Hypothalamic Contribution To Eating Behaviour, neuropeptide Y
Hypothalamic Neuropeptide Y: Effects on Food Intake and Respiratory Quotient
Date: Thursday, March 13th, 2003
Title: “Hypothalamic Neuropeptide Y: Effects on Food Intake and Respiratory Quotient”
Speaker: Paul J. Currie, Barnard College, Columbia University, NY
Neuropeptide Y (NPY) is a 36 amino acid peptide synthesized in neurons of the arcuate nucleus. These neurons send axonal projections to other hypothalamic areas including the paraventricular nucleus (PVN). Evidence for a physiological role of NPY in the control of food intake, energy balance and metabolic regulation is obtained from numerous studies (see DiBona, 2001 for overview).
NPY exerts a robust stimulatory effect on food intake and chronic infusion promotes enhanced body weight gain accompanied by hyperinsulinemia and insulin resistance. Additional work shows that, specifically within the PVN, NPY alters the release of insulin, corticosterone and glucagon, decreases brown fat thermogenesis and increases white fat lipoprotein lipase activity.
Hypothalamic NPY levels increase under conditions of food restriction, spontaneous and experimentally induced diabetes, and in several animal models of genetic obesity. These findings support the argument that elevated brain NPY levels are likely to be a major contributing factor in the obese/diabetic state.
Whereas genes encoding five NPY receptor subtypes have been identified (Y1, Y2, Y4, Y5, Y6), evidence to date suggests that Y1 and Y5 receptors are implicated in ingestive behavior (Chamorro et al., 2002; Polidori et al., 2000). Limited work indicates that the Y2 receptor is involved in feeding inhibition, possibly via a NPY Y2 autoreceptor mechanism. Until very recently, the lack of selective NPY receptor agonists and antagonists has prevented detailed investigation of the role of specific NPY receptors in appetitive behavior.
Pharmacological studies with these now available selective compounds are providing increasing support for Y1 and Y5 receptor involvement. Curiously, NPY knockout and NPY receptor knockout preparations fail to show consistent metabolic perturbations. For example, NPY deficient mice do not exhibit disturbances in food intake and body weight. However, given the redundancy of feeding regulatory pathways it is likely that these same neural systems compensate for the lack of NPY throughout development.
Energy Substrate Utilization
In addition to the effects of NPY on appetitive motivation, other work in our laboratory has focused on the role of the peptide in energy substrate utilization (Currie & Coscina, 1996; 1995). Specifically, we have investigated the effects of hypothalamic NPY on respiratory quotient (RQ; VCO2/VO2) as measured using indirect calorimetry. Under free-feeding conditions, rats typically show RQs in the light cycle of around 0.90 indicating mixed utilization of carbohydrates, fats and proteins.
Fasting increases the reliance on fat reserves and lowers RQ to reflect fat metabolism (0.7-0.8 range). We have reported that PVN NPY shifts RQ in the direction of enhanced carbohydrate oxidation (>0.90). In fact NPY typically evokes increases in RQ to values greater than 1.0, reflecting the exclusive utilization of carbohydrate as well as fat synthesis from carbohydrate. The increased catabolism of carbohydrate in favor of fat synthesis reflects an anabolic state consistent with the action of chronic PVN NPY infusion wherein a potent effect on fat deposition and body weight is observed.
Hypothalamic NPY and 5-hydroxytryptamine (5-HT)
Other investigations in our lab have focused on the interaction of 5-hydrotrypatimergic receptor mechanisms and NPY. Earlier reports had indicated that NPY hyperphagia could be blocked by fenfluramine and that hypothalamic peptide levels decrease after treatment with 5-HT agonists and increase after 5-HT antagonist administration. In our studies, the feeding effects of 5-HT1 and 5-HT2 receptor agonists injected immediately prior to NPY were examined (Currie et al., 2002; 1999; 1998).
