Spinal Interferon Regulatory Factor 8 and Brain-derived Neurotrophic Factor in the Prefrontal Cortex are Involved in Pain-induced Depression Relief via Ultrasound-guided Pulsed Radiofrequency in a Rat Spared Nerve Injury Model

BACKGROUND

Pain-depression comorbidity has become a great burden to individuals and society. Nevertheless, the mechanisms underlying comorbid diseases have still not been fully revealed. Ultrasound-guided pulsed radiofrequency (PRF) on peripheral nerves, which produces remarkable analgesia via high-frequency electromagnetic energy, has become a main, minimally invasive treatment for chronic neuropathic pain.

OBJECTIVES

The aim of this study was to explore the effect of ultrasound-guided PRF on the sciatic nerve of spared nerve injury (SNI) rats to relieve pain-induced depression.

STUDY DESIGN

Experimental trial in rats.

SETTING

The research took place in the Laboratory of The First Affiliated Hospital of Wenzhou Medical University.

METHODS

Sixty male Wistar rats were randomly divided into a sham group, an SNI group, an SNI + free-PRF group, and an SNI + PRF group. Ultrasound-guided PRF was applied to the sciatic nerve on day 7 after SNI. The basal paw mechanical withdrawal threshold (PMWT) was evaluated as a measure for pain-related behavior, and a  sucrose preference test was performed as a measure for depression-related behavior. The expression levels of spinal interferon regulatory factor 8 (IRF8) and of brain-derived neurotrophic factor (BDNF) in the prefrontal cortex (PFC) were also studied on days 21 and 42.

RESULTS

The results showed that the PMWT was significantly decreased in rats following SNI operation; the decreased levels of PMWT were reversed in the SNI + PRF group after the application of PRF on the sciatic nerve on day 7. There were no statistically significant differences among the groups in the sucrose preference rate on day 21 after SNI operation. The sucrose preference rate on day 42 was higher in the SNI + PRF group than in the SNI + free-PRF group. Western blot and reverse transcription polymerase chain reaction also demonstrated that ultrasound-guided PRF on the sciatic nerve downregulated overexpression of spinal IRF8 and increased the levels of BDNF in the PFC.

LIMITATIONS

This study was performed using only an SNI rat model which cannot represent all rodent neuropathic pain models. Only the short-term effectiveness of ultrasound-guided PRF on the sciatic nerve of SNI rats was investigated. The BDNF changes of other important brain areas were not taken into consideration in this study.

CONCLUSIONS

These findings suggest that ultrasound-guided PRF on sciatic nerve could alleviate pain-induced depression. The mechanisms of this treatment may be involved in the downregulated spinal IRF8 and the increased BDNF in PFC.

Exercise Facilitates the M1-to-M2 Polarization of Microglia by Enhancing Autophagy via the BDNF/AKT/mTOR Pathway in Neuropathic Pain

BACKGROUND

In neuropathic pain following peripheral nerve injury, microglia are rapidly activated and accumulated in the spinal cord. Physical exercise can alleviate neuropathic pain. However, the exact mechanism underlying this analgesic effect is not fully understood.

OBJECTIVES

We aimed to investigate the molecular mechanisms by which exercise alleviates neuropathic pain in relation to brain-derived neurotrophic factor (BDNF), microglia polarization, and autophagy.

STUDY DESIGN

A randomized controlled animal study divided into 2 stages. The first stage comprised 4 groups each with 6 mice, and the second stage comprised 6 groups, 3 with 18 mice and 3 with 12 mice.

SETTING

Department of Anesthesiology, Lanzhou University Second Hospital, Orthopaedics Key Laboratory of Gansu Province, Lanzhou University.

METHODS

Von Frey filaments, Western blotting, immunofluorescence, and transmission electron microscopy analyses were conducted to detect relevant markers.

RESULTS

After peripheral nerve injury, exercise training downregulated BDNF expression and reversed microglial activation, as indicated by the increased expression of the M2 marker CD206 and decreased expression of the M1 marker CD86 in the spinal dorsal horn of mice. Autophagy flux was enhanced after exercise training, as suggested by the increased expression of the autophagy markers LC3-II/LC3-I and Beclin1 and decreased expression of the autophagy adaptor protein p62. Furthermore, autophagy inhibition by 3-methyladenine aggravated M1 polarization and hyperalgesia, whereas autophagy induced by rapamycin promoted M2 polarization and reduced hyperalgesia. Intrathecal injection of BDNF significantly upregulated BDNF expression, inhibited autophagy, triggered M1 polarization of spinal microglia, and aggravated hyperalgesia. Furthermore, BDNF regulated autophagy through the AKT/mTOR pathway, thereby participating in exercise training-mediated polarization of microglia after nerve injury.

LIMITATIONS

The effect of exercise on autophagy and pain cannot be assessed in an in vitro model. The influence of intrathecal injection of BDNF on the metabolic changes in other neuronal cells and the subsequent effects on pain should be investigated. Further studies on how exercise training modulates microglial autophagy to alleviate neuropathic pain are needed.

CONCLUSIONS

Exercise training promoted the recovery of sciatic nerve injury in mice, possibly by regulating microglial polarization through BDNF/AKT/mTOR signaling-mediated autophagy flux. We confirmed the efficacy of exercise training in alleviating neuropathic pain and suggest a new therapeutic target for neuropathic pain.