Pharmacological evidence for transactivation within melatonin MT2 and serotonin 5‐HT2C receptor heteromers in mouse brain

Association of G protein‐coupled receptors into heterodimeric complexes has been reported for over 50 receptor pairs in vitro but functional in vivo validation remains a challenge. Our recent in vitro studies defined the functional fingerprint of heteromers composed of Gi‐coupled melatonin MT2 receptors and Gq‐coupled serotonin 5‐HT2C receptors, in which melatonin transactivates phospholipase C (PLC) through 5‐HT2C. Here, we identified this functional fingerprint in the mouse brain. Gq protein activation was probed by [35S]GTPγS incorporation followed by Gq immunoprecipitation, and PLC activation by determining the inositol phosphate levels in brain lysates of animals previously treated with melatonin. Melatonin concentration‐dependently activated Gq proteins and PLC in the hypothalamus and cerebellum but not in cortex. These effects were inhibited by the 5‐HT2C receptor‐specific inverse agonist SB‐243213, and were absent in MT2 and 5‐HT2C knockout mice, fully recapitulating previous in vitro data and indicating the involvement of MT2/5‐HT2C heteromers. The antidepressant agomelatine had a similar effect than melatonin when applied alone but blocked the melatonin‐promoted Gq activation due to its 5‐HT2C antagonistic component. Collectively, we provide strong functional evidence for the existence of MT2/5‐HT2C heteromeric complexes in mouse brain. These heteromers might participate in the in vivo effects of agomelatine.