TY - JOUR T1 - V3 Interneurons Are Active and Recruit Spinal Motor Neurons during <em>In Vivo</em> Fictive Swimming in Larval Zebrafish JF - eneuro JO - eNeuro DO - 10.1523/ENEURO.0476-21.2022 VL - 9 IS - 2 SP - ENEURO.0476-21.2022 AU - Timothy D. Wiggin AU - Jacob E. Montgomery AU - Amanda J. Brunick AU - Jack H. Peck AU - Mark A. Masino Y1 - 2022/03/01 UR - http://www.eneuro.org/content/9/2/ENEURO.0476-21.2022.abstract N2 - 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. ER -