Afferent and efferent connections of the paramedian reticular nucleus in the brain stem of cats

Abstract

Anatomical connections of the paramedian reticular nucleus (PRN) of the caudal medulla were investigated using a bi-directional tracer, horseradish peroxidase (HRP). The followings were observed when the tracer was microinjected to PRN: A. Both labelled neurons and terminals were found in the areas of the mesencephalic cardioinhibitory mechanism (CIM), the gigantocellular reticular nucleus (GRN), the ambiguus nucleus (AN) and the contralateral PRN. B. Only labelled terminals were demonstrated in the area of the nucleus of solitary tract (NTS) and the intermedial lateral cell column (IML) of the spinal cord. C. Only retrogradely labelled neurons were observed in the areas of the dorsal and dorsomedial medulla (DM) and ventrolateral medulla (VLM). A few labelled neurons were observed in the periaqueductal gray, the cuneiform nucleus and superior colliculus of the mesencephalon as well as the alamina spinal trigeminal nucleus. When HRP was applied to the CIM, GRN or AN structures, respectively, both labelled cells and terminals were found in the PRN area. HRP injection in the VLM showed only labelled terminals in the PRN. However, injection of HRP to DM showed neither labelling neurons nor terminals in PRN. Results suggest that PRN projects to the pressor area of DM/NTS and IML through which PRN could exert its inhibitory functions on the sympathetic pressor actions. In addition, PRN may suppress the vagal bradycardiac action through its reciprocal connections with CIM, GRN and AN. No lateralization in the PRN pathway was evident.