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Emotions help us navigate the world by assigning positive or negative value to our experiences. Whether we pursue a reward or avoid a threat depends on how our brain interprets these events. This thesis investigates the role of a brain region, the ventral tegmental area (VTA), in that process.

Traditionally the VTA has been studied by focusing on its neurotransmitter defined cell types such as dopamine and GABA neurons. This work takes a different approach by emphasizing the importance of neuronal ensembles, groups of neurons that become active together in response to specific experiences.
Using advanced tools to label and manipulate active neurons, this research identified two distinct VTA ensembles in mice: one activated by rewarding experiences and another by social stress. These ensembles comprised a mix of neuron types but consistently encoded either positive or negative valence. Stimulating them was sufficient to drive approach or avoidance behaviors, modulate anxiety behavior and alter food intake. Conversely, silencing them during the corresponding experiences disrupted those behavioral responses and demonstrated that they are both sufficient and necessary for emotional actions.
Importantly we observe that these ensembles do not act alone. They are connected to other emotion related brain regions such as the nucleus accumbens and lateral habenula and also communicate locally within the VTA.
By showing how emotional experiences are encoded by functionally defined groups of neurons rather than fixed neurotransmitter-defined cell types, this work opens new paths for understanding disorders such as depression and anxiety where emotional processing can go awry.