Modification of discharge patterns of neocortical neurons by induced oscillations of the membrane potential.

We investigated, with whole-cell recordings from rat visual cortex slices, how sinusoidal modulation of the membrane potential affects signal transmission. Subthreshold oscillations activate tetrodotoxin sensitive, transient inward currents whose threshold, phase lag and duration change with modulation frequency. These periodically recurring phases of enhanced excitability affect synaptic transmission in two ways. Weak and short lasting excitatory postsynaptic potentials evoke discharges only if they are coincident within a few milliseconds with these active membrane responses. Long-lasting, N-methyl-D-aspartate-mediated or polysynaptic excitatory postsynaptic potentials, by contrast, evoke trains of spikes, that are precisely time-locked to the oscillations and may last for more than 100 ms. Thus, oscillations impose a precise temporal window for the integration of synaptic inputs, favouring coincidence detection and they generate temporally-structured responses whose timing and amplitude are largely independent of the input. These properties are ideally suited for the synchronization of neuronal activity and the encoding of information in the precise timing of discharges. A preliminary account of these data has appeared in an abstract form [Volgushev M. et al. (1995) Eur. J Neurosci. 8, 77].