Our experiments are motivated by the conviction that progress in research on LTP and other forms of synaptic plasticity must proceed hand with gaining a better understanding of fundamental characteristics of synaptic transmission. Basic knowledge about the physiological factors determining the reliability of transmission and the amplitude of the postsynaptic response in CNS neurons is still lacking. Non-uniform behavior from one synapse to the next poses great problems for analysis of synaptic transmission in preparations where the number and location of stimulated synapses is not known. To overcome these problems, we will use two new approaches to study the behavior of single visualized synapses in hippocampal cultures: first, the use of differential uptake of antibodies against a lumenal protein marker, synaptotagmin, to monitor changes in presynaptic activity independently of postsynaptic responsiveness (Malgaroli et al., Science, in press), and second, the focal stimulation of transmitter release from single presynaptic boutons or glutamatergic responses from individual postsynaptic receptor clusters (Lie et al., nature, in press). These strategies will help us to settle some long- standing questions about normal synaptic transmission, and to study the enduring modification of transmission during LTP. Our specific goals are as follows: (1) Mechanisms governing the reliability of the synaptic response (failures or successes) and variability in quantal response size will be studied at the level of single, visualized synapses, (2) LTP will be induced at single synapses and the spread of potentiation and the relative importance of presynaptic and postsynaptic changes will be assessed by direct methods. (3) The locus and mechanisms of late long-term potentiation (L-LTP) -- potentiation lasting many hours -- will be studied with cell biological and morphological approaches, (4) Signaling mechanisms for initiation and maintenance of various forms of LTP will be analyzed separately with regard to their pre- and postsynaptic components.
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