores for all four IVD scoring parameters. Histological scores for each parameter: (b) NP structure, (c) NP clefts/fissures, (d) AF structure, and (e) AF-NP boundary. Error bars indicate the 10^th-90^th percentiles. No differences were noted for the total average score or any individual parameter. (n = 3)