Hypothalamic and hindbrain NPY, AGRP and NE increase consummatory feeding responses

Feeding behavior is comprised of both appetitive and consummatory responses to food. Appetitive responses include the motivated acquisition of food. Consummatory responses, including swallowing, are those that move the food from the mouth to the stomach. Intraoral delivery of liquid food bypasses the requirement for appetitive responses and has been used to examine consummatory responses directly in intact rats. In the present study, we administered neuropeptide Y (NPY), agouti-related protein (AGRP) and norepinephrine (NE), into the paraventricular nucleus of the hypothalamus (PVN) or into the fourth cerebral ventricle to examine their effects on the consummatory component of feeding behavior in the rat. To measure consummatory responses, milk (40% lactose free cow's milk diluted with water) was infused intraorally through a chronic cheek fistula (1 ml/min), using an alternating 5 min on − 1 min off schedule, until rejection occurred. We found that both hypothalamic and fourth ventricle injections of NPY, AGRP and NE significantly increased consumption of the intraorally-delivered milk. Our results indicate that the circuitry for modulation of consummatory ingestive responses includes NE, NPY and AGRP receptors operating in both hypothalamic and hindbrain sites.     

Insulin suppresses intake without inducing illness in sham feeding rats

Insulin's effects on consumption were investigated in rats feeding with open gastric cannulas. Insulin produced small but statistically reliable decreases in sham intake. Intake was reduced during the one-hour sham feeding interval by 11 to 18%, however, the second, third and fourth 15-min intervals were reduced by 20 to 33%. An initial increase (first 15 min of sham feeding) following insulin injection offset somewhat insulin's suppressive effects on sham intake. All reductions were obtained by comparing sham intake following 0.1 to 0.75 U insulin per rat to intake following saline. When insulin or saline injections were paired with alternatively flavored sweetened condensed milk during sham feeding, subjects preferred the insulin-paired flavor in a subsequent two-bottle test. After just 6 complete pairings, insulin-paired, flavored milk was preferred at a ratio of almost six to one. The results indicate the effectiveness of insulin in inhibiting intake in a situation in which the actions of other potential satiety mechanisms are minimized. Additionally, insulin accomplishes this without inducing a flavor aversion for the milk sham consumed.     

Involvement of brain ketone bodies and the noradrenergic pathway in diabetic hyperphagia in rats

Uncontrolled type 1 diabetes leads to hyperphagia and severe ketosis. This study was conducted to test the hypothesis that ketone bodies act on the hindbrain as a starvation signal to induce diabetic hyperphagia. Injection of an inhibitor of monocarboxylate transporter 1, a ketone body transporter, into the fourth ventricle normalized the increase in food intake in streptozotocin (STZ)-induced diabetic rats. Blockade of catecholamine synthesis in the hypothalamic paraventricular nucleus (PVN) also restored food intake to normal levels in diabetic animals. On the other hand, hindbrain injection of the ketone body induced feeding, hyperglycemia, and fatty acid mobilization via increased sympathetic activity and also norepinephrine release in the PVN. This result provides evidence that hyperphagia in STZ-induced type 1 diabetes is signaled by a ketone body sensed in the hindbrain, and mediated by noradrenergic inputs to the PVN.     

The effects on feeding of galanin and M40 when injected into the nucleus of the solitary tract, the lateral parabrachial nucleus, and the third ventricle.

