A role for reward sensitivity in the serotonergic modulation of impulsivity

Abstract

Impulsive behavior is a deleterious component of a number of mental health disorders but has few targeted pharmacotherapies. One contributing factor to the difficulty in understanding the neural substrates of disordered impulsivity is the diverse presentations of impulsive behavior. Defining the behavioral and cognitive processes which contribute to different subtypes of impulsivity is integral to understanding and treating disorders with dysregulated impulsive behavior. Our approach was to first determine what behavioral and cognitive phenotypes are associated with increased impulsive behavior, and then probe if they could causally contribute to increasing impulsivity. We used a mouse model for disordered impulsivity - mice lacking the serotonin 1B receptor (5-HT1BR) which have deficits specific to impulsive action, and not other components of impulsive behavior. Here we report, that in addition to increased impulsive action, mice lacking expression of 5-HT1BR also have increased goal-directed responding and motivation, with no differences in extinction, development of habitual behavior, delay discounting, or effort-based discounting. Interestingly, mice lacking 5-HT1BR expression did show an overall increase in the choice of higher value rewards, increased hedonic responses to sweet rewards, and responded more to cues that predict reward, compared to controls. We developed a novel paradigm to demonstrate that increasing anticipated reward value could directly increase impulsive action. Furthermore, we found that 5-HT1BR KO-induced impulsivity could be ameliorated by decreasing the reward value relative to controls, suggesting that the increased 5-HT1BR-associated impulsive action is a result of increased reward valuation. Taken together, these data show that the effects of serotonin on impulsive action are mediated through the modulation of hedonic value, which may alter the reward representations that motivate action. Additionally this work supports a role for reward valuation as an important substrate in impulsive action which may drive clinically-relevant increases in impulsivity.