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.

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.