THE PROJECTIONS OF THE RETRORUBRAL FIELD A8 TO THE HIPPOCAMPAL-FORMATION IN THE RAT

The dopaminergic neurons in the midbrain of the rat are located in thr ee groups: the ventral tegmental area (VTA-A10), substantia nigra (SN- A9)? and retrorubral field (RRF-A8). We have recently examined the org anization of the projections from the VTA and the SN to the hippocampa l formation (HF) in the rat. In the present study we characterize the projections of the RRF to the HF by using anterograde tracing, retrogr ade tracers, and tyrosine hydroxylase (TH) immunohistochemistry. Follo wing iontophoretical injections of Phaseolous vulgaris leucoagglutinin (PHA-L) into the RRF, anterograde labeling was observed primarily in the ipsilateral subiculum and adjacent CAI cell field. Sparse labeling was also observed in the CA3 cell field and dentate gyrus. The distri bution of RRF neurons projecting to the HF was examined by injecting r etrograde fluorescent tracers (fluorogold, fast blue, and nuclear yell ow) into several hippocampal areas. The retrograde tracer findings ind icate that the medial aspects of thr RRF project to the subiculum and adjacent CA I cell field of both the septal and temporal HE In order t o evaluate the percentage of dopaminergic cells of the RRF projecting to the HE the retrograde neuronal tracer fluorogold was used in combin ation with TH immunohistochemistry. The quantitative evaluation of ret rograde labeled and TH-immunoreactive (IR) cells showed that RRF proje ctions to the HF are partially (10-18%) dopaminergic. The findings sug gest that the general pattern of distribution and organization of the RRF-A8 projections to the HF is similar to that observed in our previo us studies examining hippocampal afferents from the VTA and SN. The da ta also suggest a crude topographical organization of RRF afferents to the HF and a more prominent input to the temporal than to the septal HF.