Reversal by imipramine of beta-adrenoceptor up-regulation induced in a chronic mild stress model of depression

Chronic mild stress paradigm as a rat model of depression: facts, artifacts, and future perspectives

RATIONALE

The chronic mild stress (CMS) paradigm was first described almost 40 years ago and has become a widely used model in the search for antidepressant drugs for major depression disorder (MDD). It has resulted in the publication of almost 1700 studies in rats alone. Under the original CMS procedure, the expression of an anhedonic response, a key symptom of depression, was seen as an essential feature of both the model and a depressive state. The prolonged exposure of rodents to unpredictable/uncontrollable mild stressors leads to a reduction in the intake of palatable liquids, behavioral despair, locomotor inhibition, anxiety-like changes, and vegetative (somatic) abnormalities. Many of the CMS studies do not report these patterns of behaviors, and they often fail to include consistent molecular, neuroanatomical, and physiological phenotypes of CMS-exposed animals.

OBJECTIVES

To critically review the CMS studies in rats so that conceptual and methodological flaws can be avoided in future studies.

RESULTS

Analysis of the literature supports the validity of the CMS model and its impact on the field. However, further improvements could be achieved by (i) the stratification of animals into 'resilient' and 'susceptible' cohorts within the CMS animals, (ii) the use of more refined protocols in the sucrose test to mitigate physiological and physical artifacts, and (iii) the systematic evaluation of the non-specific effects of CMS and implementation of appropriate adjustments within the behavioral tests.

CONCLUSIONS

We propose methodological revisions and the use of more advanced behavioral tests to refine the rat CMS paradigm, which offers a valuable tool for developing new antidepressant medications.

Diminished responses to monoaminergic antidepressants but not ketamine in a mouse model for neuropsychiatric lupus

BACKGROUND

A significant proportion of patients suffering from major depression fail to remit following treatment and develop treatment-resistant depression. Developing novel treatments requires animal models with good predictive validity. MRL/lpr mice, an established model of systemic lupus erythematosus, show depression-like behavior.

AIMS

We evaluated responses to classical antidepressants, and associated immunological and biochemical changes in MRL/lpr mice.

METHODS AND RESULTS

MRL/lpr mice showed increased immobility in the forced swim test, decreased wheel running and sucrose preference when compared with the controls, MRL/MpJ mice. In MRL/lpr mice, acute fluoxetine (30 mg/kg, intraperitoneally (i.p.)), imipramine (10 mg/kg, i.p.) or duloxetine (10 mg/kg, i.p.) did not decrease the immobility time in the Forced Swim Test. Interestingly, acute administration of combinations of olanzapine (0.03 mg/kg, subcutaneously)+fluoxetine (30 mg/kg, i.p.) or bupropion (10 mg/kg, i.p.)+fluoxetine (30 mg/kg, i.p.) retained efficacy. A single dose of ketamine but not three weeks of imipramine (10 mg/kg, i.p.) or escitalopram (5 mg/kg, i.p.) treatment in MRL/lpr mice restored sucrose preference. Further, we evaluated inflammatory, immune-mediated and neuronal mechanisms. In MRL/lpr mice, there was an increase in autoantibodies' titers, [3H]PK11195 binding and immune complex deposition. There was a significant infiltration of the brain by macrophages, neutrophils and T-lymphocytes. p11 mRNA expression was decreased in the prefrontal cortex. Further, there was an increase in the 5-HT_2aR expression, plasma corticosterone and indoleamine 2,3-dioxygenase activity.

CONCLUSION

In summary, the MRL/lpr mice could be a useful model for Treatment Resistant Depression associated with immune dysfunction with potential to expedite antidepressant drug discovery.

