Effect of synthetic steroids on GABAA receptor binding in rat brain

- GABAA receptor binding was tested using synthetic steroids with A/B ring low flexibility.
- Three ligands for specific binding sites were tested by quantitative in vitro autoradiography.
- Natural and synthetic steroids show different effect on the cerebral cortex and hippocampus.
- The 3α-hydroxy-6,19-epoxypregn-4-ene-20-one steroid may be used as allopregnanolone agonist.

Neuroactive steroids, like allopregnanolone (A) and pregnanolone (P), bind to specifics sites on the GABAA receptor complex and modulate receptor function. They are capable to inhibit or stimulate the binding of GABAA receptor-specific ligands, like t-butyl-bicyclophosphorothionate, flunitrazepam and muscimol. We have previously characterized a set of oxygen-bridged synthetic steroids (SS) analogs to A or P using synaptosomes. Considering that the subunit composition of the GABAA receptor throughout the central nervous system affects the magnitude of the modulation of the GABAA receptor by NAS, we evaluated the action of two selected SS, in brain sections containing the cerebral cortex (CC) and hippocampus (HC) using quantitative receptor autoradiography. Both SS affected the binding of the three ligands in a similar way to A and P, with some differences on certain CC layers according to the ligand used. One of the SS, the 3α-hydroxy-6,19-epoxypregn-4-ene-20-one (compound 5), behaved similarly to the natural neuroactive steroids. However, significant differences with compound 5 were observed on the HC CA2 region, making it steroid suitable for a specific action. Those differences may be related to structural conformation of the SS and the subunits' composition present on the receptor complex.