The impact of these same compounds on NPY-induced alterations in energy metabolism was also assessed in an attempt to characterize the potential interactive relationship of PVN NPY and 5HT on feeding and whole body calorimetry. The compounds included a variety of 5-HT1A, 5-HT1B, 5-HT1D, 5-HT2A and 5-HT2C agonists. Only DOI, a 5-HT2A/2C agonist inhibited the effects of NPY on eating and RQ. All other agonists acting on the 5-HT1A-D and 5-HT2C receptors were ineffective. The action of DOI was in turn reversed by the 5-HT2A antagonist spiperone (as well as the 5-HT2A/2C antagonists, ketanserin and mianserin) but not by a variety of other selective 5-HT2B and 5-HT2C antagonists.
It should be noted that feeding evoked by NPY injections into the perifronical or ventromedial hypothalamus was not altered by local injection of 5-HT agonists. Our data suggest that 5-HT2A receptors within the PVN may exert a modulatory role over NPY. Several recent findings support such a relationship. For example, while no data are yet available on the 5-HT receptor subtype expressed by PVN NPY neurons, in a recent report, cortical NPY immunoreactivity and 5-HT2A/2C binding were detected in response to 5-HT depletion.
Reduced 5-HT availability is also found to decrease the density of striatal NPY neurons. While it is likely that NPY-serotonergic interactions are influenced by anatomical localization and cellular dynamics such as peptide-monoamine colocalization, these findings provide continued support for a potential interactive relationship between NPY and 5-HT mechanisms.
Hypothalamic PVN Urocortin (UCN)
A separate line of our research has investigated the potential interaction of hypothalamic urocortin (UCN) and NPY systems (Currie et al., 2001). UCN is a recently identified CRH-related peptide that suppresses food intake when injected into multiple hypothalamic and extrahypothalamic sites (Heinrichs et al., 1999; Skelton et al., 2000). We have recently reported that UCN decreases RQ thereby promoting fat oxidation. It is well documented that CRH itself alters energy balance and sympathetically mediated thermogenesis in addition to suppressing food intake elicited by NPY, norepineprhine and dynorphin.
Our data indicate that PVN injections of UCN inhibit the effect of PVN NPY on food intake and RQ. Also noteworthy is the finding that ventromedial hypothalamic injections of UCN similarly blunt the eating and RQ responses of PVN NPY. Finally we observed that PVN injections of DOI potentiated the magnitude of the suppression of UCN on both the feeding and RQ effects of NPY.
Hypothalamic Neuropeptide Integration
The past decade has seen an increased focus on the role of neuropeptides in the control of ingestive behavior and energy metabolism. More importantly, perhaps, is the increased attention given to how these peptides interact with one another in the brain and periphery. For example, it is now proposed that circulating leptin, secreted from fat cells, acts on leptin receptors in the hypothalamus to suppress peptide neurotransmitters implicated in the stimulation of eating, particularly, NPY and agouti-related peptide (AGRP). In contrast, leptin is reported to facilitate the action of amelanocyte stimulating hormone (aMSH), a feeding inhibitory peptide.
Feeding is diminished, therefore, by inhibiting arcuate neurons that release NPY and AGRP in the PVN, and exciting neurons that release aMSH. (the decrease in PVN AGRP, and increase in aMSH, results in an inhibition of eating via activation of PVN melanocortin receptors) We propose that, within the PVN, 5-HT and UCN receptor mechanisms act to inhibit the effects of NPY on both energy intake and energy substrate utilization with the candidate 5-HT receptor being the 5-HT2A subtype.
As discussed above, DOI potentiates the action of UCN suggesting that both neurotransmitters jointly modulate NPY function within the PVN. Since CRH antagonism reportedly blocks the inhibitory effect of DOI on NPY-induced eating, we are currently investigating whether CRH receptor antagonism effectively blocks UCN/5-HT action on both NPY hyperphagia and RQ.
Moreover, given that leptin receptor activation elicits increases in endogenous CRH levels, one could speculate that leptin may act as a neuromodulator of PVN 5-HT and UCN containing neurons. This is an additional line of work we are currently pursuing.
