Central NPY receptor-mediated alteration of heart rate dynamics in mice during expression of fear conditioned to an auditory cue

Identifying Cardiovascular Risk by Nonlinear Heart Rate Dynamics Analysis: Translational Biomarker from Mice to Humans

BACKGROUND

The beat-by-beat fluctuation of heart rate (HR) in its temporal sequence (HR dynamics) provides information on HR regulation by the autonomic nervous system (ANS) and its dysregulation in pathological states. Commonly, linear analyses of HR and its variability (HRV) are used to draw conclusions about pathological states despite clear statistical and translational limitations.

OBJECTIVE

The main aim of this study was to compare linear and nonlinear HR measures, including detrended fluctuation analysis (DFA), based on ECG recordings by radiotelemetry in C57BL/6N mice to identify pathological HR dynamics.

METHODS

We investigated different behavioral and a wide range of pharmacological interventions which alter ANS regulation through various peripheral and/or central mechanisms including receptors implicated in psychiatric disorders. This spectrum of interventions served as a reference system for comparison of linear and nonlinear HR measures to identify pathological states.

RESULTS

Physiological HR dynamics constitute a self-similar, scale-invariant, fractal process with persistent intrinsic long-range correlations resulting in physiological DFA scaling coefficients of α~1. Strongly altered DFA scaling coefficients (α ≠ 1) indicate pathological states of HR dynamics as elicited by (1) parasympathetic blockade, (2) parasympathetic overactivation and (3) sympathetic overactivation but not inhibition. The DFA scaling coefficients are identical in mice and humans under physiological conditions with identical pathological states by defined pharmacological interventions.

CONCLUSIONS

Here, we show the importance of tonic vagal function for physiological HR dynamics in mice, as reported in humans. Unlike linear measures, DFA provides an important translational measure that reliably identifies pathological HR dynamics based on altered ANS control by pharmacological interventions. Central ANS dysregulation represents a likely mechanism of increased cardiac mortality in psychiatric disorders.

Heart rate variability in neonatal seizures: Investigation and implications for management

Many factors acting during the neonatal period can affect neurological development of the infant. Neonatal seizures (NS) that frequently occur in the immature brain may influence autonomic maturation and lead to detectable cardiovascular signs. These autonomic manifestations can also have significant diagnostic and prognostic value. The analysis of Heart Rate Variability (HRV) represents the most used and feasible method to evaluate cardiac autonomic regulation. This narrative review summarizes studies investigating HRV dynamics in newborns with seizures, with the aim of highlighting the potential utility of HRV measures for seizure detection and management. While HRV analysis in critically ill newborns is influenced by many potential confounders, we suggest that it can enhance the ability to better diagnose seizures in the clinical setting. We present potential applications of the analysis of HRV, which could have a useful future role, beyond the research setting.

P11 deficiency increases stress reactivity along with HPA axis and autonomic hyperresponsiveness

Patients suffering from mood disorders and anxiety commonly exhibit hypothalamic-pituitary-adrenocortical (HPA) axis and autonomic hyperresponsiveness. A wealth of data using preclinical animal models and human patient samples indicate that p11 deficiency is implicated in depression-like phenotypes. In the present study, we used p11-deficient (p11KO) mice to study potential roles of p11 in stress responsiveness. We measured stress response using behavioral, endocrine, and physiological readouts across early postnatal and adult life. Our data show that p11KO pups respond more strongly to maternal separation than wild-type pups, even though their mothers show no deficits in maternal behavior. Adult p11KO mice display hyperactivity of the HPA axis, which is paralleled by depression- and anxiety-like behaviors. p11 was found to be highly enriched in vasopressinergic cells of the paraventricular nucleus and regulates HPA hyperactivity in a V_1B receptor-dependent manner. Moreover, p11KO mice display sympathetic-adrenal-medullary (SAM) axis hyperactivity, with elevated adrenal norepinephrine and epinephrine levels. Using conditional p11KO mice, we demonstrate that this SAM hyperactivity is partially regulated by the loss of p11 in serotonergic neurons of the raphe nuclei. Telemetric electrocardiogram measurements show delayed heart rate recovery in p11KO mice in response to novelty exposure and during expression of fear following auditory trace fear conditioning. Furthermore, p11KO mice have elevated basal heart rate in fear conditioning tests indicating increased autonomic responsiveness. This set of experiments provide strong and versatile evidence that p11 deficiency leads to HPA and SAM axes hyperresponsiveness along with increased stress reactivity.

