Park T J, Pollak G D
Department of Zoology, University of Texas, Austin 78712.
J Neurosci. 1993 May;13(5):2050-67. doi: 10.1523/JNEUROSCI.13-05-02050.1993.
This study examined how GABAergic inhibition affected binaural properties of neurons in the mustache bat's inferior colliculus. Evaluations were made by documenting changes in acoustically evoked inhibition that occurred when GABAergic inputs were reversibly blocked by iontophoretic application of the GABAA antagonist bicuculline. We studied neurons sensitive to interaural intensity disparities (IIDs), since these are the principal cues animals use to localize high-frequency sounds. Neurons sensitive to these cues receive excitation from one ear and inhibition from the other ear, and are called EI neurons. Recordings focused on the EI region in the hypertrophied 60 kHz isofrequency contour, where the sensitivities of the EI cells to IIDs are systematically ordered, thereby creating a map of IID sensitivity. EI neurons were classified on the basis of their IID functions, of which there were two principal types. Seventy percent of the cells had conventional IID functions where the firing rate evoked by a fixed intensity at the contralateral (excitatory) ear remained constant with low intensities at the ipsilateral (inhibitory) ear and then declined progressively as the intensity at the ipsilateral ear increased. We refer to cells that had this type of IID function simply as EI neurons. The IID functions in the remaining 30% of the cells showed binaural facilitation and were classified as EI/f neurons. In these cells, increasing sound intensity at the ipsilateral (inhibitory) ear when the intensity at the contralateral (excitatory) ear was fixed, initially caused the firing rate to increase by at least 25% above the rate evoked by the sound at the contralateral ear alone. Additional intensity increases at the ipsilateral ear then resulted in a marked decline in response rate. We examined the effects of bicuculline on three binaural properties: (1) the degree of inhibition evoked by the ipsilateral ear (the maximum inhibition), (2) the IID at which the unit's discharge rate declined by 50% (the 50% point), and (3) binaural facilitation. There are three main findings. First, bicuculline substantially reduced or eliminated the inhibition evoked by the ipsilateral ear in about 40% of the cells. In the other 60% of the cells, bicuculline had little or no effect on the magnitude of the ipsilaterally evoked inhibition. The second finding is that in more than half of the cells in which there was little or no reduction in the magnitude of the ipsilaterally evoked inhibition, bicuculline changed the IID at which the ipsilaterally evoked inhibition caused the discharge rate to decline by 50%.(ABSTRACT TRUNCATED AT 400 WORDS)
本研究考察了γ-氨基丁酸（GABA）能抑制如何影响髯蝠下丘神经元的双耳特性。通过记录当GABAA拮抗剂荷包牡丹碱经离子电泳施加而使GABA能输入被可逆性阻断时，听觉诱发抑制的变化来进行评估。我们研究了对双耳强度差异（IID）敏感的神经元，因为这些是动物用于定位高频声音的主要线索。对这些线索敏感的神经元从一只耳朵接收兴奋，从另一只耳朵接收抑制，被称为EI神经元。记录聚焦于肥厚的60千赫兹等频轮廓中的EI区域，其中EI细胞对IID的敏感性呈系统性排列，从而形成一个IID敏感性图谱。EI神经元根据其IID功能进行分类，主要有两种类型。70%的细胞具有传统的IID功能，即对侧（兴奋性）耳固定强度诱发的放电率在同侧（抑制性）耳低强度时保持恒定，然后随着同侧耳强度增加而逐渐下降。我们将具有这种IID功能类型的细胞简单地称为EI神经元。其余30%细胞的IID功能表现出双耳易化，被归类为EI/f神经元。在这些细胞中，当对侧（兴奋性）耳强度固定时，同侧（抑制性）耳声音强度增加，最初会使放电率比仅由对侧耳声音诱发的放电率至少增加25%。同侧耳强度进一步增加则导致反应率显著下降。我们考察了荷包牡丹碱对三种双耳特性的影响：（1）同侧耳诱发的抑制程度（最大抑制），（2）单位放电率下降50%时的IID（50%点），以及（3）双耳易化。有三个主要发现。第一，荷包牡丹碱在约40%的细胞中大幅降低或消除了同侧耳诱发的抑制。在另外60%的细胞中，荷包牡丹碱对同侧诱发抑制的幅度几乎没有影响。第二个发现是，在超过一半的同侧诱发抑制幅度几乎没有降低的细胞中，荷包牡丹碱改变了同侧诱发抑制导致放电率下降50%时的IID。（摘要截断于400字）
