Recent studies have suggested that gamma-aminobutyric acid (GABA) inputs shape monaural and binaural neuronal response properties in the central nucleus of the inferior colliculus (CIC). CIC neurons receive major inhibitory GABAergic projections from intrinsic, commissural, and extrinsic sources. Many GABAergic projections now are thought to arise from cells that are tonotopically matched to their CIC targets. 2. We tested the hypothesis that GABA circuits are aligned primarily within the CIC target neuron's excitatory response area and therefore have their greatest effects on discharge rate mainly within that frequency domain. GABA inhibition was examined by recording families of isointensity contours before, during, and after GABAA receptor blockade. Iontophoretic application of bicuculline-methiodide (BMI) was used to block GABAA receptors. Quantitative measures of frequency bandwidth and z-score analysis of discharge rate within the excitatory receptive field were used to compare pre- and postdrug conditions. 3. Chinchilla CIC unit response properties were similar to those described for other species, with a high percentage of phasic temporal response patterns and nonmonotonic rate-intensity functions in response to monaural contralateral characteristic frequency (CF) tones. Binaural responses of most CIC neurons showed suppression of contralaterally evoked responses by ipsilateral stimulation. 4. For 85% of CIC neurons, blockade of GABAA inputs was found to increase discharge rate within the excitatory response area. Forty-five percent were classified as near-CF changes and 32% as near-CF and low side. Changes in lateral/flanking inhibition in the absence of near-CF changes were never observed. Forty-one percent of CIC neurons displayed less than a 10% increase in frequency bandwidth at 25-35 dB above threshold with BMI application. 5. These data suggest that GABA inhibition arises primarily from neurons with inhibitory fields aligned with their CIC targets. Thus the effect of the inhibition is primarily contained within or overlapping each target neuron's excitatory response area. CIC GABAergic circuits may function to adjust the gain needed for coding complex signals over a wide dynamic range.
