Cortifugal projection inhibits the auditory thalamus through the thalamic reticular nucleus

Abstract

Electrical stimulation of the auditory cortex causes mainly facilitatory effect on the lemniscal nuclei of the medial geniculate body (MGB) and inhibitory effect on the non-lemniscal nuclei of the MGB. There are three possible pathways for that corticofugal inhibition. The first possibility, cortex→MGB interneurons→MGB relay neurons, is quite low as we use guinea pig as the animal model in which interneurons count for less than 1% in the MGB. To further identify the corticofugal inhibitory pathway to the MGB among the latter two possibilities, cortex→inferior colliculus→MGB or cortex→thalamic reticular nucleus→MGB, we compared the neuronal responses of the MGB to electrical stimulation of the auditory cortex between the experimental condition in which bilateral inferior colliculi were ablated by suction and the control condition in which the inferior colliculi were kept intact. Our result showed that cortical stimulation under both conditions caused strong inhibitory effects on the MGB neurons. Under the experimental condition, eight of the 27 neurons that received corticofugal inhibitory effect were histologically confirmed to be located in the non-lemniscal MGB while five of the 12 neurons that received corticofugal facilitatory effect were histologically confirmed to be located in the lemniscal MGB. The ablation of the inferior colliculi was confirmed histologically. The result suggests that the corticofugal inhibition is likely caused by the corticoreticulothalamic pathway rather than the corticocolliculothalamic pathway. Supported by Supported by Competitive Research Grants Commission (PolyU5407/03M)