Pharmacological modulation of conditioned fear in the fear-potentiated startle test: a systematic review and meta-analysis of animal studies

RATIONALE AND OBJECTIVES

Fear conditioning is an important aspect in the pathophysiology of anxiety disorders. The fear-potentiated startle test is based on classical fear conditioning and over the years, a broad range of drugs have been tested in this test. Synthesis of the available data may further our understanding of the neurotransmitter systems that are involved in the expression of conditioned fear.

METHODS

Following a comprehensive search in Medline and Embase, we included 68 research articles that reported on 103 drugs, covering 56 different drug classes. The systematic review was limited to studies using acute, systemic drug administration in naive animals.

RESULTS

Qualitative data synthesis showed that most clinically active anxiolytics, but not serotonin-reuptake inhibitors, reduced cued fear. Anxiogenic drugs increased fear potentiation in 35% of the experiments, reduced fear potentiation in 29% of the experiments, and were without effect in 29% of the experiments. Meta-analyses could be performed for five drug classes and showed that benzodiazepines, buspirone, 5-HT1A agonists, 5-HT1A antagonists, and mGluR2,3 agonists reduced cued conditioned fear. The non-cued baseline startle response, which may reflect contextual anxiety, was only significantly reduced by benzodiazepines and 5-HT1A antagonists. No associations were found between drug effects and methodological characteristics, except for strain.

CONCLUSIONS

The fear-potentiated startle test appears to have moderate to high predictive validity and may serve as a valuable tool for the development of novel anxiolytics. Given the limited available data, the generally low study quality and high heterogeneity additional studies are warranted to corroborate the findings of this review.

The effect of SSRIs on fear learning: a systematic review and meta-analysis

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs) are considered first-line medication for anxiety-like disorders such as panic disorder, generalized anxiety disorder, and post-traumatic stress disorder. Fear learning plays an important role in the development and treatment of these disorders. Yet, the effect of SSRIs on fear learning are not well known.

OBJECTIVE

We aimed to systematically review the effect of six clinically effective SSRIs on acquisition, expression, and extinction of cued and contextual conditioned fear.

METHODS

We searched the Medline and Embase databases, which yielded 128 articles that met the inclusion criteria and reported on 9 human and 275 animal experiments.

RESULTS

Meta-analysis showed that SSRIs significantly reduced contextual fear expression and facilitated extinction learning to cue. Bayesian-regularized meta-regression further suggested that chronic treatment exerts a stronger anxiolytic effect on cued fear expression than acute treatment. Type of SSRI, species, disease-induction model, and type of anxiety test used did not seem to moderate the effect of SSRIs. The number of studies was relatively small, the level of heterogeneity was high, and publication bias has likely occurred which may have resulted in an overestimation of the overall effect sizes.

CONCLUSIONS

This review suggests that the efficacy of SSRIs may be related to their effects on contextual fear expression and extinction to cue, rather than fear acquisition. However, these effects of SSRIs may be due to a more general inhibition of fear-related emotions. Therefore, additional meta-analyses on the effects of SSRIs on unconditioned fear responses may provide further insight into the actions of SSRIs.

Anxiolytic effects of endocannabinoid enhancing compounds: A systematic review and meta-analysis

The endocannabinoid system is a promising candidate for anxiolytic therapy, but translation to the clinic has been lagging. We meta-analyzed the evidence for anxiety-reduction by compounds that facilitate endocannabinoid signaling in humans and animals. To identify areas of specific potential, effects of moderators were assessed. Literature was searched in Pubmed and Embase up to May 2021. A placebo/vehicle-control group was required and in human studies, randomization. We excluded studies that co-administered other substances. Risk of bias was assessed with SYRCLE's RoB tool and Cochrane RoB 2.0. We conducted three-level random effects meta-analyses and explored sources of heterogeneity using Bayesian regularized meta-regression (BRMA). The systematic review yielded 134 studies. We analyzed 120 studies (114 animal, 6 human) that investigated cannabidiol (CBD, 61), URB597 (39), PF-3845 (6) and AM404 (14). Pooled effects on conditioned and unconditioned anxiety in animals (with the exception of URB597 on unconditioned anxiety) and on experimentally induced anxiety in humans favored the investigational drugs over placebo/vehicle. Publication year was negatively associated with effects of CBD on unconditioned anxiety. Compared to approach avoidance tests, tests of repetitive-compulsive behavior were associated with larger effects of CBD and URB597, and the social interaction test with smaller effects of URB597. Larger effects of CBD on unconditioned anxiety were observed when anxiety pre-existed. Studies reported few side effects at therapeutic doses. The evidence quality was low with indications of publication bias. More clinical trials are needed to translate the overall positive results to clinical applications.

Cannabidiol in clinical and preclinical anxiety research. A systematic review into concentration-effect relations using the IB-de-risk tool

BACKGROUND

Preclinical research suggests that cannabidiol (CBD) may have therapeutic potential in pathological anxiety. Dosing guidelines to inform future human studies are however lacking.

AIM

We aimed to predict the therapeutic window for anxiety-reducing effects of CBD in humans based on preclinical models.

METHODS

We conducted two systematic searches in PubMed and Embase up to August 2021, into pharmacokinetic (PK) and pharmacodynamic (PD) data of systemic CBD exposure in humans and animals, which includes anxiety-reducing and potential side effects. Risk of bias was assessed with SYRCLE's RoB tool and Cochrane RoB 2.0. A control group was an inclusion criterion in outcome studies. In human outcome studies, randomisation was required. We excluded studies that co-administered other substances. We used the IB-de-risk tool for a translational integration of outcomes.

RESULTS

We synthesised data from 87 studies. For most observations (70.3%), CBD had no effect on anxiety outcomes. There was no identifiable relation between anxiety outcomes and drug levels across species. In all species (humans, mice, rats), anxiety-reducing effects seemed to be clustered in certain concentration ranges, which differed between species.

DISCUSSION

A straightforward dosing recommendation was not possible, given variable concentration-effect relations across species, and no consistent linear effect of CBD on anxiety reduction. Currently, these results raise questions about the broad use as a drug for anxiety. Meta-analytic studies are needed to quantitatively investigate drug efficacy, including aspects of anxiety symptomatology. Acute and (sub)chronic dosing studies with integrated PK and PD outcomes are required for substantiated dose recommendations.

Cannabidiol enhancement of exposure therapy in treatment refractory patients with social anxiety disorder and panic disorder with agoraphobia: A randomised controlled trial

Preclinical research suggests that enhancing CB1 receptor agonism may improve fear extinction. In order to translate this knowledge into a clinical application we examined whether cannabidiol (CBD), a hydrolysis inhibitor of the endogenous CB1 receptor agonist anandamide (AEA), would enhance the effects of exposure therapy in treatment refractory patients with anxiety disorders. Patients with panic disorder with agoraphobia or social anxiety disorder were recruited for a double-blind parallel randomised controlled trial at three mental health care centres in the Netherlands. Eight therapist-assisted exposure in vivo sessions (weekly, outpatient) were augmented with 300 mg oral CBD (n = 39) or placebo (n = 41). The Fear Questionnaire (FQ) was assessed at baseline, mid- and post-treatment, and at 3 and 6 months follow-up. Primary analyses were on an intent-to-treat basis. No differences were found in treatment outcome over time between CBD and placebo on FQ scores, neither across (β = 0.32, 95% CI [-0.60; 1.25]) nor within diagnosis groups (β = -0.11, 95% CI [-1.62; 1.40]). In contrast to our hypotheses, CBD augmentation did not enhance early treatment response, within-session fear extinction or extinction learning. Incidence of adverse effects was equal in the CBD (n = 4, 10.3%) and placebo condition (n = 6, 15.4%). In this first clinical trial examining CBD as an adjunctive therapy in anxiety disorders, CBD did not improve treatment outcome. Future clinical trials may investigate different dosage regimens.

The poly(I:C)-induced maternal immune activation model; a systematic review and meta-analysis of cytokine levels in the offspring

The maternal polyinosinic:polycytidylic acid (poly(I:C)) animal model is frequently used to study how maternal immune activation may impact neuro development in the offspring. Here, we present the first systematic review and meta-analysis on the effects of maternal poly(I:C) injection on immune mediators in the offspring and provide an openly accessible systematic map of the data including methodological characteristics.

