Corresponding author: Dr. Philip H. S. Jen, Division of Biological Sciences, University of Missouri-Columbia, MO 65211, USA. Tel: 573- 882-7479, Fax: 573-884-5020, E-mail:jenp@missouri.edu Received: January 6, 2003; Accepted: March 13, 2003. Chinese Journal of Physiology 46(2): 83-90, 2003 83 GABAergic Inhibition and the Effect of Sound Direction on Rate-Intensity Functions of Inferior Collicular Neurons of the Big Brown Bat, Eptesicus fuscus Philip H. S. Jen, Ruiben Feng 1 and Brian Chen 2 Division of Biological Sciences University of Missouri Columbia Columbia, MO 65211, USA 1 Department of Biology East China Normal University Shanghai, PRC 2 Department of Neurobiology State University of New York Stony Brook, NY, USA Abstract 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° contralateral, 0° and 60° ipsilateral relative to recording site), most collicular neurons had nonmonotonic and saturated rate-intensity functions at 60° contralateral and 0° but had monotonic rate-intensity functions at 60° ipsilateral. The dynamic range of rate-intensity functions of majority (>90%) of collicular neurons significantly decreased as the sound direction changed from 60° contralateral to 60° 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. Key Words: bat, bicuculline, GABAergic inhibition, inferior colliculus, rate-intensity function Introduction In sound analysis, frequency and intensity are two fundamental signal parameters. They are the basis of perception of pitch and loudness. For this reason, many studies have examined the frequency or intensity sensitivity of auditory neurons in different animals (25). These studies determined how a change in frequency or intensity of sound stimuli delivered from a specific direction might affect the responses of auditory neurons such as the number of impulses, latency, response probability or threshold. However, naturally occurring sounds often change direction with time. Therefore, a neuron’s intensity or frequency sensitivity to sounds broadcast from one direction cannot predict well its intensity or frequency sensitivity to sounds broadcast from different directions. For this reason, many studies have examined the effect of sound direction on responses of auditory neurons (5). These studies showed that most neurons in the central nucleus of the inferior colliculus (IC) discharge maximally or display lowest minimum threshold (MT) to sounds delivered from contralateral angles (1, 2, 4, 10, 11, 16, 17, 18, 27, 28, 30, 33, 35). They also have sharper frequency tuning curves with higher MTs at the best frequency (BF) to sounds delivered from ipsilateral than from contralateral angles. It has been shown that GABAergic inhibition, which is predominant in the IC (6, 7, 23, 26), contributes to this direction-