Heterotropic Cooperativity Between Putative Recognition Sites for Progesterone Metabolites and the Atypical Benzodiazepine Ro 5–4864

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Abstract
The binding of the cage convulsant t-butylbicyclo phosphorothionate (TBPS) and 36CI- 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 γ-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 5αpregnan-3α-ol-20-one (3α-OH-DHP) and 5α-pregnan-3α, 20α-diol(pregnanediol). The pregnanediol effect is completely GABA dependent, whereas that of 3α-OH-DHP is not. Conversely, Ro 5–4864 opposed the action of 3α-OH-DHP by increasing the IC50 for 3α-OH-DHP inhibition of [35S]TBPS binding. In cortical snaptoneurosomes, Ro 5–4864 antag onized both 3α-OH-DHP and pregnanediol enhancement of GABA-stimulated 36CI- uptake. In both binding and functional studies, pregnanediol showed limited efficacy relative to 3α-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 3α-OH-DHP and pregnanediol.