[Easing human pain: challenges to a novel therapeutic drug in neuropathic pain].

Pain control is very important since, even today, 20 million people suffer from neuropathic pain. Although many basic science and clinical researchers have made efforts to control pain, the mechanism of neuropathic pain is unfortunately still not fully understood. Morphine, a prototypical opioid, is a useful medicine to relieve severe pain. However, repeated or continuous use of morphine and other opioids are associated with a potential risk of analgesic tolerance, which requires an increase in dosage to maintain the same efficacy. In addition, morphine is not always effective in neuropathic pain. In this review we focus on: (1) the role of muscarinic receptors in the spinal cord and anterior cingulate cortex (ACC) in neuropathic pain, (2) how chemokine (C-C motif) ligand 1 (CCL-1) is involved in neuropathic pain, and (3) the novel mechanism of morphine tolerance. The findings in this study may cast new light on novel mechanism of neuropathic pain and development of novel clinical medicines in the future.