Olfactory bulbectomy increases food intake and hypothalamic neuropeptide Y in obesity-prone but not obesity-resistant rats

Obese individuals often suffer from depression. The olfactory bulbectomy (OBX) model is an animal model of depression that produces behavioral, physiological, and neurochemical alterations resembling clinical depression. The OBX model was employed to assess depression-related changes in food intake in obesity-prone, Osborne–Mendel (OM) rats and obesity-resistant, S5B/Pl rats. OBX increased food intake in OM rats beginning 7 days following surgery, however, OBX did not alter food intake in S5B/Pl rats at any time point. Fourteen days following surgery, OBX significantly increased locomotor activity (total lines crossed and rears) in the openfield test in OM and S5B/Pl rats. Fifteen days following surgery, prepro-neuropeptide Y (NPY) mRNA levels were significantly increased in the hypothalamus of bulbectomized OM rats and in the medial nucleus of the amygdala of bulbectomized OM and S5B/Pl rats. OBX decreased NPY Y2 receptor mRNA levels in the hypothalamus and medial nucleus of the amygdala in OM rats, while increasing NPY Y2 receptor mRNA levels in the medial nucleus of the amygdala of S5B/Pl rats. These data indicate that though both obesity-prone and obesity-resistant strains were susceptible to the locomotor effects of OBX, food intake and hypothalamic prepro-NPY mRNA were only increased in OM rats. Therefore, strain specific alterations in hypothalamic NPY may account for increased food intake in the obesity-prone rats following OBX, and suggests a potential mechanism to explain the comorbidity of obesity and depression.     

Mapping of hypothalamic sites involved in the effects of NPY on body temperature and food intake

The objective of the present study was to identify hypothalamic sites that might be implicated in the effects of neuropeptide Y (NPY) on both body temperature and food intake. For this purpose, the effects of direct microinjections of NPY in several doses (0.156–20 μg) into discrete hypothalamic nuclei on body temperature were examined in rats. To examine specificity of effects, food consumption of animals following injections was also measured. Results indicate that the influence of NPY on body temperature varies with the hypothalamic region where the peptide is administered. NPY had no effect on temperature after administration into the ventromedial (VMH) and the perifornical hypothalamus (PeF). However, a significant hypothermia was seen following administration into the preoptic (POA) and arcuate nucleus (Arc), and hyperthermia was seen after injection into the paraventricular nucleus (PVN). Finally, a biphasic effect was observed after injection into the lateral hypothalamus (LH): hyperthermia with relatively small doses and hypothermia with higher doses. Similar effects were obtained when administred into the third ventricle (3V) but in an inverted dose-related fashion: hypothermia at low and hyperthermia at higher doses. For feeding, NPY consistently increased food intake in all regions examined, with the strongest effect obtained after administration into the PeF. The present results clearly dissociate the effects of NPY on food intake and body temperature, and demonstrate that these effects are related to specific hypothalamic nuclei.     

Effects of NPY and NPY2–36 on body temperature and food intake following administration into hypothalamic nuclei

Our previous in vivo structure-activity studies suggested that the putative receptors mediating the effects of NPY and NPY_2–36 on food intake and body temperature are pharmacologically different [17]. In the present study, we examined and compared dose-related effects of NPY and NPY_2–36 on ad lib food intake and rectal temperature after administration into discrete hypothalamic nuclei of the rat. Results indicate that NPY and NPY_2–36 have opposite effects on body temperature to those of NPY when injected in the preoptic area (POA): hypothermia and hyperthermia, respectively. When administered in the paraventricular nucleus (PVN), both increased body temperature. When injected into the third ventricle (3V), NPY produced a biphasic effect: hypothermia at low doses and hyperthermia at high doses. Similar effects were obtained with NPY_2–36, but in an inverted dose-related fashion: hyperthermia at low and hypothermia at higher doses. In the arcuate nucleus (Arc), NPY induced a significant hypothermia whereas NPY_2–36 had no effect. Finally, neither peptide affected body temperature when injected into the ventromedial (VMH) and perifornical (PeF) nuclei. Both NPY and NPY_2–36 increased food intake after injection in all regions examined. In general, NPY was more potent and efficacious than NPY_2–36. The present results clearly dissociate the effects of NPY on food intake and body temperature. Furthermore, the data support the hypothesis that the putative receptors underlying the effects of NPY and NPY_2–36 on food intake are similar, whereas those mediating the effects on body temperature are pharmacologically different.     

