Antidepressant-like action of AGN 2979, a tryptophan hydroxylase activation inhibitor, in a chronic mild stress model of depression in rats

Effects of fluoxetine, tianeptine and olanzapine on unpredictable chronic mild stress-induced depression-like behavior in mice

AIMS

Tianeptine is an atypical antidepressant drug that has a different mechanism of action than other antidepressants. Olanzapine is an atypical antipsychotic drug used for the treatment of schizophrenia. The present study was undertaken to investigate effects of chronic administration of tianeptine or olanzapine on unpredictable chronic mild stress (UCMS)-induced depression-like behavior in mice compared to a widely used SSRI antidepressant, fluoxetine.

MAIN METHODS

Male inbred BALB/c mice were subjected to different kinds of stressors several times a day for 7weeks and were treated intraperitoneally with tianeptine (5mg/kg), olanzapine (2.5mg/kg), fluoxetine (15mg/kg) or vehicle for 5weeks (n=7-8 per group).

KEY FINDINGS

All the drugs tested prevented stress-induced deficit in coat state during UCMS procedure, in grooming behavior in the splash test, decreased the attack frequency in the resident intruder test and decreased the immobility time in the tail suspension test. In the open field test olanzapine had anxiolytic-like effects in both stressed and non-stressed mice. Tianeptine, olanzapine and fluoxetine decreased the enhanced levels of plasma ACTH and IL-6. Chronic treatment with tianeptine resulted in a significant increase in both total number and density of BrdU-labeled cells in stressed animals, while fluoxetine and olanzapine had a partial effect.

SIGNIFICANCE

The results of this study support the hypothesis that tianeptine can be as effective as fluoxetine for the treatment of depression in spite of the differences in the mechanism of action of these drugs. Moreover, olanzapine could be used effectively in psychotic patients with depression.

Effects of peony glycosides on mice exposed to chronic unpredictable stress: further evidence for antidepressant-like activity

ETHNOPHARMACOLOGY RELEVANCE

Peony, the processed root of Paeonia lactiflora Pall. (Ranunculaceae), is a component herb of many traditional formulae for the treatment of depression-like disorders.

AIM OF THE STUDY

The present study aimed to investigate whether the total glycosides of peony (TGP) could prevent depression induced by chronic stress.

MATERIALS AND METHODS

Mice were subjected to an experimental setting of chronic unpredictable stress (CUS). The effect of TGP treatment on CUS-induced depression was examined by measuring behavioral and neurochemical parameters of depression and the antioxidant status of brain tissue.

RESULTS

CUS-induced depression, as indicated by a significant increase in immobility time in the tail suspension test, was associated with increases in the activities of monoamine oxidases, depletion of reduced glutathione, and an increase in malondialdehyde level, in mice brains. TGP treatment alleviated the extent of CUS-induced depression and the associated impairment of antioxidant status in the mouse brain.

CONCLUSION

The results suggest that TGP alleviates depression induced by chronic unpredictable stress. The antidepressant-like activity of TGP is probably mediated by inhibition of monoamine oxidases and the attenuation of oxidative stress in mouse brain.

AGN-2979, an inhibitor of tryptophan hydroxylase activation, does not affect serotonin synthesis in Flinders Sensitive Line rats, a rat model of depression, but produces a significant effect in Flinders Resistant Line rats

