Pathway Specific Activation of D1 And D2 Medium Spiny Neurons in a CRE Mouse Model Reveals Distinct Roles in Reinforcement Learning and Decision Making via Closed Loop Optogenetic Neuromodulation

Objective: The basal ganglia integrate cortical and dopaminergic inputs to shape reinforcement learning, action selection, and motivation through the direct (D1) and indirect (D2) striatal pathways. While these pathways are thought to facilitate opposing behavioral outcomes, their causal roles in reinforcement-driven learning remain poorly defined. Methods: Pathway-specific optogenetic activation was used in D1-Cre and D2-Cre transgenic mice to examine the functional contributions of dorsomedial striatum (DMS) circuits. Animals were trained in an olfactory Go/No-Go discrimination task with closed-loop optogenetic stimulation, and complementary open-field and unilateral rotation assays were performed to assess motivational and motor effects. Results: In unilateral rotation tests, D1 stimulation induced contralateral turning, while D2 stimulation elicited ipsilateral rotations, validating effective pathway recruitment. Open-field assays showed that D1 stimulation drove strong preference for the stimulation zone, consistent with enhanced motivational salience, while D2 stimulation induced avoidance, reflecting behavioral inhibition. In the Go/No-Go task, pathway stimulation did not alter learning rates or accuracy. Instead, changes were observed in motivational dynamics: D1 stimulation reduced next-trial initiation times, particularly after errors, indicating increased persistence and approach drive, whereas D2 stimulation prolonged initiation times following errors, indicating reduced engagement and motivational suppression. Conclusion: These findings demonstrate that the optogenetic-driven activity in D1 and D2 pathways in the DMS do not directly encode reinforcement learning but instead modulate motivational vigor and behavioral engagement. Next-trial initiation latency and spatial preference emerge as sensitive behavioral markers of pathway-specific neuromodulation, offering translational value for disorders involving motivation and action control.