Development of afferent patterns in the inferior colliculus of the rat: Projection from the dorsal nucleus of the lateral lemniscus

The inferior colliculus (IC) receives a variety of layered afferent project ions. The purpose of the present study was to determine the development of the projection from the dorsal nucleus of the lateral lemniscus (DNLL) to t he IC in rat prior to the onset of hearing (postnatal day 12/13). Lipophili c carbocyanine dye, DiI (1,1'-dioctodecyl-3,3,3',3'-tetramethylcyanine perc hlorate), was used to trace the crossed inhibitory projection of DNLL in a developmental series of rat embryos and pups between ages embryonic day 15 (E15) and postnatal day 12 (P12). Dye-coated pins were positioned in parafo rmaldehyde-fixed brains either unilaterally in DNLL (embryonic cases), or i n the commissure of Probst where DNLL fibers cross the midline (postnatal c ases). By E15, pioneer fibers have left DNLL and crossed the midline. A few fibers have nearly reached the contralateral IC by E19. At birth (E22 = P0 ), the projection has invaded ventromedial, high-frequency layers of the IC . The vast majority of DNLL axons parallel the presumptive IC layers by P4, and by P8 the projection has segregated into a pattern of bands (afferent dense) and interband (afferent: sparse) spaces that encompasses the entire frequency axis of the IC. Adult-like patches, regions along afferent bands that exhibit the heaviest labeling, develop by P12. These results indicate that some mature projection patterns are in place prior to the onset of hea ring. Such findings suggest that evoked activity may not be required for th e initial organization of patterned projections in the ascending auditory p athway. J. Comp. Neurol. 416:368-382, 2000. a (C) 2000 Wiley-Liss, Inc.