Auditory projections to the inferior colliculus of the rat are present by birth.

Despite extensive literature on the anatomical organization of the adult mammalian auditory system, the ontogeny of internuclear connections is still obscure. We studied the postnatal development of afferent brainstem connections to the inferior colliculus in rat pups using WGA-HRP and DiI as tracers. At birth, connections to the inferior colliculus from the cochlear nuclei and nuclei of the superior olivary complexes and the lateral lemnisci are present. During successive development, there are no obvious changes in the quantity of labeled neurons or the basic labeling pattern. These results provide evidence that all input connections from auditory brainstem nuclei to the rat's inferior colliculus are established prenatally.