GABA shapes sensitivity to interaural intensity disparities in the mustache bat's inferior colliculus: implications for encoding sound location.

This study examined how GABAergic inhibition affected binaural properties of neurons in the mustache bat's inferior colliculus. Evaluations were made by documenting changes in acoustically evoked inhibition that occurred when GABAergic inputs were reversibly blocked by iontophoretic application of the GABAA antagonist bicuculline. We studied neurons sensitive to interaural intensity disparities (IIDs), since these are the principal cues animals use to localize high-frequency sounds. Neurons sensitive to these cues receive excitation from one ear and inhibition from the other ear, and are called EI neurons. Recordings focused on the EI region in the hypertrophied 60 kHz isofrequency contour, where the sensitivities of the EI cells to IIDs are systematically ordered, thereby creating a map of IID sensitivity. EI neurons were classified on the basis of their IID functions, of which there were two principal types. Seventy percent of the cells had conventional IID functions where the firing rate evoked by a fixed intensity at the contralateral (excitatory) ear remained constant with low intensities at the ipsilateral (inhibitory) ear and then declined progressively as the intensity at the ipsilateral ear increased. We refer to cells that had this type of IID function simply as EI neurons. The IID functions in the remaining 30% of the cells showed binaural facilitation and were classified as EI/f neurons. In these cells, increasing sound intensity at the ipsilateral (inhibitory) ear when the intensity at the contralateral (excitatory) ear was fixed, initially caused the firing rate to increase by at least 25% above the rate evoked by the sound at the contralateral ear alone. Additional intensity increases at the ipsilateral ear then resulted in a marked decline in response rate. We examined the effects of bicuculline on three binaural properties: (1) the degree of inhibition evoked by the ipsilateral ear (the maximum inhibition), (2) the IID at which the unit's discharge rate declined by 50% (the 50% point), and (3) binaural facilitation. There are three main findings. First, bicuculline substantially reduced or eliminated the inhibition evoked by the ipsilateral ear in about 40% of the cells. In the other 60% of the cells, bicuculline had little or no effect on the magnitude of the ipsilaterally evoked inhibition. The second finding is that in more than half of the cells in which there was little or no reduction in the magnitude of the ipsilaterally evoked inhibition, bicuculline changed the IID at which the ipsilaterally evoked inhibition caused the discharge rate to decline by 50%.(ABSTRACT TRUNCATED AT 400 WORDS)