Effects and mechanisms of auricular electroacupuncture on visceral pain induced by colorectal distension in conscious rats

Objective: To investigate the effects and mechanisms of action of auricular electroacupuncture (AEA) on visceral pain induced by colorectal distension (CRD). Methods: Twenty-nine female Sprague-Dawley rats were randomly divided into four groups: control; untreated CRD; CRD+AEA; and CRD+sham electroacupuncture (SEA). An electromyogram (EMG) was recorded for 120 min in the conscious state. After a 30 min baseline recording, CRD was performed in untreated CRD, AEA and SEA groups and lasted for 90 min. AEA and SEAwere started at 30 min and lasted for 30 min. The EMG was recorded and analysed to evaluate the severity of visceral pain, indicated by the magnitude of the vasomotor response (VMR). mRNA expression of the 5-hydroxytryptamine 1a (5-HT_1a) receptor was measured separately in the colon and raphe nuclei using real-time fluorescent quantitative PCR. Results: No differences were seen in the baseline EMG among the four groups (p>0.05). During prestimulation, VMR magnitude in the CRD, AEA and SEA groups increased compared with that in the control group (p<0.05). During stimulation, the VMR magnitude was significantly decreased in AEA but not SEA groups relative to the (untreated) CRD group. Similarly, mRNA expression of the 5-HT_1a receptor in both the colon and raphe nuclei was lower in AEA but not SEA groups compared with the CRD group (p<0.05). Conclusions: AEA can ameliorate CRD-induced visceral pain in rats, and increase mRNA expression of the 5-HT_1a receptor peripherally (in the colon) and centrally (in the raphe nuclei), suggesting a serotonergic mechanism of action.