Synaptic GABAA receptors are primary mediators of rapid inhibition in the brain and play a key role in the pathophysiology of epilepsy and other neurologic disorders. The δ-subunit GABAA receptors are expressed extrasynaptically in the dentate gyrus and contribute to tonic inhibition, promoting network shunting as well as reducing seizure susceptibility. However, the neurosteroid structure-function relationship at δGABAA receptors within the native hippocampus neurons remains unclear. Here we report a structure-activity relationship for neurosteroid modulation of extrasynaptic GABAA receptor–mediated tonic inhibition in the murine dentate gyrus granule cells. We recorded neurosteroid allosteric potentiation of GABA as well as direct activation of tonic currents using a wide array of natural and synthetic neurosteroids. Our results shows that, for all neurosteroids, the C3α-OH group remains obligatory for extrasynaptic receptor functional activity, as C3β-OH epimers were inactive in activating tonic currents. Allopregnanolone and related pregnane analogs exhibited the highest potency and maximal efficacy in promoting tonic currents. Alterations at the C17 or C20 region of the neurosteroid molecule drastically altered the transduction kinetics of tonic current activation. The androstane analogs had the weakest modulatory response among the analogs tested. Neurosteroid potentiation of tonic currents was completely (approximately 95%) diminished in granule cells from δ-knockout mice, suggesting that δ-subunit receptors are essential for neurosteroid activity. The neurosteroid sensitivity of δGABAA receptors was confirmed at the systems level using a 6-Hz seizure test. A consensus neurosteroid pharmacophore model at extrasynaptic δGABAA receptors is proposed based on a structure-activity relationship for activation of tonic current and seizure protection.
- Received September 13, 2015.
- Accepted February 5, 2016.
This research was partly supported by the National Institutes of Health National Institute of Neurologic Disorders and Stroke [Grant R01NS051398 (to D.S.R.)]. This research was also partly supported by the CounterACT Program, National Institutes of Health Office of the Director, and the National Institutes of Health National Institute of Neurologic Disorders and Stroke [Grant U01NS083460 (to D.S.R.)]. The views expressed in this article are those of the authors and do not reflect the official policy of the National Institutes of Health or the U.S. Government or the Texas A&M University.
- Copyright © 2016 by The American Society for Pharmacology and Experimental Therapeutics