Neuropeptide Y as Alternative Pharmacotherapy for Antidepressant-Resistant Social Fear

In many social anxiety disorder (SAD) patients, the efficacy of antidepressant therapy is unsatisfactory. Here, we investigated whether mice deficient for the lysosomal glycoprotein acid sphingomyelinase (ASM−/−) represent an appropriate tool to study antidepressant-resistant social fear. We also investigated whether neuropeptide Y (NPY) reduces this antidepressant-resistant social fear in ASM−/− mice, given that NPY reduced social fear in a mouse model of SAD, namely social fear conditioning (SFC). We show that neither chronic paroxetine nor chronic amitriptyline administration via drinking water were successful in reducing SFC-induced social fear in ASM−/− mice, while the same treatment reduced social fear in ASM+/− mice and completely reversed social fear in ASM+/+ mice. This indicates that the antidepressants paroxetine and amitriptyline reduce social fear via the ASM-ceramide system and that ASM−/− mice represent an appropriate tool to study antidepressant-resistant social fear. The intracerebroventricular administration of NPY, on the other hand, reduced social fear in ASM−/− mice, suggesting that NPY might represent an alternative pharmacotherapy for antidepressant-resistant social fear. These results suggest that medication strategies aimed at increasing brain NPY concentrations might improve symptoms of social fear in SAD patients who fail to respond to antidepressant treatments.

Neuropeptide Y reduces expression of social fear via simultaneous activation of Y1 and Y2 receptors

BACKGROUND

Neuropeptide Y (NPY) has anxiolytic effects and facilitates extinction of cued and contextual fear in rodents, thereby acting as a resilience factor against exaggerated fear responses after adverse events. We investigated whether NPY influences acquisition, expression and extinction of social fear in a mouse model of social fear conditioning (SFC).

METHODS

NPY was administered intracerebroventricularly before SFC or before social fear extinction with or without prior administration of Y1 and/or Y2 receptor antagonists.

RESULTS

We show that NPY affects SFC-induced social fear in a time point-dependent manner. When administered before SFC, NPY did not affect acquisition, expression and extinction of social fear. However, when administered before social fear extinction, NPY reduced expression of social fear via simultaneous activation of Y1 and Y2 receptors. As such, neither the Y1 receptor antagonist BIBO3304 trifluoroacetate nor the Y2 receptor antagonist BIIE0246 was able to block the effects of NPY completely. However, when administered in combination, they completely blocked the effects of NPY on social fear expression.

CONCLUSIONS

These findings have important clinical implications, as they suggest that although medication strategies aimed at increasing brain NPY activity are unlikely to prevent the formation of aversive memories after a traumatic social experience, they might improve the recovery from a traumatic social experience by reducing the expression of social fear.

Increased amygdalar metabotropic glutamate receptor 7 mRNA in a genetic mouse model of impaired fear extinction

RATIONALE

Post-traumatic stress disorder (PTSD) is a devastating anxiety-related disorder which develops subsequent to a severe psychologically traumatic event. Only ~ 9% of people who experience such a trauma develop PTSD. It is clear that a number of factors, including genetics, influence whether an individual will develop PTSD subsequent to a trauma. The 129S1/SvImJ (S1) inbred mouse strain displays poor fear extinction and may be useful to model this specific aspect of PTSD. The metabotropic glutamate receptor 7 (mGlu₇ receptor) has previously been shown to be involved in cognitive processes and anxiety-like behaviour placing it in a key position to regulate fear extinction processes. We sought to compare mGlu₇ receptor mRNA levels in the S1 strain with those in the robustly extinguishing C57BL/6J (B6) inbred strain using in situ hybridisation (ISH) in three brain regions associated with fear extinction: the amygdala, hippocampus and prefrontal cortex (PFC).

RESULTS

Compared to the B6 strain, S1 mice had increased mGlu₇ receptor mRNA levels in the lateral amygdala (LA) and basolateral amygdala (BLA) subdivisions. An increase was also seen in the hippocampal CA1 and CA3 subregions of S1 mice. No difference in mGlu₇ receptor levels were seen in the central nucleus (CeA) of the amygdala, dentate gyrus (DG) of the hippocampus or prefrontal cortex.

CONCLUSIONS

These data show altered mGlu₇ receptor expression in key brain regions associated with fear extinction in two different inbred mouse strains which differ markedly in their fear extinction behaviour. Altered mGlu₇ receptor levels may contribute to the deficit fear extinction processes seen in fear extinction in the S1 strain.

Reduced NPY Y1 receptor hippocampal expression and signs of decreased vagal modulation of heart rate in mice

