Fos expression in the medial geniculate body of cochlear-ablated and cortical-activated rats

We examined the time course of Fos expression in medial geniculate body (MGB) neurons after chemical activation of the auditory cortex in cochlear-ablated Sprague-Dawley rats in the present study. The cochlea of ketamine-anesthetized rats were bilaterally removed and the animals were allowed to recover for 4 hrs (group I), 24 hrs (II) , 7 days (III), 14 days (IV) and 30 days (V). The rats were then anesthetized with ketamine again. The auditory cortex on right side was injected with bicuculline methobromide (20 mM), which activates cell bodies but not fibers of passage. At 1 hr 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 ventral nucleus and the medial nucleus of the MGB ipsilateral to the activated hemisphere. In group-II rats, Fos expression was detected in the ventral, medial and dorsal nuclei of the MGB ipsilateral to the activated hemisphere. In group-III rats, Fos expression was mainly located in the dorsal and medial nuclei, some in the marginal zone of the MGB, and clearly void in the ventral nucleus of the MGB. In group IV and V rats, Fos expressions were located in the nuclei of the MGB similar to group-III rats and clearly void in the ventral nucleus of the MGB. The Fos expressions were less intensive than the group-III rats. Our results indicate that the corticofugally-induced neuronal excitability in the non-lemniscal MGB exhibits a slower time course than that in the ventral MGB, suggesting that contributions of corticofugal modulation to neural plasticity and regeneration within the thalamus are different between the lemniscal and non-lemniscal MGB