Analysis of the role of inhibition in shaping responses to sinusoidally amplitude-modulated signals in the auditory system

Abstract

Neurons in the inferior colliculus (IC) typically respond with phase-locked discharges to low rates of sinusoidal amplitude modulated (SAM) signals and fail to phase-lock to higher SAM rates. The hypothesis that these properties are shaped by the integration of phase-locked excitation and inhibition, as they are in lower nuclei, was tested. Responses were recorded from IC neurons evoked by SAM signals before and during the iontophoretic application of bicuculline, a competitive antagonist for GABAA receptors, strychnine, a competitive antagonist for glycine receptors, and the GABAB receptor blocker, phaclofen. The hypothesis that inhibition shapes responses to SAM signals in the IC was not confirmed. In more than 90% of the ICc neurons tested, the range of SAM rates to which they phase-locked was unchanged after blocking inhibition with bicuculline, strychnine, or phaclofen, applied either individually or in combination. These results illustrate that the same response property, phase-locking restricted to low SAM rates, is formed in more than one way in the auditory brainstem. In lower nuclei, the mechanism is coincidence of phase-locked excitation and inhibition, whereas in IC the same response feature is formed by a different but unknown mechanism. [Work supported by NICD.]