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. 1988 Aug;94(4):1257–1269. doi: 10.1111/j.1476-5381.1988.tb11646.x

Modulation of the GABAA receptor by depressant barbiturates and pregnane steroids.

J A Peters 1, E F Kirkness 1, H Callachan 1, J J Lambert 1, A J Turner 1
PMCID: PMC1854113  PMID: 2850060

Abstract

1. The modulation of the gamma-aminobutyric acidA (GABAA) receptor by reduced metabolites of progesterone and deoxycorticosterone has been compared with that produced by depressant barbiturates in: (a) voltage-clamp recordings from bovine enzymatically isolated chromaffin cells in cell culture, and (b) an assay of the specific binding of [3H]-muscimol to a preparation of porcine brain membranes. 2. The progesterone metabolites 5 alpha- and 5 beta-pregnan-3 alpha-ol-20-one (greater than or equal to 30 nM) reversibly and dose-dependently enhanced the amplitude of membrane currents elicited by locally applied GABA (100 microM), and over the concentration range 30 nM-100 microM stimulated the binding of [3H]-muscimol. In contrast, 5 alpha- and 5 beta-pregnan-3 beta-ol-20-one (30 nM-100 microM) had little effect in either assay, indicating a marked stereoselectivity of steroid action. 3. Scatchard analysis of the ligand binding data suggested an apparent increase in the number, rather than the affinity, of detectable [3H]-muscimol binding sites as the principle action of the active steroid isomers. 4. GABA-evoked currents were also potentiated by androsterone (1 microM) and the deoxycorticosterone metabolite 5 alpha-pregnane-3 alpha,21-diol-20-one (100 nM). 5. Secobarbitone (10-100 microM), pentobarbitone (10-300 microM) and phenobarbitone (100-500 microM) reversibly and dose-dependently potentiated the amplitude of GABA-evoked currents in the absence of any change in their reversal potential. 6. At relatively high concentrations (greater than or equal to 30 microM) secobarbitone and pentobarbitone directly elicited a membrane current. It is concluded that such currents result from GABAA receptor-channel activation since they share a common reversal potential with GABA-evoked responses (approximately 0 mV), are reversibly antagonized by bicuculline (3 microM), and potentiated by either diazepam (1 microM) or 5 beta-pregnan-3 alpha-ol-20-one (500 nM). 7. Secobarbitone (1 microM-1 mM) dose-dependently enhanced the binding of [3H]-muscimol. In common with the active steroids, an increase in the apparent number of binding sites was responsible for this effect. 8. A saturating concentration (1 mM) of secobarbitone in the ligand binding assay did not suppress the degree of enhancement of control binding produced by 5 beta-pregnan-3 alpha-ol-20-one (30 nM-100 microM). Similarly the steroid, at a concentration of 100 microM, did not influence the enhancement of [3H]-muscimol binding by secobarbitone (1 microM-1 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

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Selected References

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