Auricular Acupuncture Electrical Stimulation In The Treatment Of Opioid-induced Constipation And Related Mechanisms Of Brain-gut Axis In Rats

Background and aims:Constipation and opioid-induced constipation(OIC)are common with limited treatment options.We investigated whether a noninvasive method of auricular electroacupuncture or auricular vagal nerve stimulation(aVNS)could be used for treating OIC and explored its potential mechanisms and neural pathways in a rodent model of OIC.Materials and Methods:Sprague-Dawley were chronically implanted with one pair of auricular electrodes for aVNS.In acute study,8 normal rats were implanted with a polyethylene catheter in the proximal colon for assessing whole colon transit.In chronic study,8 rats as control,and 16 rats were treated with loperamide for a week while another 16 rats received bilateral vagotomy,then randomly treated with aVNS or sham-aVNS for a week.Feces were collected and analyzed daily,distal colon transit time(dCTT)and whole gut transit time(WGTT)were measured using previously validated methods before and after the stimulation treatment.Heart rate variability derived from the electrocardiogram was analyzed to evaluate autonomic functions.At the end of the chronic study,gastric emptying(GE)and small intestinal transit(SIT)were measured.Colon tissues were collected for measuring the protein expressions of ChAT,nNOS,GDNF by western blot,and the percentage of ChAT and nNOS in the enteric nervous system(ENS)were detected by immunohistochemistry.ICC in the gastric antrum and colon were also detected by immunohistochemistry.The brain tissues were also harvested for the assessment of immunoactivity of c-fos in the nucleus tractus solitarius(NTS)and dorsal motor nucleus of the vagus(DMV)which are the areas for vagal afferent and vagal efferent,respectively.The Plasma level of pancreatic polypeptide(PP)and noradrenaline(NE)were assessed by ELISA.Results:1)The number of fecal pellets and water content in feces increased after aVNS,compared with sham-aVNS.2)aVNS shortened colon transit time,dCTT,WGTT,GE and SIT,compared with sham-aVNS.3)In colon tissues,aVNS increased the protein expression of choline acetyltransferase,glial cell line-derived neurotrophic factor and the c-kit expression in myenteric interstitial cells of Cajal but decreased the protein expression of neural nitric oxide synthase(P<0.05 for all,vs.sham-VNS).4)The prokinetic effects of aVNS were abolished by both subdiaphragmatic vagotomy and atropine.5)aVNS increased the c-fos expression in both NTS and DMV,and increased vagal efferent activity(P<0.05,vs.sham-VNS).Conclusions:aVNS improves OIC by enhancing colon motility and restoring enteric neural functions mediated via the central and vagal efferent pathway.