ACCELERATED COMMUNICATION Enantiomers of Neuroactive Steroids Support a Specific Interaction with the GABA-C Receptor as the Mechanism of Steroid Action

Neuroactive steroids can either potentiate or inhibit a variety of membrane channels. Most studies have suggested that the effects are mediated by specific association of the steroid with the affected channel. However, a recent study of the 1 (GABA-C) receptor (Mol Pharmacol 66:56–69, 2004) concluded that the actions were consistent with an action of the steroid in the lipid bilayer to alter the lateral pressure profile in the membrane. The enantiomers of an optically active compound are expected to have identical physical properties, including interactions with hydrophobic portions of the cell membrane. We have used two pairs of enantiomers (pregnanolone and entpregnanolone, allopregnanolone and ent-allopregnanolone) and show that the ability to potentiate (allopregnanolone) or inhibit (pregnanolone) the 1 receptor is enantioselective. Therefore, these results strongly suggest that the actions of these neuroactive steroids are mediated by interactions with chiral regions of the target protein, rather than by a change in membrane properties (including lateral pressure). Many mechanisms have been proposed to underlie the actions of psychoactive compounds. In general, the mechanisms fall into two classes: mechanisms involving specific interactions with target molecules (e.g., sites on membrane proteins) and mechanisms involving more general effects on the milieu in which the target molecule is found (e.g., effects on membrane lipid packing, mobility or order). Neuroactive steroids are a particular type of compound, which can both modulate the function of membrane channels and also intercalate into biological membranes. Indeed, it seems very likely that a preferred route of access for neuroactive steroids may be through the lipid membrane (Akk et al., 2005). However, the question arises of whether effects are mediated through a direct interaction with the receptor protein. Previous studies of neuroactive steroids have shown that the enantiomer of allopregnanolone (3 5 P) is much less effective at potentiating responses of the GABA-A receptor (Wittmer et al., 1996, Covey et al., 2000), the enantiomer of 17 -estradiol is ineffective at potentiating the human nicotinic 4 2 receptor (Paradiso et al., 2001), and the enantiomer of (3 ,5 ,17 )-3hydroxyandrostan-17-carbonitrile is less effective at blocking the nicotinic 4 2 receptor (Paradiso et al., 2000). These observations are consistent with the idea that steroids interact with chiral sites on the target proteins. However, there are two caveats to this interpretation. The first is that not all steroids show enantioselectivity in actions (for example, Nilsson et al., 1998; Covey et al., 2000). The second is that biological membranes contain optically active components in the lipid milieu that might interact differently with the ste-