Bolduc, C.; Oram, C.; Donovan, S.; Bach, H.; Liu, M.; Marier, R.; Sharpe, M.; Campeau, C.; Spencer, C. D.; Martin, S. A.; Awatramani, R.; Poulin, J.-F.

Despite advances in delineating the molecular diversity and projection patterns of midbrain dopaminergic (DA) neurons, their specific contributions to locomotion and motor learning remain poorly defined. Here, we applied intersectional ablation and chemogenetic approaches to dissect the distinct roles of calbindin-expressing (CALB1+) and non-expressing (CALB1-) DA neurons in locomotion. Using newly engineered intersectional constructs, we ablated CALB1+ or CALB1- DA neurons in the mouse midbrain. Loss of either subtype led to pronounced deficits in the initiation and vigor of voluntary movements, as demonstrated by a reduction in peak speed, acceleration and deceleration of locomotor bouts. Notably, only CALB1- ablation disrupted locomotor learning. Beyond these functional effects, we observed that selective ablation of CALB1 DA neurons induced local microglial activation and was followed by a non-cell-autonomous loss of CALB1- DA neurons, suggesting that CALB1- neurons are more vulnerable to inflammation triggered by CALB1 neuron loss. We then confirmed these findings by performing acute inhibition of either population using inhibitory DREADD hM4Di. CALB1- DA neurons inhibition impaired the initial acquisition of locomotor learning, whereas inhibition of CALB1+ DA neurons disrupted the retention of acquired motor skills from previous days. Moreover, inhibition of CALB1+ DA neurons further impaired the initiation and amplitude of voluntary movements, as well as the velocity and acceleration/deceleration of locomotor bouts. Together, these findings provide causal evidence for functional specialization among molecularly distinct midbrain DA subtypes and reveal new aspects of mesostriatal circuit organization underlying locomotion and motor memory.