The Journal of Neurophysiology Vol. 88 No. 2 August 2002, pp. 1040-1050
Copyright ©2002 by the American Physiological Society

Modulatory Effect of Cortical Activation on the Lemniscal Auditory Thalamus of the Guinea Pig

 ¹Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong;  ²Laboratory for Neural Architecture, Brain Science Institute, The Institute of Physical and Chemical Research, Wako, Saitama 351-0198, Japan; and  ³Department of Physiology, The University of Hong Kong, Sassoon Road, Hong Kong

He, Jufang, Yan-Qin Yu, Ying Xiong, Tsutomu Hashikawa, and Ying-Shing Chan. Modulatory Effect of Cortical Activation on the Lemniscal Auditory Thalamus of the Guinea Pig. J. Neurophysiol. 88: 1040-1050, 2002. 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 µA current into one after another of the three implanted electrodes (101 neurons × 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.