Independence of extracellular tortuosity and volume fraction during osmotic challenge in rat neocortex

Abstract

The structural properties of brain extracellular space (ECS) are summarised by the tortuosity (l) and the volume fraction (a). To determine if these two parameters were independent, we varied the size of the ECS by changing the NaCl content to alter osmolality of bathing media for rat cortical slices. Values of l and a were extracted from diffusion measurements using the real-time ionophoretic method with tetramethylammonium (TMA + ). In normal medium (305 mosmol kg _1 ), the average value of l was 1.69 and of a was 0.24. Reducing osmolality to 150 mosmol kg _1 , increased l to 1.86 and decreased a to 0.12. Increasing osmolality to 350 mosmol kg _1 , reduced l to about 1.67 where it remained unchanged even when osmolality increased further to 500 mosmol kg _1 . In contrast, a increased steadily to 0.42 as osmolality increased. Comparison with previously published experiments employing 3000 M r dextran to measure l, showed the same behaviour as for TMA + , including the same constant l in hypertonic media but with a steeper slope in the hypotonic solutions. These data show that l and a behave differently as the ECS geometry varies. When a decreases, l increases but when a increases, l rapidly attains a constant value. A previous model allowing cellular shape to alter during osmotic challenge can account qualitatively for the plateau behaviour of l.

Aquaporin-4-Deficient Mice Have Increased Extracellular Space without Tortuosity Change

“…These impedance data [Traynelis and Dingledine (1989), their Table 2] correspond to a 1-10% increase in ␣ (assuming ␣ is proportional to impedance). This estimate can be corroborated using the osmotic data of Traynelis and Dingledine (1989) and Equation 13 of Kume-Kick et al (2002) that predicts a 2-13% increase in ␣. Thus, even a moderate expansion of the ECS may be sufficient to increase the threshold for seizure by reducing the buildup of neuroactive substances in the ECS or decreasing ephaptic interactions that normally promote synchronized firing.…”

Diffusion of Epidermal Growth Factor in Rat Brain Extracellular Space Measured by Integrative Optical Imaging

“…It is also likely that some types of pathology will further enhance the differences in observed between high and low groups of macromolecules under normal conditions. For instance, the relative increase in for TMA ϩ is significantly less than that observed for 3,000 M r dextran during both ischemia (20 and 64% increase, respectively;Hrabětová et al 2000Hrabětová et al , 2003 and osmotic stress resulting from slice incubation in 150 mOsm/kg ACSF (10 and 34% increase, respectively; Kume-Kick et al 2002;Tao 1999). Therefore, the diffusion of EGF may be significantly modified during injury or disease of the CNS, with consequences for both its normal action and its effects relative to that of larger growth factors such as NGF.…”

“…In theory, diffusion analysis using appropriate small markers can isolate the effects of tissue properties on , because the influences of binding and the physicochemical properties of the diffusing molecule are negligible. Accordingly, measurements with small ions such as tetramethylammonium have yielded ϳ 1.6 in most brain regions, both in vivo (Cserr et al 1991;Lehmenkühler et al 1993;Mazel et al 1998;Nicholson and Phillips 1981;Roitbak and Syková 1999) and in the rat brain slice preparation (Cragg et al 2001;Hrabětová and Nicholson 2000;Hrabětová et al 2002;Kume-Kick et al 2002;Pérez-Pinzón et al 1995;Rice and Nicholson 1991). Studies with dextran macromolecules have shown increases from about 1.8 for 3,000 and 10,000 M r dextrans to about 2.2 for 40,000 and 70,000 M r dextrans (Nicholson and Tao 1993), suggesting globular molecules experience an increase in hindrance above a critical size.…”

“…The hindrance experienced by molecules as they travel through the porous brain ECS and encounter obstructions is described by , a dimensionless parameter formally defined as ϭ (D/D*) 1/2 where D is the diffusion coefficient of the molecule in a free medium (water or dilute agarose) and D* is the effective diffusion coefficient of the same molecule in brain (Nicholson 2001). Whereas ␣ is a property of the tissue, is sensitive to tissue properties such as geometry (Chen and Nicholson 2000;Hrabětová et al 2003;Kume-Kick et al 2002;Tao 1999) and extracellular matrix composition (Rusakov and Kullmann 1998;Vargová et al 2003;Voříšek et al 2002), as well as to binding (Berk et al 1997) and the physicochemical properties (size, shape, and charge) of the diffusing molecule (Nicholson and Tao 1993;Prokopová-Kubinová et al 2001;Tao and Nicholson 1996). In theory, diffusion analysis using appropriate small markers can isolate the effects of tissue properties on , because the influences of binding and the physicochemical properties of the diffusing molecule are negligible.…”

“…Diffusion governs the movement of molecules through brain extracellular space (ECS) and determines, along with clearance (enzymatic breakdown or efflux across the blood-brain barrier) and cellular uptake, the distribution of molecules after their release into the ECS (Nicholson 2001;Nicholson and Syková 1998). Although the diffusion coefficients of small ions (Cserr et al 1991;Kume-Kick et al 2002;Lehmenkühler et al 1993;Nicholson and Phillips 1981;Pérez-Pinzón et al 1995) and macromolecules such as dextrans (Nicholson and Tao 1993) and albumins (Tao and Nicholson 1996) have been determined in brain tissue, there is no information about the diffusion characteristics of neurotrophic factors other than NGF (Stroh et al 2003).…”