Several reports indicate that central injection of galanin stimulates feeding, and that there is macronutrient specificity in this response. In addition, the galanin receptor antagonist, M40, reduces food intake when injected centrally. The nucleus of the solitary tract (NTS) and the lateral parabrachial nucleus (PBN) contain galanin receptors, and are involved in the control of food intake. Hence, we sought to compare the feeding response to galanin injection into these areas with that of third ventricle (3V) galanin injection. The feeding response to injection of galanin was greatest for the 3V. Hindbrain injection of galanin stimulated food intake only at the beginning of the dark period. NTS injection of M40 inhibited intake of a macronutrient diet in food-deprived rats, but was ineffective at reducing dark-onset feeding or deprivation-induced chow intake. 3V injection of M40 did not reduce deprivation-induced intake. PBN injection of galanin at dark onset had no effect in a group of fat-preferring rats. These results suggest that hindbrain galanin may contribute to feeding by inhibiting satiety, and that hypothalamic galanin receptors are involved with stimulation of intake. Furthermore, the absence of a consistent pattern of the stimulation of macronutrient intake suggests that galanin may not be a significant effector of macronutrient selection during individual meals.     

Cannabinoid agonist, CP 55,940, prevents capsaicin-induced sensitization of spinal cord dorsal horn neurons

Low doses of cannabinoids applied intrathecally attenuate capsaicin-evoked heat and mechanical hyperalgesia via CB1 receptors. Although cannabinoids produce antinociception, in part, by attenuating responses of nociceptive neurons in the spinal cord, few studies have examined the effect of cannabinoids on sensitization of spinal neurons. We therefore investigated whether a cannabinoid receptor agonist, CP 55,940, attenuated excitation and sensitization of spinal nociceptive neurons produced by intraplantar injection of 0.1% capsaicin (10 microl). In rats, wide-dynamic-range (WDR) and high-threshold (HT) neurons were classified according to responses evoked by mechanical stimuli of varying intensity. CP 55,940 (10 microg in 50 microl) or vehicle was applied directly to the spinal cord and responses to mechanical (von Frey monofilament) and heat stimuli were recorded 10 min after drug treatment. CP 55,940 alone did not alter responses to mechanical stimuli; however the enhanced responses to mechanical stimuli after injection of capsaicin into the receptive field were dose dependently attenuated in both HT and WDR neurons. Vehicle-treated neurons increased their response to 300.6 +/- 52.1% of baseline after capsaicin, whereas CP 55,940-treated neurons responded at 153.0 +/- 27.1% of baseline. The effects of CP 55,940 on sensitization to heat were less pronounced; however, CP 55,940 attenuated the capsaicin-evoked decrease in heat threshold in HT neurons. The attenuation by CP 55,940 of sensitization to mechanical stimuli was blocked by pretreatment of the spinal cord with the CB1 receptor antagonist, SR141716A. These studies demonstrate that cannabinoid application to the spinal cord prevents central sensitization.     

Evidence for a physiological role of central calcitonin gene-related peptide (CGRP) receptors in the control of food intake in rats

In the present study, we investigated the role of central calcitonin gene-related peptide (CGRP) and amylin receptors in mediating the anorectic effects of CGRP and amylin in rats chronically cannulated in the lateral brain ventricle. Intracerebroventricular (ICV) injection of the CGRP and amylin receptor antagonist CGRP(8–37) failed to influence the anorectic effects of peripherally injected CGRP and amylin. CGRP(8–37) alone, however, increased food intake in food deprived rats when administered 2 h before food presentation. Under the same experimental conditions, the more specific amylin receptor antagonists amylin(8–37) or AC 187 did not affect food intake. We therefore conclude, that CGRP is a physiological regulator of food intake within the central nervous system, acting at central CGRP receptors. Peripheral receptors, however, are likely to mediate the anorectic effects of peripherally administered amylin and CGRP.     

Effects of amygdala lesions on body weight, conditioned taste aversion, and neophobia

Female rats with posterodorsal amygdala (PDA), basolateral amygdala (BLA), or sham lesions were compared regarding ad libitum food intake, weight gain, consumption of a novel food, and acquisition of a conditioned taste aversion (CTA). While only the rats with PDA lesions evidenced substantial weight gains at 10 days after surgery eating standard lab chow (25-45 g more than the other groups), only the rats with BLA lesions demonstrated significant deficits in the CTA and neophobia paradigms. Rats with basolateral lesions, on average, took less than 30 s to begin drinking the novel sweetened condensed milk after pairing with illness while the other groups took approximately 15 min to begin drinking. Also, rats with basolateral lesions ate, on average, 5 g of the novel Froot Loops while the other groups ate approximately 2 g. It is concluded that the changes in food-motivated behavioral tests frequently observed in animals with amygdala lesions do not coexist with the hyperphagia and weight gain of animals with PDA lesions.     

