How the intricate pattern of connections between nerve cells and their targets is established is a fundamental and largely unanswered question in neurobiology. One feature of this pattern, however, which has begun to be understood: the number and distribution of an axon's contacts with postsynaptic cells. This quantitative feature of neural connectivity is regulated by a competitive process during development known as synapse elimination. I propose to investigate the mechanisms underlying synapse elimination by developing techniques that allow for light microscopic visualization of individual axonal terminals in living preparations. I plan to follow the normally occurring reduction in the number of terminals by activity dependent enzyme uptake. Preliminary experiments indicate that it is possible to identify neurally activated nerve terminals using uptake of a peroxidase enzyme and the intensification of its reaction produce in vitro. Furthermore, I have found that peroxidase-filled processes can be viewed in living preparations by using a novel histochemical assay for the enzyme: the transformation of L-DOPA into melanin. My objectives are: (1) to combine these two techniques in order to visually identify peroxidase-filled terminals in living preparations, (2) to study the competition between axonal terminals in a unique and potentially important preparation: the transversus abdominis muscle of the garter snake, and (3) to apply these techniques to a study of the pattern of innervation on adult and developing autonomic neurons. The overall aim of this work is to better understand the developmental strategies that regulate the number and distribution of axonal contacts on postsynaptic cells.
