The thalamocortical synapse is a crucial link in sensory processing by the neocortex, and its early stages of development are particularly susceptible to abnormal sensory inputs. It may also play an important role in the propagation of epileptic seizures. Thus understanding its development is essential for understanding both normal and abnormal cortical function. Nevertheless surprisingly little is known about its properties at these early stages. The development of a novel slice preparation that retains functional thalamocortical connectivity, together with the recent application of the whole-cell recording technique to the thick slice, now make it feasible to study development of this synapse at the intracellular level. Our preliminary studies in acute thalamocortical slices have revealed that functional thalamocortical contacts are first established in the deep layers (V/VI) of the cortex and that both NMDA and nonNMDA-type receptors mediate the newly formed thalamocortical connections. The establishment of intracortical inhibitory connections is delayed relative to the formation of thalamocortical excitatory connections. In this study we propose to further examine both the morphological and physiological development of the thalamocortical synapse in neonatal mouse cortex in the acute slice. Based on electrophysiological and pharmacological criteria, responses will be classified as NMDA or non-NMDA type EPSC's or as GABAergic IPSC's. The development of each type of response will be examined separately. Properties that will be examined include the spatial distribution across different layers and on different morphological classes of neurons during the first two weeks postnatal. Labelling of the population of thalamic afferents with DiI in acute slices will be done in conjunction with the physiological experiments in order to correlate the morphological distribution of fibers with the location of functional connections. We have also determined conditions under which thalamocortical slices prepared from early postnatal animals (P0/P2) survive and maintain intact fibers projecting from the thalamus to the cortex for at least 1-2 weeks in a serum-free chemically-defined medium. In these cultures we will examine the growth of thalamic afferents and patterns of arborization in the first two weeks in culture. Electrophysiological recordings will be performed to assess which thalamic fibers projecting to the cortex form functional connections. Manipulation of the growth conditions will then be done in order to identify physiological or biochemical factors involved in the process of synaptogenesis. The results of these studies on acute and cultured thalamocortical slices will contribute to our understanding of the development of both normal and abnormal cortical function.
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