Brain volume regulation: osmolytes and aquaporin perspectives

Cerebral water control is critical to maintain neuronal excitability, and to prevent injuries derived from brain swelling or shrinkage. The influence of aquaporins (AQPs) in the balance of water distribution between intracranial compartments is getting much experimental support. The importance of AQPs in fluid clearance during vasogenic brain edema seems well established but their role in cytotoxic swelling and in brain cell shrinkage is not known in detail. The main AQPs function as water channels anticipates their influence on cell volume changes as well as on the mechanisms of volume recovery, which include notably the osmolyte translocation across the cell membrane. Osmolyte fluxes permit the reestablishment of an osmotic balance and volume recovery in anisosmotic-elicited cell volume changes, but are also causal factors per se of brain cell swelling or shrinkage in pathological situations. This review aims to inform on the so far described functional interactions between AQPs and osmolyte fluxes and their volume-sensitive pathways. It also points to the coincidence of AQPs and activation of osmolyte fluxes in physiological and pathological conditions and to the importance of finding possible functional links between these two events, thus enlarging the possibilities via AQP manipulations, to prevent the adverse consequences of cell volume changes in brain.