Fear-potentiated startle response is remarkably similar in two laboratories

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.

Translational Studies in the Complex Role of Neurotransmitter Systems in Anxiety and Anxiety Disorders

Discovery of innovative anxiolytics is severely hampering. Existing anxiolytics are developed decades ago and are still the therapeutics of choice. Moreover, lack of new drug targets forecasts a severe jeopardy in the future treatment of the huge population of CNS-diseased patients. We simply lack the knowledge on what is wrong in brains of anxious people (normal and diseased). Translational research, based on interacting clinical and preclinical research, is extremely urgent. In this endeavor, genetic and genomic approaches are part of the spectrum of contributing factors. We focus on three druggable targets: serotonin transporter, 5-HT_1A, and GABA_A receptors. It is still uncertain whether and how these targets are involved in normal and diseased anxiety processes. For serotonergic anxiolytics, the slow onset of action points to indirect effects leading to plasticity changes in brain systems leading to reduced anxiety. For GABA_A benzodiazepine drugs, acute anxiolytic effects are found indicating primary mechanisms directly influencing anxiety processes. Close translational collaboration between fundamental academic and discovery research will lead to badly needed breakthroughs in the search for new anxiolytics.

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.

Electric foot shock stress: a useful tool in neuropsychiatric studies

Electric foot shock is a complex stressor with both physical and emotional components. It has been employed as an important tool to develop diverse animal models in the field of psychopharmacology. The electric foot shock paradigm includes acute or chronic exposures of shocks of varying intensity and duration on an electrified grid floor in an electric foot shock apparatus. Research evidence reveals that foot shocks of varying intensity produce behavioral and neurochemical changes reflecting depression, anxiety, and post-traumatic stress disorder (PTSD) in humans. Animals generally do not habituate to foot shocks in comparison to other stressors, including loud noise, bright light, and hot and cold temperatures. Additionally, it offers an experimental advantage of control over intensity and duration; therefore, by varying its application parameters, different disorder models have been created. Electric foot shock fear conditioning-induced ultrasonic vocalization and fear-potentiated startle have been explored to develop models of anxiety and panic. Similarly, fear conditioning in the form of foot shock exposure followed by situational reminders has been used to develop a model of PTSD. Electric foot shock-induced conflict has been explored to develop operant conflict models (Geller-Seifter and Vogel tests), which in turn are pharmacologically validated to screen potential anti-anxiety agents. Inescapable electric shock-induced ‘learned helplessness’ mimics the symptomology of depression, and this phenomenon has been employed to develop the model of depression. The present review describes the pharmacologically validated models of anxiety, depression, and PTSD involving electric foot shock as an aversive stimulus.

The brain orexin system and almorexant in fear-conditioned startle reactions in the rat

RATIONALE

The rat fear-potentiated startle (FPS) paradigm is a translational model of conditioned fear involving central amygdala pathways of the brain. Hypothalamic orexin neurons have input-output projections to the amygdala; they modulate vigilance and stress-related responses.

OBJECTIVE

To investigate whether the transient pharmacological blockade of orexin receptors moderates the conditioned fear response.

METHODS

F344 rats received acute oral treatment with the dual orexin receptor antagonist almorexant (30-300 mg/kg) or with one of the clinically effective anxiolytics diazepam (1-10 mg/kg), buspirone (10-100 mg/kg), fluoxetine (3-30 mg/kg), and sertraline (10-100 mg/kg). Drug effects on startle responses were assessed in both fear- and non-fear-conditioned rats; on forepaw grip and horizontal wire motor performance, and on elevated plus maze (EPM) behavior.

RESULTS

Diazepam and almorexant both dose-dependently decreased FPS in the presence of the fear-conditioned stimulus (CS; light) more prominently than background startle in absence of the CS (dark). Diazepam induced myorelaxation and reduced startle responses in control non-fear-conditioned rats. Almorexant had no myorelaxant effects and left startle responses under light in non-fear-conditioned rats intact. On the EPM, diazepam showed anxiolytic-like effects, almorexant not. Buspirone demonstrated anxiolytic-like effects on FPS by simultaneously reducing CS-related startle and increasing no-CS-background startle. Fluoxetine did not affect FPS, whereas sertraline showed anxiogenic-like effects.

