NEURONAL MORPHOLOGY AND EFFERENT PROJECTIONS OF THE DORSAL NUCLEUS OFTHE LATERAL LEMNISCUS IN THE RAT

The dorsal nucleus of the lateral lemniscus (DLL) is the main source o f inhibitory influence in the auditory brainstem of mammals. The cytoa rchitecture and connectional properties of DLL were established in the cat in contrast to the rat. The goal of the present study was to esta blish to what extent the anatomical properties of the rat DLL compare to those of the cat, thus providing a basis of interpretation for futu re functional studies in the rat, an animal model used more and more i n the auditory system. DLL of the rat contains four well-differentiate d neuronal types, as seen in Nissl-stained material. Type I neurons ar e large and multipolar with abundant cytoplasm and darkly stained Niss l substance. Type II neurons are large, bipolar and darkly stained in Nissl material. Type III neurons are medium in size and their soma is round or ovoid. Type IV neurons are small and round with scant cytopla sm; they seem to be also the least common neuronal type of the DLL. Af ter Phaseolus vulgaris-leucoagglutinin or biocytin injections in the D LL, fibers and terminals labeled by orthograde transport were observed in the corresponding region of the contralateral DLL and in the infer ior colliculus, bilaterally. A few labeled fibers and terminal fields were seen in the deep layers of the superior colliculus bilaterally, a s well as in the medial division of the medial geniculate body and, ev en more rostrally, in the posterior nucleus of the thalamus. Descendin g projections from DLL terminated in the periolivary regions of the ip silateral superior olivary complex. Retrograde tracing based on inject ions of horseradish peroxidase in the various targets of the DLL confi rmed the connections established with orthograde labeling. (C) 1993 Wi ley-Liss, Inc.