The role of mesolimbic circuitry in aversive signaling and opioid dependence.

Positive reinforcement via mu-opioid receptor-mediated disinhibition of ventral tegmental area (VTA) dopamine neurons is crucial in opioid use disorder (OUD). However, VTA dopamine neurons are more heterogeneous than initially thought, both at the molecular and computational levels. Besides encoding reward prediction error, subpopulations of dopamine neurons have also been proposed to encode salience and aversion. How opioid use alters these distinct encoding properties remains unclear. Negative reinforcement-learning to avoid adverse outcomes like withdrawal-also drives chronic drug use, implicating the mesolimbic dopamine system in both positive and negative reinforcement in OUD. This review explores how chronic opioid use modifies heterogeneous VTA neuron populations, enhancing sensitivity to aversive stimuli, promoting negative affect, and motivating withdrawal-avoidance behaviors. We also examine how chronic pain may amplify these effects and discuss the importance of charting circuit-level interactions between chronic pain and OUD for clinical translation.