A Hybrid Electrical/Chemical Circuit in the Spinal Cord Generates a Transient Embryonic Motor Behavior

Abstract

Spontaneous network activity is a highly stereotyped early feature of developing circuits throughout the nervous system, including in the spinal cord. Spinal locomotor circuits produce a series of behaviors during development before locomotion that reflect the continual integration of spinal neurons into a functional network, but how the circuitry is reconfigured is not understood. The first behavior of the zebrafish embryo (spontaneous coiling) is mediated by an electrical circuit that subsequently generates mature locomotion (swimming) as chemical neurotransmission develops. We describe here a new spontaneous behavior, double coiling, that consists of two alternating contractions of the tail in rapid succession. Double coiling was glutamate-dependent and required descending hindbrain excitation, similar to but preceding swimming, making it a discrete intermediary developmental behavior. At the cellular level, motoneurons had a distinctive glutamate-dependent activity pattern that correlated with double coiling. Two glutamatergic interneurons, CoPAs and CiDs, had different activity profiles during this novel behavior. CoPA neurons failed to show changes in activity patterns during the period in which double coiling appears, whereas CiD neurons developed a glutamate-dependent activity pattern that correlated with double coiling and they innervated motoneurons at that time. Additionally, double coils were modified after pharmacological reduction of glycinergic neurotransmission such that embryos produced three or more rapidly alternating coils. We propose that double coiling behavior represents an important transition of the motor network from an electrically coupled spinal cord circuit that produces simple periodic coils to a spinal network driven by descending chemical neurotransmission, which generates more complex behaviors.

Behavioral screening of the LOPAC1280 library in zebrafish embryos

“…Early spontaneous activity consists of single coils that are driven by periodic, non-chemically-mediated depolarizations, excitation spikes, and gap-junction-mediated electrical coupling within early spinal neurons (Saint-Amant and Drapeau, 2001, 2000). On the other hand, late spontaneous activity consists of side-to-side double coils that arise through the addition of chemically-mediated synapses to the existing electrical circuit and is regulated within the spinal cord as well as the hindbrain of embryonic zebrafish (Behra et al, 2002; Knogler et al, 2014; Raftery and Volz, 2015). As such, late spontaneous activity represents an intermediate form of behavior that precedes secondary motoneuron development and bridges early spontaneous activity with stimuli-induced responses observed during later stages of embryonic and larval development (Knogler et al, 2014).…”

“…Although different forms of larval and adult zebrafish locomotion have been leveraged within behavioral assays, spontaneous activity (tail contraction) – a behavior that occurs from late-segmentation (~17–19 hours post-fertilization, hpf) through early-pharyngula (~27–29 hpf) – represents an early, primitive form of motor activity within zebrafish embryos, providing a potential readout for rapid identification of neuroactive chemicals (Kokel et al, 2010; Raftery et al, 2014; Reif et al, 2015; Truong et al, 2016). Although previous studies have explored the biological basis of this behavior (Knogler et al, 2014; Knogler and Drapeau, 2014; Saint-Amant and Drapeau, 2001, 2000), it remains unclear whether spontaneous activity is responsive only to certain compounds with specific mechanisms of action.…”

Neuronal labeling patterns in the spinal cord of adult transgenic Zebrafish

“…Further studies should be also carried out on more precise architecture including synaptic interactions between cell populations. In zebrafish embryos, chx10 positive V2a interneurons have been recently shown to establish putative synaptic contacts with others and motoneurons (Knogler et al, ). We suggest that a correlation between physiological recordings and histology could help to provide answers in adults.…”

From neuron to behavior: Sensory‐motor coordination of zebrafish turning behavior

“…Zebrafish motor neurons are activated via descending interneurons such as CiD (Circumferential ipsilateral descending) and IC (ipsilateral projecting), which form electrical connections by gap junction to synchronize neural activity among them and motor neurons, and also form glutamatergic synapses that excite motor neurons (Saint‐Amant & Drapeau ; Knogler et al . ). Probably commissural axons from CoPA neuron provide sensory signal to these descending interneuron, whereas connections between CoPA and the descending neurons are not clarified yet in current studies.…”