Developmental segregation of spinal networks driving axial- and hindlimb-based locomotion in metamorphosing <em>Xenopus laevis</em>

Amphibian metamorphosis includes a complete reorganization of an organism's locomotory system from axial-based swimming in larvae to limbed propulsion in the young adult. At critical stages during this behavioural switch, larval and adult motor systems operate in the same animal, commensurate with a gradual and dynamic reconfiguration of spinal locomotor circuitry. To study this plasticity, we have developed isolated preparations of the spinal cord and brainstem from pre- to post-metamorphic stages of the amphibian Xenopus laevis, in which spinal motor output patterns expressed spontaneously or in the presence of NMDA correlate with locomotor behaviour in the freely swimming animal. Extracellular ventral root recordings along the spinal cord of pre-metamorphic tadpoles revealed motor output corresponding to larval axial swimming, whereas postmetamorphic animals expressed motor patterns appropriate for bilaterally synchronous hindlimb flexion-extension kicks. However, in vitro recordings from metamorphic climax stages, with the tail and the limbs both functional, revealed two distinct motor patterns that could occur either independently or simultaneously, albeit at very different frequencies. Activity at 0.5-1 Hz in lumbar ventral roots corresponded to bipedal extension-flexion cycles, while the second, faster pattern (2-5 Hz) recorded from tail ventral roots corresponded to larval-like swimming. These data indicate that at intermediate stages during metamorphosis separate networks, one responsible for segmentally organized axial locomotion and another for more localized appendicular rhythm generation, coexist in the spinal cord and remain functional after isolation in vitro. These preparations now afford the opportunity to explore the cellular basis of locomotor network plasticity and reconfiguration necessary for behavioural changes during development.