Optical measurement of swelling and water transport in spinal cord slices from aquaporin null mice.

Water movement between cells and interstitium in spinal cord and brain occurs during neural signal transduction and in response to injuries such as ischemia and blunt trauma. At least two aquaporin-type water channels are expressed in spinal cord: AQP1 in afferent sensory nerve fibers in the superficial layers of the dorsal horn, and AQP4 in glial cells throughout gray matter. An imaging method was developed to map thickness changes in viable spinal cord and brain slices cut by a vibratome, and applied to measure osmotically induced water transport in spinal cord slices from wildtype and aquaporin knockout mice. Spinal cord slices (300 microm thickness) were mounted in a perfusion chamber with < 2 s exchange time, and transmitted light (565 nm) was imaged by a CCD camera. Changes in slice thickness were mapped from the amount of light passing through a thin ( approximately 100 microm) layer of perfusate bathing the slice, in which hemoglobin (6 mg/ml) was added to the perfusate as an inert absorbing chromophore. In response to osmotic challenges imposed by changing perfusate osmolality by 100 mOsm, transmitted light intensity changed reversibly with approximately mono-exponential kinetics whose initial rate depended upon position in the slice. In the superficial dorsal horn where AQP1 is strongly expressed, the rate of osmotic swelling was 7.0 +/- 1.3 microm/s in wildtype mice and 2.0 +/- 0.2 microm/s in AQP1 null mice; osmotic swelling was slower in deeper lamina of dorsal horn, and was decreased in AQP4 but not AQP1 null mice. These results establish a simple imaging method to map changes in water content of spinal cord slices, and provide evidence that aquaporins facilitate osmotic water transport in functionally relevant areas of the spinal cord.