Cortical activation on temporal expression of FOS in various nuclei of themedial geniculate body of cochlear-ablated rats

Abstract

To investigate the organization of corticothalamic projection in the auditory pathway, we examined the time course of Fos expression in medial geniculate body (MGB) neurons after chemical activation of the auditory cortex in cochlearablated Sprague–Dawley rats. The cochlea of pentobarbitalanesthetized rats were bilaterally removed and the animals were allowed to recover for 4 h (group I), 24 h (group II) or 7 days (group III). These rats were then anesthetized with pentobarbital again. The auditory cortex on one side was injected with bicuculline methobromide (20 mM), which activates cell bodies but not fibers of passage. At 1 h after this injection, the brain was processed for immunohistochemical expression of Fos protein. Neuronal activation was indicated by Fos expression in cell nuclei. In group-I rats, Fos expression was mainly restricted in the central region of the ventral nucleus of the MGB ipsilateral to the activated hemisphere and the medial nucleus of the MGB. In group-II and group-III rats, Fos expression was mainly located in the marginal zone, the dorsal and medial nuclei of the MGB ipsilateral to the activated hemisphere. The spatial distribution and the number of Fos-immunoreactive MGB neurons in group-III rats were larger than those of group II. Our results indicate that the corticofugal-induced neuronal excitability in the non-ventral MGB exhibits a different time course than that in the ventral MGB after cochlear ablation.