Distribution of nerve endings and sensory neuropeptides in rat synovium, meniscus and bone

The nervous system collects information from the outer world by specific senses and from the interior milieu by somatic senses. This information is processed and stored in memory and affects various bodily functions through the efferent arm of the nervous system. The efferent chemical neuropeptide message is transported intra-axonally to the site of action, which imparts site-specificity to the peripheral, paracrine neuropeptide effects. In the present study, immunohistochemistry using the immunoperoxidase method with nickel amplification was applied to visualize the topographical distribution of articular nerve fibres and nerve endings using the markers PGP 9.5 and synaptophysin, respectively. Furthermore, to get a comprehensive idea of the sensory innervation of the articular and para-articular tissue, antisera to calcitonin gene-related peptide (CGRP) and substance P were employed. Samples were collected after fixation by perfusion followed by immersion in fixative and decalcification by a special method, which also allows studies of the bone innervation. PGP 9.5- and synaptophysin-immunoreactive type IVa and IVb nerve fibres and endings were found in the synovial lining and sublining tissue and in the vascularized peripheral parts of the menisci. Furthermore, periosteum, bone marrow and the epiphyseal growth plates were also innervated, whereas innervation of the diaphyseal and metaphyseal bone was more sparse. PGP 9.5- and synaptophysin-immunoreactive nerves were also characterized by their CRGP, and to some extent, substance P content. Because of their distribution, the peripheral peptide-containing type IVa and IVb nerve fibres and nerve endings are in a position to participate in the pathogenesis of arthritis, including aspects of nociception, tissue remodelling and neurogenic inflammation.