Origin, course and termination of the mesohabenular dopamine pathway in the rat

This study describes the organization of the mesohabenular dopamine (DA) system in the rat as revealed by fluorescence histochemistry in combination with lesions, DA uptake experiments and injections of a retrograde tracer. The DA axons were found to be aggregated in a dense terminal field located in the caudal two thirds of the medial part of the lateral habenular nucleus. Microknife lesions of the stria medullaris left this DA innervation unaffected while cuts through the fasciculus retroflexus resulted in the virtual disappearance of the DA innervation. Injections of the fluorescent retrograde tracer True Blue (TB) into the lateral habenula produced labeling of both DA and non-DA-containing cells in the ventral mesencephalon, mainly in the interfascicular nucleus ipsilateral to the injection. This study thus documents the existence of a mesohabenular DA pathway whose cell bodies are located in the ventral mesencephalon and whose axons ascend with the fasciculus retroflexus to terminate in the caudomedial part of the lateral habenular nucleus. This information, taken together with insights gained from other studies, suggests a role for the mesohabenular DA system in modulating telencephalic feedback onto the mesencephalic DA-neurons and also in regulating the output from the dorsal raphe nucleus.