Membrane capacitance of cortical neurons and glia during sleep oscillations and spike-wave seizures

Dual intracellular recordings in vivo were used to disclose relationships between cortical neurons and glia during spontaneous slow (<1 Hz) sleep oscillations and spike-wave (SW) seizures in cat. Glial cells displayed a slow membrane potential oscillation (<1 Hz), in close synchrony with cortical neurons. In glia, each cycle of this oscillation was made of a round depolarizing potential of 1.5-3 mV. The depolarizing slope corresponded to a steady depolarization and sustained synaptic activity in neurons (duration, 0.5-0.8 s). The repolarization of the glial membrane (duration, 0.5-0.8 s) coincided with neuronal hyperpolarization, associated with disfacilitation, and suppressed synaptic activity in cortical networks. SW seizures in glial cells displayed phasic events, synchronized with neuronal paroxysmal potentials, superimposed on a plateau of depolarization, that lasted for the duration of the seizure. Measurements of the neuronal membrane capacitance during slow oscillating patterns showed small fluctuations around the resting values in relation to the phases of the slow oscillation. In contrast, the glial capacitance displayed a small-amplitude oscillation of 1-2 Hz, independent of phasic sleep and seizure activity. Additionally, in both cell types, SW seizures were associated with a modulatory, slower oscillation (approximate to 0.2 Hz) and a persistent increase of capacitance, developing in parallel with the progression of the seizure. These capacitance variations were dependent on the severity of the seizure and the distance between the presumed seizure focus and the recording site. We suggest that the capacitance variations may reflect changes in the membrane surface area (swelling) and/or of the interglial communication via gap junctions, which may affect the synchronization and propagation of paroxysmal activities.