Preface by Tomasz Janecki

Opioids were among the earliest neuropeptides identified in the nervous system. They are some of the most effective pain-relieving drugs used in the clinical management of pain. In addition to their analgesic effect, opioid peptides also affect a number of physiological functions, including hormone secretion, neurotransmitter release, feeding, gastrointestinal motility, and respiratory activity. Research on opioids has been carried out for about 30 years. An enormous amount of experimental material has been accumulated. The recent identification of multiple opioid receptors, as well as the development of peptide agonist and antagonist analogs for each of these receptors has provided a framework to explore functional roles of opioids. The present issue, containing five articles, provides a brief overview on only a few aspects of opioid chemistry and physiology. The authors review the main developments that have led to potent and selective peptide analogs at three main receptors, their bioavailability and resistance against enzymatic degradation. They survey the relationship of opioid peptides to such undesirable side-effects as addiction and tolerance and discuss some biological functions of opioids, including their role in inflammatory responses and gastric function (in this case the role of other neuropeptides, not only opioids is discussed). I would like to thank the authors of this issue for their valuable contributions.