V3 Interneurons Are Active and Recruit Spinal Motor Neurons during In Vivo Fictive Swimming in Larval Zebrafish

Survival for vertebrate animals is dependent on the ability to successfully find food, locate a mate, and avoid predation. Each of these behaviors requires motor control, which is set by a combination of kinematic properties. For example, the frequency and amplitude of motor output combine in a multiplicative manner to determine features of locomotion such as distance traveled, speed, force (thrust), and vigor. Although there is a good understanding of how different populations of excitatory spinal interneurons establish locomotor frequency, there is a less thorough mechanistic understanding for how locomotor amplitude is established. Recent evidence indicates that locomotor amplitude is regulated in part by a subset of functionally and morphologically distinct V2a excitatory spinal interneurons (Type II, nonbursting) in larval and adult zebrafish. Here, we provide direct evidence that most V3 interneurons (V3-INs), which are a developmentally and genetically defined population of ventromedial glutamatergic spinal neurons, are active during fictive swimming. We also show that elimination of the spinal V3-IN population reduces the proportion of active motor neurons (MNs) during fictive swimming but does not alter the range of locomotor frequencies produced. These data are consistent with V3-INs providing excitatory drive to spinal MNs during swimming in larval zebrafish and may contribute to the production of locomotor amplitude independently of locomotor frequency.

Automated summary

The ability to find food, locate a mate, and avoid predation is crucial for the survival of vertebrate animals. These behaviors require motor control, which is determined by kinematic properties. While we have a good understanding of how locomotor frequency is established, the mechanisms for establishing locomotor amplitude are less well-known. Recent evidence suggests that a subset of excitatory spinal interneurons called V2a interneurons regulate locomotor amplitude in larval and adult zebrafish. This study provides direct evidence that V3 interneurons, a population of ventromedial glutamatergic spinal neurons, are active during swimming in larval zebrafish and may contribute to locomotor amplitude independently of locomotor frequency.