Pancreatic polypeptide and its central Y4 receptors are essential for cued fear extinction and permanent suppression of fear

Background and purpose

Avoiding danger and finding food are closely related behaviours that are essential for surviving in a natural environment. Growing evidence supports an important role of gut‐brain peptides in modulating energy homeostasis and emotional‐affective behaviour. For instance, postprandial release of pancreatic polypeptide (PP) reduced food intake and altered stress‐induced motor activity and anxiety by activating central Y₄ receptors.

Experimental approach

We characterized [K³⁰(PEG2)]hPP_2‐36 as long‐acting Y₄ receptor agonist and injected it peripherally into wildtype and Y₄ receptor knockout (Y4KO) C57Bl/6NCrl mice to investigate the role of Y₄ receptors in fear conditioning. Extinction and relapse after extinction was measured by spontaneous recovery and renewal.

Key results

The Y4KO mice showed impaired cued and context fear extinction without affecting acquisition, consolidation or recall of fear. Correspondingly, peripheral injection of [K³⁰(PEG2)]hPP_2‐36 facilitated extinction learning upon fasting, an effect that was long‐lasting and generalized. Furthermore, peripherally applied [K³⁰(PEG2)]hPP_2‐36 before extinction inhibited the activation of orexin‐expressing neurons in the lateral hypothalamus in WT, but not in Y4KO mice.

Conclusions and implications

Our findings suggests suppression of excessive arousal as a possible mechanism for the extinction‐promoting effect of central Y₄ receptors and provide strong evidence that fear extinction requires integration of vegetative stimuli with cortical and subcortical information, a process crucially depending on Y₄ receptors. Importantly, in the lateral hypothalamus two peptide systems, PP and orexin, interact to generate an emotional response adapted to the current homeostatic state. Detailed investigations of feeding‐relevant genes may thus deliver multiple intervention points for treating anxiety‐related disorders.

The role of Neuropeptide Y in fear conditioning and extinction

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.

Structure and function of the amygdaloid NPY system: NPY Y2 receptors regulate excitatory and inhibitory synaptic transmission in the centromedial amygdala

The amygdala is essential for generating emotional-affective behaviors. It consists of several nuclei with highly selective, elaborate functions. In particular, the central extended amygdala, consisting of the central amygdala (CEA) and the bed nucleus of the stria terminalis (BNST) is an essential component actively controlling efferent connections to downstream effectors like hypothalamus and brain stem. Both, CEA and BNST contain high amounts of different neuropeptides that significantly contribute to synaptic transmission. Among these, neuropeptide Y (NPY) has emerged as an important anxiolytic and fear-reducing neuromodulator. Here, we characterized the expression, connectivity and electrophysiological function of NPY and Y2 receptors within the CEA. We identified several NPY-expressing neuronal populations, including somatostatin- and calretinin-expressing neurons. Furthermore, in the main intercalated nucleus, NPY is expressed primarily in dopamine D1 receptor-expressing neurons but also in interspersed somatostatin-expressing neurons. Interestingly, NPY neurons did not co-localize with the Y2 receptor. Retrograde tract tracing experiments revealed that NPY neurons reciprocally connect the CEA and BNST. Functionally, the Y2 receptor agonist PYY3-36, reduced both, inhibitory as well as excitatory synaptic transmission in the centromedial amygdala (CEm). However, we also provide evidence that lack of NPY or Y2 receptors results in increased GABA release specifically at inhibitory synapses in the CEm. Taken together, our findings suggest that NPY expressed by distinct populations of neurons can modulate afferent and efferent projections of the CEA via presynaptic Y2 receptors located at inhibitory and excitatory synapses.

