ROLE OF GABA IN SHAPING FREQUENCY TUNING AND CREATING FM SWEEP SELECTIVITY IN THE INFERIOR COLLICULUS

1. We examined the role of gamma-aminobutyric acid (GABA)-mediated inh ibition in shaping excitatory tuning curves and creating selectivity f or frequency-modulated (FM) sweeps in 29 neurons in the central nucleu s of the inferior colliculus (ICC) of the pallid bat, with the use of single-unit recording coupled with the iontophoretic application of bi cuculline methiodide (BIC), an antagonist of GABA(A) receptors. 2. BIC increased response magnitude 2 to 6 times over pretreatment levels in >80% of neurons tested, and converted >50% of nonmonotonic intensity- rate functions to monotonic or plateaued functions, demonstrating that GABAergic input normally limited response magnitude and inhibited res ponses at higher intensities. BIC typically had little effect on respo nse thresholds, except in more specialized neurons that normally respo nded poorly to tones. In these cases, BIC disinhibited the neurons' re sponses to tones and lowered excitatory thresholds as much as 25 dB. 3 . We examined the effects of BIC application on both excitatory and in hibitory tuning curves (measured with simultaneous 2-tone inhibition) to determine whether inhibitory curves were GABA mediated and whether removal of this inhibition was accompanied by an expansion of the exci tatory curve. BIC had variable effects on the width of excitatory curv es. In most cases, excitatory curves were at least slightly broadened, and expanded into regions previously occupied by inhibitory curves. H owever, in a few cases, inhibitory curves could be eliminated without an expansion of the excitatory curve. The greatest effect was seen in neurons with closed excitatory tuning curves; blocking GABAergic input caused the curves to open, allowing the neurons to respond at higher intensities. 4. Approximately 50% of the neurons in the ICC tuned to t he spectrum of the bat's downward FM sweeping biosonar pulse respond p referentially to downward FM sweeps and not to upward sweeps, tones, o r noise. In all neurons tested, BIC at least partially destroyed selec tivity for sweep direction. This destruction could occur, however, wit hout a loss of response exclusivity; in some cases, the neurons still did not respond to tones or noise. These results suggest that response selectivity for a species-specific signal is created by GABAergic inp ut to ICC neurons. These results are used to suggest a mechanism that creates selectivity for FM sweep direction.