Serotonin regulation of neurite outgrowth in identified neurons from mature and embryonic Helisoma trivolvis.

Neurite outgrowth and growth cone motility are among the many aspects of neuronal development that can be affected by specific neurotransmitters. This was first demonstrated in experiments on identified molluscan neurons that were isolated from mature ganglia and cultured under conditions that promote the regeneration of new neurites. The application of serotonin to a regenerating Helisoma neuron B19 produced an abrupt, reversible cessation of neurite outgrowth and growth cone motility. While this type of response would subsequently be demonstrated for other neurons and neurotransmitters in many different invertebrate and vertebrate species, experiments on Helisoma neurons have continued to play a pivotal role in advancing this field. In this paper, the mechanisms and sites of serotonin action and how these responses are manifested in vivo during embryonic development are discussed. Experiments primarily on neuron B19 have shown that serotonin acts on a novel serotonin receptor that is coupled to the elevation of cyclic AMP. This intracellular messenger directly activates a class of cyclic-nucleotide-gated sodium channels, leading to sodium influx, membrane depolarization, and activation of voltage-gated calcium channels. The resulting elevation of intracellular calcium acts through a calcium/calmodulin-dependent pathway to inhibit neurite outgrowth and growth cone motility. Although the final steps have yet to be completely resolved, they undoubtedly involve calcium-dependent regulation of cytoskeletal components. Regarding the sites of serotonin action, serotonin responses have been localized to growth cones and even filopodia in specific neurons. However, some studies suggest that neurite development may actually be regulated by serotonin in a paracrine, non-localized manner in a surprisingly large percentage of Helisoma neurons. Finally, experiments on Helisoma embryos have investigated how serotonin actually regulates the in vivo development of specific neurons. Pharmacological treatments that reduce the serotonin concentration in embryos affected the neurite morphology and synaptic efficacy of neuron B19 and the amount of neurite branching in embryonic neuron C1. All of these responses were consistent with the primary action of serotonin being the inhibition of neurite outgrowth, as predicted by the original cell culture studies.