Rapid and localized alterations of neuropeptide Y in discrete hypothalamic nuclei with feeding status

Neuropeptide Y (NPY) is believed to regulate the normal eating behavior and body weight in rats via central mechanisms. We have investigated whether NPY, which stimulates food intake, may in turn be modified by the nutritional state of the animals. Thus the impact of food deprivation (FD) (48 h) and subsequent refeeding on the levels of NPY in discrete hypothalamic areas was examined in this study. The results showed site specific change in only 3 of 7 hypothalamic sites. A 5-fold increment in NPY was reported in the paraventricular nucleus (PVN) and a 10-fold increase was observed in the arcuate nucleus-median eminence (ARC-ME). While subsequent refeeding for 6 h reversed the effect of FD in the ARC-ME, the levels of NPY in the PVN remained high in the refed rats. The perifornical lateral hypothalamus displayed a different pattern, namely, a significant increase in NPY content in refed as compared to satiated and deprived rats. The NPY levels in 4 other hypothalamic sites, namely, the dorsomedian, ventromedian, supraoptic and suprachiasmatic nuclei, and two extrahypothalamic sites, namely caudate nucleus and nucleus accumbens, showed total resistance to any change following deprivation and refeeding. These data emphasize the important and specific role of the paraventricular and arcuate nuclei in NPY's regulation of food intake and provide support for the idea that the variations of hypothalamic NPY after food deprivation reflect a specific physiological response of feeding regulatory system to alterations in the animal nutritional state and body weight.     

Hypothalamic neuropeptide Y gene expression in rats on scheduled feeding regimen.

Recent evidence suggests that neuropeptide Y (NPY) is an important signal in the neural circuitry that controls feeding behavior. Previously we observed that in rats entrained to 4 h daily scheduled feeding regimen (SFR), NPY content and release in the paraventricular nucleus (PVN) was elevated but decreased rapidly in association with food consumption. In the present study, we investigated the pattern of hypothalamic NPY gene expression in SFR rats before and after food consumption by measuring the content of preproNPY mRNA in the medial basal hypothalamus (MBH). Adult male rats were maintained on either ad libitum diet (control) or on SFR. Rats were killed before food presentation at 11.00 h and at the end of 4 h food consumption at 15.00 h. The levels of preproNPY mRNA in the MBH were determined by solution hybridization/RNase protection assay using a cRNA probe complementary to rat NPY precursor mRNA. We observed that, as compared to that in control rats on ad libitum diet, preproNPY mRNA levels in the MBH were increased two-fold in the SFR rat at 11.00 h and remained elevated even after 4 h of food consumption. These results show a simultaneous enhancement in PVN NPY release and hypothalamic gene expression in advance of scheduled feeding time, but food intake rapidly decreases PVN NPY release and content, with little impact on hypothalamic gene expression.     

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.     

Gene expression of neuropeptide Y in the nucleus of the solitary tract is activated in rats under restricted daily feeding but not under 48-h food deprivation

The two neuropeptide Y (NPY) systems innervating the hypothalamic paraventrivular nucleus were examined regarding their roles in the prefeeding corticosterone peak developed under restricted daily feeding (RF). Protein and mRNA levels of NPY were measured in the arcuate nucleus (ARC) and the nucleus of the solitary tract (NST) in rats under 48-h food deprivation (48-hFD), RF, and 72-h food deprivation imposed after RF (post-RF 72-hFD) with 7 days of ad libitum feeding in between. NPY protein and mRNA levels in the ARC significantly increased with 48-hFD and decreased with re-feeding, whereas those in the NST were not changed by 48-hFD. When rats had RF imposed with free access to food from 10.00 to 12.00 h (lights on from 06.00 to 18.00 h) for 3 weeks, NPY concentrations in the ARC increased at 10.00 h, just prior to the daily meal, but those in the NST did not change significantly throughout the period examined. On the other hand, NPY mRNA levels in both the ARC and NST increased before the meal supply and remained high for 4 h after feeding. Under post-RF 72-hFD, the prefeeding peak of NPY mRNA was detected in the NST, but NPY mRNA levels in the ARC were continuously high throughout the 24-h period. These findings indicate that the NPY neurons from the NST are specifically activated by RF, whereas those from the ARC are generally stimulated by an increased food demand.     

