A PROCEDURE FOR IN-SITU HYBRIDIZATION COMBINED WITH RETROGRADE LABELING OF NEURONS - APPLICATION TO THE STUDY OF CELL-ADHESION MOLECULE EXPRESSION IN DIL-LABELED RAT PYRAMIDAL NEURONS

We have devised a simple method that combines retrograde labeling of p rojecting neurons and in situ hybridization histochemistry to examine mRNA expression in the retrogradely labeled neurons. First, projecting neurons were retrogradely labeled in vivo by injection of the lipophi lic neuronal tracer Dil. The fluorescence of the labeled neurons in th e brain slices was photoconverted into stable DAB precipitate by green light illumination. The slices were cut into thinner sections and pro cessed for detection of specific mRNA by in situ hybridization. Using this highly sensitive method, we demonstrate here that the corticospin al tract neurons in newborn rats express mRNA for the cell adhesion mo lecule L1. TAG-1 mRNA was not detected in these neurons. Therefore, th e present method provides an important tool to study the molecular exp ression of projection neurons during the development of neuronal circu itry.