Role of branchiomotor neurons in controlling food intake of zebrafish larvae

Abstract

The physical act of eating or feeding involves the coordinated action of several organs like eyes and jaws, and associated neural networks. Moreover, the activity of the neural networks controlling jaw movements (branchiomotor circuits) is regulated by the visual, olfactory, gustatory and hypothalamic systems, which are largely well characterized at the physiological level. By contrast, the behavioral output of the branchiomotor circuits, and the functional consequences of disruption of these circuits by abnormal neural development are poorly understood. To begin to address these questions, we sought to evaluate the feeding ability of zebrafish larvae, a direct output of the branchiomotor circuits, and developed a qualitative assay for measuring food intake in zebrafish larvae at 7 days post fertilization. We validated the assay by examining the effects of ablating the branchiomotor neurons. Metronidazole-mediated ablation of nitroreductase-expressing branchiomotor neurons resulted in a predictable reduction in food intake without significantly affecting swimming ability, indicating that the assay is robust. Laser-mediated ablation of trigeminal motor neurons resulted in a significant decrease in food intake, indicating that the assay is sensitive. Importantly, in larvae of a genetic mutant with severe loss of branchiomotor neurons, food intake was abolished. These studies establish a foundation for dissecting the neural circuits driving a motor behavior essential for survival.

Defective Neuronal Positioning Correlates With Aberrant Motor Circuit Function in Zebrafish

“…A reduction in the number or loss of branchiomotor neurons, including FBM neurons, can severely disrupt food intake ( Allen et al, 2017 ; Kassim et al, 2018 ). However, mispositioning of FBM neurons in landlocked mutants did not affect opercular muscle movement ( McArthur and Fetcho, 2017 ).…”

“…Our procedure is extensively described in Allen et al (2017) . Briefly, 10–15 larvae were placed in 5 ml of E3 medium containing 2 mg Larval fish food (Zeigler <100 microns) coated in yellow fluorescent microspheres (Molecular Probes/Thermo Fisher) for 3 h. At the end of the feeding period, larvae were anesthetized with 0.02% tricaine (Western Chemical Inc.), mounted laterally, and scored based on the amount of fluorescent food in the gut.…”

Zebrafish expression reporters and mutants reveal that the IgSF cell adhesion molecule Dscamb is required for feeding and survival

“…For example, the olfactory system regulates feeding behaviors in fish (Hamdani, Kasumyan, & Døving, 2001), and dscamb is expressed in both olfactory receptors neurons and the downstream olfactory bulb. Recent studies also demonstrated that molecular or anatomical ablation of branchiomotor neurons in larval zebrafish impairs food capture in a remarkably similar fashion to dscamb loss-of-function (Allen et al, 2017). dscamb is expressed in a subpopulation of larval branchiomotor neurons and their target muscles, but we did not detect obvious defects in the development of NMJs.…”

Zebrafish: from genes and neurons to circuits, behavior and disease

“…Identification of anxiety genes and circuitry in fish is a promising avenue for gaining new insights into human psychiatric disorders. Finally, Allen et al (2017) describe a qualitative but robust feeding assay, and analyze the role of branchiomotor neurons in mediating the intake of food. Ablation of the trigeminal motor neurons or the loss of branchiomotor neurons respectively diminishes or abolishes food intake, indicating that the assay is a sensitive readout of the function of these neurons.…”

“…Historically, behaviors that were amenable to analysis involved simple yet robust reflexes such as startle responses. Although these reflexive behaviors are still important in testing sensory or motor function (Nicolson, 2017;Niklaus & Neuhauss, 2017), other types of behaviors integrating sensory and motor aspects, such as dark avoidance (Wagle, Nguyen, Lee, Zaitlen, & Guo, 2017) and food intake (Allen et al, 2017) are being added to the growing list of quantifiable responses in zebrafish larvae. These developments are facilitating the discovery of novel genes or the generation of new zebrafish models of human neurological diseases.…”