Models of affective illness: chronic mild stress in the rat

Described in this unit is a chronic mild stress (CMS) procedure used for predicting an antidepressant response. Following exposure to a variety of mild stressors for a period of several weeks, rat behavior is modified in a number of ways. Among these is a substantial reduction in consumption of a 1% sucrose solution. Chronic administration of antidepressant drugs reverses diminished enthusiasm for sucrose in these subjects. While most antidepressants must be administered for at least 3 to 5 weeks to normalize behavior, there are treatments that display a more rapid onset of action. More recently, it has been shown that the CMS-induced deficit in sucrose consumption can also be reversed by second-generation antipsychotics. Based on these findings, the CMS model can be employed in discovery programs aimed at identifying antianhedonic drugs (e.g., antidepressants and antipsychotics) that act more quickly than existing agents.

Measuring behavior in mice with chronic stress depression paradigm

Many studies with chronic stress, a common depression paradigm, lead to inconsistent behavioral results. We are introducing a new model of stress-induced anhedonia, which provides more reproducible induction and behavioral measuring of depressive-like phenotype in mice. First, a 4-week stress procedure induces anhedonia, defined by decreased sucrose preference, in the majority of but not all C57BL/6 mice. The remaining 30-50% non-anhedonic animals are used as an internal control for stress effects that are unrelated to anhedonia. Next, a modified sucrose test enables the detection of inter-individual differences in mice. Moreover, testing under dimmed lighting precludes behavioral artifacts caused by hyperlocomotion, a major confounding factor in stressed mice. Finally, moderation of the stress load increases the reproducibility of anhedonia induction, which otherwise is difficult to provide because of inter-batch variability in laboratory mice. We believe that our new mouse model overcomes some major difficulties in measuring behavior with chronic stress depression models.

Chronic mild stress induces widespread decreases in thyroid hormone alpha1 receptor mRNA levels in brain--reversal by imipramine

While considerable clinical evidence implicates thyroid hormones (THs) in depressive illness, the specific nature of this involvement remains unclear. The alpha1 subtype (TR-alpha1) is the most abundant TH receptor in brain. Here we investigated changes in TR-alpha1 mRNA in the chronic mild stress (CMS) model of depression. Rats were exposed to a CMS schedule for 3 weeks, which resulted in a progressive decreases in sucrose preference (an index of anhedonia). They were then treated daily with either imipramine (IMI, 10mg/kg) or vehicle (VEH) for 2 weeks before being sacrificed for quantitative in situ hybridization analyses of TR-alpha1 mRNA throughout the brain. Results indicated that CMS followed by VEH induced widespread decreases in TR-alpha1 mRNA in brain. In contrast, CMS-exposed rats receiving IMI for the last 2 weeks prior to sacrifice showed full recovery of sucrose preference. Furthermore, brain TR-alpha1 mRNA levels in these animals were similar to those of non-stressed controls receiving either SAL or IMI. These results reveal that TR-alpha1 mRNA brain levels are very sensitive to CMS effects. The reversal of both anhedonic and TR-alpha1 effects of CMS by IMI suggests that TR-alpha1 may play a role both in stress-induced depressive symptoms and in their reversal by antidepressant interventions.

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.

Selective effects of citalopram in a mouse model of stress-induced anhedonia with a control for chronic stress

A stress-induced decrease in sucrose preference in rodents is regarded as an analog of anhedonia, a key symptom of depression. We investigated the effects of citalopram, administrated via drinking water (15 mg/kg/day), in a mouse model of stress-induced anhedonia. In this model, chronic stress induces anhedonia in a subset of C57BL/6N mice, while the remaining animals do not show a hedonic deficit or other depressive-like behaviors, although they are exposed to the same stressors as the anhedonic mice. Pre-stress and post-stress treatment with citalopram counteracted the development and maintenance of anhedonia and rescued normal floating in the forced swim test, demonstrating an antidepressant-like action. During the post-stress treatment, citalopram selectively increased sucrose preference and intake on the fourth week of treatment in anhedonic mice without affecting non-anhedonic animals. Citalopram also decreased elevated water consumption in the anhedonic group. Citalopram, administered 1 week before and during a 4-week stress procedure, decreased the percentage of anhedonic mice and reduced the increase of water intake in stressed mice. This study suggests that our chronic stress paradigm can serve as a model of anhedonia, in which antidepressant treatment is selectively effective in animals with a hedonic deficit.

