Effects on gamma-aminobutyric acid (GABA)(A) receptors of a neuroactive steroid that negatively modulates glutamate neurotransmission and augments GABA neurotransmission

Neurosteroids positively and negatively modulate gamma -aminobutyric acid ( GABA)(A) receptors and glutamate receptors, which underlie most fast inhibi tion and excitation in the central nervous system. We report the identifica tion of a neuroactive steroid, (3 alpha ,5 beta)-20-oxo-pregnane-3-carboxyl ic acid (3 alpha5 beta PC), with unique cellular actions. 3 alpha5 beta PC positively modulates GABAA receptor function and negatively modulates N-met hyl-D-aspartate (NMDA) receptor function, a combination that may be of part icular clinical benefit. 3 alpha5 beta PC promotes net GABA(A) potentiation at low steroid concentrations (<10 <mu>M) and at negative membrane potenti als. At higher concentrations, the steroid also blocks GABA receptors. Beca use this block would presumably counteract the NMDA receptor blocking actio ns of 3 alpha5 beta PC, we characterize the GABA receptor block in some det ail. Agonist concentration, depolarization, and high extracellular pH incre ase the block. The apparent pK for both potentiation and block was 6.4 to 6 .9, substantially higher than expected from carboxylated steroid in an aque ous environment. Block is not dependent on the stereochemistry of the carbo xylic acid at carbon 3 and is relatively insensitive to placement of the ca rboxylic acid at the opposite end of the steroid (carbon 24). Potentiation is critically dependent on the stereochemistry of the carboxylic acid group at carbon 3. Consistent with the pH dependence of potentiation, effects of the amide derivative (3 alpha ,5 beta)-20-oxo-pregnane-3-carboxamide, sugg est that the unionized form of 3 alpha5 beta PC is important for potentiati on, whereas the ionized form is probably responsible for block. Further ref inement of the neuroactive steroid to promote GABA potentiation and NMDA re ceptor block and diminish GABA receptor block may lead to a clinically usef ul neuroactive steroid.