Heterotropic cooperativity between putative recognition sites for progesterone metabolites and the atypical benzodiazepine Ro 5-4864.
The binding of the cage convulsant t-butylbicyclophosphorothionate (TBPS) and 36Cl- uptake by synaptoneurosomes were used to test the ability of progesterone metabolites to modulate allosterically the Ro 5-4864 (4'-chlorodiazepam) binding site that is functionally coupled to the gamma-aminobutyric acid (GABA)/benzodiazepine receptor complex (GBRC) in rat brain. Dose-dependent enhancement of [35S]TBPS binding by Ro 5-4864 occurs in rat cerebral cortex in the presence of the progesterone metabolites 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) and 5 alpha-pregnan-3 alpha, 20 alpha-diol (pregnanediol). The pregnanediol effect is completely GABA dependent, whereas that of 3 alpha-OH-DHP is not. Conversely, Ro 5-4864 opposed the action of 3 alpha-OH-DHP by increasing the IC50 for 3 alpha-OH-DHP inhibition of [35S]TBPS binding. In cortical synaptoneurosomes, Ro 5-4864 antagonized both 3 alpha-OH-DHP and pregnanediol enhancement of GABA-stimulated 36Cl- uptake. In both binding and functional studies, pregnanediol showed limited efficacy relative to 3 alpha-OH-DHP, as previously reported. These findings provide the initial evidence that the GBRC-linked Ro 5-4864 binding site is allosterically coupled to the putative progesterone metabolite recognition site and confirm the GABA-mimetic properties of 3 alpha-OH-DHP and pregnanediol.
Belelli
D
D
Division of Biological Sciences, School of Pharmacy, University of Southern California, Los Angeles 90033.
McCauley
L
L
Gee
K W
KW
D000818:Animals; D001570:Benzodiazepinones / Q000494:pharmacology*; D001665:Binding Sites; D001643:Bridged Bicyclo Compounds / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002540:Cerebral Cortex / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism; D003292:Convulsants; D008297:Male; D011276:Pregnanediol / Q000494:pharmacology; D011280:Pregnanolone / Q000494:pharmacology; D011374:Progesterone / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D001570:Benzodiazepinones; D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002712:Chlorides; D003292:Convulsants; C028513:4'-chlorodiazepam; D011374:Progesterone; D005680:gamma-Aminobutyric Acid; C037476:tert-butylbicyclophosphorothionate; D011280:Pregnanolone; D011276:Pregnanediol