Reward Processing by the Dorsal Raphe

Reward motivates animal behaviors,produces the feelings of pleasure,and guides learning and memory formation. The brain reward system – a group of interconnected brain structures – participates in various aspects of reward processing and its malfunctions are associated with numerous psychiatric disorders. Although dopamine neurons in the midbrain ventral tegmental area(VTA) receive much focus,whether and how neurons in the dorsal raphe nucleus(DRN) contribute to reward processing remain controversial. The DRN represents the major source of serotonin in the forebrain and contains also glutamate,GABA,and dopamine neurons. Here I will summarize recent evidences indicating that the DRN contributes to reward processing in a cell type-specific manner. First,I will present optogenetic studies indicating that DRN neurons produce reward signals through the release of serotonin and glutamate. Second,physiological recordings reveal that DRN serotonin neurons positively encode a wide range of reward signals and drugs of abuse that are associated with pleasure. Our recordings suggest that DRN serotonin neurons encode beneficialness signals and may be important for reward belief. The beneficialness model of serotonin contrast with VTA dopamine neurons that encode reward prediction error and crucially organize reward seeking and reinforcement learning. Finally,I will present some new data indicating that DRN dopamine neurons encode saliency and play an important role in memory expression. Therefore,the DRN contributes to various aspects of reward processing through different neuron types associated with distinct neurotransmitter phenotypes.