Bilobalide, a constituent of Ginkgo biloba, has neuroprotective properties. Its mechanism of action is unknown but it was recently found to interact with neuronal transmission mediated by
glutamate,
gamma-aminobutyric acid (
GABA) and
glycine. The goal of this study was to test the interaction of
bilobalide with
glycine in assays of neuroprotection. In rat hippocampal slices exposed to
N-methyl-D-aspartate (
NMDA), release of
choline indicates breakdown of membrane
phospholipids.
NMDA-induced
choline release was almost completely blocked in the presence of
bilobalide (10 microM).
Glycine (10-100 microM) antagonized the inhibitory action of
bilobalide in this assay. In a second assay of excitotoxicity, we measured tissue water content as an
indicator of cytotoxic
edema formation in hippocampal slices which were exposed to
NMDA. In this assay,
edema formation was suppressed by
bilobalide but
bilobalide's action was attenuated in the presence of
glycine and of D-
serine (100 microM each). To investigate
bilobalide's interaction with
glycine receptors directly, we determined 36chloride flux in rat cortico-hippocampal synaptoneurosomes.
Glycine (100 microM) was inactive in this assay indicating an absence of functional
glycine-A receptors in this preparation. [3H]
Glycine was used to assess binding at the
glycine binding site of the
NMDA receptor but
bilobalide was found to be inactive in this assay. Finally, [3H]
glycine release was monitored in hippocampal slices exposed to
oxygen-
glucose deprivation. In this model,
glycine release was induced by
ischemia, an effect that was strongly reduced by
bilobalide. We conclude that
bilobalide does not interact with
glycine receptors in neurochemical assays but it significantly reduces the release of
glycine under ischemic conditions. This effect likely contributes to
bilobalide's
neuroprotective effects in assays of excitotoxicity and
ischemia.