Effects of neuropeptide Y (NPY) and [D-Trp32]NPY on monoamine and metabolite levels in dialysates from rat hypothalamus during feeding behavior

Neuropeptide Y modifies a part of diencephalic catecholamine but not indolamine metabolism in chicks depending on feeding status

The role of the monoaminergic system in the feeding behavior of neonatal chicks has been reported, but the functional relationship between the metabolism of monoamines and appetite-related neuropeptides is still unclear. This study aimed to investigate the changes in catecholamine and indolamine metabolism in response to the central action of neuropeptide Y (NPY) in different feeding statuses and the underlying mechanisms. In Experiment 1, the diencephalic concentrations of amino acids and monoamines following the intracerebroventricular (ICV) injection of NPY (375 pmol/10 μl/chick), saline solution under ad libitum, and fasting conditions for 30 min were determined. Central NPY significantly decreased L-tyrosine concentration, the precursor of catecholamines under feeding condition, but not under fasting condition. Central NPY significantly increased dopamine metabolites, including 3,4-dihydroxyphenylacetic acid and homovanillic acid (HVA). The concentration of 3-methoxy-4-hydroxyphenylglycol was significantly reduced under feeding condition, but did not change under fasting condition by NPY. However, no effects of NPY on indolamine metabolism were found in either feeding status. Therefore, the mechanism of action of catecholamines with central NPY under feeding condition was elucidated in Experiment 2. Central NPY significantly attenuated diencephalic gene expression of catecholaminergic synthetic enzymes, such as tyrosine hydroxylase, L-aromatic amino acid decarboxylase, and GTP cyclohydrolase I after 30 min of feeding. In Experiment 3, co-injection of α-methyl-L-tyrosine, an inhibitor of tyrosine hydroxylase with NPY, moderately attenuated the orexigenic effect of NPY, accompanied by a significant positive correlation between food intake and HVA levels. In Experiment 4, there was a significant interaction between NPY and clorgyline, an inhibitor of monoamine oxidase A with ICV co-injection which implies that co-existence of NPY and clorgyline enhances the orexigenic effect of NPY. In conclusion, central NPY modifies a part of catecholamine metabolism, which is illustrated by the involvement of dopamine transmission and metabolism under feeding but not fasting conditions.

Heat Stress Biomarker Amino Acids and Neuropeptide Afford Thermotolerance in Chicks

With global warming, heat stress is becoming a pressing concern worldwide. In chickens, heat stress reduces food intake and growth, and increases body temperature and stress responses. Although it is believed that young chicks do not experience heat stress as they need a higher ambient temperature to survive, our series of studies in young chicks showed that they are sensitive to heat stress. This review summarizes current knowledge on amino acid metabolisms during heat stress, with special emphasis on the hypothermic functions of l-citrulline (l-Cit) and l-leucine (l-Leu), and the functions of neuropeptide Y (NPY) in terms of body temperature and heat stress regulation in chicks. Amino acid metabolism is severely affected by heat stress. For example, prolonged heat stress reduces plasma l-Cit in chicks and l-Leu in the brain and liver of embryos. l-Cit and l-Leu supplementation affords thermotolerance in young chicks. NPY expression is increased in the brains of heat-exposed chicks. NPY has a hypothermic action under control thermoneutral temperature and heat stress in chicks. The NPY-sub-receptor Y5 is a partial mediator of the hypothermic action of NPY. Further, NPY stimulates brain dopamine concentrations and acts as an anti-stress agent in heat-exposed fasted, but not fed chicks. In conclusion, young chicks can serve as a model animal for the study of heat stress in chickens. l-Cit, l-Leu, and NPY were identified as biomarkers of heat stress, with the potential to afford thermotolerance in chicks.

