Brain Research 862 (2000) 127–137 www.elsevier.com / locate / bres Research report The role of GABAergic inhibition on direction-dependent sharpening of frequency tuning in bat inferior collicular neurons 1 * Philip H.-S. Jen , Jiping Zhang Division of Biological Sciences, The University of Missouri –Columbia, Missouri, MO 65211, USA Accepted 1 February 2000 Abstract This study examined the role of GABAergic inhibition on direction-dependent sharpening of frequency tuning curves (FTCs) in bat inferior collicular (IC) neurons under free field stimulation conditions. The minimum threshold (MT) at the neurons best frequency (BF) and the sharpness (Q , Q , Q ) of FTCs of most IC neurons increased as the sound direction changed from contralateral azimuths to 10 20 30 ipsilateral azimuths. The application of GABA antagonist, bicuculline, lowered all MTs but the application did not abolish A direction-dependent variation in MT. MTs determined during bicuculline application at 40 ipsilateral were still significantly higher than those determined at 408 contralateral (two-tailed paired t -test, P ,0.0001). In contrast, although application of bicuculline essentially had no effect on the BFs of IC neurons, it differentially broadened neurons FTCs at different azimuths abolishing the direction-dependent sharpening of frequency tuning (i.e. Q values, two-tailed paired t -test, P ,0.01). These data indicate that GABAergic inhibition makes n an important contribution to the direction-dependent frequency tuning of most IC neurons.  2000 Elsevier Science B.V. All rights reserved. Themes: Sensory systems Topics: Auditory systems: central physiology Keywords: Bat; Bicuculline; Frequency tuning curve; GABAergic inhibition; Inferior colliculus 1. Introduction nuclei and is an obligatory relay center for higher auditory centers and, thus, is critical for central auditory processing During hunting, big brown bats (Eptesicus fuscus) utilize [4,24]. In these studies, directional sensitivity was studied an echolocation mechanism to detect insects and avoid mainly by determining how sound direction alters the obstacles [11]. In this active acoustic behavior, extraction response magnitude in terms of the minimum threshold of target feature including target direction is essential for (MT) at best frequency (BF) or a neurons number of successful prey capture. To understand the neural basis for impulses per stimulus. These studies have shown that in directional hearing in bats, many studies have been de- the IC most neurons discharge a maximal number of voted to examining directional sensitivity of auditory impulses and have the lowest MTs to contralateral sounds. neurons in the brainstem and directionality of sound The directional slope, which is obtained by dividing the pressure transformation at the pinna [8,12,15,17– difference in the number of impulses or the MT de- 20,27,28,31,33,36]. Many of these studies have concen- termined at the angles of maximal and minimal sensitivity trated on the inferior collicular (IC) because it receives by the azimuthal difference between the two angles, is excitatory and inhibitory inputs from all the lower auditory frequency-dependent and increases with stimulus fre- quency [8,12,13,18,19,28,31]. While directional responses of auditory neurons provide *Corresponding author. Tel.: 11-573-882-7479; fax: 11-573-884- information about target direction, a bat relies upon the 5020. frequency tuning characteristics of its auditory neurons for E-mail address: jenp@missouri.edu (P.H.-S. Jen) 1 fine target feature analysis. If sound direction affects the Department of Biology, East China Normal University, Shanghai, PR China. frequency tuning characteristics of auditory neurons, it 0006-8993 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993(00)02098-9