The origin of brainstem noradrenergic and serotonergic projections to the spinal cord dorsal horn in the rat.

Although it has been proposed that the locus coeruleus is the predominant, if not exclusive, brainstem origin of the noradrenergic innervation of the spinal dorsal horn, pharmacological studies argue otherwise. In this study we made localized injections of the retrograde tracer wheatgerm agglutinin conjugated to apo-horseradish peroxidase gold (WGA:apoHRP-Au), in conjunction with immunocytochemical labeling for tyrosine hydroxylase (TH) or serotonin (5-HT), to identify the brainstem source of the noradrenaline (NA) and 5-HT innervation of the dorsal horn of the rat. Our studies were concentrated in the C5 spinal segment. The pattern of labeling was only studied in animals in which the tracer injection was restricted to the dorsal horn. In these rats, TH-immunoreactive neurons in widespread regions of the brainstem, including the locus coeruleus, subcoeruleus, A5, and A7 cell groups, were found to project to the dorsal horn. In terms of absolute numbers of double-labeled cells, no one noradrenergic cell group predominated. As expected, dorsal-horn-projecting 5-HT-immunoreactive neurons were found within the 5-HT populations of the rostroventromedial medulla and caudal pons, including the nucleus raphe magnus, nucleus paragigantocellularis (PGi), and ventral portions of the nucleus gigantocellularis (Gi). The majority of retrogradely labeled 5-HT-immunoreactive cells were, however, located off the midline, in the ipsilateral PGi and ventral Gi. Finally, a large number of retrogradely labeled, non-5-HT cells were found intermingled among the 5-HT cells of this region. Our results provide evidence that the noradrenergic regulation of nociceptive transmission at the spinal cord level arises from direct spinal projections of several brainstem noradrenergic cell groups.