Role of peripheral-type benzodiazepine receptors in steroidogenesis

Peripheral-type benzodiazepine (BZ) receptors (PBRs) have been identified in various peripheral tissues as well as in glial cells in in the brain. PBRs are located mainly on the outer mitochondrial membrane and bind with high affinity the BZ Ro 5-4864 (4'-cholorodiazepam) and the non-BZ PK 11195 (an isoquinoline carboxamide derivative), but bind with very low affinity the BZ clonazepam. PBRs have been cloned from various species. PBRs are multimeric receptors composed of the 18-kDa binding site for isoquinolines, the 32-kDa voltage-dependent anion channel, and the 30-kDa adenine nucleotide carrier (which binds BZs). The expression of PBRs is especially high in steroidogenic organs. Steroid administration affects PBR density, whereas depletion of hormones by hypophysectomy in female rats, or castration (surgical or chemical) in male rats, decreases PBR density in endocrine organs, which can be elevated to normal values after administration of the appropriate hormone. PBRs are probably involved in several functions, including cell proliferation, respiration, and steroidogenesis. It has been suggested that PBRs are involved in the translocation of cholesterol from the outer to the inner membrane of the mitochondria and have an effect on the biosynthesis of steroids.