Translocator Protein as a Promising Target for Novel Anxiolytics

Neurosteroids are able to rapidly control the excitability of the central nervous system, acting as regulators of type A receptors for GABA. Over the last two decades, many authors have confirmed that neurosteroid level alterations occur in psychiatric disorders, including anxiety disorders. More recently, investigators have manipulated neurosteroidogenesis in an effort to correct neuronal excitation and inhibition imbalances, which may lie at the root of neuropsychiatric conditions. In line with this strategy, emerging data have demonstrated that a promising target for therapy of anxiety disorders is the Translocator Protein (TSPO). TSPO is a five transmembrane domain protein (18 kDa) that is expressed predominantly in steroid-synthesizing tissues. At the subcellular level, TSPO is localized at contact sites between the outer and inner mitochondrial membranes and mediates the rate-limiting step of neurosteroidogenesis. Brain concentrations of neurosteroids can be affected by selective TSPO activation. Indeed, TSPO drug ligands are able to stimulate the primary neurosteroid formations that enhance GABA(A) receptor activity, pregnenolone and allopregnenalone, both in in vitro steroidogenic cells and in vivo animal models. A spectrum of TSPO ligands has been shown to exert anxiolytic actions when administered in rodents. Some TSPO drug ligands could potentially reach clinical development. For example, recent evidence has shown that the selective TSPO ligand, XBD173 (AC-5216, Emapunil), exerts anxiolytic effects not only in animal models, but also in human volunteers. Herein, we review the current literature regarding the central nervous system biology of TSPO, a promising molecular target, in combination with its available ligands.