Orexin-induced food intake involves neuropeptide Y pathway

Orexins (orexin-A and -B) are recently identified neuropeptides, which are thought to be implicated in the regulation of feeding behavior. We used a NPY-Y1 receptor specific antagonist, BIBO3304, to examine whether NPY is involved in orexin-induced feeding behavior. Intracerebroventricular administration of orexin-A (10 nmol) induced food intake in rats (food intake for 3 h; vehicle 0.3+/-0.2 g vs. orexin-A 10 nmol, 4.0+/-0.5 g, n=4). Orexin-induced feeding behavior was partially inhibited by prior administration of BIBO3304 (3 h food intake: orexin-A 10 nmol, 4.0+/-0.5 g vs. BIBO3304 (60 microgram) + orexin-A 10 nmol, 2.2+/-0.2 g, n=4). A low dose of BIBO3304 (30 microgram) did not show a significant inhibitory effect. BIBO3457, an inactive enantiomer, used as a negative control, did not show any inhibitory effect on orexin-A-induced feeding behavior. Fos expression was observed in NPY-containing neurons in the arcuate nucleus 1 h after orexin-A (10 nmol) was administered intracerebroventricularly (control 0.3+/-0.08%, orexin-A 10.2+/-0.8%, n=5 rats/group). These observations suggest that NPY is involved in orexin-induced feeding behavior. However, BIBO3304 did not completely abolish the effect of orexin-A. These results suggest that orexin-A elicits feeding behavior partially via the NPY pathway. The NPY system could be the one of downstream pathways by which orexin-A induces feeding behavior. Another pathway may also be involved in orexin-A-induced feeding behavior, because BIBO3304 did not completely abolish orexin-A-induced feeding behavior.     

Glucokinase inhibitor glucosamine stimulates feeding and activates hypothalamic neuropeptide Y and orexin neurons

Maintaining glucose levels within the appropriate physiological range is necessary for survival. The identification of specific neuronal populations, within discreet brain regions, sensitive to changes in glucose concentration has led to the hypothesis of a central glucose-sensing system capable of directly modulating feeding behaviour. Glucokinase (GK) has been identified as a glucose-sensor responsible for detecting such changes both within the brain and the periphery. We previously reported that antagonism of centrally expressed GK by administration of glucosamine (GSN) was sufficient to induce protective glucoprivic feeding in rats. Here we examine a neurochemical mechanism underlying this effect and report that GSN stimulated food intake is highly correlated with the induction of the neuronal activation marker cFOS within two nuclei with a demonstrated role in central glucose sensing and appetite, the arcuate nucleus of the hypothalamus (ARC) and lateral hypothalamic area (LHA). Furthermore, GSN stimulated cFOS within the ARC was observed in orexigenic neurons expressing the endogenous melanocortin receptor antagonist agouti-related peptide (AgRP) and neuropeptide Y (NPY), but not those expressing the anorectic endogenous melanocortin receptor agonist alpha-melanocyte stimulating hormone (α-MSH). In the LHA, GSN stimulated cFOS was found within arousal and feeding associated orexin/hypocretin (ORX), but not orexigenic melanin-concentrating hormone (MCH) expressing neurons. Our data suggest that GK within these specific feeding and arousal related populations of AgRP/NPY and ORX neurons may play a modulatory role in the sensing of and appetitive response to hypoglycaemia.     

