WILKENS, L/A; HOFMANN, M/H; Univ. of Missouri-St. Louis; Univ. of Bonn, Germany: Electrosensory neuroanatomy in the paddlefish CNS

The paddlefish uses its electrosense for detecting planktonic food, a novel adaptation of passive electroreception based on an extensive array of ampullary receptors on its head and elongated rostrum. Juvenile fish rely on the accurate location of plankters that they capture individually at detection distances up to 8-9 cm from the fish. The primary role of the electrosense in feeding and the cathodal sensitivity of the receptors are now well established. How an electrical image is formed in the brain is the focus of current interest. This study describes the neural architecture in the mid- and hindbrain regions that process electrosensory information. At the level of the brainstem, the primary electrosensory afferents project to a single structure, the greatly enlarged dorsal octavolateral nucleus (DON). Purkinje-like cells in the DON send axons to three mesencephalic targets, the tectum, torus semicircularis, and lateral mesencephalic nucleus. In addition the pretectum receives some fibers directly from the DON. These ascending electrosensory pathways are more widespread in the paddlefish than in any other fish. In addition to the electrosensory afferents received by the DON, a large number of unmyelinated fibers that originate in the auricles of the cerebellum also terminate in the DON. They are part of a feedback loop involving the cerebellum and the nucleus preeminentialis, the latter apparently absent in the closely related sturgeon. There are a number of other differences in the paddlefish, in contrast to the sturgeon and other fishes that indicate a trend toward reduction of visual projection pathways and an elaboration of the electrosense. These results give information on brain mechanisms accompanying evolutionary trends in behavioral form and function.