Central administration of neuropeptide Y differentially regulates monoamines and corticosterone in heat-exposed fed and fasted chicks

Recently, we demonstrated that brain neuropeptide Y (NPY) mRNA expression was increased in heat exposed chicks. However, the functions of brain NPY during heat stress are unknown. This study was conducted to investigate whether centrally administered NPY affects food intake, rectal temperature, monoamines, stress hormones and plasma metabolites in chicks under high ambient temperatures (HT). Five or six-day-old chicks were centrally injected with 0, 188 or 375pmol of NPY and exposed to either HT (35±1°C) or a control thermoneutral temperature (CT; 30±1°C) for 3h whilst fed or fasted. NPY increased food intake under both CT and HT. NPY reduced rectal temperature 1 and 2h after central administration under CT, but not under HT. Interestingly, NPY decreased brain serotonin and norepinephrine concentrations in fed chicks, but increased concentrations of brain dopamine and its metabolites in fasted and fed chicks, respectively. Plasma epinephrine was decreased by NPY in fed chicks, but plasma concentrations of norepinephrine and epinephrine were increased significantly by NPY in fasted-heat exposed chicks. Furthermore, NPY significantly reduced plasma corticosterone concentrations in fasted chicks. Plasma glucose and triacylglycerol were increased by NPY in fed chicks, but triacylglycerol declined in fasted NPY-injected chicks. In conclusion, brain NPY may attenuate the reduction of food intake during heat stress and the increased brain NPY might be a potential regulator of the monoamines and corticosterone to modulate stress response in heat-exposed chicks.

Feeding Induced by Increasing Doses of Neuropeptide Y: Dual Effect on Hypothalamic Serotonin Release in Normal Rats

Endogenous neuropeptide Y (NPY) levels increase during fasting and before dark onset in rats. The feeding that follows these states elicits the release of serotonin in the lateral hypothalamus (LH), as part of the physiological mechanisms controlling satiety. With the hypothesis that exogenous NPY-induced feeding should also stimulate serotonin, we measured its release in the LH of non-fasted rats, which received a single intracerebroventricular injection of either 1.0, 2.0, or 5.0 microg of NPY. After 1.0 microg, the cumulative 2-h intake was of 13 g and serotonin release significantly increased (54% peak). These feeding and serotonergic responses were highly similar to the ones we observed in a previous study, in which feeding followed an overnight fast. Thus, the 1.0 microg NPY dose stimulated intake while preserving the normal serotonergic activation. Contrarily, as the NPY dose was increased to either 2.0 or 5.0 microg, the cumulative 2-h intakes were of 18 g, but the serotonergic stimulation was absent. It is suggested that this dual NPY effect relies on a finely tuned control mechanism, reflecting the existence of a narrow range of NPY levels within which the serotonergic stimulation resembles those seen in physiological states.

NPY receptors as potential targets for anti-obesity drug development

The neuropeptide Y system has proven to be one of the most important regulators of feeding behaviour and energy homeostasis, thus presenting great potential as a therapeutic target for the treatment of disorders such as obesity and at the other extreme, anorexia. Due to the initial lack of pharmacological tools that are active in vivo, functions of the different Y receptors have been mainly studied in knockout and transgenic mouse models. However, over recent years various Y receptor selective peptidic and non-peptidic agonists and antagonists have been developed and tested. Their therapeutic potential in relation to treating obesity and other disorders of energy homeostasis is discussed in this review.

Feeding behavior as seen through the prism of brain microdialysis

The knowledge of feeding behavior mechanisms gained through brain microdialysis is reviewed. Most of the chemical changes so far reported concern to the limbic system in rodents. A picture showing increases and decreases of extracellular neurotransmitters correlating to different aspects of feeding behavior is gradually emerging. Depending on the region, the same neurotransmitter may signal opposite aspects of feeding. Dopamine (DA) in the nucleus accumbens (NAC) correlates with food reward, stimulus saliency, and goal directed hyperlocomotion but in the ventromedial hypothalamus DA correlates with satiety and hypolocomotion. The findings accumulated in the last 25 years suggest that the control of a particular function relies on the interaction of several neurotransmitters rather than on a single neurotransmitter. The poor sensitivity of most analytical techniques hinders time and spatial resolution of microdialysis. Therefore, neurochemical correlates of short lasting behaviors are hard to figure out. As new and more sensitive analytical techniques are applied, new neurochemical correlates of feeding show up. Sometimes the proper analytical techniques are simply not available. As a consequence, critical signals such as neuropeptides are not yet completely placed in the puzzle. Despite such limitations, brain microdialysis has yielded a great deal of knowledge on the neurochemical basis of feeding.

