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

Abstract

The time course and extent of the expression of Fos protein in neurons within the medial geniculate body (MGB) were examined after chemical activation of the auditory cortex in cochlearablated Sprague-Dawley rats. Surgical ablation of the bilateral cochlea was performed in ketamine-anesthetized animals which were then allowed to recover for 4 h (Group I), 24 h (II), 7 days (III), 14 days (IV), and 30 days (V). After each of these time points, the rats were again anesthetized with ketamine. The exposed auditory cortex on the right side was administered with GABA A antagonist biccuculline methobromide (20 m M ), which in turn resulted in the activation of cells in the injected site. At 1 h after injection, the brain was processed for Fos immunostaining. Completeness of cochlea destruction was ascertained by post-mortem examination of the temporal bone. Control experiments were also conducted in cochlear-ablated animals without the injection of biccuculline. Neuronal activation was defined by Fos expression in cell nuclei. In Group-I rats, Fos labeling was mainly confined within the ventral and medial nuclei of the MGB ipsilateral to the activated hemisphere. In Group-II rats, Fos expression was found 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, with some in the marginal zone of the MGB, but absent in the ventral nucleus of the MGB. In Groups-IV and V rats, the distribution of Fos-labeled neurons in the MGB was similar to that of Group-III rats but the Fos expression was less intense. We demonstrate that corticofugal induction of neuronal excitability in the non-lemniscal MGB shows a slower time course than in the lemniscal MGB, suggesting that these two areas of the auditory thalamus are differentially modulated by corticofugal pathways.