GABA INPUTS CONTROL DISCHARGE RATE PRIMARILY WITHIN FREQUENCY RECEPTIVE-FIELDS OF INFERIOR COLLICULUS NEURONS

1. Recent studies have suggested that gamma-aminobutyric acid (GABA) i nputs 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, an d extrinsic sources. Many GABAergic projections now are thought to ari se from cells that are tonotopically matched to their CIC targets. 2. We tested the hypothesis that GABA circuits are aligned primarily with in 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 isointe nsity contours before, during, and after GABA(A) receptor blockade. Io ntophoretic application of bicuculline-methiodide (BMI) was used to bl ock GABA(A) receptors. Quantitative measures of frequency bandwidth an d z-score analysis of discharge rate within the excitatory receptive f ield were used to compare pre- and postdrug conditions 3. Chinchilla C IC unit response properties were similar to those described for other species, with a high percentage of phasic temporal response patterns a nd nonmonotonic rate-intensity functions in response to monaural contr alateral characteristic frequency (CF) tones. Binaural responses of mo st CIC neurons showed suppression of contralaterally evoked responses by ipsilateral stimulation. 4. For 85% of CIC neurons, blockade of GAB A(A) 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 inhibitio n in the absence of near-CF changes were never observed. Forty-one per cent of CIC neurons displayed less than a 10% increase in frequency ba ndwidth at 25-35 dB above threshold with BMI application. 5. These dat a suggest that GABA inhibition arises primarily from neurons with inhi bitory fields aligned with their CIC targets. Thus the effect of the i nhibition is primarily contained within or overlapping each target neu ron's excitatory response area. CIC GABAergic circuits may function to adjust the gain needed for coding complex signals over a wide dynamic range.