Increased neuropeptide Y secretion in the hypothalamic paraventricular nucleus of obese (fa/fa) Zucker rats

NPY is synthesized in the hypothalamic arcuate nucleus (ARC), and NPY injected into the paraventricular nucleus (PVN), the main site of NPY release, induces hyperphagia and reduces energy expenditure. Hypothalamic NPY and mRNA and NPY levels are increased in fatty Zucker rats, consistent with increased NPY release. This could explain the hyperphagia and reduced energy expenditure, which lead to obesity in the fatty Zucker rat. We have therefore compared NPY secretion in the PVN of conscious fatty and lean Zucker rats using push-pull sampling. The NPY secretory profile was consistently higher in fatty Zucker rats than in lean rats throughout the 3-h study period (P < 0.01), and mean NPY secretion over the whole 3 h was increased 2-fold in the fatty rats (P < 0.001). We conclude that fatty Zucker rats have increased NPY release in the PVN. This observation further supports the hypothesis that increased activity of the NPYergic ARC-PVN pathway may contribute to obesity in the fatty Zucker syndrome.     

Nicotine administration decreases neuropeptide Y expression and increases leptin receptor expression in the hypothalamus of food-deprived rats

The effects of nicotine on the expressions of neuropeptide Y (NPY) and leptin receptor in the rat hypothalamus were investigated via immunohistochemistry. The results show that NPY expression is not affected in the arcuate nucleus (ARN) and is increased only slightly in the paraventricular nucleus (PVN) by nicotine administration under normal (i.e. fed) conditions and that leptin receptor expression is decreased slightly in the ARN and not affected in the PVN following nicotine treatment under the same conditions. Food deprivation enhanced NPY and suppressed leptin receptor expression in the ARN and PVN of the hypothalamus. Nicotine administration resulted in decreased NPY and increased leptin receptor levels.     

Central insulin inhibits hypothalamic galanin and neuropeptide Y gene expression and peptide release in intact rats

The central actions of insulin, on galanin (GAL) and neuropeptide Y (NPY) in the brain, are examined in intact satiated rats. Ventricular injections of insulin reduce both GAL and NPY gene expression and immunoreactivity in different hypothalamic areas but have no effect in extra-hypothalamic sites. Insulin applied to medial hypothalamic fragments in vitro significantly reduces GAL and NPY release. This evidence suggests that insulin acts centrally and directly on hypothalamic peptide activity under normal feeding conditions.     

Developmental increases in hypothalamic neuropeptide Y content with the embryonic age of meat- and layer-type chicks

We determined central neuropeptide Y (NPY) content of meat- and layer-type chicks at embryonic days 7, 14, 20, and at post-hatching day 1. The central NPY was detectable at day 7; hypothalamic NPY content developmentally increased with a similar pattern but a different level between both types of chicks. These results were discussed with respect to feeding behavior early period after hatching.     

Changes in neuropeptide Y immunoreactivity in the arcuate nucleus during and after food restriction in lactating rats

Previous studies have demonstrated that food restriction during lactation extends the length of lactational infertility in rats. In order to determine whether Neuropeptide Y (NPY) plays a role in the increased length of lactational infertility seen in food-restricted rats, NPY immunoreactivity in the arcuate nucleus of lactating rats that were ad lib fed or food restricted from Day 8 to 14 postpartum was compared. Food-restricted rats showed higher numbers of NPY-stained cells at the end of the food-restriction period (Day 15 postpartum) than did ad lib-fed rats at the same stage of lactation. This difference persisted until Day 25 postpartum although all animals were fed ad libitum from Day 15 onwards. Switching litters between ad lib-fed and food-restricted females from Day 15 until Day 20 postpartum did not eliminate the difference in NPY immunoreactivity between the two diet conditions. Given that food restriction during lactation leads to a prolonged suppression of LH release that also persists after refeeding and is not affected by the nutritional status of the litter, these data are consistent with a role for NPY in the prolonged suppression of reproductive function seen in food restricted lactating rats.     