Central neuropeptide Y (NPY) signaling participates in the regulation of cardiac autonomic outflow, particularly via activation of NPY-Y1 receptors (Y1Rs). However, the specific brain areas and neural pathways involved have not been completely identified yet. Here, we evaluate the role of hippocampal Y1Rs in the modulation of the autonomic control of cardiac function using a conditional knockout mouse model. Radiotelemetric transmitters were implanted in 4-month-old male mice exhibiting reduced forebrain expression (rfb) of the Y1R (Npy1r^rfb, n=10) and their corresponding controls (Npy1r^2lox, n=8). ECG signals were recorded (i) during resting conditions, (ii) under selective pharmacological manipulation of cardiac vagal activity, and (iii) during acute and chronic psychosocial stress challenges, and analyzed via time- and frequency-domain analysis of heart rate variability. Npy1r^rfb mice showed a lower Npy1r mRNA density in the dentate gyrus and in the CA1 region of the hippocampus. Under resting undisturbed conditions, Npy1r^rfb mice exhibited (i) a higher heart rate, (ii) a reduced overall heart rate variability, and (iii) lower values of the indices of vagal modulation compared to Npy1r^2lox counterparts. Following pharmacological vagal inhibition, heart rate was higher in control but not in Npy1r^rfb mice compared to their respective baseline values, suggesting that tonic vagal influences on heart rate were reduced in Npy1r^rfb mice. The magnitude of the heart rate response to acute stressors was smaller in Npy1r^rfb mice compared to Npy1r^2lox counterparts, likely due to a concurrent lower vagal withdrawal. These findings suggest that reduced Y1R expression leads to a decrease in resting vagal modulation and heart rate variability, which, in turn, may determine a reduced cardiac autonomic responsiveness to acute stress challenges.

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.

Targeting the Neuropeptide Y System in Stress-related Psychiatric Disorders

Repeated, extreme, or traumatic stressors can elicit pathological effects leading to many negative physical and psychological outcomes. Stressors can precipitate the onset of psychiatric diseases, or exacerbate pre-existing disorders including various anxiety and mood disorders. As stressors can negatively impact human psychiatric health, it is essential to identify neurochemicals that may confer protection from the negative sequelae of repeated or extreme stress exposure. Elucidating the neurobiological underpinnings of stress resilience will enhance our ability to promote resilience to, or recovery from, stress-related psychiatric disease. Herein, we will review the evidence for neuropeptide Y as an endogenous mediator of resilience and its potential relevance for the treatment of stress-related psychiatric diseases.

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Overview of neuropeptide Y and receptor subtypes in the central nervous system.

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Alterations of neuropeptide Y in human stress-related psychiatric disorders.

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Evidence for neuropeptide Y in resilience to stress-related emotionality in rodent behavioral models.

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Pharmacotherapeutic implications for neuropeptide Y in the treatment of stress-related psychiatric disorders.

Central 5-HT1A receptor-mediated modulation of heart rate dynamics and its adjustment by conditioned and unconditioned fear in mice

BACKGROUND AND PURPOSE

The beat-by-beat fluctuation (dynamics) of heart rate (HR) depends on centrally mediated control of the autonomic nervous system (ANS) reflecting the physiological state of an organism. 5-HT1A receptors are implicated in affective disorders,associated with ANS dysregulation which increases cardiac risk but their role in autonomic HR regulation under physiological conditions is insufficiently characterized.

EXPERIMENTAL APPROACH

The effects of subcutaneously administered 5-HT1A receptor ligands on HR dynamics were investigated in C57BL/6 mice during stress-free conditions and emotional challenge (recall of fear conditioned to an auditory stimulus and novelty exposure) using time domain and non-linear HR analyses.

KEY RESULTS

Pre-training treatment with of 8-OH-DPAT (0.5 mg·kg(-1) , s.c.) prevented conditioned tachycardia in the retention test indicating impaired fear memory. Pretest 5-HT1A receptor activation by 8-OH-DPAT (0.5 but not 0.1 and 0.02 mg·kg(-1) ) caused bradycardia and increased HR variability. 8-OH-DPAT (0.5 mg·kg(-1) ) lowered the unconditioned and conditioned tachycardia from ∼750 to ∼550 bpm, without changing the conditioned HR response to the sound. 8-OH-DPAT induced profound QT prolongation and bradyarrhythmic episodes. Non-linear analysis indicated a pathological state of HR dynamics after 8-OH-DPAT (0.5 mg·kg(-1) ) with ANS hyperactivation impairing HR adaptability. The 5-HT1A receptor antagonist WAY-100635 (0.03 mg·kg(-1) ) blocked these effects of 8-OH-DPAT.

CONCLUSIONS AND IMPLICATIONS

Pre-training 5-HT1A receptor activation by 8-OH-DPAT (0.5 mg·kg(-1) ) impaired memory of conditioned auditory fear based on an attenuated HR increase, whereas pretest administration did not prevent the fear-conditioned HR increase but induced pathological HR dynamics through central ANS dysregulation with cardiac effects similar to acute SSRI overdose.

