RT97- and calcitonin gene-related peptide-like immunoreactivity in lumbar intervertebral discs and adjacent tissue from the rat

The innervation of rat intervertebral disc and adjacent ligamentous tissue has been investigated using 2 antibodies, RT97 and anti-calcitonin gene-related peptide. Immunoreactivity to the peptide was found in many fibres throughout the long ligaments around the intervertebral discs and in the periosteum, especially associated with vascular channels entering the vertebral bodies. Few of the immunoreactive fibres entered the annular lamellae of the disc tissue. Most of those which terminated did so as fine fibres which lay close to, or in, the interlamellar spaces of the outer annulus fibrosus. Calcitonin gene-related peptide-like immunoreactivity was also found in more complex endings in the longitudinal ligaments and rarely within the annulus fibrosus. RT97-immunoreactivity was also present in the complex endings and associated fibres. Conversely, RT97-immunoreactivity was apparent only in a few fine filamentous fibre endings. This suggested that the majority of fine filamentous, or free, nerve endings were of an unmyelinated sensory origin. Alternatively, those endings of a more complex nature, which were RT97-immunoreactive, were of a myelinated sensory origin. No immunoreactivity to either antibody was seen in the inner annular or nuclear tissue. It was therefore concluded that the sensory innervation of the rat intervertebral disc has both myelinated and unmyelinated components, the latter being more extensive. Both types of innervation appear to be restricted to the outermost rings of the annulus fibrosus.