Injection Of Neuropeptide Y Research Articles

NPY Y 2 receptor agonist, N-acetyl [Leu 28,Leu 31]NPY24–36, reduces renal vasoconstrictor activity in anaesthetised dogs

The actions of neuropeptide Y (NPY) at the autonomic neuroeffector junction have been attributed to two main receptor subtypes. At NPY Y 1 receptors, located postsynaptically, NPY has been shown to produce vasoconstriction, or to potentiate the action of other vasoconstrictor agents. At NPY Y 2 receptors, located presynaptically on nerve terminals, NPY inhibits the release of neurotransmitter from autonomic nerve terminals. In these experiments we have used the specific NPY Y 2 receptor agonist, N-acetyl [Leu 28,Leu 31]NPY, which lacks local constrictor activity, and have demonstrated inhibition of nerve-evoked vasoconstriction in the renal circulation of anaesthetised dogs in a way that suggests an intra-renal regional specificity. Under control conditions stimulation of the renal sympathetic nerves over a range of frequencies (1–5 Hz) reduced renal vascular conductance and glomerular filtration rate (GFR). Following the injection of the selective NPY Y 2 receptor agonist, N-acetyl [Leu 28,Leu 31]NPY24–36, nerve-evoked reductions in renal conductance were reduced by over 45%. At the lowest stimulation frequencies, reduced vasoconstrictor activity was associated with a marked increase in GFR in the presence N-acetyl [Leu 28,Leu 31]NPY24–36. At both higher levels of stimulation N-acetyl [Leu 28,Leu 31]NPY24–36 significantly inhibited vasoconstrictor activity and attenuated the nerve-evoked reductions in GFR. Full recovery of both variables was observed 20 min after N-acetyl [Leu 28,Leu 31]NPY24–36 injection. N-acetyl [Leu 28,Leu 31]NPY24–36 produced a similar inhibition of renal vasoconstrictor activity when the renal nerves were left intact and activated reflexly. These results suggest that NPY can act via NPY Y 2 receptors to inhibit sympathetic vasoconstrictor activity in the renal circulation of dogs. On the basis of the demonstrated dissociation of effects on vascular conductance and GFR, we suggest that this might result from a preferential action of the NPY Y 2 agonist on sympathetic nerves supplying the afferent arteriole of the kidney.

Evidence that the inhibition of luteinizing hormone secretion exerted by central administration of neuropeptide Y (NPY) in the rat is predominantly mediated by the NPY-Y5 receptor subtype.

A number of studies have indicated that neuropeptide Y (NPY) is a central regulator of the gonadotropic axis, and the Y1 receptor was initially suggested to be implicated. As at least five different NPY receptor subtypes have now been characterized, the aim of the present study was to reinvestigate the pharmacological profile of the receptor(s) mediating the inhibitory action of NPY on LH secretion by using a panel of NPY analogs with different selectivity toward the five NPY receptor subtypes. When given intracerebroventricularly (icv) to castrated rats, a bolus injection of native NPY (0.7-2.3 nmol) dose-dependently decreased plasma LH. Peptide YY (PYY; 2.3 nmol) was as potent as NPY, suggesting that the Y3 receptor is not implicated. Confirming previous data, the mixed Y1, Y4, and Y5 agonist [Leu31,Pro34]NPY (0.7-2.3 nmol) inhibited LH release with potency and efficacy equal to those of NPY. Neither the selective Y2 agonist C2-NPY (2.3 nmol) nor the selective Y4 agonist rat pancreatic polypeptide affected plasma LH, excluding Y2 and Y4 subtypes for the action of NPY on LH secretion. The mixed Y4-Y5 agonist human pancreatic polypeptide (0.7-7 nmol) as well as the mixed Y2-Y5 agonist PYY3-36 (0.7-7 nmol) that displayed very low affinity for the Y1 receptor, thus practically representing selective Y5 agonists in this system, decreased plasma LH with potency and efficacy similar to those of NPY, indicating that the Y5 receptor is mainly involved in this inhibitory action of NPY on LH secretion. [D-Trp32]NPY, a selective, but weak, Y5 agonist, also inhibited plasma LH at a dose of 7 nmol. Furthermore, the inhibitory action of NPY (0.7 nmol) on LH secretion could be fully prevented, in a dose-dependent manner (6-100 microg, icv), by a nonpeptidic Y5 receptor antagonist. This antagonist (60 microg, icv) also inhibited the stimulatory action of NPY (0.7 nmol) on food intake. The selectivity of PYY3-36, human PP, [D-Trp32]NPY, and the Y5 antagonist for the Y5 receptor subtype was further confirmed by their ability to inhibit the specific [125I][Leu31,Pro34]PYY binding to rat brain membrane homogenates in the presence of the Y1 receptor antagonist BIBP3226, a binding assay system that was described as being highly specific for Y5-like receptors. With the exception of [D-Trp32]NPY, all analogs able to inhibit LH secretion were also able to stimulate food intake. Taken together, these results indicate that the Y5 receptor is involved in the negative control by NPY of the gonadotropic axis.

