Chronic mild stress for modeling anhedonia

Facilitation of serotonin signaling by SSRIs is attenuated by social isolation

Hypofunction of the serotonergic system is often associated with major depression and obsessive compulsive disorder (OCD). Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed to treat these disorders, and require 3-6 weeks of chronic treatment before improvements in the symptoms are observed. SSRIs inhibit serotonin's transporter, and in doing so, increase extracellular serotonin concentrations. Thus, efficacy of SSRIs likely depends upon the brain's adaptive response to sustained increases in serotonin levels. Individual responsiveness to SSRI treatment may depend on a variety of factors that influence these changes, including ongoing stress. Social isolation is a passive, naturalistic form of chronic mild stress that can model depression in rodents. In this study, we examined how 20-day treatment with the SSRI citalopram (CIT) alters marble-burying (MB), open field behavior, and serotonin signaling in single- vs pair-housed animals. We used in vivo voltammetry to measure electrically evoked serotonin, comparing release rate, net overflow, and clearance. Pair-housed mice were significantly more responsive to CIT treatment, exhibiting reduced MB and facilitation of serotonin release that positively correlated with the frequency of electrical stimulation. These effects of CIT treatment were attenuated in single-housed mice. Notably, although CIT treatment enhanced serotonin release in pair-housed mice, it did not significantly alter uptake rate. In summary, we report that chronic SSRI treatment facilitates serotonin release in a frequency-dependent manner, and this effect is blocked by social isolation. These findings suggest that the efficacy of SSRIs in treating depression and OCD may depend on ongoing stressors during treatment.

Chronic stress reduces the number of GABAergic interneurons in the adult rat hippocampus, dorsal-ventral and region-specific differences

Major depressive disorder is a common and complex mental disorder with unknown etiology. GABAergic dysfunction is likely to contribute to the pathophysiology since disrupted GABAergic systems are well documented in depressed patients. Here we studied structural changes in the hippocampal GABAergic network using the chronic mild stress (CMS) model, as one of the best validated animal models for depression. Rats were subjected to 9 weeks of daily stress and behaviorally characterized using the sucrose consumption test into anhedonic and resilient animals based on their response to stress. Different subtypes of GABAergic interneurons were visualized by immunohistochemistry using antibodies for parvalbumin (PV), calretinin (CR), calbindin (CB), cholecystokinin (CCK), somatostatin (SOM), and neuropeptide Y (NPY). We used an unbiased quantification method to systematically count labeled cells in different subareas of the dorsal and ventral hippocampus. Chronic stress reduced the number of specific interneurons in distinct hippocampal subregions significantly. PV+ and CR+ neurons were reduced in all dorsal subareas, whereas in the ventral part only the CA1 was affected. Stress had the most pronounced effect on the NPY+ and SOM+ cells and reduced their number in almost all dorsal and ventral subareas. Stress had no effect on the CCK+ and CB+ interneurons. In most cases the effect of stress was irrespective to the behavioral phenotype. However, in a few specific areas the number of SOM+, NPY+, and CR+ neurons were significantly reduced in anhedonic animals compared to the resilient group. Overall, these data clearly demonstrate that chronic stress affects the structural integrity of specific GABAergic neuronal subpopulations and this should also affect the functioning of these hippocampal GABAergic networks.

Sucrose consumption test reveals pharmacoresistant depression-associated behavior in two mouse models of temporal lobe epilepsy

Among the comorbidities observed in epilepsy patients depression is the most frequent one. Likewise, depression by itself is accompanied by an increased risk to develop epilepsy. Both epilepsy and depression are characterized by a high incidence of pharmacoresistance, which might be based on overactivity of multidrug transporters like P-glycoprotein at the blood-brain barrier. Using genetically modified mice in preclinical epilepsy research is pivotal for investigating this bidirectional relationship. In the present study, we used the sucrose consumption test (SCT) in the pilocarpine and the intrahippocampal kainate mouse post-status epilepticus model to reveal anhedonic behavior, i.e. hyposensitivity to pleasure, as a key symptom of depression. Mice were repetitively investigated by SCT during early epilepsy and the chronic phase of the disease, during which response to antidepressant drug treatment was assessed. SCT revealed long-lasting anhedonia in both models. Anhedonia appeared to be pharmacoresistant, as neither chronic treatment with imipramine in the pilocarpine model nor chronic treatment with fluoxetine in the kainate model could annihilate the differences in sucrose consumption between control and epileptic mice. Moreover, knock-out of P-glycoprotein did not improve the treatment effect of fluoxetine. In conclusion, our findings show for the first time that the SCT is suited for detection of depression-like behavior in mouse models of temporal-lobe epilepsy. Both models might serve as tools to further investigate the neurobiology and pharmacology of epilepsy-associated pharmacoresistant depression.

