D. Michael Denbow

Gastrointestinal Anatomy and Physiology

Abstract The digestive tract is not only important for nutrient digestion and absorption, but it is the largest immunological organ in the body protecting against exogenous pathogens. The digestive system has adaptations designed to facilitate flight. The length of the intestinal tract is shorter in birds relative to mammals. Also, birds lack teeth and heavy jaw muscles, which have been replaced with a lightweight bill or beak. Food particles are swallowed whole and then reduced in size by the ventriculus or gizzard located within the body cavity. This chapter will not attempt to describe the many species variations in detail but will instead describe differences between birds and mammals. The reader is referred to the excellent reviews by McLelland (1975, 1979) for specific information on various species.

Intracerebroventricular injection of ghrelin and growth hormone releasing factor inhibits food intake in neonatal chicks

Growth hormone releasing factor (GRF) is known to stimulate feeding of rats. Ghrelin, a novel growth hormone (GH)-releasing acylated peptide, was recently isolated from rat stomach. It also stimulates the release of GH from the anterior pituitary through the GH secretagogue receptor (GHS-R) and feeding in the rat. We have investigated the effects of ghrelin and GRF on food intake of the neonatal chick. In Experiment 1, 0, 1.25, 2.5 and 5 microg of ghrelin were administered intracerebroventricularly (i.c.v.) to ad libitum fed birds. In Experiment 2, the effect of (i.c.v.) injection of 0, 1.25, 2.5 and 5 microg of GRF was investigated. Both peptides strongly inhibited food intake of the chick during the 2-h post-injection period. In the third experiment, 0, 0.5, 1 and 2 microg of ghrelin was injected i.c.v. in chicks previously deprived of food for 3 h. Food intake was again inhibited by ghrelin in a dose-dependent manner. These results suggest that the mechanisms for feeding of the neonatal chick through GH release are different from mammals.

Chicken ghrelin and growth hormone-releasing peptide-2 inhibit food intake of neonatal chicks.

Ghrelin is an endogenous ligand for the growth hormone secretagogue (GHS) receptor. Ghrelin stimulates feeding in rats, however, intracerebroventricular (i.c.v.) injection of rat ghrelin inhibits feeding of neonatal chicks. In the present study, the effect of i.c.v. injection of different ghrelins including chicken and bullfrog ghrelin, and synthetic GH-releasing peptide (GHRP) on feeding of neonatal chicks was investigated. Chicken ghrelin strongly suppressed feeding. To compare the inhibitory effect, chicken and rat ghrelin were examined. The suppressive effect of feeding by chicken and rat ghrelin was almost identical. Bullfrog ghrelin contains a change in the acylated amino acid from Ser to Thr, strongly suppressed feeding. The i.c.v. injection of GHRP-2 (KP-102), a synthetic GHS, also inhibited feeding. These results indicate that the chicken GHS receptor is affected by several forms of GHS, and that food intake of neonatal chicks is inhibited by GHS receptor agonists.

Inhibition of Food Intake by CRF in Chickens

The effect of intracerebroventricular (i.c.v.) injection of corticotrophin releasing factor (CRF) on food and water intake and on body temperature in chickens was determined. Both broiler and Leghorn type chickens were utilized in this experiment. A stainless steel guide cannula was surgically implanted into the right lateral ventricle of each bird. The i.c.v. injection of CRF significantly decreased food intake in both fed and overnight-fasted broilers and Leghorns. Water intake was decreased by CRF in Leghorns but not broilers. When CRF was injected into Leghorns given access to water, but not food, water intake was not affected. Body temperature was not affected by the i.c.v. injection of CRF. These results suggest that CRF acts within the central nervous system of chickens to decrease food intake while having no affect on water intake or body temperature.

Leptin effects on food and water intake in lines of chickens selected for high or low body weight.

There is an association between autonomic nervous system output and obesity. The sympathetic nervous system stimulates lipid metabolism and regulates food intake and, hence, body weight. Leptin, produced by adipocytes in proportion to their size, has been shown to directly stimulate the satiety center. In the experiment reported here, food and water intake were compared after intracerebroventricular administration of human recombinant leptin to lines of chickens that had undergone divergent selection for over 45 generations from a common White Rock base population for high (HWS) or low (LWS) body weight at 8 weeks-of-age. Leptin caused a linear decrease in food intake in chickens from the LWS line whereas no effect was observed in those from the HWS line. The HWS chickens tended to have reduced water intake post leptin administration. Others reported that leptin decreased food intake in both broiler and Leghorn chickens. Leptin concentration in the central nervous system may not contribute directly to the difference of body weight between HWS and LWS chickens.

