Cochlear Electrical Stimulation: Influence of Age of Implantation on Fos Immunocytochemical Reactions in Inferior Colliculi and Dorsal Cochlear Nuclei of the Rat WEI-CHUNG HSU, 1 ANTONIO CAMPOS-TORRES, 2 FREDERIC PORTIER, 2,3 ERIC LECAIN, 2 THIERRY VAN DEN ABBEELE, 2 CATHERINE DE WAELE, 2,3 AND PATRICE TRAN BA HUY 2,3 * 1 Department of Otolaryngology, Taiwan National University Hospital, Taipei, Taiwan 2 UPRESA-CNRS, Universite ´ Paris, Paris, France 3 Service ORL, Ho ˆpital Lariboisie `re, Paris, France ABSTRACT The influence of age at the time of implantation of a stimulating electrode unilaterally in the inner ear on central auditory pathways was investigated in rats deafened shortly after birth. Immunoreactivity for Fos served as a functional marker of neuronal activity. Electrodes were implanted in the left cochlea of rats aged 3 weeks or 4 months. Stimulation lasted 45 minutes, then rats were sacrificed and tissues processed for immunocytochemistry. The younger animals showed significantly more neurons with Fos immunoreactivity bilaterally in the dorsal cochlear nuclei (DCN) and inferior colliculi (IC) than the older rats or control animals with normal hearing receiving the same stimulation. Activity was more prominent in the left DCN and right IC. The results show that electrical stimulation of the inner ear is more effective in younger animals in eliciting gene expression associated with development of a functional network in the auditory pathways. This suggests that deaf children should be provided with cochlear implants as early as possible. J. Comp. Neurol. 438:226 –238, 2001. © 2001 Wiley-Liss, Inc. Indexing terms: cochlear implant; auditory nuclei; activity marker; sensory deprivation It is now well established that in congenital profound deafness cochlear implantation in childhood is able to restore significant hearing capacities and to allow speech acquisition (Cheng et al., 1999; Kileny et al., 2001). How- ever, ongoing debate concerns the most efficient period for implanting deaf children: Should it be performed as soon as the hearing deficit is established or not until 2 to 3 years of age (Rizer and Burkey, 1999)? From clinical, surgical, and neurophysiological points of view, the an- swer to this crucial question depends, among other fac- tors, on the status of the auditory pathways following sound deprivation. Numerous experiments using animal models of congen- ital hearing loss have revealed structural, metabolic, and functional effects of early sound deprivation upon the brain stem auditory nuclei including significant neuronal atrophy (Webster and Webster, 1977; Coleman and O’Connor, 1979; Trune, 1982; Webster, 1983; Born and Rubel, 1985; Moore and Kowalchuk, 1988), changes in protein and neurotransmitter synthesis and release (Lippe et al., 1980; Potaschner, 1983; Steward and Rubel, 1985; Trune and Kiessling, 1988; Sie and Rubel, 1992), reorganization in central structures (Nordeen et al., 1983; Trune, 1983; Moore and Kowalchuk, 1988), and altered electrophysiological activity (Kitzes and Semple, 1985; Ko- erber et al., 1986). Employing these experimental models, subsequent studies have investigated the effects of electrical stimula- tion of the cochlea upon the brain stem auditory pathways and suggested that this treatment delayed or prevented the degeneration of the primary neurons (Chouard et al., Grant sponsor: Fondation de l’Avenir; Grant sponsor: MXM Company *Correspondence to: Pr. Patrice Tran Ba Huy, Service ORL, Ho ˆpital Lariboisie `re, 2, rue Ambroise Pare ´, 75475 Paris cedex 10, France. E-mail: patrice.tran-ba-huy@lrb.ap-hop-paris.fr Received 13 October 2000; Revised 2 April 2001; Accepted 20 June 2001 THE JOURNAL OF COMPARATIVE NEUROLOGY 438:226 –238 (2001) © 2001 WILEY-LISS, INC.