CONCLUSIONS

Almorexant reduced FPS, but did not affect EPM behavior. Almorexant's overall pattern of effects on FPS was comparable to but less pronounced than that of the anxiolytic benzodiazepine diazepam. The endogenous orexin system actively contributes to fear-conditioned startle reactions in the rat.

Acute tryptophan depletion increases translational indices of anxiety but not fear: serotonergic modulation of the bed nucleus of the stria terminalis?

Serotonin is strongly implicated in the mammalian stress response, but surprisingly little is known about its mode of action. Recent data suggest that serotonin can inhibit aversive responding in humans, but this remains underspecified. In particular, data in rodents suggest that global serotonin depletion may specifically increase long-duration bed nucleus of the stria terminalis (BNST)-mediated aversive responses (ie, anxiety), but not short-duration BNST-independent responses (ie, fear). Here, we extend these findings to humans. In a balanced, placebo-controlled crossover design, healthy volunteers (n=20) received a controlled diet with and without the serotonin precursor tryptophan (acute tryptophan depletion; ATD). Aversive states were indexed by translational acoustic startle measures. Fear and anxiety were operationally defined as the increase in startle reactivity during short- and long-duration threat periods evoked by predictable shock (fear-potentiated startle) and by the context in which the shocks were administered (anxiety-potentiated startle), respectively. ATD significantly increased long-duration anxiety-potentiated startle but had no effect on short-duration fear-potentiated startle. These results suggest that serotonin depletion in humans selectively increases anxiety but not fear. Current translational frameworks support the proposition that ATD thus disinhibits dorsal raphé-originating serotonergic control of corticotropin-releasing hormone-mediated excitation of the BNST. This generates a candidate neuropharmacological mechanism by which depleted serotonin may increase response to sustained threats, alongside clear implications for our understanding of the manifestation and treatment of mood and anxiety disorders.

Loop diuretics have anxiolytic effects in rat models of conditioned anxiety

A number of antiepileptic medications that modulate GABA(A) mediated synaptic transmission are anxiolytic. The loop diuretics furosemide (Lasix) and bumetanide (Bumex) are thought to have antiepileptic properties. These drugs also modulate GABA(A) mediated signalling through their antagonism of cation-chloride cotransporters. Given that loop diuretics may act as antiepileptic drugs that modulate GABAergic signalling, we sought to investigate whether they also mediate anxiolytic effects. Here we report the first investigation of the anxiolytic effects of these drugs in rat models of anxiety. Furosemide and bumetanide were tested in adult rats for their anxiolytic effects using four standard anxiety models: 1) contextual fear conditioning; 2) fear-potentiated startle; 3) elevated plus maze, and 4) open-field test. Furosemide and bumetanide significantly reduced conditioned anxiety in the contextual fear-conditioning and fear-potentiated startle models. At the tested doses, neither compound had significant anxiolytic effects on unconditioned anxiety in the elevated plus maze and open-field test models. These observations suggest that loop diuretics elicit significant anxiolytic effects in rat models of conditioned anxiety. Since loop diuretics are antagonists of the NKCC1 and KCC2 cotransporters, these results implicate the cation-chloride cotransport system as possible molecular mechanism involved in anxiety, and as novel pharmacological target for the development of anxiolytics. In view of these findings, and since furosemide and bumetanide are safe and well tolerated drugs, the clinical potential of loop diuretics for treating some types of anxiety disorders deserves further investigation.

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.

SSRIs and conditioned fear

Among drugs that act on serotonergic neurotransmission, selective serotonin (5-HT) reuptake inhibitors (SSRIs) are now the gold standard for the treatment of anxiety disorders. The precise mechanisms of the anxiolytic actions of SSRIs are unclear. We reviewed the literature related to the effects of SSRIs and the neurochemical changes of 5-HT in conditioned fear. Acute SSRIs and 5-HT(1A) receptor agonists reduced the acquisition and expression of contextual conditioned fear. Chronic SSRI administration enhanced anxiolytic-like effects. Microinjection studies revealed the amygdala as the target brain region of both classes of serotonergic drugs, and the hippocampus as the target of 5-HT(1A) receptor agonists. These findings highlight the contribution of post-synaptic 5-HT receptors, especially 5-HT(1A) receptors, to the anxiolytic-like effects of serotonergic drugs. These results support the new 5-HT hypothesis of fear/anxiety: the facilitation of 5-HT neurotransmission ameliorates fear/anxiety. Furthermore, these behavioral data provide a new explanation of neurochemical adaptations to contextual conditioned fear: increased 5-HT transmission seems to decrease, not increase, fear.

