Utilising two point voltage-clamp techniques on Xenopus laevis oocytes expressing human (α1β1γ(2L)) recombinant GABA(A) receptors, the GABA modulatory actions of six naturally occurring neurosteroids have been determined and compared with those of known positive allosteric modulators. The anaesthetic steroids 5α- and 5β-pregnan-3α-ol-20-one produced a concentration-dependent enhancement of the GABA-evoked current. The maximal enhancement of the agonist-induced response produced by these steroids was intermediate between that of pentobarbitone and diazepam, but much greater than that caused by bretazenil. For both the 5α and 5β steroid a reduction of the 20 ketone group to form either the corresponding 20α or 20β hydroxy steroid produced, in all cases, a reduction in potency and a decrease in the maximal effect. The relationship of steroid structure to these two parameters is considered. The influence of the α subtype (α(x)β1γ(2L), where x = 1, 2 or 3) for the behaviourally active 5α-pregnan-3α,20α-diol is also determined. Although the maximal effect of the steroid is not influenced by the α subtype, the α2-containing receptor exhibits a modest decrease (approximately 6-fold) in potency compared to α1- and α3-containing receptors. The results described here are discussed in relation to the distinct behavioural actions of the neurosteroids.
- General anaesthetics
- Human recombinant GABA(A) receptors
- Xenopus laevis oocytes