STIMULATION OF BRAIN PREGNENOLONE SYNTHESIS BY MITOCHONDRIAL DIAZEPAM-BINDING INHIBITOR RECEPTOR LIGANDS IN-VIVO

Evidence that neurosteroids are potent modulators of the action of GAB A at GABA(A) 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 diazepa m binding inhibitor (DBI)-a polypeptide that is abundant in steroidoge nic cells-with glial mitochondrial DBI receptors (MDRs) is a crucial s tep in the physiological regulation of neurosteroid biosynthesis. MDRs bind 4'-chlorodiazepam (4'-CD), ,N-di-n-hexyl-2-(4-fluorophenyl)-indo l-3-acetamide (FGIN-1-27), and the isoquinoline carboxamide PK 11195 w ith high affinity, and these ligands have been used to investigate whe ther the stimulation of glial MDRs increases brain pregnenolone produc tion in vivo. Adrenalectomized and castrated (A-C) male rats (to elimi nate peripheral sources of pregnenolone) were pretreated with trilosta ne (to prevent pregnenolone metabolism to progesterone), and the pregn enolone 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 cereb ellum of A-C rats contained-4-7 ng of pregnenolone/g of tissue, and th e olfactory bulb contained 10-14 ng/g. These concentrations of brain p regnenolone 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 0.15-0.20 ng/ml in the plasma of A-C rats. I n A-C rats, treatment with the MDR ligands 4'-CD and FGIN-1-27 increas ed the pregnenolone content in the brain but failed to change the plas ma or peripheral tissue content of this steroid. The effect of 4'-CD o n brain pregnenolone content was maximal (70-1 00% increase) at the do se of 18 mumol/kg, 5-1 0 min after intravenous injection. The effect o f oral administration of FGIN-1-27 on brain pregnenolone content was m aximal (80-150% increase) at doses of 400-800, mumol/kg and peaked at approximately 1 h. That this effect of FGIN-127 was mediated by the MD R was documented by pretreatment with the MDR partial agonist PK 11195 (100 mumol/kg, i.p.). PK 11195 did not affect basal brain pregnenolon e content but prevented the accumulation of brain pregnenolone induced by FGIN-1-27. FGIN-1-27 and 4'-CD failed to increase the brain concen tration of dehydro-epiandrosterone in A-C rats. These data suggest tha t glial cell MDRs play a role in neurosteroid biosynthesis in vivo.