Fig. 1: Conformational states of the 5-HT_2AR.

The receptor conformation coupled to G-protein signalling pathways represents the functional state and is preferentially identified by agonist radioligands such as [³H]LSD. The inactive conformation of the 5-HT_2AR, which is uncoupled from G proteins, is preferentially labelled by inverse agonist radioligands such as [¹⁸F]altanserin. Active and inactive conformations are interchangeable states that stay in equilibrium¹⁷. Antagonist radioligands, such as [³H]MDL100907, bind both receptor conformations and, therefore, do not distinguish between functional and non-functional states.