Epilepsy and hippocampal neurodegeneration induced by glutamate decarboxylase inhibitors in awake rats.
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.
División de Neurociencias, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, AP 70-253, 04510 México DF, Mexico.
Patricia
P
Salazar
División de Neurociencias, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, AP 70-253, 04510 México DF, Mexico. Electronic address: rtapia@ifc.unam.mx.
Ricardo
R
Tapia
D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D004195:Disease Models, Animal; D016291:Dizocilpine Maleate / Q000627:therapeutic use; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000633:toxicity*; D004827:Epilepsy / Q000139:chemically induced*; D006624:Hippocampus / Q000473:pathology*; D008297:Male; D017551:Microdialysis; D019636:Neurodegenerative Diseases / Q000139:chemically induced* / Q000188:drug therapy; D018696:Neuroprotective Agents / Q000627:therapeutic use; D010648:Phenylacetates / Q000494:pharmacology; D011732:Pyridoxal Phosphate / Q000031:analogs & derivatives / Q000633:toxicity; D051381:Rats; D017208:Rats, Wistar; D012663:Semicarbazides / Q000633:toxicity; D013997:Time Factors; D014851:Wakefulness
D000596:Amino Acids; D004791:Enzyme Inhibitors; D018696:Neuroprotective Agents; D010648:Phenylacetates; D012663:Semicarbazides; C003257:pyridoxal phosphate gamma-glutamyl hydrazone; D011732:Pyridoxal Phosphate; C005151:thiosemicarbazide; D016291:Dizocilpine Maleate; C512803:alpha-methoxy-alpha-phenylacetic acid
Epilepsy; GABA; Glutamate; Hippocampus; Neurodegeneration; Pyridoxal phosphate