Rodent models of depression: neurotrophic and neuroinflammatory biomarkers

Rodent models are an indispensable tool for studying etiology and progress of depression. Since interrelated systems of neurotrophic factors and cytokines comprise major regulatory mechanisms controlling normal brain plasticity, impairments of these systems form the basis for development of cerebral pathologies, including mental diseases. The present review focuses on the numerous experimental rodent models of depression induced by different stress factors (exteroceptive and interoceptive) during early life (including prenatal period) or adulthood, giving emphasis to the data on the changes of neurotrophic factors and neuroinflammatory indices in the brain. These parameters are closely related to behavioral depression-like symptoms and impairments of neuronal plasticity and are both gender- and genotype-dependent. Stress-related changes in expression of neurotrophins and cytokines in rodent brain are region-specific. Some contradictory data reported by different groups may be a consequence of differences of stress paradigms or their realization in different laboratories. Like all experimental models, stress-induced depression-like conditions are experimental simplification of clinical depression states; however, they are suitable for understanding the involvement of neurotrophic factors and cytokines in the pathogenesis of the disease—a goal unachievable in the clinical reality. These major regulatory systems may be important targets for therapeutic measures as well as for development of drugs for treatment of depression states.

Association between endothelial dysfunction and depression-like symptoms in chronic mild stress model of depression

OBJECTIVE

Cardiovascular diseases have high comorbidity with major depression. Endothelial dysfunction may explain the adverse cardiovascular outcome in depression; therefore, we analyzed it in vitro. In the chronic mild stress model, some rats develop depression-like symptoms (including "anhedonia"), whereas others are stress resilient.

METHODS

After 8 weeks of chronic mild stress, anhedonic rats reduced their sucrose intake by 55% (7%), whereas resilient rats did not. Acetylcholine-induced endothelium-dependent relaxation of norepinephrine-preconstricted mesenteric arteries was analyzed in nonstressed, anhedonic, and resilient rat groups.

RESULTS

Small resistance arteries from anhedonic rats were less sensitive to acetylcholine than those of the nonstressed and resilient groups (p = .029). Pathways of endothelium-dependent relaxation were altered in arteries from anhedonic rats. Nitric oxide (NO)-dependent relaxation and endothelial NO synthase expression were increased in arteries from anhedonic rats (0.235 [0.039] arbitrary units and 155.7% [8.15%]) compared with the nonstressed (0.135 [0.012] arbitrary units and 100.0% [8.08%]) and resilient (0.152 [0.018] arbitrary units and 108.1% [11.65%]) groups (p < .001 and p = .002, respectively). Inhibition of cyclooxygenase (COX) activity revealed increased COX-2-dependent relaxation in the anhedonic group. In contrast, endothelial NO synthase- and COX-independent relaxation to acetylcholine (endothelium-dependent hyperpolarization-like response) was reduced in anhedonic rats (p < .001). This was associated with decreased transcription of intermediate-conductance Ca-activated K channels.

CONCLUSIONS

Our findings demonstrate that depression-like symptoms are associated with reduced endothelium-dependent relaxation due to suppressed endothelium-dependent hyperpolarization-like relaxation despite up-regulation of the NO and COX-2-dependent pathways in rat mesenteric arteries. These changes could affect peripheral resistance and organ perfusion in major depression.

Differential effects of acute stress on anticipatory and consummatory phases of reward processing

Anhedonia is one of the core symptoms of depression and has been linked to blunted responses to rewarding stimuli in striatal regions. Stress, a key vulnerability factor for depression, has been shown to induce anhedonic behavior, including reduced reward responsiveness in both animals and humans, but the brain processes associated with these effects remain largely unknown in humans. Emerging evidence suggests that stress has dissociable effects on distinct components of reward processing, as it has been found to potentiate motivation/'wanting' during the anticipatory phase but reduce reward responsiveness/'liking' during the consummatory phase. To examine the impact of stress on reward processing, we used a monetary incentive delay (MID) task and an acute stress manipulation (negative performance feedback) in conjunction with functional magnetic resonance imaging (fMRI). Fifteen healthy participants performed the MID task under no-stress and stress conditions. We hypothesized that stress would have dissociable effects on the anticipatory and consummatory phases in reward-related brain regions. Specifically, we expected reduced striatal responsiveness during reward consumption (mirroring patterns previously observed in clinical depression) and increased striatal activation during reward anticipation consistent with non-human findings. Supporting our hypotheses, significant Phase (Anticipation/Consumption)×Stress (Stress/No-stress) interactions emerged in the putamen, nucleus accumbens, caudate and amygdala. Post hoc tests revealed that stress increased striatal and amygdalar activation during anticipation but decreased striatal activation during consumption. Importantly, stress-induced striatal blunting was similar to the profile observed in clinical depression under baseline (no-stress) conditions in prior studies. Given that stress is a pivotal vulnerability factor for depression, these results offer insight to better understand the etiology of this prevalent disorder.

Does stress elicit depression? Evidence from clinical and preclinical studies

Exposure to stressful situations may induce or deteriorate an already existing depression. Stress-related depression can be elicited at an adolescent/adult age but evidence also shows that early adverse experiences even at the fetal stage may predispose the offspring for later development of depression. The hypothalamus-pituitary-adrenal axis (HPA-axis) plays a key role in regulating the stress response and dysregulation in the system has been linked to depression both in humans and in animal models. This chapter critically reviews clinical and preclinical findings that may explain how stress can cause depression, including HPA-axis changes and alterations beyond the HPA-axis. As stress does not elicit depression in the majority of the population, this motivated research to focus on understanding the biology underlying resilient versus sensitive subjects. Animal models of depression have contributed to a deeper understanding of these mechanisms. Findings from these models will be presented.