Roles Of Gaseous Neuromodulators NO And CO In Determining Neuronal Electrical Activity And Growth Cone Motility

Throughout neuronal development, bouts of spontaneous electrical activity are critical for the proper wiring of neuronal connections. Alterations in firing activity can affect growth cones, which tip developing and regenerating neurites and are responsible for the integration of extracellular guidance cues into pathfinding behaviors. While growing evidence implicates gaseous signaling molecules, nitric oxide (NO) and carbon monoxide (CO), as modulators of neuronal firing activity, less is understood about how they affect growth cone motility. Therefore, in this dissertation, I focus on how NO and CO affect electrical activity of developing and regenerating neurons and how these effects translate into changes at the growth cone level. The specific goals of this dissertation were to investigate 1) the neuron-type-specific effects of NO on growth cone motility; 2) the role of CO in the regulation of neuronal firing activity and excitability; and 3) the role CO plays in the regulation of growth cone motility. Using the well-established developmental model, Helisoma trivolvis, neurons were isolated in single-cell culture allowing for the maximal control over environmental conditions for the direct characterization of NO and CO. In the study of NO, differences in B5 and B19 growth cone responses to NO were due to neuron-type-specific differences in action potential duration. Moreover, the non-responsive B19 growth cones could be made responsive to NO treatment upon the pharmacological broadening of its action potentials. While NO has been found to increase firing activity, the study of CO revealed that CO had the opposite effect on electrical activity, silencing spontaneous firing activity and decreasing neuronal excitability. The study of CO on growth cone motility showed that CO increased growth cone filopodial length through a soluble guanylyl cyclase/protein kinase G/ryanodine receptor mediated pathway without inducing robust increases in growth cone calcium concentration. Taken together, this dissertation reveals new insight into how NO and CO regulate electrical activity and growth cone motility, providing evidence for these gases as important signaling messengers during for the development and regeneration of nervous system