During development, neurons must find their way to and make connections with their appropriate targets. Growth cones are dynamic, motile structures that are integral to the establishment of appropriate connectivity during this wiring process. As growth cones migrate through their environment, they encounter guidance cues that direct their migration to their appropriate synaptic targets. The gaseous messenger nitric oxide (NO), which diffuses across the plasma membrane to act on intracellular targets, is a signaling molecule that affects growth cone motility. However, most studies have examined the effects of NO on growth cone morphology when applied in large concentrations and to entire cells. In addition, the intracellular second messenger cascade activated by NO to bring about these changes in growth cone morphology is not well understood. Therefore, this dissertation addresses the effects that a spatially- and temporally-restricted application of physiological amounts of NO can have on individual growth cone morphology, on the second messenger pathway that is activated by this application of NO, and on the calcium cascades that result and ultimately affect growth cone morphology.
Helisoma trivolvis, a pond snail, is an excellent model system for this type of research because it has a well-defined nervous system and cultured neurons form large growth cones. In the present study, local application of NO to Helisoma trivolvis B5 neurons results in an increase in filopodial length, a decrease in filopodial number, and an increase in the intracellular calcium concentration ([Ca2+]i). In B5 neurons, the effects of NO on growth cone behavior and [Ca2+]i are mediated via sGC, protein kinase G, cyclic adenosine diphosphate ribose, and ryanodine receptor-mediated intracellular calcium release. This study demonstrates that neuronal growth cone pathfinding in vitro is affected by a single spatially- and temporally-restricted exposure to NO. Furthermore, NO acts via a second messenger cascade, resulting in a calcium increase that leads to cytoskeletal changes. These results suggest that NO may be a signal that promotes appropriate pathfinding and/or target recognition within the developing nervous system. Taken together, these data indicate that NO may be an important messenger during the development of the nervous system in vivo.