Central administration of NPY or an NPY-Y5 selective agonist increase in vivo extracellular monoamine levels in mesocorticolimbic projecting areas

Selective NPY-Y5 antagonists are known to reduce NPY-evoked increase of food intake under free feeding conditions and drug-reinforced operant responding in rodents suggesting that NPY-Y5 receptors can regulate reinforcers, potentially by modulating the hypothalamic-limbic reward system. However, evidence published to date has revealed a limited expression of NPY-Y5 in the limbic areas. Thus, the first aim of the present study was to investigate the distribution of NPY-Y5 receptor binding sites in rat mesocorticolimbic projection areas such as the nucleus accumbens (NAc), medial prefrontal cortex (mPFC), and lateral hypothalamus (LH). Since mesocorticolimbic release of monoamines has been typically associated to the rewarding and motivational significance of reinforcers, we then compared the ability of NPY and an NPY-Y5 selective agonist, [cPP1-7,NPY19-23,Ala31,Aib32,Gln34]hPP, to evoke changes in extracellular monoamines from these brain regions using in vivo microdialysis techniques. Intracerebral doses of each compound were selected on the basis of those previously demonstrated to trigger food intake in a separate set of animals. We found that NPY-Y5 receptors were widely distributed in both the NAc and mPFC but not in the LH nuclei. Central administration of either NPY (4.5 nmol/rat) or the NPY-Y5 agonist (0.6 nmol/rat) induced a significant increase of dopamine (DA) output of up to 150% of basal values in the NAc. In addition, NPY induced a stepped increase of norepinephrine (NE) outflow in the NAc area. Also extracellular levels of NE levels were increased by both treatments in the mPFC (150% vs basal concentration). Hypothalamic monoamine levels were unaffected by both treatments. Extracellular serotonin (5-HT) levels were also unchanged in all regions. Given the NPY-Y5 agonist paralleled the in vivo ability of NPY to increase DA, these data suggest that the release of NPY may modulate behaviours associated to accumbal DA release such reward and reinforcement by, at least in part, acting on mesocorticolimbic NPY-Y5 receptors.

Neuropeptide Y and alpha-melanocyte-stimulating hormone: interaction in obesity and possible role in the development of hypertension

AIM

Obesity and hypertension frequently coexist and both represent important risk factors for cardiovascular disease. The mechanisms implicated in the regulation of food intake have not been completely elucidated. Recent data suggests that peripheral and central neuropeptides play an important role in the maintenance of energy balance. More specifically, leptin, neuropeptide Y (NPY) and alpha-melanocyte-stimulating hormone (a-MSH) appear to be implicated in the pathogenesis of obesity and also contribute to the development of hypertension in obesity.

METHODS

Analysis of the pertinent bibliography published in PubMed database.

RESULTS

Leptin is produced in the adipose tissue directly correlated with fat tissue mass. Leptin acts on two distinct neural populations in the hypothalamus: the first expresses the orexigenic peptides NPY and agouti-related protein (AgRP), the second pro-opiomelanocortin (POMC). The activation of POMC neurons increases the production of the anorexigenic hormone a-MSH and inhibits the release of NPY and AgRP. In addition, the hypothalamus integrates the neuroendocrine systems with the autonomic nervous system and controls the activity of the latter. Stimulation of hypothalamic nuclei elicits sympathetic responses including blood pressure elevation. Both NPY and a-MSH appears to be implicated in the hypothalamic regulation of sympathetic nervous system (SNS) activity.

CONCLUSION

Alterations in leptin, NPY and a-MSH are frequently observed in obesity and might stimulate SNS activity, contributing to the development of hypertension in obese patients. These neuropeptides might provide a pathophysiologic link between excess weight and hypertension. However, more research is needed before the pharmacologic manipulation of these complex neuroendocrine systems can be applied in the treatment of obesity and hypertension.

Sigma 1 receptor-mediated increase in hippocampal extracellular dopamine contributes to the mechanism of the anticonvulsant action of neuropeptide Y

