Effects of supraspinal orphanin FQ/nociceptin

The first reported behavioral action of the endogenous ligand for the "orph an" opioid receptor was a seemingly paradoxical increased sensitivity to no ciception (i.e, hyperalgesia) after supraspinal injection into the cerebral ventricles of mice. In the continuing absence of an appropriate in vivo re ceptor antagonist, studies attempting to define the role of orphanin FQ/noc iceptin (OFQ/N) in pain modulation and other behaviors have also featured c entral injection of peptide. This article reviews the findings of such stud ies. There appears to be concordance around the observation of anti-opioid actions of supraspinally injected OFQ/N, whereas the observations of hypera lgesia and/or analgesia are much less clear. A portion of the discrepant da ta may be explained in terms of methodological issues, stress-induced analg esia accompanying experimental protocols, and genotypic variation among sub jects. Clarification of OFQ/N's role in nociception, as with other putative biologic functions, will probably depend upon the availability of a select ive receptor antagonist. (C) 2000 Elsevier Science Inc. All rights reserved .