RNA-editing of the 5-HT_2C receptor alters agonist-receptor-effector coupling specificity

1. The serotonin_2C (5-HT_2C) receptor couples to both phospholipase C (PLC)-inositol phosphate (IP) and phospholipase A₂ (PLA₂)-arachidonic acid (AA) signalling cascades. Agonists can differentially activate these effectors (i.e. agonist-directed trafficking of receptor stimulus) perhaps due to agonist-specific receptor conformations which differentially couple to/activate transducer molecules (e.g. G proteins). Since editing of RNA transcripts of the human 5-HT_2C receptor leads to substitution of amino acids at positions 156, 158 and 160 of the putative second intracellular loop, a region important for G protein coupling, we examined the capacity of agonists to activate both the PLC-IP and PLA₂-AA pathways in CHO cells stably expressing two major, fully RNA-edited isoforms (5-HT_2C-VSV, 5-HT_2C-VGV) of the h5-HT_2C receptor. 2. 5-HT increased AA release and IP accumulation in both 5-HT_2C-VSV and 5-HT_2C-VGV expressing cells. As expected, the potency of 5-HT for both RNA-edited isoforms for both responses was 10 fold lower relative to that of the non-edited receptor (5-HT_2C-INI) when receptors were expressed at similar levels. 3. Consistent with our previous report, the efficacy order of two 5-HT receptor agonists (TFMPP and bufotenin) was reversed for AA release and IP accumulation at the non-edited receptor thus demonstrating agonist trafficking of receptor stimulus. However, with the RNA-edited receptor isoforms there was no difference in the relative efficacies of TFMPP or bufotenin for AA release and IP accumulation suggesting that the capacity for 5-HT_2C agonists to traffic receptor stimulus is lost as a result of RNA editing. 4. These results suggest an important role for the second intracellular loop in transmitting agonist-specific information to signalling molecules.