Relationships Among Seizures, Extracellular Amino Acid Changes, and Neurodegeneration Induced by 4‐Aminopyridine in Rat Hippocampus: A Microdialysis and Electroencephalographic Study

Abstract

4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glutamate. We report the effect of intrahippocampal administration of 4-aminopyridine at six different concentrations through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and correlate this effect with histological changes in the hippocampus. 4-Aminopyridine induced in a concentrationdependent manner intense and frequent epileptic discharges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glutamate, aspartate, and GABA levels and profound hippocampal damage. Neurodegenerative changes occurred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammonium resulted in increased amino acid levels but no epileptic 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 releaseinducing effect of 4-aminopyridine results in excitotoxicity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsynaptic receptors, which is probably not the case after K+ depolarization.

Potassium channel blockers as an effective treatment to restore impulse conduction in injured axons

“…One contributing factor is the discrepancy between the maximum tolerable blood level of 4-AP in human patients (0.5-1 mol/L) and the most effective concentrations demonstrated in vitro (100 mol/L in acute injury and 10 mol/L in chronic injury) [4,20] . Concentrations beyond 1 mol/L may increase the probability of side effects such as respiratory distress, anxiety, and epileptiform seizures [67][68][69] .…”

Allopregnanolone Potentiates the Glutamate-Mediated Seizures Induced by 4-Aminopyridine in Rat Hippocampus in vivo

“…These results suggest that the potentiation of the 4-APinduced epilepsy by allopregnanolone may be due to a further increased concentration of glutamate in the synaptic cleft that overactivates NMDA receptors. This explanation is supported by the fact that there is a correlation between the elevation of extracellular glutamate and the excitotoxic action of 4-AP [11], and by the notable protective effect of MK-801, observed against both 4-AP alone and against 4-AP? allopregnanolone, even when MK-801 did not modify the increase in extracellular glutamate.…”

“…The microdialysis procedure in anesthetized animals was as previously described for the study of 4-AP effects and the protection by some compounds [11,12,16,17]. Briefly, rats were anesthetized with 0.4-1.5% halothane in 95% O 2 /5% CO 2 mixture and implanted with a microdialysis cannula-electrode (CMA/12 membrane with 2 mm length and 0.5 mm diameter, CMA/ Microdialysis, Acton, MA, USA) in the hippocampus (coordinates from bregma: -3.3 antero-posterior, -1.9 lateral, and -2.7 vertical [18].…”

“…The microdialysis cannula was used as an electrode, as previously described [11,12]. A Grass polygraph with a low frequency filter at 3 Hz and a high frequency filter at 100 Hz was used to record the EEG during the whole microdialysis procedure (100-140 min).…”

“…4-Aminopyridine (4-AP) is a blocker of K ? channel that induces convulsions and hippocampal neurodegeneration through the stimulation of glutamate release from nerve endings and the consequent overactivation of NMDA receptors, when administered by reverse microdialysis in rat hippocampus [11,12]. Studies in hippocampal slices have shown that 4-AP is also epileptogenic in vitro, since it produces depolarization shifts, bursting and increase in amplitude spikes [13,14].…”