Anisotropic organization of the rat superior paraolivary nucleus.

In rodents, the superior paraolivary nucleus (SPON) is one of the major nuclei of the superior olivary complex that innervate the inferior colliculus. To analyze the intrinsic organization of the SPON and to gain further insight into its relationship with the inferior colliculus, the neuroanatomical tracers biotinylated dextran and horseradish peroxidase were unilaterally injected into different regions of the central nucleus of the inferior colliculus of adult albino rats. Both tracers resulted in retrograde labeling of SPON cell bodies. In addition, biotinylated dextran rendered excellent filling of dendritic and axonal processes within the nucleus. Our results confirm that the projection from the SPON to the central nucleus of the inferior colliculus is nearly exclusively ipsilateral and strictly topographic. Furthermore, our data show that virtually all SPON neurons participate in this projection. The labeling with biotinylated dextran reveals that typical SPON neurons are medium to large multipolar cells with four to seven thick, long, scarcely branched and smooth dendrites that extend over long distances within a nearly parasagittal plane and intermingle with similarly oriented axonal plexuses. Some of the neurons located ventrally within the nucleus possess dendrites that extend ventrally beyond the limits of the SPON to penetrate into the underlying ventral nucleus of the trapezoid body. The parallel arrangement of flattened dendritic and axonal fields within the SPON is reminiscent of the fibrodendritic laminae found in other mammalian auditory nuclei. This fact and the available data about the connectivity of the nucleus stress the similarities between the SPON and the principal nuclei of the superior olivary complex.