Central administration of corticotropin releasing factor in the pig: effects on operant feeding, drinking and plasma cortisol

Four experiments were carried out to investigate the effects of corticotropin releasing factor (CRF) on ingestive behaviour and cortisol secretion in the prepubertal pig. In Experiment 1, 19-hr food-deprived animals were given intracerebroventricular (ICV) injections of 2.5, 10 or 20 micrograms CRF, or saline vehicle, 5 min after the start of a 30-min operant-feeding session. The total number of food reinforcements obtained in the posttreatment period did not change after 2.5 or 10 micrograms CRF, although the latter dose reduced intake in the final 5 min of the test; overall food consumption, however, was reduced after 20 micrograms CRF. In Experiment 2, pigs were treated intravenously with 20 micrograms CRF, or saline, using the same test situation as in Experiment 1; neither treatment significantly affected operant feeding. In Experiment 3, 19-hr water-deprived pigs were given ICV injections of 20 micrograms CRF, or saline, 15 min before the start of a 30-min operant-drinking session. Water intake was increased in the 5-min period directly after CRF injection, but there was no overall reduction in drinking. In Experiment 4, blood samples were taken at 15-min intervals for 30 min before and 90 min after ICV injection of 20 micrograms CRF, or saline, in food and water replete animals. Both treatments appeared to increase plasma cortisol levels, as determined by radioimmunoassay, but the effect was more pronounced after CRF.     

Diurnal variation for inhibition of eating by bombesin in the rat

Sprague-Dawley male albino rats ate sweet milk at the midpoint of the day or night phase of a 12:12 light/dark cycle 1 min after IP 0.9% NaCl or synthetic bombesin (BBS; 2-32 micrograms/kg) following 24-hr food deprivation. Exogenous BBS inhibited food intake in a dose-related manner during the day; a linear regression line accounted for 85% of the total variance for percentage suppression of food intake by BBS in 30 min. In contrast, inhibition of eating by BBS at night was not dose-related; a linear regression line accounted for only 16% of the variance. Rats were tested under identical conditions following 3-hr food deprivation. Exogenous BBS (4-64 micrograms/kg) inhibited food intake in a dose-related manner at night; a linear regression line accounted for 92% of the total variance. In contrast, inhibition of eating by BBS during the day was not as orderly; a linear regression line accounted for only 44% of the variance. The 8 micrograms/kg dose was twice as potent for inhibition of eating of 25% GIBCO 116EC liquid diet than it was for inhibition of eating sweet milk at the midpoint of the day phase. These results describe diurnal variation in potency of exogenous BBS for inhibition of food intake in the rat. Whether BBS is more or less potent at night than during the day depends upon the particular dose, degree of hunger of the rat, and probably the type of food being eaten.     

Effects of chronic intrahypothalamic infusion of insulin on food intake and diurnal meal patterning in the rat

In Experiment 1, rats were chronically infused with insulin (2.7, 27, or 270 ng/hr) or 0.9% saline into the ventromedial (VMH), medial perifornical (PF), or lateral (LH) hypothalamus. VMH infusions of insulin caused a significant, dose-dependent decrease in food intake and body weight; PF infusion of insulin was less effective, but significant; whereas LH infusions of insulin were ineffective. In Experiment 2, rats were chronically infused with insulin (0.54 ng/hr) or 0.9% saline into the VMH, paraventricular (PVN), or posterior (PN) hypothalamic nucleus. Subjects that received VMH or PN infusions of insulin failed to regain weight lost as a result of surgery even 2 weeks after infusion; subjects that received PVN infusions of insulin regained their preoperative weights faster than did controls. All of the groups that received insulin significantly increased their daytime food intake during the infusion period and decreased their night food intake slightly; 24-hr food intake remained unchanged.     

Angiotensin and salt appetite: physiological amounts of angiotensin given peripherally increase salt appetite in the rat

Two experiments were conducted in which adrenalectomized male Sprague-Dawley rats were maintained ad lib on distilled water, 3% saline, and sodium-free food. In Experiment 1, 45 rats were given 100, 200, 400, 800, and 1,000 micrograms/kg/day desoxycorticosterone acetate (DOCA) im for 5 days to determine the dose of DOCA that would produce the lowest voluntary saline intake, and 800 micrograms/kg/day was found to produce the nadir in saline intake. In Experiment 2, 40 rats were placed ad lib on distilled water, saline, and sodium-free food as described above, maintained on 800 micrograms/kg/day DOCA, and infused with 4, 25, 100 micrograms/kg/day angiotensin II (A II) or 0.9% saline. The three A II groups showed significant percentage changes in their saline intake above pre-A II levels; the saline control group showed no change in saline intake from pre-A II level. These results are interpreted to demonstrate the production of salt appetite in rats by peripheral administration of physiological doses of angiotensin II.     

