Neuropathic pain: a possible role for the melanocortin system?

In humans, damage to the nervous system can lead to a pain state referred to as neuropathic pain. Here, we give a short overview of the clinical picture and classification of neuropathic pain and highlight some of the currently known pathophysiological mechanisms involved, with special emphasis on neuropeptide plasticity. In this context, we discuss a specific group of neuropeptides, the melanocortins. These peptides have been demonstrated to play a role in nociception and to functionally interact with the opiate system. Recently, we demonstrated that spinal melanocortin receptors are upregulated in a rat model of neuropathic pain and that blockade of the melanocortin MC4 receptor has anti-allodynic effects in this condition, suggesting that the melanocortin system plays a role in neuropathic pain. A natural agonist of melanocortin receptors is -melanocyte-stimulating hormone (-MSH), derived from the precursor molecule pro-opiomelanocortin (POMC). Cleavage of this precursor also yields -endorphin, which is co-released with -MSH in nociception-associated areas of the spinal cord. We hypothesise that melanocortin receptor blockade attenuates a tonic influence of -MSH on nociception, thus allowing the analgesic effects of -endorphin to develop, resulting in the alleviation of allodynia. In this way, treatment with melanocortin receptor antagonists might enhance opioid efficacy in neuropathic pain, which would be of great benefit in clinical practice