The brain does not solely comprise nerve cells (neurons); roughly half of the organ is made up of so-called glial cells, which play an important role in brain development and are crucial for communication between neurons and the function of neural networks. Glial cells also include so-called star cells or “astrocytes”.
The element sodium, or rather positively charged sodium ions, are the most important electrolytes in the human body. These ions are crucial for many bodily functions. The main source thereof is table salt (NaCl), which is obtained from food.
Sodium ions are also involved in many processes in the brain, meaning that their concentration must be strictly regulated. In astrocytes, a low intracellular sodium concentration is important among other things for the regulation of neurotransmitters at the synapses – the junctions between nerve cells. It is also important for regulating the levels of other electrolytes. This enables astrocytes to ensure the functionality of nerve cells and regulate their excitability.
At the Institute of Neurobiology at HHU, the team led by Professor Dr Christine Rose has now developed a new technique, as part of a study funded by the Federal Ministry of Education and Research (the SynGluCross project), which can make the sodium content in the astrocytes and their fine processes directly visible in brain tissue for the first time. Together with researchers from Friedrich-Alexander-Universität Erlangen-Nuremberg, the University of Bonn and the University of South Florida in Tampa (USA), the neurobiologists in Düsseldorf set out to test the existing assumption that there is a similarly low concentration of sodium in all astrocytes and in all their sub-units to enable the astrocytes to perform their vital tasks reliably.
They actually established that this is not the case. Rather, they discovered differences – both between individual astrocytes and within various sub-units of these cells. Together with their colleagues from Erlangen-Nuremberg, they also demonstrated that certain transport molecules, which can be found in the cell membrane of various astrocytes in differing numbers and configurations, are responsible for these differences.
The cooperation partners from the USA implemented these findings in biophysical computer models and were able to replicate the experimental results in simulations. The findings obtained in isolated brain tissue in Düsseldorf were validated in animal models by the colleagues in Bonn.
“While textbooks assume a uniform resting concentration of sodium within and between astrocytes, the data show significant variability,” says Prof. Dr. Christian Henneberger of the Institute for Cellular Neurosciences I at the University of Bonn and the University Hospital Bonn. This is significant because many important processes, such as the reuptake of excitatory glutamate, are coupled to the sodium concentration in astrocytes. Their efficiency would then, unexpectedly, also fluctuate, adds the scientist, who is also a member of the Transdisciplinary Research Area “Life & Health” at the University of Bonn.
Dr Jan Meyer, lead author of the study: “We were also able to show that specialised functional sub-domains exist in astrocytes due to the different sodium concentrations. In each case, they react to the local needs of their neighbouring neural network.”