Cholecystokinin octapeptide and lithium produce different effects on feeding and taste aversion learning

The strengths of taste aversion induced by sulphated cholecystokinin 26-33 (CCK-8; 1,2,4 and 8 micrograms/kg IP) and lithium chloride (LiCl; 7.5, 15, 30 and 60 mg/kg IP) were determined in order to assess the relative aversiveness of the two compounds. All doses of LiCl induced strong aversion, but only the highest dose of CCK-8 induced aversion, which was mild. Effects of CCK-8 and LiCl on food intake were then compared in the hour (hr) following 8 hr of food deprivation; rats were on this food deprivation schedule for a relatively long time (78 days) throughout testing. All doses of CCK-8 reduced food intake significantly. Most doses of LiCl either did not affect or significantly increased food intake. Although 60 mg/kg LiCl did not affect food intake when administered 15 or 30 min before food presentation, it significantly increased food intake when administered 1, 2 or 3 hr before food presentation. Overeating of solid food may be an illness-induced behavior. Although a very high dose of LiCl (120 mg/kg) decreased food intake markedly, the rats were obviously distressed, not satiated. Failure of CCK-8 to affect feeding behavior like LiCl is indirect evidence that the reduction of food intake by CCK-8 is not merely the result of aversiveness, but is an extremely potent and specific behavioral effect.     

2-Mercaptoacetate does not stimulate chow intake in periweanling rats

In adult rats, administration of drugs that suppress oxidation of fatty acids, like mercaptoacetate (MA), produces increases in food intake. During development, the consequences of administration of MA are more varied. For example, in very young pups, intake of milk diets is unaffected by MA, while pups aged 12 to 15 days demonstrate increases in intake. However, in 18- and 21-day-old rats, milk intake is suppressed by administration of MA. Typically, the paradigms used to test rat pups differ significantly from those used to assess intake in adult rats. The present experiments were designed to examine whether 18-day-old pups tested with adultlike paradigms showed adultlike responses to administration of MA. In the first experiment, rat pups aged 18 days were injected with 0 or 68.4 mg/kg MA, then given 60-min tests while consuming either milk or chow diets that were novel, or to which they had previously been exposed. The results demonstrated that chow intake was not affected by administration of MA, but milk intake in experienced animals was suppressed by MA. Experiment 2 demonstrated that in contrast to administration of MA, 18-day-old pups deprived of food overnight showed increases in intake of chow and milk diets. In Experiment 3, when the effects of a range of doses of MA (22.8, 45.6, 68.4 and 91.2) on chow intake over a 4-h period were assessed, all doses of MA produced a significant suppression of chow intake in 18-day-old pups. Taken together, the data suggest that alterations in fatty acid oxidation produced by administration of MA do not stimulate chow intake in periweanling pups tested in an adultlike fashion.     

Lesions of the MPO or AV3V: influences on fluid intake

Electrolytic lesions in the MPO of rats had no significant effects on ad lib food and water intake, but impaired the drinking response to 1 M NaCl. Large MPO lesions also produced a persistent increase in plasma osmolality. In Experiment 2, we depleted neurons from the MPO of rats by iontophoretic application of the neurotoxin kainic acid (KA) which destroys nerve cell bodies without damage to fibers of passage. KA-induced neuron depletion in the MPO of rats significantly reduced the drinking response to 1.0 M saline, to 30% PG, and to 30 micrograms/kg isoproterenol. Ad lib water intake and drinking responses to food or water deprivation, to low concentrations (0.5 M) of hypertonic saline, to low concentrations (10% or 20%) of PG, and to systemic administration of 1.5 mg/kg angiotensin II were within the normal range. In Experiment 3, rats with electrolytic lesions that were strictly confined to the tissue immediately surrounding the wall of the anteroventral portion of the third ventricle (AV3V), without invading the MPO displayed normal ad lib food and water intake and plasma osmolality as well as drinking responses to water deprivation, hypertonic saline (0.5 or 1.0 M), angiotensin II (1.5 mg/kg) and isoproterenol (30 micrograms/kg).     

