Secretory pathways of neuropeptides in rat lumbar dorsal root ganglion neurons and effects of peripheral axotomy

Abstract

Using immunocytochemistry combined with confocal and electron microscopy, the secretory pathways related to substance P (SP), calcitonin gene‐related peptide (CGRP), galanin (GAL), and neuropeptide Y (NPY) were investigated in neurons in rat lumbar (L) 4 and L5 dorsal root ganglia (DRGs) before and after peripheral axotomy. All four peptides were processed through the regulated secretory pathway in many small neurons in normal DRGs, and CGRP through this pathway also in some large neurons. In many small neurons, two neuropeptides could be sorted into the same or separate large dense‐core vesicles (LDCVs). The LDCVs had a significantly larger diameter in small as compared to large DRG neurons. Fourteen days after sciatic nerve cut, the levels of SP‐ and CGRP‐like immunoreactivities (‐LIs) and the number of LDCVs containing these peptides were markedly reduced, but SP‐ and CGRP‐LIs were still seen in the regulated pathway. GAL‐LI was markedly increased in many small neurons and some large neurons and NPY‐LI mainly in large neurons. Both peptides were particularly abundant in the Golgi region. In small neurons, the number of LDCVs containing GAL‐ or NPY‐LI was increased, but did not appear to reach the numbers containing SP‐ or CGRP‐LI in normal DRG neurons. After axotomy, CGRP‐LI and GAL‐LI were often in separate LDCVs. One type of NPY‐positive large neurons showed budding off of LDCVs after axotomy, but also some “scattered” labeling in the cytoplasm. In the second type, NPY‐LI was mainly found in multivesicular bodies. In several myelinated nerve fibers a “diffuse” distribution of NPY was seen together with some LDCVs containing NPY‐LI. In contrast, in unmyelinated nerve fibers, NPY‐, GAL‐, SP‐, and CGRP‐LIs were always observed in LDCVs. Thus, both in normal and axotomized DRG neurons, peptides are processed through the regulated pathway. However, in some large neurons, NPY is, in addition, secreted through the constitutive pathway, perhaps as a consequence of limited sorting mechanisms for NPY, i.e., the plasticity of the secretory mechanisms does not match the rate of peptide synthesis after axotomy.