Characterisation of agonist binding on human 5-HT2C receptor isoforms

The 5-HT2C receptor is expressed in different isoforms as a result of mRNA editing. Both INI (unedited) and VSV (a fully edited version) isoforms are abundant in rat brain. The VSV isoform lacks the high affinity recognition site for 5-HT, which may be caused by low efficiency coupling to G-proteins . In this study we have investigated the pharmacology of the agonist bindin g site of these two isoforms of the 5-HT2C receptor. The VSV isoform was ex pressed in Chinese hamster ovary cells (CHO) and the INI isoform in both Ch inese hamster ovary cells and human embryonic kidney cells (HEK-293). Satur ation analysis using [H-3]5-HT revealed high and low affinity recognition s ites on the INI isoform in both cell types whilst the VSV isoform did not h ave the high affinity binding site for [H-3]5-HT. Displacement studies were undertaken using [H-3]5-HT to label the receptors. In these studies the af finity of agonists (5-HT, Ro600175 ((S)-2-(6-Chloro-5-fluoroindol-1-yl)-1-m ethylethylamine), MK212 (6-Chloro-2-(piperazinyl) pyrazine), mCPP (1-(m-chl orophenyl)-piperazine), TfMPP ( N-(m-trifluoromethylphenyl)piperazine). DOI (1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane). DOB (1-(4-bromo-2,5-dimet hoxyphenyl)-2-aminopropane and 8OH-DPAT (8-hydroxy-2-(di-N-propylamino)tetr alin) was higher at the INI isoform, whilst antagonist affinity (ketanserin and mesulergine) did not change between the two receptor isoforms. There w ere no differences between the INI isoform expressed in the CHO and HEK-293 . TI lis suggests that the INI isoform of the 5-HT2C receptor is pharmacolo gically similar to the VSV form of the 5-HT2C receptor but that it couples more efficiently to G-proteins. (C) 2001 Elsevier Science B.V. All rights r eserved.