Perspectives on Endogenous Opioids in Birds

Effects of Restraint Stress on Circulating Corticosterone and Met Enkephalin in Chickens: Induction of Shifts in Insulin Secretion and Carbohydrate Metabolism

This study examined the effects of acute restraint stress in the presence or absence of naltrexone on the circulating concentrations of insulin, glucose, Met-enkephalin and corticosterone in 14-week-old chickens [design: 2 sex × 2 stress/non-stress × 2 +/- naltrexone]. In chickens (five male and five females per treatment) subjected to restraint for 30 min, there were increases in the plasma concentrations of corticosterone and Met-enkephalin. The plasma concentrations of insulin and glucose were also increased in the chickens during restraint. Moreover, there were increases in the plasma concentrations of insulin and glucose in the chickens. The patterns of expression of the proenkephalin gene (PENK) in both the anterior pituitary gland and the adrenal gland were very similar to that of plasma Met-enkephalin. There were relationships between the plasma concentrations of corticosterone, Met-enkephalin, insulin and glucose after 30 min of restraint. The effects of naltrexone treatment on both untreated and stressed chickens were also examined, with naltrexone attenuating the stress-induced increases in the plasma concentrations of corticosterone, Met-enkephalin and glucose but not in those of insulin. The present study demonstrates that stress increases insulin secretion in chickens but also induces insulin resistance.

Avian opioid peptides: evolutionary considerations, functional roles and a challenge to address critical questions

The present review considers the putative hormonal opioid peptides in birds. In birds and all other vertebrates, there are four opioid related genes encoding a series of peptides. These genes are, respectively, proenkephalin (PENK), prodynorphin (PDYN), pronociceptin (PNOC) and proopiomelanocortin (POMC). Proenkephalin (PENK) encodes Met- and Leu-enkephalin together with peptides containing met enkephalin motifs in birds, mammals and reptiles. Proopiomelanocortin (POMC) encodes β endorphin together with adrenocorticotropic hormone (ACTH), and melanocyte stimulating hormone (MSH). Prodynorphin (PDYN) encoding dynorphins A and B with α- and β-neoendorphins together intermediate polypeptides across the vertebrates. Pronociceptin (PNOC) encodes nociceptin together with possibly putative avian nocistatin and a non-opioid peptide derived from the C terminal of pronociceptin. There is a high degree of identity in the sequences of enkephalin peptides, dynorphin-A and B and nociceptin in birds and, to a less extent, across vertebrates. The opioid peptides exert effects related to pain together with other biological actions such as growth/development acting via a series of opioid receptors. What is unclear, particularly in birds, is the biological roles and interactions (additivity, antagonistic and synergistic) for the individual opioid peptides, the processing of the prohormones in different tissues and the physiological relevance of the different peptides and, particularly, of the circulating forms.

Pain Recognition and Assessment in Birds

The recognition and assessment of pain in avian species are crucial tools in providing adequate supportive care in clinical, laboratory, zoologic, rehabilitation, and companion animal settings. With birds being a highly diverse class of species, there is still much to be determined regarding how to create specific criteria to recognize and assess pain in these animals. This article provides a clinical review on the physiology of pain in birds, observed behavioral and physiologic alterations with pain, how different sources and degrees of pain can alter behaviors observed, and how this information can be applied in a clinical setting.

Opioid receptor μ, not δ and κ modulate food intake induced by ghrelin in laying chickens

Evidence from animal studies suggests that opioidergic system and gherlin have a regulatory role in food intake, but their interaction(s) have not been studied in laying chickens. So in this study, 4 experiments (each included 4 groups) were designed. The first experiment was performed to evaluate the effect of ghrelin on the cumulative food intake. Experiments 2 to 4 were designed to investigate the possibility of μ, δ, or κ opioid receptors mediating ghrelin-induced hypophagia. All drugs were injected intracerebroventricularly (ICV) at 5 days of age. The results of this study showed that the ICV injection of 1.5 nmol ghrelin did not affect cumulative food intake. But ICV injection of ghrelin with doses of 3 and 6 nmol significantly reduced the cumulative food intake (p<0.05). However, co-injection of ghrelin with NTI, and nor-BNI did not show a significant change in decreased food intake compared to ghrelin. Also, opioid μ receptor gene expression significantly increased (p<0.05), but δ and κ opioid receptors gene expression did not significantly change. These results indicated the opioidergic system is involved in developing ghrelin-induced hypophagic effects in laying chickens. Accordingly, this effect of ghrelin to modify the nutritional behavior is possibly mediated by opioid μ receptor.

