Agonist-Trafficking and Hallucinogens

G protein-coupled receptors (GPCRs) are the most common target for therapeutic drugs. The traditional ternary complex model, where receptors shift between active and inactive states, has been revised because different agonists can activate distinct signaling pathways from the same receptor. This agonist-trafficking model proposes that agonists stabilize unique receptor conformations that preferentially trigger specific pathways. Hallucinogenic drugs like LSD, psilocybin, and mescaline, which act on serotonin 5-HT2A receptors, offer a useful system to study this phenomenon. Non-hallucinogenic chemicals like lisuride show similar in vitro activity at the same receptor but do not induce hallucinogenic effects, highlighting unresolved questions about how agonist-trafficking determines behavioral outcomes.

Study at a glance

Abstract

Seven transmembrane domain receptors, also termed G protein-coupled receptors (GPCRs), represent the most common molecular target for therapeutic drugs. The generally accepted pharmacological model for GPCR activation is the ternary complex model, in which GPCRs exist in a dynamic equilibrium between the active and inactive conformational states. However, the demonstration that different agonists sometimes elicit a different relative activation of two signaling pathways downstream of the same receptor has led to a revision of the ternary complex model. According to this agonist- trafficking model, agonists stabilize distinct activated receptor conformations that preferentially activate specific signaling pathways. Hallucinogenic drugs and non-hallucinogenic drugs represent an attractive experimental system with which to study agonist-trafficking of receptor signaling. Thus many of the behavioral responses induced by hallucinogenic drugs, such as lysergic acid diethylamide (LSD), psilocybin or mescaline, depend on activation of serotonin 5-HT(2A) receptors (5-HT2ARs). In contrast, this neuropsychological state in humans is not induced by closely related chemicals, such as lisuride or ergotamine, despite their similar in vitro activity at the 5-HT2AR. In this review, we summarize the current knowledge, as well as unresolved questions, regarding agonist-trafficking and the mechanism of action of hallucinogenic drugs.