Activity of pectoral fin motoneurons during two swimming gaits in the larval zebrafish (Danio rerio) and localization of upstream circuit elements.

In many animals, limb movements transition between gait patterns with increasing locomotor speed. While for tetrapod systems several well-developed models in diverse taxa (e.g., cat, mouse, salamander, turtle) have been used to study motor control of limbs and limb gaits, virtually nothing is known from fish species, including zebrafish, a well-studied model for axial motor control. Like tetrapods, fish have limb gait transitions, and the advantages of the zebrafish system make it a powerful complement to tetrapod models. Here we describe pectoral fin motoneuron activity in a fictive preparation with which we are able to elicit two locomotor gaits seen in behaving larval zebrafish: rhythmic slow axial and pectoral fin swimming and faster axis-only swimming. We found that at low swim frequencies (17-33 Hz), fin motoneurons fired spikes rhythmically and in coordination with axial motoneuron activity. Abductor motoneurons spiked out of phase with adductor motoneurons, with no significant coactivation. At higher frequencies, fin abductor motoneurons were generally nonspiking, whereas fin adductor motoneurons fired spikes reliably and nonrhythmically, suggesting that the gait transition from rhythmic fin beats to axis-only swimming is actively controlled. Using brain and spinal cord transections to localize underlying circuit components, we demonstrate that a limited region of caudal hindbrain and rostral spinal cord in the area of the fin motor pool is necessary to drive a limb rhythm while the full hindbrain, but not more rostral brain regions, is necessary to elicit the faster axis-only, fin-tucked swimming gait.