5-Hydroxytryptamine (5-HT) Cellular Sequestration during Chronic Exposure Delays 5-HT3 Receptor Resensitization due to Its Subsequent Release

• Published in: The Journal of biological chemistry Vol. 289; no. 46; pp. 32020 - 32029
• Main Authors: Hothersall, J. Daniel, Alexander, Amy, Samson, Andrew J., Moffat, Christopher, Bollan, Karen A., Connolly, Christopher N.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 14.11.2014; American Society for Biochemistry and Molecular Biology
• Subjects: Animals; Binding Sites; Chlorocebus aethiops; Cognition Disorders - metabolism; COS Cells; Cytoplasm - metabolism; Cytosol - metabolism; Female; Gastrointestinal Diseases - metabolism; Guinea Pigs; Ileum - metabolism; Inhibitory Concentration 50; Male; Membrane Biology; Muscle Contraction; Receptors, Serotonin, 5-HT3 - metabolism; Serotonin - pharmacology; Signal Transduction; Membrane Trafficking; Selective Serotonin Reuptake Inhibitor (SSRI); Serotonin; Intestine; Receptor Internalization; Pharmacology; Receptor Desensitization; Serotonin Transporter; Ion Channel; 5-HT3
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M114.594796

The serotonergic synapse is dynamically regulated by serotonin (5-hydroxytryptamine (5-HT)) with elevated levels leading to the down-regulation of the serotonin transporter and a variety of 5-HT receptors, including the 5-HT type-3 (5-HT3) receptors. We report that recombinantly expressed 5-HT3 receptor binding sites are reduced by chronic exposure to 5-HT (IC50 of 154.0 ± 45.7 µm, t½ = 28.6 min). This is confirmed for 5-HT3 receptor-induced contractions in the guinea pig ileum, which are down-regulated after chronic, but not acute, exposure to 5-HT. The loss of receptor function does not involve endocytosis, and surface receptor levels are unaltered. The rate and extent of down-regulation is potentiated by serotonin transporter function (IC50 of 2.3 ± 1.0 µm, t½ = 3.4 min). Interestingly, the level of 5-HT uptake correlates with the extent of down-regulation. Using TX-114 extraction, we find that accumulated 5-HT remains soluble and not membrane-bound. This cytoplasmically sequestered 5-HT is readily releasable from both COS-7 cells and the guinea pig ileum. Moreover, the 5-HT level released is sufficient to prevent recovery from receptor desensitization in the guinea pig ileum. Together, these findings suggest the existence of a novel mechanism of down-regulation where the chronic release of sequestered 5-HT prolongs receptor desensitization.