Journal zyxwvutsrqponmlk of Neurochemistry Lippincott Williams zyxwvutsrqponm & Wilkins, Inc., Philadelphia zyxwvutsrqpo 0 1999 International Society for Neurochemistry Relationships Among Seizures, Extracellular Amino Acid Changes, and Neurodegeneration Induced by 4-Aminopyridine in Rat Hippocarnpus: zyxw A Microdialysis and Electroencephalographic Study Fernando Peiia and Ricardo Tapia Departamento de Neurociencias, Instituto de Fisiologia Celular, Universidad Nucional Authornu de Mkxico, Mkxico, D. F., Mtfxico zyxwv Abstract: 4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glu- tamate. We report the effect of intrahippocampal admin- istration of 4-aminopyridine at six different concentra- tions through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and cor- relate this effect with histological changes in the hip- pocampus. 4-Aminopyridine induced in a concentration- dependent manner intense and frequent epileptic dis- charges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glu- tamate, aspartate, and GABA levels and profound hip- pocampal damage. Neurodegenerative changes oc- curred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammo- nium resulted in increased amino acid levels but no epi- leptic activity and no or moderate neuronal damage. These results suggest that seizure activity induced by 4-aminopyridine is due to a combined action of excitatory amino acid release and direct stimulation of neuronal firing, whereas neuronal death is related to the increased glutamate release but is independent of seizure activity. In addition, it is concluded that the glutamate release- inducing effect of 4-aminopyridine results in excitotoxic- ity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsyn- aptic receptors, which is probably not the case after K+ depolarization. Key Words: 4-Aminopyridine-Seizures- Hippocampus-Neurodegeneration-Excitotoxicity- Microdialysis. J. Neurochem. 72,2006-2014 (1999). al., 1991; Versteeg et al., 1995; Schechter, 1997), and some cerebral regions in vivo (Dawson and Routledge, 1995; Morales-VillagrBn and Tapia, 1996). 4-AP is also an efficient convulsant that induces epileptiform electri- cal discharges in rat hippocampal (Perrault and Avoli, 1991; Yonekawa et al., 1995; Avoli et al., 1996) and cortical (Siniscalchi et al., 1997) slices and in vivo pro- duces intense seizure activity in the rat (Gandolfo et al., 1989; Fragoso-Veloz and Tapia, 1992; Morales-Villa- grhn et al., 1996), mouse (Yamaguchi and Rogawsh, 1992; Cramer et al., 1994), and human (Spyker et al., 1980). Among the neurotransmitters whose release is induced by 4-AP is glutamate, and, in view of the well-estab- lished role of excitatory amino acid (EAA)-mediated synapses in convulsive and excitotoxic mechanisms (Choi, 1988; Meldrum, 1991), it has been postulated that the epileptogenic action of this drug is related to an increased glutamatergic transmission. This postulation is based on the fact that EAA receptor antagonists, of both the N-methyl-D-aspartate (NMDA) and the non-NMDA types, are effective anticonvulsants against the 4-AP- induced seizures, both in brain slices (Perrault and Avoli, 1992; Avoli et al., 1996; Siniscalchi et al., 1997) and in vivo (Gandolfo et al., 1989; Fragoso-Veloz and Tapia, 1992; Cramer et al., 1994; Morales-VillagrBn et al., 1996). Furthermore, we have demonstrated, using micro- dialysis, that 4-AP preferentially stimulates the release of glutamate in the striatum and that this release is corre- lated with intense behavioral convulsive activity (Mo- rales-VillagrBn and Tapia, 1996). An effect of Kf channel blockers, such as 4-amino- pyridine (4-AP), is to induce the release of neurotrans- mitters. 4-AP stimulates the release of both excitatory and inhibitory neurotransmitters in synaptosomes (Tapia and Sitges, 1982; Tapia et al., 1985), brain slices (Hu et Received December 21, 1998; accepted December 22, 1998. Address correspondence and reprint requests to Dr. R. Tapia at Departamento de Neurociencias, Instituto de Fisiologia Celular, Uni- versidad Nacional Aut6noma de MBxico, AP 70-253, 045 10-MBxico, D.F., MCxico. Abbreviations zyxwv used: 4-AP, 4-aminopyridine; EAA, excitatory amino acid; NMDA, N-methyl-D-aspartate; TEA, tetraethylammonium. 2006