Origin of ascending projections to inferior colliculus in the mustache bat, Pteronotus parnellii

The origins of pathways to the inferior colliculus of the mustache bat were identified by retrograde transport of horseradish peroxidase (HRP). A specific goal of this study was to obtain evidence that would help determine whether the nuclei, shown in the previous paper to have unusual cytoarchitectural features, are unique to bats, or whether they are homologous to areas that are not well differentiated in other mammals. The auditory pathways in the lower brain stem of Pteronotus appear to conform to the same basic organization as in other mammals: After injection of HRP into one inferior colliculus, labeled cells are located contralaterally in the cochlear nucleus, ipsilaterally in the medical superior olive, bilaterally in the lateral superior olive, ipsilaterally in the ventral and intermediate nuclei of the lateral lemniscus, and bilaterally in the dorsal nucleus of the lateral lemniscus. These patterns of labeling provide a basis for understanding how the specialized auditory areas of the bat may be organized within a basic plan of mammalian auditory systems. In the anteroventral cochlear nucleus the unusually small spherical cells seem to be homologous to stellate cells in the anteroventral cochlear nucleus of the cat. In the superior olive, differences in patterns of labeled cells distinguish the medial from the lateral superior olive. In the lateral lemniscus the pattern of labeled cells shows clear differences between the two special parts, intermediate and ventral nuclei, as well as between these and the dorsal nucleus of the lateral lemniscus. The results are consistent with the hypothesis that the unusual auditory nuclei of the bat have homologues in mammals whose auditory systems are not specialized for echolocation.