Pubmed and EMBASE were searched for relevant publications, yielding 45 unique papers that met inclusion criteria. We extracted data on immune outcomes and methodological characteristics, and assessed the risk of bias. The descriptive summary showed that most studies reported an absence of effect, with an equal number of studies reporting an increase or decrease in the immune mediator being studied.

Meta-analysis showed increased IL-6 concentrations in the offspring of poly(I:C) exposed mothers. This effect appeared larger prenatally than post-weaning. Furthermore, poly(I:C) administration during mid-gestation was associated with higher IL-6 concentrations in the offspring. Maternal poly(I:C) induced changes in IL-1β, Il-10 and TNF-α concentrations were small and could not be associated with age of offspring, gestational period or sampling location. Finally, quality of reporting of potential measures to minimize bias was low, which stresses the importance of adherence to publication guidelines.

Since neurodevelopmental disorders in humans tend to be associated with lifelong changes in cytokine concentrations, the absence of these effects as identified in this systematic review may suggest that combining the model with other etiological factors in future studies may provide further insight in the mechanisms through which maternal immune activation affects neurodevelopment.

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Long-term effects of maternal poly(I:C) on immune mediators in the offspring appear limited.

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Prenatal measurements and mid gestation poly(I:C) injection are associated with increases in IL-6 concentrations.

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Variety in methodological conduct hampers identification of key elements that affect cytokine concentrations.

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The quality of reporting of potential measures to minimize bias is poor.

Cannabidiol enhancement of exposure therapy in treatment refractory patients with phobias: study protocol of a randomized controlled trial

BACKGROUND

Phobic anxiety disorders are among the most prevalent psychiatric disorders and are burdensome in terms of loss of quality of life and work productivity. Evidence-based treatments are relatively successful in the majority of patients, especially exposure therapy. However, a substantial subset of patients fails to achieve or stay in remission. Preclinical and genetic research have yielded evidence that the cannabinoid system is involved in the extinction of fear, presumed to underlie the beneficial effects of exposure therapy in phobic disorders. A cannabinoid constituent that may enhance endocannabinoid signaling is cannabidiol (CBD), a non-psychoactive component of cannabis. Hence, the addition of CBD to exposure therapy is expected to strengthen effects of treatment. To determine the added benefit of CBD on exposure therapy, we conduct a randomized controlled trial, in which patients in whom previous treatment as usual has not yielded sufficient response receive either CBD or placebo preceding 8 exposure sessions in a double-blind fashion. A subsidiary aim is to explore which (combination of) clinical, behavioral and genetic profiles of patients are related to treatment response.

METHODS/DESIGN

This is an 8-week multicenter, randomized, double-blind, placebo-controlled trial. Seventy-two patients with social phobia or panic disorder with agoraphobia with incomplete response to earlier treatment will be included from outpatient clinics in the Netherlands. Patients are randomized to augmentation of exposure therapy with 300 mg CBD or placebo. The study medication is administered orally, 2 h preceding each of the eight 90 min exposure sessions. Measurements will take place at baseline, first administration of medication, every session, mid-treatment, last administration of medication, post-treatment and at 3 and 6 months' follow-up. The primary outcome measure is the score on the Fear Questionnaire (FQ). In addition, determinants of the expected treatment enhancing effect of CBD will be explored.

DISCUSSION

This is the first trial to investigate whether the addition of CBD to exposure therapy is effective in reducing phobic symptoms in treatment refractory patients with social phobia or panic disorder with agoraphobia.

TRIAL REGISTRATION

Netherlands Trial Register NTR5100 . Registered 13 March 2015. Protocol version: issue date 17 Jan 2018, protocol amendment number 7.

Activation of somatodendritic 5-HT1A autoreceptors reduces the acquisition and expression of cued fear in the rat fear-potentiated startle test

RATIONALE

Fear conditioning is an important factor in the etiology of anxiety disorders. Previous studies have demonstrated a role for serotonin (5-HT)_1A receptors in fear conditioning. However, the relative contribution of somatodendritic 5-HT_1A autoreceptors and post-synaptic 5-HT_1A heteroreceptors in fear conditioning is still unclear.

OBJECTIVE

To determine the role of pre- and post-synaptic 5-HT_1A receptors in the acquisition and expression of cued and contextual conditioned fear.

METHODS

We studied the acute effects of four 5-HT_1A receptor ligands in the fear-potentiated startle test. Male Wistar rats were injected with the 5-HT_1A receptors biased agonists F13714 (0-0.16 mg/kg, IP), which preferentially activates somatodendritic 5-HT_1A autoreceptors, or F15599 (0-0.16 mg/kg, IP), which preferentially activates cortical post-synaptic 5-HT_1A heteroreceptors, with the prototypical 5-HT_1A receptor agonist R(+)8-OH-DPAT (0-0.3 mg/kg, SC) or the 5-HT_1A receptor antagonist WAY100,635 (0-1.0 mg/kg, SC).

RESULTS

F13714 (0.16 mg/kg) and R(+)-8-OH-DPAT (0.03 mg/kg) injected before training reduced cued fear acquisition. Pre-treatment with F15599 or WAY100,635 had no effect on fear learning. In the fear-potentiated startle test, F13714 (0.04-0.16 mg/kg) and R(+)-8-OH-DPAT (0.1-0.3 mg/kg) reduced the expression of cued and contextual fear, whereas F15599 had no effect. WAY100,635 (0.03-1.0 mg/kg) reduced the overall startle response.

CONCLUSIONS

The current findings indicate that activation of somatodendritic 5-HT_1A autoreceptors reduces cued fear learning, whereas 5-HT_1A receptors seem not involved in contextual fear learning. Moreover, activation of somatodendritic 5-HT_1A autoreceptors may reduce cued and contextual fear expression, whereas we found no evidence for the involvement of cortical 5-HT_1A heteroreceptors in the expression of conditioned fear.

Nicotine improves probabilistic reward learning in wildtype but not alpha7 nAChR null mutants, yet alpha7 nAChR agonists do not improve probabilistic learning

Cognitive impairments, e.g., reward learning, are present in various psychiatric disorders and warrant treatment. Improving reward-related learning could synergistically enhance psychosocial treatments and cognition generally. A critical first step is to understand the mechanisms underlying reward learning. The dopamine system has been implicated in such learning, but less known is how indirect activation of this system may affect reward learning. We determined the role of alpha7 nicotinic acetylcholine receptors (nAChR) on a probabilistic reversal learning task (PRLT) in mice that includes reward and punishment. Male alpha7 knockout (KO), heterozygous (HT), and wildtype (WT) littermate mice (n = 84) were treated with vehicle, 0.03, or 0.3 mg/kg nicotine. Two cohorts of C57BL/6NJ male mice were treated with various alpha7 nAChR ligands, including the full agonists PNU282877 and AR-R-17779, the positive allosteric modulator CCMI, the partial agonist SSR180711, and the antagonist methyllycaconitine. All mice were then tested in the PRLT. Nicotine (0.3 mg/kg) significantly improved initial reward learning in alpha7 WT and HT mice but did not improve learning in KO mice, suggesting an involvement of the alpha7 nAChR in the pro-learning effects of nicotine. Neither alpha7 nAChR treatments (PNU282987, AR-R-17779, CCMI, SSR180711, nor methyllycaconitine) affected mouse PRLT performance however. Nicotine improved reward learning via a mechanism that may include alpha7 nAChRs. This improvement unlikely relied solely on alpha7 nAChRs however, since no alpha7 nAChR ligand improved reward learning in normal mice. Future assessments of the effects of other nAChR subtypes on reward learning are needed.

