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.