Time-dependent effects of neuropeptide Y infusion in the paraventricular hypothalamus on ingestive and associated behaviors in rats

In this study the role of neuropeptide Y (NPY) in the paraventricular nucleus of the hypothalamus (PVN) in the daily regulation of feeding, drinking, locomotor activity, and nestbox occupation was investigated. These behaviors were recorded during and after bilateral infusion of NPY into the PVN of rats during the early (E) or late (L) part of the light phase. Administration of NPY caused a significant increase in feeding behavior at E, but not at L. In contrast to the feeding at E, L feeding was associated with increased water intake following NPY infusion. While locomotor activity was similar in sCSF- and NPY-infused rats at all times of the daily cycle, administration of NPY at L, but not at E increased nestbox occupation during the first few hours of the dark phase. This increased nestbox occupation was not associated with altered food intake or drinking behavior, implying that NPY-treated rats made frequent excursions between nestbox and food hopper/water bottle. Thus, feeding-associated drinking and explorative behavior are time-dependently modulated by NPY in the PVN, independent of locomotor activity.     

Intraventricular neuropeptide Y injections stimulate food intake in lean, but not obese Zucker rats.

We examined the effect of acute third intraventricular (IVT) injections of either saline or NPY (0.95, 3.0, 9.5, or 30.0 micrograms in 1 microliter) on the 1-, 4-, and 22-hour postinjection food and water intake of female obese (fa/fa), heterozygous lean (Fa/fa), and homozygous lean (Fa/Fa) Zucker rats. None of the doses of NPY had an effect on either food or water intake of fa/fa rats. A significant increase of food intake was seen in Fa/Fa rats at 1 and 4 hours after the 3.0 micrograms injection of NPY and at 1, 4, and 22 hours after the 9.5 micrograms injection of NPY. Both 3.0 and 9.5 micrograms of NPY also stimulated 1- and 4-hour postinjection food intake of Fa/fa rats, although this effect was significant only at 4 hours after the 3.0 micrograms dose. NPY had a less reliable effect on water intake; 3.0 micrograms of NPY stimulated 1-hour postinjection water intake of Fa/fa rats and 4-hour postinjection water intake of Fa/Fa rats. These results indicate that lean, but not obese Zucker rats, respond by eating more to centrally administered NPY. This deficit is similar to the effects seen with IVT insulin injections and may be a result of a common receptor-mediated mechanism.     

Structure-function analysis of stimulation of food intake by neuropeptide Y: effects of receptor agonists.

Neuropeptide Y (NPY) is a potent natural orexigenic signal in the rat. In this study, we have compared the effects of several COOH-terminal fragments of NPY and NPY receptor agonists on cumulative food intake in male rats. Rats were implanted with permanent cannulae either into the third cerebroventricle or paraventricular nucleus (PVN). NPY1-36 and various COOH-terminal fragments of NPY, two agonist analogues [Leu31, Pro34]NPY and NPY 1-4-Aca (epsilon-amino-caproic acid)-25-36, were administered intracerebroventricularly (ICV) or directly into the PVN, and the cumulative 2-h food intake response was compared. We observed that peptides that were effective by ICV were also effective when administered into the PVN, but smaller amounts of the peptides were required after PVN injection to evoke an equivalent food intake response. Injection of NPY1-36 induced a dose-dependent increment in food intake. Surprisingly, deletion of NH2-terminal tyrosine residue did not adversely affect feeding behavior. In fact, NPY2-36 was consistently more effective than NPY1-36; the enhancement in feeding by NPY2-36 was dose-related and was higher than evoked by NPY1-36 at each dose tested. Further serial deletion of aminoacids at NH2-terminal resulted in complete loss of activity. In addition, NPY agonist analogue, NPY 1-4-Aca-25-36, failed to stimulate feeding. However, NPY Y1 receptor agonist, [Leu31, Pro34]NPY, but not Y2 receptor agonist, NPY13-36, stimulated feeding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cannabinoid agonist, CP 55,940, facilitates intake of palatable foods when injected into the hindbrain

