Activation of dorsal raphe serotonin neurons underlies waiting for delayed rewards

The serotonergic system plays a key role in the control of impulsive behaviors. Forebrain serotonin depletion leads to premature actions and steepens discounting of delayed rewards. However, there has been no direct evidence for serotonin neuron activity in relation to actions for delayed rewards. Here we show that serotonin neurons increase their tonic firing while rats wait for food and water rewards and conditioned reinforcement tones. The rate of tonic firing during the delay period was significantly higher for rewards than for tones, for which rats could not wait as long. When the delay was extended, tonic firing persisted until reward or tone delivery. When rats gave up waiting because of extended delay or reward omission, serotonin neuron firing dropped preceding the exit from reward sites. Serotonin neurons did not show significant response when an expected reward was omitted, which was predicted by the theory that serotonin signals negative reward prediction errors. These results suggest that increased serotonin neuron firing facilitates a rat's waiting behavior in prospect of forthcoming rewards and that higher serotonin activation enables longer waiting.