MODULATION OF LOCOMOTION AND ORIENTING MOVEMENTS BY D2 RECEPTOR-EXPRESSING NEURONS IN THE GIGANTOCELLULAR RETICULAR NUCLEUS

The reticular formation plays a key role in motor control, containing the command systems for initiating, stopping, and directing locomotion. These commands are transmitted to spinal circuits via reticulospinal neurons. In lampreys, reticular regions are targeted by descending dopaminergic pathways (Ryczko et al. 2020 J Neurosci). However, in mammals, the reticular neurons targeted by dopamine and their behavioral roles are unknown. Here we investigated in mice the behavioral role of neurons expressing dopaminergic D2 receptors in the gigantocellular reticular nucleus (Gi), a region involved in the control of stops and orienting movements. Using RNAscope, we identified cells expressing the D2 receptor in the Gi. To activate these Gi D2-positive neurons in vivo, we unilaterally injected a virus in the Gi of D2-Cre mice, enabling Cre-dependent expression of channelrhodopsin. An optical fiber was implanted above the injection site and video recordings were done in an open-field to track body movements. Unilateral photostimulation of Gi D2-expressing neurons reduced locomotor speed and evoked ipsilateral head movements. Using high resolution axial body tracking in a linear corridor, we found that stimulation evoked body orienting movements towards the stimulated side. Altogether, our findings indicate that activation of D2-positive neurons in the Gi decreases locomotor speed and induces ipsilateral body and head orienting movements.