Munc18-1 haploinsufficiency results in enhanced anxiety-like behavior as determined by heart rate responses in mice

Heterozygous (HZ) missense mutations in the gene encoding syntaxin binding protein 1 (Stxbp1 or Munc18-1), a presynaptic protein essential for neurotransmitter release, causes early infantile epileptic encephalopathy, abnormal brain structure and mental retardation in humans. Here we investigated whether the mouse model mimics symptoms of the human phenotype. The effects of the deletion of munc18-1 were studied in HZ and wild-type (WT) mice based on heart rate (HR) and its variability (HRV) as independent measures to expand previous behavioral results of enhanced anxiety and impaired emotional learning suggesting mild cognitive impairments. HR responses were assessed during novelty exposure, during the expression and extinction of conditioned tone-dependent fear and during the diurnal phase. Novelty exposure yielded no differences in activity patterns between the two genotypes, while maximum HR differed significantly (WT: 770 bpm; HZ: 790 bpm). Retention tests after both auditory delay and trace fear conditioning showed a delayed extinction of the conditioned HR response in HZ mice compared to WT mice. Since the HR versus HRV correlation and HR dynamics assessed by nonlinear methods revealed similar function in HZ and WT mice, the higher HR responses of munc18-1 HZ mice to different emotional challenges cannot be attributed to differences in autonomic nervous system function. Thus, in contrast to the adverse consequences of deletion of a single allele of munc18-1 in humans, C57BL/6J mice show enhanced anxiety responses based on HR adjustments that extend previous results on the behavioral level without support of cognitive impairment, epileptic seizures and autonomic dysregulation.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

Neuropeptide regulation of fear and anxiety: Implications of cholecystokinin, endogenous opioids, and neuropeptide Y

The neural circuitry of fear likely underlies anxiety and fear-related disorders such as specific and social phobia, panic disorder, and posttraumatic stress disorder. The primary pharmacological treatments currently utilized for these disorders include benzodiazepines, which act on the GABAergic receptor system, and antidepressants, which modulate the monamine systems. However, recent work on the regulation of fear neural circuitry suggests that specific neuropeptide modulation of this system is of critical importance. Recent reviews have examined the roles of the hypothalamic-pituitary-adrenal axis neuropeptides as well as the roles of neurotrophic factors in regulating fear. The present review, instead, will focus on three neuropeptide systems which have received less attention in recent years but which are clearly involved in regulating fear and its extinction. The endogenous opioid system, particularly activating the μ opioid receptors, has been demonstrated to regulate fear expression and extinction, possibly through functioning as an error signal within the ventrolateral periaqueductal gray to mark unreinforced conditioned stimuli. The cholecystokinin (CCK) system initially led to much excitement through its potential role in panic disorder. More recent work in the CCK neuropeptide pathway suggests that it may act in concordance with the endogenous cannabinoid system in the modulation of fear inhibition and extinction. Finally, older as well as very recent data suggests that neuropeptide Y (NPY) may play a very interesting role in counteracting stress effects, enhancing extinction, and enhancing resilience in fear and stress preclinical models. Future work in understanding the mechanisms of neuropeptide functioning, particularly within well-known behavioral circuits, are likely to provide fascinating new clues into the understanding of fear behavior as well as suggesting novel therapeutics for treating disorders of anxiety and fear dysregulation.

Genetic strain differences in learned fear inhibition associated with variation in neuroendocrine, autonomic, and amygdala dendritic phenotypes

Mood and anxiety disorders develop in some but not all individuals following exposure to stress and psychological trauma. However, the factors underlying individual differences in risk and resilience for these disorders, including genetic variation, remain to be determined. Isogenic inbred mouse strains provide a valuable approach to elucidating these factors. Here, we performed a comprehensive examination of the extinction-impaired 129S1/SvImJ (S1) inbred mouse strain for multiple behavioral, autonomic, neuroendocrine, and corticolimbic neuronal morphology phenotypes. We found that S1 exhibited fear overgeneralization to ambiguous contexts and cues, impaired context extinction and impaired safety learning, relative to the (good-extinguishing) C57BL/6J (B6) strain. Fear overgeneralization and impaired extinction was rescued by treatment with the front-line anxiety medication fluoxetine. Telemetric measurement of electrocardiogram signals demonstrated autonomic disturbances in S1 including poor recovery of fear-induced suppression of heart rate variability. S1 with a history of chronic restraint stress displayed an attenuated corticosterone (CORT) response to a novel, swim stressor. Conversely, previously stress-naive S1 showed exaggerated CORT responses to acute restraint stress or extinction training, insensitivity to dexamethasone challenge, and reduced hippocampal CA3 glucocorticoid receptor mRNA, suggesting downregulation of negative feedback control of the hypothalamic-pituitary-adrenal axis. Analysis of neuronal morphology in key neural nodes within the fear and extinction circuit revealed enlarged dendritic arbors in basolateral amygdala neurons in S1, but normal infralimbic cortex and prelimbic cortex dendritic arborization. Collectively, these data provide convergent support for the utility of the S1 strain as a tractable model for elucidating the neural, molecular and genetic basis of persistent, excessive fear.