The contribution of contextual fear in the anxiolytic effect of chlordiazepoxide in the fear-potentiated startle test

This study evaluated the extent to which a reduction in contextual fear contributes to the anxiolytic effect of benzodiazepines in the fear-potentiated startle response. To this end, chlordiazepoxide, an anxiolytic often used as positive control in preclinical drug studies, and zolpidem, known to have sedative properties and to be devoid of anxiolytic effects, were tested in two contexts: the same context as training had taken place and an alternative context. In addition, the level of muscle relaxation was assessed in a grip strength test. Chlordiazepoxide (2.5-10 mg/kg) decreased the fear-potentiated startle response, confirming its anxiolytic activity. In addition, it dose-dependently decreased the overall startle response in the same, but not the alternative context, and did not affect grip strength, indicating that chlordiazepoxide inhibits contextual fear in the absence of non-specific drug effects. Zolpidem (1.0-10 mg/kg) reduced the overall startle response in both contexts equally and decreased grip strength, indicating that its effects on fear-potentiated startle are due to non-specific drug effects, and not anxiolytic effects. The present findings show that chlordiazepoxide reduces contextual conditioned fear in the absence of non-specific drug effects. In addition, they show that training and testing rats in different contexts makes it possible to distinguish between cued, contextual and non-specific drug effects. As exaggerated contextual fear conditioning contributes to the fear generalization processes implicated in pathological anxiety, focus in screening of anxiolytic effects could be directed more towards the suppression of contextual fear and, therefore, this approach would be a valuable addition to standard preclinical screening.

Blockade of dopamine D1-family receptors attenuates the mania-like hyperactive, risk-preferring, and high motivation behavioral profile of mice with low dopamine transporter levels

BACKGROUND

Patients with bipolar disorder mania exhibit poor cognition, impulsivity, risk-taking, and goal-directed activity that negatively impact their quality of life. To date, existing treatments for bipolar disorder do not adequately remediate cognitive dysfunction. Reducing dopamine transporter expression recreates many bipolar disorder mania-relevant behaviors (i.e. hyperactivity and risk-taking). The current study investigated whether dopamine D1-family receptor blockade would attenuate the risk-taking, hypermotivation, and hyperactivity of dopamine transporter knockdown mice.

METHODS

Dopamine transporter knockdown and wild-type littermate mice were tested in mouse versions of the Iowa Gambling Task (risk-taking), Progressive Ratio Breakpoint Test (effortful motivation), and Behavioral Pattern Monitor (activity). Prior to testing, the mice were treated with the dopamine D1-family receptor antagonist SCH 23390 hydrochloride (0.03, 0.1, or 0.3 mg/kg), or vehicle.

RESULTS

Dopamine transporter knockdown mice exhibited hyperactivity and hyperexploration, hypermotivation, and risk-taking preference compared with wild-type littermates. SCH 23390 hydrochloride treatment decreased premature responding in dopamine transporter knockdown mice and attenuated their hypermotivation. SCH 23390 hydrochloride flattened the safe/risk preference, while reducing activity and exploratory levels of both genotypes similarly.

CONCLUSIONS

Dopamine transporter knockdown mice exhibited mania-relevant behavior compared to wild-type mice. Systemic dopamine D1-family receptor antagonism attenuated these behaviors in dopamine transporter knockdown, but not all effects were specific to only the knockdown mice. The normalization of behavior via blockade of dopamine D1-family receptors supports the hypothesis that D1 and/or D5 receptors could contribute to the mania-relevant behaviors of dopamine transporter knockdown mice.

Human fear acquisition deficits in relation to genetic variants of the corticotropin-releasing hormone receptor 1 and the serotonin transporter--revisited

We recently showed that a genetic polymorphism (rs878886) in the human corticotropin-releasing hormone receptor 1 (CRHR1) is associated with reduced fear-conditioned responses to a threat cue. This is a potentially important finding considering that the failure to acquire fear contingencies can leave an individual in a maladaptive state of more generalized anxiety. Consistent with that idea, the CRHR1-dependent fear acquisition deficit translated into heightened contextual anxiety when taking genetic variability within the serotonin transporter long polymorphic region (5-HTTLPR) into account. To replicate our previous findings, we conducted a replication study in 224 healthy medication-free human subjects using the exact same cue and context virtual reality fear-conditioning procedure as in study by Heitland et al. (2013). In the replication study, consistent with the original findings, CRHR1 rs878886 G-allele carriers showed reduced acquisition of cue-specific fear-conditioned responses compared with C/C homozygotes. Also, in this larger sample the cue acquisition deficit of G-allele carriers translated into heightened contextual anxiety, even independent of 5-HTT gene variation. In contrast to our earlier findings, there was an additional interaction effect of CRHR1 rs878886 and the triallelic 5-HTTLPR/rs25531 variant on cued fear acquisition. In summary, this study replicated the initially reported association of the CRHR1 rs878886 G-allele with cued fear acquisition deficits, albeit with a different pattern of results regarding the interaction with 5-HTT variation. This further supports the notion that the human corticotropin-releasing hormone plays a role in the acquisition of fears.

GABA(A) receptor α subunits differentially contribute to diazepam tolerance after chronic treatment

Background

Within the GABA_A-receptor field, two important questions are what molecular mechanisms underlie benzodiazepine tolerance, and whether tolerance can be ascribed to certain GABA_A-receptor subtypes.

Methods

We investigated tolerance to acute anxiolytic, hypothermic and sedative effects of diazepam in mice exposed for 28-days to non-selective/selective GABA_A-receptor positive allosteric modulators: diazepam (non-selective), bretazenil (partial non-selective), zolpidem (α₁ selective) and TPA023 (α_2/3 selective). In-vivo binding studies with [³H]flumazenil confirmed compounds occupied CNS GABA_A receptors.

Results

Chronic diazepam treatment resulted in tolerance to diazepam's acute anxiolytic, hypothermic and sedative effects. In mice treated chronically with bretazenil, tolerance to diazepam's anxiolytic and hypothermic, but not sedative, effects was seen. Chronic zolpidem treatment resulted in tolerance to diazepam's hypothermic effect, but partial anxiolytic tolerance and no sedative tolerance. Chronic TPA023 treatment did not result in tolerance to diazepam's hypothermic, anxiolytic or sedative effects.

Conclusions

Our data indicate that: (i) GABA_A-α₂/α₃ subtype selective drugs might not induce tolerance; (ii) in rodents quantitative and temporal variations in tolerance development occur dependent on the endpoint assessed, consistent with clinical experience with benzodiazepines (e.g., differential tolerance to antiepileptic and anxiolytic actions); (iii) tolerance to diazepam's sedative actions needs concomitant activation of GABA_A-α₁/GABA_A-α₅ receptors. Regarding mechanism, in-situ hybridization studies indicated no gross changes in expression levels of GABA_A α₁, α₂ or α₅ subunit mRNA in hippocampus or cortex. Since selective chronic activation of either GABA_A α₂, or α₃ receptors does not engender tolerance development, subtype-selective GABA_A drugs might constitute a promising class of novel drugs.

Models of anxiety: stress-induced hyperthermia (SIH) in singly housed mice

Described in this unit is the stress-induced hyperthermia (SIH) test in mice in a single-housed format. This protocol has proven reliable in detecting the anxiolytic properties of test compounds. In this test, SIH is quantified in singly housed mice using a rectal temperature measurement as the stressor. Rectal temperature is measured twice at a 10-min interval. Due to the stress experienced during the first temperature measurement, the temperature of the second measurement (T(2)) is ∼0.8° to 1.5°C higher than that of the first (T(1)). This difference in temperature (ΔT = T(2) - T(1)) is defined as the SIH response. The SIH response is reduced by different classes of anxiolytics. The SIH test is simple and robust, it does not require training of animals, and test compound effects on motor behavior, feeding, and nociception do not affect test outcome. Furthermore, it is one of few anxiety tests that focuses on the physiological component of anxiety.

Models of anxiety: ultrasonic vocalizations of isolated rat pups

Described in this unit is the ultrasonic distress vocalization test in rat pups. This test is a reliable method for detecting anxiolytic properties of test compounds. In this test, ultrasonic vocalizations (30 to 50 kHz) are elicited by separating rat pups of 9 to 11 days of age from their mother and littermates for a brief period of time. The test can be performed under two different stress conditions. Pups are placed in isolation in either a warm (37°C) or cold (18°C) environment for 5 min. The total number and duration of ultrasonic calls emitted by the pups during this period is used as an index of anxiety. Established anxiolytic compounds, including benzodiazepines, serotonin(1A) receptor agonists, and selective serotonin reuptake inhibitors (SSRIs), consistently reduce the number and cumulative duration of these ultrasonic distress vocalizations.

