Origin of excitation underlying locomotion in the spinal circuit of zebrafish

Abstract

Neural circuits in the spinal cord transform instructive signals from the brain into well-coordinated locomotor movements by virtue of rhythm-generating components. Although evidence suggests that excitatory interneurons are the essence of locomotor rhythm generation, their molecular identity and the assessment of their necessity have remained unclear. Here we show, using larval zebrafish, that V2a interneurons represent an intrinsic source of excitation necessary for the normal expression of the locomotor rhythm. Acute and selective ablation of these interneurons increases the threshold of induction of swimming activity, decreases the burst frequency, and alters the coordination of the rostro-caudal propagation of activity. Thus, our results argue that V2a interneurons represent a source of excitation that endows the spinal circuit with the capacity to generate locomotion.central pattern generator | premotor interneurons | motor behavior | synaptic transmission T he generation of motor behavior involves decision making, selection, initiation, and execution (1-9). The spinal cord acts as an interface to process descending commands from the brain and sensory inputs from the periphery (5,8,(10)(11)(12)(13)(14)(15)(16). Many insights into the organization and function of circuits underlying motor behavior have been gained from studies of spinal networks controlling locomotor movements (6,13,(17)(18)(19)(20)(21)(22)(23)(24). The basic locomotor activity requires the interplay of ipsilateral excitatory drive and crossed inhibition, which ensures the alternating pattern between the two sides of the spinal cord. Whereas inhibitory interneurons ensure the coordination of motoneurons controlling antagonistic muscles, excitatory interneurons are believed to represent the core components for the generation of the locomotor rhythm.Determining the identity of the interneurons at the origin of excitation necessary for the normal generation of locomotor activity is central for understanding the principles of organization and function of locomotor circuits. However, the molecular identity of these excitatory interneurons has remained unclear. A prominent class of excitatory interneurons is the V2a interneurons, defined by Chx10 expression and derived from the p2 progenitor domain with homologous counterparts across vertebrate species (21,(25)(26)(27). In larval zebrafish and newborn mice, V2a interneurons have been shown to project to motoneurons and to be recruited in a frequency-dependent manner (28-32). These properties are consistent with the possibility that this class of interneurons represents a source of excitation within the spinal locomotor circuit.In newborn mice, however, genetic elimination of V2a interneurons has been reported to have little effect on the rhythmic output of the locomotor circuits induced by pharmacological means (33, 34). The absence of tangible effects on the excitability of the locomotor circuit in the absence of V2a interneurons led to the conclusion that these interneurons are not essential fo...

Decoding the rules of recruitment of excitatory interneurons in the adult zebrafish locomotor network

“…Our results suggest that mechanisms underlying the recruitment of neurons in the spinal cord may undergo a refinement as the zebrafish develop toward adulthood. This refinement could be reflected in a reconfiguration of the organization of the swimming circuits to accommodate the changes in the swimming pattern and frequency range between early and adult developmental stages (10,30,36,37,(42)(43)(44)(45). It is not yet clear whether the scrambling of the topographic order of V2a interneuron recruitment mirrors only the displacement of these interneurons, an additional tuning of their cellular and synaptic features, or both.…”

“…Their partial ablation in larval zebrafish decreases the excitability of the spinal networks and consequently increases the threshold for inducing swimming activity (30). Their pattern of activation during locomotion has been examined in both embryonic/larval zebrafish and newborn mice (32,35).…”

“…However, motoneurons are not primarily involved in the generation of the locomotor rhythm, a task undertaken by premotor interneurons. V2a interneurons are a neuronal class of critical importance to the vertebrate locomotor network (1,8,12,15,(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35). Partial ablation of these interneurons in zebrafish decreases the excitability of the spinal networks (30).…”

Episodic swimming in the larval zebrafish is generated by a spatially distributed spinal network with modular functional organization

“…Larval zebrafish are an attractive species for studying motor systems (Fetcho 2007;Fetcho and Liu 1998;McLean and Fetcho 2011). Although the neuronal circuits for some motor behaviors such as escape (Koyama et al 2011;O'Malley et al 1996;Satou et al 2009) and early touch response (Downes and Granato 2006;Pietri et al 2009) have been characterized, the CPG circuits that control swimming in larval zebrafish are less well characterized (Eklöf-Ljunggren et al 2012;Kimura et al 2006;McLean et al 2008).…”

Generation of V2a Interneurons from Mouse Embryonic Stem Cells

“…Genetic ablation of V2a in mice leads to the loss of left-right coordination during locomotor activities [11], whereas targeted ablation of cervical V2a subpopulations leads to deficits in reaching movements [10]. Cells homologous to V2a interneurons in zebrafish have been shown to span greater than two spinal cord segments and synapse onto motoneurons [13]. Recently, V2a interneurons in the medial reticular formation of the hindbrain have been shown to stimulate excitatory signals to produce regular breathing patterns.…”