Journal of Neuroscience, Vol 12, 2960-2972, Copyright © 1992 by Society for Neuroscience
Segregation of presynaptic inputs on an identified target neuron in vitro: structural remodeling visualized over time
M Bank and S Schacher
Center for Neurobiology and Behavior, Columbia University, College of Physicians and Surgeons, New York, New York 10032.
Sensory cells of Aplysia form chemical synapses with the motor cell L7 in
culture. Under certain conditions, sensory cells will also form electrical
connections with each other. Sites of chemical synaptic interaction between
the sensory cells and L7 are located at varicosities along sensory cell
processes that overlie the main axons of L7, since these structures have
been shown ultrastructurally to contain active zones. Previous studies have
suggested that the distribution of sensory cell varicosities can be
restricted to exclusive regions of the motor cell by the presence of other
sensory cells. We wished to investigate (1) how this segregated pattern is
generated over time and (2) whether electrical coupling between sensory
cells has an effect on this segregated pattern. Using fluorescent dye
injection and low-light video microscopy, we visualized the distribution of
varicosities for each of two sensory cells growing on L7. In cases in which
sensory cells are not electrically coupled, the varicosities from these two
cells are spatially segregated on the target after 4 d in culture but not
after 2 d in culture. Examination of the varicosity distribution of the
same sensory cells on the second and third day of growth indicated both an
increased rate in the elimination of varicosities from previously occupied
areas and a restriction of varicosity formation in new areas of the target
when a second sensory cell is present. For sensory cells that are
electrically coupled, varicosities from these cells were not spatially
segregated on the target even after 4 d in culture. These observations in
vitro suggest that segregation of synaptic inputs by Aplysia sensory cells,
which show little spontaneous activity of action potentials, can emerge
over time via a process that includes both the elimination of existing
sensory varicosities and the restriction of new varicosity formation. Our
results also suggest that electrical connections between presynaptic cells
can disrupt the segregation of their varicosities on a target, resulting in
significant changes in the developing connectivity.