The potent anticonvulsant properties of neuropeptide Y (NPY) are generally attributed to a Y2 receptor-mediated inhibition of glutamatergic synaptic transmission. Independent studies have shown that NPY increases brain dopamine content, possibly via interaction with sigma 1 receptors. Recently, we showed that increased extracellular hippocampal dopamine attenuates pilocarpine-induced limbic seizures via activation of hippocampal D2 receptors. Our aim in this study was to elucidate the role of increased hippocampal dopamine in the mechanism of the anticonvulsant action of NPY and to investigate the involvement of Y2 and sigma 1 receptors in this process. Limbic seizures were evoked in freely moving rats by intrahippocampal administration of pilocarpine via a microdialysis probe. NPY was administered intracerebroventricularly, intrahippocampally via the microdialysis probe, or coadministered intrahippocampally with the D2 receptor antagonist remoxipride, the Y2 receptor antagonist BIIE0246 or the sigma 1 receptor antagonist BD1047. Changes in hippocampal extracellular dopamine were monitored, and behavioural changes indicative of seizure activity were scored. Intracerebroventricular (10 nmol/3 microL) and intrahippocampal (20-50 microm) NPY administration increased hippocampal dopamine and attenuated pilocarpine-induced seizures. Hippocampal D2 receptor blockade (4 microm remoxipride) reversed the anticonvulsant effect of NPY. Y2 receptor blockade (1 microm BIIE0246) reversed the anticonvulsant effect of NPY but did not prevent NPY-induced increases in hippocampal dopamine. Sigma 1 receptor blockade (10 microm BD1047) abolished NPY-induced increases in hippocampal dopamine and reversed the anticonvulsant effect of NPY. Our results indicate that NPY-induced increases in hippocampal dopamine are mediated via sigma 1 receptors and contribute to the anticonvulsant effect of NPY via increased activation of hippocampal D2 receptors. This novel mechanism of anticonvulsant action of NPY is separate from, and may be complementary to, the well established Y2 receptor-mediated inhibition of hippocampal excitability.

TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline) inhibits the consumption of “snacks” in mice

There is considerable evidence that alpha2-adrenergic receptor activity exerts a pivotal role in initiation of feeding behavior. The appetite suppressant and monoamine release effects of TDIQ (5,6,7,8-tetrahydro-1,3-dioxolo[4,5-g]isoquinoline), a putative selective alpha2-adrenergic compound, were compared to those of fenfluramine, a reference drug that produces an anorectic effect via presynaptic release and reuptake inhibition of serotonin. The drugs were administered to two groups of mice that had learned to consume either sweet milk or chocolate pellets (i.e. "snacks") during the low-activity/reduced-feeding "light" portion of their light/dark cycle. The selectivity of the drugs to suppress the consumption of snacks was determined by comparing doses of each drug that inhibited the animals' consumption of snacks to doses of each drug that have been shown, or were shown, to impact the motor (i.e. locomotor, rotarod, and inclined-screen side effect-like tests) or conditioned taste aversion (CTA) behavior of mice. An evaluation of TDIQ as a releaser of monoamines was determined in rodent brain synaptosomes. The administration of TDIQ or fenfluramine inhibited the consumption of the snacks, and a comparison of their ED50 doses indicated that TDIQ is about 3 times more potent than fenfluramine (1.3 mg/kg vs. 4.2 mg/kg, respectively) in the sweet milk test and almost equipotent to fenfluramine (19.4 mg/kg vs. 18.4 mg/kg, respectively) in the chocolate pellet assay. The selectivity of the appetite suppressant effect of TDIQ was differentiated from that of fenfluramine on the basis that TDIQ exhibited a wide separation between its dose-response effects that suppressed snack consumption and its minimal effects in tests that measured behavioral impairment. Moreover, TDIQ was distinguished from fenfluramine in that it displayed very low potencies as a presynaptic releaser of monoamines. Finally, TDIQ (0.3 mg/kg-30.0 mg/kg) did not induce a conditioned taste aversion. TDIQ may represent a novel chemical entity that exhibits a significantly favorable therapeutic-like (i.e. appetite suppressant) effect to side effect-like ratio.

Neuropeptide Y, alpha-melanocyte-stimulating hormone, and monoamines in food intake regulation