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.

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.

Animal models of anxiety and anxiolytic drug action

Animal models of anxiety attempt to represent some aspect of the etiology, symptomatology, or treatment of human anxiety disorders, in order to facilitate their scientific study. Within this context, animal models of anxiolytic drug action can be viewed as treatment models relevant to the pharmacological control of human anxiety. A major purpose of these models is to identify novel anxiolytic compounds and to study the mechanisms whereby these compounds produce their anxiolytic effects. After a critical analysis of "face," "construct," and "predictive" validity, the biological context in which animal models of anxiety are to be evaluated is specified. We then review the models in terms of their general pharmacological profiles, with particular attention to their sensitivity to 5-HTIA agonists and antidepressant compounds. Although there are important exceptions, most of these models are sensitive to one or perhaps two classes of anxiolytic compounds, limiting their pharmacological generality somewhat, but allowing in depth analysis of individual mechanisms of anxiolytic drug action (e.g., GABAA agonism). We end with a discussion of possible sources of variability between models in response to 5-HTIA agonists and antidepressant drugs.

Phenytoin normalizes exaggerated fear behavior in p-chlorophenylalanine (PCPA)-treated rats

Temporal lobe epilepsy may be associated with emotional difficulties such as depression and anxiety. Because the amygdala is involved in both epilepsy and emotion, common neural mechanisms in this temporal lobe structure may underlie the emotional disturbances observed in people with epilepsy. The neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) is implicated in many psychopathologies, and 5-HT also modulates amygdala excitability. Therefore, the present study uses the fear-potentiated startle (FPS) paradigm to investigate the effect of neuronal excitability on fear behavior in rats treated with p-chlorophenylalanine (PCPA) to chronically inhibit 5-HT synthesis. PCPA treatment selectively enhanced FPS in individually housed rats. The exaggerated FPS response was reduced to control level by the anticonvulsant phenytoin at 10mg/kg, and phenytoin at 30mg/kg further decreased FPS behavior. These data suggest that a subseizure state of neuronal excitability mediated by low 5-HT in brain fear circuits may be associated with pathological fear behavior.

Effects of prenatal maternal stress by repeated cold environment on behavioral and emotional development in the rat offspring

It has been reported that many types of stresses, which caused physiological and psychological alterations in dams as prenatal maternal stress, affected behavioral and emotional traits of their offspring. However, effects of environmental temperature changes, which induce various stress responses in both animals and humans, have not been assessed as prenatal maternal stress. Repeated cold stress (RCS) is a type of chronic cold stress in which environmental temperature changes rapidly and frequently several times within a day. In the present study, to investigate effects of chronic maternal stress by the RCS on behavioral and emotional development of the rat offspring (prenatal RCS rats), the RCS stress was loaded to pregnant rats between day 9 and 19 after fertilization. The prenatal RCS rats showed similar locomotor activity in an open field to control rats that were borne by non-stressed pregnant rats. On the other hand, the prenatal RCS rats showed significantly higher startle responses than the control rats in a light enhanced startle paradigm. However, treatment of diazepam decreased the startle responses in the prenatal RCS rats to the same degree as those in the control rats. The results indicated that prenatal RCS affected emotional development of the rat offspring, but not locomotor activity. Comparison of the present results with the previous studies suggests that there might be unknown common mechanisms among different prenatal maternal stresses that induce similar behavioral developmental alteration.

The effects of sex and neonatal maternal separation on fear-potentiated and light-enhanced startle

This study was based on the higher prevalence of anxiety disorders in women than in men, and on the finding that early adverse experiences are a major risk factor for the development of anxiety disorders later in life. The object of this study was to investigate in rats, the sensitivities of the light-enhanced startle (LES) and fear-potentiated startle (FPS) paradigms to sex differences and to determine the effects of maternal separation (MS) on the baseline startle magnitude and potentiated startle response in these paradigms. Pups in the MS group were separated daily from their mother for 180 min/day from postnatal day 2 (PND2) to PND14. Control litters remained undisturbed. The adult male and female progeny were tested in the FPS and LES. As predicted, females showed a significantly greater potentiation of startle than males in the FPS, and a strong trend towards greater startle potentiation in the LES. Contrary to predictions, MS had no effect on startle potentiation in the FPS and severely disrupted LES in female, but not male rats. The observed sex differences add to the validity of the FPS and LES as animal paradigms of fear and anxiety. The findings indicate that these paradigms can be used to study the biological basis of sex differences in fear and anxiety. In contrast, the effects of MS on startle potentiation argue against the idea that MS provides a robust model for the predicted influences of early adverse effects on these startle potentiation measures of fear and anxiety.