Neuropeptide Y (NPY) May Integrate Responses of Hypothalamic Feeding Systems and the Hypothalamo-Pituitary-Adrenal Axis

Neuropeptide Y (NPY) is a powerful stimulus to food intake in the rat. Exogenous NPY given into the third ventricle or into the paraventricular nucleus (PVN) of the hypothalamus stimulates both food consumption as well as the hypothalamus-pituitary-adrenal (HPA) axis. Presumably NPY activates the adrenocortical system through direct stimulation of CRF containing cells in the PVN. Food intake is also a major regulator of adrenocortical activation. Rhythms in HPA axis activity follow rhythms in food consumption, and rats that have been food deprived overnight have inhibited HPA axis responses to restraint stress and corticosteroid feedback the following morning. To investigate the interaction of NPY with both feeding and HPA axis activation three sets of experiments were performed: Animals fed ad lib were injected icv with NPY (2.5 μg) and allowed access to food or not post injection; animals were fasted overnight prior to NPY injection; finally, dose response experiments were performed to examine the relative sensitivities of feeding and HPA axis activation to exogenous NPY. Ad lib fed animals allowed access to food after NPY injection had slightly greater ACTH responses to NPY while glucocorticoid and insulin responses were not significantly different from ad lib fed animals not allowed access to food post injection. Animals allowed to eat post injection had significantly decreased food consumption the night following injection, however, total 24 h food consumption was not different between these animals and those given food 8 h post NPY injection. In overnight fasted animals NPY injections produced ACTH responses of equal magnitude to those in ad lib fed animals. Insulin responses to NPY were significantly elevated compared to CSF controls in overnight fasted animals. Dose response studies revealed that the adrenocortical system responds to icv NPY with at least as great sensitivity as feeding systems. NPY is discussed as a potential integrator of feeding and responsiveness in the HPA axis.     

Disruption in neuropeptide Y and leptin signaling in obese ventromedial hypothalamic-lesioned rats

Electrolytic lesions placed in the ventromedial hypothalamus (VMH) of rats induce instant hyperphagia and excessive weight gain. Since neuropeptide Y (NPY) is a potent hypothalamic orexigenic signal, and leptin secreted by adipocytes regulates NPY output, we tested the hypothesis that altered NPYergic-leptin signaling may underlie hyperphagia in VMH-lesioned rats. VMH-lesioned rats exhibiting hyperphagia and excessive weight gain in a time-related fashion were sacrificed on days 2, 7, and 21 post-surgery. Quite unexpectedly, NPY concentrations in the hypothalamic paraventricular nucleus (PVN), a major site of NPY release for stimulation of feeding, and in other sites, such as the dorsomedial nucleus, lateral hypothalamic area and median eminence-arcuate nucleus decreased, with the earliest diminution occurring on day 2 in the PVN only. In vitro basal and K⁺-evoked NPY release from the PVN of VMH-lesioned rats was significantly lower than that of controls. Analysis of hypothalamic NPY gene expression showed that although the daily decrease in NPY mRNA from 0800 to 2200 h occurred as in control rats, NPY mRNA concentrations were markedly reduced at these times in the hypothalami of VMH-lesioned rats. Leptin synthesis in adipocytes as indicated by leptin mRNA levels was also profoundly altered in VMH-lesioned rats. The daily pattern of increase in adipocyte leptin mRNA at 2200 h from 0800 h seen in controls was abolished, higher levels of leptin gene expression at 2200 h were maintained at 0800 h. The pattern of increase in serum leptin and insulin levels diverged in VMH-lesioned rats. Serum insulin concentration increased to maximal on day 2 and remained at that level on day 21-post-lesion; serum leptin levels on the other hand, increased slowly in a time-related fashion during this period. These results demonstrate that hyperphagia and excessive weight gain in VMH-lesioned rats are associated with an overall decrease in hypothalamic NPY and augmented leptin signaling to the hypothalamus. The divergent time course of increases in serum leptin and insulin levels suggest independent mechanisms responsible for their augmented secretion, and neither these hormones nor VMH lesions altered the daily rhythm in NPY gene expression. These observations underscore the existence of an independent mechanism controlling the daily rhythm in hypothalamic NPY gene expression and suggest that leptin feedback action requires an intact VMH.     

