Trigeminal motoneurons in frogs develop a new dendritic field during metamorphosis.

During metamorphosis neural systems that regulate larval behavior are altered to adult patterns. Identified strategies used to produce this alteration include neuronal deletions, neuronal additions, and the reconfiguration of existing neural circuits. This investigation focused on the structural alterations that occur in frog trigeminal motoneurons during metamorphic climax. These neurons are of special interest because they mediate the vastly different muscular activities associated with larval and adult food capture. We examined the development of the trigeminal motoneurons in anuran larvae, devoting special attention to the elaboration of different dendritic fields during metamorphic climax. When horseradish peroxidase (HRP) back-filled motoneurons in adults were scrutinized by Sholl analysis, they were found to have two spatially discrete dendritic domains, one extending ventrally and laterally, the other projecting dorsomedially into the periventricular cells. The ventrolateral dendritic field alone is represented in the motoneurons of premetamorphic larvae. The dorsomedial dendritic field first appears at the beginning of metamorphic climax and is rapidly elaborated during the terminal stages of larval development.