Plasticity in the development of afferent patterns in the inferior colliculus of the rat after unilateral cochlear ablation

The central nucleus of the inferior colliculus (IC) is the site of convergence for nearly all ascending monaural and binaural projections. Several of these inputs, including inhibitory connections from the dorsal nucleus of the lateral lemniscus (DNLL), are highly ordered and organized into series of afferent bands or patches. Although inputs to the IC from the contralateral DNLL are present in the rat by birth [postnatal day 0 (P0)], the earliest indications of band formation are not evident until P4. Subsequently, the initially diffuse projection segregates into a pattern of bands and interband spaces, and by P12 adult-like, afferent-dense patches are established (Gabriele et al., 2000). To determine the role of the auditory periphery in the development of bands and patches before the onset of hearing (P12/P13), unilateral cochlear ablations were performed at P2 (before any evidence of banding). Rat pups were reared to P12, at which time glass pins coated with 1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate were placed in fixed tissue in the commissure of Probst where DNLL fibers cross the midline. The results indicate that a unilateral cochlear ablation disrupts the normal development of afferent patches in the IC. Although the crossed DNLL projections labeled via commissural dye placement always mirrored each other in P12 controls, ablation cases exhibited a consistent, bilateral asymmetry in pattern formation and relative density of the labeled projections. Possible developmental mechanisms likely to be involved in the establishment of afferent bands and patches before the onset of hearing are discussed.