Trigeminal, vagal, and spinal projections of supramedullary cells in the puffer fish, Takifugu niphobles.

The supramedullary cells (SMCs) of teleosts have been studied for nearly 100 years, but their peripheral connections have remained obscure. We examined the supramedullary cells of the puffer fish, Takifugu niphobles, using horseradish peroxidase transport. Horseradish peroxidase labeling was found bilaterally after application to the trigeminal, the posterior branch of the vagal, and the spinal nerves. No labeled neurons were found after application to the anterior or visceral branches of the vagal nerve. Thus, labeled SMCs were found only after application to the nerves containing cutaneous branches. Some rostrocaudal topographical labeling was found after selective application to each of the four branches of the trigeminal nerve. Labeled neurons were more common in the rostral than in the central or caudal part of the SMC region. Some topographical labeling was also found after application to the first, second, and third spinal nerves, but the topography was not very clear, and there was considerable overlap in the distribution of labeled cells. The sum total of labeled SMCs after unilateral horseradish peroxidase application to each peripheral nerve was more than three times the total number of ipsilateral SMCs, indicating that a single SMC projects several peripheral processes into different nerves. From these results, and taking previous studies into consideration, we propose that supramedullary neurons have a phylogenetic relationship with the spinal dorsal cells of the lamprey and with the extramedullary cells of the amphibian embryo.