Circadian Mechanisms in Brain Fluid Biology

Biological Fluid Flows: Signaling Mediums for Circadian Timing

While there is extensive literature on both the neuronal circuitry of rhythms and the intracellular molecular clock, there is a large component of signaling that has been understudied: interstitial fluid (ISF)-fluid that surrounds the cells in the extracellular space of tissue. In this review, we highlight evidence in the circadian literature supporting ISF signaling as key to circadian synchronization and entrainment and propose new mechanisms of how fluid movement between the brain and periphery may act as zeitgebers by examining the main ISF pathways of the body, focusing on circadian regulation of the glymphatic and lymphatic systems. We identify key pieces of circadian research that point to ISF as an important timing medium, expand on the basics of cerebrospinal fluid (CSF) and ISF production, and outline the basic structure and function of the glymphatic and lymphatic systems.

Melatonin Regulates Glymphatic Function to Affect Cognitive Deficits, Behavioral Issues, and Blood-Brain Barrier Damage in Mice After Intracerebral Hemorrhage: Potential Links to Circadian Rhythms

BACKGROUND

Intracerebral hemorrhage (ICH) is a life-threatening cerebrovascular disorder with no specific pharmacological treatment. ICH causes significant behavioral deficits and cognitive impairments. Recent research suggests that circadian rhythm regulation could be a promising therapeutic strategy for ICH. Melatonin has been shown to alleviate glymphatic system (GS) dysfunction by regulating circadian rhythms, thereby improving depressive-like behaviors and postoperative sleep disorders in mice. However, its application in ICH treatment and specific mechanisms are not well understood.

METHODS

ICH models were created in 8-to-10-week-old mice using collagenase injection. Circadian rhythm modulation was tested with melatonin and luzindole. Behavioral and cognitive impairments were assessed with the modified neurological severity score, corner test, and novel object recognition test. Brain water content was measured by the dry/wet weight method, and cerebral perfusion was assessed by cerebral blood flow measurements. GS function was evaluated using RITC-dextran and Evans blue assays. Immunofluorescence and western blotting were used to analyze GS function and BBB permeability.

RESULTS

Melatonin restored GS transport after ICH, promoting hematoma and edema absorption, reducing BBB damage, and improving cognitive and behavioral outcomes. However, luzindole partially blocked these benefits and reversed the neuroprotective effects.

CONCLUSION

Melatonin and luzindole treatment affect GS function, BBB permeability, and cognitive-behavioral outcomes in mice with ICH. The underlying mechanism may involve the regulation of circadian rhythms.

Influencing factors of glymphatic system during perioperative period

The glymphatic system is a functional cerebrospinal fluid circulatory system that uses peri-arterial space for inflow of cerebrospinal fluid and peri-venous space for efflux of cerebrospinal fluid from brain parenchyma. This brain-wide fluid transport pathway facilitates the exchange between cerebrospinal fluid and interstitial fluid and clears metabolic waste from the metabolically active brain. Multiple lines of work show that the glymphatic system is crucial to normal brain functions, and the dysfunction of the glymphatic system is closely associated with various neurological disorders, including aging, neurodegeneration, and acute brain injury. Currently, it is common to explore the functional and molecular mechanisms of the glymphatic system based on animal models. The function of glymphatic system during perioperative period is affected by many factors such as physiological, pathological, anesthetic and operative methods. To provide a reference for the interpretation of the results of glymphatic system studies during perioperative period, this article comprehensively reviews the physiological and pathological factors that interfere with the function of the glymphatic system during perioperative period, investigates the effects of anesthetic drugs on glymphatic system function and the potential underlying mechanisms, describes operative methods that interfere with the function of the glymphatic system, and potential intervention strategies based on the glymphatic system. Future, these variables should be taken into account as critical covariates in the design of functional studies on the glymphatic system.