Effects of food deprivation on water intake induced by intermittent delivery of salted liquid food

The water intake of rats maintained through food deprivation at 70, 80, 90, and 100% of free-feeding body weights was examined under conditions where a small amount of sweetened condensed milk containing 8% sodium chloride was presented at one-minute intervals. For each of two subjects, number of licks and milliliters of water consumed varied inversely with body weight. This finding agrees with earlier reports concerning the effects of food deprivation on adjunctive behaviors induced by intermittent delivery of dry food.     

Sodium appetite in lactating rats

Lactating rats that were given free access to sodium-deficient food, water, and 0.51 M NaCl solution showed no evidence of sodium appetite. The estimated daily loss of 1-2 mEq Na in milk was replaced by basal daily intake of 2-5 ml of saline. Sodium loss in urine was minimal, but milk sodium concentration was unchanged, and pups grew normally. Saline intake was enhanced when lactating rats that had been maintained on standard laboratory chow were injected with 30% polyethylene glycol solution to reduce plasma volume but no more so than when virgin female rats or male rats were similarly colloid-treated. Lactating rats markedly increased their intake of NaCl solution after simply depriving them of dietary sodium for 4 days, whereas male and virgin female rats did not. These findings indicate that pronounced sodium appetite does not invariably accompany lactation in rats, although it can occur whenever such animals become hypovolemic or sodium deficient.     

Modulation of the satiating effect of amylin by central ghrelin, leptin and insulin

Amylin is a pancreatic hormone that is considered to be a satiating signal acting on neurons of the area postrema (AP) in the hindbrain. The adiposity signals leptin and insulin act in the hypothalamus to influence feeding. They also enhance the hindbrain's responsivity to satiating signals, e.g. cholecystokinin (CCK). The orexigenic hormone ghrelin is thought to use the same hypothalamic pathways as leptin and insulin, with opposite actions on feeding behaviour. In fact, CCK and ghrelin also seem to interact in the control of feeding. Because CCK's anorectic effect depends on endogenous amylin, the aim of this study was therefore to evaluate a possible functional interaction between amylin and these hormones on short-term food intake in rats. The experiments were performed with male Wistar rats. Intracerebroventricular injection (i3vt) of an orexigenic dose of ghrelin (5 ng/5 microl) reduced but did not completely reverse the intraperitoneal amylin (5 microg/kg)-induced inhibition of food intake. In comparison, administration of a sub-threshold dose of ghrelin (3 ng/5 microl) did not affect the anorexigenic action of peripheral amylin. Leptin administered into the third ventricle (i3vt; 3.5 microg/5 microl) and intraperitoneal amylin (5 microg/kg) synergistically reduced food intake in chow-fed rats. I3vt insulin, administered at a sub-threshold dose (0.5 mU/5 microl), significantly enhanced the response to peripheral amylin. These results indicate that the lipostatic signals leptin and insulin may synergize with amylin to reduce food intake. In contrast, under the conditions tested, the orexigenic hormone ghrelin does not seem to influence the feeding response to peripheral amylin.     

