Neuropeptide Y in trigeminal ganglion following chronic constriction injury of the rat infraorbital nerve: is there correlation to somatosensory parameters?

The aim of this study was to investigate neuropeptide Y (NPY) levels in trigeminal ganglia following infraorbital nerve injury. Two experimental procedures were performed in three groups of rats: a unilateral chronic constriction injury (CCI) to the infraorbital nerve (n=13), nerve manipulation without CCI (n=13) and unoperated controls (n=8). All rats underwent baseline and regular assessment of mechanical withdrawal threshold (Von Frey) and reaction to pin prick as well as free behavior evaluations. CCI to the infraorbital nerve induced significant hyperalgesia and allodynia within 9-12 days. At 6 days seven rats were euthanized and trigeminal ganglia harvested for immunocytochemical (ICC) studies. The study was ended at 14 days when all rats were euthanized and their ganglia harvested for ICC and radioimmunoassay (RIA) studies. An increase in NPY levels was seen in the ipsilateral ganglia of manipulated and CCI rats at 6 days, when rats displayed no pain-related behavior. At 14 days, ICC and RIA both detected significant increases in NPY levels in the ipsilateral ganglia of CCI and manipulated rats but not in unoperated controls. The possible roles of NPY in pain modulation and nerve injury are discussed in light of these findings.