[Role of Median Raphe Serotonergic Neurons in Positive and Negative Information Processing]

Serotonergic neurons play a critical role in processing reward and aversive information. Rewarding stimuli activate serotonergic neurons in the dorsal raphe nucleus (DRN), whereas optogenetic activation of DRN serotonergic neurons induces reward-like effects. However, the pharmacological enhancement of serotonin neurotransmission does not induce rewarding or aversive effects. These findings suggest the presence of another serotonergic neuron that plays a role opposite to that of the DRN in processing reward and aversion information. Previous reports suggested that the median raphe nucleus (MRN) processes negative emotional stimuli. To elucidate the function of MRN serotonergic neurons in these processes, we recorded the changes in serotonergic activity in mice in response to rewarding and aversive stimuli. We also used optogenetic manipulation to determine whether these changes could induce rewarding and aversive behaviors. The activity of MRN serotonergic neurons decreased in response to rewarding stimuli and increased after aversive stimuli. Optogenetic inhibition of MRN serotonergic neurons induced reward-related behavior, while optogenetic stimulation induced aversion-related behavior. Furthermore, we found that the projection pathway from MRN serotonergic neurons to the interpeduncular nucleus is crucial for these processes. These results indicate that MRN serotonergic neurons play a pivotal role in processing reward and aversive information, functioning oppositely to DRN neurons.