Protracted morphine withdrawal corresponds with sex-specific alterations to motivated behavior and mesoaccumbal subcircuit dopamine cell plasticity

Abstract

Background: Opioid use disorder is associated with enduring psychological withdrawal symptoms believed to contribute to drug abuse. Amongst these are shifts in motivational states, wherein pursuit of drug consumption exceeds that of non-drug rewards, reinforcing escalated opioid use and relapse vulnerability. A critical regulator of behavioral reinforcement, the mesoaccumbal dopamine (DA) system is thought to be both necessary and sufficient for opioid motivation. However, previous research into its involvement in opioid withdrawal has been limited to acute vs protracted timepoints, global neuroadaptations vs those in subcircuits, and overwhelmingly focused on males vs females. Methods: Evaluations of effort-based motivated behavior for both sucrose and morphine reward were combined with patch clamp electrophysiological assessments of synaptic plasticity within lateral vs medial DA neurons projecting to the lateral vs medial nucleus accumbens shell during protracted morphine withdrawal in male and female mice. Further effects of mesoaccumbal subcircuit inhibition on motivated behavior for sucrose were also measured. Results: Protracted morphine withdrawal was found to be associated with elevations in morphine seeking, intake, and motivation compared to saline controls in both sexes. Escalation of intake was paralleled by a male-exclusive reduction in motivation for the non-drug reward, sucrose. Male-exclusive neuroadaptations during protracted withdrawal were also found, with reductions in neuronal excitability and increased inhibitory (GABAAR-dependent) synaptic transmission found in lateral ventral tegmental area (VTA) DA neurons projecting to the lateral nucleus accumbens shell, though not in medial DA projections to the medial shell. Finally, chemogenetic inhibition of the lateral but not medial subcircuit was found to significantly reduce motivated responding for sucrose in male morphine-naive mice. Conclusions: These data suggest that protracted opioid withdrawal is associated with a sex-independent increase in opioid consumption and motivation. They also suggest that male-specific reductions in motivation for non-drug reward during protracted withdrawal may be driven by a hypoactive state in a lateral mesoaccumbal DA subcircuit driven in part by increased inhibition of DA cells. These insights may be useful in development of therapies that temper withdrawal-associated psychological states predisposed towards prolonged and escalated opioid intake, a major treatment goal for OUD patients.