Spinal afferent projections to subnucleus reticularis dorsalis in the rat.

Small amounts of the retrograde tracer WGA-apoHRP-Au complex were injected in the caudal medulla to study the spinal afferents to the subnucleus reticularis dorsalis (SRD). Labelled neurones were found at all levels of the spinal cord: the highest numbers were in the ipsilateral cervical spinal cord (mainly laminae I, V, VI, VII, VIII and X), the lowest were at the thoracic and lumbar levels bilaterally, while an intermediate density was found bilaterally at the sacral level. When injection sites were located in the underlying subnucleus reticularis ventralis (SRV), labelling was bilateral and mainly in the deep layers of the cervical spinal cord. Together with our previous electrophysiological and anatomical data, this study suggests that the SRD provides a link in spino-reticulo-spinal loops implicated in the processing of pain.