1. The importance of studying individual variation
Several limbic brain regions play a key role in both reward and sleep. The dopamine-mediated mesolimbic circuitry responsible for reward and reinforcement is also heavily involved in the regulation of sleep/wake states and is strongly affected by sleep loss. In humans, a single night of sleep deprivation can decrease D2/D3 dopamine receptor availability in the ventral striatum (Volkow et al. 2008; Volkow et al. 2012; Wiers et al. 2016), which is associated with a greater propensity for risk-taking behavior (Linnet et al. 2011) and an increased risk for compulsive drug consumption (Dalley et al. 2007). Furthermore, sleep disturbances have been shown to mediate the reduced D2/D3 receptor availability that has been observed in chronic cocaine abusers (Wiers et al. 2016). However, in human populations, there is tremendous individual variation in the degree to which sleep deprivation impairs cognitive performance and enhances reward sensitivity. For example, genetic variation in the human dopamine transporter (DAT) gene has been shown to influence neural responses to sleep loss; with individuals with the DAT allele that is linked to higher phasic dopamine activity demonstrating greater striatal responses to monetary reward after sleep deprivation (Greer et al. 2016).
Preclinical studies in rodents are frequently used to investigate the relationship between sleep and reward processing; however, the impact of individual variation has rarely been addressed in these models. The study of sleep and reward processing may benefit from incorporating the rodent models of individual differences, particularly the sign-tracker/goal-tracker model of incentive salience, as it identifies underlying phenotypic differences in dopamine transmission and mesolimbic functioning that render some rats hyper-responsive to reward-paired cues. Here, we review evidence for a link between the brain regions involved in sleep and those responsible for the motivational impact of reward cues. We argue that by examining populations of rats that show natural phenotypic variation in the degree to which food cues engage mesolimbic circuitry, we can learn more about the role of sleep in the emotional and motivational states that are triggered by cues.