Connections between the nucleus isthmi and the tectum in larval and post‐metamorphic axolotls

The nucleus isthmi (NI) is the primary relay for the frog's ipsilateral visuotectal projection. Using electrophysiological methods, ipsilateral visuotectal activity has been recorded in thyroxine‐treated, postmetamorphic axolotls but not in larval axolotls. In order to determine whether changes in isthmotectal projections are responsible for this change in electrophysiological responsiveness, we have investigated the connections between the tectum and the NI using horseradish peroxidase. Our results indicate that the axolotl's isthmotectal pathways are strikingly similar to those of the frog NI, and that the NI sends bilateral projections to the tecta in both larval and thyroxine‐treated, postmetamorphic axolotls. Thus, the anatomical connections underlying the ipsilateral visuotectal projection are present during larval stages, despite the lack of electrophysiological evidence for the larval ipsilateral visuotectal projection. We hypothesize that thyroxine‐induced metamorphosis produces changes in the terminal arborizations of the crossed isthmotectal projection which allow them to be detected by presynaptic electrophysiological techniques.