Influence of auditory deprivation upon the tonopic organization in the inferior colliculus: a Fos immunocytochemical study in the rat

The frequency organization in the inferior colliculus of neonatally-deafened rats was investigated using electrical stimulation of the cochlea and immunoreactivity for Fos as a marker of neuronal activity. An electrode implanted either at the base or at the apex of the right cochlea delivered a unique 45-min stimulation at two different level intensities and at two time points, i.e. either at 4 weeks or at 4 months. In 4-week-old rats stimulated at 5x threshold, a site-for-site organization was observed since basal or apical stimulation induced a strong labelling in the ventro-medial or in the dorsolateral part of the left inferior colliculus, respectively. In 4-month-old rats, stimulation of the base induced an extremely weak Fos labelling without any specific location in the left inferior colliculus while stimulation of the apex induced a diffuse labelling with two discrete bands being distinguishable in the left inferior colliculus. In 4-week-old rats stimulated at 15x threshold, basal stimulation elicited a diffuse Fos-like immunoreactivity in the left inferior colliculus while apical stimulation yielded a response restricted to the dorsal part of the left inferior colliculus. In 4-month-old rats, no response was detected in the left inferior colliculus after stimulation of the basal part of the cochlea. Stimulation of the apex could still induce a labelling in the dorsolateral left inferior colliculus. Thus, the inferior colliculus exhibits an adult-like tonotopic organization early on independently of any acoustic stimulation. Prolonged absence of auditory input dramatically alters this organization in the inferior colliculus, especially for high frequencies. From a clinical standpoint, these results could argue for early implantation in deaf children.