The effect of a neuropeptide Y antagonist, BIBP 3226, on short-term arterial pressure control in conscious unrestrained rats with congestive heart failure

The effects of a neuropeptide Y (NPY) Y1-receptor antagonist (BIBP 3226) on mean arterial pressure (MAP) and heart rate were investigated in conscious unrestrained rats with chronic congestive heart failure. The rats were randomly assigned to 2 groups, and received either BIBP 3226 or its inactive enantiomer (BIBP 3435) as an intravenous infusion (6 mg/kg/h for 1.5 h, respectively). Before, during and after the infusion, rats were stressed with a jet of air and received a bolus injection of NPY (2 nmol/kg iv.). There was no difference between the 2 groups in resting MAP and heart rate before, during or after infusion (BIBP 3226 vs. BIBP 3435). The effects of exogenous NPY on MAP were significantly attenuated in BIBP 3226 group during and 1 h after the infusion (p < 0.05). The tissue NPY levels in heart, adrenal gland and kidney in heart failure rats were not different from those in sham-operated rats. The results suggest that Y1-receptor mechanisms are of minor importance in the short-term control of basal MAP and heart rate in conscious unrestrained rats with congestive heart failure.

5-HT(2A/2C) receptor antagonists in the paraventricular nucleus attenuate the action of DOI on NPY-stimulated eating.

Hypothalamic neuropeptide Y (NPY) and serotonin (5-HT)-containing neurons are believed to exert an interactive effect on ingestive behavior. The present study examined the ability of two serotonergic antagonists, spiperone (SPIP), a 5-HT2A antagonist, and mianserin (MIAN), a 5-HT(2A/2C) antagonist, to block the inhibitory action of the 5-HT(2A/2C) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on NPY-stimulated eating. Drugs were injected directly into the paraventricular nucleus (PVN), the perifornical (PFH) or the ventromedial hypothalamus (VMH) at the onset of the dark cycle. PVN, PFH and VMH injections of NPY potentiated food intake although only PVN pretreatment with DOI (5-20 nmol) suppressed NPY-induced eating. SPIP or MIAN, injected immediately prior to PVN DOI, reversed the suppressive effect of DOI on NPY feeding. These findings are consistent with other recent data showing that 5-HT2A receptors within the PVN modulate NPY's effect on food intake at the start of the nocturnal period.

Effects of Neuropeptide Y on the Discriminative Stimulus and Antinociceptive Properties of Morphine

Previous research indicates that opioid receptor blockade diminishes the effects of neuropeptide Y (NPY) on feeding and memory. Conversely, NPY attenuates naloxone-precipitated morphine withdrawal. The present study evaluated the effects of NPY on the discriminative stimulus and antinociceptive effects produced by the prototypical mu opioid, morphine. Rats were trained to discriminate 5.6 mg/kg morphine (IP) from saline using a standard two-lever, food-reinforced, drug discrimination procedure. Across a range of doses (3.0, 5.0, and 10 μg), intracerebroventricular (ICV) injection of NPY failed to substitute for, antagonize, or potentiate the discriminative stimulus effects of morphine. A warm-water tail-withdrawal procedure was used to examine the antinociceptive effects of morphine and NPY, alone and in combination. NPY (3.0 and 10 μg, ICV) failed to alter tail-withdrawal latencies from 52° and 56°C water, whereas morphine (1.0–30 mg/kg, IP) produced a dose-related increase in latencies at both water temperatures. A 10-μg dose of NPY also failed to alter the antinociceptive effects of morphine. This study does not support the idea that the discriminative stimulus and antinociceptive effects of morphine are dependent on an NPYergic pathway.