NPY Y2 receptors in the central amygdala reduce cued but not contextual fear

The amygdala is fundamental for associative fear and extinction learning. Recently, also the central nucleus of the amygdala (CEA) has emerged as a site of plasticity actively controlling efferent connections to downstream effector brain areas. Although synaptic transmission is primarily mediated by glutamate and GABA, neuropeptides critically influence the overall response. While neuropeptide Y (NPY) acting via postsynaptic Y1 receptors exerts an important anxiolytic and fear-reducing action, the role of the predominantly presynaptic Y2 receptors is less defined. To investigate the role of Y2 receptors in the CEA we employed viral-vector mediated over-expression of the Y2 selective agonist NPY3-36 in fear conditioning and extinction experiments. NPY3-36 over-expression in the CEA resulted in reduced fear expression during fear acquisition and recall. Interestingly, this effect was blocked by intraperitoneal injection of a brain-penetrant Y2 receptor antagonist. Furthermore, over-expression of NPY3-36 in the CEA also reduced fear expression during fear extinction of CS-induced but not context-related fear. Again, fear extinction appeared delayed by peripheral injection of a Y2 receptor antagonist JNJ-31020028. Importantly, mice with over-expression of NPY3-36 in the CEA also displayed reduced spontaneous recovery and reinstatement, suggesting that Y2 receptor activation supports a permanent suppression of fear. Local deletion of Y2 receptors in the CEA, on the other hand, increased the expression of CS-induced freezing during fear recall and fear extinction. Thus, NPY inhibits fear learning and promotes cued extinction by reducing fear expression also via activation of presynaptic Y2 receptors on CEA neurons.

Patterns of mRNA and protein expression for 12 GABAA receptor subunits in the mouse brain

The GABA_A receptor is the main inhibitory receptor in the brain and its subunits originate from different genes or gene families (α1–α6, β1–β3, γ1–γ3, δ, ε, θ, π, or ρ1–3). In the mouse brain the anatomical distribution of GABA_A receptor subunit mRNAs so far investigated is restricted to subunits forming benzodiazepine-sensitive receptor complexes (α1–α3, α5, β2, β3 and γ2) in the forebrain and midbrain as assessed by in situ hybridization (ISH). In the present study the anatomical distribution of the GABA_A receptor subunits α1–α6, β1–β3, γ1–γ2 and δ was analyzed in the mouse brain (excluding brain stem) by ISH and immunohistochemistry (IHC). In several brain areas such as hippocampus, cerebellum, bulbus olfactorius and habenula we observed that mRNA levels did not reflect protein levels, indicating that the protein is located far distantly from the cell body. We also compared the distribution of these 12 subunit mRNAs and proteins with that reported in the rat brain. Although in general there is a considerable correspondence in the distribution between mouse and rat brains, several species-specific differences were observed.

NPY controls fear conditioning and fear extinction by combined action on Y₁ and Y₂ receptors

BACKGROUND AND PURPOSE

Neuropeptide Y (NPY) and its receptors have been implicated in the control of emotional-affective processing, but the mechanism is unclear. While it is increasingly evident that stimulation of Y₁ and inhibition of Y₂ receptors produce prominent anxiolytic and antidepressant effects, the contribution of the individual NPY receptor subtypes in the acquisition and extinction of learned fear are unknown.

EXPERIMENTAL APPROACH

Here we performed Pavlovian fear conditioning and extinction in NPY knockout (KO) and in NPY receptor KO mice.

KEY RESULTS

NPY KO mice display a dramatically accelerated acquisition of conditioned fear. Deletion of Y₁ receptors revealed only a moderately accelerated acquisition of conditioned fear, while lack of Y₂ receptors was without any effect on fear learning. However, the strong phenotype seen in NPY KO mice was reproduced in mice lacking both Y₁ and Y₂ receptors. In addition, NPY KO mice showed excessive recall of conditioned fear and impaired fear extinction. This behaviour was replicated only after deletion of both Y₁ and Y₂ receptors. In Y₁ receptor single KO mice, fear extinction was delayed and was unchanged in Y₂ receptor KO mice. Deletion of NPY and particularly Y₂ receptors resulted in a generalization of conditioned fear.

