Mauthner neurons survive metamorphosis in anurans: a comparative HRP study on the cytoarchitecture of Mauthner neurons in amphibians.

Giant medullary neurons were revealed in adult Xenopus laevis and Rana esculenta following HRP injections to the spinal cord. These neurons were identified as Mauthner neurons because (1) they have the same position and orientation as the larval Mauthner neurons, i.e., they lie at the level of the VIIIth nerve root. (2) they have two large dendritic trees that for each species are similar to those of the larval Mauthner neurons, (3) they are clearly distinguishable from other large reticular neurons, (4) they have close connections with the VIIIth nerve afferents, (5) they have a decussating descending axon, the largest of the adult brainstem, and (6) they have a reduced axon cap, i.e., a dense neuropil without cap dendrites. Mauthner neurons were also identified in adult Xenopus muelleri, Hymenochirus curtipes, Hyla cinerea, Rhacophorus leucomystax, and Kalula pulchra. No Mauthner neuron was identified in larval or adult Bombina bombina or Bufo bufo or in adult Bufo terrestris, Bombina orientalis, or Scaphiopus holbrookii. Cytoarchitecture and the elaborate dendritic trees were also revealed in urodeles. Mauthner neurons of terrestrial forms (Ambystoma tigrinum, Paramesotriton chinensis) exhibit a less-extensive branching of the "lateral line" dendrite but otherwise show no signs of involution or regression compared to aquatic forms (Siredon mexicanum, Cynops pyrrhogaster). No Mauthner neuron was identified in larval or adult Ichthyophis kohtaoensis. Besides the topographical relations between Mauthner neurons and afferent fibers from the lateral line and the VIIIth nerve we could demonstrate a projection of trigeminal and mesencephalic neurons onto the soma of the amphibian Mauthner neuron.