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Growth hormone secretagogues (GHS), including ghrelin, are known to stimulate feeding behavior in mammals. In contrast, intracerebroventricular (ICV) injection of rat or chicken ghrelin, strongly inhibits food intake in chicks. The difference prompted us to compare the effect of growth hormone releasing peptide-6 (GHRP-6), one of synthetic GHSs on feeding behavior of chicks with that of ghrelin. Gastric motility stimulating action of GHRP-6 was also examined in vivo measuring retention of food in digestive tracts. ICV injection of GHRP-6 (0.38nmol-1.5nmol) dose-dependently inhibited food intake of neonatal chicks under both ad libitum and fasting conditions. The anorexic effect of GHRP-6 was attenuated by D-Lys3-GHRP-6 (GHS receptor antagonist) in a dose dependent manner. Central injection of GHS agonist and/or antagonist did not affect food retention in crop, proventriculus and gizzard. These results suggest that central injection of synthetic GHS, GHRP-6 inhibits feeding behavior as dose ghrelin without affecting gastrointestinal motility in layer chicks.

Chicks were offered a choice between diets added with or without quinine hydro-chloride (HCl) to elucidate the role and efficacy of feeder color (gray or orange) or feeder position (fix or change) in the two-choice preference test. In the first experiment, chicks were given a choice between a basal diet and a diet containing 0.4% quinine HCl by two feeders with different colors for 24h. They had a strong aversion to the quinine- added diet, regardless of fix or change of feeder positions. After replacing the quinine-added diet by the basal diet next morning, chicks initially avoided to eat the diet from feeders previously delivering the quinine-added diet. However, they consumed the diet from both feeders with time. From these results, it was shown that chicks were able to differentiate between the gray and orange feeders, and associated these colors with the diet contained in each feeder. At the same time, chicks occasionally sampled the diet in the other feeders. Feeder color itself had little effect on feed intake when the basal diet was given by two feeders. In the next experiment, supplemental level of quinine HCl was reduced to 0.2%. As a result, the individual differences in sensitivity to quinine became apparent. Some chicks had an aversion to the quinine-added diet from the first 2h, but others not. The aversion to quinine became evident, especially in the latter chicks when feeders with different colors were used or feeder positions were fixed. Therefore, feeder color or feeder position acts as a cue in the choice feeding and it is effective in minimizing the individual differences among choice-fed chicks.

Quinine is known as a bitter compound for humans and also used in animal experiment. However, there is no evidence that animals share the same taste sensation of quinine with humans. The purpose of this experiment was to elucidate how chicks perceive quinine. First, 14-d-old chicks were freely fed diets varying in quinine HCl (0, 0.2, 0.4 and 0.8%) for 3 or 4d as a single diet, or given a choice between the quinine-free basal diet and a diet containing 0.2% quinine HCl for 4d. Feed intake of quinine added diets depressed within the first 4h after the start of the experiment, after which it increased with time. Especially, chicks became accustomed to 0.2% quinine HCl from the second d of the experiment. As contrasted with this result, choice-fed chicks did not differentiate between the basal diet and the diet containing 0.2% quinine HCl during the first 4h, after which they had the aversion to the quinine-added diet over 4d. Chicks fed 0.4% quinine HCl increased water intake as compared with those fed the basal diet, irrespective of the lower body weight and feed intake. Next, chicks previously fed diets containing 0, 0.2 or 0.8% quinine HCl were given a choice between the basal diet and the diet containing 0.2% quinine HCl for 24h. Chicks previously fed 0.2% quinine HCl was more sensitive to the dietary quinine than those fed the basal diet previously, but chicks fed 0.8% quinine HCl for 4d did not discriminate between the two diets. The adverse effect of quinine was not carried over, because chicks previously given the quinine-added diets ate more diet than those previously fed the basal diet immediately after the start of the experiment when they were offered a chance to eat the basal diet in the choice feeding. These results indicated that the aversive effect of quinine on chicks was associated with the uncomfortable taste sensation as humans perceived.