The discriminative stimulus effects of neuropeptide Y

Neuropeptide Y (NPY), an endogenous peptide which strongly induces food intake, is demonstrated to have discriminative stimulus properties when administered intracerebroventricularly. Rats rapidly learned to press the appropriate lever during training. NPY discrimination was dose-dependent. NPY's discriminative stimulus properties were compared to those of two doses of Peptide YY (PYY) and 24 and 48 h of food deprivation, conditions which also increase feeding. Both doses of PYY generalized to NPY, supporting previous findings that PYY has effects similar to NPY. Although food deprivation increases feeding in a manner similar to NPY, food deprivation did not result in NPY-appropriate responding.     

Neuropeptide Y and food intake in fasted rats: effect of naloxone and site of action

Central administration of neuropeptide Y (NPY) induces food intake in freely feeding animals and this effect is mediated by hypothalamic sites. Little is known, however, about the effect of NPY on food intake and site of action in food-deprived animals. To examine this further, 24-h fasted rats received injections of saline or NPY into the lateral cerebral ventricle (10 micrograms/10 microliters; n = 8) or into the lateral (LH) or ventromedial hypothalamus (VMH) (1 microgram/0.5 microliters; n = 44). In addition, intracerebroventricular (i.c.v.) injections of NPY were carried out with or without i.c.v. naloxone (25 micrograms), a specific opioid receptor antagonist. During the first 40 min food intake was not different with or without NPY. After 60 and 120 min, food intake was 5.9 +/- 0.4 g and 8.3 +/- 0.6 g with i.c.v. saline which was significantly augmented by i.c.v. NPY to 8.7 +/- 0.9 g and 14.4 +/- 1.5 g, respectively (P less than 0.05). This increase in food consumption was due to a prolongation of feeding time. The opioid receptor antagonist naloxone significantly augmented latency to feed, both in the absence and presence of NPY (8.0 vs 1.7 min or 14.7 vs 2.8 min, respectively) and abolished the NPY-induced increase in food intake. Following intrahypothalamic injection of NPY, an increase in food intake (greater than 20%) was observed in 50% of the histologically identified LH and VMH sites, but only in 15% of the injection sites outside the LH/VMH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypothalamic neuropeptide Y, its gene expression and receptor activity: relation to circulating corticosterone in adrenalectomized rats

Previous evidence has suggested a possible relationship between the adrenal steroid, corticosterone (CORT) and neuropeptide Y (NPY) in the brain. To provide a more systematic analysis of this interaction, the present study employed a variety of techniques, including in sity hybridization to measure NPY gene expression, radioimmunoassay to examine peptide levels and radioligand [¹²⁵I]peptide YY (PYY) binding for analysis of peptide receptors. The results show that adrenalectomy (ADX), which caused a decline in CORT to levels < 0.3 μg%, has generally little impact on the hypothalamic NPY projection system under normal, basal conditions. This includes peptide gene expression or content in the area of its cell bodies (arcuate nucleus, ARC), in addition to peptide binding at its receptor sites. While it also includes peptide content at most hypothalamic terminal sites, there are three notable exceptions, namely, the medial paraventricular (PVN) and dorsomedial nuclei and medial preoptic area, where NPY nerve terminals and glucocorticoid receptors are particularly dense and the decline in CORT through ADX markedly reduces NPY content. In contrast, evidence obtained from CORT replacement in ADX rats shows that this steroid has profound impact on all components of the hypothalamic NPY system. This peptide-steroid interaction is apparent at the level of the cell body (ARC), as well as at the nerve terminal or receptor site (PVN and ARC), where CORT levels > 10 μg% strongly potentiate NPY gene expression, peptide content and radioligand binding. These and other findings suggest that this CORT-NPY interaction in the hypothalamus occurs physiologically under conditions, e.g., at the onset of the active feeding cycle, when circulating CORT normally rises.     

