Differential Responses of Expressed Recombinant Human γ‐Aminobutyric AcidA Receptors to Neurosteroids

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Abstract
: Neuroactive steroids, in particular 3α‐hydroxypregnanes, are allosteric modulators of the γ‐aminobutyric acidA (GABAA) receptor. Regionally selective expression of receptor subunit subtypes may account for differential responsiveness of tissues to GABAergic inhibition and ncurosteroid modulatory effects. the effect of 5α‐pregnan‐3α‐ol‐20‐one (epiallopregnanolone) on heterotropic cooperativity on the GABAA receptor complex has been studied in three subtypes of expressed recombinant human receptors and in cat brain and spinal cord. Steroid potentiation of [3H]flunitrazepam binding was greatest for the α1β1γ2 receptor complex, whereas α1β1γ2 and α2β1γ2 complexes showed < 100% enhancement in binding. Previous studies suggest that the spinal cord is devoid of α1, whereas cerebellum is rich in α1 subunits. Correspondingly, a differential enhancement of [3H]flunitrazepam binding in spinal cord (51%) versus cerebellum (28%) was also observed. The structure of neuroactive steroids is important in determining the extent of neuromodulatory activity. The 5β‐pregnanes, 5β‐pregnan‐3α‐ol‐20‐one (epipregnanolone) and 5β‐pregnan‐3α,21‐diol‐20‐one (5β‐tetrahydrodeoxycorticosterone), were both less potent than their corresponding 5α derivatives. A 3α‐hydroxyl group is essential for neuromodulatory activity in the expressed receptors, as demonstrated by the observation that 5α‐preg‐nan‐3β‐ol‐20‐one (allopregnanolone) and 4‐pregnen‐3,20‐dione (progesterone) were both inactive. The ability to screen synthetic molecules using expressed human receptors that selectively contain individual subunit subtype combinations may prove to be a powerful tool in the development of therapeutic agents that act as allosteric modulators of the GABAA receptor and other neurotransmitter receptors as well.