Effects of electroacupuncture on expression of c-fos protein and N-methyl-D-aspartate receptor 1 in the rostral ventromedia medulla of rats with chronic visceral hyperalgesia

Acupuncture has been clinically proved to be effective in treating abdominal pain in patients with irritable bowel syndrome (IBS). However, its neurobiological mechanism remains largely unexplored. The aim of this study was to investigate the effect of electroacupuncture (EA) in relieving chronic visceral hyperalgesia and the possible involvement of N-methyl-D-aspartate receptor 1 (NR1) in rostral ventromedial medulla (RVM) of the brain in an IBS rat model.To establish the IBS rat model, male Sprague-Dawley neonatal rats received colon mechanical irritation on a daily basis from the 9th to the 22nd day after their birth. After a resting period of another two to four weeks, behavioral tests of pain threshold pressure (PTP) and abdominal withdrawal reflex (AWR) responding to colorectal distention (CRD) stimuli were conducted to judge the colorectal sensitive situation. Then administration of EA at acupoints of Zusanli (ST36) and Shangjuxu (ST37) bilaterally in the hind limbs was repeated four times every other day, while sham-EA was done by inserting needles at similar acupoints without electrical stimulation. Immunohistochemical method was used to display the expression of proto-oncogene protein c-fos and NR1 in RVM of rats.The results demonstrated that the PTP values and AWR scores, in response to the CRD stimuli, significantly decreased and increased, respectively (P<0.01, P<0.01), while the number of immunoreactive neurons of c-fos protein and NR1 significantly increased in nucleus reticularis gigantocellularis (Gi), nucleus lateralis paragigantocellularis (LPGi), nucleus reticularis gigantocellularis pars alpha (GiA) and nucleus raphe magnus (NRM) of RVM in IBS model rats compared with the normal rats (P<0.05). After EA treatment, PTP values and AWR scores significantly increased and decreased, respectively (P<0.01, P<0.05); the number of immunoreactive neurons of c-fos and NR1 significantly decreased respectively in Gi, LPGi and GiA and in Gi, LPGi, GiA and NRM (P<0.05). No such effects on PTP values, AWR scores and the number of immunoreactive neurons of c-fos and NR1 were observed after sham-EA treatment.These data provide the evidence that EA can relieve chronic visceral hyperalgesia in rats with IBS, and such an effect may be correlated with inhibitory modulation of hyperactivity of neurons by means of down-regulating the high expression of NR1 in RVM of IBS model rats.