Effect of neurosteroids on glutamate binding sites and glutamate uptake in rat hippocampus

Effects of some neurosteroids on the binding of 3H]-glutamate, 3H]-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and 3H]-MK-801, as well as on the 3H]-glutamate uptake were examined in rat hippocampus. The following compounds were evaluated: (a) positive modulators of the GABA(A) receptor: 5alpha-pregnan-3alpha-ol-20-one (allopregnanolone), 5alpha-pregnane-3alpha,21-diol-20-one (allotetrahydrodeoxycorticosterone), 5alpha-pregnan-3alpha-ol-11,20-dione (alphaxalone) and 5alpha-androstan-3alpha-ol-17-one (androsterone); (b) compounds showing GABA(A)-antagonistic and/or N-methyl-D-aspartic acid (NMDA)-agonistic properties: dehydroepiandrosterone sulfate and pregnenolone sulfate; (c) a substance which, apart from its GABA(A)-agonistic potency, has a NMDA-antagonistic action: 5beta-pregnan-3alpha-ol-20-one. None of those neurosteroids tested at concentrations of 0.001-100 microM affected the binding of 3H]-glutamate, 3H]-AMPA and 3H]-MK-801 or the glutamate uptake. The present study suggests that the previously reported inhibitory effects of neurosteroids on excitatory amino acid-induced seizures and neurotoxicity can be linked neither to the direct interaction of these compounds with the above binding sites on glutamate receptor complexes, nor to the glutamate uptake mechanism.