Modulation of frequency selectivity by Na+- and K+-conductances in neuronsof auditory thalamus

In thalamic neurons, frequency-filter properties arise from intrinsic membr ane properties which transform sensory inputs to thalamocortical signals. T hey also contribute to the tendency for the membrane to generate synchroniz ed oscillations. We studied the frequency selectivities of thalamocortical neurons in the rat ventral medial geniculate body (MGBv) in vitro, using wh ole-cell recording techniques, sinewave (swept 'ZAP' or single) current inp uts and pharmacological blockade of membrane currents. In a voltage range t hat was subthreshold to spike genesis, the frequency responses below 20 Hz were voltage-dependent; they exhibited lowpass characteristics at depolariz ed potentials and bandpass resonance (near 1 Hz) in the activation range (s imilar to -65 to -50 mV) of the low-threshold Ca2+-current (I-T). A tempera ture increase of > 10 degrees C in 3 neurons did not change this voltage-de pendence and increased the frequency of maximum resonance to 2 Hz. The remo val of extracellular Ca2+, its equimolar substitution with Mg2+ or blockade of I-T With Ni2+ (0.5 mM) completely blocked the resonance at hyperpolariz ed potentials or rest, as well as the low-threshold Ca2+-spike (LTS). Block ade of high threshold Ca2+-currents with Cd2+ (50 mu M) did not affect the resonance. These data implied that, like the LTS, an activation of TT produ ced the membrane resonance. An increased ZAP-current input evoked action po tentials near the resonant frequency as well as Cd2+-sensitive high-thresho ld Ca2+-spikes at depolarized membrane potentials and very low frequencies. By blocking a persistent Na+-current (I-NaP), tetrodotoxin (300 nM) reduce d the magnitude of the frequency response without affecting the frequency p reference. The response was larger in amplitude, especially at frequencies lower than the maximum resonant frequency, when we used 4-aminopyridine (0. 05-0.1 and 1-2 mM), Ba2+ (0.2 mM) or Cs+ (3 mM) to block voltage-dependent K+-currents. From these data, we suggest that A-type (I-A and I-AS) and inw ardly rectifying (I-KIR) K+-currents modulate resonance, changing the quali ty of the lowpass filter function. We conclude that the generation of membr ane resonance in MGBv neurons depends critically on I-T-activation while th e quality of the frequency response is subject to modulation by voltage-dep endent conductances. The frequency selectivities in MGBv may contribute to lowpass filter functions for auditory transmission during wakefulness and o scillations observed during sleep. (C) 1999 Elsevier Science B.V. All right s reserved.