He Jufang, Yu Yan-Qin, Xiong Ying, Hashikawa Tsutomu, Chan Ying-Shing
Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong.
J Neurophysiol. 2002 Aug;88(2):1040-50. doi: 10.1152/jn.2002.88.2.1040.
In the present study, we investigated the point-to-point modulatory effects from the auditory cortex to the thalamus in the guinea pig. Corticofugal modulation on thalamic neurons was studied by electrical activation of the auditory cortex. The modulation effect was sampled along the frontal or sagittal planes of the auditory thalamus, focusing on the ventral division (MGv) of the medial geniculate body (MGB). Electrical activation was targeted at the anterior and dorsocaudal auditory fields, to which the MGv projects and from which it assumptively receives reciprocal projections. Of the 101 MGv neurons examined by activation of the auditory cortex through passing pulse trains of 100-200 microA current into one after another of the three implanted electrodes (101 neurons x 3 stimulation sites = 303 cases), 208 cases showed a facilitatory effect, 85 showed no effect, and only 10 cases (7 neurons) showed an inhibitory effect. Among the cases of facilitation, 63 cases showed a facilitatory effect >100%, and 145 cases showed a facilitatory effect from 20-100%. The corticofugal modulatory effect on the MGv of the guinea pig showed a widespread, strong facilitatory effect and very little inhibitory effect. The MGv neurons showed the greatest facilitations to stimulation by the cortical sites, with the closest correspondence in BF. Six of seven neurons showed an elevation of the rate-frequency functions when the auditory cortex was activated. The comparative results of the corticofugal modulatory effects on the MGv of the guinea pig and the cat, together with anatomical findings, hint that the strong facilitatory effect is generated through the strong corticothalamic direct connection and that the weak inhibitory effect might be mainly generated via the interneurons of the MGv. The temporal firing pattern of neuronal response to auditory stimulus was also modulated by cortical stimulation. The mean first-spike latency increased significantly from 15.7 +/- 5.3 ms with only noise-burst stimulus to 18.3 +/- 4.9 ms (n = 5, P < 0.01, paired t-test), while the auditory cortex was activated with a train of 10 pulses. Taking these results together with those of previous experiments conducted on the cat, we speculate that the relatively weaker inhibitory effect compared with that in the cat could be due to the smaller number of interneurons in the guinea pig MGB. The corticofugal modulation of the firing pattern of the thalamic neurons might enable single neurons to encode more auditory information using not only the firing rate but also the firing pattern.
在本研究中，我们调查了豚鼠听觉皮层对丘脑的点对点调节作用。通过电刺激听觉皮层来研究皮质传出纤维对丘脑神经元的调节作用。沿着听觉丘脑的额平面或矢状平面采样调节效果，重点关注内侧膝状体（MGB）的腹侧部（MGv）。电刺激的靶点是MGv投射的前听觉场和背尾侧听觉场，并且推测MGv从这些区域接收反向投射。通过将100 - 200微安电流的脉冲序列依次通入三个植入电极中的一个来激活听觉皮层，以此检测101个MGv神经元（101个神经元×3个刺激位点 = 303例），其中208例显示促进作用，85例无作用，仅10例（7个神经元）显示抑制作用。在促进作用的案例中，63例促进作用大于100%，145例促进作用为20% - 100%。豚鼠MGv的皮质传出纤维调节作用表现为广泛、强烈的促进作用，抑制作用非常小。MGv神经元对皮层位点的刺激表现出最大的促进作用，在最佳频率上对应性最强。当听觉皮层被激活时，7个神经元中有6个的率 - 频函数升高。豚鼠和猫的MGv皮质传出纤维调节作用的比较结果以及解剖学发现提示，强烈的促进作用是通过强大的皮质 - 丘脑直接连接产生的，而微弱的抑制作用可能主要是通过MGv的中间神经元产生的。神经元对听觉刺激的时间发放模式也受到皮层刺激的调节。在仅给予噪声爆发刺激时，平均首次放电潜伏期为15.7±5.3毫秒，当用一串10个脉冲激活听觉皮层时，平均首次放电潜伏期显著增加至18.3±4.9毫秒（n = 5，配对t检验，P < 0.01）。综合这些结果与之前对猫进行的实验结果，我们推测与猫相比，豚鼠相对较弱的抑制作用可能是由于豚鼠MGB中中间神经元数量较少。丘脑神经元发放模式的皮质传出纤维调节可能使单个神经元不仅能利用发放率，还能利用发放模式来编码更多的听觉信息。
