SEROTONIN ACTIVATION OF A CYCLIC-AMP-DEPENDENT SODIUM CURRENT IN AN IDENTIFIED NEURON FROM HELISOMA-TRIVOLVIS

The mechanisms by which neurotransmitters regulate neurite extension a nd growth cone motility have been extensively studied using identified Helisoma neurons regenerating in cell culture. Specific neurons, such as buccal neuron B19, display a complex response to the addition of 5 -HT involving the generation of action potentials, influx of extracell ular calcium, and cessation of neurite extension and growth cone motil ity. While several studies have addressed the role of calcium in this cascade, little is known about the mechanism underlying the 5-HT-induc ed excitation of neuron B19. Therefore, we have begun to characterize the ion currents, receptors, and second messengers involved in the 5-H T-dependent depolarization of B19. Exposure of B19 to 5-HT resulted in the activation of a maintained inward current. Ion substitution exper iments revealed that this current was carried mainly by sodium ions. T he use of 8-bromo-cAMP, forskolin, or the phosphodiesterase inhibitor isobutyl methylxanthine (IBMX) to increase intracellular cAMP levels a ll resulted in inward current activation in the absence of 5-HT. Moreo ver, preloading the neuron with 8-bromo-cAMP was sufficient to prevent further current activation by 5-HT. In addition, the IBMX-activated c urrent was greatly enhanced when induced in the presence of 5-HT. Prot ein kinase inhibitors failed to prevent 5-HT activation of sodium curr ent, suggesting that cAMP may directly activate the current, independe nt of phosphorylation. Pharmacological experiments showed the B19 5-HT receptor has an EC50 of approximately 10(-7) m and can be activated b y various indole analogs of 5-HT. Furthermore, methysergide displayed partial agonist activity. These results have identified receptors and ion currents that may play crucial roles in the regulation of nervous system development during embryogenesis in Helisoma.