[Changes in aminopeptidase N located on neutrophils derived from patients with chronic pain]

It is known that aminopeptidase N (APN) and neutral endopeptidase (NEP) in the central nervous system (CNS) regulate opioid peptides, leading to pain modulation. To examine whether these enzymes located on human neutrophils (PMNs) play a role in several modalities of pain, we measured the activity of these enzymes located on PMNs derived from patients with chronic pain and compared this with that of healthy volunteers. APN activity in the group of patients with chronic pain was significantly increased compared with that in group of healthy volunteers (4.25 +/- 0.17, n = 36 vs 3.53 +/- 0.21, n = 24, nmol.min-1.10(6) cells, P > 0.05, mean +/- SE). But NEP activity showed no differences in two groups. These results suggest that APN located on PMNs from patients with chronic pain may act as an indicator of continuous painful condition and there may be a pain-modulating system in the blood.