Studies on the role of serotonin receptor subtypes in the effect of sibutramine in various feeding paradigms in rats.

The effect of the 5-hydroxytryptamine (5-HT) and noradrenaline (NA) reuptake inhibitor sibutramine was studied in food deprived, neuropeptide Y (NPY)- or muscimol-injected rats. Sibutramine dose-dependently reduced feeding caused by food-deprivation (ED50 = 5.1+/-0.8 mg kg(-1)) or by NPY injection into the paraventricular nucleus of the hypothalamus (ED50 = 6.0+/-0.5 mg kg(-1)). The increase in food intake caused by muscimol injected into the dorsal raphe was not modified by sibutramine (1-10 mg kg(-1)). The hypophagic effect of 5.1 mg kg(-1) sibutramine in food-deprived rats was studied in rats pretreated with different serotonin receptor antagonists. Metergoline (non-selective, 0.3 and 1.0 mg kg(-1)), ritanserin (5-HT2A/2C, 0.5 and 1.0 mg kg(-1)) and GR127935 (5-HT1B/1D), 0.5 and 1.0 mg kg(-1)) did not modify the hypophagic effect of sibutramine, while SB206553 (5-HT2B/2C, 5 and 10 mg kg(-1)) slightly but significantly reduced it (Fint(2.53) = 3.4; P<0.05). The reduction in food intake caused by 6.0 mg kg(-1) sibutramine in NPY-injected rats was not modified by GR127935 (1.0 mg kg(-1)). The results suggest that, with the possible exception of a partial involvement of 5-HT2B/2C receptors in sibutramine's hypophagia in food-deprived rats, 5-HT1 and 5-HT2 receptor subtypes do not play an important role in the hypophagic effect of sibutramine, at least in the first 2 h after injection.

Opposite effects on feeding of suprachiasmatic nucleus neuropeptide Y administration in rats

The effects of injecting or infusing neuropeptide Y (NPY) into the suprachiasmatic nucleus of rats on patterns of individual macronutrient and water intake were examined during the following 2 h and also across 12 and 24 h light/dark cycles. Increased total energy intake (218 and 170%) and energy intake from the dextrin/sucrose diet (499 and 247%) were observed in the 2 h following injection of 100 pmol NPY at early light and early dark, respectively, and in the following 24 h (total energy: 67%, dextrin/sucrose: 73%). Nocturnal casein energy intake was also increased (258%) following NPY injection. Continuous infusion of 10 pmol/h of NPY suppressed nocturnal total energy (36%) and dextrin/sucrose intake (36%) as well as 24 h energy intake from casein (43%). These results demonstrate divergent effects of NPY subsequent to different mode of administration.

Neuropeptide Y restores appetite and alters concentrations of GH after central administration to endotoxic sheep.

