Development of topographic connections between the isthmic nuclei and optic tecta in the frog Limnodynastes dorsalis.

In the frog Limnodynastes dorsalis, the pattern of topographic connections between the isthmic nuclei and optic tecta was determined by anterograde and retrograde transport of horseradish peroxidase from localised tectal regions. In both larvae and adults, reciprocal mapping of the uncrossed isthmo-tectal and tecto-isthmal projections was evidenced by the juxtaposition of labelled tecto-isthmal terminations with labelled cells in the cortex and medulla of the ipsilateral isthmic nucleus. The crossed isthmo-tectal projection was revealed by labelled cells in the cortex and medulla of the nucleus contralateral to the injection. In adults, rostral tectal areas projected to rostral and ventral regions of the ipsilateral isthmic nucleus. Following more caudal tectal injections, labelled cells were found in progressively more dorsal locations within the nucleus. Labelled cells in the contralateral nucleus were found in the rim cortex abutting a neuropil and in medullary cells adjacent to this region. Connections between ventral isthmic regions and most rostral tectum and between dorsomedial nucleus and caudomedial tectum were similar in both nuclei. However, for isthmic areas projecting to rostromedial and mid-tectum, the location of labelled cells in the contralateral nucleus was inverted with respect to the ipsilateral nucleus. This inversion would allow both nuclei to project to visually corresponding regions of each tectum. During larval stages the basic adult topography was established despite the continued neurogenesis of both isthmic nuclei and optic tecta. In late larval stages a rim neuropil appeared adjacent to the cortical region in the isthmic nuclei where labelled cells of the crossed isthmotectal projection were found. Prior to this stage labelled cells abutted labelled medullary cells. The appearance of this neuropil was approximately temporally correlated with the onset of electrophysiologically detectable responses in the ipsilateral visuotectal projection. Formation of the rim neuropil may relate to maturation of the tecto-isthmo-tectal connections which underlie this visual projection.