The role of V3 neurons in speed-dependent interlimb coordination during locomotion in mice

Abstract

Speed-dependent interlimb coordination allows animals to maintain stable locomotion under different circumstances. The V3 neurons are known to be involved in interlimb coordination. We previously modeled the locomotor spinal circuitry controlling interlimb coordination (Danner et al., 2017). This model included the local V3 neurons that mediate mutual excitation between left and right rhythm generators (RGs). Here, our focus was on V3 neurons involved in ascending long propriospinal interactions (aLPNs). Using retrograde tracing, we revealed a subpopulation of lumbar V3 aLPNs with contralateral cervical projections. V3OFF mice, in which all V3 neurons were silenced, had a significantly reduced maximal locomotor speed, were unable to move using stable trot, gallop, or bound, and predominantly used a lateral-sequence walk. To reproduce this data and understand the functional roles of V3 aLPNs, we extended our previous model by incorporating diagonal V3 aLPNs mediating inputs from each lumbar RG to the contralateral cervical RG. The extended model reproduces our experimental results and suggests that locally projecting V3 neurons, mediating left–right interactions within lumbar and cervical cords, promote left–right synchronization necessary for gallop and bound, whereas the V3 aLPNs promote synchronization between diagonal fore and hind RGs necessary for trot. The model proposes the organization of spinal circuits available for future experimental testing.

Widespread innervation of motoneurons by spinal V3 neurons globally amplifies locomotor output in mice

“…Finally, to examine whether V3 neurons are necessary for controlling the gain of motoneuron output, we genetically silenced them by crossing Sim1 cre+/mice with VGlut2 flx/flx mice to stop glutamate transmission (V3OFF mice). These mice, without functional V3 neurons, were able to function relatively normally, but they were slow 35 , clumsy (with wide meandering stepping; as in Zhang et al, 2008) 30 , and only explored a new cage for an unusually short period (n = 10/10 mice). This seemed to be due to an overall weakness in these mice, but the weakness proved hard to quantify because V3OFF mice have intact voluntary supraspinal control and accordingly adopt compensatory behaviours, like rarely fully relaxing, so muscles are partly pre-activated in case they have to move.…”

“…When we forced V3OFF mice to walk on a level treadmill with increasing speed, they failed reach the high speeds normal wildtype mice can reach (~<20 m/s for V3OFF mice with EMG implants, vs <80 cm/s in wildtype mice; Zhang et al, 2019) 35 , and when the treadmill was inclined 17.5°, they failed to keep up with the slow treadmill speed and so gradually drifted to the bottom of the treadmill, essentially walking slower than wildtype mice. Furthermore, when placed in water, they swam with a slower forward velocity with rather weak propulsive motions, compared to in wildtype mice (see Supplementary Video).…”

“…V3 neurons are highly heterogeneous, with molecularly, anatomically and electrophysiologically distinct properties, ultimately forming functionally distinct subpopulations with dorsal, intermediate and ventral locations and differing molecular identities [31][32][33][34] . Although we have recently begun to study the role of the more dorsal V3 neurons with ascending projections 35 , we know less about the function of the ventral V3 neurons (V3v), which make up the majority of the total V3 population 34 . V3v neurons are located in lamina VIII and X, and mostly form long commissural axonal projections that travel across the entire contralateral lumbosacral spinal cord.…”

Operation regimes of spinal circuits controlling locomotion and role of supraspinal drives and sensory feedback

“…without rhythmic or patterned external inputs) generate the complex coordinated pattern of locomotor activity 4,6,7,15,17,18,35,36 . Following our previous computational models [24][25][26][28][29][30] , we assume that the circuitry of the spinal locomotor CPG includes (a) rhythm-generating (RG) circuits that control states and rhythmic activity of each limb, and (b) rhythm-coordinating circuits that mediate neuronal interactions between the RG circuits and define phase relationships between their activities (coupling, synchronization, alternation). Also, following the classical view, we assume that each RG consists of two halfcenters that mutually inhibit each other and define the two major states/phases of the RG, the flexor and extensors phases, in which the corresponding sets of limb muscles are activated 6,7,15,35 .…”

Sensory Feedback and Central Neuronal Interactions in Mouse Locomotion

“…Central interactions between RGs controlling each limb are provided by commissural interneurons (CINs, projecting their axons to the opposite side of the spinal cord) and long descending and ascending propriospinal neurons (LPN, projecting their axons from the cervical to the lumbar enlargement or vice-versa) that mediate interactions between left-right and cervical-lumbar (fore-hind) circuits. Recent molecular genetic studies led to the identification of candidate CINs and LPNs for limb coordination [3,30,40,41,[102][103][104]. Specifically, the genetically-defined V0 CINs (V0D and V0V types) are involved in left-right alternation in a speed-dependent manner.…”