Differential effect of fasting on hypothalamic expression of genes encoding neuropeptide Y, galanin, and glutamic acid decarboxylase

The effect of caloric deprivation to stimulate hypothalamic neuropeptide Y (NPY) gene expression is hypothesized to represent a physiologically important adaptation in body weight homeostasis. To evaluate the specificity of this response, we used in situ hybridization histochemistry to measure hypothalamic expression of mRNA encoding NPY, galanin, and the two isoforms of glutamic acid decarboxylase (GAD₆₇ and GAD₆₅) in male Wistar rats either fed ad lib or deprived of food for 24 or 48 h. As expected, food deprivation for 24 and 48 h increased preproNPY mRNA levels in the arcuate nucleus by 43 ± 13% (p = NS) and 127 ± 29% (p < 0.05 vs. both fed and 24-h fasted groups) when compared to ad lib-fed controls, and hypothalamic preproNPY mRNA levels were significantly correlated to the percent change in body weight over the three groups of rats (r = −0.72; p < 0.05). In contrast, no significant effects of either 24 or 48 h of fasting were observed on hypothalamic levels of preprogalanin, GAD₆₇, or GAD₆₅ mRNA, and no relationship between percent change in body weight and expression of any of these mRNA species could be demonstrated. In conclusion, fasting increases preproNPY mRNA levels in the arcuate nucleus but does not alter expression of other hypothalamic mRNA species pertinent to feeding behavior. This supports the hypothesis that stimulation of NPY gene expression represents an important component of the hypothalamic response to caloric deprivation.     

Feeding and drinking elicited by central injection of neuropeptide Y: Evidence for a hypothalamic site(s) of action

Neuropeptide Y (NPY), which exists in very high concentrations in the brain, has been shown to elicit a powerful feeding response and a small drinking response in satiated rats. In order to delineate the brain sites sensitive to these effects, NPY was injected through chronic guide cannulas into seven different brain regions, and the food and water intake of satiated rats was measured one hr postinjection. Injection of NPY (78 pmoles) into hypothalamic areas, namely the paraventricular nucleus (PVN), ventromedial hypothalamus (VMH), and lateral hypothalamus (LH), elicited a strong feeding response; in contrast, injections into extra-hypothalamic areas, namely the amygdala, thalamus, and periaqueductal gray, were completely ineffective. Administration of NPY into the PVN and VMH also elicited a small drinking response; however, all other areas, including the LH, were insensitive to this effect. The findings that NPY was effective in the hypothalamus, as opposed to sites anterior, posterior, lateral or dorsal to this structure, suggest a hypothalamic site(s) of action for this neuropeptide.     

Increased feeding and neuropeptide Y (NPY) but not NPY mRNA levels in the hypothalamus of the rat following central administration of the serotonin synthesis inhibitorp-chlorophenylalanine

