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Front. Physiol. | doi: 10.3389/fphys.2018.01842

Perspectives on Endogenous Opioids in Birds

  • 1Department of Poultry Science, University of Arkansas, United States
  • 2Department of Animal Physiology and Endocrinology, University of Agriculture of Krakow, Poland

The present review summarizes the state of knowledge of endogenous opioids in birds. Endogenous opioid peptides acts in a neuromodulatory, hormonal and paracrine manner to mediate analgesic and other physiological functions. These peptides act through specific G-protein coupled receptors. Opioid receptors consist of a family of four closely-related proteins. The three types of opioid receptors are the mu (MOR or μ), delta (DOR or δ), and kappa (KOR or κ) opioid receptor proteins. The role of the fourth member of the opioid receptor family, the nociceptin or orphanin FQ receptor (ORL), is not clear. The ligands for opioid receptors are: β–endorphin (MOR), Met-enkephalin, Leu-enkephalin (DOR) and dynorphin (KOR), together with probably endomorphins 1 and 2. In spite of long history of research on endogenous opioid peptides, there are no studies of endogenous opioids per se in wild birds and few in poultry species. β-endorphin is present in all birds investigated and there is close agreement between the structures of β-endorphin in different birds. Plasma concentrations of β- endorphin are increased by ether stress in geese. There is evidence that β- endorphin plays a role in the control of luteinizing hormone release in chickens. Met-enkephalin is present in tissues such as the retina, hypothalamus, pituitary gland and adrenals together with circulation of birds. Stresses such as crowding and withholding water increase circulating concentrations of Met-enkephalin in chickens. The structures of chicken dynorphin A and B have been deduced from cDNA. What is missing are comprehensive studies of plasma concentrations and expression of the full array of endogenous opioids in multiple avian species under different situations. Also, what is not known is the extent to which circulating or locally released or intra-cellular Met-enkephalin influence physiological process in birds. Thus, there is considerable scope for investigation of the physiology of endogenous opioids in birds.

Keywords: beta-Endorphin, Met-enkephalin, Endogenous opioids, Opioid Receptor, stress

Received: 08 Mar 2018; Accepted: 06 Dec 2018.

Edited by:

Rocco V. Carsia, Rowan University School of Osteopathic Medicine, United States

Reviewed by:

Gregoy Y. Bedecarrats, University of Guelph, Canada
Takeshi Ohkubo, Ibaraki University, Japan  

Copyright: © 2018 Scanes and Pierzchala-Koziec. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Prof. Krystyna Pierzchala-Koziec, University of Agriculture of Krakow, Department of Animal Physiology and Endocrinology, Krakow, 31-120, Poland, rzkoziec@cyf-kr.edu.pl