Ultrastructural localization of increased neuropeptide immunoreactivity in the axons and cells of the gracile nucleus following chronic constriction injury of the sciatic nerve.

Neuropeptide plasticity in the gracile nucleus is thought to play a role in the development of neuropathic pain following nerve injury. Two weeks after chronic constriction injury of adult rat sciatic nerve, galanin, neuropeptide Y and calcitonin gene-related peptide immunoreactivities were increased in fibers and cells in the gracile nucleus ipsilateral to injury. At the electron microscopic level, this increased neuropeptide immunoreactivity was localized in myelinated axons, boutons, dendrites, neurons and glial cells. Galanin-, neuropeptide Y- and calcitonin gene-related peptide-immunoreactive boutons were frequently presynaptic to dendrites of both immunoreactive and non-immunoreactive neurons. However, no neuropeptide Y, galanin and calcitonin gene-related peptide messenger RNA was detected in the injured side gracile nuclei by in situ hybridization. These results show that partial nerve injury to the sciatic nerve induces increases in the content of galanin, neuropeptide Y and calcitonin gene-related peptide immunoreactivities in synaptic terminals within the gracile nucleus, which suggests that there may be increased release of these neuropeptides following sensory or spontaneous stimulation of large-diameter primary afferents following partial nerve injury, perhaps one mechanism involved in neuropathic pain. We also show an apparent transfer of these neuropeptides to the cells of the gracile nucleus, both neurons and glial cells, an intriguing phenomenon of unknown functional significance.