A mouse model of high trait anxiety shows reduced heart rate variability that can be reversed by anxiolytic drug treatment

Increasing evidence suggests that specific physiological measures may serve as biomarkers for successful treatment to alleviate symptoms of pathological anxiety. Studies of autonomic function investigating parameters such as heart rate (HR), HR variability and blood pressure (BP) indicated that HR variability is consistently reduced in anxious patients, whereas HR and BP data show inconsistent results. Therefore, HR and HR variability were measured under various emotionally challenging conditions in a mouse model of high innate anxiety (high anxiety behaviour; HAB) vs. control normal anxiety-like behaviour (NAB) mice. Baseline HR, HR variability and activity did not differ between mouse lines. However, after cued Pavlovian fear conditioning, both elevated tachycardia and increased fear responses were observed in HAB mice compared to NAB mice upon re-exposure to the conditioning stimulus serving as the emotional stressor. When retention of conditioned fear was tested in the home cage, HAB mice again displayed higher fear responses than NAB mice, while the HR responses were similar. Conversely, in both experimental settings HAB mice consistently exhibited reduced HR variability. Repeated administration of the anxiolytic NK1 receptor antagonist L-822429 lowered the conditioned fear response and shifted HR dynamics in HAB mice to a more regular pattern, similar to that in NAB mice. Additional receiver-operating characteristic (ROC) analysis demonstrated the high specificity and sensitivity of HR variability to distinguish between normal and high anxiety trait. These findings indicate that assessment of autonomic response in addition to freezing might be a useful indicator of the efficacy of novel anxiolytic treatments.

In vivo assessment of neurocardiovascular regulation in the mouse: principles, progress, and prospects

A growing body of evidence indicates that a number of common complex diseases, including hypertension, heart failure, and obesity, are characterized by alterations in central neurocardiovascular regulation. However, our understanding of how changes within the central nervous system contribute to the development and progression of these and other diseases remains unclear. As with many areas of cardiovascular research, the mouse has emerged as a key species for investigations of neuroregulatory processes because of its amenability to highly specific genetic manipulations. In parallel with the development of increasingly sophisticated murine models has come the miniaturization and advancement in methodologies for in vivo assessment of neurocardiovascular end points in the mouse. The following brief review will focus on a number of key direct and indirect experimental approaches currently in use, including measurement of arterial blood pressure, assessment of cardiovascular autonomic control, and evaluation of arterial baroreflex function. The advantages and limitations of each methodology are highlighted to allow for a critical evaluation by the reader when considering these approaches.

Neuropeptide Y (NPY) gene: Impact on emotional processing and treatment response in anxious depression

Neuropeptide Y (NPY) has been found to play a role in the pathomechanism of both anxiety and depression. Thus, NPY is a promising candidate in the investigation of the clinical phenotype of "anxious depression". Five NPY gene variants were investigated for an influence on antidepressant treatment response in a sample of 256 patients with depression. Additionally, NPY gene impact on amygdala activation during facial emotion processing was analyzed in a subsample of 35 depressed patients. Particularly in anxious depression, the less active NPY rs16147 -399C allele conferred slow response after 2weeks and failure to achieve remission after four weeks of treatment. The rs16147 C allele was further associated with stronger bilateral amygdala activation in response to threatening faces in an allele-dose fashion. The present results point towards a possible influence of functional NPY gene variation on antidepressant treatment response in anxious depression, potentially conveyed by altered emotional processing.

Effect of pharmacological manipulations of neuropeptide Y and corticotropin-releasing factor neurotransmission on incubation of conditioned fear

We recently developed a procedure to study fear incubation in which rats given 100 tone-shock pairings over 10 days show low fear 2 days after conditioned fear training and high fear after 30 or 60 days. Here, we studied the role of the stress-related peptides, neuropeptide Y (NPY) and corticotropin-releasing factor (CRF), in fear incubation. We gave rats either 10 or 100 30-s tone-0.5-s footshock pairings over 1 day (short training) or 10 days (long training) and then assessed tone-cue-induced conditioned suppression of lever responding 2 days after short training or 2 days and 1 month after long training. Prior to testing, we injected NPY (5-10 microg, i.c.v.), the NPY Y1 receptor antagonist BIBO3304 (20-40 microg, i.c.v.), the NPY Y2 receptor antagonist BIIE0246 (2.5-5 mg/kg s.c.), the non-selective CRF receptor antagonist D-Phe CRF(12-41) (10 microg, i.c.v.), or the CRF1 receptor antagonist MTIP (10-20 mg/kg s.c.). Conditioned suppression after long training was higher after 1 month than after 2 days (fear incubation); conditioned suppression was robustly expressed 2 days after short training (non-incubated fear). Both incubated and non-incubated fear responses were attenuated by NPY. In contrast, D-Phe CRF(12-41), MTIP, BIBO3304, or BIIE0246 had no effect on conditioned fear at the different time points. Results confirm previous work on the potent effect of exogenous NPY administration on conditioned fear, but the negative results with BIBO3304 and BIIE0246 question whether endogenous NPY contributes to incubated (or non-incubated) fear. Results also suggest that CRF receptors are not involved in cue-induced fear in the conditioned suppression procedure.

