The septointerpeduncular projection in the rat: tracing with the carbocyanine dye Dil

The fluorescent carbocyanine dye Dil has been used to retrogradely label the neuronal connections between the forebrain septal area and the interpeduncular nucleus. Previous works based on retrograde horseradish peroxidase transport have identified that only the diagonal band nucleus is a source of the septointerpeduncular projections, but anterograde tracing with labeled amino acids and selective lesions with colchicine have shown that also the posterior septal nuclei project to the interpeduncular nucleus. In the present study, the retrograde labeling in septal nuclei after placing the carbocyanine Dil in the interpeduncular nucleus resulted in the fluorescent labeling of numerous neurons of the diagonal band nucleus. Our results, in addition, showed the labeling of some scattered neurons in the ventral portion of the triangular nucleus of the septal area and in the septofimbrial nucleus, confirming the presence of a previously controversial septointerpeduncular projection.