Firing rhythmicity in the neocortex in vivo and in vitro under sustained iontophoretic excitation

• Published in: Journal of Comparative Physiology A Vol. 178; no. 1; pp. 63 - 74
• Main Author: Karnup, S.V
• Format: Journal Article
• Language: English
• Published: 1996
• Subjects: aspartic acid; autocorrelation; brain; cats; correlation analysis; electroencephalograms; electrophysiology; firing pattern; glutamic acid; guinea pigs; iontophoretic depolarization; membrane potential; neurons; potassium; spike train
• ISSN: 0340-7594; 1432-1351
• DOI: 10.1007/BF00189591

Extracellular recordings were made from the cat intact neocortex and guinea-pig neocortical slices during microiontophoretic application of amino acid neurotransmitters. Spike train autocorrelation analysis showed a high stability of firing patterns in the intact neocortex. When excitation of a cell was increased in a step-wise manner with glutamate iontophoresis only an enhancement of the rate of firing was observed. The rhythmic component, which was mainly due to periodic multiple discharges, remained up to the highest firing frequencies. In contrast to the in vivo observation, glutamate, aspartate or K+ iontophoresis in cortical slices resulted in firing pattern alternations (always from bursts or irregular activity to regular spike firing) as well as an increase in firing rate. In slices the periodic component was typically due to single-spike regularity and its frequency rose with an increase of firing rate. The comparison of autocorrelogram alternations in vivo and in vitro suggests that the temporal organization of spike trains in the intact cortex is under tight external control and is defined mainly by neuronal interactions, whereas virtually all the neurons in vitro are very sensitive to the same iontophoretic influences and their individual outputs easily chance according to the excitation (depolarization) level. The coincidence of the lowest frequencies of single-spike regularity in the in vitro preparation (5-7 Hz and 8-10 Hz) with theta and alpha-rhythms in the electroencephalogram (EEG), and with single unit firing rhythmicity in the whole brain, may represent the basis of a unit-circuit resonance and provide a high stability of these EEG-rhythms.