Chromosome 4q31-34 panic disorder risk locus: association of neuropeptide Y Y5 receptor variants

There is strong evidence for a genetic contribution to the pathogenesis of panic disorder, with a recent linkage study pointing toward a risk locus on chromosome 4q31-q34 [Kaabi et al., 2006]. Since the neuropeptide Y (NPY) system has been reported to be involved in the pathophysiology of anxiety and in particular panic disorder and the genes coding for NPY Y1, Y2, and Y5 receptors are located in the suggested risk region (4q31-q32), variants in the NPY, NPY Y1, Y2, and Y5 genes were investigated for association with panic disorder in a sample of 230 German patients with panic disorder and matched healthy controls. A synonymous (Gly-426-Gly) NPY Y5 coding variant (rs11946004) as well as haplotypes including rs11946004 and an intronic NPY Y5 variant (rs11724320) were significantly associated with panic disorder (P = 0.027), with the effect originating from the subgroup of female patients (P = 0.030), particularly with concurrent agoraphobia (P = 0.002-0.019). No association was observed for any variants located in the genes coding for NPY, NPY Y1, or Y2. The present results provide preliminary support for an influence of NPY Y5 receptor variants on the etiology of panic disorder in a potentially gender-specific manner further strengthening the evidence for a risk locus on chromosome 4q31-q34 in anxiety disorders. However, in order to allow for conclusive evaluation of the present finding and to exclude a false positive result, further studies in larger, independent, preferably family based samples are warranted.

The genetic contribution to heart rate and heart rate variability in quiescent mice

Recent studies have suggested a genetic component to heart rate (HR) and HR variability (HRV). However, a systematic examination of the genetic contribution to the variation in HR and HRV has not been performed. This study investigated the genetic contribution to HR and HRV using a wide range of inbred and recombinant inbred (RI) mouse strains. Electrocardiogram data were recorded from 30 strains of inbred mice and 29 RI strains. Significant differences in mean HR and total power (TP) HRV were identified between inbred strains and RI strains. Multiple significant differences within the strain sets in mean low-frequency (LF) and high-frequency (HF) power were also found. No statistically significant concordance was found between strain distribution patterns for HR and HRV phenotypes. Genomewide interval mapping identified a significant quantitative trait locus (QTL) for HR [LOD (likelihood of the odds) score = 3.763] on chromosome 6 [peak at 53.69 megabases (Mb); designated HR 1 (Hr1)]. Suggestive QTLs for TP were found on chromosomes 2, 4, 5, 6, and 14. A suggestive QTL for LF was found on chromosome 16; for HF, we found one significant QTL on chromosome 5 (LOD score = 3.107) [peak at 53.56 Mb; designated HRV-high-frequency 1 (Hrvhf1)] and three suggestive QTLs on chromosomes 2, 11 and 15. In conclusion, the results demonstrate a strong genetic component in the regulation of resting HR and HRV evidenced by the significant differences between strains. A lack of correlation between HR and HRV phenotypes in some inbred strains suggests that different sets of genes control the phenotypes. Furthermore, QTLs were found that will provide important insight to the genetic regulation of HR and HRV at rest.

Viral vector-induced amygdala NPY overexpression reverses increased alcohol intake caused by repeated deprivations in Wistar rats

Acute administration of neuropeptide Y (NPY) modulates alcohol intake in genetic and chemical models of high intake, while leaving intake unaffected during 'normal' or baseline conditions. In non-selected, normal rat lines, alcohol consumption can be increased by prolonged exposure to alcohol, and it is unclear what effect a constitutive increase in NPY function will have on alcohol intake. The purpose of the present study was to examine the effects on alcohol intake of an inducible, constitutive overexpression of NPY, one of the most abundant neuropeptides in the central nervous system. A liquid diet was used in combination with repeated alcohol deprivation sessions to increase alcohol intake in normal Wistar rats. We then examined the effect of NPY overexpression in the amygdala on excessive alcohol intake produced by prolonged exposure to alcohol and alcohol deprivation. Repeated withdrawal increased alcohol consumption in a 24-h continuous access two-bottle choice model. Both the number of withdrawals as well as the length of the withdrawal periods affected alcohol consumption with an increased intake resulting from multiple withdrawals and the alcohol deprivation effect being enhanced by longer periods of abstinence. The increase in intake following repeated abstinence was blunted by intra-amygdala administration of a Sindbis viral vector containing NPY cDNA. Amygdala NPY overexpression also was demonstrated to be anxiolytic in the open field test. Repeated withdrawal in combination with a history of alcohol consumption significantly elevated alcohol intake, and the amygdala may mediate the transition to high-drinking states in this model.

