Activation of dorsal raphe serotonin neurons is necessary for waiting for delayed rewards

Abstract

The forebrain serotonergic system is a crucial componentin the control of impulsive behaviors. We previously reported that the activity of serotonin neurons in the midbrain dorsal raphe nucleus increased when rats performed a task that required them to wait for delayed rewards. However, the causal relationship between serotonin neural activity and the tolerance for the delayed reward remained unclear. Here, we test whether the inhibition of serotonin neural activity by the local application of the 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin inthe dorsal raphe nucleus impairs rats' tolerance for delayed rewards. Rats performeda sequential food-water navigation task that required them to visit food and water sites alternately via a tone site to get rewards at both sites after delays. During the short (2s) delayed reward condition, the inhibition of serotonin neural activity did not significantly influence the numbersofreward choice errors (nosepoke at an incorrect reward site following a conditioned reinforcer tone), reward wait errors (failure to wait for the delayed rewards), or total trials (sum of reward choice errors, reward wait errors, and acquired rewards). By contrast, during the long (7-11 s) delayed reward condition, the number of wait errors significantly increased while the numbers of total trials and choice errors did not significantlychange.Theseresultsindicatethat the activation of dorsal rapheserotoninneurons is necessaryfor waitingfor long delayed rewards and suggest that elevated serotonin activity facilitates waiting behavior when there is the prospect of forthcoming rewards. © 2012 the authors.