Amazing stuff!
"Chronic pain and depression are mutually linked. Ding et al. explored the mechanistic link between chronic pain and depression in humans and rodents ... In patients and an animal model, early stages of chronic pain were found to be associated with an increase in hippocampal volume, whereas later stages were associated with decreased hippocampal volume.
These changes in rats were followed by the development of depressive-like behavior. Mechanistically, the authors showed that microglial activation leads to dysregulation of hippocampal neurogenesis (producing volumetric changes), shifts in hippocampal physiology, and onset depressive-like behaviors. The study provides valuable insights into the role of the hippocampus in the development of comorbid depression in the context of chronic pain."
From the abstract of the Perspective:
"Pain is classically defined as a sensory experience, yet it also engages emotional and cognitive processes. When pain becomes chronic—persists or recurs beyond 3 months from injury—it is frequently accompanied by disability and emotional dysregulation. Among these comorbidities, depression, anxiety, and sleep disorders are the most prevalent, yet the mechanistic relationships linking pain chronicity to these affective consequences remain poorly understood. On page 1235 and 1236 of this issue, Wei et al. (3) and Ding et al. (4), respectively, report evidence of pain-related brain regulation at two opposite ends of the temporal spectrum. Wei et al. describe a neural circuit that accounts for daily fluctuations in pain sensitivity. Meanwhile, Ding et al. identify cellular and structural brain changes that contribute to the emergence of depression in chronic pain. These observations offer new insight into how chronic pain becomes coupled to affective and cognitive comorbidities."
From the abstract:
"Structured Abstract
INTRODUCTION
Chronic pain is a leading risk factor for depression and anxiety, yet the brain mechanisms that convert persistent sensory distress into affective dysfunction remain unclear. Neuroimaging studies have implicated the hippocampus in both pain and mood regulation, but it is unknown whether hippocampal alterations precede, accompany, or result from the emergence of affective symptoms. Resolving this temporal and mechanistic relationship is essential for explaining individual vulnerability to depression in chronic pain and for identifying intervention points that can prevent this transition.
RATIONALE
We hypothesized that chronic pain induces a staged remodeling process, rather than a uniform degenerative change, within the hippocampus. Specifically, we proposed that the dentate gyrus serves as a critical gate where persistent nociceptive input is initially accommodated through adaptive plasticity but later diverted into maladaptive circuit destabilization by interactions between adult-born neurons and microglia.
RESULTS
Integrating longitudinal human neuroimaging data from the UK Biobank with rodent models of neuropathic pain, we identified a conserved biphasic trajectory of hippocampal remodeling. During early stages of chronic pain, hippocampal volume increased and hippocampal-dependent cognitive performance improved, consistent with an adaptive response. As pain persisted, this phase transitioned to hippocampal atrophy, cognitive decline, and the emergence of anxiety- and depression-like behaviors.
At the cellular level, early chronic pain selectively increased activity of newborn neurons within the dentate gyrus and triggered targeted recruitment and remodeling of microglia in the neurogenic niche. These cell-type–specific changes progressively amplified local circuit excitability and disrupted network balance, marking a transition from adaptive hippocampal plasticity to maladaptive circuit remodeling. Functionally, distinct modes of dentate gyrus modulation produced divergent outcomes: Suppressing newborn neuron activity alleviated affective symptoms but impaired cognition, whereas microglial modulation prevented anxiety- and depression-like behaviors while preserving cognitive function. Together, these findings identify microglia as a key regulator of the pain-to-depression transition.
CONCLUSION
By resolving distinct modes of dentate gyrus modulation, we show that microglia act as critical and therapeutically tractable regulators of the transition from chronic pain to affective disorders. Our findings reveal that this transition is governed not by hippocampal hyperactivity per se but rather by microglia-dependent remodeling that determines whether adaptive plasticity is sustained or diverted into maladaptive circuit states. Targeting microglial activation preserves hippocampal structure and cognitive function while preventing affective pathology, positioning microglia as a selective leverage point for interrupting the progression from chronic pain to mood disorders."
Pain across time (Perspective, no public access)
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