Dopaminergic projection from nucleus raphe dorsalis to neostriatum in the rat.

The existence of a dopamine (DA) projection from nucleus raphe dorsalis (RD) to neostriatum was demonstrated in the rat by combined tyrosine hydroxylase (TH) immunohistochemistry and radioautography after retrograde axonal transport of [3H]noradrenaline ([3H]NA). Intrastriatal injections of [3H]NA were carried out in normal rats or after ipsilateral destruction of the nigrostriatal DA system by injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra. Some 1,000 TH-positive nerve cell bodies were counted within the confines of RD as defined by its content in serotonin (5-HT) neurons. These DA neurons occupied the upper third of the RD and they were part of its small cell population. In all cases, a small proportion of the TH-immunoreactive nerve cell bodies in RD were retrogradely radiolabeled. Radiolabeled but immunonegative cells were exceedingly rare. The double-labeled neurons were generally more numerous after elimination of the nigrostriatal DA innervation than in normal rats. They mostly lay within the ventral portion of the medial subdivision of RD and always predominated on the [3H]NA- injected side. Some were also present in nucleus linearis caudalis. It was concluded that [3H]NA had been taken up and retrogradely transported exclusively by catecholamine neurons; part of the DA cell group in RD projects to the neostriatum; and that most if not all non-5-HT neurons projecting from RD to neostriatum are likely to be dopaminergic.