GABAergic inhibition and the effect of sound direction on rate-intensity functions of inferior collicular neurons of the big brown Bat, Eptesicus fuscus.

GABAergic inhibition shapes many auditory response properties of neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus. This study examined the role of GABAergic inhibition on direction-dependent rate-intensity functions of bat inferior collicular neurons. When plotted at three sound directions (60 degrees contralateral, 0 degrees and 60 degrees ipsilateral relative to recording site), most collicular neurons had nonmonotonic and saturated rate-intensity functions at 60 degrees contralateral and 0 degrees but had monotonic rate-intensity functions at 60 degrees ipsilateral. The dynamic range of rate-intensity functions of majority (>90%) of collicular neurons significantly decreased as the sound direction changed from 60 degrees contralateral to 60 degrees ipsilateral. Bicuculline application increased or decreased the dynamic range of IC neurons in different degrees with sound direction and abolished direction-dependent intensity sensitivity of these IC neurons. Possible mechanisms for these observations are discussed.