Organization of the ascending projections from the ventral tegmental area: a multiple fluorescent retrograde tracer study in the rat

The projections from the ventral tegmental area of Tsai (VTA) to the frontal cortex (FC), lateral septum (LS), were investigated in the rat by means of the double retrograde fluorescent tracer technique. True blue and fast blue were used in combination with nuclear yellow as retrograde tracers. After combined injections placed into two different terminal fields, many singly and some doubly labeled neurons were seen in the midbrain. In all cases the labeled cells were observed in the ipsilateral VTA, while after injections placed into the LS and Acc some fluorescent neurons were also seen in the contralateral VTA. The patterns of distribution of the labeled neurons showed a topographic organization of the VTA efferent pathways. However, some degree of overlapping was evident in the distribution of cells retrogradely labeled from different terminal fields. The number of the doubly labeled neurons varied according to the sites of combined injections, but in each experiment it never exceeded 10% of the total number of labeled perikarya. Doubly labeled neurons were particularly numerous after combined injections placed into the FC, LS, or LH; on the contrary, very few doubly labeled cells were observed after combined injections placed into the CPu and LS or LH. The organization of the ascending VTA projections suggests that they are probably integrated into different anatomical sets.