Fujita I, Konishi M
Division of Biology, California Institute of Technology, Pasadena 91125.
J Neurosci. 1991 Mar;11(3):722-39. doi: 10.1523/JNEUROSCI.11-03-00722.1991.
The barn owl uses interaural time differences (ITDs) to localize the azimuthal position of sound. ITDs are processed by an anatomically distinct pathway in the brainstem. Neuronal selectivity for ITD is generated in the nucleus laminaris (NL) and conveyed to both the anterior portion of the ventral nucleus of the lateral lemniscus (VLVa) and the central (ICc) and external (ICx) nuclei of the inferior colliculus. With tonal stimuli, neurons in all regions are found to respond maximally not only to the real ITD, but also to ITDs that differ by integer multiples of the tonal period. This phenomenon, phase ambiguity, does not occur when ICx neurons are stimulated with noise. The main aim of this study was to determine the role of GABAergic inhibition in the processing of ITDs. Selectivity for ITD is similar in the NL and VLVa and improves in the ICc and ICx. Iontophoresis of bicuculline methiodide (BMI), a selective GABAA antagonist, decreased the ITD selectivity of ICc and ICx neurons, but did not affect that of VLVa neurons. Responses of VLVa and ICc neurons to unfavorable ITDs were below the monaural response levels. BMI raised both binaural responses to unfavorable ITDs and monaural responses, though the former remained smaller than the latter. During BMI application, ICx neurons showed phase ambiguity to noise stimuli and no longer responded to a unique ITD. BMI increased the response magnitude and changed the temporal discharge patterns in the VLVa, ICc, and ICx. Iontophoretically applied GABA exerted effects opposite to those of BMI, and the effects could be antagonized with simultaneous application of BMI. These results suggest that GABAergic inhibition (1) sharpens ITD selectivity in the ICc and ICx, (2) contributes to the elimination of phase ambiguity in the ICx, and (3) controls response magnitude and temporal characteristics in the VLVa, ICc, and ICx. Through these actions, GABAergic inhibition shapes the horizontal dimension of the auditory receptive fields.
仓鸮利用双耳时间差（ITD）来定位声音的方位角位置。ITD由脑干中一条在解剖学上不同的通路进行处理。对ITD的神经元选择性在层状核（NL）中产生，并传递到外侧丘系腹侧核的前部（VLVa）以及下丘的中央（ICc）和外侧（ICx）核。对于音调刺激，发现所有区域的神经元不仅对真实的ITD反应最大，而且对与音调周期的整数倍不同的ITD也反应最大。这种现象，即相位模糊，在用噪声刺激ICx神经元时不会出现。本研究的主要目的是确定GABA能抑制在ITD处理中的作用。在NL和VLVa中对ITD的选择性相似，而在ICc和ICx中有所提高。选择性GABAA拮抗剂甲基荷包牡丹碱（BMI）的离子电渗疗法降低了ICc和ICx神经元的ITD选择性，但不影响VLVa神经元的ITD选择性。VLVa和ICc神经元对不利ITD的反应低于单耳反应水平。BMI提高了对不利ITD的双耳反应以及单耳反应，尽管前者仍小于后者。在应用BMI期间，ICx神经元对噪声刺激表现出相位模糊，并且不再对唯一的ITD做出反应。BMI增加了反应幅度并改变了VLVa、ICc和ICx中的时间放电模式。离子电渗应用的GABA产生了与BMI相反的效果，并且这些效果可以通过同时应用BMI来拮抗。这些结果表明，GABA能抑制（1）增强了ICc和ICx中的ITD选择性，（2）有助于消除ICx中的相位模糊，以及（3）控制VLVa、ICc和ICx中的反应幅度和时间特征。通过这些作用，GABA能抑制塑造了听觉感受野的水平维度。