Pancreatic procolipase propeptide, enterostatin, specifically inhibits fat intake

Pancreatic procolipase is activated by trypsin forming colipase, a cofactor for pancreatic lipase involved in intestinal fat digestion and a pentapeptide named enterostatin. Enterostatin with the sequence Val-Pro-Asp-Pro-Arg (VPDPR) was previously shown to decrease food intake in rats both after peripheral and central injection. In this work enterostatin has been shown to reduce specifically the consumption of a high-fat diet as opposed to a low-fat diet after central injection of Sprague-Dawley rats. After starvation for 18 hours the rats were given a free choice of a low-fat diet (5.2% fat by weight; 14.1% by energy) and a high-fat diet (17.8% fat by weight; 32.8% by energy) in separate containers. After injection of 200 ng of VPDPR into the lateral ventricle, the rats selectively decreased the intake of the high-fat diet by 45% (p less than 0.005), while the intake of the low-fat diet was unaffected compared to saline injection. VPDP after intracerebroventricular injection had totally lost the selective effect on the consumption of a high- fat and a low-fat diet. It is suggested that enterostatin formed during fat digestion from pancreatic procolipase may provide a feed-back signal for the intake of lipid.     

Area postrema lesions block feeding induced by systemic injections of monosodium glutamate

Glutamate is an amino acid neurotransmitter capable of producing widespread receptor-mediated neuronal excitation. Recently we reported that high doses of monosodium glutamate (MSG) given systemically stimulate food intake in a dose-related fashion. Since glutamate does not cross the blood-brain barrier, it seems possible that feeding was stimulated by an action of glutamate on neurons within circumventricular organs (CVOs), areas of the brain in which the blood-brain barrier is deficient. In this experiment, we tested the hypothesis that systemic MSG stimulates feeding by an action on the area postrema (AP), a CVO in the caudal hindbrain. AP-lesioned rats (APLs) and sham-operated controls (shams) were injected with saline or MSG (2 and 6 g/kg, SC, one dose per week). Food intake was measured for 3 hr immediately following the injection. Shams increased their food intake significantly in response to both doses of MSG, but APLs did not. This result suggests that systemic glutamate may stimulate feeding by an action on the AP.     

Central administration of zinc reduces salt intake in rats

The aim of the present study was to investigate the effect of third ventricle injections of zinc on salt intake in rats in the three different experimental models where sodium appetite is increased: fluid deprivation, central angiotensinergic stimulation and sodium depletion. Adult Wistar male rats received third ventricle injections of Zn(Ac)2 in three different doses (0.03, 0.3 and 3.0 nmol/rat). Central angiotensinergic stimulation was achieved by third ventricle injections of angiotensin II in the dose of 25 ng/rat 30 min before central zinc administration. As expected, fluid deprivation, central angiotensinergic stimulation and sodium depletion significantly increased sodium appetite. Water intake was also enhanced after fluid deprivation and central angiotensinergic stimulation. After sodium depletion, no increase in water intake was observed. Third ventricle injections of zinc inhibited salt intake in all three experimental models studied. Water intake was also inhibited by central zinc administration after fluid deprivation and central angiotensinergic stimulation. Conversely, third ventricle injections of zinc were unable to modify food intake or body temperature. It is suggested that zinc, acting on central structures related to the control of body fluid homeostasis, inhibits the drive for salt intake that is normally observed during fluid deprivation, central angiotensinergic stimulation and sodium depletion.     

Capsaicin and its effects upon meal patterns, and glucagon and epinephrine suppression of food intake

The neurotoxin capsaicin has been shown to selectively interfere with unmyelinated sensory fibers, as well as leading to depletions of substance P and other peptides. Meal pattern analysis was performed both before and after treatment with capsaicin in twelve adult male Sprague-Dawley rats. Capsaicin treatment only briefly altered feeding patterns. No long term effect on body weight was noted. These same animals were then tested for the appetite suppressing effects of IP injections of glucagon (125 micrograms/kg) and epinephrine (30 micrograms/kg). Capsaicin treated rats decreased their intake of sweetened condensed milk during a 30 minute test in response to glucagon and epinephrine. Controls failed to suppress intake in response to glucagon, but drank less milk after epinephrine than did capsaicin treated rats. Efficacy of capsaicin treatment was obtained using similarly treated animals subject to histological evaluation within 2 days of capsaicin treatment. These results suggest that peripherally generated information relayed to the CNS via small-diameter sensory neurons is not a necessary component of the normal hunger/satiety sequence, nor body weight regulation.     

