Despite numerous experiments showing that administration of neuropeptide Y (NPY) to rodents stimulates feeding and obesity, whereas acute interference with NPY signaling disrupts feeding and promotes weight loss, NPY-null mice have essentially normal body weight regulation. These conflicting observations suggest that chronic lack of NPY during development may lead to compensatory changes that normalize regulation of food intake and energy expenditure in the absence of NPY. To test this idea, we used gene targeting to introduce a doxycycline (Dox)-regulated cassette into the Npy locus, such that NPY would be expressed until the mice were given Dox, which blocks transcription. Compared with wild-type mice, adult mice bearing this construct expressed Ϸ4-fold more Npy mRNA, which fell to Ϸ20% of control values within 3 days after treatment with Dox. NPY protein also fell Ϸ20-fold, but the half-life of Ϸ5 days was surprisingly long. The biological effectiveness of these manipulations was demonstrated by showing that overexpression of NPY protected against kainateinduced seizures. Mice chronically overexpressing NPY had normal body weight, and administration of Dox to these mice did not suppress feeding. Furthermore, the refeeding response of these mice after a fast was normal. We conclude that, if there is compensation for changes in NPY levels, then it occurs within the time it takes for Dox treatment to deplete NPY levels. These observations suggest that pharmacological inhibition of NPY signaling is unlikely to have long-lasting effects on body weight. body weight regulation ͉ conditional knockout ͉ feeding behavior N umerous studies using a variety of approaches have implicated neuropeptide Y (NPY) in the regulation of body weight. Injection of NPY or NPY-related peptides intracranially (either into the hypothalamus or cerebral ventricles) stimulates robust feeding (1-6); conditions of energy deprivation (starvation) or increased energy demand (lactation) induce NPY expression in the hypothalamus (7,8); most genetic models of obesity exhibit hyperphagia and an increase in hypothalamic NPY levels (9-13); and various pharmacological treatments that enhance feeding are accompanied by an increase in NPY (14). In addition, various methods (antisense oligonucleotides, antibodies, or receptor antagonists) aimed at blocking NPY actions in the hypothalamus have been shown to inhibit feeding (15)(16)(17)(18)(19)(20)(21)(22). Moreover, there is anatomical and electrophysiological evidence that NPY neurons in the arcuate (ARC) region of the hypothalamus have direct inhibitory inputs onto proopiomelanocortin (POMC) cells that form part of the melanocortin-signaling pathway, which suppresses feeding (23). Thus, current models for hypothalamic control of energy balance suggest that activation of POMC cell signaling to cells bearing melanocortin-4 receptors (MC4R) suppresses feeding, whereas NPY and various other hormones and neurotransmitters stimulate feeding by antagonizing this melanocortin-signaling pathway. NPYexpressing n...