Two major distinct subpopulations of neurokinin-3 receptor-expressing neurons in the superficial dorsal horn of the rat spinal cord

This study was designed to provide evidence for elucidating the mechanisms of neurokinin-3 receptor (NK3) in spinal pain modulation. First, colocalization of NK3 with the µ-opioid receptor (MOR1) was studied in the spinal dorsal horn of the rat. Confocal microscopy showed that about 44% of NK3-expressing neurons in laminae I and II were immunoreactive for MOR1, which corresponded to about 93% of the total population of MOR1-containing neurons in these laminae. Second, the relationship between NK3/MOR1-coexpressing neurons and those that express nitric oxide synthase (NOS) was examined by using a triple immunofluorescent staining method. About 37% of NK3-immunoreactive neurons were also NOS-immunoreactive, which constituted about 82% of NOS-immunoreacitve neurons in the superficial laminae. However, no triple-labelled neurons were detected. The present results indicate that there are two major distinct subpopulations of NK3-expressing neurons in the superficial dorsal horn, which suggests that the involvement of NK3 receptor in spinal nociception could be mediated by two distinct mechanisms, i.e. opioid and nitric oxide.