ORGANIZATION OF SEROTONINERGIC PROJECTIONS FROM THE RAPHE NUCLEI TO THE ANTERIOR THALAMIC NUCLEI IN THE RAT - A COMBINED RETROGRADE TRACINGAND 5-HT IMMUNOHISTOCHEMICAL STUDY

We combined retrograde transport of horseradish peroxidase (HRP) with 5-hydroxytryptamine (5-HT) immunohistochemistry to study serotoninergi c projections to the anterior thalamic nuclei (ATN) of the rat. Small iontophoretic injections of HRP into the anterodorsal thalamic nucleus resulted in double-labelled neurons predominantly in the ventromedial and also in the ventrolateral part of the ipsilateral dorsal raphe (D R). A smaller number of double-labelled neurons was also found in the dorsomedial part of the nucleus, predominantly ipsilaterally, and in t he median raphe nucleus (MnR), close to the midline. After injection i nto the medial subdivision of the anteroventral thalamic nucleus, the pattern of labelling in DR and MnR was similar to that detected follow ing injections into the anterodorsal thalamic nucleus. However, inject ion into the posterior subdivision of the anteroventral thalamic nucle us resulted in bilateral retrograde labelling of a few 5-HT-containing neurons in the dorsolateral part of the DR. Labelling in the ventrome dial, ventrolateral and dorsomedial regions of DR and MnR was similar to that detected after injections into the medial subdivision of the a nteroventral thalamic nucleus. After all injections into the ATN, doub le-labelled cells were found throughout the rostrocaudal extent of MnR and throughout the rostral two-thirds of DR. The caudal extension of DR was devoid of double-labelled cells. Although double-labelled cells were observed bilaterally in the dorsolateral part of the DR, the pro jection from DR to ATN was predominantly ipsilateral. These results sh ow that there is an internal organization within DR such that subnucle i of the DR can be defined on the basis of their efferent projections to specific subdivisions of the ATN.