小鼠初级听皮质神经元的强度调谐特性与机制分析

强度是声音的基本参数之一，听神经元的强度调谐在听觉信息处理方面具有重要意义．以往研究发现γ-氨基丁酸(γ-aminobutyric acid, GABA)能抑制性输入在强度调谐的形成过程中起重要作用，但对抑制性输入与局部神经回路之间的关系并不清楚．本实验通过在体细胞外电生理记录和神经药理学方法，分析了小鼠初级听皮质神经元的强度调谐特性，结果显示：单调型神经元在声刺激强度自中等强度增高时潜伏期缩短(P < 0.05)且发放持续时间延长(P < 0.05)，非单调型神经元在声刺激强度自最佳强度增高时潜伏期不变且发放持续时间缩短(P < 0.01)．注射GABA能阻断剂荷包牡丹碱(bicuculline, Bic)后，39.3%的神经元强度调谐类型不变，42.9%的神经元非单调性减弱，17.9%的神经元非单调性增强．表明GABA能抑制并非是形成非单调性的唯一因素，兴奋性输入本身的非单调性和高阈值非GABA能抑制的激活也可能在其中发挥作用．推测由兴奋性和抑制性输入所构成的局部神经功能回路及其整合决定了听皮质神经元的强度调谐特性．

Intensity Tuning of Neurons in The Primary Auditory Cortex of Albino Mouse

Cortical neurons that are tuned to sound intensity (non-monotonic neurons) are very important for processing auditory information. Considering the fact that all auditory nerve fibers have monotonical responses, inhibition in the primary auditory cortex (AI) is essential for intensity tuning. By using free field sound stimulation and in vivo extracellular recording, the present study investigated the intensity-tuning properties in AI neurons of mouse (Mus musculus, Km). We also examined the effect of cortical application of the GABAa receptor antagonist bicuculline on AI intensity tuning in order to indentify the possible source of inhibition. The intensity-tuning curves were recorded in 72 AI neurons among which 28 showed monotonic responses and 44 showed non-monotonic responses. In non-monotonic neurons, there was no change in the first spike latency but a decrease in the firing duration (P < 0.01) when the sound intensity increased from best intensity to highest intensity. After AI application of bicuculline by electrophoresis (n = 28), the non-monotonic responses did not change in 39.3% neurons, was weakened in 42.9% neurons and was enhanced in 17.9% neurons. It suggested that GABAergic inhibition must be involved in AI intensity tuning but was not the only source. Non-monotonic excitatory input and non-GABAergic inhibition should also be considered. A postulated model was presented to explain the mechanism of intensity-tuning in AI neurons.