Q. Do the NPY-Y2-/- mice become obese?
A. Yes, they do.
Q. So does the NPY-Y2 receptor act as a type of ‘satiety’ receptor?
A. In a sense, since the Y2 receptor apparently behaves as a presynaptic, auto-reuptake receptor. However, both Y1 and Y5 receptors are, together, believed to exert the predominant feeding-stimulatory effects of NPY.
Q. Do you use swivel cannulae?
A. No; we infuse the animals and then return them to their cages.
Q. Rats normally increase locomotion when a meal is about to start; is there something abnormal about your rats-that is, is locomotion somehow suppressed?
A.No; they are just unaware that food is present.
Q. What are their RQ values?
A. The RQs at the start of the dark cycle typically range from 0.90 to 0.94.
Q. If an RQ is greater than 1, what could that mean?
A. When RQ exceeds 1.0, this indicates not only carbohydrate oxidation but also the synthesis of fat. The overall effect is to promote fat storage.
Q. What would be the pattern of food intake over time if RQ were increased?
A. We would see an increase of approximately 4 grams over 1.5-2 hours, but this amount is significantly influenced by the available diet and latency to eat.
Q. Might agonists specific to RQ responses also explain some of these feeding responses?
A. Yes; we hope to use these as well in future work.
Q. What are the metabolic effects, if any, of serotonergic agents?
A. A number of 5-HT compounds have been reported to alter metabolic rate. We have not found DOI to alter EE, RQ or metabolic rate at the doses we administered. With respect to indirect agonists, fenfluramine and fluoxetine both reduce NPY expression, via 5-HT2A and 5-HT2C receptor-mediated action, and they may alter metabolic activity.
Q. Is DOI specific to feeding, or does it have other effects?
A. At the subthreshold doses we used, there was no suppression of food intake. DOI has been reported to inhibit eating at higher doses although the specificity of this action (using a behavioral satiety sequence) has been questioned.
Q. Did you give urocortin to fasted animals?
A. No; we tested free-feeding animals at dark-onset.
Q. Did you notice any anxiogenic behavior?
A. Yes, in the lateral septum, but not in the PVN.
Q. Is the combination of DOI, urocortin and NPY additive or interactive?
A. Since DOI and urocortin each had no effect on their own, but together exert a significant effect, their combination appears to result in an interaction.
Q. How long does RQ remain elevated?
A. The effect is potent and runs out after about 2 hours at very low doses. At higher doses we have observed RQ effects lasting up to 6 hrs or longer.
Q. What happens to sham-fed animals if they are given NPY?
A. I’m not sure, but it would be interesting to determine whether NPY enhances hedonic effects rather than altering post-ingestive consequences.
Q. What is the effect of chronically elevating one’s RQ?
A. Although we have not assessed chronic manipulations, it follows that prolonged carbohydrate oxidation and decrease fat oxidation might contribute to enhanced body weight gain.
Q. Are there specific effects of NPY on catecholamines/monoamines?
A. Yes. NPY-catecholamine/monoamine interactions in the hypothalamus are well documented as reported from microdialysis, microinjection and lesion studies. For example, blockade of 5-HT synthesis, or lesions to 5-HT neurons, elevates hypothalamic NPY levels. Moreover, specific interactions with 5-HT2A and 5-HT2C receptors have been reported in the cortex, but have yet to be investigated in the PVN.
Q. To what degree is urocortin related to CRH?
A. Urocortin binds CRH receptors. It exhibits higher affinity for these receptors- particularly for CRH2.
Q. Basal levels of CRH in the PVN are low, but when stimulated, they shoot up; could urocortin have a similar effect?
A. Yes, this is certainly a possibility and one which we are currently exploring.
Q. Where is urocortin expressed?
A. Major cellular sites of expression in the rat brain include the E-W nucleus, the lateral superior olive, the lateral hypothalamus and the supraoptic nucleus. More recent evidence has included the PVN which itself has high concentrations of CRH2.
Q. How does leptin effect decerebrate animals?
A. It’s unknown.