Weight loss in rats treated with intracerebroventricular cobalt protoporphyrin is not specific to the neuropeptide Y system

Cobalt protoporphyrin (CoPP) reduces food intake and body weight following intracerebroventricular (i.c.v.) administration in rats. We injected 0.2 μmol CoPP per kg body weight i.c.v. and monitored body weight and daily food intake for 7 days. The body weight and 24 h food intake of CoPP-treated animals was significantly lower than that of vehicle-treated animals in all studies (P < 0.01) from day 2 to day 7. The 2 h feeding response (CoPP vs. vehicle-treated) to 10 μg neuropeptide Y (NPY) (4.0 vs. 7.1 g;P < 0.05), the 1 h feeding response to 10 μg galanin (1.3 vs. 3.2 g;P < 0.05) and 30 μg norepinephrine (0.6 vs. 1.9 g;P < 0.05) in CoPP-treated animals were all reduced compared to the vehicle-treated group. In addition there was no change in hypothalamic NPY mRNA in CoPP-treated animals. I.c.v. COP decreases sensitivity to exogenous NPY, galanin and norepinephrine. The effect of COP is not specific to NPY as previously described.     

Involvement of hippocampal serotonin and neuropeptide Y in depression induced by chronic unpredicted mild stress

Accumulated evidence indicates a role of the hippocampal 5-hydroxy-tryptamine (5-HT) and neuropeptide Y (NPY) in the response to stress and modulation of depression, but it is unclear whether and how the hippocampal 5-HT and NPY systems make contributions to chronic unpredicted mild stress (CUMS)-induced depression. Here we observed that rats receiving a variety of chronic unpredictable mild stressors for 3 weeks showed a variety of depression-like behavioral changes, including a significant reduction in body weight, sucrose preference, and locomotion, rearing and grooming in open field test, and a significant increase in immobility time in forced swimming test. These CUMS-induced behavioral changes were suppressed or blocked by intra-hippocampal injection of 5-HT (31.25 μg/μl) or NPY (10 μg/μl). These data suggest a critical role of reduced hippocampal 5-HT and NPY neurotransmission in CUMS-induced depression.     

Evidence of a physiological role for neuropeptide Y in ventromedial hypothalamic lesion-induced hyperphagia

We evaluated the role of neuropeptide Y (NPY), a potent endogenous orexigenic signal, in the ventromedial hypothalamic (VMH) lesion-induced hyperphagia in rats. To produce hyperphagia and excessive weight gain, adult female rats received bilateral electrolytic or sham lesions in the VMH. Concurrently, a permanent intracerebroventricular cannula was implanted in the third ventricle of the brain. After a recovery period, these rats were passively immunized against NPY to evaluate the role of endogenous NPY on hyperphagia. The results showed that intraventricular administration of NPY antibodies abolished the hyperphagia in VMH-lesioned rats. These revelations are in agreement with the notion that altered hypothalamic NPY release or action may underlie the hyperphagia and excessive weight gain seen in response to structural damage in the VMH.     

Paradoxical sleep deprivation activates hypothalamic nuclei that regulate food intake and stress response

A large body of evidence has shown that prolonged paradoxical sleep deprivation (PSD) results in hypothalamic–pituitary–adrenal (HPA) axis activation, and in loss of body weight despite an apparent increase of food intake, reflecting increased energy expenditure. The flowerpot technique for PSD is an efficient paradigm for investigating the relationships among metabolic regulation and stress response. The purpose of the present study was to examine the mechanisms involved in the effects of 96 h of PSD on metabolism regulation, feeding behaviour and stress response by studying corticotrophin-releasing hormone (CRH) and orexin (ORX) immunoreactivity in specific hypothalamic nuclei. Once-daily assessments of body weight, twice-daily measurements of (spillage-corrected) food intake, and once-daily determinations of plasma adrenocorticotropic hormone (ACTH) and corticosterone were made throughout PSD or at corresponding times in control rats (CTL). Immunoreactivity for CRH in the paraventricular nucleus of the hypothalamus and for ORX in the hypothalamic lateral area was evaluated at the end of the experimental period. PSD resulted in increased diurnal, but not nocturnal, food intake, producing no significant changes in global food intake. PSD augmented the immunoreactivity for CRH and plasma ACTH and corticosterone levels, characterizing activation of the HPA axis. PSD also markedly increased the ORX immunoreactivity. The average plasma level of corticosterone correlated negatively with body weight gain throughout PSD. These results indicate that augmented ORX and CRH immunoreactivity in specific hypothalamic nuclei may underlie some of the metabolic changes consistently described in PSD.     

