5-HT2A receptors: Pharmacology and functional selectivity

Abstract

Serotonin 5-HT_2A receptors were one of the first serotonin receptors to be pharmacologically characterized. In mammals, they are expressed throughout the body in nearly every cell and tissue type, with the highest density in cortical layer V of the brain. They are involved in several aspects of normal physiological processes and behaviors and have been implicated in the etiology of neuropsychiatric diseases such as schizophrenia. Atypical antipsychotics have targeted blockade of 5-HT_2A receptors as part of their therapeutic mechanism. More recently, 5-HT_2A receptors have come to prominence for their role as the primary target for psychedelic drugs, which activate this receptor subtype to produce their characteristic behavioral effects. 5-HT_2A receptor agonists like psilocybin, dimethyltryptamine, and lysergic acid diethylamide have each demonstrated long-lasting therapeutic efficacy in clinical trials for psychiatric disorders such as major depression and substance use disorders. There is a significant effort in both academia and industry to develop new agonists of 5-HT_2A receptors with therapeutic efficacy. There are 3 primary scaffolds for agonists: tryptamines, ergolines, and phenylalkylamines, each engaging different subsets of amino acid residues in the receptor binding pocket. Differences can lead to differential responses between ligands for functionally selective outcomes. Here, we provide a historical perspective on 5-HT_2A receptors, their key structural features and motifs involved in ligand-receptor interactions, and how these interactions can affect signaling pathways downstream of the receptor. Understanding how ligands interact with the 5-HT_2A receptor will fundamentally inform future drug discovery to optimize therapeutics for a variety of disorders. SIGNIFICANCE STATEMENT: Psychedelic drugs have demonstrated long-lasting therapeutic efficacy for several conditions in multiple clinical trials. Their target, serotonin 5-HT_2A receptors, are GPCRs with complex pharmacology. Having knowledge of how ligands interact with 5-HT_2A receptors in the orthosteric binding pocket at the structural level to induce specific signal transduction pathways will inform on efforts to design and develop functionally selective drugs to potentially treat a variety of diseases.

Graphical abstract

Fig. 1

Alignment of 5-HT_2A receptors across commonly used research species. The protein sequences of 5-HT_2A receptors aligned using the clustal algorithm in UGENE software (Unipro), along with phylogenetic tree generated from the alignment with the same software. There is a high degree of homology between species. A serine at residue 5.46(242) confers an order of magnitude better affinity and potency for agonists. Note that the most common research animals (rodents) have an alanine at this position.

Fig. 2

Alignment of human 5-HT₂ receptor sequences. Key amino acid residues involved in ligand binding as described in the main text are highlighted. Of note, only 5-HT_2A has a serine at position 5.46(242), whereas 5-HT_2B/C each have an alanine. Shaded boxes represent TM domains I–VII, the unshaded box represents alpha helix IIX.

Fig. 3

Structural rearrangements of the helices between the active and inactive state for the 5-HT_2A receptor. The gold structure represents the active, and the blue the inactive conformation (cross-eyed stereo view) of the 5-HT_2A receptor based upon published crystal structures. The active conformation has outward displacements of TM5 and TM6, and a slight inward movement of TM7.

Fig. 4

5-HT_2A receptor with the agonist ligand 25CN-NBOH interacts with D155^3.32. The side chain of D3.32(155) forms a salt bridge with the nitrogen on the side chain of 25CN-NBOH. This is a common anchor formed between this residue and the nitrogen of ligands. The toggle switch W6.48(336) is also shown (cross eyed-stereo view).

Fig. 5

LSD bound to the 5-HT_2A receptor. Cross-eyed stereo view of LSD (green) in the OBP of the 5-HT_2A receptor. The “lid” structure from ECL2 (orange) can be seen capping the opening to the binding pocket, with D3.32(155), S5.46(242), and W6.48(336) highlighted.

Fig. 6

Comparison of the calculated HOMO energies for the aromatic system of a potent 5-HT_2A agonist ligand and for the aromatic system of serotonin. Calculations are for the ground state in vacuo. The side chains have been truncated to methyl groups to simplify calculation. The calculations were performed in the Spartan ’24 software (Wavefunction, Inc) using Hartree-Fock 6-31G∗ potentials.

Fig. 7

Risperidone binding the 5-HT_2A receptor. The antagonist risperidone is shown in relation to key residues in the OBP including W6.48(336). (cross-eyed stereo view).

Fig. 8

Additional interactions between the 5-HT_2A receptor and 25CN-NBOH. In the receptor, S3.36(159) hydrogen bonds with 25CN-NBOH at 2 different locations on the ligand (cross-eyed stereo view).

Fig. 9

Known signaling pathways downstream of 5-HT_2A receptors. Several pathways including canonical Gαq and noncanonical β-arrestin can be activated by ligands interacting with the receptor. These can initiate a variety of physiological changes within cells ranging from changes in membrane polarity to gene expression. Only some of the known signaling pathways downstream of 5-HT_2A receptor activation are shown here.