The neurotransmitter, serotonin, is involved in several brain functions, including both normal, physiological functions, and pathophysiological functions. Alterations in any of the normal parameters of serotonergic neurotransmission can produce several different psychiatric disorders, including major depression. In many instances, brain neurochemical variables are not able to be studied properly in humans, thus making the use of good animal models extremely valuable. One of these animal models is the Flinders Sensitive Line (FSL) of rats, which has face, predictive and constructive validities in relation to human depression. The objective of this study was to quantify the effect of the tryptophan hydroxylase (TPH) activation inhibitor, AGN-2979, on the FSL rats (rats with depression-like behaviour), and compare it to the effect on the Flinders Resistant Line (FRL) of rats used as the control rats. The effect was evaluated by measuring changes in regional serotonin synthesis in the vehicle treated rats (FSL-VEH and FRL-VEH) relative to those measured in the AGN-2979 treated rats (FSL-AGN and FRL-AGN). Regional serotonin synthesis was measured autoradiographically in more than 30 brain regions. The measurements were performed using alpha-[(14)C]methyl-l-tryptophan as the tracer. The results indicate that AGN-2979 did not produce a significant reduction of TPH activity in the AGN-2979 group relative to the vehicle group (a reduction would have been observed if there had been an activation of TPH by the experimental setup) in the FSL rats. On the other hand, there was a highly significant reduction of synthesis in the FRL rats treated by AGN-2979, relative to the vehicle group. Together, the results demonstrate that in the FSL rats, AGN-2979 does not affect serotonin synthesis. This suggests that there was no activation of TPH in the FSL rats during the experimental procedure, but such activation did occur in the FRL rats. Because of this finding, it could be hypothesized that TPH in the FSL rats cannot be easily activated. This may contribute to the development of depressive-like symptoms in the FSL rats ("depressed" rats), as they cannot easily modulate their need for elevated amounts of this neurotransmitter, and possibly other neurotransmitters. Further, because these rats represent a very good model of human depression, one can hypothesize that humans who do not have readily activated TPH may be more prone to develop depression.

An animal model of a behavioral intervention for depression

Although conditioned inhibition of fear (or learned safety) is a learning process critical for preventing chronic stress, a predisposing factor for depression and other psychopathologies, little is known about its functional purposes or molecular mechanisms. To obtain better insight into learned safety, we investigated its behavioral and molecular characteristics and found that it acts as a behavioral antidepressant in two animal models. Learned safety promotes the survival of newborn cells in the dentate gyrus of the hippocampus, while its antidepressant effect is abolished in mice with ablated hippocampal neurogenesis. Learned safety also increases the expression of BDNF in the hippocampus and leads to downregulation of genes involved in the dopaminergic and neuropeptidergic but not the serotonergic system in the basolateral amygdala. These data suggest that learned safety is an animal model of a behavioral antidepressant that shares some neuronal hallmarks of pharmacological antidepressants but is mediated by different molecular pathways.

Chronic mild stress impairs cognition in mice: from brain homeostasis to behavior

Exposure to chronic stress in rodents and psychosocial stress in humans has been shown to alter cognitive functions and has been linked to the pathophysiology of mood disorders. The purpose of the present study was to investigate effects and possible mechanisms of a chronic mild stress (CMS) procedure on cognitive behaviors in Swiss albino mice using the object recognition test (ORT) and object location test (OLT). Results showed that CMS exposure impaired cognitive performance and produced amnesia of acquired information in both ORT and OLT. Furthermore, the cognitive impairment was coexistent with increased plasma levels of interleukin-1beta (IL-1beta), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha), as well as with enhanced plasma levels of corticosterone (CORT), corticotrophin-releasing hormone (CRH) and adrenocorticotrophic hormone (ACTH). In addition, severe neuronal cell damage was found, as bromodeoxyuridine (BrdU) positive cells and the expression of brain derived neurotrophic factor (BDNF) in dentate gyrus (DG) of hippocampus were decreased after 5 weeks CMS procedure. Taken together, these findings indicated that CMS exposure-induced impairment of cognitive behaviors might be attributed to the stress-related alterations in brain homeostasis that were reflected in changes in the neuroimmune and neuroendocrine systems as well as in neurogenesis.

Functional implications of decreases in neurogenesis following chronic mild stress in mice

Numerous data from human and animal studies suggest that hippocampal plasticity might be a key element in depression. However, the connection remains loose at best and further data are needed. Human studies are of necessity limited, but animal models can help providing further insight. Unpredictable chronic mild stress (UCMS) is a commonly used model because it mimics depression-like phenotypes satisfactorily. Its rationale is based on the underlying stress-induced difficulties found in many depressed patients. We therefore studied learning and hippocampal neurogenesis in mice from three different inbred strains subjected to UCMS. Learning was assessed in different hippocampus-dependent and independent tasks. The rate of survival of newly generated brain cells was determined in behaviorally-naive animals. Results demonstrated a dramatic reduction of surviving new brain cells in both the hippocampus and the subventricular zone of UCMS-treated animals. This reduction was observed both for neurons and for other cells of the hippocampus. Behavioral data demonstrated an impairment of hippocampus-dependent learning, whereas hippocampus-independent learning was spared. However, the specific results were strongly dependent on strain and sex so that there does not appear to be a direct causative relationship between the deficits in neurogenesis and behavior.

