Expression of neuropeptides – galanin and Reg2 - in subcategories of nociceptors after nerve injury

Abstract

Neuropeptides show dynamic changes in expression after nerve injury, and have been implicated in chronic pain states. I sought to compare expression of two peptides which are upregulated after nerve injury, galanin and Reg2, and to determine the subtypes of dorsal root ganglion (DRG) cells in which they are expressed. Rats underwent unilateral, L5 spinal nerve ligation (SNL) with recovery for 1 or 7 days. Galanin and Reg2 were expressed in 20% and 8% of DRG cells at 1 day, and 49% and 5% at 7 days, respectively. At 1 day, galanin was mainly expressed in CGRPpositive DRG cells (62% of galanin cells); this expression was maintained at 7 days. At 1 day, galanin was expressed in few IB4-positive DRG cells (14% of galanin cells), but expression in IB4 cells significantly increased by 7 days (30% of galanin cells). Galanin was also coexpressed with TRPV2 (9%). Reg2 was coexpressed in 48% of galanin cells (7 days) but not coexpressed with TRPV2 and sparsely with CGRP. Using isolated DRG cultured in BSF2 supplemented with 2% foetal calf serum (FCS), galanin and Reg2 were expressed in 36.5% and 23.3% after 72hrs. Addition of IL6 (40 ng/mL in FCSsupplemented BSF2) increased galanin expression after 24 and 48hrs growth by 6.7% and 13.1%, respectively. ED1-positive cells were shown infiltrating the spinal nerve and associated with the ganglion by 7 days post-SNL. These results show Reg2 is more selectively expressed after L5 SNL than galanin. Both peptides show a complex temporal profile of expression. Galanin is upregulated initially in CGRP-cells and then in IB4/P2X3-cells, whereas Reg2 is expressed initially in IB4/P2X3-cells and then in larger cells. The factors that differentially regulate galanin, and potentially Reg2, in these cell populations remain unknown but it is hypothesized that IL6, or a related cytokine, drives-up galanin expression in IB4/P2X3-cells by 7 days but that the early upregulation in CGRP-cells is a direct consequence of axotomy.