Dopamine requires unique residues to signal via the serotonin 2A receptor

Serotonin is an important neurotransmitter and neuromodulator. Disruption of the serotonergic system has been implicated in various psychiatric disorders such as schizophrenia and bipolar disorder. Most of the drugs targeting these neurotransmitter systems are classified primarily as agonists or inverse agonists/antagonists, with their described function being limited to activating the canonical signaling pathway(s), or inhibiting the pathway(s) respectively. Previous work with the human 5-HT_2A has shown the receptor to be activated by dopamine, also an endogenous ligand. Dopamine is the cognate ligand of the dopaminergic system, which significantly overlaps with the serotonergic system in the brain. The two systems innervate many of the same brain areas, and the central serotonergic systems also regulate dopamine functions. Our aim was to investigate the downstream signaling set up by the receptor on being activated by dopamine. We show that dopamine is a functionally selective ligand at 5-HT_2A and have examined dopamine as a ligand with respect to some receptor-dependent phenotypes. Our results show that dopamine acts as an agonist at the human serotonin 2A receptor and brings about its activation and internalization. Using in vitro assays, we have established differences in the signaling pathways set up by dopamine as compared to serotonin. Using site-specific mutagenesis we have identified residues important for this functional selectivity, shown by dopamine at this receptor. Our identification of specific residues important in the functional selectivity of dopamine at 5-HT_2A could have far reaching implications for the field of GPCR signaling and drug-design. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.