Neurons containing serotonin (5-HT), a potent anorexic agent, come into contact with neuropeptide Y-ergic neurons, that project from the arcuate nucleus (ARC) to the paraventricular nucleus (PVN). NPY powerfully stimulates feeding and induces obesity when injected repeatedly into the PVN. We hypothesize that 5-HT tonically inhibits the ARC-PVN neurons and that balance between the two systems determines feeding and energy homeostasis. This study aimed to determine whether central injection of the 5-HT synthesis inhibitor p-chlorophenylalanine (pCPA), which increases feeding, increased hypothalamic NPY and NPY mRNA levels. pCPA (10 mg/kg in 3 μl) was administered into the third ventricle either as a single injection (n = 8) or daily for 7 days (n = 8). Control rats received a similar injection of saline. pCPA significantly increased food intake compared with controls after both single and repeated injections (P < 0.05). NPY levels were measured by radioimmunoassay in microdissected hypothalamic extracts. NPY levels in the acutely treated group were significantly increased in the paraventricular nucleus (PVN; by 41%,P = 0.01), anterior hypothalamic area (AHA; by 34%,P < 0.01) and lateral hypothalamic area (LHA; by 41%,P < 0.02). In the 7-day-treated group, NPY levels were also increased in the same areas, i.e. PVN (by 24%,P < 0.01), AHA (by 30%,P < 0.01) and LHA (by 38%,P = 0.01). There were no significant changes in the ARC or any other region or in hypothalamic NPY mRNA levels. pCPA administration increased NPY levels in several regions notably the PVN. This is a major site of NPY release, where NPY injection induces feeding. We suggest that the hyperphagia induced by pCPA is mediated by increased NPY levels and secretion in the PVN. This is further evidence for interactions between NPY and 5-HT in the control of energy homeostasis.     

Evidence that hypothalamic neuropeptide Y gene expression and NPY levels in the paraventricular nucleus increase before the onset of hyperphagia in experimental diabetes

Neuropeptide Y (NPY) is the most potent endogenous orexigenic signal. Several lines of evidence indicate that the site of NPY action in transducing feeding signal may reside in the paraventricular nucleus (PVN) and neighboring sites in the hypothalamus. To test the hypothesis that an increase in NPY activity in the ARC-PVN pathway precedes the onset of diabetic hyperphagia, we evaluated NPY levels in seven hypothalamic nuclei and NPY gene expression in the hypothalamus at 48, 72 or 96 h after streptozotocin (STZ) treatment in rat. In STZ-treated diabetic rats, NPY gene expression in the hypothalamus and NPY levels only in the PVN significantly elevated at 48 h, while hyperphagia occurred sometimes after 48 h post-injection. These results show that augmentation in NPY neuronal activity in the ARC-PVN axis precedes the onset of increased food intake produced by STZ-induced insulinopenia. These findings affirm the hypothesis that increased NPY neurosecretion in the PVN may underlie the diabetes-induced hyperphagia.     

Diurnal variations in the feeding responses to norepinephrine, neuropeptide Y and galanin in the PVN.

The feeding responses elicited by injection of norepinephrine (NE), neuropeptide Y (NPY) and galanin (GAL) into the paraventricular nucleus (PVN) were studied at two different times of the dark (active) cycle in male Sprague-Dawley rats maintained ad lib on pure nutrient diets. The feeding response elicited by NE in the PVN, characterized by a potent and selective stimulatory effect on ingestion of the carbohydrate diet, was significantly stronger during the early dark period (+11.7 kcal over vehicle baseline) relative to the late dark period (+7.6 kcal). A similar pattern of effects was observed with NPY in the PVN, which also selectively potentiated carbohydrate ingestion. The effects of GAL were different from those observed with NE and NPY. Whereas the total amount of food consumed after PVN GAL injection was similar in the early and late dark periods, the macronutrient selection patterns exhibited at these two times were different. During the early dark period, PVN GAL had a small stimulatory effect on carbohydrate, in addition to a strong enhancement of fat intake; in the late dark period, in contrast, GAL stimulated intake only of the fat diet. These findings may reflect differential functions of these hypothalamic neurotransmitters in controlling nutrient ingestion at different periods of the circadian cycle.     