Fear-potentiated startle and light-enhanced startle models in drug discovery

Described in this unit are the fear-potentiated startle (FPS) and light-enhanced startle (LES) tests. These protocols have proven reliable in detecting the anxiolytic properties of test compounds. The principle of these tests is that the magnitude of the acoustic startle reflex is an index of anxiety. The FPS test includes two training sessions in which an intrinsically aversive foot shock is paired with a neutral cue light. In the test session presentation of this cue light is subsequently used to elicit startle potentiation. In the LES test startle reactivity is increased by presentation of bright light. Because LES is based on the innate aversion of rodents for bright light it does not require training sessions. Although LES has been used less frequently than FPS for screening compounds, it has an advantage in that drug effects on startle potentiation are independent of memory retrieval. Further, the contextual anxiety measured in the LES test could be more relevant for pathological anxiety than the conditioned fear associated with the FPS test.

Lifelong CRF overproduction is associated with altered gene expression and sensitivity of discrete GABA(A) and mGlu receptor subtypes

RATIONALE

Repeated activation of corticotropin-releasing factor (CRF) receptors is associated with increased anxiety and enhanced stress responsivity, which may be mediated via limbic GABAergic and glutamatergic transmission.

OBJECTIVE

The present study investigated molecular and functional alterations in GABA(A) receptor (GABA(A)R) and metabotropic glutamate receptor (mGluR) responsivity in transgenic mice that chronically overexpress CRF.

METHODS

CRF(1) receptor, GABA(A)R, and mGluR sensitivity were determined in CRF-overexpressing mice using the stress-induced hyperthermia (SIH) test. In addition, we measured mRNA expression levels of GABA(A)R α subunits and mGluRs in the amygdala and hypothalamus.

RESULTS

CRF-overexpressing mice were less sensitive to the anxiolytic effects of the CRF(1) receptor antagonists CP154,526 and DMP695, the GABA(A)R α(3)-selective agonist TP003 (0-3 mg/kg) and the mGluR(2/3) agonist LY379268 (0-10 mg/kg) in the SIH test. The hypothermic effect of the non-selective GABA(A)R agonist diazepam (0-4 mg/kg) and the α(1)-subunit-selective GABA(A)R agonist zolpidem (0-10 mg/kg) was reduced in CRF-overexpressing mice. No genotype differences were found using the GABA(A)R α(5)-subunit preferential compound SH-053-2'F-R-CH(3) and mGluR(5) antagonists MPEP and MTEP. CRF-overexpressing mice showed decreased expression levels of GABA(A)R α(2) subunit and mGluR(3) mRNA levels in the amygdala, whereas these expression levels were increased in the hypothalamus. CRF-overexpressing mice also showed increased hypothalamic mRNA levels of α(1) and α(5) GABA(A)R subunits.

CONCLUSIONS

We found that lifelong CRF overproduction is associated with altered gene expression and reduced functional sensitivity of discrete GABA(A) and mGluR receptor subtypes. These findings suggest that sustained over-activation of cerebral CRF receptors may contribute to the development of altered stress-related behavior via modulation of GABAergic and glutamatergic transmission.

The amphetamine-chlordiazepoxide mixture, a pharmacological screen for mood stabilizers, does not enhance amphetamine-induced disruption of prepulse inhibition

In rodents, administration of a mixture of the psychostimulant d-amphetamine and the benzodiazepine chlordiazepoxide results in supra-additive hyperlocomotion, a phenomenon used to identify mood stabilizers. In an attempt to determine whether the d-amphetamine/chlordiazepoxide assay could extend to other behaviors that are affected in mania, we evaluated the effects of the mixture on prepulse inhibition. In addition, we combined chlordiazepoxide with the selective dopamine reuptake inhibitor GBR 12909 or the noradrenergic stimulant (-) ephedrine, and tested these alternative mixtures in locomotor activity and prepulse inhibition tests. Chlordiazepoxide (3mg/kg) robustly potentiated amphetamine-induced hyperactivity, but did not change the amphetamine-induced disruption of prepulse inhibition. This indicates that the d-amphetamine-chlordiazepoxide-induced hyperlocomotion does not extend to other dopamine-driven behaviors. GBR 12909 (16mg/kg) and (-) ephedrine (50mg/kg) both enhanced locomotor activity and disrupted PPI, but combined treatment of either of these compounds with chlordiazepoxide had no significant additive effect on locomotor activity or prepulse inhibition. These findings suggest that the effect of the d-amphetamine/chlordiazepoxide mixture cannot be accounted for by the dopamine enhancing properties of amphetamine alone. Last, valproic acid (120-240mg/kg) did not reduce the GBR-induced hyperactivity. Therefore, further pharmacological evaluation of GBR 12909-induced hyperactivity is warranted to determine its pharmacological potential to model mania-like behavior. Based on the current results, it is concluded that the utility of the pharmacological d-amphetamine/chlordiazepoxide assay as a tool to study brain mechanisms relevant to mania is limited.

5-HT(1A) receptor sensitivity in 5-HT(1B) receptor KO mice is unaffected by chronic fluvoxamine treatment

The 5-HT(1B) receptor has been implicated in disorders such as depression, anxiety and obsessive-compulsive disorder. In mice lacking the 5-HT(1B) receptor (5-HT(1B) knockout mice), important changes in physiology and behavior exist. In the absence of presynaptic 5-HT(1B) receptor inhibition, chronic SSRI treatment may differentially affect 5-HT(1A) receptor functionality. The present studies tested the hypothesis that chronically reducing 5-HT transporter (5-HTT) function with selective serotonin reuptake inhibitor (SSRI) treatment would accelerate 5-HT(1A) receptor desensitization in 5-HT(1B) knockout mice. Moreover, as 5-HT(1B) knockout mice have been found to display exaggerated autonomic and locomotor responses to environmental stressors, the effects of chronic SSRI treatment on the hyperreactive phenotype of 5-HT(1B) knockout mice were investigated. The stress-reducing effect of the 5-HT(1A) receptor agonist flesinoxan on increases in body temperature, heart rate and locomotor activity was similar in wild type and 5-HT(1B) knockout mice before and after chronic 21-day treatment with the SSRI fluvoxamine, indicating no apparent alteration of 5-HT(1A) receptor sensitivity in 5-HT(1B) knockout mice. Also, chronic SSRI treatment did not alter the increased stress reactivity to mild environmental stressors in 5-HT(1B) knockout mice. We demonstrate that no apparent differences in 5-HT(1A) receptor sensitivity occur between 5-HT(1B) knockout and wild type mice after chronic fluvoxamine treatment. Also, the hyperreactive phenotype of 5-HT(1B) knockout mice is unresponsive to chronic SSRI treatment. Taken together, these results indicate that constitutive absence of 5-HT(1B) receptors does not result in adaptive changes in 5-HT(1A) receptor functionality and that chronic SSRI treatment does not modify stress reactivity in 5-HT(1B) knockout mice.

Corticotropin-releasing factor (CRF) over-expression down-regulates hippocampal dopamine receptor protein expression and CREB activation in mice

BACKGROUND

Stress results in hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, characterized by increased central CRF activity, elevated circulating glucocorticoid levels, impaired glucocorticoid-mediated negative feedback and abnormal hippocampal functions, possibly contributing to the development of behavioral pathologies, such as depression. The hippocampus is critically involved in the control of the HPA axis as well as in explicit memory, contextual aspects of fear, organization of the behavioral response to environmental novelty and in habituation. We have previously shown that mice that over-express CRF in the brain exhibit impaired novelty detection and altered psychophysiological and behavioral habituation, functions linked to dopamine receptor-dependent hippocampal plasticity.

OBJECTIVE AND METHODS

Therefore, the aim of the present study was to measure D1 and D2 dopamine receptor expression and related signaling, such as CREB and ERK protein levels and phosphorylation, in the hippocampus and other brain regions of mice with post-natal CRF over-expression (CRF-OE mice).