Cannabinoids have been shown to influence food intake, and until recently, the neural pathways mediating these effects have remained obscure. It has been previously shown that intracerebroventricular injection of delta-9-tetrahydrocannabinol (Δ⁹-THC) causes increased consumption of palatable foods in rats, and we postulated the involvement of the hindbrain in this cannabinoid-induced food intake. Cannulated rats (both female and male groups) trained to consume sweetened condensed milk received either lateral or fourth ventricle injections of CP 55,940 and were presented with sweetened condensed milk 15 min after injection. Rats were injected over a range of doses between 100 pg and 10 μg per rat. Milk intake was recorded for a total of 3 h. Lateral ventricle injection of CP 55,940 increased milk intake at doses in the microgram range. However, CP 55,940 was effective in increasing food intake at nanogram doses when injected into the fourth ventricle. Finally, male rats appeared to be more sensitive to CP 55,940 than female rats inasmuch as milk consumption was increased at the 1 ng dose in male rats, whereas only the 10 ng dose was effective in females. These results indicate that CP 55,940 may act in the hindbrain to influence feeding behavior in rats.     

Some determinants of intake and patterns of feeding in the guinea pig

Regulation of food and water intake was studied in young and adult Guinea pigs by continuously monitoring ad lib feeding and drinking patterns and by examining the response to levels of celluflour dilution that ranged from 20–75%. Meal size and the duration of the intermeal interval were not systematically related to the sizes of preceding meals or interval lengths. Variations in food intake were mediated almost entirely by changes in meal size. This parameter increased during growth, was elevated in the first meal following food deprivation, decreased when water was withheld and increased when water was returned. The probability of drinking within 20 min of eating was 0.80. Celluflour dilution led to a decrease in apparent caloric intake without a proportional decrease in body weight. Regulation of energy balance and meal to meal control of feeding in this monogastric, herbivorous rodent is discussed in relation to the Guinea pig's digestive physiology and ecological niche.     

Neuropeptide Y stimulates food consumption through multiple receptors in goldfish

In this study, the acute effects of brain intracerebroventricular (icv) injections of mammalian neuropeptide Y (NPY) Y1 ([31Leu,34Pro]NPY), Y2 (NPY2-36) and Y5 ([D-32Trp]NPY) receptor subtype agonists on food intake in goldfish were examined. Icv injection of Y1 and Y5 receptor agonists in dosages of 1 and 5 ng/g exhibited dose-dependent effects on food intake; however, higher dosages of both receptor subtypes had desensitising effects on food intake, and caused a decrease in food intake in comparison to the lower dosages. At 10 and 20 ng/g, Y1 receptor agonist-treated fish exhibited feeding significantly lower than intact and saline-injected fish; fish treated with the same dosages of Y5 agonist exhibited food intake similar to intact and saline-injected fish. Y2 agonist had no effects on food intake. Co-icv administration of Y1 and Y5 agonists in dosages of 1 and 5 ng/g caused enhanced food intake that was additive of the individual doses alone. However, desensitising one receptor did not influence the responsiveness of the other. Co-icv injection of Y1 receptor agonist in desensitizing high dosages (10 and 15 ng/g) with Y5 receptor agonist in lower doses (1 and 5 ng/g, respectively) or vice versa, resulted in a food intake similar to the dosages of Y1 and Y5 receptor agonists at 1 and 5 ng/g given alone. Overall, this study demonstrates that NPY acts centrally through Y1 and Y5 receptors to stimulate food intake in goldfish. The Y1 and Y5 receptors appear to act independently in the stimulation of food intake in goldfish.     

