Dopamine also encodes salience or a teaching signal, which may contribute to the learned component of substance abuse. For example, dopamine neurons initially fire with reinforcer delivery, but with time dopamine neuron firing becomes time-locked with predictive conditioned stimuli that precede an expected reinforcer (the unconditioned stimulus) rather than the reinforcer, itself (Schultz, 1997). Similar findings have also been observed in detoxified cocaine abusers. Specifically, presentation of drug-associated cues increase dopamine levels in brain regions that participate in habit circuitry (Belin, Jonkman, Dickinson, Robbins, & Everitt, 2009; Volkow, Wang, Fowler, Tomasi, & Telang, 2011; Volkow et al., 2006) to a level greater than the drug itself (Volkow, et al., 2011; Volkow, et al., 2006). This dopamine signal is correlated with self-reported craving (Drevets, et al., 2001; Heinz et al., 2014). After repeated or habitual use, previously neutral stimuli become imbued with the drug experience and eventually acquire the ability to increase dopamine in anticipation of reward. This dopamine signal can elicit strong motivation to pursue a drug of abuse (Owesson‚ White et al., 2009; Salamone, Correa, Farrar, & Mingote, 2007).
