Abstract
Electroacupuncture (EA) is an efficacious treatment for alleviating visceral pain, but the underlining mechanisms are not fully understood. This study investigated the role of medullary subnucleus reticularis dorsalis (SRD) neurons in the effects of EA on visceral pain. We recorded the discharges of SRD neurons extracellularly by glass micropipettes on anesthetized rats. The responses characteristics of SRD neurons to different intensities of EA (0.5, 1, 2, 4, 6, and 8 mA, 0.5 ms, and 2 Hz) on acupoints “Zusanli” (ST 36) and “Shangjuxu” (ST 37) before and during noxious colorectal distension (CRD) were analyzed. Our results indicated that SRD neurons responded to either a noxious EA stimulation ranging from 2 to 8 mA or to noxious CRD at 30 and 60 mmHg by increasing their discharge frequency at an intensity-dependent manner. However, during the stimulation of both CRD and EA, the increasing discharges of SRD neurons induced by CRD were significantly inhibited by 2–8 mA of EA. Furthermore, SRD neurons can encode the strength of EA, where a positive correlation between current intensity and the magnitude of neuronal responses to EA was observed within 2–6 mA. Yet, the responses of SRD neurons to EA stimulation reached a plateau when EA exceeded 6 mA. In addition, 0.5–1 mA of EA had no effect on CRD-induced nociceptive responses of SRD neurons. In conclusion, EA produced an inhibiting effect on visceral nociception in an intensity-dependent manner, which probably is due to the somatovisceral interaction at SRD neurons.
Highlights
Visceral pain is one of the most common symptoms in patients with gastrointestinal disorders (Sach et al, 2002)
We found that subnucleus reticularis dorsalis (SRD) neurons did not respond to low intensity of EA stimulation (0.5–1 mA), but were significantly activated by high intensity of EA stimulation (2–8 mA)
EA (Electroacupuncture) (G) A histogram showing the dicharges of SRD neurons (n = 7) were significantly activated by EA at 2–8 mA. ∗p < 0.05 compared with baseline; #p < 0.05 compared with 0.5 mA EA; +p < 0.05 compared with 1 mA EA
Summary
Visceral pain is one of the most common symptoms in patients with gastrointestinal disorders (Sach et al, 2002). It is usually associated with impaired health-related quality of life and a significant health care burden (Chang, 2004; Spiegel et al, 2004). It is generally accepted that EA analgesia is an integrative process of afferent impulses between pain regions and acupoints at convergence neurons and this process involves different levels of central structures, such as spinal dorsal horn (Rong et al, 2005), nucleus tractus solitarius (Liu et al, 2014) and periaqueductal gray (Wang et al, 2014). The involvement of other convergence neurons in EA analgesia is still unknown
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