Allopregnanolone-stimulated GABA-mediated chloride ion flux is inhibited by 3β-hydroxy-5α-pregnan-20-one (isoallopregnanolone)

Abstract

Altered gamma-aminobutyric acid (GABA)-ergic function is associated with neurological and psychiatric disorders. Certain progesterone metabolite 3α-hydroxy-5α-pregnan-20-one, or allopregnanolone (ALLO), increases the GABA-mediated chloride ion (Cl −) flux through GABA A receptors in a similar fashion as benzodiazepines and barbiturates. We have studied the effect of its 3β diastereomer, 3β-hydroxy-5α-pregnan-20-one, or isoallopregnanolone (ISO), on the Cl − flux and investigated the interaction between ISO and ALLO on GABA-mediated Cl − uptake in cortical homogenates from adult male Wistar rats. We found that ISO from 1 μM to 1 mM does not affect baseline Cl − uptake in rat cortical homogenates. Neither does ISO at dose range of 100 nM to 100 μM interact with 10 μM GABA in the Cl − uptake assay. In addition, ISO at the dose range of 1–30 μM does not affect flunitrazepam and pentobarbital-induced increase of Cl − uptake. We conclude that ISO selectively inhibits the ALLO-induced Cl − uptake with respect to ALLO concentrations. The IC 50 of ISO inhibition on 1 μM ALLO-induced Cl − uptake was calculated to be 12.25 μM. On the other hand, we have studied the effect of 30 μM ISO on ALLO (0.01 nM to 1 μM) induced displacement of tert-butylbicyclophosphorothionate (TBPS) binding. We did not note any interaction between ALLO and ISO on TBPS binding assay. These results indicate that ISO may be useful functional blocker of GABA A receptor potentiating steroid ALLO when used at concentrations that do not affect baseline GABAergic neurotransmission.