The effects of chronic fluoxetine treatment on chronic mild stress-induced cardiovascular changes and anhedonia

BACKGROUND

Depression has a complex bidirectional association with heart disease. Previously we have shown notable cardiovascular changes in the chronic mild stress (CMS) rodent model of depression. Here we investigated the effects of a serotonin-specific reuptake inhibitor on a behavioral index of depression (anhedonia) and cardiac function in rats exposed to CMS.

METHODS

Male Sprague-Dawley rats were exposed to either 4 weeks of control conditions or CMS, consisting of unpredictable periods of mild stressors, while being treated concurrently with 4 weeks of daily fluoxetine (10 mg/kg, sc) or vehicle.

RESULTS

Chronic fluoxetine treatment prevented anhedonia in rats exposed to CMS, versus the CMS group treated with vehicle. However, treatment with fluoxetine in the CMS group only partially prevented specific cardiovascular changes associated with CMS, including elevated resting heart rate (HR), exaggerated pressor and HR responses to air jet stress, reduced cardiac output and stroke volume, and HR exaggerated responses to beta-adrenergic receptor blockade.

CONCLUSIONS

These findings provide evidence that 4 weeks of fluoxetine treatment can prevent behavioral responses and can partially prevent cardiovascular changes associated with CMS, providing insight into the role of serotonin in the link between depression and cardiovascular dysfunction.

The tryptophan hydroxylase activation inhibitor, AGN-2979, decreases regional 5-HT synthesis in the rat brain measured with alpha-[14C]methyl-L-tryptophan: an autoradiographic study

Many experimental conditions are stressful for animals. It is well known that stress induces tryptophan hydroxylase (TPH) activation, resulting in increased serotonin (5-HT) synthesis. In our experimental procedure to measure 5-HT synthesis using alpha-[(14)C]methyl-L-tryptophan (alpha-MTrp) autoradiographic method, the hind limbs of animals are restrained using a loose-fitted plaster cast such that the forelimbs of the animal remain free. The objective of the present investigation was to evaluate the changes, if any, in 5-HT synthesis, after injecting these restrained rats with the TPH activation inhibitor AGN-2979. The effect on regional 5-HT synthesis was studied using the alpha-MTrp autoradiographic method. The hypothesis was that the TPH activation inhibitor would reduce 5-HT synthesis, if TPH activation was induced by this restraint. The rats received injection of AGN-2979 (10 mg/kg, i.p.) or distilled water vehicle (1 mL/kg, i.p.) 1 h prior to tracer administration. The free- and total tryptophan concentrations were not significantly different between the treatment and control groups. The results demonstrate that 5-HT synthesis in AGN-2979 treated rats is significantly decreased (-12 to -35%) in both the raphe nuclei and their terminal areas when compared to the control rats. These findings suggest that restrained conditions, such as those used in our experimental protocol, induce TPH activation resulting in an increased 5-HT synthesis throughout the brain. The reduction in 5-HT synthesis in the AGN-2979 group is not related to a change in the plasma tryptophan. Because there was no activation in the pineal body, the structure having a different isoform of TPH, we can propose that it is only the brain TPH that becomes activated with this specific restraint.

Chronic mild stress induces behavioral and physiological changes, and may alter serotonin 1A receptor function, in male and cycling female rats

RATIONALE

Interactions among stress, serotonin 1A (5-HT(1A)) receptors, and the hypothalamic-pituitary-adrenocortical (HPA) system have been proposed to influence the development of depression in humans. The investigation of depression-relevant behaviors and physiological responses to environmental stressors in animal models of depression may provide valuable insight regarding these mechanisms.

OBJECTIVES

The purpose of these experiments was to investigate the interactions among central 5-HT(1A) receptors, endocrine function, and behavior in an animal model of depression, chronic mild stress (CMS).

