Multiple molecular
targets have been identified to mediate
membrane-delimited
and nongenomic effects of natural and synthetic steroids, but the
influence of steroid metabolism on neuroactive steroid signaling is
not well understood. To begin to address this question, we set out
to identify major metabolites of a neuroprotective synthetic steroid
20-oxo-5β-pregnan-3α-yl l-glutamyl 1-ester (pregnanolone
glutamate, PAG) and characterize their effects on GABAA and NMDA receptors (GABARs, NMDARs) and their influence on zebrafish
behavior. Gas chromatography–mass spectrometry was used to
assess concentrations of PAG and its metabolites in the hippocampal
tissue of juvenile rats following intraperitoneal PAG injection. PAG
is metabolized in the peripheral organs and nervous tissue to 20-oxo-17α-hydroxy-5β-pregnan-3α-yl l-glutamyl 1-ester (17-hydroxypregnanolone glutamate, 17-OH-PAG),
3α-hydroxy-5β-pregnan-20-one (pregnanolone, PA), and 3α,17α-dihydroxy-5β-pregnan-20-one
(17-hydroxypregnanolone, 17-OH-PA). Patch-clamp electrophysiology
experiments in cultured hippocampal neurons demonstrate that PA and
17-OH-PA are potent positive modulators of GABARs, while PAG and 17-OH-PA
have a moderate inhibitory effect at NMDARs. PAG, 17-OH-PA, and PA
diminished the locomotor activity of zebrafish larvae in a dose-dependent
manner. Our results show that PAG and its metabolites are potent modulators
of neurotransmitter receptors with behavioral consequences and indicate
that neurosteroid-based ligands may have therapeutic potential.