The objective of this study was to determine whether neuropeptide Y (NPY) and recombinant human interleukin-1 receptor antagonist (IL-1ra) would: first, increase food intake; secondly, decrease concentrations of GH; thirdly, reduce GHRH-induced release of GH; and fourthly, reduce changes to concentrations of IGF-I in plasma during experimental endotoxemia in sheep. Six treatments were given to six castrated male sheep in a 6x6 Latin square treatment order. Osmotic mini-pumps were implanted at 0 h and a jugular vein was cannulated. Each sheep was continuously infused with saline (0.9%) or lipopolysaccharide (LPS) (20 micrograms/kg per 24 h, s.c.) at 10 microliters/h for 72 h via the osmotic mini-pumps. Blood samples (3 ml) were collected at 15-min intervals from 24 to 33 h. At 26 h, one of three treatments (artificial cerebrospinal fluid, NPY or IL-1ra) was injected i.c.v. within 30 s (0.3 microgram/kg), then infused i.c.v. from 26 to 33 h (600 microliters/h) at 0.3 microgram/kg per h. GHRH was injected i.v. (0.075 microgram/kg) at 32 h after which blood samples were collected at 5, 10, 15, 30, 45 and 60 min. Feed intake was reduced up to 50% for 48 h in LPS-treated compared with non-LPS-treated sheep. NPY restored feed intake in LPS-treated sheep and induced hyperphagia in non-LPS-treated sheep from 24 to 48 h. In contrast, IL-1ra did not affect appetite. Injection of NPY increased concentrations of GH from 26 to 27 h, while IL-1ra had no effect. Infusion of NPY suppressed GHRH-induced release of GH. However, no treatment altered pulse secretion parameters of GH. Concentrations of IGF-I were 20% higher at 72 h in LPS-treated sheep given NPY than in sheep treated with LPS alone, and this may reflect increased appetite from 24 to 48 h. We concluded that reduced appetite during endotoxemia is due to down-regulation of an NPY-mediated mechanism. Furthermore, NPY stimulates release of GH in healthy sheep, does not reduce pulse secretion parameters of GH, but does suppress GHRH-induced release of GH in endotoxic sheep. Therefore, NPY may be an important neurotransmitter linking appetite with regulation of GH during endotoxemic and healthy states in sheep.

Cardiorespiratory responses to intracerebroventricular injection of neuropeptide Y in anaesthetised dogs

Cardiovascular and respiratory effects of intracerebroventricular (icv) administration of neuropeptide Y (NPY) and separate, preferential agonists for NPY Y 1 and Y 2 receptors were observed in anaesthetised dogs. Central injections of NPY resulted in significant cardiac slowing and decreases in arterial pressure. These cardiovascular effects were blocked by central injection of the NPY Y 1-preferring antagonist 1229U91. Central injection of NPY did not have a significant effect on ventilation, but the NPY Y 1 antagonist 1229U91 administered alone caused a significant increase in ventilation. The NPY Y 1-receptor agonist [Leu 31Pro 34] NPY significantly decreased ventilation while the NPY Y 2 receptor agonist N-acetyl [Leu 28Leu 31] NPY 24–36 significantly increased it. A similar inverse relationship was seen with respect to blood pressure, with the NPY Y 1-receptor agonist [Leu 31Pro 34] NPY significantly decreasing blood pressure, while the NPY Y 2 receptor agonist N-acetyl [Leu 28Leu 31] NPY 24-36 significantly increased it. These findings suggest a role for NPY Y 1 receptors in pathways mediating decreases in ventilation and blood pressure, and for NPY Y 2 receptors in those mediating increased ventilation and blood pressure.

Injection of neuropeptide Y into the third cerebroventricle differentially influences pituitary secretion of luteinizing hormone and growth hormone in ovariectomized cows

Hypothalamic neurons that control the luteinizing hormone (LH) and growth hormone (GH) axes are localized in regions that also express neuropeptide Y (NPY). Increased hypothalamic expression of NPY due to diet restriction has been associated with suppressed secretion of LH and enhanced secretion of GH in numerous species. However, these physiological relationships have not been described in cattle. Thus, two studies were conducted to characterize these relationships using ovariectomized (Experiment 1) or ovariectomized estrogen-implanted (Experiment 2) cows. In Experiment 1, four well-nourished, ovariectomized cows received third cerebroventricular (TCV) injections of 50 and 500 μg of NPY in a split-plot design. Venous blood was collected at 10-min intervals from −4 hr (pre-injection control period) to +4 hr (postinjection treatment period) relative to TCV injection. NPY suppressed (P ⩽ 0.04) tonic secretion of LH irrespective of dose and tended to stimulate (P ⩽ 0.10) an increase in tonic secretion of GH. In Experiment 2, six ovariectomized cows that were well nourished and implanted with estradiol received TCV injections of 0, 50, or 500 μg of NPY in a replicated 3 × 3 Latin Square. Both doses of NPY suppressed (P < 0.06) mean concentration of LH relative to the 0-μg dose. The 50-μg dose of NPY tended (P < 0.10) to increase the amplitude of GH pulses. In conclusion, TCV injection of NPY suppressed pituitary secretion of LH and simultaneously tended to increase pituitary secretion of GH.

