Reward deficiency is reported to be an important factor involved in dependence behaviour and has been found to linked to lower expression of dopamine D2 receptors, resulting in a decrease in the number of physiological enforcers that are responsible for activating the reward pathway [46]. This reduction in activation circuits is hypothesized to predispose an individual to drug use as a means of compensating for the lack of reward [46]. Low dopamine receptor concentrations have been associated with impulsivity, as demonstrated by findings of escalated self-administration of cocaine in rat models [33]. This relationship highlights an important increase in the vulnerability to compulsive drug intake in humans leading to dependence, which stems from being in a reward-deficient state [33]. Sensation-seeking is another characteristic often associated with acquiring self-administration after initial exposure to a drug [33]. Deficient self-regulation mechanisms and difficulty controlling one’s desires is often characterized as impulsivity, which has been hypothesized to contribute to the onset of substance use disorders [47, 48].
