Neurons Generate Synchronized Rhythmic Waves in Brain’s Interstitial Fluid To Help Clear Metabolic Waste

A new investigation provides evidence that neurons in the brain act as master organizers for clearing the brain of metabolic waste and that they do so by synchronizing their actions to create large rhythmic waves in the interstitial fluid (ISF) during sleep. The study, recently published in Nature, was conducted by researchers from Washington University in St. Louis and partially funded by the National Center for Complementary and Integrative Health.

Neuronal activity in the brain creates metabolic waste products. The accumulation of metabolic waste is a leading cause of many neurological disorders. Although the glymphatic system has been identified as the brain’s pathway for clearing waste, there is only limited knowledge about how the brain cleans itself.

In the glymphatic system, fresh cerebrospinal fluid (CSF) pulsates along arteries to enter the brain, infuses the brain tissue through channels in astrocyte cells, and then flushes out metabolic waste products along venous pathways. There is substantial knowledge about the fluid flow in the spaces along the arteries and veins in the glymphatic system. But much less is known about the fluid dynamics occurring within the brain tissue, called the parenchyma, which includes neurons, glial cells, and the surrounding interstitial space.

In this study, researchers conducted multiple experiments on mice to investigate the influx and efflux of the glymphatic system and determine the effect of neurons on CSF-to-ISF perfusion within the parenchyma. In many of the experiments, researchers placed electrodes outside neurons in the interstitial space to perform in vivo extracellular electrophysiological recordings of neuron action potentials and ionic dynamics. At the same time, the researchers monitored the global brain state by electroencephalography and electromyography.

The researchers initially conducted experiments under ketamine anesthesia, which enhances the glymphatic system to a level seen in natural sleep. Findings showed that ketamine anesthesia suppressed neuronal activity but that the combined coordination of action potentials from a network of neurons resulted in the generation of large-amplitude, rhythmic ionic waves in the ISF. The researchers suggested that this strong wave energy might facilitate the movement of small molecules and peptides within the interstitial space.

The researchers then conducted similar experiments during natural sleep. Findings showed that action potential rates of neurons increased during sleep, which differed from the ketamine anesthesia condition, but there was again a coordinated action of neuronal networks that generated strong wave energy in the ISF.

When the researchers used chemogenetics to inhibit the neurons, the ionic waves in the interstitial space flattened, and both CSF infiltration into the brain parenchyma and clearance of molecules from the parenchyma were largely prevented. The reduction in CSF perfusion occurred in both wakefulness and sleep but was much more pronounced during sleep, suggesting that rhythmic neuronal activity during sleep is indeed vital for brain CSF-to-ISF perfusion and clearance of waste. 

When the researchers used transcranial optogenetics to generate synthesized waves in the ISF, the CSF-to-ISF perfusion increased. The researchers noted that employing this type of noninvasive optogenetic stimulation might offer a new possibility for enhancing brain clearance in the future.

Other future directions, according to the researchers, will be to identify the intricate interactions between neurons and the various other components of the glymphatic system and investigate how neural circuits are shaped by the demand of waste removal during sleep.