This project is designed to provide information about the organization of neuronal systems in the mammalian spinal cord that are involved in the neural control of movement. Current work uses the isolated brain stem and spinal cord of neonatal mice or amphibians studied in vitro. During FY 2002, we completed a study of post-natal changes in low-frequency synaptic depression of group Ia monosynaptic excitatory postsynaptic potentials (EPSPs) produced in lumbosacral spinal motoneurons by repetitive stimulation of dorsal roots between postnatal ages of two (P2) to twelve (P12) days. We have refined an empirical model based on these data that assumes: 1) depletion of two presynaptic pools: 1) readily-releasable transmitter (RRT); and 2) release-ready release sites (RRS), that are renewed by independent processes with exponential time constants; 2) a rapidly decaying facilitation of transmitter release probability, and; 3) a more slowly decaying augmentation of the rate of renewal of the RRT compartment. The model suggests that the release fraction systematically decreases with postnatal age, and the incremental weights and decay time constants of the facilitation and augmentation processes also change, but in more complex ways. A full report on data from P2-3 mice is in press in the Journal of Neurophysiology. Data analysis is being completed now on age-related differences in the effects of altered external calcium concentration and of blockade of N and P/Q type calcium channels. We have tested our synaptic model in collaboration with scientists at the Australian National University in Canberra, using data from the giant end bulb of Held in the brain stem auditory nucleus, AVCN. Although this synaptic system has a totally different morphology and function from spinal group Ia synapses, the model fits data from the AVCN very well, with rather similar ranges for the majority of parameters. The major difference between the two synaptic systems is that the renewal time constant for the RRS compartment is two orders of magnitude shorter than in spinal cord group Ia synapses. The AVCN work has suggested that the renewal time constant for the RRS compartment may also be use-dependent and the same appears to be true for the data from the spinal cord. A second project is designed to elucidate spinal cord mechanisms that control rhythmic walking movements in a relatively primitive amphibian, Necturus. This animal is a useful bridge between primitive fish and in the much more complex spinal cord of mammals for work on the spinal central pattern generator for locomotion. The anatomy of the cervical spinal cord in this animal has never been described. We needed information about the shapes, sizes, and locations of motoneurons and interneurons in the cervical cord in order to design intracellular recording experiments. We have filled motoneurons, interneurons, primary afferents, and lateral white matter axons with fluorescent tracers, using retro- and orthograde transport. The results demonstrate that the Necturus spinal cord is remarkably different from mammals, and even other amphibians. When visualized by confocal microscopy, cell bodies of both motoneurons and interneurons line the lateral and ventral borders of the gray matters, immediately adjacent to the white matter into which their dendrites project. Labeling lateral white matter tracts fill interneurons located on both sides of the cord. The axons to contralateral interneurons travel in a thin sheet beneath the massive central canal region. There is no neuropil (i.e., neuronal dendrites, axons, and associated synapses) anywhere within the central portion of the gray matter. Instead the central region is occupied by a sparse network of thick and thin fibers with large interstitial spaces that appear to empty of solid structures. The most unexpected finding is that the distal terminations of motoneuron dendrites reach the lateral limits of the white matter, where they expand into elaborate, parasol-like tangles of thin branches that appear to be applied to the inside of the pial surface. The possible nature of these remarkable structures will be explored using intracellular recording and ultrastructural methods.