Activation of the brain 5-HT2C receptors causes hypolocomotion without anxiogenic-like cardiovascular adjustments in mice

The present study evaluated whether hypolocomotion elicited by subcutaneous administration of the non-specific 5-HT/preferential 5-HT(2C) receptor agonist mCPP during novelty exposure was due to an enhanced anxiety-like state. The effects of mCPP on exploratory behavior during exposure to a new environment (novelty) were studied in male C57BL/6N mice. Subcutaneous injection of mCPP (1 and 3mg/kg) and the preferential 5-HT(2C) receptor agonist MK212 (0.7 and 1mg/kg) induced hypolocomotion during novelty exposure. The selective 5-HT(2C) receptor antagonist SB242084 (0.3mg/kg) reversed the mCPP-induced hypolocomotion into hyperlocomotion. In contrast, MK212 induced hypolocomotion that was blocked by SB242084, indicating a specific 5-HT(2C) receptor involvement. When injected intracerebroventricularly, mCPP (30microg) elicited hypolocomotion, whereas the same dose mildly increased locomotion when injected into the dorsal hippocampus. Since anxiety affects autonomic functions, effects of mCPP on cardiovascular function were studied by radio-telemetry in the home cage of unrestrained mice. Subcutaneous injection of mCPP (3mg/kg) had no significant effect on heart rate and mean arterial blood pressure. In summary, in view of lack of autonomic effects, and the lack of hypoactivity upon forebrain stimulation, the hypolocomotion induced by systemic mCPP cannot be explained by an enhanced anxiety-like state.

CCK and NPY as anti-anxiety treatment targets: promises, pitfalls, and strategies

Short CCK peptides elicit panic attacks in humans and anxiogenic-like effects in some animal models, but CCK receptor antagonists have not been found clinically effective. Yet CCK overactivity appears to be involved in submissive behaviour, and CCKB receptor expression and binding are increased in suicide victims and animal models of anxiety. Preliminary data suggest that involvement of CCK and its receptor subtypes in anxiety can be better described when focusing on distinct endophenotypes, and considering environmental contingencies and confounds originating from interactions with dopamin-, opioid- and glutamatergic neurotransmission. In contrast, NPY is an anti-anxiety peptide with robust effects in various animal models when administrated into several brain regions. Studies with non-peptide antagonists selective for receptor subtypes have revealed the role of endogenous NPY in active coping. At least Y1, Y2 and Y5 receptors in various brain regions are involved, with the strongest evidence for contribution of Y1.

The Leu7Pro polymorphism of preproNPY is associated with decreased insulin secretion, delayed ghrelin suppression, and increased cardiovascular responsiveness to norepinephrine during oral glucose tolerance test

CONTEXT

Neuropeptide Y (NPY) plays a role in angiogenesis, cardiovascular regulation, and hormone secretion. The leucine7 to proline7 (Leu7Pro) polymorphism of preproNPY is associated with vascular diseases and has an impact on hormone levels in healthy subjects.

OBJECTIVE

The current study investigated the role of the Leu7Pro polymorphism in metabolic and cardiovascular autonomic regulation.

DESIGN AND SUBJECTS

A 5-h oral glucose tolerance test was performed on 27 healthy volunteers representing two preproNPY genotypes (Leu7/Pro7 and Leu7/Leu7) matched for age, sex, body mass index and physical activity.

MAIN OUTCOME MEASURES

Simultaneously we performed cardiovascular autonomic function tests and plasma measurements of sympathetic transmitters, glucose, insulin, and ghrelin.

RESULTS

The subjects with Leu7/Pro7 genotype had decreased plasma NPY, norepinephrine (NE), and insulin concentrations and insulin to glucose ratios. The suppression of ghrelin concentrations after glucose ingestion was delayed in these subjects. They also had increased heart rate variability indices and baroreflex sensitivity. However, they displayed significant negative association of NE concentration with variability of low-frequency R-R-intervals and with baroreflex sensitivity.

CONCLUSIONS

The Leu7Pro polymorphism of preproNPY is related to decreased level of basal sympathetic activity, decreased insulin secretion, and delayed ghrelin suppression during oral glucose tolerance test. The increased responsiveness of autonomic functions to NE associated with the polymorphism may be connected to increased cardiovascular vulnerability.

Dissociation of temporal dynamics of heart rate and blood pressure responses elicited by conditioned fear but not acoustic startle

Fear-inducing stimuli were hypothesized to elicit fast heart rate (HR) responses but slow mean arterial blood pressure (MAP) responses and thus were studied in auditory fear conditioning and acoustic startle at high temporal resolution in freely moving mice and rats. Fear-induced instantaneous acceleration of HR reaching maximum physiological values and subsequent recovery to baseline were observed. The MAP response consisted of an immediate, mild, and transient increase followed by a sluggish, profound elevation and slow recovery. HR and MAP responses served as reliable indicators of conditioned fear in mice with dissociated temporal dynamics. Unconditioned auditory stimuli, including acoustic startle stimuli, elicited only fast, mild, and transient MAP and HR elevations in mice and rats, reflecting arousal and attention under these experimental conditions.

