Stimulation of Brain Pregnenolone Synthesis by Mitochondrial Diazepam Binding Inhibitor Receptor Ligands In Vivo

Abstract

Evidence that neurosteroids are potent modulators of the action of GABA at GABAA receptors has prompted the investigation of the mechanism that controls brain neurosteroid synthesis by glial cell mitochondria in vivo. In vitro studies suggest that the interaction of the diazepam binding inhibitor (DBI)--a polypeptide that is abundant in steroidogenic cells--with glial mitochondrial DBI receptors (MDRs) is a crucial step in the physiological regulation of neurosteroid biosynthesis. MDRs bind 4'-chlorodiazepam (4'-CD), N,N-di-n-hexyl-2-(4-fluorophenyl)-indol-3-acetamide (FGIN-1-27), and the isoquinoline carboxamide PK 11195 with high affinity, and these ligands have been used to investigate whether the stimulation of glial MDRs increases brain pregnenolone production in vivo. Adrenalectomized and castrated (A-C) male rats (to eliminate peripheral sources of pregnenolone) were pretreated with trilostane (to prevent pregnenolone metabolism to progesterone), and the pregnenolone content in brain regions dissected after fixation with a 0.8-s exposure to microwave irradiation focused to the head was determined by HPLC followed by specific radioimmunoassay. The forebrain and cerebellum of A-C rats contained 4-7 ng of pregnenolone/g of tissue, and the olfactory bulb contained 10-14 ng/g. These concentrations of brain pregnenolone are only 30-40% lower than those of sham-operated rats. In contrast, the plasma pregnenolone content of sham-operated rats was 2-3 ng/ml, but it was only 0.1…