Aquaporin-4-deficient mice have increased extracellular space without tortuosity change

Abstract

Aquaporin-4 (AQP4) is the major water channel expressed at fluid-tissue barriers throughout the brain and plays a crucial role in cerebral water balance. To assess whether these channels influence brain extracellular space (ECS) under resting physiological conditions, we used the established real-time iontophoresis method with tetramethylammonium (TMA+) to measure three diffusion parameters: ECS volume fraction (α), tortuosity (λ), and TMA+ loss (k′). In vivo measurements were performed in the somatosensory cortex of AQP4-deficient (AQP4-/-) mice and wild-type controls with matched age. Mice lacking AQP4 showed a 28% increase in λ (0.23 ± 0.007 vs 0.18 ± 0.003) with no differences in λ (1.62 ± 0.04 vs 1.61 ± 0.02) and k′ (0.0045 ± 0.0001 vs 0.0031 ± 0.0009 s-1). Additional recordings in brain slices showed similarly elevated α in AQP4-/- mice, and no differences in λ and k′ between the two genotypes. This is the first direct comparison of ECS properties in adult mice lacking AQP4 water channels with wild-type animals and demonstrates a significant enlargement of the volume fraction but no difference in hindrance to TMA+ diffusion, expressed as tortuosity. These findings provide direct evidence for involvement of AQP4 in modulation of the ECS volume fraction and provide a basis for future modeling of water and ion transport in the CNS. Copyright © 2008 Society for Neuroscience.