Taste reactivity as a dependent measure of the rapid formation of conditioned taste aversion: a tool for the neural analysis of taste-visceral associations

Several explanations may account for deficits in the ability of animals to form taste aversions following neural manipulations. These encompass impairments in conditioned stimulus (CS) and unconditioned stimulus (US) processing, conditioned response (CR) measurement, and expression, memory, and taste-visceral integration. A behavioral procedure that aids in the distinction between some of these possibilities is presented. In Experiment 1, 10 rats received seven intraoral (IO) infusions of sucrose (30 s, 0.55 ml) spaced every 5 min starting immediately after the injection of 3.0 mEq/kg of lithium chloride (LiCl). Control rats (n = 12) were treated identically except that they were injected with sodium chloride (NaCl). Oromotor and somatic taste reactivity behaviors were videotaped and analyzed. Lithium-injected rats systematically decreased their ingestive taste reactivity behavior over time, whereas aversive behavior increased. Control rats maintained high and stable levels of ingestive responding and demonstrated virtually no aversive behavior over the 30-min period following sodium injection. Rats were tested several days later for the presence of a conditioned taste aversion (CTA). Rats previously injected with lithium during sucrose infusions demonstrated significantly more aversive behavior than the control group, which demonstrated none. There were no differences in the level of ingestive behavior displayed by the two groups on the CTA test. Experiment 3 revealed that when similarly treated rats were tested for a CTA while in a lithium-induced state, a difference in the ingestive behavior between the two groups was observed. In Experiment 2, naive rats were injected with either NaCl or LiCl but did not receive their first sucrose infusion until 20 min later. These rats also received sucrose infusions at 25 and 30 min postinjection. There were no differences in the taste reactivity behavior displayed by lithium- or sodium-injected rats during any of the sucrose infusions. Collectively, these findings indicate that rats dramatically change their oromotor responses to sucrose during the period following LiCl administration, provided that the infusions start immediately after injection. Furthermore, this time-related behavioral change is predominantly attributable to associative processes. This paradigm can be useful in distinguishing between neural manipulations that affect the establishment of taste-visceral associations from others that affect the animal's ability to retain such associations over the commonly employed 24-hr conditioning-test interval.     

The effect of cholecystokinin on food intake in gonadectomized and intact rats: the influence of sex hormones

Cholecystokinin (CCK) suppresses food intake in a number of animal models, but appears to be less effective in females [5,23]. We studied the effect of CCK on food intake in female rats on each day of the estrous cycle. In addition, we evaluated the effect of sex hormones on food intake in intact and castrate male rats which had been injected daily with oil or testosterone propionate + oil and ovariectomized female rats injected daily with oil, estradiol, progesterone or estradiol + progesterone. Food intake in intact, castrate and castrate + testosterone replaced male rats was decreased by CCK (5, 10 and 20 micrograms/kg) IP (p less than 0.05). Food intake was decreased by CCK (20 micrograms/kg) only during diestrous and metestrus in cycling female rats. During metestrus, a period of low estradiol in the presence of progesterone, food intake was also suppressed by CCK (5 and 10 micrograms/kg). CCK failed to decrease food intake in ovariectomized females receiving oil, estradiol and estradiol + progesterone. However, animals receiving progesterone alone responded to the high dose of CCK (20 micrograms/kg). Our data suggest that the effect of CCK on food intake in female rats may be dependent on the presence of progesterone. The lack of sensitivity to CCK during proestrus and estrus suggests that estradiol may be modulating the "permissive" action of progesterone on CCK's satiety effect.     

Feeding behavior in growing rats fed diets containing sorbose

The effect of dietary sorbose on food and water consumption was investigated in growing rats. Rats (26-day old) were fed diets containing 0, 100, 200 or 300 g sorbose/kg diet for 5 weeks in Experiment 1. Daily food and water intakes were measured at day 0, 7, 14, 21, 28 and 35. Absolute food intake (g) until day 21 decreased linearly with increasing sorbose levels and so did relative food intake (g/100 g b.wt.) until day 7. In contrast, relative water intake (ml/100 g b.wt.) and water:food intake ratio (ml/g) remained high by day 7 with increasing sorbose levels. In Experiment 2, the effect of sorbose on the short-term food intake was compared with those of glucose, sucrose and maltitol in growing rats (25-day old) at a level of 100 g/kg diet in order to investigate how quickly reduced food intake would be induced by sorbose consumption. Cumulative food intake was determined every hour for the first ten hours, then at two-hour intervals thereafter during the 24-hour period that followed feeding. As rapid as 6 hours after feeding, cumulative food intake significantly decreased in sorbose-fed animals compared with other dietary groups. It was concluded that sorbose consumption decreased the food intake of growing rats from 6 hours to a few weeks after feeding, but this inhibitory effect disappeared afterwards.     