RESULTS

We found a region-specific down-regulation of both D1 and D2 protein expression, without altered CRF receptor protein expression, in the hippocampus in CRF-OE mice. This was accompanied by an impaired phosphorylation of hippocampal CREB, but not ERK1 and ERK2, in the same animals.

CONCLUSIONS

These results suggest that post-natal onset CRF over-expression results in an impairment of dopamine signaling in the hippocampus, which may underlie cognitive and motivational aspects of stress-related, CRF-driven mood disorders.

5-HT1A receptor blockade reverses GABA(A) receptor alpha3 subunit-mediated anxiolytic effects on stress-induced hyperthermia

RATIONALE

Stress-related disorders are associated with dysfunction of both serotonergic and GABAergic pathways, and clinically effective anxiolytics act via both neurotransmitter systems. As there is evidence that the GABA(A) and the serotonin receptor system interact, a serotonergic component in the anxiolytic actions of benzodiazepines could be present.

OBJECTIVES

The main aim of the present study was to investigate whether the anxiolytic effects of (non-)selective alpha subunit GABA(A) receptor agonists could be reversed with 5-HT(1A) receptor blockade using the stress-induced hyperthermia (SIH) paradigm.

RESULTS

The 5-HT(1A) receptor antagonist WAY-100635 (0.1-1 mg/kg) reversed the SIH-reducing effects of the non-alpha-subunit selective GABA(A) receptor agonist diazepam (1-4 mg/kg) and the GABA(A) receptor alpha(3)-subunit selective agonist TP003 (1 mg/kg), whereas WAY-100635 alone was without effect on the SIH response or basal body temperature. At the same time, co-administration of WAY-100635 with diazepam or TP003 reduced basal body temperature. WAY-100635 did not affect the SIH response when combined with the preferential alpha(1)-subunit GABA(A) receptor agonist zolpidem (10 mg/kg), although zolpidem markedly reduced basal body temperature.

CONCLUSIONS

The present study suggests an interaction between GABA(A) receptor alpha-subunits and 5-HT(1A) receptor activation in the SIH response. Specifically, our data indicate that benzodiazepines affect serotonergic signaling via GABA(A) receptor alpha(3)-subunits. Further understanding of the interactions between the GABA(A) and serotonin system in reaction to stress may be valuable in the search for novel anxiolytic drugs.

Elucidating GABAB and GABAB Receptor Functions in Anxiety Using the Stress-Induced Hyperthermia Paradigm: A Review

Exposure to acute psychological or physical stress increases core body temperature (stress-induced hyperthermia, SIH) which is part of the autonomic stress response. SIH is used as a putative rodent anxiety paradigm in which anxiolytic drugs reduce the SIH response. The predictive validity of the SIH paradigm has proven to be good, making it suitable to detect the putative anxiolytic properties of drugs. So far, GABA receptor agonists including benzodiazepines and hypnotics have proven to attenuate the SIH response. The GABAA receptor has been known to be closely involved in the acute stress response. Also, the recent development of compounds with selective efficacy for different subunits at the benzodiazepine site of the GABAA receptor has renewed interest for the therapeutic potential of GAB ergic drugs. Moreover, metabotropic (GABAB) receptor agonists reduce the SIH response. GABAB receptors are ubiquitously expressed in the central nervous system, and there is evidence for a role of the GABAB receptor in anxiety. Thus, both drugs acting on the GABAA and the GABAB receptor are generally able to attenuate the SIH response, and this review presents a detailed overview of the effects of both drug classes on the SIH response. As the GABA receptor family is diverse and complex, this paradigm may contribute to the elucidation of the putative effects of GABA ergic drugs in emotional disorders such as anxiety and depression

Cocaine-induced changes in affective state modulate the light-enhanced startle response

In order to evaluate the influence of changes in affective state on light-enhanced startle, the effects of positive affect, induced by acute cocaine administration, and the effect of negative affect, induced by spontaneous cocaine withdrawal-induced anxiety, were studied. Acute cocaine administration decreased LES, whereas withdrawal from chronic cocaine administration exacerbated LES 24h after withdrawal, an effect indicative of increased anxiety. This exacerbated LES was reduced, but not back to normal, 4 days after withdrawal. The finding that both cocaine-induced positive and negative affect can be detected in LES, suggests that this may be a valuable tool in studying affect regulation in rodents.

Fear-potentiated startle, but not light-enhanced startle, is enhanced by anxiogenic drugs

RATIONALE AND OBJECTIVES

The light-enhanced startle paradigm (LES) is suggested to model anxiety, because of the non-specific cue and the long-term effect. In contrast, the fear-potentiated startle (FPS) is suggested to model conditioned fear. However, the pharmacological profiles of these two paradigms are very similar. The present study investigated the effects of putative anxiogenic drugs on LES and FPS and aimed at determining the sensitivity of LES for anxiogenic drugs and to potentially showing a pharmacological differentiation between these two paradigms.

METHODS

Male Wistar rats received each dose of the alpha(2)-adrenoceptor antagonist yohimbine (0.25-1.0mg/kg), the 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP, 0.5-2.0mg/kg) or the GABA(A) inverse receptor agonist pentylenetetrazole (PTZ, 3-30mg/kg) and were subsequently tested in either LES or FPS.

RESULTS

None of the drugs enhanced LES, whereas mCPP increased percentage FPS and yohimbine increased absolute FPS values. Furthermore, yohimbine increased baseline startle amplitude in the LES, while mCPP suppressed baseline startle in both the LES and FPS and PTZ suppressed baseline startle in the FPS.

CONCLUSIONS

In contrast to findings in the FPS paradigm, none of the drugs were able to exacerbate the LES response. Thus, a clear pharmacological differentiation was found between LES and FPS.

Early-life blockade of 5-HT(1A) receptors alters adult anxiety behavior and benzodiazepine sensitivity

BACKGROUND

Early-life stress may affect 5-HT(1A) receptor circuitry, which could result in increased anxiety in later life. An increased anxiety phenotype in 5-HT(1A) receptor KO mice (1AKO) mice has been ascribed to 5-HT(1A) receptor absence during the early postnatal period. Thus, subtle and transient serotonergic changes during the early postnatal period may lead to an increased risk for developing stress-related disorders during adulthood.

METHODS

Wildtype and 1AKO mice on a Swiss-Webster (SW) background were treated during the early postnatal period with vehicle or the 5-HT(1A) receptor antagonist WAY-100,635.

RESULTS

Pharmacologic 5-HT(1A) receptor blockade during the early postnatal period induced long-lasting effects on anxiety and benzodiazepine sensitivity in adolescent and adult mice on a Swiss-Webster background and resembles the SW 1AKO phenotype. Furthermore, WAY-100,635-treated mice had increased cortical gamma-aminobutyric acid-A receptor (GABA(A)R) alpha(1) and alpha(3) subunit levels and increased hippocampal GABA(A)R alpha(2) subunit levels.

CONCLUSIONS

Absence of 5-HT(1A)R signaling during early stages of brain maturation predisposes an organism to affective dysfunction later in life. Because early-life treatment with WAY-100,635 in Swiss-Webster mice reduced diazepam sensitivity and increased GABA(A)R alpha subunit levels in the prefrontal cortex and hippocampus, our data suggest a putative link between early-life disruption of the serotonergic system and the emergence of increased anxiety and decreased benzodiazepine responsivity at adult age. Moreover, early-life 5-HT(1A) receptor functionality appears to be essential for the development of normal GABA(A)R functionality. This study may have clinical implications for psychoactive drug use during pregnancy and for the pharmacogenetic background of benzodiazepine sensitivity.