METHODS

The current study examined behavioral responses to a pleasurable stimulus (sucrose), estrous cycle length (in female rats), and plasma hormone levels following systemic administration of a selective 5-HT(1A) receptor agonist [(+)8-hydroxy-N,N-dipropyl-2-aminotetralin hydrobromide (8-OH-DPAT); 40 mug/kg, s.c.; administered 15 min prior to sacrifice], in male and female rats exposed to 4 weeks of CMS.

RESULTS

Four weeks of CMS produced a reduction in the intake of 1% sucrose (anhedonia), as well as attenuated adrenocorticotropic hormone (ACTH) responses to 8-OH-DPAT in both male and female rats (22 and 18% lower than the control groups, respectively). Corticosterone and oxytocin responses to 8-OH-DPAT were not altered by exposure to CMS. In female rats, CMS induced a lengthening of the estrous cycle by approximately 40%.

CONCLUSIONS

CMS produces minor HPA disruptions along with behavioral disruptions. Alterations in 5-HT(1A) receptor function in specific populations of neurons in the central nervous system may be associated with the CMS model. The current findings contribute to our understanding of the relations that stress and neuroendocrine function have to depressive disorders.

Neuroendocrine and cytokine profile of chronic mild stress-induced anhedonia

A bidirectional relationship exists between depression and cardiovascular disease. Patients with major depression are more likely to develop cardiac events, and patients with myocardial infarction and heart failure are more likely to develop depression. A feature common to both clinical syndromes is activation of proinflammatory cytokines and stress hormones, including the hypothalamic-pituitary-adrenal axis and the renin-angiotensin-aldosterone system. In the present study we examined the hypothesis that exposure to chronic mild stress (CMS), an experimental model of depression that induces anhedonia in rats, is sufficient to activate the production of proinflammatory cytokines and stress hormones that are detrimental to the heart and vascular system. Four weeks of exposure of male, Sprague-Dawley rats to mild unpredictable environmental stressors resulted in anhedonia which was operationally defined as a reduction in sucrose intake without a concomitant effect on water intake. Humoral assays indicated increased plasma levels of tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), plasma renin activity, aldosterone, and corticosterone in the CMS exposed rats. Tissue TNF-alpha and IL-1beta were increased in the hypothalamus, and TNF-alpha was increased in the pituitary gland. These humoral responses to CMS, associated with anhedonia as an index of depression in the rat, are likely to be associated with neurohumoral mechanisms that may contribute to adverse cardiac events. The findings provide a basis for examining more directly the interactions among the central, endocrine, and immune systems in depression associated with heart disease.

Pavlovian autoshaping procedures increase plasma corticosterone and levels of norepinephrine and serotonin in prefrontal cortex in rats

Pavlovian autoshaping procedures provide for pairings of a small object conditioned stimulus (CS) with a rewarding substance unconditioned stimulus (US), resulting in the acquisition of complex sequences of CS-directed skeletal-motor responses or autoshaping conditioned responses (CRs). Autoshaping procedures induce higher post-session levels of corticosterone than in controls receiving CS and US randomly, and the enhanced post-session corticosterone levels have been attributed to the appetitive or arousal-inducing effects of autoshaping procedures. Enhanced corticosterone release can be induced by aversive stimulation or stressful situations, where it is often accompanied by higher levels of norepinephrine (NE) and serotonin (5-HT) in prefrontal cortex (PFC) but not in striatum (ST). Effects of autoshaping procedures on post-session corticosterone levels, NE contents in PFC, and 5-HT contents in PFC and ST were investigated in male Long-Evans rats. Post-session blood samples revealed higher corticosterone levels in the CS-US Paired group (n = 46) than in the CS-US Random control group (n = 21), and brain samples revealed higher levels of PFC NE and 5-HT in CS-US Paired group. Striatal 5-HT levels were unaltered by the autoshaping procedures. Autoshaping procedures provide for appetitive stimulation and induce an arousal-like state, as well as simultaneous stress-like changes in plasma corticosterone and monoamine levels in PFC. Autoshaping, therefore, may be useful for the study of endocrine and central processes associated with appetitive conditions.