Obesity is increasing in severity and prevalence in the United States and represents a major public health issue. No effective pharmacologic treatment leading to sustained weight loss currently exists. The growing interest in the regulation of food intake stems from the current drug treatments for obesity, almost all of which interfere with the monoamine system. Our knowledge of potential interactions between the orexigenic and anorexigenic pathways is limited and fragmented, making the development of targeted drug therapy for obesity difficult. The present review of the interaction of neuropeptides and monoamines emphasizes the complexity of the central mechanisms that regulate feeding behavior. Two main systems are implicated in food intake regulation: neuropeptide Y (NPY) and pro-opiomelanocortin. alpha-Melanocyte-stimulating hormone is a tridecapeptide cleaved from pro-opiomelanocortin that acts to inhibit food intake. The predominant NPY orexigenic receptors are NPY-Y1 and NPY-Y5, and the two anorexigenic melanocortin receptors involved in hypothalamic food intake control are MC3-R and MC4-R. Both neuropeptides interact with monoamines in the hypothalamus to control physiologic states such as hunger, satiation, and satiety. Serotonin suppresses food intake and body weight, acting mainly through the serotonin 1B receptor. Dopamine regulates hunger and satiety by acting in specific hypothalamic areas, through the D1 and D2 receptors. Noradrenaline activation of alpha1- and beta2-adrenoceptors decreases food intake, and stimulation of the alpha2-adrenoceptor increases food intake. A better understanding of the detailed mechanisms underlying the pathogenesis of hyperphagia and hypophagia is needed to develop new therapeutic approaches to obesity.

Norepinephrine and the control of food intake

The focus of the present review is to reconsider the role of endogenous norepinephrine (NE) in brain, specifically within the hypothalamic paraventricular nucleus (PVN), with regard to its potential role in eliciting eating or satiety. The PVN is innervated by NE fibers and is a site at which infusion of exogenous NE elicits eating at low doses. Two subtypes of alpha-adrenergic receptors within the PVN exert antagonistic actions on eating in the rat: activation of PVN alpha(2)-adrenoceptors increases eating, whereas activation of PVN alpha(1)-adrenoceptors suppresses eating. Pharmacologic manipulations that elevate NE can increase or decrease food intake, depending on the site and type of NE manipulation. Certain antiobesity drugs may act to reduce eating via release of NE and subsequent activation of alpha(1)-adrenoceptors. The PVN exhibits a reliable rhythm in the secretion of endogenous NE over the dark-and-light cycle, and this rhythm may interact with changes in numbers of PVN alpha(1)- and alpha(2)-adrenoceptors to modulate eating during the dark-and-light cycle. Push-and-pull and microdialysis studies indicate that NE secretion is strongly associated with eating, particularly at the start of the dark phase. The present review considers potential interactions of NE with substances such as leptin and neuropeptide Y that alter eating.

[D-Trp(34)] neuropeptide Y is a potent and selective neuropeptide Y Y(5) receptor agonist with dramatic effects on food intake

The neuropeptide Y (NPY) Y(5) receptor has been proposed to mediate several physiological effects of NPY, including the potent orexigenic activity of the peptide. However, the lack of selective NPY Y(5) receptor ligands limits the characterization of the physiological roles of this receptor. Screening of several analogs of NPY revealed that [D-Trp(34)]NPY is a potent and selective NPY Y(5) receptor agonist. Unlike the prototype selective NPY Y(5) receptor agonist [D-Trp(32)]NPY, [D-Trp(34)]NPY markedly increases food intake in rats, an effect that is blocked by the selective NPY Y(5) receptor antagonist CGP 71683A. These data demonstrate that [D-Trp(34)]NPY is a useful tool for studies aimed at determining the physiological roles of the NPY Y(5) receptor.

Increased NPY activity in the PVN contributes to food-restriction induced reductions in blood pressure in aortic coarctation hypertensive rats

We hypothesized that hypothalamic NPYergic mechanisms mediate the blood pressure lowering effect of caloric restriction in hypertensive rats. Aortic coarctation-induced (AC) hypertensive rats (n=25) were assigned to either an ad libitum fed control group (AL) or food restricted group (FR; 60% of AL consumption) for 3 weeks. Rats were instrumented chronically with vascular catheters and bilateral guide cannulae directed at the paraventricular hypothalamic nuclei (PVN). Blood pressure (BP) and heart rate (HR) responses to bilateral PVN microinjection of saline (200 nl) or the putative NPY receptor antagonists [D-Trp32]NPY(1-36) (3.3 micrograms/200 nl) and [D-Tyr27,36 Thr32]NPY(27-36) (D-NPY(27-36); 3.3 micrograms/200 nl) were determined. The FR rats were then refed and cardiovascular responses to PVN injections of NPY receptor antagonists were again determined. FR rats had significantly reduced resting BP (159+/-4 vs. 129+/-4 mmHg) and HR (360+/-11 vs. 326+/-9 bpm) compared to AL controls. Refeeding restored BP and HR of FR rats to levels similar to AL (BP=153+/-4 mmHg, HR=359+/-11 bpm). PVN administration of [D-Trp32]NPY produced foraging behavior and concurrent increases in BP and HR in FR, AL and Re-fed rats. The behavioral activation suggests that [D-Trp32]NPY(1-36) produced activation of NPY receptors. In contrast, D-NPY (27-36) did not produce any behavioral response or affect BP or HR in AL or Re-fed rats. In FR rats, D-NPY (27-36) produced significant increases in BP (peak=15+/-3 mmHg) which partially reversed the effect of FR on BP. Thus, in FR rats with reduced BP, PVN administration of an NPY receptor antagonist increases BP. NPY blockade in the PVN accounted for about 50% of the BP effect of food restriction, thus other mechanisms are likely to be involved. These findings are consistent with the hypothesis that NPYergic mechanisms may contribute to the reduction of BP produced by food restriction.

