Neuropeptide Y

Discovery

Following the isolation of neuropeptide-y (NPY) from the porcine hypothalamus in 1982, researchers began to speculate about the involvement of NPY in hypothalamic-mediated functions. In a 1983 study, NPY-ergic axon terminals were located in the paraventricular nucleus (PVN) of the hypothalamus, and the highest levels of NPY immunoreactivity was found within the PVN of the hypothalamus.

Six years later, in 1989, Morris et al. homed in on the location of NPYergic nuclei in the brain. Furthermore, in situ hybridization results from the study showed the highest cellular levels of NPY mRNA in the arcuate nucleus (ARC) of the hypothalamus.

In 1989, Haas & George reported that local injection of NPY into the PVN resulted in an acute release of corticotropin-releasing hormone (CRH) in the rat brain, proving that NPYergic activity directly stimulates the release and synthesis of CRH.

The latter became a hallmark paper in NPY studies. A significant amount of work had already been done in the 1970s on CRH and its involvement in stress and eating disorders such as obesity. These studies, collectively, marked the beginning of the role of NPY in orexigenesis or food intake.

Role in food intake

Behaviorial assays in orexigenic studies, in which rats are the model organism, have been done collectively with immunoassays and in situ hybridization studies to confirm that elevating NPY-ergic activity does indeed increase food intake. In these studies, exogenous NPY, an NPY agonist such as dexamethasone or N-acetyl [Leu 28, Leu31] NPY (24-36) are injected into the third ventricle or at the level of the hypothalamus with a cannula.

Furthermore, these studies unanimously demonstrate that the stimulation of NPYergic activity via the administration of certain NPY agonists increases food intake compared to baseline data in rats. The effects of NPYergic activity on food intake is also demonstrated by the blockade of certain NPY receptors (Y1 and Y5 receptors), which, as was expected, inhibited NPYergic activity; thus, decreases food intake. However, a 1999 study by King et al. demonstrated the effects of the activation of the NPY autoreceptor Y2, which has been shown to inhibit the release of NPY and thus acts to regulate food intake upon its activation. In this study a highly selective Y2 antagonist, BIIE0246 was administered locally into the ARC. Radioimmunoassay data, following the injection of BIIE0246, shows a significant increase in NPY release compared to the control group. Though the pharmacological half-life of exogenous NPY, other agonists, and antagonist is still obscure, the effects are not long lasting and the rat body employs an excellent ability to regulate and normalize abnormal NPY levels and therefore food consumption.

Role in obesity

Dryden et al., conducted a study in 1995 using genetically obese rats to demonstrate the role of NPY in eating disorders such as obesity. The study revealed four underlying factors that contributed to obesity in rats:

In obesity chronically elevated levels of NPY can be seen, this has been seen in rats fed on a high fat diet for 22 weeks and resulted in a genetic mutation increasing NPY release due to a defective leptin signal compared to control rats. In humans increased levels of free NPY were found in obese women and not in their leaner counterparts, analysing human hypothalamus' for NYP concentration however is more difficult than rats. During weaning in rats there is an early expression of gene mutations that increase hypothalamic release of NPY in rats, however in humans multiple genes are commonly associated with the results of obesity and metabolic syndrome. In most obesity cases the increased secretion of NPY is a central / hypothalamic resistance to energy excess hormone signals such as leptin, that can be a result of a variety of reasons in the CNS. In rodents resistant to obesity when fed on an obesogenic diet they had a significantly lower amount of NPY receptor in the hypothalamus suggesting an increased activity of NPY neurones in obese rats meaning that the reduction in the release of NPY may be beneficial to the reduction of obesity incidence alongside the consumption of a healthy diet and exercise. This would need to be seen in human research before looking at this avenue of weight loss although currently there is some evidence that suggests NPY is a significant predictor in weight regain after weight loss to maintain old levels of energy storage.

Furthermore, these factors correlate with each other. The sustained high levels of glucocorticosteroids stimulate gluconeogenesis, which subsequently causes an increase of blood glucose that activates the release of insulin to regulate glucose levels by causing its reuptake and storage as glycogen in the tissues in the body. In the case of obesity, which researchers speculate to have a strong genetic and a dietary basis, insulin resistance prevents high blood glucose regulation, resulting in morbid levels of glucose and diabetes mellitus. In addition, high levels of glucocorticosteroids causes an increase of NPY by directly activating type II glucocorticosteroids receptors (which are activated only by relatively high levels of glucocorticosteroids) and, indirectly, by abolishing the negative feedback of corticotropin-releasing factor (CRF) on NPY synthesis and release. Meanwhile, obesity-induced insulin resistance and the mutation of the leptin receptor (ObRb) results in the abolition of inhibition of NPYergic activity and ultimately food intake via other negative feedback mechanisms to regulate them. Obesity in rats was significantly reduced by adrenalectomy or hypophysectomy.

Correlation with stress and diet

Studies of mice and monkeys show that repeated stress—and a high-fat, high-sugar diet—stimulate the release of neuropeptide Y, causing fat to build up in the abdomen. Researchers believe that by manipulating levels of NPY, they could eliminate fat from areas where it was not desired and accumulate at sites where it is needed.

Conversely, higher levels of NPY may be associated with resilience against and recovery from posttraumatic stress disorder and with dampening the fear response, allowing individuals to perform better under extreme stress.

Alcoholism

Two results suggest that NPY might protect against alcoholism:

Receptors

The receptor protein that NPY operates on is a G protein-coupled receptor in the rhodopsin like 7-transmembrane GPCR family. Five subtypes of the NPY receptor have been identified in mammals, four of which are functional in humans. Subtypes Y1 and Y5 have known roles in the stimulation of feeding while Y2 and Y4 seem to have roles in appetite inhibition (satiety). Some of these receptors are among the most highly conserved neuropeptide receptors.