A Nonsteroidal Reversal Agent Inhibits Allopregnanolone Modulation of α1β3δ GABAA Receptors

The neurosteroid allopregnanolone is a positive allosteric modulator of GABA(A) receptors, which has proved beneficial in the treatment of major depressive disorder and epilepsies. It also has a role in treating the mood swings that are associated with fluctuations in its level during the menstrual cycle. Nonetheless, a subset of women do not tolerate high levels of allopregnanolone. Iso-allopregnanolone, a negative allosteric modulator, as well as synthetic steroid antagonists are used to treat such conditions. However, steroid-based medications are difficult to deliver and their specificity of action can be unclear. Recently introduced novel nonsteroidal agents that, like iso-allopregnanolone, can reverse the action of positive allosteric modulators without changing the positive action of GABA, might provide an alternative. We surveyed a number of them on human 1{beta}3{delta} GABAARs using a [3H]muscimol binding assay. A 6-membered ring spiro-hydantoin, DKD99, allosterically reversed the positive allosteric action of allopregnanolone over a wide concentration range (6 to 1,000 nM). DKD99 shifted allopregnanolones modulation curve 10-fold to the right. Furthermore, it has a much lower affinity when exerting similar actions on 1{beta}3{gamma}2 receptors. Agents such as this have utility for elucidating underlying mechanisms and may offer an alternative pathway for the development of nonsteroidal therapies against the positive allosteric modulatory actions of neurosteroids.