Floating Rodents and Stress-Coping Neurobiology

Long-Term Impact of Early-Life Stress on Serotonin Connectivity

BACKGROUND

Chronic childhood stress is a prominent risk factor for developing affective disorders, yet mechanisms underlying this association remain unclear. Maintenance of optimal serotonin (5-HT) levels during early postnatal development is critical for the maturation of brain circuits. Understanding the long-lasting effects of early-life stress (ELS) on serotonin-modulated brain connectivity is crucial to develop treatments for affective disorders arising from childhood stress.

METHODS

Using a mouse model of chronic developmental stress, we determined the long-lasting consequences of ELS on 5-HT circuits and behavior in females and males. Using FosTRAP mice, we cross-correlated regional c-Fos density to determine brain-wide functional connectivity of the raphe nucleus. We next performed in vivo fiber photometry to establish ELS-induced deficits in 5-HT dynamics and optogenetics to stimulate 5-HT release to improve behavior.

RESULTS

Adult female and male mice exposed to ELS showed heightened anxiety-like behavior. ELS further enhanced susceptibility to acute stress by disrupting the brain-wide functional connectivity of the raphe nucleus and the activity of 5-HT neuron population, in conjunction with increased orbitofrontal cortex (OFC) activity and disrupted 5-HT release in medial OFC. Optogenetic stimulation of 5-HT terminals in the medial OFC elicited an anxiolytic effect in ELS mice in a sex-dependent manner.

CONCLUSIONS

These findings suggest a significant disruption in 5-HT-modulated brain connectivity in response to ELS, with implications for sex-dependent vulnerability. The anxiolytic effect of the raphe-medial OFC circuit stimulation has potential implications for developing targeted stimulation-based treatments for affective disorders that arise from early life adversities.

Diet enriched with high-phenolic cocoa potentiates hippocampal brain-derived neurotrophic factor expression and neurogenesis in healthy adult micewith subtle effects on memory

Cocoa is widely known for its health benefits, but its neurocognitive impact remains underexplored. This preclinical study aimed to investigate the effects of cocoa and cocoa polyphenols on hippocampal neuroplasticity, cognitive function and emotional behavior. Seventy young-adult C57BL/6JRj male and female mice were fed either a standard diet (CTR) or a diet enriched with 10% high-phenolic content cocoa (HPC) or low-phenolic content cocoa (LPC) for at least four weeks. In a first experiment, behavioral tests assessing exploratory behavior, emotional responses and hippocampal-dependent memory were conducted four weeks into the diet, followed by animal sacrifice a week later. Adult hippocampal neurogenesis and brain-derived neurotrophic factor (BDNF) expression in the hippocampus and prefrontal cortex were evaluated using immunohistochemistry and western blot. In a different experiment, hippocampal synaptic response, long-term potentiation and presynaptic-dependent short-term plasticity were studied by electrophysiology. Cocoa-enriched diets had minimal effects on exploratory activity and anxiety-like behavior, except for reduced locomotion in the LPC group. Only the HPC diet enhanced object recognition memory, while place recognition memory and spatial navigation remained unaffected. The HPC diet also increased adult hippocampal neurogenesis, boosting the proliferation, survival and number of young adult-born neurons. However, both cocoa-enriched diets increased immobility in the forced swimming test and hippocampal BDNF expression. Hippocampal electrophysiology revealed no alterations in neuroplasticity among diets. The results were mostly unaffected by sex. Overall, the HPC diet demonstrated greater potential regarding cognitive and neuroplastic benefits, suggesting a key role of cocoa flavanols in dietary interventions aimed at enhancing brain health.

