Graphical abstractHighlights► The distribution of GABAA receptor subunits is highly heterogeneous. ► The distribution of mRNAs corresponds to that of proteins. ► The distribution in the mouse correlates largely to that in rats although there are distinct differences.

Anxiety is integrated in the amygdaloid nuclei and involves the interplay of the amygdala and various other areas of the brain. Neuropeptides play a critical role in regulating this process. Neuropeptide Y (NPY), a 36 aa peptide, is highly expressed in the amygdala. It exerts potent anxiolytic effects through cognate postsynaptic Y1 receptors, but augments anxiety through presynaptic Y2 receptors. To identify the precise anatomical site(s) of Y2-mediated anxiogenic action, we investigated the effect of site-specific deletion of the Y2 gene in amygdaloid nuclei on anxiety and depression-related behaviors in mice. Ablating the Y2 gene in the basolateral and central amygdala resulted in an anxiolytic phenotype, whereas deletion in the medial amygdala or in the bed nucleus of the stria terminalis had no obvious effect on emotion-related behavior. Deleting the Y2 receptor gene in the central amygdala, but not in any other amygdaloid nucleus, resulted in an added antidepressant-like effect. It was associated with a reduction of presumably presynaptic Y2 receptors in the stria terminalis/bed nucleus of the stria terminalis, the nucleus accumbens, and the locus ceruleus. Our results are evidence of the highly site-specific nature of the Y2-mediated function of NPY in the modulation of anxiety-and depression-related behavior. The activity of NPY is likely mediated by the presynaptic inhibition of GABA and/or NPY release from interneurons and/or efferent projection neurons of the basolateral and central amygdala.

Highlights d Central amygdala NPY neurons control feeding d Stress combined with a high-caloric diet increases NPY expression in the central amygdala d Insulin controls NPY expression in central amygdala neurons d Stress combined with a high-caloric diet causes insulin resistance in central amygdala

While anxiety disorders are the brain disorders with the highest prevalence and constitute a major burden for society, a considerable number of affected people are still treated insufficiently. Thus, in an attempt to identify potential new anxiolytic drug targets, neuropeptides have gained considerable attention in recent years. Compared to classical neurotransmitters they often have a regionally restricted distribution and may bind to several distinct receptor subtypes. Neuropeptide Y (NPY) is a highly conserved neuropeptide that is specifically concentrated in limbic brain areas and signals via at least 5 different G-protein-coupled receptors. It is involved in a variety of physiological processes including the modulation of emotional-affective behaviors. An anxiolytic and stress-reducing property of NPY is supported by many preclinical studies. Whether NPY may also interact with processing of learned fear and fear extinction is comparatively unknown. However, this has considerable relevance since pathological, inappropriate and generalized fear expression and impaired fear extinction are hallmarks of human post-traumatic stress disorder and a major reason for its treatment-resistance. Recent evidence from different laboratories emphasizes a fear-reducing role of NPY, predominantly mediated by exogenous NPY acting on Y1 receptors. Since a reduction of fear expression was also observed in Y1 receptor knockout mice, other Y receptors may be equally important. By acting on Y2 receptors, NPY promotes fear extinction and generates a long-term suppression of fear, two important preconditions that could support cognitive behavioral therapies in human patients. A similar effect has been demonstrated for the closely related pancreatic polypeptide (PP) when acting on Y4 receptors. Preliminary evidence suggests that NPY modulates fear in particular by activation of Y1 and Y2 receptors in the basolateral and central amygdala, respectively. In the basolateral amygdala, NPY signaling activates inhibitory G protein-coupled inwardly-rectifying potassium channels or suppresses hyperpolarization-induced I(h) currents in a Y1 receptor-dependent fashion, favoring a general suppression of neuronal activity. A more complex situation has been described for the central extended amygdala, where NPY reduces the frequency of inhibitory and excitatory postsynaptic currents. In particular the inhibition of long-range central amygdala output neurons may result in a Y2 receptor-dependent suppression of fear. The role of NPY in processes of learned fear and fear extinction is, however, only beginning to emerge, and multiple questions regarding the relevance of endogenous NPY and different receptor subtypes remain elusive. Y2 receptors may be of particular interest for future studies, since they are the most prominent Y receptor subtype in the human brain and thus among the most promising therapeutic drug targets when translating preclinical evidence to potential new therapies for human anxiety disorders.

Neuropeptide Y (NPY) acting through Y1 receptors reduces anxiety-and depression-like behavior in rodents, whereas Y2 receptor stimulation has the opposite effect. This study addressed the implication of Y4 receptors in emotional behavior by comparing female germ line Y4 knockout (Y42/2) mice with control and germ line Y22/2 animals. Anxiety-and depression-like behavior was assessed with the open field (OF), elevated plus maze (EPM), stress-induced hyperthermia (SIH) and tail suspension tests (TST), respectively. Learning and memory were evaluated with the object recognition test (ORT). In the OF and EPM, both Y42/2 and Y22/2 mice exhibited reduced anxiety-related behavior and enhanced locomotor activity relative to control animals. Locomotor activity in a familiar environment was unchanged in Y42/2 but reduced in Y22/2 mice. The basal rectal temperature exhibited diurnal and genotype-related alterations. Control mice had temperature minima at noon and midnight, whereas Y42/2 and Y22/2 mice displayed only one temperature minimum at noon. The magnitude of SIH was related to time of the day and genotype in a complex manner. In the TST, the duration of immobility was significantly shorter in Y42/2 and Y22/2 mice than in controls. Object memory 6 h after initial exposure to the ORT was impaired in Y22/2 but not in Y42/2 mice, relative to control mice. These results show that genetic deletion of Y4 receptors, like that of Y2 receptors, reduces anxiety-like and depression-related behavior. Unlike Y2 receptor knockout, Y4 receptor knockout does not impair object memory. We propose that Y4 receptors play an important role in the regulation of behavioral homeostasis. There is evidence that both Y1 and Y2 receptors are relevant to emotional behavior. Intracerebroventricular injection of NPY reduces anxiety-and depression-related behavior in several animal models, this action being primarily mediated by Y1 receptors (Heilig 2004;Kask et al. 2002;Primeaux et al. 2005;Redrobe et al. 2002). Neuropeptide Y acting through Y2 receptors enhances anxiety-and depression-like behavior as deduced from the behavioral characterization of Y2 receptor knockout (Y2À/À) mice (Redrobe et al. 2003;Tschenett et al. 2003). In addition, Y2 receptors are relevant to cognitive functions, given that Y2À/À mice exhibit impaired performance in the Morris water maze and object recognition tests (ORT) (Redrobe et al. 2004b).The possible role of Y4 receptors in the control of affective behavior has not yet been examined. Albeit less widely distributed in the brain than Y1 and Y2 receptors, the presence of Y4 receptors in hypothalamus, limbic system and medullary brainstem (Dumont et al. 1998;Fetissov et al. 2004;Heilig 2004;Kask et al. 2002;Parker & Herzog, 1999;Stanic et al. 2006) is consistent with a putative role of Y4 receptors in emotional and stress-related behavior. As Y4 receptor-selective antagonists are not yet available, the first and major aim of the present study was to evaluate anxietylike and depression-related behavior in Y4 receptor kno...