Properties and synaptic mechanisms of bicuculline-induced epileptiform bursts in neocortical slices from children with intractable epilepsy

1. Bicuculline-induced epileptiform bursts in slices of neocortical tissue resected from children (3 mo to 14 yr) undergoing neurosurgical treatment for intractable epilepsy were studied with conventional intracellular recording techniques. The purposes of this study were to characterize the bursts evoked in immature human neocortical slices, to gain further insight to how N- methyl-D-aspartate (NMDA) and non-NMDA receptors contribute to the genesis of the bursts, and to determine whether the characteristics of the bursts were related to patient age or clinically defined abnormality of the tissue. 2. Epileptiform bursts evoked by focal stimulation of the underlying white or gray matter in bicuculline (10 μM) were all-or-none events. Once evoked, the bursts in a given neuron appeared very similar to one another, regardless of stimulus intensity. Stronger stimuli only decreased the onset latency of the bursts. The bursts evoked with relatively weak stimuli (<2-3 times the threshold), particularly those from stimulation of a distant site (4-5 mm), were variable in onset latency. The bursts from stimulation of a close site (0.5-2 mm) with stronger stimuli (>3 times the threshold) were invariable in onset latency. 3. Across different cells, particularly across the cells in different slices, the bursts were quite variable in terms of their morphology and duration. When measured at one-half of the amplitude of the underlying depolarization (~20-50 mV), the duration of the bursts ranged from 20 to 775 ms (n = 80 cells). In 23% of the cases (18 of 80 cells), afterdischarges lasting for tens of milliseconds to a few seconds followed the bursts. 4. The selective NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5, 50-100 μM) blocked completely and abruptly the bursts evoked with relatively low stimulus intensities, particularly those from stimulation of distant sites. On the other hand, AP5 (even at 100 μM) had limited effects on the bursts evoked by stimulating at sites close to the recording electrode with relatively high intensities; AP5 mainly depressed the late phase of the burst, resulting in the reduction of burst duration but only slight decrease in the amplitude of the underlying depolarization. The effects of AP5 developed gradually, were concentration dependent (maximum effect: ~30 μM) and reversible, and were not associated with any significant changes in the baseline membrane potential or input resistance of postsynaptic neurons. 5. A non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX, 10-30 μM), blocked gradually and completely the early, AP5-resistant portion of the high-intensity stimulus-evoked bursts. The effects of DNQX were not associated with any significant changes in membrane potential or input resistance of postsynaptic neurons. 6. To determine the relation between the magnitude of the NMDA- and non-NMDA-receptor-mediated components of a burst and the magnitude of the burst, we examined the correlations among the original burst duration and the duration of the AP5-sensitive portion (r = 0.951, P < 0.001). Also, a correlation was found between the original burst duration and the duration of the AP5-resistant portion (r = 0.852, P < 0.001). In addition, a weak, but significant correlation existed between the durations of the AP5-sensitive and -resistant portions (r = 0.648, P < 0.02). 7. Whether burst duration and the presence of afterdischarges were related to patient age or clinically defined abnormality of the tissue (i.e., 'least abnormal' vs. 'most abnormal') was examined. The hypothesis that cells in the younger or 'most abnormal' tissue group would exhibit longer bursts or more frequent afterdischarges (i.e., that these cells would be more 'epileptic') was not confirmed. Burst duration was not significantly different between the youngest and the oldest patients (i.e., 4-24 mo vs. >5 yr old) or between least abnormal and most abnormal tissues within subjects. Afterdischarges appeared independent of patient age or tissue status. 8. These results suggest the following. 1) Epileptiform bursts in immature human neocortical slices have very similar characteristics to those in adult human or animal neocortical slices. 2) The NMDA receptors located on local neurons, upstream to the recorded neurons, play a critical role in the generation of polysynaptically evoked epileptiform bursts. The non-NMDA and NMDA receptors at postsynaptic sites contribute to the initiation and maintenance of monosynaptically evoked epileptiform bursts, respectively. 3) Burst duration is an indirect measure for the size of both NMDA- and non-NMDA-receptor- mediated components of each epileptiform burst, which may covary. 4) Burst duration and the presence of afterdischarges are unrelated to patient age or degree of tissue abnormality.