Supramedullary afferents of the nucleus raphe magnus in the rat: A study using the transcannula HRP gel and autoradiographic techniques

Afferents of the nucleus raphe magnus (NRM) were retrogradely la‐belled by using a transcannula HRP gel technique in conjunction with tetramethylbenzidine nuerohistochemistry to determine the sources of in‐puts to the nucleus which could potentially influence the descending antio‐ciceptive raphe‐spinal system. Large numbers of HRP‐labelled neurons were seen in the frontal cortex, dorsomedial nucleus of the hypothalamus, zona incerta, nucleus parafascicularis prerubralis (NPfPr), pretectum, dorsal and lateral periaqueductal gray, nucleus cuneiformis (NC), deep superior col‐liculus (dSC), a paraoculomotor cell group which may be the medial acces‐sory nucleus of Bechterew, dorsal column nuclei, and spinal trigeminal nucleus. Smaller numbers of labelled cells were also observed in the preoptic area, nucleus of Darkschewitsch, ventral peri(third)ventricular gray, nu‐cleus reticularis pontis oralis and caudalis, medial and lateral vestibular nuclei, and a subdivision of the hypoglossal nucleus. Confirmational an‐terograde autoradiographic studies were performed by injecting tritiated leucine into two of the principal sources of afferents to NRM: NPfPr, and dSC/NC. The results are compared with control HRP gel implants in the inferior olive, spinal cord, nucleus reticularis paragigantocellularis, and medial fa‐cial nucleus. Comments are also made concerning the parcellation of the ventromedial medulla and the possible role of both NRM and its afferents in central analgesic mechanisms.