The putative antidepressant DOV 216,303, a triple reuptake inhibitor, increases monoamine release in the prefrontal cortex of olfactory bulbectomized rats

The first line of antidepressant treatment nowadays are selective serotonin reuptake inhibitors. Although they are relatively safe to use, selective serotonin reuptake inhibitors (SSRIs) can induce severe side effects. New promising antidepressants may be the triple monoamine reuptake inhibitors, which not only enhance serotonin and norepinephrine neurotransmission, but also increase brain dopamine levels. Recently it has been shown that one of the triple reuptake inhibitors, DOV 216,303 has antidepressant-like effects in the olfactory bulbectomy (OBX) model of depression, but the alterations in monoaminergic neurotransmission in these animals are still unknown. In the present study we investigated not only the effect of acute, but also chronic treatment of DOV 216,303 in OBX rats on monoamine and metabolite levels. The main results are decreased baseline dopamine levels in the prefrontal cortex one day after OBX, while 38days after OBX no difference could be observed in monoamine levels after vehicle treatment. Treatment with DOV 216,303 leads to increased extracellular levels of serotonin and norepinephrine neurotransmission, but also increased dopamine levels in OBX animals as well as their controls. This increase could be observed after one single administration, but also after chronic treatment. However, a DOV 216,303 challenge in chronically treated animals resulted in lower monoamine concentrations than the same challenge in untreated animals. More research is needed to investigate this seemingly hyporesponsivity to chronic DOV 216,303 treatment.

Disrupted startle modulation in animal models for affective disorders

Affective startle modulation is used to study emotional reactivity in humans, and blunted affective startle modulation has been reported in depressed patients. To determine whether blunted affective startle modulation is also a common feature in animal models for affective disorders, light-enhanced startle was studied in three models: inescapable foot shock (IFS), repeated restraint stress (RRS) and olfactory bulbectomy (OBX). In addition, prepulse inhibition was studied in these models. Light-enhanced startle was blunted following IFS and OBX and RRS decreased overall startle responding. Prepulse inhibition, however, was unaffected. These findings indicate that induction models for affective disorders may be associated with long term effects on affective startle modulation. The lack of changes in sensory motor gating suggests that these changes can be ascribed to alterations in emotional reactivity. In conclusion, our results indicate that the blunted affective startle modulation seen in animal models for affective disorders may be used to examine the mechanisms underlying altered emotional reactivity.

Medial amygdala lesions differentially influence stress responsivity and sensorimotor gating in rats

BACKGROUND

The amygdala is involved in the coordination of stress but is also an important gatekeeper involved in the regulation of vigilance. The amygdala is structurally complex, consisting of several nuclei with specific functions in the affective response to environmental stimuli. There are indications that the medial amygdaloid nucleus may be a pivotal player in acute responses to emotional environmental stimuli.

METHODS

The present study therefore aimed to study the effects of bilateral electrolytic lesions of the medial amygdala on unconditioned anxiety-related behavior as well as a sensorimotor gating parameter (prepulse inhibition, PPI) in rats. Anxiety-related behavior was assessed with the use of stress-induced hyperthermia (SIH), light-enhanced startle (LES) and open field behavior.

RESULTS

Bilateral electrolytic lesions of the medial amygdala decreased the SIH response and anxiety-related open field behavior. In contrast, lesioned animals displayed augmented LES and disrupted PPI. No changes in basal locomotor activity, body temperature and acoustic startle were found between lesioned and sham animals.

CONCLUSIONS

The present study suggests that the medial amygdala is an important player in response to acute environmental stimuli. Decreased unconditioned psychological stress responses were found, whereas LES was enhanced and sensorimotor processing was disrupted. However, considering the existing data on basolateral amygdala involvement in PPI and bed nucleus of the stria terminalis involvement in LES, local infusion studies into the MeA should be performed to further substantiate these findings.

Activation of the septohippocampal system differentiates anxiety from fear in startle paradigms

It has been suggested that different brain areas are involved in the modulation and expression of fear and anxiety. In the present study we investigated these potential differences by using the fear-potentiated-startle (FPS) and light-enhanced-startle (LES) paradigms to differentiate between fear and anxiety, respectively. Male Wistar rats were tested in the FPS and LES paradigm and perfused 1 h after the test session. Fos immunoreactivity (IR) was quantified in 21 brain areas and compared between FPS, LES and four control conditions. Both FPS and LES procedures significantly enhanced the acoustic startle response. A principal component analysis of Fos-IR-data showed that 70% of the changes in Fos-IR could be explained by three independent components: an arousal-component, identifying brain areas known to be activated under conditions of vigilance, arousal and stress, a LES- and an FPS-component. The LES component comprised the septohippocampal system and functionally interrelated areas including nucleus accumbens, anterior cingulate cortex, lateral habenula and supramammillary areas, but not the dorsolateral part of the bed nucleus of the stria terminalis. The central amygdaloid nucleus and the dorsolateral part of the bed nucleus of the stria terminalis loaded exclusively on the FPS component. Analysis of the separate brain areas revealed significantly higher Fos-IR in LES relative to FPS in the anterior cingulate cortex, nucleus accumbens shell, lateral septum, lateral habenula and area postrema. We conclude that the neural circuitry activated during FPS and LES shows clear differences. In anxiety as induced by LES, activation of the septohippocampal system and related areas seems to play a major role. In fear as induced by FPS, the central amygdaloid nucleus and the dorsolateral part of the bed nucleus of the stria terminalis loaded on the same component, but Fos-IR observed in these brain regions did not differentiate between anxiety and fear. Furthermore, principal-component analysis appears a useful tool in detecting and describing correlated changes in patterns of neuronal activity.

Antidepressant effects of pramipexole, a dopamine D3/D2 receptor agonist, and 7-OH-DPAT, a dopamine D3 receptor agonist, in olfactory bulbectomized rats

Treatment with pramipexole, a dopamine D(3)/D(2) receptor agonist, reduces depressive symptoms in patients suffering from Parkinson's disease. To test the putative antidepressant quality of pramipexole, its effects were assessed in one of the most attractive animal models of depression, the olfactory bulbectomized (OBX) rat. Two experiments studied the effects of pramipexole on bulbectomy-induced hyperactivity. In experiment I, pramipexole was tested at 0.3 and 1.0 mg/kg together with the reference dopamine D(3) receptor agonist 7-OH-DPAT (0.1 mg/kg) and the tri-cyclic antidepressant imipramine (10 mg/kg). In experiment II, pramipexole was tested at lower doses: 0.03 and 0.1 mg/kg, with the same reference compounds. All animals were tested in the open field on days one (acute), seven (sub-chronic) and fourteen (chronic) of administration, as well as one week after cessation of treatment. Pramipexole, in a U-shaped dose response, reduced bulbectomy-induced hyperactivity after (sub) chronic but not acute administration (like imipramine and 7-OH-DPAT). The highest dose of pramipexole (1.0 mg/kg) did not reduce OBX hyperactivity during treatment. However, one week after cessation of treatment, all pramipexole (including the 1.0 mg/kg dose), 7-OH-DPAT and imipramine groups showed a reduction in OBX-induced hyperactivity. Pramipexole and 7-OH-DPAT exert an antidepressant profile in the OBX-rat model in normalizing bulbectomy-induced hyperactivity during (sub) chronic treatment. Moreover, treatment with both these compounds induced long-lasting changes in the bulbectomized brain similar to established antidepressants, strongly predicting antidepressant activity in major depression.

Stress-induced hyperthermia is reduced by rapid-acting anxiolytic drugs independent of injection stress in rats

BACKGROUND

Stress-induced hyperthermia (SIH) is the transient rise in body temperature after encountering a stressor. The SIH response can be blocked by administration of various anxiolytic drugs prior to inducing stress. However, a drug injection involves handling and injection stress and therefore induces a SIH response itself. In the standard SIH test, drugs are therefore injected 60 min before stress induction to allow injection-induced hyperthermia to decline. This makes it difficult to study putative anxiolytic compounds with a short half-life. The present study therefore aimed to compare the effects of standard (stressful) and stress-free anxiolytic drug administration on the subsequent SIH response with a 10-minute injection-stressor interval.

METHODS

Anxiolytic drugs with short half-lives (midazolam, 8-OH-DPAT, nicotine) were injected subcutaneously in rats using either a stressful (manual injection) or stress-free injection (subcutaneous cannula) method 10 min before novel cage stress. Body temperature and locomotor activity were measured using telemetric transmitters.

