👤 Milind J Umekar

Chronic stress is increasingly acknowledged as a pivotal precipitating factor in the pathogenesis of neuropsychiatric and neurodegenerative disorders, notably including depression and Alzheimer's disease (AD). Astrocytes, which constitute the predominant population of glial cells involved in the maintenance of synaptic homeostasis, the recycling of neurotransmitters, and the provision of metabolic support, display a pronounced susceptibility to sustained exposure to stress. The deleterious effects of astrocytic dysfunction instigate a series of neuroinflammatory and synaptic modifications that undermine both cognitive and emotional resilience. This review articulates the mechanistic interactions between stress-induced astrocyte dysfunction, neuroinflammatory signaling, and compromised neuroplasticity, underscoring the converging pathways that are implicated in both depression and AD. A thorough synthesis of the literature from 2020 to 2025 was conducted utilizing databases such as PubMed, Scopus, and Web of Science, with an emphasis on molecular, in vitro, in vivo, and translational studies that examine the modulation of astrocytic function under conditions of chronic stress and its pertinence to depression and AD. The chronic activation of the hypothalamic-pituitary-adrenal (HPA) axis precipitates morphological alterations, diminished expression of glutamate transporters (GLT-1/EAAT2), disrupted brain-derived neurotrophic factor (BDNF) signaling, and an augmented release of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) from astrocytes. These biochemical alterations exacerbate excitotoxicity, disturb monoaminergic and glutamatergic neurotransmission, and hasten synaptic degeneration. In the context of depression, this phenomenon is manifested as impaired mood regulation and a decline in neurogenesis. In AD, it synergistically interacts with amyloid-beta and tau pathologies to facilitate progressive cognitive impairment. Both conditions exhibit a common feature of diminished neurosignaling plasticity, which limits the brain's capacity for adaptation and repair. Astrocyte dysfunction constitutes a central mechanistic nexus wherein chronic stress, neuroinflammation, and synaptic pathology intersect to promote the progression of depression and AD. The targeting of astrocytic health via the modulation of reactive astrocyte phenotypes, the restoration of glutamate homeostasis, and the enhancement of neurotrophic signaling emerges as a promising therapeutic avenue for alleviating stress-related neurodegeneration and mood disorders. Show less

Neuroplasticity, the brain's capacity to adapt and reorganize in response to experiences and environmental changes, is fundamental to cognitive aging. As individuals age, cognitive functions such as memory, processing speed, and executive function commonly decline, driven largely by changes in neuroplasticity mechanisms like synaptic plasticity, neurogenesis, and functional reorganization. Synaptic plasticity is a well-established mechanism supporting learning and memory across the lifespan, whereas adult neurogenesis, robustly demonstrated in rodents, remains highly limited and controversial in the adult and aged human brain, with evidence largely restricted to rare post-mortem observations and injury-associated conditions. Functional reorganization allows the brain to adapt to structural changes, helping to preserve cognitive function despite age-related decline. Several factors, including oxidative stress, neuroinflammation, and hormonal shifts, exacerbate the decline in neuroplasticity, accelerating cognitive deterioration. Various interventions, including cognitive training, physical exercise, and pharmacological approaches, have demonstrated the potential to promote neuroplasticity and support cognitive health in aging populations. However, one of the major challenges is tailoring these interventions to the unique needs of individuals, as well as identifying novel therapeutic targets for intervention. To effectively address the cognitive decline associated with aging, future research should focus on developing personalized strategies and innovative techniques to enhance or modulate specific neuroplasticity-related processes under defined conditions in the aging brain. These advancements may provide better tools for delaying, mitigating, or even reversing age-related cognitive decline, improving quality of life for older individuals. Show less

Extended periods of microgravity during orbital flights can impair astronauts' cognitive abilities, including learning and memory, posing a persistent health concern in the field of aerospace medicine. The study examined the pharmacological effects of agmatine and its influence on simulated neurobehavioral changes in rats under microgravity conditions. Rats were exposed to simulated microgravity (SMG) conditions using the hindlimb unloading (HU) model for 28 days and evaluated for behavioural alterations using the open field test, elevated plus maze, and forced swim test, and cognitive deficits using the novel object recognition test and Morris water maze. Further, brain agmatine levels, neurochemical and structural alterations in the hippocampus, and prefrontal cortex were examined. Chronic agmatine treatment dose-dependently (40 and 80mg/kg) and its endogenous modulation by l-arginine, and aminoguanidine prevented behavioral and cognitive deficits by improving exploratory behaviour, reducing anxiety-depressive-like symptoms, and enhancing cognitive performance. Our findings reported a significant reduction in agmatine levels in the hippocampus and prefrontal cortex in SMG conditions. Agmatine administration and its modulation normalized neurotransmitter imbalances, especially by restoring the reduced levels of gamma-aminobutyric acid, dopamine, and serotonin, along with a reduction of elevated levels of glutamate in SMG conditions. Moreover, agmatine decreased reactive oxygen species production, enhanced hippocampal antioxidant enzyme activities, suppressed pro-inflammatory cytokines (TNF-α, IL-6), and improved IL-10 and brain-derived neurotrophic factor levels in HU rats. Moreover, agmatine and its endogenous modulation preserved neuronal cells of the hippocampus and prefrontal cortex. In conclusion, the present study suggests that agmatine administration and modulation of endogenous agmatine levels effectively mitigate SMG-induced neurological dysregulation through neuroprotection and neuromodulation. Understanding the neurobiological mechanisms underlying these effects opens up new possibilities for creating novel interventions targeting agmatinergic signaling in spaceflight conditions and associated complications. Show less