Cyclodextrins sequester neuroactive steroids and differentiate mechanisms that rate limit steroid actions

Background and purpose: Neuroactive steroids are potent modulators of GABAA receptors and are thus of interest for their sedative, anxiolytic, anticonvulsant and anaesthetic properties. Cyclodextrins may be useful tools to manipulate neuroactive effects of steroids on GABAA receptors because cyclodextrins form inclusion complexes with at least some steroids that are active at the GABAA receptor, such as (3α,5α)‐3‐hydroxypregnan‐20‐one (3α5αP, allopregnanolone). Experimental approach: To assess the versatility of cyclodextrins as steroid modulators, we investigated interactions between γ‐cyclodextrin and neuroactive steroids of different structural classes. Key results: Both a bioassay based on electrophysiological assessment of GABAA receptor function and optical measurements of cellular accumulation of a fluorescent steroid analogue suggest that γ‐cyclodextrin sequesters steroids rather than directly influencing GABAA receptor function. Neither a 5β‐reduced A/B ring fusion nor a sulphate group at carbon 3 affected the presumed inclusion complex formation between steroid and γ‐cyclodextrin. Apparent dissociation constants for interactions between natural steroids and γ‐cyclodexrin ranged from 10‐60 μM. Although γ‐cyclodextrin accommodates a range of natural and synthetic steroids, C11 substitutions reduced inclusion complex formation. Using γ‐cyclodextrin to remove steroid not directly bound to GABAA receptors, we found that cellular retention of receptor‐unbound steroid rate limits potentiation by 3α‐ hydroxysteroids but not inhibition by sulphated steroids. Conclusions and implications: We conclude that γ‐cyclodextrins can be useful, albeit non‐specific, tools for terminating the actions of multiple classes of naturally occurring neuroactive steroids. British Journal of Pharmacology (2007) 150, 164–175. doi:10.1038/sj.bjp.0706973