Differential effects of the selective orexin-1 receptor antagonist SB-334867 and lithium chloride on the behavioural satiety sequence in rats

Recent studies have shown that acute systemic administration of the selective orexin-1 receptor antagonist SB-334867 significantly reduces food intake in rats. Although this anorectic action of orexin-1 receptor blockade is associated with an acceleration in the transition from eating to resting, it is widely recognised that the behavioural indices of satiety are not dissimilar to those of illness. In this context, Experiment 1 confirmed a significant anorectic effect of 90 (but not 60) mg/kg lithium chloride (LiCl) in male rats presented with palatable mash in the home-cage environment. Experiment 2 employed a continuous monitoring technique to contrast the effects of LiCl (90 mg/kg) and SB-334867 (10 and 30 mg/kg) on food intake and behaviour during a 1-h test with palatable mash. SB-334867 dose-dependently inhibited food intake, with the higher dose producing a comparable degree of appetite suppression (approximately 40%) to that seen with LiCl. Despite equivalent anorectic action, the two compounds produced very different effects on behaviour. LiCl reduced active behaviours (locomotion, rearing, grooming and sniffing), slowed the rate of eating and disrupted the behavioural satiety sequence (BSS). In contrast, SB-334867 (30 mg/kg) decreased the duration of feeding and grooming, and modestly accelerated the transition between eating and resting. Furthermore, whereas LiCl failed to alter posttreatment bodyweight gain, SB-334867 (30 mg/kg) produced a significant weight loss in the 24-h period immediately following injection. Overall, the divergent profiles obtained with equianorectic doses of LiCl and SB-334867 provide convincing evidence for the behavioural selectivity of SB-334867-induced anorexia.     

The effect of portal and jugular infused glucose, mannitol and saline on food intake in dogs

Hepatic glucoreceptors have been hypothesized to have an important role in determining normal hunger and satiety. In the present study 23 dogs were fitted with chronic hepatic portal and jugular vein cannulas. The dogs were fed for 1 hr/day. On infusion days (total of 318 infusions) the animals were infused into the portal or jugular veins with a 30% glucose solution (2.4 or 3.6 g/kg, b.wt.), 0.9% NaCl as a volume control or 30% mannitol as an osmotic control and then fed 10 minutes later. The data showed that the dog's food consumption was similar after they received glucose or the appropriate control infusion regardless of the infusion site. Some dogs had blood samples taken for glucose and insulin determinations prior to infusion, at the middle and end of infusion, just prior to food presentation and at the end of the feeding period. Saline and mannitol infusions did not alter plasma glucose or insulin concentrations; whereas there were marked increases in plasma glucose (6-8 x) and insulin (18-19 x) following glucose infusions. Postinfusion glucose values indicated approximately 72% of the infused dose glucose (approximately 43 g) had left the plasma prior to food presentation. Despite the large increases in plasma glucose and insulin, as well as glucose storage and/or oxidation, the dogs consumed amounts of food similar to that eaten after control infusions. Similarly, prefeeding the dogs 20% of their average daily intake prior to infusion did not alter the animals subsequent intake. These data are in agreement with earlier work from our laboratory and question the role of the hypothesized hepatic glucose satiety receptors.     

The effect of cholecystokinin octapeptide and glucagon (1-29) on food intake of weanling rats

Cholecystokinin octapeptide (CCK-8) and glucagon (GLG) decrease food intake of a number of species. However, the responsiveness of rats to the food intake effects of these peptides may develop differentially due to sex, age and/or developmental state. Male and female weanling rats decreased early dark cycle food intake following the administration of 5 and 10 micrograms/kg CCK-8 and male rats were more responsive than female rats, p less than 0.05. GLG did not decrease early dark cycle food intake of either male or female weanling rats. Weanling male rats increased plasma glucose and insulin levels in response to GLG administration, p less than 0.05. Male rats were retested with GLG (250 and 500 micrograms/kg) at 6, 9 and 22 weeks of age. GLG did not decrease food intake of these rats until they reached 9 weeks of age and they were still responsive at 22 weeks of age, p less than 0.05.     

