In vivo measurement of brain extracellular space diffusion by cortical surface photobleaching

Abstract

Molecular diffusion in the brain extracellular space (ECS) is an important determinant of neural function. We developed a brain surface photobleaching method to measure the diffusion of fluorescently labeled macromolecules in the ECS of the cerebral cortex. The ECS in mouse brain was labeled by exposure of the intact dura to fluorescein-dextrans (M(r) 4, 70, and 500 kDa). Fluorescein-dextran diffusion, detected by fluorescence recovery after laser-induced cortical photobleaching using confocal optics, was slowed approximately threefold in the brain ECS relative to solution. Cytotoxic brain edema (produced by water intoxication) or seizure activity (produced by convulsants) slowed diffusion by >10-fold and created dead-space microdomains in which free diffusion was prevented. The hindrance to diffusion was greater for the larger fluorescein-dextrans. Interestingly, slowed ECS diffusion preceded electroencephalographic seizure activity. In contrast to the slowed diffusion produced by brain edema and seizure activity, diffusion in the ECS was faster in mice lacking aquaporin-4 (AQP4), an astroglial water channel that facilitates fluid movement between cells and the ECS. Our results establish a minimally invasive method to quantify diffusion in the brain ECS in vivo, revealing stimulus-induced changes in molecular diffusion in the ECS with unprecedented spatial and temporal resolution. The in vivo mouse data provide evidence for: (1) dead-space ECS microdomains after brain swelling; (2) slowed molecular diffusion in the ECS as an early predictor of impending seizure activity; and (3) a novel role for AQP4 as a regulator of brain ECS.

Figure 1.

Invivoloading of brain ECS by fluorescein-dextrans. A, Brain surface exposure after craniectomy showing cortical blood vessels and intact dura. B, Transdural loading of brain ECS showing a cylindrical dam containing aCSF solution of fluorescein-dextran. C, Fluorescence image of cortical surface after dye loading. Scale bar, 1 mm. D, Coronal 300 μm brain slice obtained ex vivo after loading demonstrates fluorescence loading of the cortex. Scale bar, 1 mm. A gradient of fluorescence signal is observed from the cortical surface (top left) to the dorsal hippocampus (bottom right). Arrowhead, Cortical blood vessel; asterisk, white matter.

Figure 2.

Brain ECS diffusion measured by cortical surface photobleaching. A, Schematic of apparatus for cortical surface photobleaching measurements in vivo. B, Photograph showing glass window positioned at the dural surface to dampen cardiorespiratory brain oscillations. C, Representative fluorescence recovery curves for 4 kDa fluorescein-dextran in aCSF (red), brain cortex (black), and aCSF containing 30% glycerol having viscosity ∼2.7 centipoise (blue; spot size, ∼5 μm). D, Left, Relationship between t_1/2 (half-time) for fluorescence recovery and relative brain versus aCSF diffusion coefficient (D/D_o; top x-axis) deduced from photobleaching measurements on fluorescein-dextran-containing solutions made viscous with glycerol(bottomx-axis). See Results for details. Right, Fluorescencere covery curves for a CSF containing 4kDa fluorescein-dextran and indicated concentrations of glycerol.

Figure 3.

Diffusion of fluorescein-dextrans in the brain ECS before and after cytotoxic brain edema. A, Fluorescence recovery curves for indicated fluorescein-dextrans before (black) and at 10 min (blue) and 20 min (green) after intraperitoneal water injection producing cytotoxic brain edema. Both short and long time scales are shown. B, Averaged t_1/2 (left) and deduced D/D_o (right) (mean ± SE) from data as in A. The top axis shows equivalent tortuosity (λ = [D_o/D]^1/2). C, Percentage of fluorescence recovery (mean ± SE) at 10 sec after photobleaching before and after water intoxication.

Figure 4.

Reduced macromolecular diffusion in the brain ECS after glutamate- and seizure-induced neuronal activity. A, Fluorescence recovery curves for 70 kDa fluorescein-dextran before and 5 min after application of glutamate (1 mm) to the cortical surface and 10 min after intraperitoneal injection of mannitol (40 gm/kg). Data are representative of three mice. B, Top, Electroencephalographic recordings before and after intraperitoneal injection of PTZ (100 mg/kg). Bottom, Fluorescence recovery curves for 70 kDa fluorescein-dextran before PTZ administration, after PTZ but before electroencephalographic seizure activity, and after seizure activity. The percentage of fluorescence recovery (mean ± SE; 3 mice) at 10 sec after photobleaching is summarized at the bottom.

Figure 5.

Enhanced ECS diffusion in mice lacking glial water channel AQP4. A, Fluorescence recovery curves for indicated fluorescein-dextrans in the brain cortex of wild-type (black) and AQP4-/- (gray) mice. B, Averaged t_1/2 (left) and deduced D/D_o (right) (mean ± SE) from data as in A. The top axis on the right shows equivalent tortuosity (λ = [D_o/D]^1/2). ^*p<0.01 compared with wild type. C, Averaged t_1/2 (mean ± SE) before and at 10 and 20 min after intraperitoneal water injection.