Regulation of [3H]mazindol binding to subhypothalamic areas: involvement in glucoprivic feeding

The distribution of low-affinity sodium-sensitive binding sites of [3H]mazindol were studied in rat hypothalamic nuclei. Using microdissection methods, it was demonstrated that the highest level of [3H]mazindol binding is localized to the paraventricular nucleus (PVN) and the lowest binding is observed in the lateral hypothalamus. Following food deprivation, a significant decrease in [3H]mazindol binding in the PVN and ventromedial hypothalamus (VHM) were observed. Refeeding food-deprived rats resulted in restoration of the level of binding in the PVN, and this was correlated with changes in blood glucose levels. Thus, changes in the binding of [3H]mazindol in the PVN may reflect local changes in glucose levels. In related studies, the involvement of the PVN in the regulation of food deprivation or 2-deoxyglucose (2-DG)-induced food intake was studied. Application of amphetamine (20 micrograms) into the PVN had no effect on food deprivation induced feeding, but significantly inhibited 2-DG induced (glucoprivic) feeding. The PVN may play an important role in the glucostatic regulation of feeding and in mediating the anorectic action of amphetamine and related anorectic drugs on glucoprivic feeding.     

Evidence that NPY-containing neurons in the brainstem project into selected hypothalamic nuclei: implication in feeding behavior

Recent studies show that bilateral neural transections (NT) at the level of dorsal tegmentum in the mesencephalon significantly increase food intake and decrease latency to onset of feeding behavior in response to neuropeptide Y (NPY). The increased responsiveness to NPY may be due to denervation-induced hypersensitivity to NPY in hypothalamic sites that mediate feeding behavior in rats. To test this hypothesis we have studied the effect of NT on NPY concentrations in 7 neural sites of male rats. Two weeks after NT, NPY levels in 3 hypothalamic nuclei—suprachiasmatic nucleus, arcuate nucleus and ventromedial hypothalamus—were not altered by NT thereby suggesting that NPY innervations in these nuclei may be derived mainly from NPY perikarya in the ARC and elsewhere in the diencephalon. On the other hand, NPY concentrations were markedly decreased (50–60%) in the medial preoptic area, paraventricular nucleus, median eminence and dorsomedial nucleus indicating that a substantial number of neurons in the brainstem, which show coexistence of NPY and adrenergic transmitters, project into these 4 diencephalic nuclei. These findings indicate that NPY-containing neurons in the brainstem may project into selected hypothalamic sites and the reduction in the NT rats of NPY levels, especially in the paraventricular nucleus, may be responsible for the reported increase in sensitivity of the NPY-induced feeding response.     

Anorexic action of a new potential neuropeptide Y antagonist [ -Tyr, -Thr]-NPY (27–36) infused into the hypothalamus of the rat

Neuropeptide Y (NPY) produces a vigorous feeding response in several species when it is injected into hypothalamic structures involved in eating behavior. The purpose of this study was to determine whether a unique carboxy terminal fragment of NPY would alter the pattern of eating induced in the rat either by NPY injected into the hypothalamus or by a 24-h period of food deprivation. In this case, two -tyrosine residues and one t.-threonine residue of the NPY_27–36 fragment were transformed to their D-conformation to produce [ -Tyr^27,36, -Thr³²]-NPY (27–36), i.e., D-NPY_27–36. Guide cannulae for microinjection were implanted stereotaxically just dorsal to the paraventricular nucleus (PVN) or ventromedial hypothalamus (VMH) of 24 adult male Sprague-Dawley rats. Following postoperative recovery, a microinjection of artificial CSF or 1.1 jig or 3.3 μg of a peptide was made directly into the PVN or VMH as follows; native NPY; D-NPY_27–36; or [L-Tyr^27,36 L-Thr³²]-NPY (27–36), i.e., L-NPY_27–36. Food intakes were measured at intervals of 0.25, 0.5, 1.1, 2.0, 4.0, and 24 h. When D-NPY_27–36 was microinjected at NPY reactive sites in the PVN or VMH of the rat 15 min before a similar microinjection of NPY, the intense eating response induced by the peptide was reduced significantly. Not only was the effect dose dependent, but D-NPY_27–36 also augmented the latency to feed. A mixture of the two doses of NPY and DNPY_27–36 injected at the same hypothalamic loci did not attenuate the intake of food but tended to enhance the feeding response in the rats. After the rats were deprived of food for 24 h, D-NPY_27–36 microinjected in the same hypothalamic sites similarly caused a dose-dependent suppression of normal feeding behavior. However, the CSF control vehicle and L-NPY_27–36 microinjected in the PVN or VMH were without effect on the pattern of eating. Further, D-NPY_27–38 injected in the same hypothalamic sites affected neither body temperature nor water intakes of the rats significantly. These results demonstrate that the D substitution of this C-fragment of the NPY molecule, i.e., D-NPY_27–36, serves to inhibit feeding evoked in the rat by hypothalamic NPY as well as the natural eating response to food deprivation. Thus, the D-NPY_27–36 molecule may act as an antagonist at one or more subtypes of the NPY receptor in the brain of the rat.     

