Regulation of locomotor speed and selection of active sets of neurons by V1 neurons

Abstract

During fast movements in vertebrates, slow motor units are thought to be deactivated due to the mechanical demands of muscle contraction, but the associated neuronal mechanisms for this are unknown. Here, we perform functional analyses of spinal V1 neurons by selectively killing them in larval zebrafish, revealing two functions of V1 neurons. The first is the long-proposed role of V1 neurons: they play an important role in shortening the cycle period during swimming by providing in-phase inhibition. The second is that V1 neurons play an important role in the selection of active sets of neurons. We show that strong inhibitory inputs coming from V1 neurons play a crucial role in suppressing the activities of slow-type V2a and motor neurons, and, consequently, of slow muscles during fast swimming. Our results thus highlight the critical role of spinal inhibitory neurons for silencing slow-component neurons during fast movements.

Functional Diversity of Glycinergic Commissural Inhibitory Neurons in Larval Zebrafish

“…In order to selectively ablate dI6dmrt3a neurons, we generated Tg[dmrt3a:lRl-DTA] (DTA [diphtheria toxin A subunit]). The fish were crossed to Tg[hoxa9a-3 0 enhancer:Cre] (hereafter called Tg[hox:Cre]) in which Cre recombinase expression is localized to the trunk (Kimura and Higashijima, 2019). This enabled targeting of DTA expression to the spinal cord.…”

“…Tg[glyt2:lRl-GFP] and Tg[dmrt3a:loxP-DsRed-loxP-DTA (lRl-DTA)] transgenic fish were generated using the CRISPR/Cas9-mediated knock-in method with the hsp70 promoter (Kimura et al, 2014). Tg[hoxa9a-3 0 enhancer:Cre] was described in Kimura and Higashijima (2019).…”

“…Neurons were labeled with 0.01% Alexa Fluor 633 or 647 hydrazide (Thermo Fisher Scientific) in the intracellular solution used for patching. In some of the recordings for examining spiking patterns of V0d neurons, we performed loose-patch technique instead of whole cell patch-clamp (Kimura and Higashijima, 2019). Fictive locomotion was elicited either by applying a brief electric shock near the tail (stimulus strength of 7-20 V for a duration of 0.2-1.0 ms) or by changing the illumination intensity.…”

“…After finding a neuron that consistently fired in most of the swim cycles during regular fictive swimming, we targeted the same neuron for whole-cell voltage-clamp recording. The voltage-clamp recordings for measuring inhibitory currents were performed as described in Kimura and Higashijima (2019). The intracellular solution contained (in mM) 140 CsMeSO 4 , 1 QX314-Cl, 1 TEA-Cl, 3 MgCl 2 , 10 HEPES, 1 EGTA, 4 Na 2 -ATP, and was adjusted to pH 7.2 with CsOH.…”

Spatiotemporal Transition in the Role of Synaptic Inhibition to the Tail Beat Rhythm of Developing Larval Zebrafish

“…Recent studies identified V1 interneurons marked by engrailed-1 expression and V2b interneurons defined by expression of Lhx3 and Gata3 as ipsilaterally projecting inhibitory interneurons that play contrasting roles in locomotor control. V1 interneurons seem to shorten the duration of swimming episodes and derecruit slow-muscle innervating MNs (Kimura and Higashijima, 2019), whereas V2b interneurons provide a break to locomotor speed (Callahan et al, 2019). How the combined activity of these populations shapes the rhythm of tail beats during swimming episodes, and whether their contributions change as fish transition from burst swimming to beat-and-glide swimming remains to be investigated.…”