Comparative Proteomic Characterization of Ventral Hippocampus in Susceptible and Resilient Rats Subjected to Chronic Unpredictable Stress

Hippocampal sharp wave ripples underlie stress susceptibility in male mice

The ventral hippocampus (vHC) is a core brain region for emotional memory. Here, we examined how the vHC regulates stress susceptibility from the level of gene expression to neuronal population dynamics in male mice. Transcriptome analysis of samples from stress-naïve mice revealed that intrinsic calbindin (Calb1) expression in the vHC is associated with susceptibility to social defeat stress. Mice with Calb1 gene knockdown in the vHC exhibited increased stress resilience and failed to show the increase in the poststress ventral hippocampal sharp wave ripple (SWR) rate. Poststress vHC SWRs triggered synchronous reactivation of stress memory-encoding neuronal ensembles and facilitated information transfer to the amygdala. Suppression of poststress vHC SWRs by real-time feedback stimulation or walking prevented social behavior deficits. Taken together, our results demonstrate that internal reactivation of memories of negative stressful episodes supported by ventral hippocampal SWRs serves as a crucial neurophysiological substrate for determining stress susceptibility.

Ventral hippocampal CA1 modulates pain behaviors in mice with peripheral inflammation

Chronic pain is one of the most significant medical problems throughout the world. Recent evidence has confirmed the hippocampus as an active modulator of pain chronicity, but the underlying mechanisms remain unclear. Using in vivo electrophysiology, we identify a neural ensemble in the ventral hippocampal CA1 (vCA1) that shows inhibitory responses to noxious but not innocuous stimuli. Following peripheral inflammation, this ensemble becomes responsive to innocuous stimuli, representing hypersensitivity. Mimicking the inhibition of vCA1 neurons using chemogenetics induces chronic pain-like behaviors in naive mice, whereas activating vCA1 neurons in mice with peripheral inflammation results in a reduction of pain-related behaviors. Pathway-specific manipulation of vCA1 projections to basolateral amygdala (BLA) and infralimbic cortex (IL) shows that these pathways are differentially involved in pain modulation at different temporal stages of chronic inflammatory pain. These results confirm a crucial role of the vCA1 and its circuits in modulating the development of chronic pain.

Fraxetin attenuates disrupted behavioral and central neurochemical activity in a model of chronic unpredictable stress

Purpose: Chronic unpredictable stress (CUS) induces long-term neuronal and synaptic plasticity with a neurohormonal disbalance leading to the development of co-existing anxiety, depression, and cognitive decline. The side effects and delayed onset of current clinically used antidepressants has prompted a quest for antidepressants with minimum drawbacks. Fraxetin is a natural coumarin derivative with documented antioxidant and neuroprotective activity though its effects on stress are unknown. This study therefore aimed to investigate any possible acute effect of fraxetin in behavioral tests including a CUS paradigm in correlation with brain regional neurochemical changes. Methods: Mice were subjected to a series of mild stressors for 14 days to induce CUS. Furthermore, behavioral performance in the open field test, forced swim test (FST), Y-maze and elevated plus-maze were evaluated. Postmortem frontal cortical, hippocampal and striatal tissues were analyzed via high-performance liquid chromatography (HPLC) for neurochemical changes. Result: Acute administration of fraxetin (20-60 mg/kg, orally) decreased depression-like behavior in the FST and behavioral anxiety in both the open field test and elevated plus-maze. Memory deficits induced during the CUS paradigm were markedly improved as reflected by enhanced Y maze performance. Concurrent biochemical and neurochemical analyses revealed that only the two higher fraxetin doses decreased elevated serum corticosterone levels while diminished serotonin levels in the frontal cortex, striatum and hippocampus were reversed, though noradrenaline was only raised in the striatum. Concomitantly, dopamine levels were restored by fraxetin at the highest dose exclusively in the frontal cortex. Conclusion: Acute treatment with fraxetin attenuated CUS-induced behavioral deficits, ameliorated the increased corticosterone level and restored altered regional neurotransmitter levels and this may indicate a potential application of fraxetin in the management of anxiety and depression modeled by CUS. However, further studies are warranted regarding the chronic effects of fraxetin behaviorally and neurochemically.

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.