5-HT Cellular Sequestration During Chronic Exposure Delays 5-HT3 Receptor Resensitization Due to its Subsequent Release. [Neurobiology]

October 3rd, 2014 by Hothersall, J. D., Alexander, A., Samson, A. J., Moffat, C., Bollan, K. A., Connolly, C. N.

The serotonergic synapse is dynamically regulated by serotonin (5-HT) with elevated levels leading to the downregulation of the serotonin transporter (SERT) and a variety of 5-HT receptors, including the 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, t1/2 = 28.6 min). This is confirmed for 5-HT3 receptor-induced contractions in the guinea pig ileum which are downregulated following 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 downregulation is potentiated by SERT function (IC50 of 2.3 ± 1.0 μM, t1/2 = 3.4 min). Interestingly, the level of 5-HT uptake correlates with the extent of downregulation. 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 downregulation where the chronic release of sequestered 5-HT prolongs receptor desensitization.