Glutamic acid decarboxylase (GAD), the
enzyme responsible for
GABA synthesis, requires
pyridoxal phosphate (PLP) as a cofactor.
Thiosemicarbazide (
TSC) and γ-glutamyl-
hydrazone (
PLPGH) inhibit the free PLP-dependent
isoform (GAD65) activity after systemic administration, leading to
epilepsy in mice and in young, but not in adult rats. However, the competitive GAD inhibitor
3-mercaptopropionic acid (MPA) induces convulsions in both immature and adult rats. In the present study we tested comparatively the epileptogenic and neurotoxic effects of
PLPGH,
TSC and MPA, administered by microdialysis in the hippocampus of adult awake rats. Cortical EEG and motor behavior were analyzed during the next 2h, and
aspartate,
glutamate and
GABA were measured by HPLC in the microdialysis-collected fractions. Twenty-four hours after
drug administration rats were fixed for histological analysis of the hippocampus.
PLPGH or
TSC did not affect the motor behavior, EEG or cellular morphology, although the extracellular concentration of
GABA was decreased. In contrast, MPA produced intense wet-dog shakes, EEG epileptiform discharges, a >75% reduction of extracellular
GABA levels and remarkable neurodegeneration of the CA1 region, with >80% neuronal loss. The systemic administration of the
NMDA glutamate receptor antagonist MK-801 30 min before MPA did not prevent the MPA-induced
epilepsy but significantly protected against its neurotoxic effect, reducing neuronal loss to <30%. We conclude that in adult awake rats, drugs acting on PLP availability have only a weak effect on
GABA neurotransmission, whereas direct GAD inhibition produced by MPA induces hyperexcitation leading to
epilepsy and hippocampal neurodegeneration. Because this degeneration was prevented by the blockade of
NMDA receptors, we conclude that it is due to
glutamate-mediated excitotoxicity consequent to disinhibition of the hippocampal excitatory circuits.