Choice feeding has been used to compare likes or dislikes of feedstuff for animals. In the two-choice preference test, it is expected that chicks eat their diets equally from two feeders when they were given the diets with the same palatability. The present experiment was conducted to make sure whether this basic principle is true or not in the short-term choice feeding. When chicks were given the same diet by two feeders, they ate the diet predominantly from one or the other of two feeders for a certain time. Half of chicks ate the diet from right feeders (right-handed chicks) and the other from left feeders (left-handed chicks) so that feed intake from right and left feeders was the same. These results indicated that the favorite feeder position was not confined to one side. Chicks given the choice between diets with the same nutritive value (added with or without 1% cholesterol) did not show the special preference or aversion to cholesterol, and they initially ate the diet from particular feeders, regardless of the presence or absence of cholesterol. Such undesirable feeding behavior in the choice feeding disappeared as the experiment progressed, or by using older chicks or the acclimation to the choice feeding prior to the experiment.

Some kinds of saponins are considered to depress body weight gain in chicks by reducing feed intake. Although the mechanism by which saponins exert such effects is unclear, tea saponin (TS) is known to delay the crop emptying. The purpose of the present experiment was to compare effects of TS, Quillaja saponin (QS) and Yucca saponin (YS) on growth rate and crop emptying using force-fed chicks. Chicks fed ad libitum a diet containing 0.5% TS, 0.5% QS or 2% YS depressed body weight gain and feed intake in a 7-d experiment. These adverse effects were alleviated by the concomitant addition of cholesterol. The growth retardation did not occur when feed intake of the diet containing 0.5% QS or 2% YS for 10d was equalized to that of the basal diet by force-feeding. On the other hand, it was difficult to continue the force-feeding of the diet containing 1% TS more than 3d because of the severe crop distention due to the ingesta. YS at the 2% in the diet also delayed the crop emptying, extent of which was milder than 1% TS. The concomitant addition of cholesterol alleviated the delayed crop emptying due to TS or YS. Force-feeding the diet containing 1% QS had little effect on the crop emptying, although the adverse effect of QS on feed intake was similar to that of TS or greater than that of YS. On the other hand, half of chicks force-fed the diet containing 1% QS died within 24-36h after feeding. These results indicate that the decreased feed intake plays an important role in the growth retardation due to saponins, and they also suggest that the delayed crop emptying is associated with the decrease in feed intake due to some saponins but not to all saponins.

Octopamine, an important neurotransmitter in invertebrates, has noted several similarities with noradrenaline (NA) in mammals. To compare the effect of octopamine and NA in the central nervous system, food intake and categorized postures of fasted chicks were investigated after intracerebroventricular (ICV) injection. We found that ICV injection of NA significantly inhibited food intake of fasted chicks at 30 min postinjection, but not octopamine. Although chicks treated NA showed a tendency to be sleep-like behavior, octopamine significantly induced hyperactivity when compared with NA. The results presented here suggest that octopamine may also act on nervous system(s) without noradrenergic system in the brain of chicks.

Tea saponin is known to depress growth rates of chicks by decreasing feed intake. Since tea saponin has bitter taste, gustatory sensation was proposed to be responsible for the decreased feed intake. However, inconsistent observations are also reported. The present experiment was conducted with chicks to evaluate the involvement of factors other than taste in the decreased feed intake due to saponin. Chicks freely fed diets containing tea saponin or quinine sulfate at 0.5 and 1% levels for 12 h decreased feed intake within 4 h. Chicks given a choice between the basal diet and the diet containing 0.5 or 1% quinine sulfate for 12 h showed strong aversion to quinine-added diets within 4 h. Chicks preferred the basal diet to the diet containing 0.5 or 1% saponin in a similar two-choice preference test, but there were no statistical differences in feed intake of the first 4 h between the basal diet and the diet containing 0.5% saponin. When chicks were given a free access to the basal diet for 6 h a day in a 12-d feeding experiment, daily feed intake during 0-2 h was kept constant, whereas feed intake during 2-6 h gradually increased as the experiment progressed. On the other hand, feed intake during 0-2 and 2-6 h in chicks fed the diet containing 0.5% saponin was always suppressed to the same extent throughout the experiment. Force-feeding this saponin-added diet delayed the crop emptying, which was reversed by the concomitant addition of cholesterol. These results suggest that the decreased feed intake due to tea saponin is related to the crop distention by the ingesta remained in this organ rather than taste.

