Anterograde and retrograde tracing of projections from the ventral tegmental area to the hippocampal formation in the rat.

Employing anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L), and a triple labeling protocol using retrogradely transported fluorescent tracers, we examined the projections from the ventral tegmental area (VTA-A10) to the hippocampal formation (HF) in the rat. Injections of PHA-L into VTA resulted in labeling in the ventral subiculum (stratum oriens and molecular layer) and in the adjacent CA1 field (stratum oriens, pyramidal, suprapyramidal and molecular layers) of HF. Additional labeling was observed in the stratum oriens of CA3 and in the hilus of fascia dentata. In the dorsal HF labeling was present in the subicular and CA1 field polymorphic layers. The distribution of VTA neurons projecting to the HF was also examined by injecting retrograde fluorescent tracers (Fluoro Gold, Fast Blue, and Nuclear Yellow) in several hippocampal areas. The most abundant VTA-HF projections originate from the upper and lower edges and the lower half of the VTA. These terminal fields in the HF match with the hippocampal areas projecting to the nucleus accumbens. The VTA, via projections to interconnected regions of the HF and nucleus accumbens, may modulate the hypothesized functional link between the limbic system and basal ganglia.