ORGANIZATION OF THE EFFERENT PROJECTIONS FROM THE SPINAL CERVICAL ENLARGEMENT TO THE PARABRACHIAL AREA AND PERIAQUEDUCTAL GRAY - A PHA-L STUDY IN THE RAT

The organization of efferent projections from the spinal cervical enla rgement to the parabrachial (PB) area and the periaqueductal gray (PAG ) was studied in the rat by using microinjections of Phaseolus vulgari s-leucoagglutinin (PHA-L) into different laminae around the C7 level. The results demonstrated two areas of cervical enlargement which proje ct in different ways to the PB area and PAG. First, the superficial la minae (I, II) showed a very dense projection, with a clear contralater al dominance at the coronal level where the inferior colliculus merges with the pens, to a restricted ''superficial'' portion of the PB area , namely the lateral crescent area, the dorsal lateral, the superior l ateral (PBsl), and the outer portion of the external lateral PB subnuc lei. Less dense projections were observed in the Kolliker-Fuse nucleus (KF) and in the ventrolateral/lateral quadrant of the caudal and mid PAG. By contrast, the labeling was weak or absent in the other PB subn uclei and the outer adjacent regions; in particular, no, or very littl e, labeling was found in the cuneiform nucleus. The PB area appeared t o be the supraspinal target that received the densest projection from laminae I and II. Projections were less dense in the PAG and the thala mus and markedly less in other sites such as the ventrolateral medulla , the subnucleus reticularis dorsalis, and the nucleus of the solitary tract. Second, the reticular portion of lamina V, the medial portion of laminae IV-VI up to X and lamina VIII, showed bilateral projections with a weak ipsilateral dominance and a high to medium density on a v ery restricted portion of the PB area, namely the internal lateral PB subnucleus. A lesser projection was also observed in the adjacent port ion of the PBsl, the KF, and the lateral quadrant of the PAG. These re sults suggest that signals carried by neurons from lamina I-II converg e on a restricted superficial portion of the PB area and the ventral p art of the lateral quadrant of the PAG. These results are discussed in the context of the role of the spino-PB and spino-PAG pathways in noc iception. (C) 1995 Wiley-Liss, Inc.