In order to evaluate the involvement of taste in the decreased feed intake due to tea saponin, the preference for a diet containing saponin or cholesterol was studied in chicks given diet selection. When chicks were fed the basal diet added with 1% saponin ad libitum for 4-6h, feed intake decreased within 2h and the decrease in feed intake was became severer with time. Chicks given the choice between the basal and saponin-added diets for 12h selected the basal diet exclusively during 4-12h, but there were no significant differences in feed intake between the two diets during the first 4h. Chicks preferred the basal diet to the saponin-added diet when the two-choice preference test was prolonged to 8d. However, there were large individual differences in the feeding response among chicks; some chicks selected the basal diet from the start of the test, but others needed more time to select the basal diet. These individual differences were disappeared with fixing the feeder positions rather than changing the feeder positions every day. The deleterious effect due to tea saponin did not occur when equal amounts of cholesterol were supplemented to the saponin-added diet. However, chicks did not show the special preference for cholesterol, when they were given the choice between the basal diet and a diet containing 1% cholesterol for 12h or between the basal diet and a diet containing saponin plus cholesterol at 1% levels for 8d. These results suggest that chicks can recognize the presence of saponin, but taste is not a major determinant responsible for the depression in feed intake due to saponin.

Tea saponin is known to suppress chick growth rate by decreasing feed intake, although the mechanism by which tea saponin decreases feed intake remains obscure. Since tea saponin has bitter taste, palatability was suggested to be a major factor affecting feed intake. However, there are some experimental results showing another possibility. The present experiment was conducted with chicks to evaluate the involvement of taste in the adverse effect of tea saponin on feed intake. When Single Comb White Leghorn male chicks were given a casein diet for at least 4h in experiments 1 and 2, 1% tea saponin depressed feed intake. The decrease in feed intake immediately after feeding (0-2h) was rather mild and became severer with time (2-4h). Removal of saponin from the diet increased feed intake, and this beneficial effect was manifested for the 2-4h of the feeding period as compared with the first 2h. In experiment 3, 40mg of tea saponin was administered directly to the proventriculus by capsules to avoid the effect of taste or orally as a 1% solution. Despite the fact that chicks were given free access to the saponin-free diet after the administration, the depression in feed intake due to saponin was lasted until 8h had passed, irrespective of different dosing methods. These results show that the aversion to the saponin-added diet does not occur immediately after the ingestion of tea saponin and that the adverse effect of tea saponin on feed intake is carried over. Thus, it seems unlikely that taste is a primary factor decreasing feed intake in chicks given tea saponin.

Effects of casein (CAS) and casein hydrolysate (CHY) as the dietary protein sources on serum and liver cholesterol concentrations in chicks were compared in the first experiment. Next, effects of CAS, CHY and soybean protein isolate (SPI) on the cholesterol-lowering action of Quillaja or tea saponin were investigated. Seven-day-old male chicks were fed experimental diets added with cholesterol for 10 days. In experiment 1, chicks were given diets containing 20 and 30% crude protein. Increasing protein content in CAS and CHY diets suppressed the elevation of serum total cholesterol and liver cholesterol, but CHY was less effective than CAS in lowering LDL-cholesterol concentrations. When beef tallow was added to diets containing 20% crude protein in experiment 2, chicks fed CAS diet had lower serum total cholesterol concentrations than those fed CHY diet. CHY was water-soluble, and approximately half of total nitrogen was amino-acid nitrogen. These results indicate that the hypocholesterolemic effect of dietary CAS relative to CHY is related to the structure of protein. Serum total cholesterol and liver cholesterol concentrations were much higher in chicks fed CHY diet than those fed SPI diet. However, by the cholesterol-lowering action of saponins, there were no differences in these concentrations between both groups. The two saponins also lowered serum total cholesterol and liver cholesterol concentrations in chicks fed CAS diet, but their concentrations were still higher than those of chicks given another protein diets. Thus, in addition to the interference between saponins and steroids in the intestine, it seems likely that dietary proteins affect the cholesterol-lowering action of saponins.