Nitric oxide controls feeding behavior in the chicken.

To investigate the effect of nitric oxide (NO) on food intake in the chicken, L-NG-nitro-arginine methyl ester HCl (L-NNA), an inhibitor of NO synthase, was applied. When i.p. administered, L-NNA significantly inhibited the food intake of broiler chickens in a dose response manner. Food ingestion was also depressed by the i.c.v. injection of L-NNA in a dose response fashion. The effect of L-NNA was attenuated by i.c.v. administration of L-arginine. These results suggest that central NO may control feeding behavior in the chicken.

Intracerebroventricular injection of l-serine analogs and derivatives induces sedative and hypnotic effects under an acute stressful condition in neonatal chicks

Four experiments were conducted to clarify the central functions of L-serine and its analogs on an acute stressful condition. Intracerebroventricular (i.c.v.) injection of L-serine (0.21, 0.42 and 0.84 micromol) attenuated stress responses in a dose-dependent fashion, as well as induced sleep, in Experiment 1. The effects of L- and D-serine in Experiment 2, those of L-serine, phosphoserine, acetylserine and L-cysteine in Experiment 3 and those of L-serine, glycine and lysophosphatidylserine in Experiment 4 were compared at an equimolar basis (0.84 micromol). D-Serine, proposed as an endogenous agonist of N-methyl-D-aspatate (NMDA) receptor, did not have sedative and hypnotic effects as observed with L-serine. In contrast, all the analogs and derivatives of L-serine had a sedative effect, although with a different manner in several behavioral markers of stress such as spontaneous activity and distress vocalizations. No significant changes in plasma corticosterone concentration were observed in any experiment. Taken together, the i.c.v. injection of L-serine analogs and its derivatives have sedative and hypnotic effects under an acute stressful condition, which does not involve the hypothalamic-pituitary-adrenal axis. In conclusion, L-serine may be effective in improving anxiety or sleep disorders induced by psychological stressor.

Relationships between feeding and locomotion behaviors after central administration of CRF in chicks

The effect of intracerebroventricular injection of corticotropin-releasing factor (CRF) on various behaviors in chicks was determined at 15-min intervals over a 30-min period. Food intake of chicks was significantly decreased, and pecking rhythm was significantly delayed by CRF during the first 15-min post-injection. The similar tendencies were observed in the second 15-min post-injection, but not significantly different. Stepping, as an indicator of locomotion, was not different at 15-min post-injection, but was increased by CRF, thereafter. These results suggest that CRF acts within the central nervous system to decrease food intake and increase locomotion in the chick.

Effect of central administration of carnosine and its constituents on behaviors in chicks

Even though their contents in the brain are high, the function of brain carnosine and its constituents has not been clarified. Both carnosine and anserine inhibited food intake in a dose dependent fashion when injected intracerebroventricularly. The constituents of carnosine, beta-alanine (beta-Ala) and l-histidine (His), also inhibited food intake, but their effects were weaker than carnosine itself. Co-administration with beta-Ala and His inhibited food intake similar to carnosine, but also altered other behaviors. Injection of carnosine induced hyperactivity and increased plasma corticosterone level, whereas beta-Ala plus His induced hypoactivity manifested as sleep-like behavior. This later effect seemed to be derived from beta-Ala, not His. These results suggest that central carnosine may act in the brain of chicks to regulate brain function and/or behavior in a manner different from its constituents.

Central administration of cocaine- and amphetamine-regulated transcript inhibits food intake in chicks.

The present study was done to clarify whether intracerebroventricular (ICV) injection of cocaine- and amphetamine-regulated transcript (CART) affects feeding in chicks. ICV injection of CART significantly inhibited fasting-induced feeding of broiler chicks. In layer chicks, on the other hand, CART inhibited food intake in birds with ad libitum access to feed but only weakly affected intake of fasted birds. In addition, the ICV injection of CART attenuated neuropeptide Y (NPY)-induced feeding in both broiler and layer chicks. These results indicate that CART is one of the important regulators of feeding in chicks, but the suppressive effect on feeding is somewhat different between strains. Furthermore, the present study also demonstrates that CART interacts with NPY in the central nervous system to regulate feeding in chicks.