Brain Research. 417 (1987) 199-203 199 Elsevier BRE 22408 Seizure-like discharges induced by lowering [Mga+] o in the human epileptogenic neocortex maintained in vitro M. Avoli, J. Louvel*, R. Pumain* and A. Olivier Montreal Neurological Institute, Department of Neurology and Neurosurgerv, McGill University, Montreal, Qu& (Canada) (Accepted 28 April 1987) Key words: Seizure-like discharge; Neocortex; Epilepsy; N-methyl-D-aspartate Seizure-like discharges were observed in slices of human epileptogenic neocortex maintained in vitro when [Mg2+]owas lowered near to zero. This type of epileptiform activity: (1) could occur spontaneously or following extracellular focal stimuli: (2) resembled the electrographic pattern associated with tonic-clonic seizures; (3) was accompanied by increases in [K+]o(maximally 6.2 mM from a baseline of 3.25 mM) and decreases in [Ca2+]o (maximally 0.23 mM from a baseline of 1.8 mM). Application of the selective antagonist of N-methyl-D-aspartate (NMDA) receptors, DL-2-amino-5-phosphonovalerate, suppressed in a reversible manner both spontaneous and stimulus-induced seizure-like discharges, suggesting that NMDA-activated conductances are important for the genesis of pro- longed epileptiform discharges generated by human epileptogenic neocortical slices. The dicarboxylic amino acids L-glutamate and L- aspartate, which are excitatory neurotransmitters in the mammalian central nervous system, exert their effects through the activation of at least 3 distinct re- ceptor types named after the sensitivity to the agon- ists kainate, quisqualate and N-methyl-D-aspartate (NMDA) 25. While kainate and quisqualate receptors appear to be involved in fast excitatory transmission at cortical synapses 4"614, NMDA-activated conduc- tances are associated with specific mechanisms of neuronal excitability such as the initiation of long- term potentiation 11'14 or experimentally induced epi- leptiform activity 3'5"8"1°'15"22. Furthermore, we have recently shown that the bursts of action potentials generated by human neocortical neurons in response to extracellular focal stimulations in vitro slices ob- tained from epileptogenic cortex are blocked by the selective antagonist of NMDA receptors, DL-2-ami- no-5-phosphonovalerate (APV), at concentrations which have no effect on the normal excitatory post- synaptic potential 2. NMDA receptors are coupled to an ionophore which is permeable to cations such as Ca 2÷, Na ÷ and K + and is gated in a voltage-dependent manner by Mg 2+ normally present in the extracellular space 7'9' 16-19,21. Thus, during gradual depolarization from ne- gative membrane potential , responses to NMDA show first an increase (as the block by Mg 2+ de- clines), then a decrease in amplitude (as the mem- brane potential approaches the equilibrium potential for the activated channel, around +2 mV). In keep- ing with such behavior, decreasing [Mg2+]o in rat cor- tical slices maintained in vitro uncovers conductances activated by NMDA receptors and causes epilepti- form discharges to appear 13'24. The present experiments were designed to study the possible involvement of NMDA receptors in hu- man epileptogenesis. Here we report that by perfus- ing slices of human epileptogenic neocortex with MgZ+-free artificial cerebrospinal fluid, it is possible to evoke prolonged, seizure-like discharges which are mainly associated with Ca 2+ sinks located in the middle cortical layers and are blocked by the antago- nist of NMDA receptors APV. Human cortical tissue was obtained during resec- tion of epileptogenic tissue carried out for the control * Permanent address: INSERM U-97, Paris, F-75014, France. Correspondence: M. Avoli, Montreal Neurological Institute, 3801 University Street, Montr6al, Qu6bec, H3A 2B4, Canada. 0006-8993/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)