Expression of orphanin FQ/nociceptin and its receptor in rat peripheral ganglia and spinal cord.

Expression of the neuropeptide orphanin FQ/nociceptin (OFQ/N) and its receptor, the opioid receptor-like receptor (ORL1), have been found to have a wide distribution in the central nervous system, and in brain areas involved in sensory perception in particular. The effects of OFQ/N on, e.g., sensory transmission are very complex, and a modulatory effect on pain perception has been suggested. We therefore wanted to investigate the distribution of OFQ/N and ORL1 in the spinal cord and DRG, and also in SCG and some other peripheral tissues. The methods used were in situ hybridization, immunohistochemistry and ligand binding. We found that OFQ/N and ORL1 mRNA are expressed in DRG; primarily in small and large neurons, respectively. In spinal cord, mRNA for OFQ/N and ORL1 is expressed in neurons in laminae I, II and X, and in ventral horn neurons. Further, immunoreactivity for OFQ/N is observed in fibers and neurons in the superficial laminae of the dorsal horn and around the central canal, and also in neurons in the ventral horn of the spinal cord. Receptor ligand binding to the spinal cord grey matter is demonstrated, primarily concentrated to the dorsal horn and around the central canal, and also to medium and large size DRG neurons. These findings on the morphological distribution pattern of OFQ/N and ORL1 at the cellular level may support the notion that OFQ/N is involved in modulating pain transmission. Further, expression of OFQ/N and ORL1 mRNA was also found in SCG, whereas expression was undetectable in skin.