Peripheral Therapeutic Ultrasound Stimulation Alters the Distribution of Spinal C-Fos Immunoreactivity Induced by Early Or Late Phase of Inflammation

Abstract

The purpose of this investigation was to examine the central modulated effects of therapeutic ultrasound (US) on neuronal activity in the spinal cord on early and late phases of inflammation. In this study, induction of c-Fos protein, which reflects neuronal activation (particularly inflammatory nociception), was investigated in the lumbar spinal cord with immunohistochemistry. Inflammatory monoarthritis was induced in 20 male Wistar rats (weighing 250–300 g) via intra-articular injection of complete Freund’s adjuvant (CFA) into the tibiotarsal joint. Two phases of arthritis, early phase (18 h after adjuvant injection) and late phase (7 d after adjuvant injection), were studied in the rats. Pulsed-mode US (1 MHz, the spatial average temporal average intensity [I SATA] = 0.5 W/cm 2, 50% duty cycle) was applied for 5 min. The effects of US and sham treatments against these phases of arthritis were demonstrated by spinal c-Fos–like immunoreactivity (c-Fos-LI). All data were evaluated statistically with the paired t-test or analysis of variance with Bonferroni corrections. c-Fos-LI neurons were abundant (average 264.2 ± 11.9) in the L3 and L4 neurons of the spinal cord in areas ipsilateral to the CFA-induced arthritic leg in the early phase, but few were present (average 40.4 ± 4.5) in the late phase in sham-treated animals. Bonferroni corrections to the alpha level were used to check the group differences in spinal c-Fos expression, and significance was reached when p < 0.025. In the early inflammatory phase, US treatment significantly suppressed the increased number of c-Fos-LI neurons associated with CFA-induced arthritis in superficial laminae, nucleus proprius, deep laminae and ventral horn of the spinal cord. However, during the late inflammatory phase, US significantly triggered c-Fos expression in most laminae, particularly in the nucleus proprius, deep laminae and ventral horn of the spinal cord. The results of our study suggest that administration of US causes a reduction of early nociceptive inflammatory processing, as shown by a decrease in CFA-induced c-Fos-LI neurons at the level of the spinal cord. In contrast, the US did not suppress, but rather enhanced, the number of c-Fos-LI neurons during the late inflammatory phase. The peripheral influences of US on the central modulation of the spinal nociceptive processing system is suggested and may reflect the work being done through the neuroplasticity of spinal cord in response to peripheral stimulation of US. Therefore, we propose a difference in spinal expression of c-Fos-LI neurons between effects of peripheral US stimulation in arthritic models that underlie early and late inflammatory pain. (E-mail: sherrie@sunrise.hk.edu.tw)