In the first talk of this day, Vincent Rehder talked about the role of nitric oxide (NO) in growth cone development in cultured Heliosoma neurons. We learned that giving NO into the culture medium increased filiopodial elongation within about 5-10min. The molecular pathway for this elongation involved guanulate cyclase which generates cGMP and leads to calcium increase in the neuron. The calcium comes first from the ER and then from calcium channels in the plasma membrane. Another effect of NO is a slowdown of neurite outgrowth in a dose-dependent manner: the more NO the less outgrowth.
NO synthase is only expressed in few neurons in the buccal ganglia and only some neurons express guanylate cyclase. This suggests that not all neurons respond equally to NO signaling. NO signaling in turn takes place locally at the growth cone. This was shown by applying NO only to one of several growth cones of a single neuron and leading to filopodial elongation only in this growth cone. The NO effects were suggested to be physiological by showing the filopodial elongation occurred in response to a single neuron producing NO close to the growth cone. The distance to the growth cone needs to be at least 100 microns or less.
NO application also depolarizes the membrane potential and stimulates spiking activity in the neurons. Any spikes produced after the NO increase are slightly broadened. This is mediated in part by NO decreasing a slow-activating outward current.