The melanocortin circuit in obese and lean strains of chicks

Abstract

Agonists of membranal melanocortin 3 and 4 receptors (MC3/4Rs) are known to take part in the complex control mechanism of energy balance. In this study, we compared the physiological response to an exogenous MC3/4R agonist and the hypothalamic expression of proopic melanocortin (POMC) gene, encoding few MC3/4R ligands, between broiler and layer chicken strains. These strains, representing the two most prominent commercial strains of chickens grown for meat (broilers) and egg production (layers), differ in their food intake, fat accumulation, and reproductive performance and, therefore, form a good model of obese and lean phenotypes, respectively. A single i.v. injection of the synthetic peptide melanotan-II (MT-II; 1 mg/kg body weight) into the wing vein of feed-restricted birds led to attenuation of food intake upon exposure to feeding ad libitum in both broiler and layer chickens. A study of the POMC mRNA encoding the two prominent natural MC3/4R agonists, alpha-MSH and ACTH, also revealed a general similarity between the strains. Under feeding conditions ad libitum, POMC mRNA levels were highly similar in chicks of both strains and this level was significantly reduced upon feed restriction. However, POMC mRNA down-regulation upon feed restriction was more pronounced in layers than in broilers. These results suggest: (i) a role for MC3/4R agonists in the control of appetite; (ii) that the physiological differences between broilers and layers are not related to unresponsiveness of broiler chickens to the satiety signal of MC3/4R ligands. Therefore, these findings suggest that artificial activation of this circuit in broiler chicks could help to accommodate with their agricultural shortcomings of overeating, fattening, and impaired reproduction.

Figure 1

Effect of a single administration of MT-II on food in take of broiler and layer chicks. Injection of 1 mg MT-II/kg BW (n=6) or PBS (n= 6) into the wing vein of 3-week-old female broiler and layer chicks and measurement of food intake under feeding ad libitum were performed following 4 days of feed restriction to 70% (A and B). Food intake of (A) broiler and (B) layer chicks. (C and D). Cumulative food intake of (C) broilers and (D) layers. Values are means ± s.e.m. (*P <0·05; **P <0·01; ***P <0·001 Student’s t-test).

Figure 2

Effect of MT-II administration on percent daily food intake. Cumulative food intake (shown in Fig. 1C and D) was normalized by dividing the cumulative values by the mean daily intake, measured before feed restriction. Values are means ± s.e.m.

Figure 3

Effect of MT-II administration on BW of broiler and layer chicks (A and B). Cumulative BW change as measured for the chicks described in Fig. 1. (A) Broilers; (B) layers; (C) normalization of the changes in BW to individual BW before injection. Values are means ± S.E.M. (*P < 0·05; **P < 0·01; ***P < 0·001 Student’s t-test).

Figure 4

Effect of MT-II administration on drinking in (A) broiler and (B) layer chicks, water consumption measured for the chicks described in Fig. 1. Values are means ± s.e.m.

Figure 5

Northern analysis of POMC gene expression in the hypothalamus of broiler and layer chicks. One-month-old female broiler and layer chicks had access to feed ad libitum (FF, n = 4) or were restricted to 50% of their food consumption (FR, n = 4) for 1 week prior to sampling. (A) Ethidium bromide staining of the gel before blotting. (B) Hybridization with ^[32]P-labeled probe and exposure to OMX film (Kodak) for 4 days. (C) Relative expression as calculated by comparing POMC/28S RNA band intensities, shown in B, relative to this value obtained for the fed broilers ad libitum. Vertical lines represent standard error, calculated based on the present Northern analysis and an additional one with broilers and layers under feeding ad libitum, n = 4 for each group, not shown.