title: 5-Hydroxytryptamine (5-HT) cellular sequestration during chronic exposure delays 5-HT3 receptor resensitization due to its subsequent release.
abstract: 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.
authors.affiliation: Medical Research Institute, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom.
authors.forename: J Daniel
authors.initials: JD
authors.lastname: Hothersall
authors.affiliation: Medical Research Institute, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom.
authors.forename: Amy
authors.initials: A
authors.lastname: Alexander
authors.affiliation: Medical Research Institute, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Samson
authors.affiliation: Medical Research Institute, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom.
authors.forename: Christopher
authors.initials: C
authors.lastname: Moffat
authors.affiliation: Medical Research Institute, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom.
authors.forename: Karen A
authors.initials: KA
authors.lastname: Bollan
authors.affiliation: Medical Research Institute, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom. Electronic address: c.n.connolly@dundee.ac.uk.
authors.forename: Christopher N
authors.initials: CN
authors.lastname: Connolly
mesh_terms: D000818:Animals; D001665:Binding Sites; D019556:COS Cells; D002522:Chlorocebus aethiops; D003072:Cognition Disorders / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D005260:Female; D005767:Gastrointestinal Diseases / Q000378:metabolism; D006168:Guinea Pigs; D007082:Ileum / Q000378:metabolism; D020128:Inhibitory Concentration 50; D008297:Male; D009119:Muscle Contraction; D044406:Receptors, Serotonin, 5-HT3 / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology*; D015398:Signal Transduction
chemical_list: D044406:Receptors, Serotonin, 5-HT3; D012701:Serotonin
keywords: 5-HT3; Intestine; Ion Channel; Membrane Trafficking; Pharmacology; Receptor Desensitization; Receptor Internalization; Selective Serotonin Reuptake Inhibitor (SSRI); Serotonin; Serotonin Transporter