Corresponding authors: Dr. Fei-Jian Wu, College of Life Sciences, Central China Normal University, Wuhan 430079, Hubei, PRC; E-mail: wufj@mail.ccnu.edu.cn and Dr. Philip H.-S. Jen, Division of Biological Sciences and Interdisciplinary Neuroscience Program, University of Missouri-Columbia, MO, USA. Tel: (573)-882-7479, Fax: (573)-884-5020, E-mail: jenp@missouri.edu Received: May 4, 2007; Revised: June 5, 2007; Accepted: June 11, 2007. 2008 by The Chinese Physiological Society. ISSN : 0304-4920. http://www.cps.org.tw Chinese Journal of Physiology 51(1): 13-26, 2008 13 GABA-Mediated Modulation of the Discharge Pattern and Rate-Level Function of Two Simultaneously Recorded Neurons in the Inferior Colliculus of the Big Brown Bat, Eptesicus Fuscus Fei-Jian Wu 1 and Philip H.-S. Jen 2 1 College of Life Sciences Central China Normal University Wuhan 430079, Hubei, PRC and 2 Division of Biological Sciences and Interdisciplinary Neuroscience Program University of Missouri-Columbia Missouri, USA Abstract Neurons in the central nucleus of the inferior colliculus (IC) receive excitatory and inhibitory inputs from both lower and higher auditory nuclei. Interaction of these two opposing inputs shapes response properties of IC neurons. In this study, we examine the interaction of excitation and inhibition on the responses of two simultaneously recorded IC neurons using a probe and a masker under forward masking paradigm. We specifically study whether a sound that serves as a probe to elicit responses of one neuron might serve as a masker to suppress or facilitate the responses of the other neuron. For each pair of IC neurons, we deliver the probe at the best frequency (BF) of one neuron and the masker at the BF of the other neuron and vice versa. Among 33 pairs of IC neurons recorded, this forward masking produces response suppression in 29 pairs of IC neurons and response facilitation in 4 pairs of IC neurons. The degree of suppression decreases with recording depth, sound level and BF difference between each pair of IC neurons. During bicuculline application, the degree of response suppression decreases in the bicuculline-applied neuron but increases in the paired neuron. Our data indicate that the forward masking of responses of IC neurons observed in this study is mostly mediated through GABAergic inhibition which also shapes the discharge pattern of these neurons. These data suggest that interaction among individual IC neurons improves auditory sensitivity during auditory signal processing. Key Words: bat, bicuculline, discharge pattern, facilitation, inferior colliculus, masker, probe, rate-level function, suppression Introduction In the auditory pathway, the central nucleus of the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many lower auditory nuclei as well as from the auditory cortex (1, 2, 15, 16, 44, 45, 48). The predominant inhibitory inputs to the IC are either GABAergic, which originates extrinsically and intrinsically, or glycinergic, which originates extrinsically (12, 39, 43). Many studies have shown that the interplay between excitation and GABAergic and/or glycinergic inhibition shapes auditory response properties and multi-parametric selectivity of IC neurons ( e.g. , duration, frequency,