GABAergic Inputs Shape Responses to Amplitude Modulated Stimuli in the Inferior Colliculus

The inferior colliculus (IC) is an important auditory processing center receiving inputs from lower brainstem nuclei, higher auditory and nonauditory structures, and contralateral IC. The IC, along with other auditory structures, is involved in coding information about the envelope of complex signals. Biologically relevant acoustic signals, including animal vocalizations and speech, are spectrally and temporally complex and display amplitude and frequency variations over time. Certain IC neurons respond selectively over a narrow range of modulation frequencies to sinusoidally amplitude modulated (SAM) stimuli. Responses to SAM stimuli can be measured in terms of discharge rate, with rate plotted against the modulation frequency to generate rate modulation transfer functions (rMTF). A role for the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), in shaping selective responses to SAM stimuli has been suggested. The present study examined the role of GABA in shaping responses to SAM stimuli in the IC of anesthetized chinchilla. Responses from 94 IC neurons were obtained before, during and after iontophoretic application of the GABA(A) receptor antagonist bicuculline methiodide. Complete responses to SAM stimuli were obtained from 55 extensively tested neurons, displaying band-pass (38) and low-pass rMTFs (17). For neurons showing band-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for 14 units, increased discharge near the best modulation frequency for 12 units. For neurons showing low-pass rMTFs, GABA(A) receptor blockade selectively increased discharge rate at low modulation frequencies for nine units. GABA(A) receptor blockade consistently reduced peak modulation gain, producing low-pass gain functions in a subset of IC neurons. In support of previous findings suggesting that selective temporal responses to SAM stimuli are coded in lower brainstem nuclei, temporal responses to SAM stimuli were relatively unaffected by GABA(A) receptor blockade. These findings support a role for GABA in shaping selective rate responses to SAM stimuli for a subset of chinchilla IC neurons.