Delayed response of the median raphe serotonin neurons projecting to the ventral hippocampus to aversive stimuli

Appropriate processing of aversive information is essential for survival. We previously demonstrated that serotonin neurons in the median raphe nucleus (MRN) play a key role in such processing; however, MRN responses to predictive cues of aversive events remain unclear. Here, we found that the MRN serotonin neurons were activated by aversive air-puff stimuli but not by auditory cues predicting the air puff. Moreover, delayed activation of the ventral hippocampus-projecting MRN serotonin neurons, together with subsequent 5-HT_2A receptor signaling, was required for aversion. These findings shed light on the roles of the hippocampus-projecting MRN serotonin neurons and elucidate their molecular mechanisms.