Balancing positive and aversive emotional states -or hedonic control- is essential for animal survival. The habenular complex (Hb) has recently emerged as a central brain node shaping innate and learned reward- and aversion-related behaviors. Both medial (MHb) and lateral (LHb) Hb sub-divisions have been proposed to control reward and aversive states and are conventionally studied as separate neural circuits. In particular, until today, Hb functions of the µ opioid receptor (MOR), a key regulator of hedonic states and the molecular target of morphine, were mainly investigated in MHb as it was believed that MORs expression was restricted to this subdivision. Our findings show that MORs and MOR neurons instead straddle medial and lateral territories. This raises intriguing questions on whether MHb and LHb MOR-expressing neurons have different physiological roles or represent a functionally defined cross-habenular neuronal population bidirectionally modulating affective states. During our project HABEmood, we will test the hypothesis that both LHb and MHb Hb-MOR neurons are crucial for hedonic balance. Based on our complementary expertise in Hb and opioid physiology, our consortium will combine circuit tracking, electrophysiology, in vivo imaging, whole-brain connectivity and behavioral read-outs to 1. Map the anatomical and physiological organization of Hb-MOR neurons in LHb and MHb and 2. Functionally characterize this specific neuronal population. Supported by our preliminary data, our cutting-edge expertise promises an important advancement on the neuronal bases of hedonic control in mammals. Moreover, in contrast to the current anatomical subdivision-defined vision of Hb function, we propose a Hb cell-specific understanding of the neural bases for balancing positive and aversive emotional states with crucial implications for mental health (e.g. addiction and mood disorders).