Major depressive disorder (MDD) is a chronic, recurrent, and severe psychiatric disorder with high mortality and medical comorbidities. Stress-related pathways have been directly involved in the pathophysiology and treatment of MDD. The present paper provides an overview on the stress system as a model to understand key pathophysiological paradigms in MDD. These mechanisms involve behavioral, cognitive, and systemic manifestations and are also associated with the mechanisms of action of effective antidepressants. Aspects such as depression subtypes, inflammation, insulin resistance, oxidative stress, and prothrombotic states in critical brain circuits and periphery are critically appraised. Finally, new strategies for approaching treatment-resistant major depression and potential adverse effects associated with this complex and intricate network are highlighted. The authors used PubMed as the database for this review. Each author extracted relevant data and assessed the methodological quality of each study.
Major depression and bipolar disorder are heterogeneous conditions in which there can be dysregulation of (1) the stress system response, (2) its capacity for counterregulation after danger has passed and (3) the phase in which damaging molecules generated by the stress response are effectively neutralized. The response to stress and depressed mood share common circuitries and mediators, and each sets into motion not only similar affective and cognitive changes, but also similar systemic manifestations. We focus here on two highly interrelated processes, parainflammation and endoplasmic reticulum (ER) stress, each of which can potentially interfere with all phases of a normal stress response in affective illness, including adaptive neuroplastic changes and the ability to generate neural stem cells. Parainflammation is an adaptive response of the innate immune system that occurs in the context of stressors to which we were not exposed during our early evolution, including overfeeding, underactivity, aging, artificial lighting and novel foodstuffs and drugs. We postulate that humans were not exposed through evolution to the current level of acute or chronic social stressors, and hence, that major depressive illness is associated with a parainflammatory state. ER stress refers to a complex program set into motion when the ER is challenged by the production or persistence of more proteins than it can effectively fold. If the ER response is overwhelmed, substantial amounts of calcium are released into the cytoplasm, leading to apoptosis. Parainflammation and ER stress generally occur simultaneously. We discuss three highly interrelated mediators that can effectively decrease parainflammation and ER stress, namely the central insulin, klotho and peroxisome proliferator-activated receptor-γ (PPAR-γ) systems and propose that these systems may represent conceptually novel therapeutic targets for the amelioration of the affective, cognitive and systemic manifestations of major depressive disorder.
Exercise represents a physical stress that challenges homeostasis. In response to this stressor, the autonomic nervous system and hypothalamus-pituitary-adrenal axis are known to react and participate in the maintenance of homeostasis and the development of physical fitness. This includes elevation of cortisol and catecholamines in plasma. However, physical conditioning is associated with a reduction in pituitary-adrenal activation in response to exercise. On the other hand, highly trained athletes exhibit chronic mild hypercortisolism at baseline that may be an adaptive change to chronic exercise. In addition the proinflammatory cytokine, IL-6 is also activated, probably via catecholamines. On the other hand, the stress of chronic exercise induces certain changes to the thyroid axis. Peripheral thyroid hormone metabolism suppression is observed, and the result is a hormonal status similar to that of euthyroid sick syndrome (ESS), with suppression of T3 and elevation of rT3 plasma levels. One mechanism proposed involves exercise-activated pathways participating in the pathogenesis of ESS. This is realized through norepinephrine's activation of NF-kappaB. Neuroendocrine response to exercise stress involves activation of NF-kappaB resulting in inactivation of T3-dependent 5'-deiodinase gene expression and enzyme activity. Thus, ESS is generated in the periphery. On the other hand, activation and nuclear translocation of NF-kappaB leads to increased transcription of proinflammatory genes responsible for the expression of proinflammatory cytokines such as TNF-alpha and IL-6. These cytokines could activate cortisol, which in turn inhibits NF-kappaB activation through IkappaB and finally shuts down this cycle.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.