Effect of lateral cerebroventricular injection of the appetite-stimulating neuropeptide, orexin and neuropeptide Y, on the various behavioral activities of rats

The effect of lateral cerebroventricular injection of the appetite-stimulating neuropeptide, orexin and neuropeptide Y (NPY), on the behavior of rats was investigated. An immediate increase in face washing activity was observed after injection of orexin A or orexin B, but not NPY. Orexin A had a more potent effect on face washing behavior than orexin B. Grooming and burrowing activities also increased significantly after injection of orexin A, whereas, orexin B significantly increased burrowing and searching behavior. Feeding behavior and food consumption increased dramatically within 10 min of injection of NPY. Although the significant increase in feeding behavior was also observed after injection of orexin A, total food intake did not change significantly. These results suggest that orexin may be involved in the regulation of several other behavioral activities in rats, besides feeding.

Hypothalamic NPY status during positive energy balance and the effects of the NPY antagonist, BW1229U91, on the consumption of highly palatable energy-rich diet

We have studied the hypothalamic activity of the neuropeptide Y (NPY) system in dietary-induced obese male Wistar rats and examined whether the NPY antagonist, BW1229U91, can inhibit the hyperphagia during positive energy balance associated with feeding rats an energy-rich, highly palatable diet. Rats given a highly palatable, high-fat diet became obese after 8 weeks and exhibited hyperinsulinemia and hyperleptinemia, as compared to lean rats fed on standard pellet laboratory diet. Hypothalamic NPY mRNA concentrations were significantly reduced by approximately 70% in dietary-obese rats compared with lean controls, and the former were hypersensitive to intracerebroventricular injections of NPY, possibly as a result of NPY receptor up-regulation. Intracerebroventricular injections of BW1229U91, that inhibits food intake in starved rats, did not alter food intake in either control or obese rats fed either standard pellet diet or the highly palatable diet, respectively. We conclude that dietary-obese rats have underactive hypothalamic NPYergic neurons compared to lean controls, possibly as a result of increased plasma concentrations of leptin and/or insulin that directly inhibit the NPY neuronal activity. The lack of effect of BW1229U91 on the increased caloric intake of dietary-obese rats suggests that the hyperphagia is not NPY-driven and supports the data indicating reduced synaptic activity of the hypothalamic NPY system.

Comparative vascular responses in elasmobranchs to different structures of neuropeptide Y and peptide YY

The vascular responses to neuropeptide Y (NPY) and peptide YY (PYY) were tested in several species of elasmobranchs to assess whether changes in sequence in these neuropeptides from elasmobranchs to mammals are associated with different physiological responses. NPY-like immunoreactivity was detected in the gut and in nerve fibres surrounding some, but not all, blood vessels of six species. Intravenous injection of dogfish, frog and human NPY in anaesthetised fish caused similar vasopressor effects in the three species tested, except human NPY which lowered blood pressure in one of the three. Dogfish NPY and PYY were equipotent pressor agents in two species, but PYY was significantly more potent than NPY in one species. NPY and PYY both contracted isolated gut arteries from three species, but had no effect on isolated efferent arteries tested. In conclusion, differential vascular responses in elasmobranchs are not associated with changes in NPY sequence across vertebrates, but may be with changes in PYY in some species.

The effects of central administration of neuropeptide Y on behavior of tenascin-gene knockout mice

Tenascin-C (TN), an extracellular matrix glycoprotein, plays a pivotal role in the regulation of neuronal pattern formation. TN-gene deficient mice produced by homologous recombination show hyperlocomotive activities. We have recently demonstrated that neuropeptide Y (NPY) mRNA expression is reduced in the limbic area of the TN-gene knockout mouse brain, compared to that in wild-type mice. We now report the effect of NPY on the behavior of TN mutant mice. Intracerebroventricular injection of NPY transiently decreased the hyperlocomotion of TN mutant mice. Concurrently, mice given NPY showed anxiolytic behavioral change as assessed by an exploratory model. The results presented here suggest that the hyperlocomotion of TN mutant mice is partially derived from severe tensity.