Expression of beta-adrenergic receptor up-regulation is mediated by two different processes

Mechanisms of up-regulation of beta-adrenergic receptors (beta-ARs) induced by sustained exposure to 10(-8) M nadolol, a non-selective beta-AR antagonist, were examined using mouse cerebrocortical neurons. Nadolol dose- and time-dependently increased [3H]CGP-12177 bindings to the particulate fractions. This increase occurred 6 h and attained its plateau 12 h after the exposure, whereas beta1- and beta2-AR mRNA significantly increased 24 h and attained their plateaus 3 days after the exposure. Scatchard analysis revealed that the increased bindings were due to increase of receptor density. The [3H]CGP-12177 bindings to beta1- and beta2-ARs increased both 12 h and 5 days after the exposure. Although cycloheximide (CHX) decreased the bindings with or without nadolol, the extent of increase of the bindings induced by nadolol was not affected by CHX. Actinomycin D (AD) with nadolol showed no affects on the bindings 12 h after nadolol exposure, while AD treated 6 h after nadolol exposure significantly reduced the bindings 48 h after nadolol exposure. During 24 h after nadolol exposure, the increase in proteins of beta1- and beta2-ARs in the neuronal membrane was due to the increased receptor protein translocation from cytosol to membrane. These results indicate that the up-regulation of beta-ARs induced by nadolol is mediated by, at least, two different processes, one is increase in translocation of receptor proteins from cytosol to membrane with no changes in synthesis of receptor proteins and their mRNA and another is dependent on receptor protein synthesis with increased synthesis of their mRNA.

Chronic mild stress (CMS) revisited: consistency and behavioural-neurobiological concordance in the effects of CMS

The chronic mild stress (CMS) model of depression has high validity but has in the past been criticized for being difficult to replicate. However, a large number of recent publications have confirmed that CMS causes behavioural changes in rodents that parallel symptoms of depression. This review summarizes studies from over sixty independent research groups that have reported decreases in reactivity to rewards, and a variety of other depression-like behaviours, in rats or mice, following exposure to CMS. Together, these changes are referred to as a 'depressive' behavioural profile. Almost every study that has examined the effects of chronic antidepressant treatment in these procedures has reported that antidepressants were effective in reversing or preventing these 'depressive' behavioural changes. (The single exception is a study in which the duration of treatment was too brief to constitute an adequate trial.) There are also a handful of reports of CMS causing significant effects in the opposite direction, termed here an 'anomalous' behavioural profile. There are six neurobiological parameters that have been studied in both 'anhedonic' and 'anomalous' animals: psychostimulant and place-conditioning effects of dopamine agonists; dopamine D2 receptor number and message; inhibition of dopamine turnover by quinpirole, and beta-adrenergic receptor binding. On all six measures, CMS caused opposite effects in animals displaying 'depressive' and 'anomalous' profiles. Thus, there is overwhelming evidence that under appropriate experimental conditions, CMS can cause antidepressant-reversible depressive-like effects in rodents; however, the 'anomalous' profile that is occasionally reported appears to be a genuine phenomenon, and these two sets of behavioural effects appear to be associated with opposite patterns of neurobiological changes.

Alpha2- and beta-adrenoceptors differentially modulate GABAA- and GABAB-mediated inhibition of red nucleus neuronal firing

In mesencephalic red nucleus (RN), GABA-induced inhibition of neuronal firing is modulated by noradrenaline acting on alpha2- and beta-adrenoceptors. Since both GABAA and GABAB receptors are present in the rat RN, we have recorded the firing activity of RN neurons in vivo from anaesthetized rats to study how GABAA- and GABAB-mediated effects are modulated by either alpha2- or beta-adrenoceptor activation. Both the GABAA agonist isoguvacine and the GABAB agonist baclofen depressed the firing of RN neurons. During simultaneous application of clonidine, an alpha2-adrenoceptor agonist, half of the isoguvacine- and baclofen-mediated responses were modified: isoguvacine-mediated inhibition was enhanced by 97% without any change in effect duration, whereas baclofen responses were either increased or slightly reduced in the same number of cases. Application of isoprenaline, a beta-adrenoceptor agonist, increased isoguvacine effect in 66% of neurons without modifying effect duration; the amount of increase (43%) was significantly lower than that induced by clonidine. On the other hand, in the presence of isoprenaline, baclofen response was reduced in 72% of neurons with respect to both the amount (52%) and the duration (34%) of effect. Taken together, these results indicate that alpha2-adrenoceptors mainly enhance GABAA-induced inhibition and induce mixed effects on GABAB response; on the other side, beta-adrenoceptors exert an opposite modulation on GABA effects, respectively, enhancing and depressing GABAA- and GABAB-mediated responses.

