Locomotor speed control circuits in the caudal brainstem

In the mouse caudal brainstem, functionally distinct neuronal subpopulations, which are distinguishable by neurotransmitter identity, connectivity and location, regulate locomotion parameters. Locomotion is a universal behaviour of many animal species, controlled by multiple regions and circuits in the brain and spinal cord. Silvia Arber and colleagues now show that in the mouse caudal brainstem, functionally distinct neuronal subpopulations, distinguishable by their neurotransmitter identity, connectivity and location, influence locomotion parameters. One subpopulation, glutamatergic neurons within the lateral paragigantocellular nucleus (LPGi), is essential for high-speed locomotion, and can tune the speed of locomotion via inputs from the midbrain locomotor region. Conversely, glycinergic neurons arrest locomotor behaviour. These distinct subpopulations communicate with separate circuits in the spinal cord, providing substrates for distinct pathways with opposing functions for tuning locomotor behaviour. Locomotion is a universal behaviour that provides animals with the ability to move between places. Classical experiments have used electrical microstimulation to identify brain regions that promote locomotion1,2,3,4,5, but the identity of neurons that act as key intermediaries between higher motor planning centres and executive circuits in the spinal cord has remained controversial6,7,8,9,10,11,12,13,14. Here we show that the mouse caudal brainstem encompasses functionally heterogeneous neuronal subpopulations that have differential effects on locomotion. These subpopulations are distinguishable by location, neurotransmitter identity and connectivity. Notably, glutamatergic neurons within the lateral paragigantocellular nucleus (LPGi), a small subregion in the caudal brainstem, are essential to support high-speed locomotion, and can positively tune locomotor speed through inputs from glutamatergic neurons of the upstream midbrain locomotor region. By contrast, glycinergic inhibitory neurons can induce different forms of behavioural arrest mapping onto distinct caudal brainstem regions. Anatomically, descending pathways of glutamatergic and glycinergic LPGi subpopulations communicate with distinct effector circuits in the spinal cord. Our results reveal that behaviourally opposing locomotor functions in the caudal brainstem were historically masked by the unexposed diversity of intermingled neuronal subpopulations. We demonstrate how specific brainstem neuron populations represent essential substrates to implement key parameters in the execution of motor programs.