SECRETORY PATHWAYS OF NEUROPEPTIDES IN RAT LUMBAR DORSAL-ROOT GANGLION NEURONS AND EFFECTS OF PERIPHERAL AXOTOMY

Using immunocytochemistry combined with confocal and electron microsco py, the secretory pathways related to substance P (SP), calcitonin gen e-related peptide (CGRP), galanin (GAL), and neuropeptide Y (NPY) were investigated in neurons in rat lumbar (L) 4 and L5 dorsal root gangli a (DRGs) before and after peripheral axotomy. All four peptides were p rocessed through the regulated secretory pathway in many small neurons in normal DRGs, and CGRP through this pathway also in some large neur ons. In many small neurons, two neuropeptides could be sorted into the same or separate large dense-core vesicles (LDCVs). The LDCVs had a s ignificantly larger diameter in small as compared to large DRG neurons . Fourteen days after sciatic nerve cut, the levels of SP- and CGRP-li ke immunoreactivities (-LIs) and the number of LDCVs containing these peptides were markedly reduced, but SP- and CGRP-LIs were still seen i n the regulated pathway. GAL-LI was markedly increased in many small n eurons and some large neurons and NPY-LI mainly in large neurons. Both peptides were particularly abundant in the Golgi region. In small neu rons, 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 norm al DRG neurons. After axotomy, CGRP-LI and GAL-LI were often in separa te LDCVs. One type of NPY-positive large neurons showed budding off of LDCVs after axotomy, but also some ''scattered'' labeling in the cyto plasm. In the second type, NPY-LI was mainly found in multivesicular b odies. In several myelinated nerve fibers a ''diffuse'' distribution o f NPY was seen together with some LDCVs containing NPY-LI. In contrast , in unmyelinated nerve fibers, NPY-, GAL-, SP-, and CGRP-LIs were alw ays observed in LDCVs. Thus, both in normal and axotomized DRG neurons , peptides are processed through the regulated pathway. However, in so me large neurons, NPY is, in addition, secreted through the constituti ve 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. (C) 1995 Wiley-Liss, Inc.