Lack of response of proenkephalin A and sympathetic nervous system in chronic pain associated with lung cancer.

Since the discovery of the link between peripheral endogenous opioid peptides and pain regulation, these substances have been studied in relation to certain pain conditions. In order to elucidate the effect of chronic pain on both peripheral opioid system and sympathetic nervous activity, we assayed plasma met-enkephalin (ME), neutrophil met-enkephalin containing peptides (NMECP) and plasma free and conjugated catecholamines (CA) in lung cancer patients with chronic pain related to bone metastases and without pain. No significant difference was found in ME levels when the pain cancer group (0.36 +/- 0.06 pmol/ml) was compared to the pain-free group (0.37 +/- 0.04 pmol/ml); results were similar for NMECP levels (14.1 +/- 1.66 pmol/mg prot and 18.41 +/- 1.93 pmol/mg prot, respectively). CA levels in both groups were also similar. These results differ from those we have reported previously for acute pain, suggesting that a non-permanent painful stimulus may be necessary for peripheral opioid system stimulation.