Dopamine release in the medullary reticular formation is related to locomotor velocity

Dopamine is classically recognized for modulating locomotion through basal ganglia circuits. However, it also modulates motor circuits in the brainstem and spinal cord. The medullary reticular formation (MRF) integrates supraspinal motor commands and relays them to the spinal cord, yet the role of dopamine in this region is poorly understood. Using the genetically encoded dopamine sensor dLight1.4, we observed that dopamine release in the MRF decreased during the initiation of locomotion and showed an inverse correlation with locomotor velocity. Dopamine release was also modulated during directional head turns. RNA in situ hybridization revealed D₂R mRNA expression is present in about one-third of glutamatergic reticulospinal neurons (RSNs), whereas D₄R mRNA expression is nearly absent. Retrograde viral tracing identified the A11 region, encompassing dopaminergic TH⁺ cells, as the primary source of dopaminergic input to the MRF, with minimal contributions from other dopaminergic nuclei. Anterograde tracing demonstrated that A11 neurons project ipsilaterally throughout the rostrocaudal MRF and form putative synaptic contacts with neuronal populations that project to the spinal cord. These findings reveal discrete dopaminergic projections from the A11 region to the MRF and lead to the hypothesis that the A11-MRF dopaminergic pathway acts as a gain-control mechanism setting the excitability of D₂ receptor-expressing reticulospinal neurons, thereby providing state-dependent control of locomotor speed.Significance Statement The medullary reticular formation (MRF) is a critical hub for controlling diverse motor behaviors; however, the source and organization of its dopaminergic inputs remain unclear. Here we identify A11 dopamine neurons as the principal dopaminergic projection to the MRF. Dopamine signaling in the MRF correlates inversely with locomotor velocity and is modulated during ipsilateral head turns. Furthermore, the expression of D₂Rs on glutamatergic reticulospinal neurons identifies these cells as primary targets for dopaminergic modulation.