Aberrant trajectory of entorhino-dentate axons in the mutant Shaking Rat Kawasaki: a DiI-labelling study

The Shaking Rat Kawasaki (SRK) is a neurological mutant that exhibits abnor malities of cell migration and lamination, with many similarities to the mo use reeler mutant. We recently used lamina-specific antibody staining to sh ow that despite severe aberrations in the laminar organization of the SRK d entate gyrus, the entorhinal terminal field in the outer dentate molecular layer appeared relatively normal (Woodhams & Terashima, 1999, J. Comp. Neur ol. 409 p57). However, neurofilament immunostaining suggested that entorhin o-dentate afferents take an abnormal trajectory in reaching their appropria te targets, the granule cells dendrites. In the present study, anterograde tracing with the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylind ocarbocyanine perchlorate (DiI) has been used to delineate directly the pat h that entorhinal axons take to the dentate gyrus, confirming that in SRK e ntorhinal axons do indeed reach their appropriate terminal fields in the mo lecular layer, with laminar segregation between projections from the latera l and medial entorhinal cortices. However, these fibres fail to cross the h ippocampal fissure between the subiculum and the dentate gyrus, coursing in stead parallel to it until they curve round the deepest point of the fissur e in field CA3. Similar findings were seen in the murine reeler mutant. Ins ertion of DiI crystals into the entorhinal cortex of neonatal rats also ret rogradely labelled the developmentally transient Cajal-Retzius cells at the hippocampal fissure; these survive for longer in SRK than in normal litter mates. The presence of a marked astrogliosis at the SRK hippocampal fissure may play a part in determining the abnormal trajectory taken by entorhino- dentate afferents in this mutant.