Corticothalamic synchronization leads to c-fos expression in the auditory thalamus

In this study, we investigated the relationship between c-fos expression in the auditory thalamus and corticofugal activation. The contribution of neurotransmitters and related receptors, the involvement of thalamic reticular nucleus (TRN), and the role of neuronal firing patterns in this process were also examined. The principal nuclei of the medial geniculate body (MGB) showed c-fos expression when the auditory cortex (AC) was activated by direct injection of bicuculline methobromide. However, no expression was detectable with acoustic stimuli alone. This indicated that c-fos expression in the principal nuclei of the MGB was triggered by the corticofugal projection. c-fos expression could be elicited in the MGB by direct injection of glutamate. Direct administration of acetylcholine, alternatively, had no effect. Bicuculline methobromide injection in the AC also triggered synchronized oscillatory activities sequentially in the AC and MGB. Cortically induced c-fos expression in the MGB was not mediated by a pathway involving the TRN because it remained intact after a TRN lesion with kainic acid. The present results also conclude that c-fos expression is not simply associated with firing rate, but also with neuronal firing pattern. Burst firings that are synchronized with the cortical oscillations are proposed to lead to c-fos expression in the principal nuclei of the MGB.