Tegmental kappa-opioid receptor neurons modulate opioid withdrawal via the periaqueductal gray

Opioid withdrawal is a common and distressing manifestation of opioid dependence which, if left untreated, frequently results in relapse, accidental overdose and suicide. While much is known about the role of mesolimbic and mesocortical dopaminergic neurons in mu opioid receptor-mediated reward, much less is understood about the impact of chronic opioid use on parallel aversive pathways mediated by the kappa opioid receptor and its endogenous ligand dynorphin. In the present study, we interrogated kappa opioid receptor-expressing ventral tegmental area neurons and their dynorphinergic inputs in vitro and in vivo to elucidate the circuit mechanisms by which chronic opioids promote withdrawal behaviours in mice. Through a combination of genetic, molecular and custom machine learning analytical approaches, we uncovered the effects of chronic morphine on kappa opioid receptor mRNA expression in the ventral tegmental area as well as dynorphin mRNA expression in several retrogradely traced dynorphinergic input regions. We found that chronic morphine exposure diminishes opioid-induced c-Fos expression selectively in midbrain kappa opioid receptor-expressing neurons. In addition, chemogenetic activation of kappa opioid receptor-expressing ventral tegmental area neurons is sufficient to attenuate diverse opioid withdrawal-associated behaviours, negative affect and gastrointestinal distress in mice. Finally, we uncovered a glutamatergic subpopulation of kappa opioid receptor-expressing ventral tegmental area neurons projecting to the ventrolateral periaqueductal gray which, when activated, selectively relieves opioid withdrawal-associated gastrointestinal distress. These discoveries highlight a critical role for midbrain kappa opioid receptor-expressing neurons and the downstream ventrolateral periaqueductal gray in opioid withdrawal-associated disruption of hedonic homeostasis and gastrointestinal regulation.