Differences in the morphology of spinal V2a neurons reflect their recruitment order during swimming in larval zebrafish

Abstract

Networks of neurons in spinal cord generate locomotion. However, little is known about potential differences in network architecture that underlie the production of varying speeds of movement. In larval zebrafish, as swimming speed increases, Chx10-positive V2a excitatory premotor interneurons are activated from ventral to dorsal in a topographic pattern that parallels axial motoneuron recruitment. Here, we examined if differences in the morphology and synaptic output of V2a neurons reflect their recruitment order during swimming. To do so, we used in vivo single cell labeling approaches to quantify the dorso-ventral distribution of V2a axonal projections and synapses. Two different classes of V2a neurons are described; cells with ascending and descending axons, and cells that are only descending. Among the purely descending V2a cells, more dorsal cells project longer distances than ventral ones. Proximally, all V2a neurons have axonal distributions that suggest potential connections to cells at and below their own soma positions. At more distal locations, V2a axons project dorsally, which creates a cumulative intersegmental bias to dorsally located spinal neurons. Assessments of the synapse distribution of V2a cells, reported by synaptophysin expression, support the morphological observations and also demonstrate that dorsal V2a cells have higher synapse densities proximally. Our results suggest that V2a cells with more potential output to spinal neurons are systematically engaged during increases in swimming frequency. The findings help explain patterns of axial motoneuron recruitment and set up clear predictions for future physiological studies examining the nature of spinal excitatory network connectivity as it relates to movement intensity.

Regenerated Interneurons Integrate Into Locomotor Circuitry Following Spinal Cord Injury

“…5A-B). All GFP+ neurons in uninjured 14 dpf larvae exhibited typical V2a morphologies previously documented in zebrafish (Kimura et al 2008, Menelaou et al 2014 with axons that projected ventrally, then caudally across multiple segments and lateral dendritic processes at the level of the cell body (Fig. 5A).…”

“…Locomotor activity requires several distinct subtypes of spinal interneurons that are conserved throughout diverse vertebrate species. These include V0 commissural interneurons that function in left-right alternation (Björnfors & El Manira 2016), V2a interneurons that provide both premotor excitation (Kimura et al 2013, McLean et al 2007, Menelaou et al 2014, and contralateral inhibition (Menelaou & McLean 2019) and V2b interneurons that provide ipsilateral inhibition for speed control (Callahan et al 2019). We therefore hypothesized that these interneuron subtypes would be regenerated during the recovery of swimming behavior between 3 and 9 dpi.…”

Selective Responses to Tonic Descending Commands by Temporal Summation in a Spinal Motor Pool

“…The MeM and MeLc both had extensive axon collaterals that reached the dorsal-most aspect of the axial motor column (Figure 2B1 and C1). There were also extensive ventral collaterals (Figure 2B1 and C1), where motoneurons are more numerous (Menelaou et al, 2014). Collaterals throughout the dorso-ventral axis penetrated the soma layer, which is most obvious from a coronal view (Figure 2B2 and C2).…”

Peeling back the layers of locomotor control in the spinal cord

“…At larval stages, premotor V2a neurons vary in their participation in swimming based on their spatial location; at the highest frequencies subsets of ventrally located V2a neurons are inhibited as more dorsal ones are engaged [12,14,22,23]. A recent anatomical study in larvae has demonstrated that more dorsal V2a neurons have the potential to make systematically more connections to less-excitable motor neurons by virtue of convergent intersegmental axonal projections [24]. This could provide the observed bias in rhythmic excitation during faster swimming [19].…”

“…A new model (bottom) based on [19] has maximal drive (grey arrows) weighted to neuronal excitability across the motor pool. (b) In larval and juvenile zebrafish, V2a neurons comprise the UBG and provide appropriately biased drive to the motor pool [24,27]. V2a neurons recruited at slow speeds (top, filled purple circles) are biased in their connectivity to motor neurons active at those speeds (filled black circles).…”