Serotonin 5-HT2C receptor RNA editing alters receptor basal activity:: Implications for serotonergic signal transduction

Rat and human serotonin 5-HT2C receptor isoforms were evaluated for agonist-independent activation of inositol phosphate production in COS-7 cells. The nonedited isoform (5-HT2C-INI) displayed the greatest basal activity, stimulating inositol phosphate production fourfold over the fully edited isoform (5-HT2C-VGV). All of the other isoforms tested displayed intermediate levels of basal activity. Decreasing receptor expression levels by 50% produced a parallel decrease in basal activity. 5-HT stimulated inositol phosphate production twofold over basal levels through the 5-HT2C-INI receptor and eightfold over basal levels through the 5-HT2C-VGV receptor but produced similar maximal levels of inositol phosphate, 5-HT competition for [H-3]mesulergine binding to 5-HT2C-INI best fit a two-site analysis with K-H = 7.6 nM and K-L = 160 nM, whereas 5-HT2C-VGV best fit a one-site model with K-i = 163 nM. [H-3]5-HT labeled 36% of the total population of 5-HT2C-INI receptors labeled by [H-3]mesulergine but only 12% of 5-HT2C-VGV receptors. [H-3]5-HT K-D values increased from 5.1 nM for 5-HT2C-INI to 20 nM for 5-HT2C-VGV. [H-3]Mesulergine K-D values were the same for both isoforms, 5-HT EC50 values for inositol phosphate production increased from 6.1 nM for 5-HT2C-INI to 30 nM for 5-HT2C-VGV. These results demonstrate that RNA editing decreases 5-HT2C receptor basal activity, agonist affinity, and potency, indicating that RNA editing may play a role in regulating serotonergic signal transduction and response to drug therapy.