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L Friedman, TT Gibbs and DH Farb
Department of Pharmacology and Experimental Therapeutics, Boston University School of Medicine, Massachusetts 02118-2394.
Pregnanolone [5 beta-pregnan-3 alpha-ol-20-one (5 beta 3 alpha)] and allopregnanolone [5 alpha-pregnan-3 alpha-ol-20-one (5 alpha 3 alpha)] are neuroactive steroids that are reduced metabolites of progesterone. Both 5 beta 3 alpha and 5 alpha 3 alpha are potent positive modulators of the gamma-aminobutyric acid response that enhance the binding of [3H]flunitrazepam ([3H] FNZ) to the gamma-aminobutyric acid type A receptor. Chronic (48 hr) exposure of brain neurons in culture to 5 beta 3 alpha or 5 alpha 3 alpha abolishes potentiation of [3H]FNZ binding by these steroids. This uncoupling, or loss of allosteric interactions between steroid and benzodiazepine recognition sites, is dose dependent, stereospecific, and reversible. The number and affinity of [3H]FNZ binding sites are unaffected. In contrast, the steroids 5 beta-pregnan-3 beta-ol-20-one, beta-estradiol, testosterone, progesterone, deoxycorticosterone, and dexamethasone, which show little capacity to potentiate [3H]FNZ binding, are also much less effective in inducing uncoupling of steroid and benzodiazepine recognition sites. These results suggest a mechanism whereby neurons could become refractory to long term modulation by neuroactive steroids. The results are discussed in terms of their possible relevance to premenstrual anxiety and enhanced frequency of seizures in certain women.
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