An indirect telencephalo-cerebellar pathway and its relay nucleus in teleosts

In order to clarify telencephalo-cerebellar connections in teleosts, horseradish peroxidase (HRP) was injected into the telencephalon and the corpus cerebelli of 6 species which belong to Cyprinidae, Holocentridae, Scorpaenidae and Balistidae. In fishes belonging to Holocentridae, Scorpaenidae, and Balistidae, some fibers of the telencephalo-tectal tract were observed to terminate ipsilaterally in a longitudinal nucleus, which is located dorsolaterally to the posterior commissure, while the remaining fibers terminated in the tectum. Because the nucleus has not been identified as an independent cell-group or nucleus in previous papers, it was named the nucleus paracommissuralis in the present study. HRP injections into the ipsilateral side of the corpus cerebelli resulted in labeled cells in the nucleus paracommissuralis. Fibers from the nucleus pass through the tractus mesencephalo-cerebellaris anterior. Cytoarchitecture of the nucleus paracommissuralis was studied by means of the Bodian, Nissl and Golgi-Cox methods. The telencephalo-cerebellar fiber system could not be identified in cyprinoid species. Other orthograde- and retrograde-labeled areas were also reported following telencephalic and cerebellar injections with respect to the species difference.