Elevated hypothalamic neuropeptide Y levels in rats with dorsomedial hindbrain lesions

Lesions centered on the area postrema (AP) and adjacent nucleus of the solitary tract (AP/mNTS-lesions) are reported to result in increased consumption of highly palatable diets. Recent studies suggest that neuropeptide Y (NPY) may cause a preference for carbohydrate-rich diets. Thus, it is possible that NPY may play a role in the enhanced intake of highly palatable diets by AP/mNTS-lesioned rats. In the studies reported here, we found that lesions centered on the AP result in increased levels of NPY-immunoreactivity in the paraventricular nucleus of the hypothalamus. Additionally, steady-state NPY mRNA in the basomedial hypothalamus including the arcuate nucleus was elevated. Enhanced NPY was not found throughout the hypothalamus however, as NPY-immunoreactivity was not elevated in the lateral hypothalamus or the tissue bordering the anteroventral third ventricle. These data suggest the possibility that elevated hypothalamic NPY, particularly in the arcuate and paraventricular nuclei, may contribute to the altered food intake and energy balance observed in rats with lesions centered on the AP.     

c-fos expression in the rat brain following central administration of neuropeptide Y and effects of food consumption

Administration of neuropeptide Y (NPY) intracerebroventricularly (i.c.v.) results in the release of a number of hypothalamic and pituitary hormones and stimulation of feeding and suppression of sexual behavior. In this study, we sought to identify cellular sites of NPY action by evaluating perikaryal Fos-like immunoreactivity (FLI), a marker of cellular activation, in those hypothalamic and extrahypothalamic sites previously implicated in the control of neuroendocrine function and feeding behavior. Additionally, we compared the topography of FLI in these brain sites when food was either available ad libitum or withheld after NPY injection (1 nmol/3 μl, i.c.v.). The results showed that one hour after NPY injection a large number of cells in the parvocellular region of the paraventricular nucleus (PVN) were FLI-positive in the absence of food consumption. However, in association with food intake, a significant number of cells were intensely stained in the magnocellular region of the PVN. An analogous increase in FLI in association with feeding was apparent in the supraotic nucleus (SON), the dorsomedial nucleus and the bed nucleus of the stria terminalis in the hypothalamus. Anong the extrahypothalamic sites, feeding facilitated FLI in a large number of cells located in the lateral subdivision of the central amygdaloid nucleus and the lateral subdivision of the solitary tract. FLI was observed in a moderate number of cells in the hypothalamic arcuate nucleus (ARC) and ventromedial nucleus, and this response was not changed by feeding. Cumulatively, these results show that neurons in a number of discrete hypothalamic and extrahypothalamic sites, previously implicated in the control of neuroendocrine function and feeding behavior, are activated by NPY and further, a divergent pattern of c-fos expression emerged in some of these sites if feeding occurs after NPY injection. Stimulation of FLI in cells of the PVN, SON and ARC by NPY imply the presence of NPY target cells that play a role in the neuroendocrine control of pituitary function. The finding that NPY induced FLI in cells located in the parvocellular subdivision of the PVN even in the absence of feeding, imply that cells involved in initiation of food intake by NPY may reside in this subdivision of the PVN. On the other hand, the appearance of Fos-cells in the magnocellular subdivision of the PVN in response to feeding, suggests neural mechanisms that operate during the post-ingestion period, including those associated with termination of NPY-induced feeding, may impinge upon this subdivision of the PVN.     

Antinociceptive effects induced by intra-periaqueductal grey administration of neuropeptide Y in rats

Hindpaw withdrawal latency (HWL) to thermal and mechanical stimulation increased dose-dependently after intra-periaqueductal grey (PAG) injection of neuropeptide Y (NPY). Furthermore, the NPY-induced increases in HWLs were attenuated by intra-PAG injection of the Y1 receptor antagonist NPY28-36. The results demonstrated that NPY plays an important role in antinociception in PAG, in which Y1 receptor is involved in.