The role of GABAergic inhibition on direction-dependent sharpening of frequency tuning in bat inferior collicular neurons

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 (Q₁₀, Q₂₀, Q₃₀) of FTCs of most IC neurons increased as the sound direction changed from contralateral azimuths to ipsilateral azimuths. The application of GABA(A) antagonist, bicuculline, lowered all MTs but the application did not abolish direction-dependent variation in MT. MTs determined during bicuculline application at 40 ipsilateral were still significantly higher than those determined at 40°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(n) values, two-tailed paired t-test, P<0.01). These data indicate that GABAergic inhibition makes an important contribution to the direction-dependent frequency tuning of most IC neurons. Themes: Sensory systems. Topics: Auditory systems: central physiology. Copyright (C) 2000 Elsevier Science B.V.