Natural auditory environment consists of multiple sound sources that are embedded in ambient strong and weak noise. For effective sound communication and signal analysis, animals must somehow extract biologically relevant signals from the inevitable interference of ambient noise. The present study examined how a weak noise may affect the amplitude sensitivity of neurons in the mouse central nucleus of the inferior colliculus (IC) which receives convergent excitatory and inhibitory inputs from both lower and higher auditory centers. Specifically, we studied the amplitude sensitivity of IC neurons using a probe (best frequency pulse) and a masker (weak noise) under simultaneous masking paradigm. For most IC neurons, weak noise masking increases the minimum threshold and decreases the number of impulses. Noise masking also increased the slope and decreased the dynamic range of the rate amplitude function of these IC neurons. The strength of this noise masking was greater at low than at high sound amplitudes. This variation in the amplitude sensitivity of IC neurons in the presence of the weak noise was mostly mediated through GABAergic inhibition. These data indicate that in the real world the ambient weak noise improves amplitude sensitivity of IC neurons through GABAergic inhibition while inevitably decreases the range of overall auditory sensitivity of IC neurons.

本实验使用了9只成年健康的大棕蝠（Eptesicus fuscus）。采用双声刺激和多管电极电泳导入荷包牡丹碱（bicuculline，Bic）的方法，研究了γ-氨基丁酸（γ-aminobutyric acid，GABA）能抑制在锐化听皮层（primary auditory cortex，AC，即初级听皮层）神经元频率调谐中的作用。结果发现：正常AC神经元的频率调谐曲线表现出单峰开放式、多峰开放式和单峰封闭式3种类型；用双声刺激方法研究证实，至AC神经元的抑制性输入能被抑制性声刺激所激活，且这种神经抑制有自身的最佳频率，根据其对兴奋反应的影响程度和系统地改变抑制性声刺激的强度，可在兴奋性频率调谐曲线或兴奋区的高频边或/和低频边测出抑制性频率调谐曲线或抑制区；当这种抑制性输入被抑制性声刺激激活后，能降低阈上10dB声强引起的兴奋反应的发放率，抑制效率随抑制声刺激强度的增强而加强；电泳GABAa受体拮抗剂荷包牡丹碱Bic后，可不同程度地去GABA能抑制，扩宽频率调谐曲线，使多峰调谐曲线变成单峰，封闭型变成开放型。表明GABA能抑制参与构成至AC神经元的抑制性输入，在正常情况下这种抑制有助于提高中枢听神经元的信号/噪声比和频率分析能力，并锐化频率调谐。因此本结果提示，声音的各参量中所包含的信息从外周传入中枢后，随着中枢的升级，逐级抽提整合成若干特征，直至在AC形成某种“声像（sound image）”，对大多数AC神经元而言，GABA能抑制在该过程中起关键作用。

有关听中枢神经元纯音前掩蔽效应的神经表征已进行了大量研究，但是，噪声前掩蔽尤其是间断噪声前掩蔽效应的神经表征却鲜有报道。本研究观察了自由声场条件下，昆明小鼠下丘神经元在持续与间断噪声前掩蔽条件下对纯音探测声的反应。共记录到96个下丘神经元，测量了其中51个神经元在不同声刺激条件下的强度-放电率函数。结果显示，掩蔽声强度分布较广（探测声阈下21dB至阈上19dB之间）。在将近一半的神经元中，间断噪声的前掩蔽效应比持续噪声强（Ⅰ型，45.10%，P<0.001），但也有少数神经元其间断噪声的掩蔽效应较持续噪声的弱（Ⅲ型，17.65%，P<0.001)，部分神经元无显著性差异（Ⅱ型，37.25%，P>0.05）。无论Ⅰ型还是Ⅲ型神经元，持续噪声和间断噪声均在探测声强度较低时产生较强的抑制效应，随着探测声强度的升高，抑制效应逐渐降低（P<0.001）； 同时，持续噪声和间断噪声之间前掩蔽效应差异亦不复存在（P>0.05）。此外，当掩蔽声由持续噪声换为间断噪声后，部分Ⅰ型神经元掩蔽时相的类型发生转变，其中最主要的转变为由前期抑制转变为均衡抑制（53.85%，7/13）。对下丘神经元声反应的时间域以及强度域，持续与间断噪声具有分化性前掩蔽效应，提示噪声前掩蔽并非简单的神经元发放压抑源，某些主动性神经调制机制可能参与了噪声条件下时相声信息的编码过程。

自由声场刺激条件下，采用单单位胞外微电极记录方法，研究了一种未被研究过的恒频/调频（CF/FM）蝙蝠——菲菊头蝠（Rhinolophus pusillus）的下丘神经元基本声反应特性，其结果发现，在所得的110个下丘神经元中，发放类型包括相位型（54.5%）、紧张型（25.5%）、持续型（7.3%）、梳齿型（7.3%）和暂停型（5.4%）等5种类型。记录深度在208～1855（829.0±328.1）μm之间，最佳频率在16.7～75.6（38.9±15.7）kHz之间，最小阈值在5～74（34.7±13.6）dB SPL之间，阈上10dB SPL潜伏期在5.0～27.5（15.2±3.9）ms之间。最佳频率随记录深度的增加而增大(r=0.9578，P<0.001)；记录的54个频率调谐曲线（FTCs）均为开放型，其中52个为单峰型，2个为双峰型。52个单峰型FTC的Q10-dB值介于1.56～31.61之间，并且大部分是狭窄型（Q10-dB值>5），占69.2%（36/52），少部分为宽阔型（Q10-dB值<5），占30.8%（16/52）。2个双峰型神经元FTC在低频处为宽阔型，高频处为狭窄型，Q10-dB值分别为1.95、8和2.89、6.51。共获得34个神经元的强度_发放率函数（RIFs），可分为单调型、非单调型和饱和型。结合先前所研究的FM蝙蝠——普通伏翼蝠（Pipistrllus abramus）下丘神经元的基本声反应特性，比较分析了CF/FM蝙蝠与FM蝙蝠下丘神经元的声反应差异及其行为学意义。

The effect of pulse repetition rate on auditory sensitivity of the big brown bat, Eptesicus fuscus, was studied by determining the minimum threshold, response latency and recovery cycle of inferior collicular neurons at different repetition rates under free field stimulation conditions. In general, collicular neurons shortened the response latency and increased the number of impulses monotonically or non-monotonically with stimulus intensity. They recovered at least 50% when the interpulse interval was 10-57 ms. In addition, they increased the minimum threshold, lengthened the response latency, and reduced the number of impulses discharged to each pulse with increasing repetition rate. The increase in minimum threshold with repetition rate is partly because the neuron can not recover from previous stimulation when the interpulse interval is shortened. This increase reduces a neuron's response sensitivity and thus diminishes its number of impulses to each presented pulse. This increase also reduces the effectiveness of a given stimulus intensity which contributes to the lengthening of the neuron's response latency. Data obtained from single neuron recordings are used to highlight these observations. Implications of present findings regarding the bat's echolocation are also discussed.