Behavioural and biochemical studies of citalopram and WAY 100635 in rat chronic mild stress model

Reversal of chronic mild stress (CMS)-induced decrease of sucrose consumption has been studied in rats after 2, 7, 14, and 35 days treatment with imipramine, citalopram (both 10 mg/kg per day, i.p.), WAY 100635 (0.2 mg/kg sc, b.i.d.), and citalopram plus WAY 100635. Bmax, Kd, and functional status [cyclic AMP (cAMP) generation] of beta1-adrenoceptors were assessed in cortical tissue at the same time points. Citalopram reversed CMS-induced reduction of sucrose intake at an earlier time point than imipramine. WAY 100635 was not effective and did not potentiate the effect of citalopram. CMS produced increase of Bmax. Imipramine decreased Bmax in controls (Days 2, 7, 14, and 35) and normalised Bmax in stressed animals (Day 35). Citalopram, WAY 100635, and the combination increased Bmax in stressed animals and controls (Days 14 and 35). Inconsistent changes of Kd values and of cAMP responses to noradrenaline (NA) stimulation were observed. Thus stress- and drug-induced effects on beta1-adrenoceptors do not appear to be a common biochemical marker of antidepressant-like activity in the CMS model.

A proximal promoter domain containing a homeodomain-binding core motif interacts with multiple transcription factors, including HoxA5 and Phox2 proteins, and critically regulates cell type-specific transcription of the human norepinephrine transporter gene

Expression of the norepinephrine transporter (NET), which mediates the reuptake of norepinephrine into presynaptic nerve terminals, is restricted to noradrenergic (NA) neurons. We have demonstrated previously that the 9.0 kb upstream sequences and the first intron residing in the 5' untranslated area are critical for high-level and NA cell-specific transcription. Here, using transient transfection assays, we show that 4.0 kb of the 5' upstream sequences contains sufficient genetic information to drive reporter gene expression in an NA cell type-specific manner. Three functional domains appear to be potentially important for the regulation of human NET (hNET) gene transcription: an upstream enhancer region at -4.0 to -3.1 kb, a proximal domain at -133 to -75 bp, and a middle silencer region between these two domains. DNase I footprinting analysis of the proximal promoter region shows that a subdomain at -128 to -80 bp is protected in a cell-specific manner. We provide evidence that multiple protein factors interact with the proximal promoter domain to critically regulate the transcriptional activity of the hNET gene. In the middle of this proximal subdomain resides a homeodomain (HD)-binding core motif, which interacts with HD factors, including Phox2a and HoxA5, in an NA-specific manner. Cotransfection analyses suggest that HoxA5 and Phox2a may transactivate the hNET gene promoter. Together with previous studies indicating direct activation of dopamine beta-hydroxylase transcription by Phox2a/2b, the present results support a model whereby Phox2 proteins may coordinately regulate the phenotypic specification of NA neurons by activating both NA biosynthetic and reuptake genes.

