Dr. Minmin Luo’s lab revealed how serotonin neurons encode reward related signals.

    On January 28, 2016,  Dr. Minmin Luo’s lab published a paper titled “Serotonin neurons in the dorsal raphe nucleus encode reward signals” in Nature communication. This research paper shows that serotonin neurons in the dorsal raphe nucleus encode both reward expectation and reward acquisition.

    Natural rewards, such as food, sexual activities, and social interactions, are important for animals to survive and reproduce. Decrease in the ability to experience pleasure from natural reward (anhedonia) is a characteristic of mental disorders including depression. The midbrain dorsal raphe nucleus (DRN) project widely over the brain and strongly interconnect with several reward-related brain areas. Besides serotonergic neurons which releases serotonin (5-hydroxytryptamine; 5-HT), many other neuron types such as GABAergic (inhibitory neurons) are also located in the DRN. The mixed distribution of these different neuron types makes it difficult for the accurate understanding of the role of serotonergic neurons in reward encoding. Some conclusions are even misleading.

    Here, we addressed this question using fiber photometry and single-unit recording from serotonin (5-HT) neurons and GABA neurons in the DRN of behaving mice. Rewards including sucrose, food, sex, and social interaction rapidly activate 5-HT neurons, but aversive stimuli including quinine and footshock do not. Bedsides the activation by unexpected rewards, 5-HT neurons are also activated during reward expectation after mice learns to wait for reward delivery. In this case, most 5-HT neurons fire tonically during waiting and then phasically upon reward acquisition. Finally, GABA neurons are activated by aversive stimuli but inhibited when mice are seeking rewards. Thus, DRN 5-HT neurons positively encode a wide range of reward signals during anticipatory and consummatory phases of reward responses. Moreover, GABA neurons may play a complementary role in reward processing.

    These results provide direct theoretical evidence for selective serotonin reuptake inhibitors (SSRIs) in treating depression and support the theory that DRN is one of the important centers for reward processing in the brain.

http://www.nature.com/ncomms/2016/160128/ncomms10503/pdf/ncomms10503.pdf