Histamine-elicited drinking in weanling and adult rats

A total of 260 male and female adult (60-70 days of age) and weanling (22-25 days of age) Sprague-Dawley derived rats were used in these experiments. Subcutaneous administration of histamine (HA) elicited drinking in a dose-dependent manner for both ages tested, although the threshold dose varied with age. A dose of 5.0 mg/kg HA elicited significant increases in water intake for adults, whereas for weanlings a dose of 20 mg/kg HA was necessary. Adult rats exhibited decreased latency to drink after all doses of HA tested, whereas for weanlings, decreased latency was evident only after doses of HA sufficient to elicit increases in water intake. Combined antagonism of H1 and H2 receptors for HA, using dexbrompheniramine and cimetidine, respectively, inhibited HA-elicited drinking in adults and weanlings. Further investigation of the ontogeny of histamine- and food-related drinking may provide a useful approach to examine the physiological mechanisms underlying fluid consumption in adult animals and as they are gradually elaborated during ontogeny.     

Chronic hypodipsia to intraperitoneal and subcutaneous hypertonic saline after vagotomy

Recent studies suggest that the decreased drinking response to hypertonic saline produced by bilateral subdiaphragmatic vagotomy (VGX) is a function of the route of saline administration and the length of postoperative recovery. We determined the effects of VGX on drinking during the two hours after intraperitoneal and subcutaneous injections of 0.25, 0.5 and 1.0% body weight doses of 1 M NaCl 30 weeks after surgery. Regardless of the route of injection of saline, VGX rats took longer to initiate drinking and drank less water after the two highest doses than controls. Although VGX rats drank less than controls after both routes of injection, the decrease in water intake was greater after intraperitoneal administration. We conclude that, since both deficits were obtained regardless of the route of saline injection 30 weeks after surgery, route of administration and length of postoperative recovery are not important factors for demonstrating impairments in the drinking elicited by hypertonic saline after total abdominal vagotomy under our experimental conditions.     

Central bombesin inhibits food intake and the orexigenic effect of neuropeptide Y in the neonatal chick

It is well known that central injection of bombesin (BN) suppresses feeding in mammalian and avian species, but the anorexigenic effect of central BN are still open with special reference to the chick. The dose response (0, 0.1 and 0.5 μg) of intracerebroventricular (ICV) injection of BN was examined in Experiment 1. ICV injection of BN inhibited food intake in a dose-dependent manner. Experiment 2 was done to determine whether BN interacts with the orexigenic effect of neuropeptide Y (NPY) in the neonatal chick. Central administration of NPY (2.5 μg) greatly enhanced food intake, but co-injection of BN (0.5 μg) suppressed food intake. The dose response of NPY (2.5 μg) co-injected with three levels of BN (0, 0.1 and 0.5 μg) was examined in Experiment 3. ICV injection of BN attenuated the hyperphagia by NPY in a dose-related fashion. It is suggested that central BN may interact with NPY for the regulation of feeding in the neonatal chick.     

Dose-related stimulation of feeding by systemic injections of monosodium glutamate

Monosodium glutamate (MSG) is an excitotoxin capable of both stimulating and lesioning neurons in circumventricular organs (CVOs) after systemic administration. In this study, MSG and equiosmotic concentrations of NaCl were administered subcutaneously to adult rats in order to observe the effects on food and water intake. MSG (0.5, 1, 2 and 6 g/kg), but not NaCl, stimulated feeding. The magnitude of the feeding was dose-related. After the highest dose, rats consumed 4.4 g of pelleted food. Since MSG does not cross the blood-brain barrier, we conclude that feeding was stimulated by an action of glutamate on CVOs. Doses of MSG that stimulated feeding did not alter blood glucose concentration. Neonatal MSG treatment, which is known to be more damaging to circumventricular neurons than adult treatment, greatly reduced or abolished subsequent MSG-induced stimulation of feeding in adults. Both MSG and NaCl stimulated drinking. Since the magnitude of the drinking response was similar for both solutes and was directly related to the osmotic strength of the solutions, we conclude that the drinking response after MSG was mediated by cellular dehydration.     

