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 2001, we continued a study of low-frequency synaptic depression of monosynaptic excitatory postsynaptic potentials (EPSPs) produced in lumbosacral spinal motoneurons by short trains (10 pulses) of stimuli to dorsal roots at 0.0125 - 8.0 Hz in neonatal mice at postnatal ages of two (P2) to twelve (P12) days at room temperature (24 deg C). The NMDA receptor blocker, AP5, was added to the bathing solution (100 microM) to suppress spontaneous and stimulus-evoked background discharges. All responses were normalized by that of the first response (R1). Changing external calcium concentrations and increasing bath temperature to 32 deg C produced systematic alterations in EPSP amplitudes and in the depression curves. Based on these data from young mice (P2-3), we developed an empirical model that assumes: 1) depletion of two presynaptic compartments (N and S) 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 N compartment. This model gives excellent fits to the data from all external Ca and temperature conditions. The model suggests that the release fraction and the incremental weights of the facilitation and augmentation processes change in parallel with Ca concentration, without change in model time constants. In contrast, model fits to the 32 deg C data imply that the process time constants all decreased by 45 to 50% while the calcium-dependent weighting factors were unchanged. A full report on data from P2-3 mice has been published in the Journal of Neurophysiology. Application of the synaptic model to data from older animals shows systematic changes in some parameters with increasing postnatal age (P7, 10, 11, and 12). In particular, the release fraction shows a steady decrease with age, while there are complex changes in the activity-dependent process that speeds transmitter renewal. However, there is surprisingly little change in the time constants of renewal for the two depleting compartments. The work will continuing with an exploration of the effect of metabotropic glutamate receptor agonists and antagonists on synaptic depression at different ages. This work will enable us to develop a profile of synaptic development during early postnatal life. We have begun a collaboration with scientists at the Australian National University to examine whether our synaptic model can also be used to define mechanisms that operate at a very different synaptic system in the auditory pathway of neonatal mice. During FY2001 we continued a project designed to elucidate spinal cord mechanisms that control rhythmic walking movements in a relatively primitive amphibian, Necturus. We have examined the role of different spinal segments on the quality of rhythmic stepping in the in vitro Necturus spinal cord, in order to define the optimum preparation to use for intra- and extracellular recording studies of the spinal circuits that produce locomotion, as well as the cellular anatomy of spinal neurons in the cervical segments. The Necturus model is a useful bridge between primitive fish and in the much more complex spinal cord of mammals for studying the spinal central pattern generator for locomotion.