RESULTS

Stressful and stress-free drug administration resulted in comparable drug effects on the stress-induced hyperthermia and locomotor responses in rats.

CONCLUSION

The present study shows that both stressful and stress-free drug injection shortly before a stressor results in reproducible attenuation of the SIH response in rats. In rats, a short injection-stressor interval can therefore be applied using the SIH model, enabling the study of putative anxiolytic drugs with short half-lives.

Dissociating anxiolytic and sedative effects of GABAAergic drugs using temperature and locomotor responses to acute stress

RATIONALE

The stress-induced hyperthermia (SIH) model is an anxiety model that uses the transient rise in body temperature in response to acute stress. Benzodiazepines produce anxiolytic as well as sedative side effects through nonselective binding to GABA(A) receptor subunits. The GABA(A) receptor alpha(1) subunit is associated with sedation, whereas the GABA(A) receptor alpha(2) and alpha(3) subunits are involved in anxiolytic effects.

OBJECTIVES

We therefore examined the effects of (non)subunit-selective GABA(A) receptor agonists on temperature and locomotor responses to novel cage stress.

RESULTS

Using telemetric monitoring of temperature and locomotor activity, we found that nonsubunit-selective GABA(A) receptor agonist diazepam as well as the alpha(3) subunit-selective receptor agonist TP003 dose-dependently attenuated SIH and locomotor responses. Administration of GABA(A) receptor alpha(1)-selective agonist zolpidem resulted in profound hypothermia and locomotor sedation. The GABA(A) receptor alpha(1)-selective antagonist betaCCt antagonized the hypothermia, but did not reverse the SIH response attenuation caused by diazepam and zolpidem. These results suggest an important regulating role for the alpha(1) subunit in thermoregulation and sedation. Ligands of extrasynaptic GABA(A) receptors such as alcohol and nonbenzodiazepine THIP attenuated the SIH response only at high doses.

CONCLUSIONS

The present study confirms a putative role for the GABA(A) receptor alpha(1) subunit in hypothermia and sedation and supports a role for alpha(2/3) subunit GABA(A) receptor agonists in anxiety processes. In conclusion, we show that home cage temperature and locomotor responses to novel home cage stress provide an excellent tool to assess both anxiolytic and sedative effects of various (subunit-selective) GABA(A)ergic compounds.

Translational aspects of pharmacological research into anxiety disorders: the stress-induced hyperthermia (SIH) paradigm

In anxiety research, the search for models with sufficient clinical predictive validity to support the translation of animal studies on anxiolytic drugs to clinical research is often challenging. This review describes the stress-induced hyperthermia (SIH) paradigm, a model that studies the activation of the autonomic nervous system in response to stress by measuring body temperature. The reproducible and robust SIH response, combined with ease of testing, make the SIH paradigm very suitable for drug screening. We will review the current knowledge on the neurobiology of the SIH response, discuss the role of GABA(A) and serotonin (5-HT) pharmacology, as well as how the SIH response relates to infectious fever. Furthermore, we will present novel data on the SIH response variance across different mice and their sensitivity to anxiolytic drugs. The SIH response is an autonomic stress response that can be successfully studied at the level of its physiology, pharmacology, neurobiology and genetics and possesses excellent animal-to-human translational properties.

CRF1 not glucocorticoid receptors mediate prepulse inhibition deficits in mice overexpressing CRF

BACKGROUND

Both corticotropin-releasing factor (CRF) and glucocorticoid receptors (GR) are implicated in the psychotic symptoms of psychiatric disorders. Correspondingly, it is of interest to determine their respective involvement in the sensorimotor gating deficits displayed by transgenic mice overexpressing CRF. These mice reveal lifelong elevations of CRF and corticosterone levels.

METHODS

Effects of the GR antagonists ORG34517 (5-45 mg/kg by mouth [PO]) and mifepristone (5-45 mg/kg PO) and the CRF(1) receptor antagonists CP154,526 (20-80 mg/kg intraperitoneally [IP]) and DMP695 (2.5-40.0 mg/kg IP) on prepulse inhibition (PPI) of the acoustic startle response were studied in mice overexpressing CRF and in their wild-type littermates. In addition, PPI was measured in both genotypes 2 weeks after adrenalectomy with or without exogenous corticosterone administration via subcutaneous pellet implant (20 mg corticosterone).

RESULTS

ORG34517 and mifepristone did not influence perturbation of PPI in mice overexpressing CRF; reducing corticosterone levels by adrenalectomy likewise did not improve PPI. Further, elevation in corticosterone levels by pellet implantation did not disrupt PPI in wild-type mice. Conversely, both CRF(1) receptor antagonists, CP154,526 (40-80 mg/kg IP) and DMP695 (40 mg/kg IP), significantly restored PPI in CRF-overexpressing mice.

CONCLUSIONS

Sustained overactivation of CRF(1) receptors rather than excessive GR receptor stimulation underlies impaired sensorimotor gating in CRF-overexpressing mice. CRF(1) receptors thus may play a role in the expression of psychotic features in stress-related psychiatric disorders.

Altered behavioural adaptation in mice with neural corticotrophin-releasing factor overexpression

Overproduction of corticotrophin-releasing factor (CRF), the major mediator of the stress response, has been linked to anxiety, depression and addiction. CRF excess results in increased arousal, anxiety and altered cognition in rodents. The ability to adapt to a potentially threatening stimulus is crucial for survival, and impaired adaptation may underlie stress-related psychiatric disorders. Therefore, we examined the effects of chronic transgenic neural CRF overproduction on behavioural adaptation to repeated exposure to a non-home cage environment. We report that CRF transgenic mice show impaired adaptation in locomotor response to the novel open field. In contrast to wild-type (WT) mice, anxiety-related behaviour of CRF transgenic mice does not change during repeated exposure to the same environment over the period of 7 days or at retest 1 week later. We found that locomotor response to novelty correlates significantly with total locomotor activity and activity in the centre at the last day of testing and at retest in WT but not in CRF transgenic mice. Mice were divided into low responders and high responders on the basis of their initial locomotor response to novelty. We found that differences in habituation and re-exposure response are related to individual differences in locomotor response to novelty. In summary, these results show that CRF transgenic mice are fundamentally different from WT in their ability to adapt to an environmental stressor. This may be related to individual differences in stress reactivity. These findings have implications for our understanding of the role of CRF overproduction in behavioural maladaptation and stress-related psychiatric disorders.

Long-term behavioral changes after cessation of chronic antidepressant treatment in olfactory bulbectomized rats

BACKGROUND

Olfactory bulbectomy (OBX) in rats causes several behavioral and neurochemical central nervous system changes, reminiscent of symptoms of human depression. Moreover, depression-like behavior after OBX can be reversed with antidepressant drugs. However, the lasting effects of these antidepressant drugs on behavior after cessation of treatment have never been studied.

METHODS

Male rats received OBX or sham surgery. After recovery, animals received 14 consecutive daily doses of imipramine (20 mg/kg), escitalopram (5 and 10 mg/kg), or vehicle. Animals were tested in an open field after acute, sub-chronic, and chronic injections, as well as 1, 2, 6, and 10 weeks after cessation of treatment.

RESULTS

The OBX-induced hyperactivity was normalized after sub-chronic administration of imipramine and escitalopram. Two weeks after treatment, activity of OBX animals was comparable to sham-treated animals, but after 6 weeks, OBX animals treated with both doses of escitalopram had returned to pre-treatment hyperactivity levels. The OBX animals treated with the high imipramine dose (20 mg/kg) retained activity levels comparable to sham-treated animals until 10 weeks after cessation of treatment.

CONCLUSIONS

Chronic but not acute administration of imipramine and escitalopram normalizes OBX-induced hyperactivity. This effect continues for up to 10 weeks after cessation of treatment in a dose dependant manner.