Alpha 2-adrenoceptor blockade does not block feeding induced by neuropeptide Y in sheep

We tested the hypothesis that blockade of central alpha 2-adrenergic receptors would prevent neuropeptide Y (NPY)-induced feeding. Nine young female sheep were fitted with lateral ventricula cannulas. Bolus intracerebroventricular (ICV) injection of 3 nmol of NPY increased feed intake after 30 min between 45 and 153% in three experiments. A bolus ICV injection of 400 or 100 nmol of the alpha 2-antagonist, yohimbine, either 5 or 30 min before NPY injection, did not attenuate this response. Instead, yohimbine increased feed intake over NPY-induced feeding by 52 to 55%. We interpret these data as evidence that the putative NPY feeding pathway in feed-sated sheep is not dependent on the type of alpha 2-adrenergic mechanism which can be blocked by ICV injection of yohimbine.     

Drinking by senescent and adult rats in response to regulatory challenges

The regulation of water and electrolyte balance was elevated in senescent (greater than 31 months) and adult (5-10 months) rats of several strains. Weekly food and water intake, drinking induced by 24-hr water deprivation and drinking induced by injection of hypertonic saline were roughly the same in old and adult rats. However, senescent rats drank less after injection of the beta-adrenergic agonist isoprenaline than adult rats. There were no appreciable strain differences in drinking in response to these regulatory challenges although baselines sometimes differed between strains.     

Time course and response specificity of prolactin-induced hyperphagia in ring doves

Intracranial injections of prolactin (PRL) have been previously shown to elevate food and water intake in ring doves. In an attempt to further characterize these PRL-induced behavioral responses and the time course of PRL action, food and/or water intake were measured as frequent intervals in male doves given a single intracerebroventricular (ICV) injection of ovine PRL (44 pmoles) or vehicle under food deprivation, water deprivation, or nondeprivation conditions. PRL increased food consumption by 35-50% over baseline levels in water deprived and nondeprived doves, although response latencies (10 hr) and durations (greater than 24 hr) were considerably longer than those reported for other orexigenic peptides. Behavioral observations of nondeprived doves further revealed that PRL significantly increased total time spent feeding and average feeding bout duration. In contrast to this pattern, water intake remained unchanged in food deprived doves and was only marginally increased in nondeprived doves following PRL treatment. Collectively, these results suggest that PRL promotes a selective and long-lasting hyperphagia which may in turn augment drinking activity.     

Ingestion in the satiated rat: role of alpha and beta receptors in mediating effects of hypothalamic adrenergic stimulation.

Anterior perifornical hypothalamic injection of l-norepinephrine in satiated rats elicits a brief, vigorous drinking response followed within a minute or two by a vigorous feeding response. These adrenergically elicited responses, which bear striking similarities to a rat's naturally motivated ingestive behaviors, were examined in the present series of experiments. It was found that: (1) Both responses could be elicited by perifornical hypothalamic injection of l-epinephrine, which was actually found to be more potent than l-norepinephrine. In contrast, only feeding could be elicited by the alpha-stimulant metaraminol, and neither feeding nor drinking could be elicited by hypothalamic injection of d-norepinephrine. l-isoproterenol, or dopamine. (2) The threshold doses of l-epinephrine for eliciting reliable ingestive responses were quite low, namely, 0.8 nmole (0.15 mug) for drinking and 0.2 nmole (0.04 mug) for feeding. (3) Pharmacological analysis of the ingestive behaviors induced by l-norepinephrine or l-epinephrine indicated that the eating response was mediated by alpha-adrenergic receptors, whereas the drinking response involved the synergistic action of both alpha- and beta-adrenergic receptors. No evidence for the involvement of dopaminergic or cholinergic (muscarinic) receptors was obtained. (4) A third adrenergically elicited phenomenon, namely, a suppression of drinking, was observed during and after the period of induced feeding. Analysis of this effect revealed its dependence solely upon alpha-adrenergic receptor activity.     

