Neurotransmitter-specific projection neurons revealed by combining PAP immunohistochemistry with retrograde transport of HRP.

The use of PAP immunohistochemistry in combination with HRP retrograde transport is described, allowing the transmitter characterization of identified projection neurons. To assess the validity of this procedure the dorsal raphe nucleus has been studied. It has been possible in single sections to stain for 5-HT immunoreactivity cells which have been retrogradely labeled following injection of HRP into the striatum. The presence of such neurons and their distribution in the dorsal raphe demonstrated with this dual staining technique agrees very well with previous results obtained from separately performed retrograde labeling and histochemical or immunohistochemical staining. The procedure described has some advantages over other traditional and recently described methods and in addition should be applicable to electron microscopic studies.