GABA Inputs Control Discharge Rate Primarily Within Frequency Receptive Fields of Inferior Colliculus Neurons. Palombi, Peggy S. and Donald M. Caspary. Department of Pharmacology, SIU School of Medicine, Springfield, IL 62702.
APStracts 2:0366N, 1995.
SUMMARY AND CONCLUSIONS
1. Recent studies have suggested that [delta]-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 are now thought to arise from cells which are tonotopically matched to their CIC targets. 2. We tested the hypothesis that GABA circuits are primarily aligned 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 prior to, during, and after GABA A receptor blockade. Iontophoretic application of bicuculline-methiodide (BMI) was used to block GABA A receptors. Quantitative measures of frequency bandwidth and Z- score analysis of discharge rate within the excitatory receptive field were used to compare pre- and post-drug 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 GABA 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 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 to 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. 

Received 11 July 1995; accepted in final form 26 November 1995.
APS Manuscript Number J440-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95