Neurotransmitter-mediated control of neuronal firing in the red nucleus of the rat: reciprocal modulation between noradrenaline and GABA

The electrical activity of neurons from the red nucleus, a mesencephalic structure involved in motor control, is under the influence of several neurotransmitters released from afferent fibers and/or from local interneurons. We have investigated the combined effects of gamma-aminobutyric acid (GABA) and noradrenaline (NA), both present at high levels in the red nucleus, on the firing activity of single rubral neurons recorded extracellularly in vivo on anesthetized adult rats. NA inhibited the firing activity of a large part of rubral neurons and induced excitatory or biphasic inhibitory/excitatory effects in a smaller group of cells. Neuronal firing was also inhibited by GABA in all the cells studied. When the effect of GABA was tested during continuous applications of NA, the magnitude of GABA response was modified in 58% of the cells: the effect of GABA was potentiated by NA in half of the responding neurons and was decreased in the remaining half. NA-induced potentiation of GABA response was mimicked by the alpha(2)-adrenoceptor agonist clonidine and was abolished by the alpha(2)-adrenoceptor antagonist yohimbine. On the other side, the decrease of GABA response was reproduced by the beta-adrenoceptor agonist isoprenaline and was blocked by timolol, an antagonist of beta-adrenoceptors. Neuronal firing activity was reduced by nipecotic acid, an inhibitor of GABA reuptake mechanism, and was instead increased during application of the GABA(A) receptor antagonist bicuculline, suggesting that rubral neurons in vivo were under tonic control by endogenous GABA. Both the inhibitory and the excitatory effects of NA were reduced in the presence of nipecotic acid and were instead potentiated during application of bicuculline, suggesting that NA responses were modified by endogenous GABA. Taken together, our results indicate a reciprocal modulation between the effects of GABA and NA on neuronal firing activity in the red nucleus of the rat: GABA depresses the responsiveness of rubral neurons to NA, whereas NA is able either to potentiate or to decrease the effects of GABA by activation of alpha(2)- and beta-adrenoceptors, respectively. The functional significance of such interaction, as well as the possible implication in diseases affecting motor control, will be discussed.

Heterogeneous effects of rapid eye movement sleep deprivation on binding to alpha- and beta-adrenergic receptor subtypes in rat brain

Quantitative receptor autoradiography was used to map alterations in binding to alpha1-, alpha2-, beta1- and beta2-adrenergic receptors throughout the brain of rats deprived of rapid eye movement sleep for 96 h. Binding of [3H]prazosin to alpha1 sites, while not significantly different in any of 46 brain regions examined, showed a clear overall tendency towards decreased values after sleep deprivation. [3H]UK-14,314-labeled alpha2 binding sites were not significantly affected by sleep deprivation in any of 91 brain regions analysed, despite a trend towards increased values. In contrast, beta-adrenergic binding was significantly reduced throughout the brain. Binding to beta1 sites labeled by [125I]iodopindolol in the presence of ICI-11855 was significantly reduced in 13 of 69 brain areas examined; binding to beta2 sites labeled by [125I]iodopindolol in the presence of CGP-20712A was likewise reduced throughout the brain and significantly so in 25 of the 72 brain areas analysed. Rank ordering of the binding changes indicated that reductions in beta1 vs beta2 binding were maximal in different brain areas. This pattern of results may reflect a particular configuration of effects specifically associated with sleep loss stress. The results are consistent with evidence of persisting noradrenergic cell activity during sleep deprivation. The observed heterogeneity of effects suggests that not all norepinephrine receptors are equally affected by rapid eye movement sleep deprivation.

Preliminary comparison of behavioral and biochemical effects of chronic transcranial magnetic stimulation and electroconvulsive shock in the rat

To confirm the assumption that repetitive rapid-rate transcranial magnetic stimulation (TMS) induces the functional and structural changes analogous to those which are evoked during electroconvulsive shock (ECS), we compared now the effects of treatments with TMS and ECS on the behavioral responses in rats. We also tested the reactivity of the cyclic adenosine monophosphate (AMP) generating system in cerebral cortical slices. TMS similarly to ECS shortened the immobility time in the forced swimming test and produced a depression of responsiveness of the noradrenaline-stimulated cyclic AMP generating system, although the significance of the latter effect was borderline. In contrast to ECT, TMS produced no such immediate behavioral effects as analgesia and depression of the early phase of locomotor activity. The data suggest that TMS produces in rats some responses that are regarded as predictive for antidepressant activity, similar to those produced by ECS, but less adverse effects.