Opioid Peptides and Their Receptors in Chickens: Structure, Functionality, and Tissue Distribution

Opioid peptides, derived from PENK, POMC, PDYN and PNOC precursors, together with their receptors (DOR, MOR, KOR and ORL1), constitute the opioid system and are suggested to participate in multiple physiological/pathological processes in vertebrates. However, the question whether an opioid system exists and functions in non-mammalian vertebrates including birds remains largely unknown. Here, we cloned genes encoding opioid system from the chicken brain and examined their functionality and tissue expression. As in mammals, 6 opioid peptides encoded by PENK (Met-enkephalin and Leu-enkephalin), POMC (β-endorphin), PDYN (dynorphin-A and dynorphin-B) and PNOC (nociceptin) precursors and four opioid receptors were found to be highly conserved in chickens. Using pGL3-CRE-luciferase and pGL4-SRE-luciferase reporter systems, we demonstrated that chicken opioid receptors (cDOR, cMOR, cKOR and cORL1) expressed in CHO cells, could be differentially activated by chicken opioid peptides, and resulted in the inhibition of cAMP/PKA and activation of MAPK/ERK signaling pathways. cDOR is potently activated by Met-enkephalin and Leu-enkephalin, and cKOR is potently activated by dynorphin-A, dynorphin-B and nociceptin, whereas cORL1 is specifically activated by nociceptin. Unlike cDOR, cKOR and cORL1, cMOR is moderately/weakly activated by enkephalins and other opioid peptides. These findings suggest the ligand-receptor pair in chicken opioid system is similar, but not identical to, that in mammals. Quantitative real-time PCR revealed that the opioid system is mainly expressed in chicken central nervous system including the hypothalamus. Collectively, our data will help to facilitate the better understanding of the conserved roles of opioid system across vertebrates.

Birds Drinking Alcohol: Species and Relationship with People. A Review of Information from Scientific Literature and Social Media

Ethanol is a natural by-product of the fermentation process of fruit sugars and is occasionally consumed by fruit-eating and tree sap drinking birds. Information on this form of alcohol consumption features in the scientific literature. However, as pets or as wild animals living close to humans in urban habitats, birds have increasing possibilities to consume alcohol from beverages, such as beer, wine or spirits. Some observations have been discussed in a light-hearted manner in mass media and social media, but without any generalization of why some bird species drink the beverages intentionally or unintentionally provided by humans. To check which species and in what circumstances birds drink alcohol and how this is evaluated by humans, we reviewed the scientific literature and analysed videos from YouTube. In total we found and analysed 8 scientific papers and 179 YouTube videos, from which we identified at least 55 species (in some cases not all birds were identified to species level), 11 in the scientific literature and 47 in videos. The distribution of these species over the avian phylogenetic tree suggests that the origin of this convergent behaviour is mainly by human influence. The two data sources differed in the species covered. Videos typically presented interactions of birds with human-provided alcoholic beverages, and were dominated by two groups of intelligent birds: parrots and corvids. The popularity of YouTube videos for a particular species was positively correlated with the general popularity of the species as measured by the number of hits (results listed) on Google. Human responses to the videos were generally very positive and we analysed how the responses were influenced by factors derived from viewing the videos. Moreover, YouTube videos also provide information on at least 47 new bird species not previously mentioned as using alcohol, and our results suggest that parrots in particular can be potentially good candidates for future restricted laboratory studies on the effect of ethanol on birds and their relationship with humans.