Defensive burying following injections of cholecystokinin, bombesin, and LiCl in rats.

Past research indicates that feeding is reduced for animals injected with cholecystokinin and bombesin. One explanation for this effect suggests that these peptides act as natural satiety signals; an opposing view asserts that bombesin and cholecystokinin reduce feeding through malaise. The present experiment tested the basic assumptions associated with these positions using the defensive burying procedure. Groups of rats were given sweetened condensed milk followed by IP injections of bombesin (6, 16, and 32 micrograms/kg), cholecystokinin (0.7, 1.4, and 2.9 micrograms/kg), LiCl (6.4 mg/ml), or saline. The results showed that animals injected with cholecystokinin, bombesin, and LiCl developed learned aversions to the milk and actively buried the milk spout with their bedding. The findings provide further support for the view that bombesin and cholecystokinin induce malaise rather than satiety.     

Patterns of increased and decreased ingestive behavior after injections of lithium chloride and 2-deoxy-D-glucose

In previous research, 750 mg/kg 2DG yielded a number of effects suggesting a postdrug nausea; and the present experiments revealed that the illness-inducing agent LiCl similarly produced taste aversion learning at 7.5 mg/kg, inhibited drinking in water-deprived animals at 30 mg/kg, depressed feeding in hungry rats at 60 mg/kg, and evoked food intake and pica at 120 mg/kg. The appearance of eating and pica at the same dosage suggested that rats may eat food as well as a nonnutritive substance as a species-specific reaction to illness and that postdrug feeding, including that observed after 2DG, is an insufficient condition for concluding that a treatment produces no internal distress. A liquid diet that reportedly ameliorates the glucoprivic feeding deficits produced by lateral hypothalamic and zona incerta lesions theoretically could produce its effects if lesions made rats more reactive to 2DG-induced malaise and if this diet were more palatable to animals experiencing internal distress. However, this liquid diet failed to facilitate food intake after LiCl, nor did it reduce the inhibited eating produced by LiCl in food-deprived subjects. Liquid diet effects in lesioned animals, therefore, may not be explained by factors related to a 2DG-induced malaise.     

Tolbutamide affects food ingestion in a manner consistent with its glycemic effects in the rat

The sulphonylurea tolbutamide possesses the ability to stimulate insulin release, produce hypoglycemia and increase food intake; however, no study has investigated the effects of moderate doses which do not produce frank hypoglycemia. Forty male rats received injections of tolbutamide at 0, 5, 15, 25 or 50 mg/kg body weight. The injections terminated a 2-hr fast and occurred at light offset, insuring a meal. Food intakes were then recorded for two hr following injection. Tolbutamide at 5 and 15 mg doses decreased food intake during the first half-hour or hour, respectively. In parallel experiments, 10 rats were sampled for blood prior to injection of tolbutamide or saline at doses cited above, and again at 10 and 40 min following injection in the absence of food. Plasma was then analyzed for insulin and glucose. Both 5 and 15 mg tolbutamide produced a mild, reliable increase in insulin accompanied by a decrease of 5 to 15 mg/dl in plasma glucose. On the other hand, the 50 mg dose produced a marked increase in insulin and a decrease of approximately 25% in plasma glucose. Thus, the present studies suggest that when endogenous insulin levels are modestly raised by tolbutamide, such that only moderate reductions of circulating glucose were observed, decreases in food intake occur.     

Predigestion and suppression of food intake by gastric loads

The effects of gastric loads of 0.9% (W/V) NaCl, 5.25% glucose, 5% casein hydrolysate and Complan (a nutritionally complete diet) on food intake of rats, as measured by an operant response for food, were compared with the effects of gastric loads of the same materials after they had been predigested in donor animals. Predigestion was achieved by perfusion of the load materials through the stomach or duodenum of anesthetized donor rats. Gastric predigestion significantly reduced the inhibitory effect of the Complan load on food intake, and duodenal predigestion significantly increased the inhibitory effect of the saline load on food intake. Smaller nonsignificant changes in the effects of the other loads on food intake were also observed. The results are consistent with the hypothesis that gastric loads depress subsequent food intake at least partly because of the consequences of placing nonphysiological, i.e., undigested material in the stomach or duodenum of experimental animals.     