Thioperamide, a histamine H3 receptor antagonist, powerfully suppresses peptide YY-induced food intake in rats

BACKGROUND

Whether or not peptide YY (PYY)-induced hyperphagia is modified by the histaminergic system in the brain is not yet known.

METHODS

We investigated the effect on feeding of intracerebroventricular (ICV) administration of a specific histamine H3 receptor antagonist prior to ICV administration of PYY in rats.

RESULTS

PYY (1, 3, and 10 micrograms/10 microL) strongly induced feeding behavior in a dose-dependent manner in sated rats. The 4-hour food intake induced by 3 micrograms/10 microL of PYY was equal to that induced by a 16-hour fast. The ICV administration of thioperamide (40.8, 122.4, and 408.5 micrograms/10 microL) did not suppress the 4-hour food intake induced by 16-hour fasting; however, thioperamide produced dose-dependent and strong inhibition of hyperphagia induced by a 3-microgram dose of PYY.

CONCLUSIONS

These results suggest that the effect of PYY on appetite is different than that induced by fasting and may involve a histaminergic mechanism.

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.

Thioperamide, a histamine H3 receptor antagonist, suppresses NPY-but not dynorphin A-induced feeding in rats

Whether or not neuropeptide Y (NPY)-induced feeding in rats is influenced by the histaminergic system in the brain was investigated by intracerebroventricular (i.c.v.) administration of a selective histamine H3 receptor antagonist prior to i.c.v. administration of NPY. NPY (10 microg/10 microl) strongly induced feeding in sated rats during the light phase of the day. Dynorphin A1-17 (10 microg/10 microl), a kappa-opioid agonist, and rat pancreatic polypeptide (rPP, 30 microg/10 microl) also stimulated ingestive behavior in sated rats, but food intake in both cases was less than that induced by NPY. Thioperamide maleate, a specific histamine H3 receptor antagonist (408.5 microg/10 microl) reduced the feeding response to NPY by 52% (P < 0.0001), but not to dynorphin A1-17 and rPP. Thioperamide at i.c.v. doses of 40.8-408.5 microg/10 microl had no effect on food intake in sated rats. These results suggest that the thioperamide may have a specific effect on NPY receptor-mediated neuronal systems related to feeding.

The pharmacology of neuropeptide Y (NPY) receptor-mediated feeding in rats characterizes better Y5 than Y1, but not Y2 or Y4 subtypes

Thirteen neuropeptide Y (NPY) agonists were administered intracerebroventricularly (i.c.v.) in rats (full dose-response curves) to estimate their half-effective dose (ED50) on feeding. These values were compared to their binding affinities (IC50) for rat NPY receptor subtypes Y1, Y2, Y4 and Y5 in vitro. Correlations between in vivo ED50 and in vitro IC50 were strong for the Y5 (r = 0.87; P < 0.01), weak for the Y1 (r = 0.48; P < 0.04) and non-significant for the Y2 and Y4 receptor subtypes. In vitro, h[D-Trp32]NPY was found to be a Y5-selective ligand and a full agonist in Y5-expressing cells. In vivo, it dose-dependently stimulated feeding, but failed to induce the full maximal response observed with pNPY. It did not antagonize pNPY-induced feeding and overfeeding in 24 h fasted rats. These findings demonstrate a role for the Y5, or possibly Y5 in combination with Y1, but not Y2 or Y4 receptor subtypes in feeding. No evidence was found for the existence of an additional, as yet undescribed, NPY feeding receptor.

