Topographic alterations in TMP and peptide immunostaining in the substantia gelatinosa after injury to sciatic nerve branches

Numerous immunohistochemical and histochemical studies indicate that peripheral nerve injury decreases the staining in the substantia gelatinosa (SG) of SP, CGRP, and endomorphin-2 (EM2) from peptidergic afferents, thiamine monophosphatase (TMP) from non-peptidergic afferents, and increases NPY from large-diameter afferents. Here we further investigate this “neuroanatomical plasticity” in terms of topographic relationships, time course, and behavioral signs of allodynia/ hyperalgesia after injury to the tibial (T), T + common peroneal (T&CP), CP + sural (CP&S), or no branches (sham) of the sciatic nerve. We used Metamorph software to assess loss of TMP staining at 14d post-surgery. We calculated the mediolateral boundaries of L4 SG TMP staining as follows: medial tibial = medial 34%; common peroneal = central 20%; sural = centrolateral 21%; lateral tibial = 8%; saphenous = most lateral 15%. We then applied these regional boundaries to sections stained for SP, CGRP, and EM, and found a remarkably similar staining pattern. For example, T transection selectively reduced SP staining in the medial (29±8%) but not mediolateral (106±8%) regions 2 wk post-surgery. Remarkably, the medial staining recovered (86±4%) 6 m post-surgery, as did mechanical hyperalgesia and cold allodynia. By contrast, T transection produced long-lasting (6 mo) reductions in TMP intensity, increases in NPY immunoreactivity, and persistent mechanical allodynia. We conclude that our method of semi-quantitative immunohistochemistry powerfully and precisely estimates the regional loss of immunohistochemical staining across the mediolateral extent of the SG. This method is uniquely suited for studies in spared nerve injury models of neuropathic pain involving section of one or two branches of the sciatic nerve. We introduce the hypothesis that non-peptidergic afferents contribute to mechanical allodynia, while peptidergic afferents contribute to mechanical hyperalgesia and cold allodynia. Supported by DA10356, NS43383 (BKT) and the VA (JEZ).