Agonists and Allosteric Modulators of G Protein-Coupled Receptors as Promising Psychotropic Drugs

During the last half a century, a lot of psychoactive agents have been developed and utilized as therapeutic drugs for mental disorders such as schizophrenia, mood disorders, anxiety disorders, and dementia. Based on the major target illnesses or symptoms, they have been classified into antipsychotics, antidepressants, mood stabilizers, anxiolytics, hypnotics, and nootropics. From a pharmacological point of view, it is well established that most of these psychotropic drugs alter neural transmission via classical neurotransmitters, e.g., dopamine, serotonin (5-HT), norepinephrine (NE), glutamate, γ-aminobutyric acid (GABA), and acetylcholine, all of which are implicated in the maintenance and control of higher brain function and human behavior. One major molecular target of these psychotropic drugs is a G protein-coupled receptor (GPCR), at which the neurotransmitter is specifically bound. In most cases, the psychotropic drugs behave as antagonists at the GPCR. For instance, all classical antipsychotics are antagonists of dopamine D₂ receptors. The recent approval and great success of aripiprazole (a partial dopamine D₂ agonist) as an effective antipsychotic drug in many countries, has paved the way for the concept that some GPCR agonists have the potential to treat psychiatric illnesses. Interestingly, the prototypic atypical antipsychotic clozapine (or its active metabolite N-desmethylclozapine) behaves as an agonist at several GPCRs. It is also well known that 5-HT_1A receptor agonists, such as buspirone and tandospirone, are efficacious anxiolytics. Another major progress in psychopharmacology in recent years is the recognition of multiple allosteric sites for many GPCRs, and many novel allosteric modulators of GPCRs have been synthesized. Though still preliminary, many studies have indicated that these allosteric modulators are promising as novel effective psychotropic drugs. In this chapter, the author will provide an update of the recent development of psychoactive agents that act as agonists or allosteric modulators at several GPCR subtypes, which are potentially useful as therapeutic drugs for mental disorders.

Keywords: Antipsychotic, antidepressant, anxiolytic, G protein-coupled receptor (GPCR), agonist, intrinsic activity, antagonist, allosteric modulator, dopamine, serotonin (5-HT), acetylcholine, glutamate.