新生ニワトリヒナにおける摂食量の中枢制御に関する研究 : 特にL-アルギニン代謝産物とニューロペプチドYの役割について

概要

The control of food intake is a crucial mean for the animals to adjust nutrition availability for the maintenance of energy homeostasis. Domestic chickens demonstrate well-developed feeding behavior from the neonatal stage that is assumed to be similar with mammals in term of the control mechanism. Central functions of amino acids and their metabolites in sedative and hypnotic effects as well as excitatory effects are widely investigated in neonatal chicks. In this study, an investigation of the precise feeding behavior in neonatal chicks and the functional relationship between feeding behavior and brain free amino acid metabolism has been demonstrated. To deeply extend the understanding about the mechanism underneath, the interactions between appetite-related neuropeptide and amino acids and their metabolites were also elucidated.

Findings of the present study confirmed that feeding behaviors of neonatal chicks were characterized by short resting periods between very brief time spent for food intake and initially showed some quick metabolic changes in amino acid and monoamine concentrations in the brain associated with their frequent food intake. Accordingly, the relationships between regulated appetite induced by fasting and subsequent short-term re-feeding and central amino acid metabolism were intensively investigated. Re-feeding mainly induced a quick increment in concentrations of several amino acids in the brain of chicks that may contribute as satiety signals. L-Arginine (L-Arg) and L-ornithine (L-Orn) in all brain parts, except mesencephalic L-Arg and cerebellar L-Orn, as well as amino adipic acid and L-proline (L-Pro) were increased in a time-dependent manner following re-feeding. The results reported the inhibition of food intake by exogenous central administration of L-Orn in a dose-dependent manner. It is implied that endogenous L-Orn may be a physiological satiety signal in neonatal chicks. In addition, the results showed differential central effects of L-Arg and its enantiomer, D-Arg, on neonatal chicks during isolation stress implying that the central effects of L-Arg may not associated with physiological feeding behavior, but with stress response and its enantiomer, D-Arg, may not contribute to the central pathway of L-Arg.

On the other hand, the hypothesized collaboration between brain nutrient signals like amino acids and appetite-related neuropeptides in neonatal chicks was investigated. The obtained result showed that L-Orn produced a significant dose-dependent suppression of neuropeptide Y (NPY)-elicited eating in accordance with the dose proportional elevation of the co-injected L-Orn detected in the brain. Brain L-Orn is shown to interact with NPY in the regulation of feeding behavior of neonatal chicks through several inhibitory signaling pathways generated from brain amino acid metabolism including the elevation of L-Pro as L-Orn metabolite, the reduction of glycine and L-methionine concentrations regarding the generation of glutathione, and the catabolism of L-tryptophan. The results also suggested that NPY stimulates food intake through the inhibition of several inhibitory signals in feeding of chicks such as the pipecolic acid pathway and L-Orn. It is demonstrated in the present study that N-methyl-D-aspartate receptor is a novel inhibitory factor in feeding of chicks; however, its regulating in feeding behavior may be independent with both NPY and L-Orn. In the present study, the observed reduction in the concentrations of amino acid in plasma after short period of feeding time following central injection of NPY and L-Orn in neonatal chicks supports for a putative role of NPY in the stimulation of protein synthesis in chicks. The present study further found that NPY also interact with brain monoamine system, in particular, the modulation of NPY in the catecholaminergic system may be mediated by nutrients through feeding.

In conclusion, findings of the present study demonstrated that endogenous neuropeptides and endogenous and/or exogenous nutrients like amino acids simultaneously collaborate to regulate feeding behavior of neonatal chicks. The important role of amino acids in overall network of the central nervous system to regulate feeding behavior in neonatal chicks confirmed here benefits to better adjust dietary amino acids for ideal performance of poultry production and provides important keys for study in human appetite biology.