Immunoreactive neuropeptides in nerves in ligamentous tissue. An experimental neuroimmunohistochemical study

A search for neuropeptide nerves in the healing of the experimentally ruptured medial collateral ligament (MCL) of the rabbit knee used specific antisera to the neuropeptides Substance P, calcitonin gene-related peptide (CGRP), and galanin. Sutured and unsutured MCLs were studied four and 14 weeks postoperatively. Both fluorescent thin nerve strands and small dotlike nerve terminals were regularly seen in the healing zone and in the adjacent normal ligamentous tissue, suggesting innervation of such structures by neuropeptide nerves. All three neuropeptides were more abundant in sutured ligaments than in unsutured ligaments, which may suggest beneficial effects of the apposition of the torn ligament ends on local nerve regeneration. Active involvement of the neural elements in the healing process was also suggested by kinetic studies showing a decrease in Substance P and CGRP staining as well as an increase in galanin staining during the study period. These changes in the periphery parallel the reactive changes earlier described in the dorsal root ganglion and dorsal horn cells occurring after a peripheral nerve injury. This may depend on the antidromic transport to the periphery of neuropeptides synthesized in the central nervous system. This experimental neuroimmunohistochemical mapping study and the known effects of neuropeptides on blood vessels, macrophages, and fibroblasts should stimulate further work on the role of innervation in ligamentous healing.