Anorexia in rats caused by a valine-deficient diet is not ameliorated by systemic ghrelin treatment

Rodents exhibit aversive behavior toward a diet that lacks at least one of the essential amino acids. We sought to determine whether the particular form of anorexia caused by such diets could be ameliorated by the administration of orexigenic peptides while simultaneously analyzing the neural mechanisms underlying anorexia. Rats were fed a valine-deficient diet, which induced severe anorexia (reducing food consumption by 80%). The severe anorexia was associated with a significant decrease in the cerebrospinal fluid valine concentration and hyper-ghrelinemia. Between 6 and 12 days after initiation of the valine-deficient diet, we injected rats twice daily with valine and/or an orexigenic peptide (ghrelin, neuropeptide Y, or agouti-related protein) either i.p. or i.c.v.. We then measured dietary intake. An i.c.v. valine injection allowed earlier food intake compared with an i.p valine injection and increased the density of c-Fos-positive ependymal cells lining the third ventricle. Whereas an i.c.v. injection of ghrelin or neuropeptide Y increased consumption of the valine-deficient diet, i.p injection of ghrelin or i.c.v. injection of agouti-related protein did not. Following i.c.v. administration of either valine or ghrelin, we did not observe complete recovery of consumption of the valine-deficient diet. This may be due to the ineffectiveness of peripheral ghrelin and central agouti-related protein and/or to conditioned aversion to the valine-deficient diet. Since ghrelin is known to be involved in food anticipatory activities, whether the hyper-ghrelinemia observed in valine-deficient rats play role in foraging behavior other than food intake is the future study to be investigated.     

Area postrema/nucleus of the solitary tract ablations: Analysis of the effects of hypophagia

The effect of hypophagia following lesions of the area postrema and caudal-medial aspect of the nucleus of the solitary tract (AP/cmNTS) on body-weight, water intake and preference for palatable diets was examined. Following AP/cmNTS ablation, rats reduced pelleted-food intake to a degree which was sufficient to account for the weight loss and increased water:food ratios observed. Restricting food intakes of intact rats to levels taken by lesioned animals resulted in similar weight losses and increased water:food ratios. When offered both pelleted food and milk, lesioned rats took more calories as milk than did previously food-restricted intact rats. Thus, the hypophagia of AP/cmNTS lesioned rats does not account for their increased preference for milk diets. Lesioned rats ate less high-fat diet than did intact or sham-lesioned controls and did not increase their intakes when this diet was sweetened. At autopsy, retroperitoneal and epididymal fat-pad weights accounted for less of the total body weight of lesioned animals than controls suggesting that body-fat levels are reduced following AP/cmNTS ablation.     

The effect of TNFα on food intake and central insulin sensitivity in rats

Circulating and tissue levels of the proinflammatory cytokine tumor necrosis factor α (TNFα) are elevated in obesity. TNFα interferes with insulin signaling in many tissues and also plays a causal role in the anorexia that accompanies severe challenges to the immune system. The interactions between TNFα and insulin in the control of eating are less well known. The present study evaluated the role of TNFα in the central nervous system control of food intake by insulin in adult male Long Evans rats. We first determined the ability of several doses of TNFα injected into the 3rd cerebral ventricle (i3vt) to reduce food intake in male rats. Subsequently, we assessed the ability of a subthreshold dose of TNFα to modulate the effect of i3vt insulin on food intake in male rats fed a low-fat chow or a high-fat (HF) diet. TNFα administered i3vt dose-dependently reduced food intake in rats fed a standard low-fat chow diet. Moreover, a low, sub-threshold dose of TNFα diminished the reduction in food intake by insulin in rats maintained on a chow diet, but enhanced insulin action in rats maintained on a HF diet. These data suggest that the interaction of TNFα with central insulin varies with nutritional and/or dietary conditions.     

