Ultrastructure and synaptic targets of the raphe-nigral projection in the rat

A major input to the substantia nigra is from the 5-hydroxytryptamine-containing neurons in the dorsal raphe nucleus. In order to examine the morphology and distribution of this projection, rats were given injections of the anterograde tracers, Phaseolus vulgaris-leucoagglutinin or biocytin, in the dorsal raphe nucleus and the substantia nigra was examined at both the light and electron microscopic levels. In addition, sections of the substantia nigra were immunostained for 5-hydroxytryptamine and examined in both the light and electron microscopes. Since dopaminergic neurons of the substantia nigra are known to be responsive to stimulation of the raphe and to applied 5-hydroxytryptamine, sections that contained anterogradely labelled terminals were further processed to reveal tyrosine hydroxylase immunoreactivity to determine whether the raphe input makes direct synaptic contact with dopaminergic neurons. Light microscopic analysis revealed that all divisions of the substantia nigra received input from the dorsal raphe which, in agreement with previous observations, showed a topographical organization. In that formed asymmetrical synaptic contact with dendritic shafts and spines. The synaptic boutons were often associated with subjunctional dense bodies. Terminals that displayed immunoreactivity for 5-hydroxytryptamine had a similar morphology, synaptic specialisations and postsynaptic targets to the anterogradely labelled terminals. In those sections that were stained for both anterogradely labelled terminals and tyrosine hydroxylase, the raphe-nigral terminals were seen to form asymmetrical synaptic contact with the dendrites of the dopaminergic neurons. It is concluded that dendrites of dopaminergic neurons in the substantia nigra pars reticulata represent at least one of the synaptic targets of the raphe-nigral projection and that these contacts provide an anatomical substrate for the effects of the dorsal raphe, and presumably 5-hydroxytryptamine, on dopaminergic systems in the substantia nigra.