When a good taste turns bad: Neural mechanisms underlying the emergence of negative affect and associated natural reward devaluation by cocaine

An important feature of cocaine addiction in humans is the emergence of negative affect (e.g., dysphoria, irritability, anhedonia), postulated to play a key role in craving and relapse. Indeed, the DSM-IV recognizes that social, occupational and/or recreational activities become reduced as a consequence of repeated drug use where previously rewarding experiences (e.g., food, job, family) become devalued as the addict continues to seek and use drug despite serious negative consequences. Here, research in the Carelli laboratory is reviewed that examined neurobiological mechanisms that may underlie these processes using a novel animal model. Oromotor responses (taste reactivity) were examined as rats learned that intraoral infusion of a sweet (e.g., saccharin) predicts impending but delayed access to cocaine self-administration. We showed that rats exhibit aversive taste reactivity (i.e., gapes/rejection responses) during infusion of the sweet paired with impending cocaine, similar to aversive responses observed during infusion of quinine, a bitter tastant. Critically, the expression of this pronounced aversion to the sweet predicted the subsequent motivation to self-administer cocaine. Electrophysiology studies show that this shift in palatability corresponds to an alteration in nucleus accumbens (NAc) cell firing; neurons that previously responded with inhibition during infusion of the palatable sweet shifted to excitatory activity during infusion of the cocaine-devalued tastant. This excitatory response profile is typically observed during infusion of quinine, indicating that the once palatable sweet becomes aversive following its association with impending but delayed cocaine, and NAc neurons encode this aversive state. We also review electrochemical studies showing a shift (from increase to decrease) in rapid NAc dopamine release during infusion of the cocaine-paired tastant as the aversive state developed, again, resulting in responses similar to quinine infusion. Collectively, our findings suggest that cocaine-conditioned cues elicit a cocaine-need state that is aversive, is encoded by a distinct subset of NAc neurons and rapid dopamine signaling, and promotes cocaine-seeking behavior. Finally, we present data showing that experimentally induced abstinence (30 days) exacerbates this natural reward devaluation by cocaine, and this effect is correlated with a greater motivation to lever press during extinction. Dissecting the neural mechanisms underlying these detrimental consequences of addiction is critical since it may lead to novel treatments that ameliorate negative affective states associated with drug use and decrease the drive (craving) for the drug