The proposed research will increase understanding of how the vertebrate brain processes sensory information and translates it into appropriate behaviors. A very unique animal and sensory mechanism is the subject of the project, the American paddlefish, which detects and captures its planktonic prey by sensing the weak electric signals from tiny water fleas. No other sensory modalities are necessary for successful feeding. The extremely small electric fields are detected by skin receptors and relayed to the posterior (primitive) region of the brain where they are processed and sent forward to the midbrain where motor commands are generated that govern swimming and feeding. This research will build on what is already known about how the paddlefish brain interprets electrical signals in terms of the physiological response properties of nerve cells in the hindbrain and their anatomical features in projecting to the midbrain. Dyes are microinjected into the brain that are transported along axons and dendrites of the cells, morphological features that constitute the wiring diagrams of the brain. Microelectrodes inserted into the brain allow the action potentials or spike activity of individual neurons to be recorded in response to sensory stimuli. Using these experimental procedures the current phase of the study will focus on how cells in the midbrain collect information from electrosensory and visual inputs, how individual cells come to represent complex features of the external environment, and the projection of these cells into the various processing centers (brain nuclei) of the hindbrain and spinal cord that activate the muscles triggering behavioral responses. This research has already had a significant impact on understanding the role of electrical signals, which humans do not experience, in paddlefish biology and corresponding conservation efforts. The study has also contributed to the research education and training of students in high school, college, and graduate school.
Our earlier research on the paddlefish, involving both behavioral and physiological studies, established the fact that the elongated paddle or rostrum of the fish serves as a functional electrosensory antenna rebutting long-held popular beliefs that it was used to excavate substrate for feeding. We demonstrated that the rostrum, invested with an extensive population of ampullae of Lorenzini known from other primitive fish as highly sensitive electroreceptors, provides for a spatial distribution of such receptors on the elongated rostrum that enables the paddlefish to detect, locate and capture tiny zooplankton (Daphnia) that serve as their primary food. This system is essential for a fish that feeds by filtering small prey difficult to detect visually in the turbid environments of the Mississippi River and its many tributaries. Behavioral studies established that juvenile paddlefish capture individual daphnids when all sensory systems except the electrosense are blocked, in spatially defined patterns equivalent to those without sensory interference. In fact, paddlefish attempt to capture and "feed" on electrode tips in the water that deliver electric fields similar to those surrounding the small plankton. Physiologically, we recorded from the primary sensory afferent nerves that carry information (nerve action potentials or spikes) from the ampullae to the brain and have demonstrated that they readily respond to live plankton near the surface of the rostrum. Thus, we have shown that the electrosensory system of the paddlefish, its rostral "antenna", is the primary sensory mechanism whereby the fish captures and feeds on its planktonic prey. We also discovered that the paddlefish nervous system is exquisitely amenable to histological staining techniques that allow one to trace pathways of nerves leading to the brain and to follow the connections that nerves make within the brain. In addition, as a cartilaginous fish, it is amenable as a preparation for electrophysiological recording of neuronal signals within the brain by securing the anesthetized fish, exposing the brain, and inserting electrodes into defined regions (nuclei). As a result, the paddlefish represents an ideal model preparation for the study of brain function. The results of experiments supported by this grant represent the first stages of a long-term study with the aim of understanding how the paddlefish processes the sensory information from its rostral antenna leading to motor outputs that underlie successful feeding behaviors. Neuronal staining and recording initially focused on the dorsal octaval nucleus (DON) in the hindbrain, the target for the primary afferent nerves carrying inputs from the electroreceptors. This nucleus is homologous with brain regions in related electrosensory fish (sharks, skates and rays), but is proportionally much larger in correspondence with the elongated rostrum and much greater numbers of ampullary receptors. In conjunction with other novel features of the paddlefish, the DON itself turns out to be unique in the fact that there seems to be no spatial distribution of sensitivity in parallel with the location of receptors along the rostum. Such "somatotopic" organization is well know from a variety of sensory systems in many vertebrate organisms whereby recordings from adjacent neurons in the brain correspond to adjacent receptive fields peripherally. It was also shown that the sensitivity of DON neurons, the first stage of central processing, was very much equivalent to the sensory profiles of the input fibers themselves as determined by temporal analysis of spiking activity in response to anodal and/or cathodal stimuli delivered proximate to the receptor ampulla. The major results of the present grant involve the topography of neural projections from the hindbrain DON to the precessing centers of the midbrain. One of the prominent regions of the midbrain in lower vertebrates is the tectum, frequently referred to as the "optic tectum", which reflects that the tectum is a major center for processing visual information. However, the eyes of the paddlefish are very small and the fish displays limited visual behavior. As a result, the tectum in the paddlefish seems to be dominated by electrosensory input and indeed a spatially topographic map of peripheral receptors is restored in the tectum. Some of the details of projections to the tectum involve a specialized nucleus within the tectum, the torus semicircularis (TS). From staining and recording experiments we have determined that the left and right TS nuclei receive bilateral projections from the DON. In contrast direct connections from the DON to the tectum involve only projections from the opposite side of the hindbrain, i.e., contralateral projections. An important question was whether individual DON neurons send collateral connections to both TS nuclei and our results show that they do. Further, TS neurons appear to innervate both sides of the tectum, individually buy not via collateral branches. These results suggest that the TS nucleus is a critical center for computing source location, such as the location of planktonic prey. Experiments have also traced descending pathways that underlie the motor commands for successful prey capture.
