Utilising two point voltage-clamp techniques on Xenopus laevis oocytes
expressing human (alpha(1) beta(1) gamma(2L)) recombinant GABA(A) rec
eptors, the GABA modulatory actions of six naturally occurring neurost
eroids have been determined and compared with those of known positive
allosteric modulators. The anaesthetic steroids 5 alpha- and 5 beta-pr
egnan-3 alpha-ol-20-one produced a concentration-dependent enhancement
of the GABA-evoked current. The maximal enhancement of the agonist-in
duced response produced by these steroids was intermediate between tha
t of pentobarbitone and diazepam, but much greater than that caused by
bretazenil. For both the 5 alpha and 5 beta steroid a reduction of th
e 20 ketone group to form either the corresponding 20 alpha or 20 beta
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 or subtype
(alpha(x) beta(1) gamma(2L), where x = 1, 2 or 3) for the behavioural
ly active 5 alpha-pregnan-3 alpha,20 alpha-diol is also determined. Al
though the maximal effect of the steroid is not influenced by the alph
a subtype, the alpha(2)-containing receptor exhibits a modest decrease
(approximately 6-fold) in potency compared to alpha(1)- and alpha(3)-
containing receptors. The results described here are discussed in rela
tion to the distinct behavioural actions of the neurosteroids. Copyrig
ht (C) 1996 Elsevier Science Ltd