Characterization of Novel Serotonin Metabolites in Selected Model Neuronal Systems

Abstract

Novel 5-HT pathways were identified in Aplysia and Pleurobranchaea californica and Rattus norvegicus, via a capillary electrophoresis system with wavelength-resolved fluorescence detection. This method allows sensitive indole detection from samples as small as single cells. Compounds that natively fluoresce are identified based on their emission spectra and electrophoretic migration time. Furthermore, tryptophan is sensitively detected, and its detection can provide information on the integrity of single peptidergic vesicles, helping to determine whether peptide neurotransmitters are aggregated in a solid or dissolved state. Using the CE system, several indole peaks were observed from single serotonergic invertebrate cells that did not match known 5-HT metabolites but possessed similar fluorescence emission properties. As a result, 5-HT metabolism was surveyed across a variety of tissue types, which demonstrated that metabolism differs even in physically adjacent tissues. The molecules, which include 5-HT-sulfate and gamma-glutamyl-5-HT-sulfate, were characterized by HPLC and ESI-MS. 5-HT-sulfate has been identified in the circulation of Pleurobranchaea californica, and its concentration oscillates with the animal's circadian rhythm, suggesting potential biological significance for this metabolite. Moreover, these metabolites can be produced from mammalian enzymes, suggesting that they may form in the mammalian CNS. Recent results have suggested that novel products can form from the aldehyde derived from serotonin oxidation via MAO. In addition, the potential exists for nitric oxide, a freely diffusible gaseous neurotransmitter, to react with 5-HT in the synapse and modify the resulting signal. Elucidation of novel 5-HT pathways in a location-dependent manner is useful for the development of pharmaceuticals intended to preserve serotonergic signaling.