Pregastric food-contingent stimulation elicits drinking in the absence of systemic dehydration in the rat

Adult male Sprague-Dawley rats surgically fitted with a stainless steel gastric cannula were prepared following 24-h food deprivation for sham feeding liquid diet with open gastric fistula. Sham feeding (pregastric food-contingent stimulation) of 5, 10, 20 or 40 ml of liquid diet elicited water intake that was not in proportion to volume of liquid diet sham fed. Rats sham feeding between 1 and 68 ml of sweetened milk showed no evidence of cellular or extracellular dehydration as measured by plasma osmolality and hematocrit, respectively. Subcutaneous injection of 100 mg/kg captopril, a dose sufficient to block conversion of angiotensin I to angiotensin II in brain and periphery, inhibited drinking elicited by sham feeding without effect on sham feeding. These results demonstrate that pregastric food-contingent stimulation elicits water intake that is not in proportion to amount of liquid diet sham fed and that occurs in the apparent absence of systemic dehydration. That such drinking could depend upon endogenous angiotensin II provokes consideration of a role for angiotensin II in the mediation of a pregastric (perhaps histaminergic) mechanism that initiates drinking in advance of postprandial dehydration.     

Full amino acid sequence of centrally administered NPY required for maximal food intake response

Central administration of NPY (1-36) potently increases food intake and it has been hypothesized that biological activities of NPY are related to its ability to form an alpha-helix, represented by the fragment NPY (14-31). In this experiment the necessity of N-terminal fragments for increasing food intake was evaluated. Two-h fasted male rats were administered 0, 0.2, 1.0 or 5.0 nmol NPY (1-36) or NPY fragments in 5 microliters saline ICV and intake of lab chow pellets was measured for 22 h. Fragments containing all or part of the polyproline-like helix [NPY (1-8)] antiparallel to the alpha-helix dose-relatedly increased food intake for 4 hours after injection. Five nmol NPY (1-36) and NPY (2-36) increased 4-hour food intake 486 and 219%, respectively (p less than 0.05). Fragments excluding the first 8 amino acids but including all of the alpha-helix also increased food intake, but the response was much reduced. Five nmol NPY (9-36) and NPY (14-36) increased 4-hour food intake 128% (p = 0.02) and 62% (NS), respectively. When all or part of the alpha-helix was excluded, no activity was detected, i.e., NPY (21-36) and NPY (32-36). Substitution of dPro for lPro in position 2 increased potency but not efficacy of NPY since food intake was increased at the 0.2 and 1.0 but not 5.0 nmol doses and the percent increase was not more than to 5 nmol NPY (1-36). Thus the maximum food intake response to NPY requires both C-terminal and N-terminal fragments as well as the alpha-helix.     

Possible common central pathway for resistin and insulin in regulating food intake

Aim: Adipose tissue has been the object of intense research in the field of obesity and diabetes diseases in the last decade. Examination of adipocyte‐secreted peptides led to the identification of a unique polypeptide, resistin (RSTN), which has been suggested as a link between obesity and diabetes. RSTN plays a clearly documented role in blocking insulin (INS)‐induced hypoglycaemia. As brain injection of INS affects feeding behaviour, we studied the possible interaction between INS and RSTN in food‐deprived rats, measuring effects on food intake. In addition, we examined how RSTN might affect neuropeptide Y (NPY)‐induced feeding, as studies have shown that rat RSTN can interfere with the NPY system. Methods: Overnight food‐deprived rats were injected into the third brain ventricle (3V) with either INS (10 or 20 mUI), RSTN (0.1–0.4 nmol/rat), or saline before access to food. Another group of rats was injected into the 3V with RSTN alone, NPY alone or RSTN plus NPY. Their food intake and body weight were measured. Results: Our results confirm the hypophagic effect of RSTN on food deprivation‐induced food intake, and more importantly, show that RSTN neither potentiates nor blocks the effects of INS on food intake, but does reduce the hyperphagic effect of NPY. Conclusion: The observation that RSTN does not modify feeding INS‐induced hypophagia, but does influence NPY‐induced feeding, points to the possibility that RSTN may be involved in control of food intake through an NPY‐ergic mechanism as INS.     

