Long-term changes in neocortical activity after chemical kindling with systemic pentylenetetrazole: An in vitro study

1. Rats were chemically kindled by systemic administration of pentylenetetrazol (FTZ) every 48 h. An initially subthreshold dose that did not elicit a motor response when first applied caused severe epileptiform seizures when the animal was kindled. Once kindled, animals continued to respond to the initially subthreshold dose with a full-blown seizure for >2 mo, even when regular administration ceased for ≥1 mo. 2. In neocortical slices taken from kindled rats, low-intensity electrical stimulation evoked generation of prolonged (hundreds of milliseconds) paroxysmal extracellular field potentials and intracellular depolarizing potentials, indicating synchronized activity of large populations of neurons. This hyperexcitability usually appeared as an all-or-none event of variable latency. In a few cases it increased gradually with increasing stimulus intensity. The intensity of the paroxysmal response was greatly enhanced by application of γ- aminobutyric acid-A (GABAa) receptor blockers to the bath. 3. Intracellular recordings revealed that PTZ-kindled cells differ from normal cells in their higher input resistance (42.4 + 13.6 vs. 26.4 + 9.2 MΩ, mean ± SE). Spikes generated by kindled cells differed significantly from those in normal cells in that they were of longer duration (1.65 + 0.3 vs. 1.40 + 0.15 ms) and had a slower maximal rate of fall (103 + 29.7 vs. 126 + 20.8 volts/s). 4. Injection of the lidocaine derivative QX-314 to the recorded neurons (100 mM) blocked the fast Na⁺ spikes. Under these conditions slow spikes, probably Ca²⁺ mediated, were evoked from the soma in neurons from kindled but not from normal cortex. 5. The role of N-methyl-D-aspartate (NMDA) receptors in generating paroxysmal events was evaluated by application of 20 μM 2-amino- 5-phosphonovaleric acid, a specific blocker of this glutamate receptor type. Blockage of NMDA receptors cut short the paroxysmal field potentials but did not prevent their generation. Intracellularly recorded paroxysmal responses were also cut short but not abolished by intracellular hyperpolarization. 6. In slices from kindled animals intracellular responses in neurons of deeper layers differed markedly from those of superficial cells. In deep neurons, responses resembled those generated by neocortical neurons exposed to GABAergic blockers. A low-intensity stimulus to the white matter evoked an excitatory postsynaptic potential (EPSP) followed with variable latency by a paroxysmal depolarizing shift that reversed at suprathreshold membrane potentials and on which superimposed repetitive firing was always evident. By contrast, in superficial (layer II/III) neurons the same stimulus evoked an EPSP that was followed by a prolonged response whose late component reversed at subthreshold membrane potentials (between -50 and -80 mV). These cells rarely fired more than a single spike throughout the response. 7. Repetitive stimulation at relatively low frequencies (0.3-1 Hz) caused a gradual change in the synchronized responses of the superficial but not deep neurons. The reversal potential of the response shifted toward suprathreshold membrane potentials and subsequently superimposed repetitive firing became evident. 8. Directly hyperpolarizing or depolarizing the neuron with intracellularly applied current did not affect the rate at which the gradual shift transpired, suggesting that the frequency-dependent changes in the response were due to changes in synaptic drive and not postsynaptic factors. 9. The data indicate that although systemic PTZ kindling shares many features in common with electrical limbic kindling, it primarily affects the neocortex. The development of hyperexcitability in the PTZ-kindled cortex appears to be a multifactorial process, entailing changes in intrinsic neuronal properties as well as synaptic properties. Although activation of NMDA receptors does apparently contribute to the abnormal responsiveness, they are not essential for its existence in this kindling model.