Regional and topographic organization of the olivocerebellar projection in homozygous staggerer (sg/sg) mutant mice: an anterograde and retrograde tracing study.

The purpose of this investigation was to examine the organization of the olivocerebellar projection in the homozygous staggerer mouse. The adult stagger cerebellum is devoid of most Purkinje cells which are the primary target neurons for olivocerebellar fibers. Many of the remaining Purkinje cells are ectopic. In the first experiments, a small injection of a tritiated amino acid was placed into the staggerer inferior olivary complex. In coronal sections, longitudinal strips of labeled axons and olivocerebellar fiber terminals were separated by similarly oriented regions that contained little or no olivocerebellar fiber label. Within a given orthogonal band, olivocerebellar fiber terminal label was visualized around Purkinje cell soma and primary dendrites. The labeled olivocerebellar fiber axons were usually located just ventral to these labeled olivocerebellar fiber terminals. Analysis of the distribution of olivocerebellar fiber terminals in the staggerer cases indicated the presence of 10-11 distinct zones, which is slightly less than that reported in normal mice. In the next set of experiments, a small injection of horseradish peroxidase conjugated with wheat germ agglutinin was made into different mediolateral cerebellar regions. These results demonstrated that staggerer olivocerebellar fibers are entirely contralateral and are also organized topographically in a manner similar to the pattern seen in the normal animal. Thus, severe depletion and ectopia of staggerer Purkinje cells does not greatly alter olivocerebellar fiber organization.