Deep imaging in the brainstem reveals functional heterogeneity in V2a neurons controlling locomotion

Abstract

V2a neurons are a genetically defined cell class that forms a major excitatory descending pathway from the brainstem reticular formation to the spinal cord. Their activation has been linked to the termination of locomotor activity based on broad optogenetic manipulations. However, because of the difficulties involved in accessing brainstem structures for in vivo cell type–specific recordings, V2a neuron function has never been directly observed during natural behaviors. Here, we imaged the activity of V2a neurons using micro-endoscopy in freely moving mice. We find that as many as half of the V2a neurons are excited at locomotion arrest and with low reliability. Other V2a neurons are inhibited at locomotor arrests and/or activated during other behaviors such as locomotion initiation or stationary grooming. Our results establish that V2a neurons not only drive stops as suggested by bulk optogenetics but also are stratified into subpopulations that likely contribute to diverse motor patterns.

Freely Behaving Mice Can Brake and Turn During Optogenetic Stimulation of the Mesencephalic Locomotor Region

“…Interestingly, a recent study uncovered that unilateral stimulation of reticulospinal Gi Chx10positive neurons projecting to the lumbar spinal cord decreases locomotor speed without inducing turning (Usseglio et al, 2020). In vivo calcium imaging showed that Gi Chx10-positive neuron activity increases during locomotor stops in mice (Schwenkgrub et al, 2020). Locomotor pauses and rhythm resetting were also reported when photoactivating Vglut2positive neurons in the Gi in vivo in mice (Lemieux and Bretzner, 2019).…”

Intrinsic brainstem circuits comprised of Chx10-expressing neurons contribute to reticulospinal output in mice

“…Calcium imaging has recently been used to study the activity of Chx10 neurons in the GRN, finding that about one half are active during locomotor stop events. But there is diversity in function: that is, individual neurons are not active during every stop event, and some Chx10 neurons were active at the onset of and during locomotion ( 28 ). This finding is consistent with the in vitro demonstration of differential responses of Chx10 GRN neurons to stimulation of the mesencephalic locomotor region ( 15 ).…”

“…This finding is consistent with the in vitro demonstration of differential responses of Chx10 GRN neurons to stimulation of the mesencephalic locomotor region ( 15 ). And some of these neurons are not active during locomotion, but are during grooming, or when the animal is stationary ( 28 ). Similarly, three functional types of glutamatergic RSNs were described in the lamprey, in a similar region of the reticular formation (one that receives inputs from the mesencephalic locomotor region).…”

“…It is likely that, similar to the heterogeneity of spinal Chx10 interneurons (e.g. 26,27), Chx10 reticular formation neurons are also not homogeneous (24,28). Furthermore, opto-or chemo-genetic studies would not capture the intrinsic organisation of the GRN, and whether local circuits regulate the output -that is, whether this region is simply an integrator of inputs, or whether it has the circuitry to refine descending output.…”

Optical Imaging of Pancreatic Innervation

“…Neurons of the dorsal vagal complex can be labeled by tracing from the pancreas (14,69,86) or by activity-dependent genetic labeling (87) induced by pancreas-specific stimulation. Once labeled, these neurons can be studied using imaging approaches in vivo or in brainstem slices (88,89).…”