Cellular and molecular coding of habit-driven behavior in the dorsal raphe nucleus.

Chronic stress has potent effects on reward-seeking behavior, reducing hedonic drive but also biasing individuals toward habitual responding at the expense of goal-directed action. While prior studies have implicated corticostriatal circuits in these shifts, the contribution of the dorsal raphe nucleus (DRN)-a central neuromodulatory hub that regulates motivational states-remains poorly understood. Here, we identify a key role for DRN neurons in mediating stress-induced transitions from goal-directed to habit-driven behavior. Using two-photon calcium imaging in awake behaving mice, we show that DRN neurons encode representations of both effort expenditures and reward acquisition that are integrated at the single-neuron level. Chronic stress selectively enhances reward-related responses in habit-driven individuals, and deficits in reward-related activity in DRN neurons predict future habit formation after chronic stress. Single-nucleus transcriptional profiling revealed that chronic stress drives habit-related gene expression changes in GABAergic DRN neurons, including upregulation of genes implicated in synapse function and inhibitory signaling, and identified potential pharmacological treatment targets. Optogenetic inhibition of DRN GABA neurons rescued stress effects on habit formation, driving stable shifts in operant responding that persisted for days. Together, these findings establish the DRN as a critical modulator of stress-induced behavioral inflexibility, and identify a new role for DRN GABAergic neurons in determining stress susceptibility and driving stable and persistent switches in operant responding during recovery from chronic stress.