Neuropeptide Y (NPY) plays an important role in stress, anxiety, obesity, and energy homeostasis via activation of NPY-Y1 receptors (Y1Rs) in the brain. However, global knockout of the Npy1r gene has low or no impact on anxiety and body weight. To uncover the role of limbic Y1Rs, we generated conditional knockout mice in which the inactivation of the Npy1r gene was restricted to excitatory neurons of the forebrain, starting from juvenile stages (Npy1r rfb ). Npy1r rfb mice exhibited increased anxiety and reduced body weight, less adipose tissue, and lower serum leptin levels. Npy1r rfb mutants also had a hyperactive hypothalamic-pituitaryadrenocortical axis, as indicated by higher peripheral corticosterone and higher density of NPY immunoreactive fibers and corticotropin releasing hormone immunoreactive cell bodies in the paraventricular hypothalamic nucleus. Importantly, through fostering experiments, we determined that differences in phenotype between Npy1r rfb and Npy1r 2lox mice became apparent when both genotypes were raised by FVB/J but not by C57BL/6J dams, suggesting that limbic Y1Rs are key targets of maternal care-induced programming of anxiety and energy homeostasis. where it is involved in the regulation of anxiety, stress reactions, energy balance, circadian rhythms, and cognition (1-3). Clinical studies suggest that NPY plays an important role in the response to stress and in psychiatric disorders (4). In humans, NPY haploinsufficiency is correlated with characteristic brain responses to emotional and stress challenges and with trait anxiety (5). Intracerebroventricular injection of NPY reduces both anxiety-and stress-related behavior in several animal models, an effect that is primarily mediated by Y1 receptors (Y1Rs) expressed in amygdala, hippocampus, and locus coeruleus (6-9). The implications of a role of endogenous NPY in acting via Y1R to control emotionality, mood, and stress reactions have been probed with Y1R-selective antagonists and antisense oligonucleotides (1). NPY exerts its anxiolytic-like effect in the brain via interactions with the hypothalamic-pituitaryadrenocortical (HPA) axis and corticosteroids. Indeed, a functional antagonism between NPY and corticotropin releasing hormone (CRH) has been demonstrated in various CNS nuclei along the stress/anxiety circuits (10).In addition to its crucial role in emotional behavior, NPY potently stimulates feeding, reduces energy expenditure, and induces obesity via the activation of Y1R expressed in the hypothalamus (1). However, global Npy1r gene knockout mice showed only minor deficiencies in energy homeostasis, feeding, and anxiety (11)(12)(13)(14).To study the function of Y1R expressed in the limbic system and to exclude effects induced by the Npy1r gene inactivation in early development, we restricted the ablation of Y1R to excitatory neurons in the postnatal forebrain of mice by using the Cre-loxP system (Fig. 1A). In addition, because early postnatal environment can modulate NPY levels (15), gene-targeted pups were reared by two diffe...