Intrahypothalamic Discriminative Stimulus Effects of Neuropeptide Y

O’HARE, E., J. CLEARY, D. T. WELDON, J. D. POMONIS, C. J. BILLINGTON AND A. S. LEVINE. Intrahy-pothalamic discriminative stimulus effects of neuropeptide Y. PHARMACOL BIOCHEM BEHAV 59(2) 375–378, 1998.—Neuropeptide Y (NPY) is one of the most ubiquitous neurotransmitters in the CNS and has been implicated in a variety of psychological and physiological functions. The current study investigated whether intrahypothalamic (IH) administrations of NPY were behaviorally discriminable from saline injections. Rats were trained to differentially respond based on whether they received IH injections of NPY (0.5 μg/0.5 μl) or saline (0.5 μl 0.9% NaCl). Subjects demonstrated discriminative control (85% correct in 8 out of 10 consecutive sessions) after a mean of 32 sessions. The ability of subjects to discriminate IH NPY from saline was dose dependent, with the lowest NPY dose tested (0.03 μg/0.5 μl) generalizing to saline. The opioid antagonist naloxone blocked the discrimination of NPY when administered IP (3.0 mg/kg) or IH (50 μg/0.5 μl).

Growth hormone inhibits its own secretion by acting on the hypothalamus through its receptors on neuropeptide Y neurons in the arcuate nucleus and somatostatin neurons in the periventricular nucleus.

GH secretion is regulated by hypothalamic somatostatin and GH-releasing factor. It has been postulated that GH feeds back on the hypothalamus and regulates its own secretion. We focused our attention on the action of GH in the hypothalamus in relation to GH secretion. Adult male rats were used throughout the studies, and the observation was made in conscious rats. Systemic administration of human GH induced c-fos gene expression, a marker of neuronal activity, in the hypothalamic arcuate nucleus (ARC) and the periventricular nucleus (PeV) in hypophysectomized male rats. The major cells in which c-fos gene expression was induced were neuropeptide Y (NPY) neurons in the ARC and somatostatin neurons in the PeV. GH receptor mRNA was demonstrated to be present in these neurons by in situ hybridization. The injection of a small dose of rat GH into the ARC or PeV inhibited GH secretion, whereas microinjection of IGF-I into these nuclei did not. Intracerebroventricular injection of NPY suppressed GH secretion, and this effect was abolished by anterolateral deafferentation of the medial basal hypothalamus (MBH), a procedure which disrupts the somatostatinergic input to the MBH. Taken together, these findings suggest that GH acts on NPY neurons in the ARC and somatostatin neurons in the PeV through GH receptor, and the activation of these neurons augments somatostatin release and inhibits GH secretion.

Stimulation of 5-HT(2A/2C) receptors within specific hypothalamic nuclei differentially antagonizes NPY-induced feeding.

The present study examined the impact of the 5-HT(2A/2C) agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on the feeding-stimulant action of neuropeptide Y (NPY) injected into three hypothalamic sites, the paraventricular nucleus (PVN), the perifornical hypothalamus (PFH) and the ventromedial nucleus (VMH). Injections of NPY (100-200 pmol) into each region potentiated food intake. Pretreatment with DOI (5-20 nmol) into the PVN effectively suppressed feeding elicited by PVN NPY (100 pmol). In contrast, DOI injections into the PFH and VMH failed to modify NPY-stimulated eating. This suggests that 5-HT(2A/2C) receptor modulation of NPY feeding within the hypothalamus is localized to the PVN.

Inhibition of food intake by neuropeptide Y Y5 receptor antisense oligodeoxynucleotides.

