Neurons expressing mu opioid receptors of the habenula promote negative affect in a projection-specific manner

BACKGROUND The mu opioid receptor (MOR) is central to hedonic balance, and produces euphoria by engaging reward circuits. MOR signaling may also influence aversion centers, and notably the medial habenula (MHb) where the receptor is highly dense, however this was not investigated. Our prior data suggest that the inhibitory activity of MOR in the MHb limits aversive states. Here we therefore tested the hypothesis that neurons expressing MOR in the MHb (MHb-MOR neurons) promote negative affective states. METHODS Using Oprm1 -Cre knock-in mice, we combined tracing and optogenetics with behavioral testing to investigate consequences of MHb-MOR neuron stimulation in approach/avoidance (real-time place preference), anxiety-related responses (open field, elevated plus maze and marble burying) and despair-like behavior (tail suspension). RESULTS Opto-stimulation of MHb-MOR neurons elicited avoidance behavior, demonstrating that these neurons promote aversive states. Anterograde tracing showed that, in addition to the interpeduncular nucleus (IPN), MHb-MOR neurons project to the dorsal raphe nucleus (DRN), uncovering a yet unreported connection of MHb to a main mood center. Opto-stimulation of MHb-MOR/IPN neurons triggered avoidance and despair-like responses with no anxiety-related effect, whereas light-activation of MHb-MOR/DRN neurons increased levels of anxiety with no effect on other behaviors, revealing two dissociable pathways controlling negative affect. CONCLUSIONS This study demonstrates aversive activity of MHb neurons that respond to MOR opioids. We propose that inhibition of these neurons by endogenous or exogenous opioids relieves negative affect via two distinct MHb microcircuits, contributing to despair-like behavior (MHb-MOR/IPN) and anxiety (MHb-MOR/DRN). This mechanism has implications for hedonic homeostasis and addiction. ### Competing Interest Statement The authors have declared no competing interest.