Evolution of anisotropic diffusion in the developing rat corpus callosum

Changes in the ability of substances to diffuse in the intersticial space of the brain are important factors in the pathophysiology of cerebrovascular diseases. Extracellular space (ECS) volume fraction alpha (alpha = ECS volume/ total tissue volume), tortuosity lambda (lambda 2 = free diffusion coefficient/apparent diffusion coefficient), and nonspecific uptake (k')-three diffusion parameters of brain tissue were studied in cortex and subcortical white matter (WM) of the developing rat during anoxia. Changes were compared with the rise in extracellular potassium concentration ([K+]e), extracellular pH (pHe) shifts, and anoxic depolarization (AD). Diffusion parameters were determined from extracellular concentration-time profiles of tetramethylammonium (TMA+) or tetraethylammonium (TEA+), TMA+, TEA+, K+, and pH changes were measured using ion-selective microelectrodes. In the cortex and WM of animals at 4-12 postnatal days (P4-P12), the volume fraction, alpha, is larger than that of animals at > or = P21. Anoxia evoked by cardiac arrest brought about a typical rise in [K+]e to approximately 60-70 mM, AD of 25-30 mV, decrease in alpha, increase in lambda, and increase in k'. At P4-P6, alpha decreased from approximately 0.43 to 0.05 in cortical layer V and from approximately 0.45 to 0.5 in WM. Tortuosity, lambda, increased in the cortex from 1.50 to 2.12 and in WM from approximately 1.48 to 2.08. At P10-P12 and at P21-P23, when alpha in normoxic rats is lower than at P4-P6 by approximately 25 and 50%, respectively, the final changes in values of alpha and lambda evoked by anoxia were not significantly different from those in P4-P6. However, the younger the animal, the longer the time course of the changes. On P4-P6 final changes in alpha, lambda and k' in cortex and WM were reached after 37 +/- 3 min and 54 +/- 2 min; on P10-P12, after 24 +/- 2 and 27 +/- 3 min; and on P21-P23 at 15 +/- 1 and 17 +/- 3 min, respectively (mean +/- SE, n = 6). The time course of the changes was longer in WM than in gray matter (GM), particularly during the first postnatal week, i.e., in the period during which WM is largely unmyelinated. Changes in diffusion parameters occurred in three phases. The first slow and second fast changes occurred simultaneously with the rise in [K+]e and AD. Peaks in [K+]e and AD were reached simultaneously; the younger the animal, the longer the time course of the changes. The third phase outlasted the rise in [K+]e and AD by 10-15 min and correlated with the acid shift in pHe. Linear regression analysis revealed a positive correlation between the normoxic size of the ECS volume and the time course of the changes. Slower changes in ECS volume fraction and tortuosity in nervous tissue during development can contribute to slower impairment of signal transmission, e.g., due to lower accumulation of ions and neuroactive substances released from cells and their better diffusion from the hypoxic area in uncompacted ECS.