Light-enhanced and fear-potentiated startle: temporal characteristics and effects of alpha-helical corticotropin-releasing hormone

BACKGROUND

It has been suggested that the light-enhanced startle paradigm (LES) is an animal model for anxiety, because of the unconditioned and nonspecific cue and the long-term effect. In contrast, the fear-potentiated startle (FPS) is suggested to model fear. In the present study, we assessed in detail the time course of LES and FPS and investigated whether corticotropin-releasing hormone (CRH) is differentially involved in these two models.

METHODS

In experiment 1, the amplitude of the startle response was tracked in the presence of the light and after light offset, in both models. In experiment 2, the effects of intracerebroventricular administration of the CRH-receptor antagonist alpha-helical CRH (0, 1, 5, and 25 microg) on LES and FPS were studied.

RESULTS

In LES, light onset resulted in a long-lasting potentiation of the startle response and a slow return to baseline after light offset. In FPS, the potentiation of the startle response returned to baseline almost immediately after light offset. Alpha-helical CRH reduced the potentiation in LES at the 5-microg dose but not at 25 microg. In FPS, alpha-helical CRH had no effect.

CONCLUSIONS

The results show that the time course of LES is markedly different from that of FPS, which together with the differences in eliciting stimuli suggest that they model anxiety and fear, respectively. Moreover, the results suggest that CRH is involved in LES and not in FPS.

Contextual fear conditioning and baseline startle responses in the rat fear-potentiated startle test: a comparison of benzodiazepine/gamma-aminobutyric acid-A receptor agonists

In the rat, fear-potentiated startle (FPS) test animals are first trained to associate brief light presentations with a mild electric footshock and then tested for startle responses to acoustic stimuli, delivered either in darkness (i.e. baseline startle) or after the conditioning stimulus. Following light presentation the magnitude of the startle response is markedly increased, and the test is commonly used to distinguish anxiolytic drug effects (i.e. a reduction in FPS) from non-specific effects such as sedation/muscle relaxation. However, recent studies suggest that the environment in which the animal is trained may also contribute towards the acquisition of a conditioned fear response (i.e. contextual fear conditioning) and that this may elevate startle responses recorded in the dark. In the present study, therefore, we have compared the benzodiazepine/gamma-aminobutyric acid-A receptor agonist chlordiazepoxide with the partial agonists FG 8205 and bretazenil, which are known to have a reduced propensity to produce sedation/myorelaxation, using two different FPS procedures: (i) conditioning and testing in stabilimeter chambers, and (ii) conditioning and testing in different environments. The results show that FPS can be demonstrated in both procedures and that treatment with chlordiazepoxide, FG 8205 or bretazenil dose-dependently attenuates the response. However, animals conditioned and tested in stabilimeter chambers also showed a significant increase in dark-startle amplitudes compared with non-shocked rats, suggesting that this response was elevated by contextual fear conditioning. Furthermore, despite clear differences in side-effect liabilities, FG 8205 and bretazenil significantly reduced dark-startle responses, suggesting that this measure is also sensitive to the anxiolytic effects of benzodiazepines. In contrast, when animals were conditioned and tested in different environments, dark-startle responses were not significantly different from those recorded in non-shocked rats and treatment with FG 8205 or bretazenil had no effect. Thus, conditioning and testing animals in different environments may provide a more effective means of distinguishing anxiolytic from non-specific drug effects in the rat FPS test.

