Multiple projections from the ventral nucleus of the trapezoid body in the rat

An analysis of the central projections of the ventral nucleus of the trapezoid body (VNTB) in the rat, a region of the superior olivary complex known for its neuronal heterogeneity, was made using two anterograde axonal tracers, [3H]leucine and biotinylated dextran amine (BDA). A mixture of these tracers was injected iontophoretically into the VNTB and the results analyzed by first assessing magnitudes of autoradiographic signal in nuclei receiving projections and then identifying the axons and terminals responsible for this signal in parallel sets of sections processed for BDA. Our analysis showed that in addition to its projections to each cochlea via the olivocochlear bundle, the VNTB has 3 major central sites of axonal terminations: (1) the cochlear nucleus, particularly the molecular layer of the contralateral dorsal cochlear nucleus, (2) the contralateral lateral superior olive, and (3) the ipsilateral inferior colliculus. Other sites receiving projections from the VNTB included the VNTB itself and the nuclei of the lateral lemniscus. Significantly, the relative magnitudes of labeling within the nuclei receiving inputs from the VNTB varied consistently as a function of the dorsoventral location of the injection site, confirming previous work showing that there is a partial segregation within this nucleus of neurons according to their projections. Our data also revealed an orderly topographic pattern of projections to the cochlear nuclei, lateral superior olive and the inferior colliculus which is consistent with the known tonotopic organization both of the VNTB and these projection targets. Methodologically, the co-injection of two tracers was advantageous in that patterns of silver grains in autoradiographs could be used to confirm whether axons and terminals labeled with BDA had originated from labeled somata at the injection site or were the result of uptake of BDA by fibers of passage.