Neuroanatomical and histochemical evidence for the presence of common lateral line and inner ear efferents and of efferents to the basilar papilla in a frog, Xenopus laevis.

The claimed absence of efferents to the basilar papilla in frogs was reexamined in Xenopus laevis with the use of acetylcholinesterase (AChE) histochemistry. We also tested the presence of common lateral line and inner ear efferents by employing neuroanatomical tract tracing techniques. Our data show some AChE-positive fibers to the basilar papilla and all other sensory epithelia of the inner ear in larval and postmetamorphic frogs. Labeling of anterior lateral line and inner ear fibers with different fluorescing dextran amines resulted in a few double labeled efferent cells in the brainstem. Examination of the branching patterns of anterior lateral line efferents revealed collaterals to the inner ear, predominantly to the saccule and the lagena. In addition, two animals showed common efferents between the anterior lateral line and the basilar papilla. In derived anurans, such as ranids, which reportedly lack efferents to the basilar papilla, the basilar papillary afferents have a peripheral course that differs from that in salamanders and Xenopus. If such efferents are, indeed, absent, we propose that changed cues for pathway selection may have enabled only the afferents, and not the efferents, to reach the basilar papilla in derived frogs.