Anorexic action of a new potential neuropeptide Y antagonist [d-Tyr, d-Thr]-NPY (27–36) infused into the hypothalamus of the rat

Neuropeptide Y (NPY) produces a vigorous feeding response in several species when it is injected into hypothalamic structures involved in eating behavior. The purpose of this study was to determine whether a unique carboxy terminal fragment of NPY would alter the pattern of eating induced in the rat either by NPY injected into the hypothalamus or by a 24-h period of food deprivation. In this case, two l-tyrosine residues and one t.-threonine residue of the NPY_27–36 fragment were transformed to their D-conformation to produce [d-Tyr^27,36,d-Thr³²]-NPY (27–36), i.e., D-NPY_27–36. Guide cannulae for microinjection were implanted stereotaxically just dorsal to the paraventricular nucleus (PVN) or ventromedial hypothalamus (VMH) of 24 adult male Sprague-Dawley rats. Following postoperative recovery, a microinjection of artificial CSF or 1.1 jig or 3.3 μg of a peptide was made directly into the PVN or VMH as follows; native NPY; D-NPY_27–36; or [L-Tyr^27,36 L-Thr³²]-NPY (27–36), i.e., L-NPY_27–36. Food intakes were measured at intervals of 0.25, 0.5, 1.1, 2.0, 4.0, and 24 h. When D-NPY_27–36 was microinjected at NPY reactive sites in the PVN or VMH of the rat 15 min before a similar microinjection of NPY, the intense eating response induced by the peptide was reduced significantly. Not only was the effect dose dependent, but D-NPY_27–36 also augmented the latency to feed. A mixture of the two doses of NPY and DNPY_27–36 injected at the same hypothalamic loci did not attenuate the intake of food but tended to enhance the feeding response in the rats. After the rats were deprived of food for 24 h, D-NPY_27–36 microinjected in the same hypothalamic sites similarly caused a dose-dependent suppression of normal feeding behavior. However, the CSF control vehicle and L-NPY_27–36 microinjected in the PVN or VMH were without effect on the pattern of eating. Further, D-NPY_27–38 injected in the same hypothalamic sites affected neither body temperature nor water intakes of the rats significantly. These results demonstrate that the D substitution of this C-fragment of the NPY molecule, i.e., D-NPY_27–36, serves to inhibit feeding evoked in the rat by hypothalamic NPY as well as the natural eating response to food deprivation. Thus, the D-NPY_27–36 molecule may act as an antagonist at one or more subtypes of the NPY receptor in the brain of the rat.     

