SUBTHRESHOLD OSCILLATIONS AND RESONANT-FREQUENCY IN GUINEA-PIG CORTICAL-NEURONS - PHYSIOLOGY AND MODELING

1. Intracellular recordings were made from neurons in slices from guin ea-pig frontal cortex. In 50% of the cells, sustained subthreshold vol tage oscillations were evoked by long (> 6 s) depolarizing pulses. The peak-to-peak amplitude of these oscillations was less than 5 mV and t he frequency was voltage dependent, increasing with depolarization fro m 4 (near rest) to 20 Hz (at 30 mV depolarization). 2. The impedance-f requency relationship of both oscillating and non-oscillating cells wa s studied by intracellular injection of sinusoidal current with linear ly changing frequency. In most cells, a peak in the impedance magnitud e (resonant behaviour) was observed at depolarized levels. The frequen cy of the peak impedance (peak frequency) increased with depolarizatio n from 3 (near rest) to 15 Hz (at 30 mV depolarization). 3. Applicatio n of TTX (10(-6) M) significantly decreased the impedance magnitude ne ar the peak frequency. The subthreshold oscillations, however, as well as the action potentials, were fully blocked by TTX. On the other han d, TEA (15 mm) and Cs+ (5 mar) abolished both the subthreshold oscilla tions and the resonant behaviour. Replacing Ca2+ with Co2+ (5 mM) or N i2+ (1 mM) did not abolish the subthreshold oscillations. The peak in the frequency-response curve was only slightly reduced. 4. An isopoten tial membrane model, consisting of a leak current, a fast persistent s odium current, a slow non-inactivating potassium current (with the kin etics of the M-current) and membrane capacitance, is sufficient to pro duce both voltage oscillations and resonant behaviour. The kinetics of the K+ current by itself is sufficient to produce resonance behaviour . The Na+ current amplifies the peak impedance magnitude and is essent ial for the generation of subthreshold oscillation. The model correctl y predicted the behaviour of the frequency response before and after T TX and TEE application, as well as the relation between the expected p assive impedance and the experimental impedance. 5. We speculate that the tendency of the neurons to generate voltage signals at a certain f requency (as a result of the subthreshold oscillations) and to prefere ntially respond to inputs arriving at the same frequency (the resonanc e behaviour) promotes population activity at that preferred frequency.