In baby chickens, injecting the peptide kisspeptin-10 (called metastin) straight into the brain makes them eat more. This appetite‑boosting effect disappears when a drug that blocks mu‑opioid receptors is given, but not when drugs that block other opioid receptors are used. The result suggests kisspein‑10 works through mu‑opioid pathways in birds.

Metastin, an RFamide peptide, has been isolated from human placenta and possesses several physiological actions in mammals. However, little is known about this bioactive peptide in avian species. This study was conducted to assess the effect of metastin on feeding behavior of chicks (Gallus gallus). The food intake of chicks is significantly increased by the intracerebroventricular injection of metastin. Beta-funaltrexamine, a mu-opioid receptor antagonist, significantly attenuates metastin-induced food intake in chicks. In contrast, delta- and kappa-opioid receptor antagonists did not show any influence on metastin-induced food intake in chicks. In addition, administration of N(G)-nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor, did not influence metastin-induced food intake. Taken together, this study shows the orexigenic effect of metastin in chicks and suggests that this effect is mediated by mu-opioid receptor.