Effect of antianxiety drugs on fear and stress

Anxiolytic profile of fluoxetine as monitored following repeated administration in animal rat model of chronic mild stress

Background: Fluoxetine, a selective serotonin re-uptake inhibitor (SSRI), has been proposed to be more effective as an antidepressive drug as compared to other SSRIs. After chronic SSRI administration, the increase in synaptic levels of 5-HT leads to desensitization of somatodentritic 5-HT autoreceptors in the raphe nuclei. Chronic stress may alter behavioral, neurochemical and physiological responses to drug challenges and novel stressors. Methods: Twenty four male rats were used in this study. Animals of CMS group were exposed to CMS. Animals of stressed and unstressed group were administrated with fluoxetine at dose of 1.0 mg/kg s well as 5.0 mg/kg repeatedly for 07 days 1 h before exposed to CMS. The objective of the present study was to evaluate that repeated treatment with fluoxetine could attenuate CMS-induced behavioral deficits. Results: Treatment with fluoxetine attenuated CMS-induced behavioral deficits. Fluoxetine administration induced hypophagia in unstressed as well as CMS rats. Acute and repeated administration of fluoxetine increased motor activity in familiar environment but only repeated administration increased exploratory activity in open field. Anxiolytic effects of fluoxetine were greater in unstressed rats. These anxiolytic effects were produced as result of repeated administration not on acute administration of fluoxetine at 1.0 mg/kg as well as 5.0 mg/kg. Conclusion: The present study demonstrated that CMS exposure resulted into behavioral deficits and produced depressive-like symptoms. Fluoxetine, an SSRI, administration attenuated behavioral deficits induced by CMS. Anxiolytic effects of repeated fluoxetine administration were greater in unstressed than CMS animals.

Models and mechanisms of anxiety: evidence from startle studies

RATIONALE

Preclinical data indicates that threat stimuli elicit two classes of defensive behaviors, those that are associated with imminent danger and are characterized by flight or fight (fear), and those that are associated with temporally uncertain danger and are characterized by sustained apprehension and hypervigilance (anxiety).

OBJECTIVE

The objectives of the study are to (1) review evidence for a distinction between fear and anxiety in animal and human experimental models using the startle reflex as an operational measure of aversive states, (2) describe experimental models of anxiety, as opposed to fear, in humans, (3) examine the relevance of these models to clinical anxiety.

RESULTS

The distinction between phasic fear to imminent threat and sustained anxiety to temporally uncertain danger is suggested by psychopharmacological and behavioral evidence from ethological studies and can be traced back to distinct neuroanatomical systems, the amygdala and the bed nucleus of the stria terminalis. Experimental models of anxiety, not fear, are relevant to non-phobic anxiety disorders.

CONCLUSIONS

Progress in our understanding of normal and abnormal anxiety is critically dependent on our ability to model sustained aversive states to temporally uncertain threat.

Metyrapone pretreatment prevents the behavioral and neurochemical sequelae induced by stress

In the present study, we examined the effect of metyrapone, an inhibitor of corticosterone (CS) synthesis, on the behavioral and neurochemical sequelae induced by a brief restraint session. A 15-min stress period induced an anxiogenic-like behavior on the elevated plus-maze (EPM), which was reversed with metyrapone (75 mg/kg i.p.) injected 3 h prior to the stress event. It was further demonstrated that metyrapone pretreatment normalized the decrease in maximal chloride uptake following GABA stimulation observed in brain cortex tissue obtained from animals exposed to both restraint and the EPM. In addition, plasma CS levels were assessed both after restraint and following EPM exposure. Furthermore, the administration of both CS (2.5 mg/kg s.c. at a dose that mimics CS levels induced by restraint) or dexamethasone (DEXA, 1.25 microg/kg s.c) resulted in an anxiogenic response in the EPM comparable to that induced by restraint. Taken together, all these evidence suggest that CS released in response to stress seems to be associated with functional changes at the GABAergic supramolecular complex which could underlie the enhanced anxiety observed following the exposure to an aversive experience.

Benzodiazepine receptors in rat brain are altered by adrenalectomy

The effects of adrenalectomy on benzodiazepine receptors in discrete regions of rat brain were examined using [3H]flunitrazepam as binding ligand. The concentration of benzodiazepine receptors was significantly increased by 25, 50 and 71% in hippocampus, striatum and hypothalamus, respectively, after adrenalectomy. In contrast, adrenalectomy did not affect the concentration of benzodiazepine receptors in cerebral cortex, olfactory bulb and cerebellum. No significant differences in the apparent binding affinity (Kd) values were seen following adrenalectomy in any brain region examined. The adrenalectomy-induced increases in [3H]flunitrazepam binding sites were completely reversed by glucocorticoid replacement with dexamethasone. These results demonstrate that adrenalectomy is capable of selectively modulating benzodiazepine receptors in brain regions presumably involved with glucocorticoid negative feedback. The data further suggest additional mechanisms by which endogenous hypothalamic-pituitary-adrenocortical hormones may affect 'anxiety' levels.