The recently discovered rat neuropeptide Y (NPY) receptor, the Y5 subtype, has been proposed to mediate the NPY-induced feeding response and therefore plays a central role in the regulation of food intake. These conclusions were based on studies with peptidic agonists. We now report studies in which phosphothioate end-protected antisense oligodeoxynucleotides (ODNs) targeted to prepro NPY (prepro NPY antisense ODNs) or to the Y5 receptor (Y5 antisense ODNs) were used to assess the functional importance of this novel receptor subtype in vivo. NPY antisense ODNs given intracerebroventricularly to rats prevented the increase in hypothalamic NPY levels during food deprivation and inhibited fasting-induced food intake. Likewise, repeated intracerebroventricular injections of Y5 antisense ODNs prevented fasting-induced food intake in rats. Moreover, two Y5 antisense ODNs, targeted to different sequences of the receptor, significantly decreased basal food intake and inhibited the increase in food intake after intracerebroventricular injection of NPY. These effects proved to be selective, since the feeding response to galanin was not affected. Analysis of the structure of feeding behavior revealed that prepro NPY and Y5 receptor antisense ODNs reduced food intake by inducing decreases in meal size and meal duration analogous to the orexigenic effects of NPY that are mediated by increases in these parameters. Although changes in Y5 receptor density could not be measured, the results with Y5 antisense ODNs strongly suggest that this receptor subtype mediates the feeding response to exogenous and endogenous NPY. Selective Y5 antagonists may therefore be of therapeutic value for the treatment of obesity and eating disorders.

Increased receptor sensitivity to neuropeptide Y in the hypothalamus may underlie transient hyperphagia and body weight gain

Disruption of neural signaling by microinjection of a neurotoxin, colchicine (COL), in the ventromedial hypothalamus (VMH) of rats results in rapid and transient hyperphagia and body weight gain. Since neuropeptide Y (NPY) is a potent hypothalamic orexigenic signal and continuous NPY receptor activation by intracerebroventricular (icv) NPY infusion results in hyperphagia and obesity, we tested the hypothesis that altered NPYergic signaling may underlie the transient hyperphagia in COL-injected rats. Immediately following COL (4 μg) microinjections in the ventromedial nucleus (VMN) rats displayed hyperphagia both during the lights-on and lights-off periods. Concomitant with hyperphagia, preproNPY mRNA levels in the arcuate nucleus and NPY levels in the paraventricular nucleus decreased in a time-dependent manner. However, food intake in response to intracerebroventricular injections of NPY (29, 117 and 470 pmole) was significantly higher in COL-injected rats and the latency to initiation of feeding was markedly reduced as compared to controls. The smallest dose of NPY which was virtually ineffective in control rats, evoked near maximal intake in COL-injected rats. This enhanced response lasted for only 4 days paralleling the transient hyperphagia. The NPY Y1 receptor antagonist 1229U91 (5 or 30 μg/rat, icv) significantly suppressed feeding in COL-treated rats thereby indicating that hyperphagia in these rats was dependent upon endogenous NPY. Overall, these studies demonstrate that not only high levels, but low levels of NPY may also result in hyperphagia and increased body weight and this hyperphagia may be attributed to the rapid development of NPY Y1 receptor hypersensitivity.

Effects of an intravenous injection of NPY on leptin and NPY-Y1 receptor mRNA expression in ovine adipose tissue

Neuropeptide Y (NPY) is highly expressed in hypothalami of undernourished and genetically obese animals, and is a potent regulator of food intake and reproduction. Leptin, a protein expressed by adipocytes, has been reported to reduce hypothalamic NPY expression. We recently detected (by ribonuclease protection assay [RPA]) expression of the NPY receptor subtype Y1 (but not Y2) mRNA in adipose tissue. Based on these observations we hypothesized that NPY-Y1 receptors in adipose may represent a peripheral mechanism by which NPY can regulate leptin expression in a direct and rapid manner. To test this hypothesis, adipose samples were biopsied from the tailhead region of 48 ± 3 kg wether lambs immediately before and 30 min after a single intravenous injection of 50 μg porcine NPY (“treated” animals, n = 5), or vehicle (“control” animals, n = 4). Injection of NPY resulted in an increase in expression (P = 0.013; as measured by RPA) of both leptin and NPY-Y1 mRNA. In treated animals, negative correlations were found between response in leptin expression and body weight ( r = −0.82, P = 0.092), and between leptin response and initial leptin mRNA levels ( r = −0.81, P = 0.097). These data provide evidence of a peripheral mechanism by which NPY may regulate adipocyte expression of both leptin and NPY-Y1 receptor mRNA.