gamma-Aminobutyric acidergic and glycinergic inputs shape coding of amplitude modulation in the chinchilla cochlear nucleus

Amplitude modulation is a prominent acoustic feature of biologically releva nt sounds, such as speech and animal vocalizations. Enhanced temporal codin g of amplitude modulation signals is found in certain dorsal and posteroven tral cochlear nucleus neurons when they are compared to auditory nerve. Alt hough mechanisms underlying this improved temporal selectivity are not know n, involvement of inhibition has been suggested. gamma-Aminobutyric acid- a nd glycine-mediated inhibition have been shown to shape the dorsal cochlear nucleus and posteroventral cochlear nucleus response properties to other a coustic stimuli. In the present study, responses to amplitude modulation to nes were obtained from chinchilla dorsal cochlear nucleus and posteroventra l cochlear nucleus neurons. The amplitude modulation carrier was set to the neuron's characteristic frequency and the modulating frequency varied from 10 Hz. Rate and temporal modulation transfer functions were compared acros s neurons. Bandpass temporal modulation transfer functions were observed in 74% of the neurons studied. Most cochlear nucleus neurons (90%) displayed flat or lowpass rate modulation transfer functions to amplitude modulation signals presented at 25-40 dB (re: characteristic frequency threshold). The role of inhibition in shaping responses to amplitude modulation stimuli wa s examined using iontophoretic application of glycine or gamma-aminobutyric acid(A) receptor agonists and antagonists. Blockade of gamma-aminobutyric acid(A) or glycine receptors increased stimulus-evoked discharge rates for a majority of neurons tested. Synchronization to the envelope was reduced, particularly at low and middle modulating frequencies, with temporal modula tion transfer functions becoming flattened and less bandpass in appearance. Application of glycine, gamma-aminobutyric acid or muscimol increased the modulation gain over the low- and mid-modulation frequencies and reduced th e discharge rate across envelope frequencies for most neurons tested. These findings support the hypothesis that glycinergic and gamma-aminobutyric ac idergic inputs onto certain dorsal cochlear nucleus and posteroventral coch lear nucleus neurons play a role in shaping responses to amplitude modulati on stimuli and may be responsible for the reported preservation of amplitud e modulation temporal coding in dorsal cochlear nucleus and posteroventral cochlear nucleus neurons at high stimulus intensities or in background nois e. (C) 1999 Elsevier Science B.V. All rights reserved.