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 converge nce for nearly all ascending monaural and binaural projections. Several of these inputs, including inhibitory connections from the dorsal nucleus of t he lateral lemniscus (DNLL), are highly ordered and organized into series o f afferent bands or patches. Although inputs to the IC from the contralater al DNLL are present in the rat by birth [postnatal day 0 (P0)], the earlies t indications of band formation are not evident until P4. Subsequently, the initially diffuse projection segregates into a pattern of bands and interb and 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 wit h 1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate were p laced in fixed tissue in the commissure of Probst where DNLL fibers cross t he midline. The results indicate that a unilateral cochlear ablation disrup ts the normal development of afferent patches in the IC. Although the cross ed DNLL projections labeled via commissural dye placement always mirrored e ach other in P12 controls, ablation cases exhibited a consistent, bilateral asymmetry in pattern formation and relative density of the labeled project ions. Possible developmental mechanisms likely to be involved in the establ ishment of afferent bands and patches before the onset of hearing are discu ssed.