dizocilpine-maleate and thiosemicarbazide

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.

Topics: Amino Acids; Animals; Disease Models, Animal; Dizocilpine Maleate; Dose-Response Relationship, Drug; Enzyme Inhibitors; Epilepsy; Hippocampus; Male; Microdialysis; Neurodegenerative Diseases; Neuroprotective Agents; Phenylacetates; Pyridoxal Phosphate; Rats; Rats, Wistar; Semicarbazides; Time Factors; Wakefulness

Seizures induced with Thiosemicarbaside, Pentylenetetrasole, N-methyl-D,L-aspartate were used as models. The NO content increased 4-5-fold in the brain cortex at the peak of seizures. The increase could be prevented by pre-treatment with N-nitro-L-arginine and the seizures were weakened. Anticonvulsant drugs reduced the seizure manifestations and partially prevented the NO generation enhancement. The latter seems to be involved in pathophysiological mechanisms underlying the seizures.

Topics: Animals; Anticonvulsants; Arginine; Convulsants; Dizocilpine Maleate; Electron Spin Resonance Spectroscopy; Electroshock; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Male; N-Methylaspartate; Nitric Oxide; Nitroarginine; Pentylenetetrazole; Rats; Rats, Wistar; Receptors, N-Methyl-D-Aspartate; Seizures; Semicarbazides