PGC-1α in the hippocampus mediates depressive-like and stress-coping behaviours and regulates excitatory synapses in the dentate gyrus in mice

Decreased hippocampal synaptic plasticity is an important pathological change in stress-related mood disorders, including major depressive disorder. However, the underlying mechanism is unclear. PGC-1α, a transcriptional coactivator, is a key factor in synaptic plasticity. We investigated the relationships between changes in hippocampal PGC-1α expression and depressive-like and stress-coping behaviours, and whether they are related to hippocampal synapses. Adeno-associated virus was used to alter hippocampal PGC-1α expression in male C57BL/6 mice. The sucrose preference test and forced swimming test were used to assess their depressive-like and stress-coping behaviours, respectively. Immunohistochemistry and stereology were used to calculate the total number of excitatory synapses in each hippocampal subregion (the cornu ammonis (CA) 1, CA3, and dentate gyrus). Immunofluorescence was used to visualize the changes in dendritic structure. Western blotting was used to detect the expression of hippocampal PGC-1α and mitochondrial-associated proteins, such as UCP2, NRF1 and mtTFAs. Our results showed that mice with downregulated PGC-1α expression in the hippocampus exhibited depressive-like and passive stress-coping behaviours, while mice with upregulated PGC-1α in the hippocampus exhibited increased stress-coping behaviours. Moreover, the downregulation of hippocampal PGC-1α expression resulted in a decrease in the number of excitatory synapses in the DG and in the protein expression of UCP2 in the hippocampus. Alternatively, upregulation of hippocampal PGC-1α yielded the opposite results. This suggests that hippocampal PGC-1α is involved in regulating depressive-like and stress-coping behaviours and modulating the number of excitatory synapses in the DG. This provides new insight for the development of antidepressants.

Polyphenols as novel interventions for depression: exploring the efficacy, mechanisms of action, and implications for future research

Numerous animal and human studies have assessed the relationship between polyphenols and outcomes related to depression. However, no comprehensive synthesis of the main findings has been conducted. The aim of this manuscript was to systematically review the available evidence from animal and human studies on the association and the effects of dietary polyphenols on depression and provide recommendations for future research. We based our review on 163 preclinical animal, 16 observational and 44 intervention articles assessing the relationship between polyphenols and outcomes related to depression. Most animal studies demonstrated that exposure to polyphenols alleviated behaviours reported to be associated with depression. However, human studies are less clear, with some studies reporting and inverse relationship between the intake of some polyphenols, and polyphenol rich foods and depression risk and symptoms, while others reporting no association or effect. Hence, while there has been extensive research conducted in animals and there is some supporting evidence in humans, further human studies are required, particularly in younger and clinical populations.

Abnormal compositions of gut microbiota and metabolites are associated with susceptibility versus resilience in rats to inescapable electric stress

BACKGROUND

Increasing evidence suggests the role of gut microbiota in resilience versus vulnerability after stress. However, the role of gut microbiota and microbiome-derived metabolites in resilience versus susceptibility in rodents exposed to stress remains unclear.

METHODS

Adult male rats were exposed to inescapable electric stress under the learned helplessness (LH) paradigm. The composition of gut microbiota and metabolites in the brain and blood from control (no stress) rats, LH resilient rats, and LH susceptible rats were examined.

RESULTS

At the genus level, the relative abundances of Asaccharobacter, Eisenbergiella, and Klebsiella in LH susceptible rats were significantly higher than that of LH resilient rats. At the species level, the relative abundances of several microbiome were significantly altered between LH susceptible rats and LH resilient rats. Furthermore, there were several metabolites in the brain and blood altered between LH susceptible rats and LH resilient rats. A network analysis showed correlations between the abundance of several microbiome and metabolites in the brain (or blood).

LIMITATIONS

Detailed roles of microbiome and metabolites are unclear.

CONCLUSIONS

These findings suggest that abnormal compositions of the gut microbiota and metabolites might contribute to susceptibility versus resilience in rats subjected to inescapable electric foot shock.

