Central mechanisms of prolactin-releasing peptides orexigenic effect in chickens

Prolactin-releasing peptide (PrRP) is an endogenous hypothalamic neuropeptide that when exogenously injected increases food intake in chickens, but decreases it in rodents and goldfish. We designed three sets of experiments to elucidate the mechanisms of PrRP's orexigenic effect in chicks. In experiment one, food and water intake were evaluated in chicks after receiving intracerebroventricular (ICV) injection of the vehicle, 0.75, 3, 12, 47 or 188 pmol PrRP. The administration of 12 and 47 pmol PrRP increased food intake for up to 120 min after injection, and 188 pmol increased it for up to 180 min. The lowest effective dose was 3 pmol, which increased food intake for up to 60 min after injection. Water intake was not affected. To investigate the molecular mechanisms, c-Fos immunohistochemistry was performed and mRNA expression of some appetite-associated neurotransmitters was measured in chicks that received either vehicle or 188 pmol of PrRP. The rostral paraventricular nucleus (PVN) was activated which coincided with increased neuropeptide Y (NPY) mRNA expression in the whole hypothalamus. In experiment two, food and water intake were evaluated in chicks fed a high carbohydrate (HC), high fat (HF) or high protein (HP) diet after ICV injection of vehicle, 3 or 188 pmol PrRP. Chicks fed the HP diet increased food intake at a lower dose than chicks fed HF and HP diets after ICV PrRP injection. In addition, ICV injection of vehicle, 3 and 188 pmol PrRP were performed in chicks fed all three diets, and ICV PrRP injection induced preferential intake of the HP diet over HC and HF diets. The expression of some appetite-associated neuropeptides in the hypothalamus was also measured in chicks fed the HC, HF or HP diet after ICV injection of vehicle or 188 pmol PrRP. There was a diet effect on mRNA abundance of all appetite-associated genes measured (P < 0.05), with greater expression in chicks fed the HF or HP than HC diet. While neuropeptide Y (NPY) mRNA abundance was similar between vehicle and PrRP-injected chicks that consumed HP or HF diets, expression was greater (P < 0.05) in PrRP- than vehicle-injected chicks that consumed the HC. In experiment three, the orexigenic effect of PrRP was tested in chicks selected for low (LWS) and high (HWS) body weight after central administration of vehicle, 24, 94 and 375 pmol PrRP. The LWS chicks had a lower threshold and higher magnitude of food intake increase in response to PrRP injection.

Results demonstrate that PrRP is a potent orexigenic factor in chickens and that effects are likely mediated through the hypothalamus. The orexigenic effect of PrRP was influenced by dietary macronutrient composition, and diet in turn influenced the food intake response to PrRP. These results may contribute to a novel understanding of appetite regulation.