5-HT(3) receptor antagonists and anxiety; a preclinical and clinical review

The present paper reviews the evidence for anxiolytic activity of 5-HT(3) receptor antagonists in animal models of anxiety and in clinical trials in humans. Compared to the established anxiolytics (benzodiazepine receptor agonists and, to a lesser extent, 5-HT(1A) receptor agonists) 5-HT(3) receptor antagonists display a different anxiolytic profile. They are anxiolytic in a limited number of animal anxiety models. If active, they often are very potent and display bell-shaped dose response curves, whereas the ratio between therapeutic activity and side effects appears remarkably large. 5-HT(3) receptor antagonists remain active after chronic dosing and no indications for tolerance, dependence or rebound effects were found, which seems to make these drugs an attractive alternative to the benzodiazepines. However, the large body of animal data indicating a complete lack of psychotropic activity of 5-HT(3) receptor antagonists weakens the prediction of anxiolytic activity in these drugs. Human data are also controversial; some investigators have reported positive effects in anxiety disorders (panic disorder, GAD), others did not. It can be concluded that 5-HT(3) receptor antagonists do not represent a breakthrough in the treatment of various anxiety disorders, as initially suggested.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

The anxiolytic effect on the fear-potentiated startle is not due to a non-specific disruption

In the present study, the effects of alprazolam, chlordiazepoxide, ethanol and haloperidol in the strychnine-potentiated startle response paradigm were investigated. Because strychnine increases control startle levels without fear-conditioning, no central state of fear exists. When anxiolytic drugs reduce the fear-induced potentiation of the startle response without reducing the strychnine-induced startle potentiation, their attenuating effect on startle potentiation in the fear-potentiated startle response paradigm can more likely be attributed to their anxiolytic properties. The selected drugs, which effectively reduced the fear-induced startle potentiation in an earlier study, indeed did not selectively affect the strychnine-induced startle potentiation, supporting the notion that the dose-dependent reduction of startle potentiation due to anxiolytic drugs in the fear-potentiated startle response paradigm does not reflect a "non-specific" disruption of startle behavior.

Anxiolytic effects of flesinoxan in the stress-induced hyperthermia paradigm in singly-housed mice are 5-HT1A receptor mediated

In the stress-induced hyperthermia paradigm in singly-housed male mice, two sequential rectal temperature measurements reveal the basal temperature (T1) and, 10 min later, an enhanced body temperature (T2), due to the stress of the first rectal measurement. The difference T2 - T1 (deltaT) is the stress-induced hyperthermia and putatively reflects a stress-induced anxiogenic response. The full 5-HT1A receptor agonist flesinoxan ((+)-enantiomer), its (-)-enantiomer and the racemic mixture reduced stress-induced hyperthermia effects, indicating putative anxiolytic properties. The ratio of their potencies to reduce stress-induced hyperthermia was similar to their potency in receptor binding affinities for 5-HT1A receptors, supporting that the anti-hyperthermia effects are mediated by the 5-HT1A receptor. This was further substantiated when the 5-HT1A receptor antagonists WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexane carboxamine trihydrochloride) and DU 125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide, monomesylate) both were able to antagonize the anti-stress-induced hyperthermia effects of flesinoxan. The stress-induced hyperthermia paradigm in singly-housed mice represents a simple and robust paradigm to measure putative anxiolytic effects of drugs.

Effects of 5-HT1A receptor ligands in a modified Geller-Seifter conflict model in the rat

In a modified Geller-Seifter conflict procedure, rats were trained to lever-press for food under a multiple variable interval-fixed ratio (VI30: food; FR10: food + shock) schedule of reinforcement. The ability to antagonize response suppression in the punished period is considered a good predictor for anxiolytic activity. Chlordiazepoxide and alprazolam increased punished responding. The 5-HT1A receptor agonists flesinoxan (R(+)-N-[2[4-(2,3-dihydro-2-2-hydroxymethyl-1,4-benzodioxin-5-yl)- 1-piperazinyl]ethyl]-4-fluorobenzoamide; 0.1-10.0 mg/kg) and 8-OH-DPAT (8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.03-0.5 mg/kg) significantly increased punished responding, supporting a role of the 5-HT1A receptor in anxiety. 8-OH-DPAT and flesinoxan also reduced unpunished responding. The anxiolytic effects of 8-OH-DPAT and flesinoxan could only be antagonized with a high dose (1.0 and 3.0 mg/kg respectively) of the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride). All doses of WAY-100635 antagonized the 5-HT1A-induced effects on unpunished responding. The dissimilarity in dose-response curve of WAY-100635 on punished and unpunished behaviour poses questions about the mediation of these effects.