Corticotropin-releasing factor receptor 1 and central heart rate regulation in mice during expression of conditioned fear

The present study was performed to 1) determine heart rate (HR) effects mediated through central corticotropin-releasing factor receptor subtypes 1 (CRF(1)) investigate and 2 (CRF(2)) and 2) to the contribution of endogenous CRF to baseline HR and its fear-induced adjustment in freely moving mice. CRF ligands were injected into both lateral ventricles (i.c.v.) 15 min before the presentation of a conditioned auditory fear stimulus (CS). Initial behavioral results suggest an ovine CRF (oCRF)-mediated enhanced baseline fear and mildly enhanced conditioned auditory fear. In contrast, i.c.v. injection of oCRF (35-210 ng/mouse) dose-dependently decreased baseline HR, increased HR variability, and attenuated the CS-induced tachycardia. This effect is suggested to depend on a combined activation of sympathetic and parasympathetic activity referred to as enhanced sympathovagal antagonism. An extreme bradycardia was elicited by oCRF injection into the lower brainstem. All HR effects were probably mediated by CRF(1) because injection of the CRF(2)-selective agonist mouse urocortin II was ineffective, and the baseline bradycardia by i.c.v. CRF was preserved in CRF(2)-deficient mice. Injection of various CRF receptor antagonists including the CRF(2)-selective antisauvagine-30 did not affect the conditioned HR response. This finding suggests that endogenous CRF does not contribute to the fear-mediated tachycardia. Thus, the hypothesis of an involvement of CRF in HR responses of mice to acute aversive stimulation is rejected. Pharmacological evidence points at the involvement of CRF(1) in enhanced sympathovagal antagonism, a pathological state contributing to elevated cardiac risk, whereas the physiological role of the brain CRF system in cardiovascular regulation remains to be determined.

Anxiolytic-like actions of centrally-administered neuropeptide Y, but not galanin, in C57BL/6J mice

Neuropeptide Y (NPY) and galanin (GAL) are densely localized in brain regions subserving stress, fear and anxiety. While previous research supports a role for both neuropeptides in the mediation of rodent emotional behaviors, there is currently a lack of information on the effects of central administration of NPY and GAL on fear- and anxiety-related behaviors in mice. In the present study, the effects of intracerebroventricularly administered NPY and GAL were assessed in C57BL/6J mice on a battery of tests for fear- and anxiety-related behavior. NPY (0.5, 1.0 nmol) produced clear anxiolytic-like effects in the elevated plus-maze and light<-->dark exploration test, whereas GAL (0.5, 1.0 nmol) was without effect. NPY (0.5 nmol) also increased locomotor activity in the open field test. In the fear conditioning paradigm, NPY administered prior to training reduced freezing to context (0.5, 1.0 nmol) and auditory cue (1.0 nmol). Pre-training GAL (0.5 nmol) treatment reduced freezing to context. Taken together, results demonstrate robust effects of centrally-administered NPY, but not GAL, on anxiety-like behaviors and fear conditioning in mice. These findings provide a basis for future studies of mice with targeted gene mutations, directed at delineating the anatomical regions and receptor subtypes mediating the effects of NPY and GAL on emotion.

Heart rate dynamics and behavioral responses during acute emotional challenge in corticotropin-releasing factor receptor 1-deficient and corticotropin-releasing factor-overexpressing mice

The role of corticotropin-releasing factor in autonomic regulation of heart rate, heart rate variability and behavior responses was investigated in two genetic mouse models: corticotropin-releasing factor receptor 1-deficient mice, and corticotropin-releasing factor-transgenic mice overexpressing corticotropin-releasing factor. Heart rate was recorded by radio-telemetry during novelty exposure and auditory fear conditioning. Locomotor activity and freezing served as behavioral indices. Locomotor activity and heart rate were invariably increased in response to novelty exposure in both corticotropin-releasing factor receptor 1-deficient mice and littermate wild-type controls. The heart rate responses during retention of conditioned auditory fear and the exponential relationship between heart rate and heart rate variability were unaffected by genotype. Moreover, conditioned fear responses inferred from multiple behavioral measures including freezing did not differ between corticotropin-releasing factor receptor 1-deficient and corticotropin-releasing factor receptor 1 wild-type control mice. Corticotropin-releasing factor-transgenic mice exhibited markedly reduced locomotor activity during novelty exposure when compared with littermate wild-type controls. Baseline and novelty-driven heart rate was slightly elevated in corticotropin-releasing factor-transgenic mice, whereas the novelty-induced increase of heart rate was not different between genotypes. In contrast, corticotropin-releasing factor-transgenic mice did not display a heart rate response indicative of conditioned auditory fear. It is concluded that corticotropin-releasing factor receptor 1-deficiency does not affect heart rate adjustment and behavioral responses to acute fearful stimuli. The resiliency of behavioral and cardiovascular patterns elevation argues against the involvement of corticotropin-releasing factor receptor 1 in acute emotional regulation on these two functional levels despite an absent corticosterone elevation in corticotropin-releasing factor receptor 1-deficient mice. It is hypothesized that the lack of a conditioned heart rate response in corticotropin-releasing factor-transgenic mice is attributable to an impairment of cognitive function. The results are compared with those of corticotropin-releasing factor receptor 2-deficient mice, and the role of the corticotropin-releasing factor system in cardiovascular regulation is discussed.