Inhibitory potentials in neurons of the deep layers of the in vitro neocortical slice

Bursting in human epileptogenic neocortex is depressed by an N-methyl-D-aspartate antagonist

Intracellular recordings were performed in human neocortical neurons in 'in vitro' slices of brain samples excised during surgical treatment of epilepsy. In 14 of 38 neurons obtained from cortex exhibiting interictal spiking, bursts of action potentials arising from a synaptic depolarizing potential could be elicited by extracellular focal stimulation of adequate strength. The N-methyl-D-aspartate (NMDA) antagonist 2-amino-5-phosphonovalerate (APV) was capable of reducing and eventually blocking these bursts without affecting the repetitive firing evoked by depolarizing intracellular pulses or the membrane input resistance. These data suggest a role played by NMDA receptors in the bursting activity displayed by human neurons from spiking cortical areas and demonstrate a potential use of NMDA antagonists as antiepileptic drugs.

Intracellular recordings in pericruciate neurons during spike and wave discharges of feline generalized penicillin epilepsy

Concurrent EEG and intracellular recordings from pericruciate neurons of cats obtained before and after i.m. injection of penicillin inducing the syndrome of feline generalized penicillin epilepsy (FGPE) characterized by spike and wave (SW) discharge in the EEG, display large excitatory postsynaptic potentials (EPSPs) at the time of the EEG 'spike' which alternate with hyperpolarizing potentials occurring in coincidence with the EEG 'wave' component of the SW complex. The large EPSPs trigger discharges of single or multiple high-frequency action potentials which do not show a progressive decrement in amplitude nor an appreciable increase in duration. These bursts thus differ in some respects from typical paroxysmal depolarization shifts. The hyperpolarizing potentials show an early phase which is reversed by intracellular Cl- injection or diffusion and thus behaves like a classical inhibitory postsynaptic potential (IPSP). The late phase is unaffected by Cl-. Hyperpolarizing potentials of pericruciate neurons induced by antidromic activation of the cerebral peduncle (CP) or by direct cortical stimulation are not altered after i.m. injections of penicillin at doses sufficient to induce generalized SW discharge. The early phase of hyperpolarization both before and after i.m. penicillin is reversed by intracellular Cl- injection or diffusion, the late phase remains unchanged. The early phase thus represents a classical IPSP, which does not appear to be affected by the low brain penicillin concentrations sufficient to induce generalized SW discharge. It is concluded that this form of epileptic discharge cannot be attributed to blockage of phasic (presumably somatic) postsynaptic inhibition by penicillin. These results indicate that to regard all forms of epileptic discharge as the consequence of a blockage of gamma-aminobutyric acid-mediated phasic postsynaptic inhibition acting on the soma represents an unduly restrictive view of epileptogenesis.

A GABAergic depolarizing potential in the hippocampus disclosed by the convulsant 4-aminopyridine

Intracellular recordings from hippocampal pyramidal cells in the CA1 subfield of the 'in vitro' slice in the presence of 4-aminopyridine (4-AP, 5-50 microM) revealed a long-lasting (up to 1.5 s) depolarizing potential which occurred either spontaneously or following orthodromic stimulation. This potential was: capable of blocking both direct and synaptic activation of the cell; sensitive to bath application of low concentrations of bicuculline methiodide; and associated to an extracellular current sink in the dendrites as suggested by the extracellular field potentials recorded at different levels along an axis perpendicular to the stratum pyramidale. It is concluded that the long-lasting depolarizing potential evoked by 4-AP is caused by the activation of GABA receptors localized in the dendritic region of the CA1 subfield.