Localization of substance P and neurofilament immunoreactive fibers in the lumbar facet joint capsule and supraspinous ligament of the rabbit.

An indirect immunofluorescence method was utilized to identify substance P-like immunoreactive (SPLI) and neurofilament protein immunoreactive (NFIR) fibers in the lumbar facet joint capsule and supraspinous ligament of the rabbit. The results demonstrated a large population of NFIR fibers, indicating that these tissues are richly innervated, and a smaller population of SPLI fibers. In some fibers, neurofilament protein and substance P were colocalized. The data suggest that the facet joint capsule and the supraspinous ligament contain SPLI nociceptive fibers that could be a source of low back pain.