Objective To observe the role of large-diameter fibers of infraorbital nerve (ION) in "Sibai" (ST 2)-electroacupuncture(EA)induced analgesia in visceral pain (VP) rats. Methods A total of 36 SD rats were randomized into control

VP

EA

ION transaction

snake venom(SV) and saline groups

with 6 rats in each group.EA(2 Hz/20 Hz) was applied to bilateral "Sibai" (ST 2) for 20 min.VP model was established by intraperitoneal injection of 0.6% acetic acid(10 mL/kg).Bilateral ION were transacted or pretreated by regional application of snake venom to selectively destroy A fibers

respectively.Behavior reactions were assessed by counting abdominal muscular contractions. Meanwhile

c-fos expression in the nucleus of the solitary tract (NTS) and paratrigeminal nucleus (PTN) was displayed by immunohistochemistry. Results In comparison with control group

the numbers of abdominal muscular contraction

and c-fos immuno-reaction(IR)positive neurons in both NTS and PTN increased significantly in VP group( P<0.001 );while in comparison with VP group

the numbers of the abdominal contraction

and c-fos IR-positive neurons of both NTS and PTN in EA and SV and saline groups decreased considerably( P<0.05

P<0.01 ).No significant differences were found between ION transaction and VP groups in the abdominal contraction number

and c-fos IR-positive neurons in both NTS and PTN areas

and among EA and SV and saline groups in the numbers of abdominal contraction and c-fos IR-positive neurons of both NTS and PTN ( P>0.05 ). Conclusion The large-diameter (A) fibers of ION are not the major afferent fibers affecting EA-ST 2 induced analgesia in visceral pain rats;and somatic sensory afferents from orofacial areas and visceral pain input converge in the NTS and PTN

which may be the basis of the EA analgesia in the present study.