Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D(3) Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D(2) receptors (D(2)Rs) and D(3) receptors (D(3)Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D(2)Rs and D(3)Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D(3)Rs and D(2)Rs, only very few can significantly occupy D(3)Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D(3)Rs receptor affinity and selectivity as determinant factors for in vivo D(3)Rs occupancy by antipsychotics, are also discussed.