In vivo regulation of peripheral-type benzodiazepine receptor and glucocorticoid synthesis by Ginkgo biloba extract EGb 761 and isolated ginkgolides.

Glucocorticoid excess has broad pathogenic potential including neurotoxicity, neuroendangerment, and immunosuppression. Glucocorticoid synthesis is regulated by ACTH, which acts by accelerating the transport of the precursor cholesterol to the mitochondria where steroidogenesis begins. Ginkgo biloba is one of the most ancient trees, and extracts from its leaves have been used in traditional medicine. A standardized extract of Ginkgo biloba leaves, termed EGb 761 (EGb), has been shown to have neuroprotective and antistress effects. In vivo treatment of rats with EGb, and its bioactive components ginkgolide A and B, specifically reduces the ligand binding capacity, protein, and messenger RNA expression of the adrenocortical mitochondrial peripheral-type benzodiazepine receptor (PBR), a key element in the regulation of cholesterol transport, resulting in decreased corticosteroid synthesis. As expected, the ginkgolide-induced decrease in glucocorticoid levels resulted in increased ACTH release, which in turn induced the expression of the steroidogenic acute regulatory protein. Because ginkgolides reduced the adrenal PBR expression and corticosterone synthesis despite the presence of high levels of steroidogenic acute regulatory protein, these data demonstrate that PBR is indispensable for normal adrenal function. In addition, these results suggest that manipulation of PBR expression could control circulating glucocorticoid levels, and that the antistress and neuroprotective effects of EGb are caused by to its effect on glucocorticoid biosynthesis.