Acetic Acid-Induced Writhing Is Temporally Dissociated from Gastrointestinal Motility in Freely Moving Rats

The acetic acid-induced writhing test is a standard model for visceral pain and analgesic screening. While writhing behavior may have been implicitly assumed to be associated with gastrointestinal motor activity, whether gastrointestinal motility directly triggers writhing behavior has remained untested. We simultaneously recorded antral and duodenal motility using strain gauge transducers in freely moving rats during acetic acid-induced writhing events. Our analysis, employing a hidden Markov model, found no evidence that antral or duodenal motility exhibited time-locked changes relative to writhing events. Furthermore, writhing behavior persisted even when gastrointestinal motility was pharmacologically suppressed with scopolamine butylbromide, providing direct evidence that writhing occurs independently of gut motor activity. Thus, acetic acid-induced writhing is temporally dissociated from gastrointestinal contraction, indicating that writhing reflects visceral nociceptive signaling distinct from gut motility.