Background Anatomic and electrophysiological studies have revealed that the neurons located in the media vestibularnuclei (MVN) receive most of the sensory vestibular input coming from the ipsilateral labyrinth and the responses of MVNneurons to caloric stimulation directly reflect changes in primary vestibular afferent activity.The aim of this study was toclarify the intrinsic characteristics of serotonin (5-hydroxytryptamine,5-HT) release in the MVN during the period ofvertigo induced by caloric stimulation.Methods We used an in vivo microdialysis technique to examine the effects of caloric stimulation on the serotoninergicsystem in MVN.Twenty four guinea pigs were randomly divided into the groups of irrigation of the ear canal with hotwater (n=6),ice water (n=6) and 37℃ water (n=4),and the groups of irrigation of the auricle with hot water (n=4) and icewater (n=4),according to different caloric vestibular stimulation.We examined the animal’s caloric nystagmus with atwo-channel electronystagmographic recorder (ENG),and meanwhile examine serotonin (5-hydroxytryptamine,5-HT)level in the MVN with microdialysis technique after caloric stimulation.Results In the caloric test the hot water (44℃) irrigation of the right external auditory canal induced horizontalnystagmus towards the right side lasting about 60 seconds and the ice water irrigation of the right external auditory canalinduced it towards the left side lasting for about 90 seconds.No nystagmus was induced by 37℃ water irrigation of theexternal ear canal.Therefore,it was used as a negative control stimulation to the middle ear.The MVN 5-HT levelssignificantly increased in the first 5-minute collecting interval and increased to 254% and 189% of the control group in thesecond collecting interval in response to caloric vestibular stimulation with ice water and hot water respectively.Theserotonin release was not distinctly changed by the irrigation of the auricle with ice water or hot water.Conclusions Neither somato-sensory stimulation of the middle ear nor nonspecific cold or hot stress affects theserotonin release.The rise of 5-HT in MVN may be involved in the mechanism of vertigo induced by caloric stimulation. Background Anatomic and electrophysiological studies have revealed that the neurons located in the media vestibularnuclei (MVN) receive most of the sensory vestibular input coming from the ipsilateral labyrinth and the responses of MVNneurons to caloric stimulation directly reflect changes in primary vestibular afferent activity.The aim of this study was toclarify the intrinsic characteristics of serotonin (5-hydroxytryptamine, 5-HT) release in the MVN during the period of vertigo induced by caloric stimulation. Methods We used an in vivo microdialysis technique to examine the effects of caloric stimulation on the serotoninergicsystem in MVN.Twenty four guinea pigs were randomly divided into groups of irrigation of the ear canal with hotwater (n = 6), ice water (n = 6) and 37 ° C water (n = 4), and the groups of irrigation of the auricle with hot water (n = 4) and icewater (n = 4), according to different caloric vestibular stimulation. We examined the animal’s caloric nystagmus with atwo-channel ele ctronystagmographic recorder (ENG), and meanwhile examining serotonin (5-hydroxytryptamine, 5-HT) level in the MVN with microdialysis technique after caloric stimulation. Results In the caloric test the hot water (44 ° C) irrigation of the right external auditory canal induced horizontalnystagmus towards the right side lasting about 60 seconds and the ice water irrigation of the right external auditory canalinduced it towards the left side lasting for about 90 seconds. No nystagmus was induced by 37 ° water irrigation of theexternal ear canal.Therefore, it was used as a negative control stimulation to the middle ear. The MVN 5-HT level is slightly fixedly increased in the first 5-minute collecting interval and increased to 254% and 189% of the control group in thesecond collecting interval in response to caloric vestibular stimulation with ice water and hot water respectively.Theserotonin release was not distinctly changed by the irrigation of the auricle with ice water or hot water. Conclusions Neith er somato-sensory stimulation of the middle ear nor nonspecific cold or hot stress affects the serotonin release. The rise of 5-HT in MVN may be involved in the mechanism of vertigo induced by caloric stimulation.