Delay aversion: Effects of 7-OH-DPAT, 5-HT1A/1B-receptor stimulation and d-cycloserine

Impulsive individuals often display an aversion to waiting for rewards. Delay aversion can be quantified in rats in a delayed reward task, in which animals choose between an immediately available, small reward, and a large reward available after a delay. In previous research conducted at our laboratory and in literature, positive correlations between delay aversion and aggression, substance abuse and persistence during extinction of conditioned responses were found. The correlations suggest a possible shared pharmacology. Therefore, we tested drugs with known effects on these behaviors for possible effects on delay aversion: the dopamine D(3)-receptor agonist 7-OH-DPAT, the 5-HT(1A)-receptor agonist flesinoxan, the 5HT(1A/1B)-receptor agonist eltoprazine, and the NMDA-receptor agonist d-cycloserine. The results show that 7-OH-DPAT slightly decreased choice for the large reward. Flesinoxan disrupted task execution by lowering choice for the large reward even at a delay of 0 s. Eltoprazine slightly increased choice for the large reward, but the 5-HT(1B)-antagonist GR127935 had no effect. Administration of D-cycloserine also had no effect on choice behavior. The data suggest the dopamine D(3)-receptor and the 5-HT(1B)-receptor are interesting targets for treating delay aversion impulsivity. These targets were correctly predicted by the positive correlation between delay aversion and aggressive behavior and the intimate links between delay aversion and substance abuse disorders.

Relationship of delay aversion and response inhibition to extinction learning, aggression, and sexual behaviour

Impulsivity is an important symptom of many psychiatric disorders, and can be divided into two subtypes: response inhibition deficits and delay aversion. In the present study, we investigated the relationship between delay aversion and response inhibition, both to each other and to locomotion, extinction of conditioned responses, sexual behaviour, and aggressive behaviour. To that end, we quantified the behaviour of 24 rats in several tests. To measure response inhibition, rats were trained in a stop-signal task. In this operant task, rats were rewarded food if they inhibited execution of a response after presentation of an audible stop-signal. Delay aversion was measured in an operant task in which rats made a choice between a small, immediately available reward and a large reward available after a delay. The results showed that delay aversion and response inhibition were independent. Responses during extinction and various measures of aggressive behaviour were positively correlated to delay aversion. The speed of go-trials in the stop-task was correlated to non-aggressive behaviour. We conclude that the role of response inhibition in various behaviours is small, but delay aversion in particular contributes to several other behaviours, such as aggressive behaviour and extinction.

Effects of genetic background and null mutation of 5-HT1A receptors on basal and stress-induced body temperature: modulation by serotonergic and GABAA-ergic drugs

The stress-induced hyperthermia procedure, in which effects of drugs on basal (T(1)) and stress-induced body temperature (T(2)) are measured, predicts anxiolytic drug effect. Serotonergic drugs alter these responses and here, we studied the role of 5-HT(1A) receptors in stress-induced hyperthermia by using 5-HT(1A) receptor knockout mice. Three strains (129/Sv, Swiss Webster and C57Bl6) were used because genetic background can significantly modulate the null phenotype. We found that GABA-ergic drugs with an anxiolytic profile and stimulate alpha(2) subunit containing GABA(A) receptors, including diazepam and L838,417, result in reduced DeltaT (DeltaT=T(2)-T(1)). The alpha(1) subunit containing GABA(A) receptor was found to be primarily involved in regulation of basal body temperature T(1) and its stimulation can induce hypothermia. In addition, stimulation of 5-HT(1A) receptors by buspirone results in a reduced DeltaT, while stimulation of 5-HT(7) receptors primarily results in hypothermia. The null mutation of 5-HT(1A) receptors resulted in differences in drug-sensitivity that was further modulated by the genetic background. In particular, the null mutation on the SW and C57Bl6 backgrounds resulted in differential diazepam/L838,417 and 5-CT responses respectively. This indicates an interaction between the 5-HT(1A) receptor and genetic background and demonstrates the importance of selecting the background strain in a receptor knockout model.

Spontaneously hypertensive rats do not predict symptoms of attention-deficit hyperactivity disorder

The validity of the Spontaneously Hypertensive rat (SHR) as a model for Attention Deficit Hyperactivity Disorder (ADHD) is explored by comparing the SHR with Wistar-Kyoto (WKY) and Wistar rats in a number of different tests. In the open field, SHR are hyperactive compared to both Wistar and WKY, but only at specific ages. At those ages, methylphenidate (1mg/kg) did not attenuate hyperactivity. Subsequently, a dose response study of methylphenidate (0.1-10mg/kg) was conducted in the Differential Reinforcement of Low-rate responding (DRL)-72s and five-choice serial reaction time tests (5-CSRTT). Compared to WKY but not Wistar rats, SHR performed worse on the DRL-72s. Performance was not improved by methylphenidate (0.1-1.0mg/kg). In the 5-CSRTT, attentional performance was similar for all rat strains, but Wistar rats made more impulsive responses than both the SHR and the WKY. Methylphenidate only attenuated impulsivity in Wistar rats. Because SHR do not consistently display symptoms of ADHD across the different tests, and methylphenidate effects were observed in both WKY and Wistar rats, but not in SHR, we conclude that SHR is not a representative animal model for ADHD.

Mouse strain differences in autonomic responses to stress

In humans, anxiety disorders are often accompanied by an overactive autonomic nervous system, reflected in increased body temperature (BT) and heart rate (HR). In rodents, comparable effects are found after exposure to stress. These autonomic parameters can give important information on stress and anxiety responses in mice. In the present experiments, stress reactivity of three frequently used mouse strains [129 Sv/Ev, Swiss Webster (SW) and C57 BL/6] was assessed using their autonomic stress responses. BT, HR and activity were telemetrically measured. Undisturbed circadian rhythms already showed clear differences between the mouse strains. Hereafter, autonomic responses to stressors with increasing intensity were measured. Strain differences were found in magnitude and duration of the stress responses, especially after high-intensity stressors. Generally, C57BL/6 mice showed the largest autonomic response, SW the lowest and the 129Sv/Ev the intermediate response. Interestingly, the observed ranking in autonomic stress response does not match the behavioral stress responsivity of these strains. Finally, sensitivity to the anxiolytic diazepam (0, 1, 2, 4 and 8 mg/kg) was tested using the stress-induced hyperthermia paradigm. Pharmacological sensitivity to diazepam differed between the strains with the 129Sv/Ev being most sensitive. These studies show that simultaneous measurement of behavioral and autonomic parameters under stressful conditions contributes considerably to a better interpretation of anxiety and stress levels in mice.

Distribution and expression of CRF receptor 1 and 2 mRNAs in the CRF over-expressing mouse brain

Corticotropin-releasing factor (CRF) acts through CRF 1 and CRF 2 receptors (CRF1, CRF2). To test the hypothesis that CRF controls the expression of these receptors in a brain site- and receptor-type specific manner, we studied CRF1 mRNA and CRF2 mRNA expressions in mice with central CRF over-expression (CRF-OE) and using in situ hybridization. CRF1 and CRF2 mRNAs appear to be differentially distributed across the brain. The brain structures expressing the receptors are the same in wild-type (WT) and in CRF-OE mice. We therefore conclude that chronically elevated CRF does not induce or inhibit expression of these receptors in structures that normally do not or do, respectively, show these receptors. However, from counting cell body profiles positive for CRF1 and CRF2 mRNAs, clear differences appear in receptor expression between CRF-OE and WT mice, in a brain-structure-specific fashion. Whereas some structures do not differ, CRF-OE mice exhibit remarkably lower numbers of CRF1 mRNA-positive profiles in the subthalamic nucleus (-38.6%), globus pallidus (-31.5%), dorsal part of the lateral septum (-23.5%), substantia nigra (-22,8%), primary somatosensory cortex (-18.9%) and principal sensory nucleus V (-18.4%). Furthermore, a higher number of CRF2 mRNA-positive profiles are observed in the dorsal raphe nucleus (+32.2%). These data strongly indicate that central CRF over-expression in the mouse brain is associated with down-regulation of CRF1 mRNA and up-regulation of CRF2 mRNA in a brain-structure-specific way. On the basis of these results and the fact that CRF-OE mice reveal a number of physiological and autonomic symptoms that may be related to chronic stress, we suggest that CRF1 in the basal nuclei may be involved in disturbed information processing and that CRF2 in the dorsal raphe nucleus may play a role in mediating stress-induced release of serotonin by CRF.