Homeostatic disruption and depression

A putative model is presented for the nature of depression. It relies upon changes in core and defense homeostatic activity, subjecting the person either to minor or major depression. The former corresponds with the interaction between assessed brain emotion and altered organic elements which disrupt homeostasis. This is in contrast to the latter, which corresponds with the disruption of several homeostatic systems. We suggest that the frequency and severity of depression may be regulated by biological markers that represent the homeostatic baseline of biochemical, biophysical and mutual data system appraisals. In this framework, the relationship of the coefficient to the baseline of the homeostatic threshold levels evaluates the intensity of depression. The type of depression depends on the properties of the negative feedback mechanism which are replaced either by abnormal (retarded depression), or by a positive feedback mechanism (agitated depression). Homeostasis eliminates depression.

Indices of brain beta-adrenergic receptor signal transduction in the learned helplessness animal model of depression

Both stress response and antidepressant drug action may be mediated by beta-adrenergic receptors (beta AR). Since learned helplessness is a stress-induced animal model of depression, beta AR are relevant to investigate in this model. To date, studies have measured changes in total receptor density (RT), but have not examined more detailed aspects of signal transduction mechanisms such as coupling of the receptor to GS protein. We have investigated brain beta AR coupling in the frontal cortex, hippocampus and hypothalamus of rats exposed to inescapable shock and then tested for learned helplessness, and in both tested and naive controls using [125I]-iodocyanopindolol (ICYP) as the ligand. Both antagonist-saturation and agonist-displacement experiments were conducted, and the specificity for the beta AR was optimized by excluding ICYP binding to 5HT1B receptors. The percentage receptor density in the high-conformational state (%RH) and the ratio of agonist (isoproterenol) dissociation constant from the receptor in the low-/high-conformational states (KL/KH) were used as indices of coupling to GS protein. No significant differences were found between rats developing learned helplessness and non-helpless rats after inescapable stress in any parameter measured in any brain region. In the frontal cortex, exposure to inescapable shock induced beta AR uncoupling from GS protein as suggested by a low KL/KH ratio both in helpless and non-helpless rats but not in either control group. In the hypothalamus, there were trends for higher RL, RT and KL/KH ratio in helpless rats and stressed controls compared to naive controls. These findings suggest that beta AR binding parameters in frontal cortex, hippocampus or hypothalamus did not differentiate between helpless and non-helpless rats. Changes in beta AR coupling observed in these brain regions may reflect effects of stress, which appeared to be region-specific, rather than stress-induced behavioral depression.

Biochemical and behaviour changes induced by acute stress in a chronic variate stress model of depression: the effect of amitriptyline

This paper examines the biochemical and behaviour changes induced by an acute stress (five 10-s, 1-mA foot-shocks) in three groups of rats: (1) never stressed, (2) subjected to chronic variate stress for 20 days, (3) subjected to the same chronic stress and treated with 5 mg/kg per day amitriptyline. After 15 min, acute stress led to a marked reduction in cortical beta-adrenoceptor and 5-HT2 receptor density, whereas the density of the 5-HT1A receptors was unchanged. Chronic stress also increased beta-adrenoceptor and 5-HT2 receptor density and had no effect on 5-HT1A. Acute stress diminished the density of beta-adrenoceptors in chronically stressed animals, but did not alter that of the two 5-HT populations. Amitriptyline alone reduced beta-adrenoceptor and 5-HT2 receptor densities only. Acute stress applied to animals treated with amitriptyline reduced 5-HT1A receptors, and caused a further beta-adrenoceptor decrease, but had no further effect on the 5-HT2 receptors. On behaviour, chronic stress diminished reactivity to the acute stress. This reduction was fully abolished by amitriptyline. An open-field study showed that acute stress reduced motor activity, increased latency times and diminished rearing in the controls, whereas chronic stress reduced motor activity only. No significant changes in behaviour were induced by the acute stress in animals subjected to chronic stress. The combination of chronic stress with amitriptyline was accompanied by a diminution of exploratory activity that persisted after the acute stress.(ABSTRACT TRUNCATED AT 250 WORDS)