Sonic/vocal-acousticolateralis pathways in teleost fishes: a transneuronal biocytin study in mochokid catfish.

Mochokid catfish have two sound producing (sonic) organs--a pectoral spine stridulatory apparatus and a swimbladder whose vibration is established by nearby "drumming" muscles. Dextran-biotin or biocytin application to sonic nerves or muscles identified topographically separated motoneuron pools. Pectoral spine-related motoneurons are located within the ventral motor column whereas swimbladder motoneurons lie just ventral to the central canal or fourth ventricle. Axons of both groups of motoneurons exit the brain and spinal cord via ventral roots of occipital (swimbladder and pectoral) and spinal (swimbladder only) nerves. Transneuronal biocytin transport identified an extensive premotor network only for the swimbladder motor nuclei. Premotoneuron somata are located ipsilaterally in 1) a dorsolateral region of the sonic motor nucleus (SMN); motoneurons were clustered in the ventromedial region of the SMN and 2) the ventromedial medulla at the rostral pole of the SMN. Biocytin-filled fibers and less frequently premotoneuron somata were also found in the contralateral SMN. Biocytin-labeled fibers were continuous farther rostrally with 1) a commissural bundle that terminated bilaterally in the medial reticular formation near the caudal pole of the descending octaval nucleus and 2) a lateral brainstem bundle that terminated ipsilaterally in regions of the medulla and cerebellum considered to subserve acoustic and lateral line functions. Together with other data in distantly related teleost fishes, the results support the hypotheses that 1) central pathways linking sound-generating (sonic or vocal) and acoustic regions of the brain are traits common to both teleosts fishes and tetrapods that actively generate sounds, and 2) sonic/vocal pathways in teleosts have a conserved pattern of organization suggestive of common developmental origins.