GABA SHAPES SENSITIVITY TO INTERAURAL INTENSITY DISPARITIES IN THE MOUSTACHE BATS INFERIOR COLLICULUS - IMPLICATIONS FOR ENCODING SOUND LOCATION

This study examined how GABAergic inhibition affected binaural propert ies of neurons in the mustache bat's inferior colliculus. Evaluations were made by documenting changes in acoustically evoked inhibition tha t occurred when GABAergic inputs were reversibly blocked by iontophore tic application of the GABA(A) antagonist bicuculline. We studied neur ons 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 in hibition from the other ear, and are called EI neurons. Recordings foc used on the EI region in the hypertrophied 60 kHz isofrequency contour , where the sensitivities of the EI cells to IIDs are systematically o rdered, thereby creating a map of IID sensitivity. EI neurons were cla ssified on the basis of their IID functions, of which there were two p rincipal types. Seventy percent of the cells had conventional IID func tions where the firing rate evoked by a fixed intensity at the contral ateral (excitatory) ear remained constant with low intensities at the ipsilateral (inhibitory) ear and then declined progressively as the in tensity at the ipsilateral ear increased. We refer to cells that had t his type of IID function simply as EI neurons. The IID functions in th e remaining 30% of the cells showed binaural facilitation and were cla ssified as EI/f neurons. In these cells, increasing sound intensity at the ipsilateral (inhibitory) ear when the intensity at the contralate ral (excitatory) ear was fixed, initially caused the firing rate to in crease by at least 25% above the rate evoked by the sound at the contr alateral ear alone. Additional intensity increases at the ipsilateral ear then resulted in a marked decline in response rate. We examined th e 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 inhibitio n evoked by the ipsilateral ear in about 40% of the cells. In the othe r 60% of the cells, bicuculline had little or no effect on the magnitu de of the ipsilaterally evoked inhibition. The second finding is that in more than half of the cells in which there was little or no reducti on in the magnitude of the ipsilaterally evoked inhibition, bicucullin e changed the IID at which the ipsilaterally evoked inhibition caused the discharge rate to decline by 50%. In most cells, a more intense si gnal at the ipsilateral ear was required to produce the criterion inhi bition with bicuculline than in the predrug condition. The third findi ng is that bicuculline affected binaural facilitation in 80% of EI/f c ells, and in more than half of the EI/f cells bicuculline totally elim inated the facilitation. One implication of the present study is that the collicular map of IID sensitivity is formed to a substantial degre e in the colliculus by GABAergic innervation that likely originates in the dorsal nuclei of the lateral lemniscus. The GABAergic innervation contributes to the establishment of the map in at least three ways. T he first is by the creation of the EI properties in some collicular ce lls through the convergence of excitatory and GABAergic inhibitory inp uts. This not only establishes the maximum inhibition of the cell, but also establishes the cell's 50% point. A second way occurs in other c ells where GABAergic inputs adjust 50% points and thus adjusts the neu ron's sensitivity to IIDs. A third way occurs in yet other cells in wh ich the inhibition produced by inputs from the ipsilateral ear was inc reased. Another implication of this study is that GABAergic inhibition within the colliculus appears to create or reinforce binaural facilit ation in most collicular EI/f cells.