Fermented Perilla frutescens Ameliorates Depression-like Behavior in Sleep-Deprivation-Induced Stress Model

Excessive stress plays a critical role in the pathogenesis of mood disorders such as depression. Fermented natural products have recently attracted attention because of their health benefits. We evaluated the antidepressant-like efficacy of fermented Perilla frutescens (FPF), and its underlying mechanisms, in sleep deprivation (SD)-induced stress mice. SD-stressed mice revealed a remarkable increase in the immobility time in both forced swimming test and tail suspension test; this increase was ameliorated by treatment with FPF at doses of 100 and 150 mg/kg. FPF treatment also reduced the level of stress hormones such as corticosterone and adrenocorticotropic hormone. Additionally, FPF increased the levels of serotonin and dopamine which were significantly decreased in the brain tissues of SD-stressed mice. The increased expression of proinflammatory cytokines, such as TNF-α and IL1β, and the decreased expression of brain-derived neurotrophic factor (BDNF) in the stressed mice were significantly reversed by FPF treatment. Furthermore, FPF also increased phosphorylation of tropomyosin receptor kinase B (TrkB), extracellular regulated protein kinase (ERK), and cAMP response element binding protein (CREB). Among the six components isolated from FPF, protocatechuic acid and luteolin-7-O-glucuronide exhibited significant antidepressant-like effects, suggesting that they are major active components. These findings suggest that FPF has therapeutic potential for SD-induced stress, by correcting dysfunction of hypothalamic-pituitary-adrenal axis and modulating the BDNF/TrkB/ERK/CREB signaling pathway.

Luteolin-7-O-Glucuronide Improves Depression-like and Stress Coping Behaviors in Sleep Deprivation Stress Model by Activation of the BDNF Signaling

Stress exposure is a major risk factor for mental disorders such as depression. Because of the limitations of classical antidepressants such as side effects, low efficacy, and difficulty in long-term use, new natural medicines and bioactive molecules from plants with greater safety and efficacy have recently attracted attention. Luteolin-7-O-glucuronide (L7Gn), a bioactive molecule present in Perilla frutescens, is known to alleviate severe inflammatory responses and oxidative stress in macrophages. However, its antistress and antidepressant effects have not been elucidated. The present study aims to explore the antidepressant the effect of L7Gn on stress-induced behaviors and the underlying mechanism in a mouse sleep deprivation (SD) model. L7Gn treatment improved depression-like and stress coping behaviors induced by SD stress, as confirmed by the tail suspension test and forced swimming test. Furthermore, L7Gn treatment reduced the blood corticosterone and hippocampal proinflammatory cytokine levels which were increased by SD stress, and L7Gn also increased the mRNA and protein levels of hippocampal brain-derived neurotrophic factor (BDNF) which were reduced by SD stress. Additionally, treatment with L7Gn resulted in increases in the phosphorylation of tropomyosin-related kinase B (TrkB), extracellular signal-regulated kinase (ERK), and cAMP response element-binding protein (CREB), which are downstream molecules of BDNF signaling. These findings suggest that L7Gn have therapeutic potential for SD-induced stress, via activating the BDNF signaling.

MTEP, a Selective mGluR5 Antagonist, Had a Neuroprotective Effect but Did Not Prevent the Development of Spontaneous Recurrent Seizures and Behavioral Comorbidities in the Rat Lithium-Pilocarpine Model of Epilepsy

Metabotropic glutamate receptors (mGluRs) are expressed predominantly on neurons and glial cells and are involved in the modulation of a wide range of signal transduction cascades. Therefore, different subtypes of mGluRs are considered a promising target for the treatment of various brain diseases. Previous studies have demonstrated the seizure-induced upregulation of mGluR5; however, its functional significance is still unclear. In the present study, we aimed to clarify the effect of treatment with the selective mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) on epileptogenesis and behavioral impairments in rats using the lithium–pilocarpine model. We found that the administration of MTEP during the latent phase of the model did not improve survival, prevent the development of epilepsy, or attenuate its manifestations in rats. However, MTEP treatment completely prevented neuronal loss and partially attenuated astrogliosis in the hippocampus. An increase in excitatory amino acid transporter 2 expression, which has been detected in treated rats, may prevent excitotoxicity and be a potential mechanism of neuroprotection. We also found that MTEP administration did not prevent the behavioral comorbidities such as depressive-like behavior, motor hyperactivity, reduction of exploratory behavior, and cognitive impairments typical in the lithium–pilocarpine model. Thus, despite the distinct neuroprotective effect, the MTEP treatment was ineffective in preventing epilepsy.