Targeting Mu opioid receptor neurons of the habenula to limit naloxone aversion

BACKGROUND: Opioid use disorder is a chronic, relapsing condition that continues to rise worldwide. Naloxone (Narcan®), an opioid antagonist, reverses overdose but triggers strong negative affect. The neuronal circuits underlying these aversive effects remain unclear. We previously identified mu opioid receptor-expressing habenular neurons (Hb-MOR) as key encoders of negative emotional states and hypothesized that they also mediate naloxone aversion. METHODS: We combined behavioral models of naloxone aversion with in vivo fiber photometry to monitor Hb-MOR activity in opioid-naïve and dependent mice. Causality was tested through chemogenetic silencing of Hb-MOR neurons. Finally, we targeted GPR139, an anti-opioid orphan receptor enriched in the habenula, to pharmacologically modulate naloxone responses. RESULTS: In opioid-naïve mice, high-dose naloxone induced conditioned place aversion (CPA) and increased Hb-MOR neuron activity, both of which were prevented by chemogenetic inhibition. In dependent animals, low-dose naloxone was sufficient to activate Hb-MOR neurons and produce CPA. Silencing Hb-MOR neurons abolished these effects and alleviated somatic withdrawal signs, indicating enhanced sensitivity of these neurons in dependence. Targeting GPR139 provided a therapeutic approach: the GPR139 antagonist JD-1 reduced Hb-MOR responses to naloxone and attenuated both somatic and affective withdrawal symptoms in morphine-dependent mice. CONCLUSION: Hb-MOR neurons are key of naloxone-induced aversion and withdrawal. Pharmacological modulation of GPR139 represents a promising strategy to limit adverse effects associated with naloxone and opioid withdrawal.