Resonant behavior and frequency preferences of thalamic neurons.

1. We studied the voltage responses of thalamocortical neurons to a periodic current input of variable frequency, in slices of mediodorsal thalamus (guinea pig). The ratio of the Fourier transform of the voltage response to the Fourier transform of the oscillatory current input was used to calculate the frequency response of the neurons at different resting and imposed membrane potentials. 2. Most neurons displayed a resonant hump in the frequency response curve. A narrow band of low-frequency (2-4 Hz) resonance occurred near the resting level [-66 +/- 8 mV (SD)] and at imposed membrane potentials in a range of -60 to -80 mV. An additional wide band (12-26 Hz) of peak resonant frequencies was observed at depolarized levels. 3. The low-frequency resonance was insensitive to tetrodotoxin (TTX) application in concentrations (0.5-1 microM) that blocked a depolarization activated inward rectifier and Na(+)-dependent action potentials. TTX, however, eliminated the wide-band resonant hump centered at 12-26 Hz that we observed at depolarized membrane potentials. 4. Application of Ni2+ (0.5-1 mM) reversibly blocked all slow spikes and greatly reduced the low-frequency resonant humps, without changing the resting potential. Octanol in concentrations of 50 microM had similar effects. 5. Application of Cs+ (3-5 mM), a blocker of the hyperpolarization activated inward rectifier, produced a 5- to 10-mV depolarization and completely blocked the rectification. Cs+ did not alter the low-frequency resonant hump or its dependence on membrane voltage.(ABSTRACT TRUNCATED AT 250 WORDS)