The effect of bicuculline application on azimuth-dependent recovery cycle of inferior collicular neurons of the big brown bat, Eptesicus fuscus.

The recovery cycle of auditory neurons is an important neuronal property, which determines a neuron's ability to respond to pairs of sounds presented at short inter-sound intervals. This property is particularly important for bats, which rely upon analysis of returning echoes to extract the information about targets after emission of intense orientation sounds. Because target direction often changes throughout the course of hunting, the changing echo direction may affect the recovery cycle and thus temporal processing of auditory neurons. In this study, we examined the effect of sound azimuth on the recovery cycle of inferior collicular (IC) neurons in the big brown bat, Eptesicus fuscus, under free-field stimulation conditions. Our study showed that the recovery cycle of most IC neurons (42/49, 86%) was longer when determined with sounds delivered at 40 degrees ipsilateral (i40 degrees ) than at 40 degrees contralateral (c40 degrees ) to the recording site. To study the contribution of GABAergic inhibition to sound azimuth-dependent recovery cycle, we compared the recovery cycle of IC neurons determined at two sound azimuths before and during iontophoretic application of bicuculline, an antagonist for GABA(A) receptors. Bicuculline application produced a greater decrease of the recovery cycle of these neurons at i40 degrees than at c40 degrees. As a result, the azimuth-dependent recovery cycle of these neurons was abolished or greatly reduced. Possible mechanisms underlying these observations and biological relevance to bat echolocation are discussed.