Electroacupuncture-Induced Muscular Inflammatory Pain Relief Was Associated With Activation of Low-Threshold Mechanoreceptor Neurons and Inhibition of Wide Dynamic Range Neurons in Spinal Dorsal Horn

Abstract

Acupuncture is an effective alternative therapy for pain management. Evidence suggests that acupuncture relieves pain by exciting somatic afferent nerve fibers. However, the mechanism underlying the interaction between neurons in different layers of the spinal dorsal horn induced by electroacupuncture (EA) remains unclear. The aim of this study was to explore the mechanism of EA relieving inflammatory muscle pain, which was associated with activation of the spontaneous firing of low-threshold mechanoreceptor (LTM) neurons and inhibition of wide dynamic range (WDR) neuronal activities in the spinal dorsal horn of rats. Inflammatory muscle pain was induced by injecting complete Freund’s adjuvant into the right biceps femoris muscle. EA with intensity of threshold of A fibers (Ta) in Liangqiu (ST34) muscle considerably inhibited the abnormal spontaneous activities of electromyography (EMG) due to muscle inflammation. While EA with intensity of C-fiber threshold (Tc) increased the abnormal activities of EMG. EA with Ta also ameliorated the imbalance of weight-bearing behavior. A microelectrode array with 750-μm depth covering 32 channels was used to record the neuronal activities of WDR and LTM in different layers of the spinal dorsal horn. The spontaneous firing of LTM neurons was enhanced by EA-Ta, while the spontaneous firing of WDR neurons was inhibited. Moreover, EA-Ta led to a significant inverse correlation between changes in the frequency of WDR and LTM neurons (r = −0.64, p < 0.05). In conclusion, the results indicated that EA could alleviate inflammatory muscle pain, which was associated with facilitation of the spontaneous firing of LTM neurons and inhibition of WDR neuronal activities. This provides a promising evidence that EA-Ta could be applied to relieve muscular inflammatory pain in clinical practice.

The Effect of Pre-Electroacupuncture on Nociceptive Discharges of Spinal Wide Dynamic Range Neurons in Rat

“…44 In addition, the underlying mechanisms of the segmental analgesia of acupuncture have been suggested to be associated with the gate control theory. 10,16,45 In this study, pre-EA with low intensity T Aβ inhibited nociceptive discharges of WDR neurons, indicating that anti-nociceptive effects of pre-EA may also be related to the gate control theory. However, the phenomena only occurred when pre-EA with T Aβ was administered at adjacent non-RFs.…”

“…Previous studies determined the intensity of EA by recordings of nociceptive reflexes (RIII reflex). 10,11,16 As the C-fiber reflex was a kind of nociceptive response, 59,60 it was difficult to find out the threshold of Aβ-fiber. Other studies employed single-unit recording of primary afferent fibers or sensory neurons to determine the intensity.…”

“…Neurons responded to both innocuous and noxious pressure but not brushing were identified as WDR neurons with muscular RFs (Figure 2C). 16,25 A total of 106 WDR neurons were recorded from 70 rats. Most WDR neurons were distributed at a depth of 650-1300 μm below the dorsal surface of the spinal cord, corresponding to laminae IV-VI of the SDH (Figure 2D).…”

“…14,15 A recent study also showed that EA inhibited spontaneous firing of WDR neurons by activating A-fibers in muscular inflammatory pain in rats. 16 Moreover, EA intervention at homotopic or heterotopic acupoints produced ameliorated effects on visceral pain via suppressing nociceptive discharges of WDR neurons in an intensity-dependent manner. [17][18][19] These studies indicated that the inhibitory effects of EA intervention on WDR neurons were closely related to the activation of different afferent fibers.…”

Local analgesia of electroacupuncture is mediated by the recruitment of neutrophils and released β-endorphins

“…For example, in inflammatory muscle pain rats, low-strength electroacupuncture (EA) has been shown to reduce the introduction of pain information through the spinal gate-control mechanism, significantly alleviating pain and inhibiting the abnormal electromyography. 7 Previous studies have also demonstrated that heterotopic EA stimulation triggers the pain-inhibiting effect of diffuse noxious inhibitory controls (DNICs). 15 In addition, Han 9 found that acupuncture applied at specific frequencies to certain body sites can stimulate the release of endogenous opioid peptides in the central nervous system and activate opioid receptors to induce antinociception.…”