The ability to predict reward promotes animal survival. Both dopamine neurons in the ventral tegmental area and serotonin neurons in the dorsal raphe nucleus (DRN) participate in reward processing. Although the learning effects on dopamine neurons have been extensively characterized, it remains largely unknown how the response of serotonin neurons evolves during learning. Moreover, although stress is known to strongly influence reward-related behavior, we know very little about how stress modulates neuronal reward responses. By monitoring Ca2+ signals during the entire process of Pavlovian conditioning, we here show that learning differentially shapes the response patterns of serotonin neurons and dopamine neurons in mice of either sex. Serotonin neurons gradually develop a slow ramp-up response to the reward-predicting cue, and ultimately remain responsive to the reward, whereas dopamine neurons increase their response to the cue but reduce their response to the reward. For both neuron types, the responses to the cue and the reward depend on reward value, are reversible when the reward is omitted, and are rapidly reinstated by restoring the reward. We also found that stressors including head restraint and fearful context substantially reduce the response strength of both neuron types, to both the cue and the reward. These results reveal the dynamic nature of the reward responses, support the hypothesis that DRN serotonin neurons signal the current likelihood of receiving a net benefit, and suggest that the inhibitory effect of stress on the reward responses of serotonin neurons and dopamine neurons may contribute to stress-induced anhedonia.

SIGNIFICANCE STATEMENT Both serotonin neurons in the dorsal raphe and dopamine neurons in the ventral tegmental area are intimately involved in reward processing. Using long-term fiber photometry of Ca2+ signals from freely behaving mice, we here show that learning produces a ramp-up activation pattern in serotonin neurons that differs from that in dopamine neurons, indicating complementary roles for these two neuron types in reward processing. Moreover, stress treatment substantially reduces the reward responses of both serotonin neurons and dopamine neurons, suggesting a possible physiological basis for stress-induced anhedonia.

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