Control of feeding and sexual behaviors by neuropeptide Y: physiological implications

Recent evidence suggests that a variety of hypothalamic neuropeptides may mediate interneuronal communication to coordinate diverse neuroendocrine and behavioral functions. In this work, we describe the effects of neuropeptide Y (NPY) on feeding and sexual behaviors. We observed that central administration of bolus NPY stimulated a robust, dose-related feeding response in satiated male and female rats. Continuous NPY receptor activation also evoked dose-related, intermittent feeding in a manner normally observed during nocturnal feeding. It appears that the paraventricular nucleus in the hypothalamus may be the primary site of NPY action because the anticipated reciprocal changes in NPY concentrations, in response to food deprivation followed by ad libitum food intake, occurred only in this site. Additional findings revealed that NPY-induced feeding may follow either substantial reduction or complete restraint of an inhibitory influence on feeding mediated by alpha 2-adrenoreceptor systems in satiated rats. Further, NPY was found to suppress male and female sexual behaviors. The suppressive effects on sexual behavior were apparent prior to or at the time of the onset of feeding after NPY administration. These observations may provide a neurochemical basis for clinical and animal studies on disorders of feeding associated with diminished reproductive functions.     

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.     

Observations on the Orexigenic Hypothalamic Neuropeptide Y-System in Neonatally Overfed Weanling Rats

Early postnatal overnutrition is a risk factor for obesity in juvenile and adult life. Underlying pathophysiological mechanisms are still unclear. Hypothalamic neuropeptides are decisively involved in the regulation of body weight and food intake. In this study, we investigated consequences of early postnatal overnutrition, as compared to normo-and undernutrition, on NPY within the arcuate nucleus and paraventricular nucleus (PVN). The normal litter size of Wistar rats was adjusted on the third day of life from 10 pups (normal litters, NL; normonutrition) to only three newborns (small litters, SL; overnutrition) or 18 pups per mother (large litters, LL; undernutrition). SL rats developed clear overweight until the day 21 of life (P<0.0001), as well as hyperleptinaemia (P<0.001), and hyperinsulinaemia (P<0.01). LL rats were underweight and had decreased leptin and insulin concentrations. Using radioimmunoassay, NPY contents were determined in hypothalamic micropunches, and immunocytochemistry for NPY was performed in serial hypothalamic sections on day 21 of life. While in the underweight, hypoleptinaemic, and hypoinsulinaemic LL rats increased concentrations of NPY in the arcuate nucleus and PVN were observed, no decrease in NPY content was found in the overweight, hyperleptinaemic, and hyperinsulinaemic SL rats. Moreover, the percentage of NPY-immunopositive neurones per total number of neurones was increased not only in the LL rats, but also in the SL rats. Since the NPY system is functionally mature already at this age, these findings might indicate an acquired resistance of the hypothalamic NPY system to increased levels of insulin and/or leptin in early postnatally overfed SL rats.     

Neuropeptide Y (NPY) in the area of the hypothalamic paraventricular nucleus activates the pituitary-adrenocortical axis in the rat

Immunocytochemical studies have documented the presence of neuropeptide Y (NPY) in the hypothalamic paraventricular nucleus (PVN) which harbours a large number of neurones that contain corticotrophin-releasing factor (CRF). In this study the close morphological association between NPY fibres and CRF cell bodies in the PVN was confirmed. The localization of NPY terminals in the vicinity of CRF neurones forms a morphological basis for an action of NPY in the hypothalamic control of the pituitary-adrenocortical axis. We therefore microinjected NPY into the area of the PVN of both conscious, freely moving and anaesthetized rats and noted a powerful stimulatory effect on adrenocorticotropic hormone (ACTH) and corticosterone release as measured by radioimmunoassay. In experiments with conscious, freely moving rats, higher ACTH and corticosterone levels were detected following injection of NPY into the area of the PVN than following control injection (desamidated NPY). Intracerebroventricular injection of NPY produced a small, albeit significant, increase in circulating corticosterone levels as compared to control (saline-injected) rats. Anaesthetized rats responded to NPY (but not to saline) injected into the area of the PVN with elevated ACTH and corticosterone levels, while injection of NPY into the neocortex failed to affect the blood concentration of either ACTH or corticosterone. In conclusion, we have demonstrated an activating effect of NPY on the pituitary-adrenocortical axis both in conscious and anaesthetized rats which may reflect the anatomical relationship between NPY fibres and CRF neurones in the PVN.