Central gustatory lesions: II. Effects on sodium appetite, taste aversion learning, and feeding behaviors.

Intake and taste reactivity tests were used to determine the effects of bilateral lesions of the gustatory portions of the nucleus of the solitary tract (NST), the parabrachial nucleus (PBN), and the ventral posteromedial nucleus of the thalamus (VPMpc) on several complex ingestive behaviors. In the 1st experiment, lesions of the PBN and the NST blocked, and VPMpc lesions impaired, the behavioral expression of salt appetite. In the 2nd experiment, alanine was paired with injections of LiCl. Control rats as well as rats with NST and VPMpc lesions acquired the taste aversion, but rats with PBN lesions did not. In the 3rd experiment, all animals increased their food intake after injections of 2 U/kg insulin and 250 mg/kg 2-deoxy-D-glucose, and their food intake was suppressed after nutritive stomach loads.     

Regulation of antral gastrin content

Five groups of rats were fasted for 3 days and injected with either NaCl or 5, 10, 20, or 40 micrograms/kg bombesin every 8 h. The animals were killed, and their serum and antral gastrin levels were compared with those of normally fed rats. Fasting reduced serum gastrin to 14% of control; antral gastrin was reduced to 21% of control. All doses of bombesin significantly increased serum gastrin in fasted rats, and 20 and 40 micrograms/kg significantly increased antral gastrin. A group of normally fed rats was also compared with one fed a liquid diet for 7 days. Half of each of these was injected with 20 micrograms/kg bombesin (3 times/day) and the other half with NaCl. Bombesin significantly increased serum and antral gastrin in the rats fed solid food. The liquid diet lowered serum and antral gastrin to 17 and 59% of control values, respectively. Bombesin injection totally prevented these decreases. These data indicate that food in the gastrointestinal tract is not required for either gastrin release or synthesis. Furthermore, the data suggest that gastrin synthesis is regulated primarily by gastrin release or by direct stimulation by bombesin rather than by specific food products.     

The orexigenic effect of peripheral ghrelin differs between rats of different age and with different baseline food intake, and it may in part be mediated by the area postrema

Ghrelin is mainly secreted during fasting. While an orexigenic effect of peripherally injected ghrelin has been reported, reproducing this effect has often proven difficult. Here, we hypothesized that ghrelin's effect to increase food intake may depend on the experimental conditions (e.g., age of animals). We therefore investigated the effect of an IP ghrelin injection (100 microg/kg) on food intake in rats of different age and at different times during the light-dark cycle, i.e. with different levels of baseline food intake. Ghrelin injected at dark onset in ad libitum fed young rats (body weight [BW] 92 g) slightly increased feeding while no such effect was observed in 12 h food deprived rats (BW 150 g). In the middle of the light phase, ghrelin significantly increased feeding up to 2 h after injection in ad libitum fed rats (BW 130 g; food intake 1 h after injection: NaCl 0.4 +/- 0.2 g versus ghrelin 1.2 +/- 0.3 g [p < 0.05]). In various subsequent experiments, older rats (BW 300-490 g) tested under the same conditions did not respond to a single ghrelin injection. However repeated ghrelin injection (15 microg/kg/day once daily at light onset) over 10 days significantly increased food intake in rats (BW 400-460 g) starting from day 4 of the experiment (24 h food intake: NaCl approx. 19.5 g, ghrelin 22.5 g). Interestingly, the latter effect was completely abolished in rats lesioned in the area postrema (AP). Cumulative food intake was also increased in SHAM but not in AP-X animals (e.g., after 7 days: SHAM/NaCl 135.1 +/- 5.3 g versus SHAM/ghrelin 149.7 +/- 3.5 g [p < 0.05], AP-X/NaCl 127.2 +/- 16.4 versus AP-X/ghrelin 127.9 +/- 5.3). We conclude that ghrelin's effect to increase food intake can best be demonstrated when basal food intake is low. Ghrelin increases feeding mainly in young, fast growing animals. Ghrelin may therefore link the high energy needs to body growth in young individuals. In older animals, peripheral ghrelin increased feeding when injected repeatedly over several days. At least under these conditions, ghrelin's effect was mediated by the AP/NTS region. Using repeated administration, ghrelin might be an interesting tool to increase feeding in patients suffering from wasting diseases such as cancer anorexia.