Central 5-HT(3) receptors and water intake in rats

In the present paper, we studied in rats the effect of third ventricle administration of m-chlorophenylbiguanide hydrochloride (1-(3-chlorophenyl)biguanide (m-CPBG), a selective 5-HT(3) agonist, on water intake induced by three different physiological stimuli: water deprivation, acute salt load and hypovolemia. Central acute m-CPBG injections in the doses of 80 and 160 nmol significantly reduced water intake elicited by an acute salt load. Third ventricle injections of m-CPBG in the dose of 160 nmol significantly inhibited water intake in hypovolemic animals, whereas third ventricle injections of m-CPBG in a higher dose (320 nmol) were necessary to decrease water intake in water-deprived rats. Pretreatment with 1-methyl-N-[8-methyl-8-azabicyclo(3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584), a selective 5-HT(3) antagonist, abolished the inhibitory effect on water intake seen after central administration of m-CPBG in all groups studied. The central administration of m-CPBG was also able to inhibit water intake induced by pharmacological activation of central cholinergic and angiotensinergic pathways. Third ventricle injections of m-CPBG in the highest dose employed in this study (320 nmol) were unable to modify food intake in food-deprived rats. An aversion test has shown that acute third ventricle injections of m-CPBG do not induce illness-like effects that could explain the water intake inhibition here observed. Also, central administration of m-CPBG did not modify the intake of a "dessert" meal consisting of diluted condensed milk. It is concluded that central 5-HT(3) receptor activation exerts a specific inhibitory effect on water intake.     

Characterization of central actions of neuropeptide Y on food and water intake in rabbits

The effects of a 36-amino acid peptide, neuropeptide Y (NPY), on feeding and drinking behaviors were studied in young and adult rabbits. Intraventricular injection of NPY to adult rabbits induced feeding and drinking in a dose-related manner. While the lowest doses tested (0.2 micrograms) was without effect, other doses (0.5 and 1 microgram) elicited feeding and drinking almost instantaneously. When 1, 5 and 10 microgram doses were injected into young rabbits, immediate increases in feeding and drinking were evident, but differences in the magnitude of responses among these dosages were significant only in water consumption. Unlike studies in rats, in these rabbits NPY elicited a more pronounced response in drinking than in feeding. The drinking response after NPY administration was not a consequence of food intake because it occurred in the absence of food. With ad lib feeding, the majority of enhanced food consumption was confined to the first 30-min after NPY injection; however, an increased motivation to eat was retained for at least 2 hr after NPY when food was withheld and then returned. These observations are consistent with specific stimulatory effects of NPY on food and water intake.     

Extracellular GABA in the medial hypothalamus is increased following hypocretin-1 administration

AIM: Hypocretin 1 is an hypothalamic neuropeptide that induces an increase in food intake when administered into the cerebral lateral ventricle. As it is well known that the medial hypothalamus (MH) is involved in the feeding behaviour also through GABAergic circuits, the aim of this experiment was to investigate the effect of an hypocretin 1 intracerebroventricular (icv) injection on the extracellular levels of GABA in the MH. METHODS: GABA levels in the MH were evaluated in six rats by microdialysis and high performance liquid chromatography-electrochemical detection 30 min before and every 30 min for an over all period of 6 h after an icv injection of hypocretin 1. The same procedure was used in another group of six rats but saline was injected into the lateral ventricle as control. RESULTS: The results show that extracellular GABA increases in the MH after the injection of hypocretin 1 at 60 min and at 3 h after the injection. CONCLUSION: This finding suggests a possible mechanism by which hypocretin 1 should induce hyperphagia in the first hour after injection. As it is already known that the inhibition of the MH by injection of GABA causes an increase of food intake, it is possible that hypocretin 1 causes an increase in food intake by increasing the GABA release in the MH. The lack of an increase in the GABA level after the fourth hour is consistent with the lack of an increase in food intake at this time, as we observed in previous experiments. The finding of a biphasic increase in the GABA level, at 60 min and at 3 h, was unexpected and should be further investigated.