Morphological evidence for a dopaminergic terminal field in the hippocampal formation of young and adult rat.

We have visualized the dopaminergic innervation of the hippocampal formation of the rat using two morphological methods: (1) tyrosine hydroxylase immunocytochemistry on noradrenaline-depleted animals and (2) fluorescence histochemistry after the uptake and storage of dopamine on hippocampal slices in vitro. The noradrenergic hippocampal terminal fields were destroyed by neonatal neurotoxin pretreatment and the validity of the lesion checked by the absence of dopamine beta-hydroxylase immunoreactivity. As observed on early postnatal ages, dopaminergic axons reached the hippocampal formation through the fimbria and the alveus, but also through the supracallosal bundle and the ventral amygdaloid area-entorhinal cortex. The temporal (ventral and caudal) part of the hippocampal formation received the bulk of the dopaminergic innervation whereas no fibers were observed in the septal pole. Very few positive axons were visualized in the hilus of the gyrus dentatus and CA3 field, only near the temporal pole. CA1 field (stratum oriens) was innervated throughout its ventral part. The most innervated area was the ventral part--especially the deep layers--of the subiculum, in particular the prosubiculum. The dorsal part of the subiculum displayed some positive axons, although to a lesser extent. The pre- and parasubiculum contained a few positive axons. In addition, some immunoreactive axons were observed in the anterior hippocampal continuation and the indusium griseum. The ventral junction prosubiculum-CA1 field appears to be the main target area for the hippocampal dopaminergic innervation. It is interesting that the same areas are characterized by their projections to the nucleus accumbens which receives dopaminergic afferents. Thus, the hippocampostriatal projections, that represent a link between the limbic and central motor mechanisms, could be under dopaminergic influence.