[Dopaminergic innervation of the parahippocampal and hippocampal regions in the rat].

In spite of extensive investigations of the rat's meso-cortico-limbic system, the dopaminergic (DA) innervation of the hippocampal formation (HF) has not heretofore been visualized by histochemical techniques. However pharmacological and biochemical studies strongly suggested its existence. We used tyrosine-hydroxylase (TH) as an immunocytochemical marker of DA neurons in rats in which the noradrenaline cortical innervation was previously destroyed by neurotoxins. The absence of noradrenergic axons was routinely controlled with dopamine-beta-hydroxylase immunocytochemistry. TH-positive axons, thus DA axons, reach the HF primarily through the fimbria in which they occupy a specific lateral sector. They innervate the ventral and caudal parts of the HF, the subiculum and the adjacent CA1 hippocampal field being the main targets. This DA terminal field in the HF, matches with the area projecting toward the nucleus accumbens. Thus the hippocampo-striatal projections which represent a link of functional importance between the limbic and central motor systems might be modulated by the dopaminergic meso-cortico-limbic pathway. The present immunocytochemical study confirms the very dense innervation of the entorhinal cortex (EC): DA axons are organized in dense fiber islands and occupy primarily the superficial layers (I-II-III). Da innervation predominates in the lateral part of EC, which is the site of multimodal cortical afferences and therefore relays information between the whole neocortex and the hippocampus. Thus the DA system could play an important role of modulation on hippocampal and parahippocampal functions.