THE CONTRALATERALLY PROJECTING NEURONS OF THE ISTHMIC NUCLEUS IN 5 ANURAN SPECIES - A RETROGRADE TRACING STUDY WITH HRP AND COBALT

The morphology of projection neurons of the isthmic nucleus was studie d in Rana esculenta, R. nigromaculata, Bufo marinus, B. bufo gargariza ns, and Xenopus laevis from a comparative anatomical point of view. Th e main point of this work was to provide an anatomical basis for elect rophysiological studies. Neurons projecting to the ipsilateral optic t ectum were labeled by retrograde transport of horseradish peroxidase a nd cobaltous lysine complex injected into the optic tectum. Contralate rally projecting cells were filled by injecting the tracer substances into the crossed isthmotectal tract. Cells of the anterior nonrim cort ex and the rostral part of the medulla project to the ipsilateral tect um. A band of cells in the middle of the medulla, a few cells in the c audal part of the medulla, and most of the neurons in the rim cortex p roject to the contralateral tectum. Five types of neurons were disting uished in the rim cortex of R. esculenta. Most of them have piriform p erikarya and their dendrites arborize in the rim neuropil. In the medu lla of the isthmic nucleus of R. esculenta, seven types of neurons wer e distinguished. Most of these neurons also exist in the other species . Medullary cells are piriform, fusiform, or multipolar, and are varia ble in size and in dendritic arborization. The isthmic neurons of the two Ranae and Bufo species are similar. The dominant cell types in Xen opus are multipolar with extensive dendritic arborization, which occup ies more space in the nucleus than in the other species. Neurons with narrow dendritic trees may represent a system of fine resolution, and those neurons with extensive dendritic arborization may belong to a co arser system. (C) 1994 Wiley-Liss, Inc.