SEROTONIN-CONTAINING PROJECTIONS TO THE THALAMUS IN THE RAT REVEALED BY A HORSERADISH-PEROXIDASE AND PEROXIDASE ANTIPEROXIDASE DOUBLE-STAINING TECHNIQUE

The retrograde transport of horseradish peroxidase (HRP) was used in combination with peroxidase antiperoxidase (PAP) immunocytochemistry to investigate serotonin[5-hydroxytryptamine]-containing projections to the thalamus of the rat. Sections were histochemically stained to reveal retrogradely transported HRP and then PAP immunostained using a monoclonal anti-serotonin (5-HT) antibody. Following HRP injections into the ventral thalamus, retrogradely labeled cells were observed in a number of sites in the brainstem and including areas known to be rich in 5-HT-containing neurons. At rostral levels of the dorsal raphe nucleus, retrogradely labeled cells were observed both on the midline and in a distinct lateral group extending diffusely into the periaqueductal gray (PAG). In both of these areas many 5-HT-immunoreactive HRP retrogradely labeled neurons were observed. Except for the most rostral levels of the dorsal raphe nucleus, such double-labeled cells represented only a small proportion of the total population of 5-HT-immunoreactive neurons. In the lateral group, the retrograde labeling was mainly unilateral to the injection site but some contralateral labeling was also seen. At caudal levels of the dorsal raphe nucleus, retrogradely labeled cells were observed predominantly in the lateral group. At the level of the dorsolateral tegmental nucleus, few 5-HT or 5-HT/HRP labeled cells were observed in the lateral group, although HRP retrogradely labeled neurons were present. Double-stained cells were detected also in the medial raphe nucleus (corresponding to the B8 cell group according to the nomenclature of Dahlstrom and Fuxe), among the fibers of the medial lemniscus (B9), and in nucleus raphe points (B5).