Neurokinin B peptide-2 neurons project from the hypothalamus to the thoracolumbar spinal cord of the rat.

The presence and location of CNS neurokinin B peptide-2-like immunoreactive neurons that project to the spinal cord were studied by a combination of retrograde transport of fluorescent dye (FluoroGold) and fluorescence immunocytochemistry. After injections of FluoroGold into the thoracic or lumbar segments of the rat spinal cord, serial sections of brain were stained with antisera directed against neurokinin B peptide-2. The results of the study showed that neurokinin B peptide-2-like immunoreactive neurons were located in the nucleus arcuate, median eminence, ventral and external bed nuclei of the stria terminalis, dorsal hypothalamic area, and medial habenula. Neurokinin B peptide-2 neurons that give rise to the long descending projections from the hypothalamus to thoracolumbar spinal cord were found only in the dorsal hypothalamic area. Approximately 36% of the neurokinin B peptide-2 neurons in the dorsal hypothalamic area projected to the spinal cord, whereas about 28% of the spinal projecting neurons in the dorsal hypothalamic area contained neurokinin B peptide-2-like immunoreactivity. Most of the spinal projecting neurokinin B peptide-2 neurons in the dorsal hypothalamic area had a cell size of 15 x 25 microns. In the spinal cord, immunoreactive neurokinin B peptide-2 fibers and terminals were distributed mainly in the superficial dorsal horn and the central autonomic area, with the highest density in laminae II and X, with less density in laminae IV and V. A few neurokinin peptide-2 fibers and terminals were also found in the ventral horn of the spinal cord. The results of the present study show that hypothalamic neurokinin B peptide-2 neurons are the main source of the spinal neurokinin B peptide-2.