CONCLUSIONS AND IMPLICATIONS

Our data demonstrate that NPY delays the acquisition, reduces the expression of conditioned fear while promoting fear extinction. Although these effects appear to be primarily mediated by Y₁ receptors, the pronounced phenotype of Y₁Y₂ receptor double KO mice suggests a synergistic role of Y₂ receptors in fear acquisition and in fear extinction.

Altered GABA transmission in a mouse model of increased trait anxiety

Anxiety disorders are the most prevalent central nervous system diseases imposing a high social burden to our society. Emotional processing is particularly controlled by GABA-ergic transmission in the amygdala. Using in situ hybridization and immunohistochemistry we now investigated changes in the expression of GABA synthesizing enzymes (GAD65 and GAD67), GABA_A (α1–5, β1–3, γ1–2) and GABA_B receptor subunits (GBBR1, GBBR2) in amygdaloid nuclei of high anxiety-related behavior (HAB) mice in comparison to mice selected for normal anxiety-related behavior (NAB). Levels of GAD65 and GAD67 mRNAs and protein, as well as those of GABA were increased in the amygdala of HAB mice. Relative to NAB controls, mRNA expression of the GABA_A receptor subunits β1, β2 and γ2 was specifically increased in the basolateral amygdala of HAB mice while transcription of α5 and γ1 subunits was reduced in the central and medial amygdala. On the protein level, increases in β2 and γ2 subunit immunoreactivities were evident in the basolateral amygdala of HAB mice. No change in GABA_B receptor expression was observed. These findings point towards an imbalanced GABA-ergic neurotransmission in the amygdala of HAB mice. On the other hand, FosB, a marker for neuronal activity, was increased in principal neurons of the basolateral amygdala in HAB mice, reflecting activation of excitatory neurons, possibly as a consequence of reduced GABA-ergic tonic inhibition through α5 and γ1 containing receptors. Ultimately these mechanisms may lead to the compensatory activation of GABA transmission, as indicated by the increased expression of GAD65/67 in HAB mice.

Increased novelty-induced motor activity and reduced depression-like behavior in neuropeptide Y (NPY)-Y4 receptor knockout mice

There is growing evidence that neuropeptide Y (NPY) acting through Y1 and Y2 receptors has a prominent role in modulating anxiety- and depression-like behavior in rodents. However, a role of other Y-receptors like that of Y4 receptors in this process is poorly understood. We now investigated male Y2, Y4 single and Y2/Y4 double knockout mice in behavioral paradigms for changes in motor activity, anxiety and depression-like behavior. Motor activity was increased in Y2, Y4 and Y2/Y4 knockout mice under changing and stressful conditions, but not altered in a familiar environment. Y4 and Y2 knockout mice revealed an anxiolytic phenotype in the light/dark test, marble burying test and in stress-induced hyperthermia, and reduced depression-like behavior in the forced swim and tail suspension tests. In Y2/Y4 double knockout mice, the response in the light/dark test and in the forced swim test was further enhanced compared with Y4 and Y2 knockout mice, respectively. High levels of Y4 binding sites were observed in brain stem nuclei including nucleus of solitary tract and area postrema. Lower levels were found in the medial amygdala and hypothalamus. Peripheral administration of pancreatic polypeptide (PP) induced Y4 receptor-dependent c-Fos expression in brain stem, hypothalamus and amygdala. PP released peripherally from the pancreas in response to food intake, may act not only as a satiety signal but also modulate anxiety-related locomotion.

Reduced anxiety-like and depression-related behavior in neuropeptide Y Y4 receptor knockout mice

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 (Y4-/-) mice with control and germ line Y2-/- 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 Y4-/- and Y2-/- mice exhibited reduced anxiety-related behavior and enhanced locomotor activity relative to control animals. Locomotor activity in a familiar environment was unchanged in Y4-/- but reduced in Y2-/- mice. The basal rectal temperature exhibited diurnal and genotype-related alterations. Control mice had temperature minima at noon and midnight, whereas Y4-/- and Y2-/- 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 Y4-/- and Y2-/- mice than in controls. Object memory 6 h after initial exposure to the ORT was impaired in Y2-/- but not in Y4-/- 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.