Energy homeostasis is the process by which adipose tissue, the stored body energy, is kept constant over time. Obviously, feeding behavior plays a crucial role in energy homeostasis. Therefore, to maintain a constant energy balance, energy (food) intake should match energy expenditure. A number of humoral factors are involved in signaling the body need in energy to modulate food intake. Among these hormones, leptin and insulin are secreted proportionally to the amount of body fat. They act in the central nervous system to regulate the action and expression of a variety of neurotransmitters and then modify energy balance by changing food intake and weight gain. The neurotransmitter neuropeptide Y (NPY) is a key player in the modulation of the leptin signal (1). In particular, NPY promotes weight gain via its stimulatory effect on food intake in the hypothalamus. In addition, more evidence suggests that NPY also acts on specific target organs at the periphery to modify body fat stores. At least five different NPY receptor subtypes have been identified (2). It has been hypothesized that each of these receptors might be associated with specific NPY functions.