The Journal of Neurophysiology Vol. 78 No. 2 August 1997, pp. 591-596
Copyright ©1997 The American Physiological Society

Isoflurane Attenuates Resonant Responses of Auditory Thalamic Neurons

Frank Tennigkeit^{1, 3}, Craig R. Ries^{1, 2}, Dietrich W. F. Schwarz³, and Ernest Puil^{1, 2}

¹ Department of Pharmacology and Therapeutics, ² Department of Anesthesia and The Rotary Hearing Centre, ³ Department of Surgery, Faculty of Medicine, The University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada

Tennigkeit, Frank, Craig R. Ries, Dietrich W. F. Schwarz, and Ernest Puil. Isoflurane attenuates resonant responses of auditory thalamic neurons. J. Neurophysiol. 78: 591-596, 1997. In thalamocortical neurons, sensory signals are transformed differently during various states of consciousness. We investigated the effects of a general anesthetic, isoflurane, on the frequency responses of neurons in the ventral medial geniculate body, the primary nucleus of the auditory thalamus. Using slice preparations, whole cell current-clamp recording techniques, and frequency-domain analyses with oscillatory inputs, we observed a resonance in the hyperpolarized voltage range, implying a frequency preference near 1 Hz in the subthreshold frequency responses of medial geniculate neurons. As in other thalamocortical neurons, an interaction of a T-type Ca²⁺ current with passive membrane properties generates the resonant responses. The frequency preference shapes the input-output signal transformation, coupling oscillatory inputs at preferred frequencies to firing. Thus resonance may contribute to the rhythmic synchronization of the output to the cortex. In a concentration range of 0.5-3%, isoflurane application reversibly decreased the resonant responses of medial geniculate neurons. Throughout the subthreshold voltage range, it reduced impedance at frequencies <10 Hz. At depolarized potentials near 60 mV, isoflurane reduced the low-pass filter selectivity of the neuron membrane. At rest near 70 mV or at hyperpolarized potentials, isoflurane had a greater effect on resonance (centered at ~1 Hz), reducing the peak impedance more than the magnitudes at other frequencies. At concentrations of 2%, isoflurane completely blocked the resonance peak, thereby imposing low-pass characteristics of poor quality throughout the subthreshold voltage range. Application of isoflurane reversibly increased membrane conductance and the current threshold for firing evoked by depolarizing pulses from potentials between 60 and 90 mV. The neurons discharged in a tonic pattern on depolarization from about 60 mV and in a phasic (burst) mode from potentials negative to about 70 mV. An increase in current amplitude compensated the suppression of tonic firing much more readily than that of the burst firing on a low-threshold Ca²⁺ spike. Although a reduction in T-type Ca²⁺ channel activationmay occur during isoflurane application, the depression of resonance is consistent with an interaction of a greatly increased leak conductance with the low-threshold Ca²⁺ current and the membrane capacitance. In the intact animal, this would tend to disrupt synchronized neural oscillations and the transfer of auditory information.

This article has been cited by other articles:

				V. Tohidi and F. Nadim Membrane Resonance in Bursting Pacemaker Neurons of an Oscillatory Network Is Correlated with Network Frequency J. Neurosci., May 20, 2009; 29(20): 6427 - 6435. [Abstract] [Full Text] [PDF]

				R.-P. Behrendt Dysregulation of thalamic sensory 'transmission' in schizophrenia: neurochemical vulnerability to hallucinations J Psychopharmacol, May 1, 2006; 20(3): 356 - 372. [Abstract] [PDF]

				I Erchova, G Kreck, U Heinemann, and A. V. M Herz Dynamics of rat entorhinal cortex layer II and III cells: characteristics of membrane potential resonance at rest predict oscillation properties near threshold J. Physiol., October 1, 2004; 560(1): 89 - 110. [Abstract] [Full Text] [PDF]

				F. Tennigkeit, D. W. F. Schwarz, and E. Puil Effects of Metabotropic Glutamate Receptor Activation in Auditory Thalamus J Neurophysiol, August 1, 1999; 82(2): 718 - 729. [Abstract] [Full Text] [PDF]

				C. R. Ries and E. Puil Mechanism of Anesthesia Revealed by Shunting Actions of Isoflurane on Thalamocortical Neurons J Neurophysiol, April 1, 1999; 81(4): 1795 - 1801. [Abstract] [Full Text] [PDF]

				C. R. Ries and E. Puil Ionic Mechanism of Isoflurane's Actions on Thalamocortical Neurons J Neurophysiol, April 1, 1999; 81(4): 1802 - 1809. [Abstract] [Full Text] [PDF]