The anatomical structure of the lumbar vertebrae of rats is similar to that of human lumbar vertebrae. The commonly used rat sciatica models can be divided into spontaneous models and induced models. The spontaneous sciatica rat models use special physical methods, such as weight bearing and strain, to accelerate the process of lumbar degeneration[28]. Though, these models can simulate the pathological model of naturally degenerative sciatica of lumbar spine. There were some disadvantages, such as long feeding time, inaccurate pathological model, and inaccurate evaluation of the specific time of modeling, so it has been abandoned gradually. The induced sciatica models can be divided into inflammatory stimulation model and mechanical compression model. However, the mechanism of mechanical compression of root sciatica has not been unanimously recognized, due to the rudeness of this method [29]. Using autologous caudal pulposus nucleus transplantation to create a rat model of discogenic sciatica can successfully simulate the physiological and pathological development of human lumbar discogenic sciatica[30].The animal model induced by autologous caudal pulposus nucleus transplantation avoiding directly compression of dorsal root was employed in this study to mimic the inflammation-induced pathological environment.
On Day 0, there was no significant difference in the left plantar mechanical pain threshold between groups, indicating that the rats in each group were comparable. On Day 2, the mechanical pain threshold was decreased in all groups. M, EA and NA groups was decreased much greater than that in FM and SEA groups. The sham operation without pulposus nucleus placement can also cause mild mechanical nociceptive sensitization, though the sensitization degree was not as high as that of the surgical model with pulposus nucleus placement[31]. This situation started from Day 2 and continued to Day 8, which may be related to the surgical trauma[32]. Previous studies have shown that exposed nucleus pulposus induces local inflammatory response and autoimmune response, which are important pathogenesis of lumbar discogenic sciatica[33].
During this period from Day10 to Day18, EA gradually played an effective role in improving the plantar mechanical pain threshold in the rat model of lumbar discogenic sciatica via constant acupoint stimulation at Shenshu (BL23), Huantiao (GB30), Weizhong (BL40) and Yanglingquan (GB34). Specific stimulation signals produced by EA intervention can inhibit the excitability of sensory neurons in the dorsal horn of the spinal cord and reduce the pain transmission[34].
From Day 20 to Day 28, the mechanical pain threshold in the EA group gradually recovered to the preoperative level[35]. At Day 28, there was no significant difference in the mechanical pain threshold between the EA, FM and SEA groups. EA treatment can restore the abnormal reduction of plantar mechanical pain threshold in model rats.
Sensory neurons in the spinal dorsal horn belong to the afferent neurons of peripheral pain, which play an important role in the reception and conduction of peripheral sensation (Fig. 4C). The spinal cord is the primary regulatory hub of nociceptive signal afferents. The surrounding nociceptive information is transmitted from DRG to the dorsal horn of the spinal cord through peripheral nociceptors, which plays an important role in the regulatory mechanism of pain sensitization[36]. Autologous caudal pulposus nucleus transplantation, as a "foreign body" around the lumbar nerve roots, damages its original physiological and immune environment, and can cause inflammatory reactions around the nerve roots. Strong inflammatory reactions lead to central spinal cord sensitization, which is the main pathological mechanism of neuropathic pain[37]. Central sensitization of the spinal cord plays a key role in the development of neuropathic pain. The ectopic firing of spinal dorsal horn neurons may be the origin of root sciatica pain.
EA can induce corresponding changes in the excitability of sensory neurons in the dorsal horn of the spinal cord in rats with lumbar discogenic sciatica[25]. Choosing 2Hz, 1mA low-frequency electrical pulses was feasible to adjust the nociceptive discharges of neurons by activating Aδ nerve fiber bundle[38].
Through tracking and detecting the expression level of inflammatory mediators closely related to the inflammatory response of dorsal root in rat model of sciatica, it was confirmed that postoperative sterile inflammatory response can reduce the pain threshold and form hyperalgesia. The levels of inflammatory mediators IL-6, IL-1β, PLA2 and NLRP3 were continuously increased in the model rats[39–41]. EA intervention is unable to reverse this strong spontaneous immune inflammatory response. But in the later disease stages for EA intervention group, the increased expression level of specific inflammatory mediators was controlled. This is consistent with previous research[42]. The exposure of the nucleus pulposus during surgery can stimulate the autologous immune response for a long time, mainly reflected in the continuous increase of TNF-α, IL-1β, IL-6, and PLA2 expression level.
Based on careful comparation, we can learn that there are two inflammatory factors that show significant change trends in the EA group. One is PGE2 and the other is NLRP3 as Fig. 3 exhibited. Compared with M and NA groups, the relative expression of inflammatory factor PGE2 in the FM, EA and SEA groups was significantly increased, which may be related to the specific isoforms of PGE2. There are four protein isoforms of PGE2 receptor, namely -EP1, -EP2, -EP3 and -EP4[43]. These four isoforms can bind to different prostaglandins respectively during the occurrence and development of inflammation, and play different regulatory modes in different stages of inflammation development, so that PGE2 can play roles in both pro-inflammatory and anti-inflammatory aspects[44]. Electroacupuncture may increase the expression of different PGE2 isoforms which has anti-inflammatory effect. A more detailed study on the correlation between electroacupuncture intervention and expression levels of PGE2 isoforms should be conducted in the future.
As one of the most active inflammasome, NLRP3 induces the maturation of IL-1β, mediates the activation of IL-6. Through binding to the interleukin receptor, NLRP3 induces the apoptosis of inflammatory cells, which finally produces anti-inflammatory effects. In this study, electroacupuncture intervention could achieve anti-inflammatory effects by inhibiting the activation of the NLRP3 inflammasome to reduce the production of IL-1β and IL-6[45].
There are also some shortcomings in the design of this experiment, that is, the lack of normal rat group. However, due to the introduction of sham operation group (FM) and sham operation electroacupuncture group (SEA), the experimental data results are also comparable and reliable by comparing with the model (M) group, electroacupuncture group (EA), and electroacupuncture sham point group (NA).