Ascending spinal systems in the fish, Prionotus carolinus

The fin rays of the pectoral fin of the sea robins (teleostei) are speciali zed chemosensory organs heavily invested with solitary chemoreceptor cells innervated only by spinal nerves. The rostral spinal cord of these animals is marked by accessory spinal lobes which are unique enlargements of the do rsal horn of the rostral spinal segments receiving input from the fin ray n erves. Horseradish peroxidase (HRP) and 1,1'-dioctadecyl-3,3,3',3'-tetramet hylindocarbocyanine perchlorate (diI) were used as anterograde and retrogra de tracers to examine the connectivity of these accessory lobes and the ass ociated ascending spinal systems in the sea robin, Prionotus carolinus. The majority of dorsal root fibers terminate within the accessory lobes at or nearby their level of entrance into the spinal cord. A few dorsal root axon s turn rostrally in the dorsolateral fasciculus to terminate in the lateral funicular complex situated at the spinomedullary junction. The lateral fun icular complex also receives a heavy projection from the ipsilateral access ory lobes. In addition, it contains a few large neurons that project back o nto the accessory lobes. Injections of either diI or HRP into the lateral f unicular complex label fibers of the medial lemniscus which crosses the mid line in the caudal medulla to ascend along the ventral margin of the contra lateral rhombencephalon. Within the medulla, fibers leave the medial lemnis cus to terminate in the inferior olive and in the ventrolateral medullary r eticular formation. Upon reaching the midbrain, the medial lemniscus turns dorsally to terminate heavily in a lateral division of the torus semicircul aris, in the ventral optic tectum, and in the lateral subnucleus of the nuc , preglomerulosus of the thalamus. Lesser projections also reach the poster ior periventricular portion of the posterior tubercle with a few fibers ter minating along the ventral, posterior margin of the ventromedial (VM) nucle us of the thalamus. The restricted projection to the ventral tectum is note worthy in that this part of the tectum maintains the representation of the ventral visual held, that is, the area in which the fin rays lie. A promine nt spinocerebellar system is also evident. Both direct and indirect spinoce rebellar fibers can be followed through the dorsolateral fasciculus, with o r without relay in the lateral funicular nucleus and terminating in a restr icted portion of the granule cell layer of the ipsilateral corpus cerebelli . The similarities in connectivity of the spinal cord between the sea robin s and other vertebrates are striking. It is especially notable because sea robins utilize the chemosensory input from the fin rays to localize food in the environment. Thus, although these fish use their spinal chemosense as other fishes use their external taste systems, the spinal chemosense appare ntly relies on the medial lemniscal system to guide this chemically driven feeding behavior. (C) 2000 Wiley-Liss, Inc.