Stimulation of feeding in lean but not in obese Zucker rats by a selective neuropeptide Y Y5 receptor agonist

Obese Zucker rats are characterized by a reduced hypothalamic NPY receptor density. We tested the effects of intracerebroventricular injections of human NPY (hNPY) and [D-Trp32]NPY, a weak but selective NPY Y5 receptor agonist, on food intake in lean and obese Zucker rats. The effect of a maximal dose of hNPY (10 microg) on feeding was more pronounced in lean than in obese rats. [D-Trp32]NPY (10 microg) stimulated feeding in lean but not in obese Zucker rats. It did not affect the feeding response to hNPY, excluding the activation of an inhibitory receptor. These results are in favor of a down-regulation of the NPY 'feeding' receptor in the obese rat, which is suggested to be the Y5 subtype.

Evidence for involvement of neuropeptide Y receptors in the regulation of food intake: studies with Y1-selective antagonist BIBP3226

1. Experiments were conducted to evaluate the effects of the novel non-peptide neuropeptide Y Y1 receptor antagonist, BIBP3226 (N2-(diphenylacetyl)-N-[(4-hydroxy-phenyl)methyl]-D-arginine amide) on spontaneous, fasting-induced and NPY-induced food intake in rats. In addition to consumption of regular chow, the effects of BIBP3226 on consumption of highly palatable sweetened mash were monitored in a 1 h test on first exposure and after familiarization with novel food. 2. BIBP3226 (10.0 nmol, i.c.v.) had no effect on the consumption of regular chow, but reduced significantly the intake of highly palatable diet and the food intake stimulated by fasting (24 h). Neuropeptide Y (NPY, 1.0 nmol, i.c.v.) significantly increased the consumption of regular rat chow. This orexigenic effect of NPY was blocked by BIBP3226 (10.0 nmol, administered i.c.v. 5 min before NPY) at 30 min and 4 h, but not at 1 and 2 h. When animals were pretreated with diazepam (0.5 mg kg(-1), i.p., 20 min before NPY), BIBP3226 failed to suppress NPY-induced feeding. 3. An NPY Y1 and Y3 receptor agonist, [Leu31,Pro34]NPY and a Y5 receptor agonist human peptide YY3-36 (hPYY3-36, both 30 pmol), microinjected into the paraventricular nucleus of the hypothalamus (PVN) increased the consumption of regular rat chow. BIBP3226 (0.4 nmol, into the PVN) completely blocked the stimulatory effect of [Leu31,Pro34]NPY but not that of hPYY3-36. BIBP3226 (0.4 nmol) alone failed to modify the consumption of the regular chow. Higher doses of BIBP3226 (1.0 and 2.0 nmol) injected into the vicinity of the PVN reduced the consumption of the sweetened mash. 4. These results suggest that both the NPY Y1 and Y5 receptors in the PVN are involved in the regulation of food intake. The stimulatory effect of exogenous NPY is probably mediated through an NPY receptor subtype that is not identical with the Y1 receptor (possibly Y5 receptor). However, the NPY Y1 receptors may mediate the effect of endogenous NPY in conditions of increased energy demand or on intake of highly palatable diets.

Regulation of neurotransmission in the arcuate nucleus of the rat by different neuropeptide Y receptors

We examined the effects of peptides of the neuropeptide Y (NPY)/pancreatic polypeptide (PP) family on synaptic transmission in the arcuate nucleus in rat hypothalamic slices. Application of NPY produced two effects. In some cells NPY produced an outward current that had the properties of a K+ current. NPY also inhibited the evoked glutamatergic EPSC recorded in these arcuate neurons by a presynaptic mechanism. Although the effects of NPY on the K+ current reversed within a few minutes of washout of the peptide, its effects on the EPSC frequently were longer lasting (>30 min). Similar effects were observed using peptide YY or the NPY analog [Leu31, Pro34]NPY. Although K+ current activation by [Leu31,Pro34]NPY was blocked by the selective Y1 antagonist BIBP 3226, inhibition of the EPSC was blocked only partially. Other NPY-related peptides such as NPY(13-36), PP, and [D-Trp32]NPY also inhibited the EPSC. However, none of these peptides produced activation of the K+ current. Thus, activation of more than one NPY receptor produces synaptic inhibition in the arcuate nucleus. A Y1 receptor activates a K+ current postsynaptically, and several receptor types appear to inhibit the EPSC by a presynaptic mechanism.