Effects of GABA-mediated Inhibition on Direction-dependent Frequency Tuning in the Frog Inferior Colliculus

Earlier studies from our laboratory have shown that the frequency selectivity of neurons in the frog inferior colliculus is direction dependent. The goal of this study was to test the hypotheses that gamma-aminobutyric acid or GABA (but not glycine)-mediated synaptic inhibition was responsible for the direction-dependence in frequency tuning, and that GABA acted through creation of binaural inhibition. We performed single unit recordings and investigated the unit's free-field frequency tuning, and/or the unit's response to the interaural level differences (under dichotic stimulation), before and during local applications of antagonists specific to gamma-aminobutyric acid A and glycine receptors. Our results showed that application of bicuculline produced a broadening of free-field frequency tuning depending on sound direction, i.e., more pronounced at azimuths at which the unit exhibited narrower frequency tuning under the pre-drug condition, thereby typically abolishing direction dependence in tuning. Application of strychnine produced no change in frequency tuning. The results from dichotic stimulation further revealed that bicuculline typically elevated and/or flattened the unit's interaural-level-difference response function, indicating a reduction in the strength of binaural inhibition. Our study provides evidence that gamma-aminobutyric acid-mediated binaural inhibition is important for direction dependence in frequency tuning.