Cell volume regulation in the nervous system

There is good evidence that the three main compartments of the brain, i.e. extracellular space, neurones and glial cells, change their volume during physiological and pathophysiological neuronal activity. However, there is strikingly little knowledge about the mechanisms underlying such alterations in cell volume. For this purpose, a better understanding of the electrophysiological behavior of the neurones and glial cells during volume changes is necessary. Examples are discussed for which changes in cell volume can be derived from the underlying changes in membrane permeabilities. Volume regulatory mechanisms in the brain have not been described under isotonic conditions. However, a rapid volume regulatory decrease is occurring in cultured glial cells during exposure to hypotonic solutions. In contrast, in these cells no volume regulatory increase was found during superfusion with hypertonic media. On the other hand, the entire brain is able to compensate chronic hypertonic perturbations within hours to days. Interestingly, not only ion fluxes induce cellular volume changes but, in turn, water movements can also influence ion fluxes in both neurones and glial cells. With respect to this it should be considered that volume regulatory membrane processes might not exclusively be activated by changes in transmembranal ion gradient, but also by changes of membrane surface shape. Future studies on cellular mechanisms of volume regulation in the brain should imply a combined use of recent techniques such as computerized video-imaging, radiotracer flux measurements and ion-sensitive microelectrodes in defined cell cultures. Optical monitoring and ion-sensitive microelectrodes should enable measurements of volume changes in identified cellular elements of intact nervous structures such as brain slices.