Effects of intracerebroventricular injection of muscimol or GABA on operant feeding in pigs

Young pigs were prepared with lateral intracerebroventricular (ICV) cannulae. They were housed individually in cages fitted with operant panels and could obtain food and water ad lib. The GABA-A receptor agonist muscimol (25-200 nmol) ICV produced an increase in food intake in which the dose-response relation was most obvious 30-60 min after dosing. The 25-nmol dose had no effect on feeding. However, muscimol (50 nmol) caused a significant increase in feeding (p less than 0.01) during the first 30 min after injection, while the 100- and 200-nmol doses increased food intake (p less than 0.01) during the first 60 min. The effect of muscimol (100 nmol) on food intake was completely abolished by the simultaneous administration of the GABA-A receptor antagonist bicuculline (100 nmol). GABA (40-1600 nmol) ICV also produced a dose-related increase in food intake (p less than 0.01) in the 15 min after injection. Only doses of 800 nmol and above were effective. The effects of GABA (1600 nmol) were completely abolished by the simultaneous administration of bicuculline (50 nmol). Neither muscimol nor GABA influenced food intake for the 24-hr time period or water intake during any time period. The results indicate that stimulation of central GABA-A receptors induces operant feeding in the satiated pig.     

Control of feeding during saline consumption and fluid deprivation in hamsters

Cellular dehydration induced by water deprivation or hypertonic saline injection reduces feeding in a variety of species. Normal feeding in rats is maintained during isotonic saline consumption by increasing the intake of saline compared to the usual intake of water. Hamsters do not show the spontaneous preference for isotonic saline noted in rats, even after adrenalectomy. In the present investigation, feeding by hamsters was depressed during both isotonic and hypertonic saline consumption compared to the usual feeding with water. Saline intakes did not exceed water intakes under similar conditions. When fluid intakes were elevated by prior fluid deprivation, feeding rates increased at all concentrations of saline after a delay proportional to the osmolality of the solution. Positive 24-hr sodium balances were always associated with saline consumption. Water and hypertonic saline injections reduced feeding, and the fluid loads were excreted very slowly. When hamsters were fluid deprived prior to injections, saline totally suppressed feeding, while water increased feeding compared to sham injected controls. It is concluded that cellular dehydration produces a reduction of feeding in hamsters drinking isotonic or hypertonic saline. Reduced feeding with isotonic saline consumption results from the failure of hamsters to increase their ad lib intake of that solution. The prolonged retention of both sodium and fluid after saline consumption or injection suggests that further saline intake may be inhibited by an expansion of the extracellular space.     

Neuropeptide Y-related compounds and feeding

Neuropeptide Y (NPY) and related compounds increase short-term feeding. Previous studies have used different animal models, feeding schedules, sources of the compounds, and time and routes of administration. These differences in methodology are important in the variability reported on the potency of NPY-related compounds. To obtain reliable data on the relative efficacy, we tested NPY, NPY 3-36, and pancreatic polypeptide (PP) using an identical protocol and the same commercial source. These three NPY-related compounds were tested using the intracerebroventricular (i.c.v., into the third ventricle) administration, and the profile of the feeding enhancement including the dose response and potency was determined. Compounds were tested in parallel on at least 2 successive days. NPY, NPY 3-36, and PP exhibited different potencies in enhancing 2-h food intake. Comparison of their dose responses (using 0.1, 0.25, 0.5, 1.0, 2.5, and 5.0 microg/rat) demonstrated an overall potency of NPY 3-36 > NPY > PP for the high doses. To study ligand interactions, we examined the effects of various combinations of NPY-related compounds administered concomitantly. These combinations were justified based on the data obtained from the individual dose responses. The data show that the effects of NPY plus NPY 3-36 or NPY 3-36 plus PP were less than additive. When compared to the individual responses, the effects of NPY 3-36 were almost identical to those induced by the combinations using low doses of NPY plus NPY 3-36, or low and high doses of PP plus NPY 3-36. The results support the notion that NPY and its analogues induce a short-term feeding response by activating multiple receptor subtypes.