ANISOTROPIC AND HETEROGENEOUS DIFFUSION IN THE TURTLE CEREBELLUM - IMPLICATIONS FOR VOLUME TRANSMISSION

1. Measurements of extracellular diffusion properties were made in thr ee orthogonal axes of the molecular and granular layers of the isolate d turtle cerebellum with the use of iontophoresis of tetramethylammoni um (TMA+) combined with ion-selective microelectrodes. 2. Diffusion in the extracellular space of the molecular layer was anisotropic, that is, there was a different value for the tortuosity factor, lambda(i), associated with each axis of that layer. The x- and y-axes lay in the plane parallel to the pial surface of this lissencephalic cerebellum w ith the x-axis in the direction of the parallel fibers. The z-axis was perpendicular this plane. The tortuosity values were lambda(x) = 1.44 +/- 0.01, lambda(y) = 1.95 +/- 0.02, and lambda(z) = 1.58 +/- 0.01 (m ean +/- SE). By contrast, the granular layer was isotropic with a sing le tortuosity value, lambda(Gr) = 1.77 +/- 0.01. 3. These data confirm the applicability of appropriately extended Fickian equations to desc ribe diffusion in anisotropic porous media, including brain tissue. 4. Heterogeneity between the molecular and granular layer was revealed b y a striking difference in extracellular volume fraction, alpha, for e ach layer. In the molecular layer alpha = 0.31 +/- 0.01, whereas in th e granular layer alpha = 0.22 +/- 0.01. 5. Volume fraction and tortuos ity affected the time course and amplitude of extracellular TMA+ conce ntration after iontophoresis. This was modeled by the use of the avera ge parameters determined experimentally, and the nonspherical pattern of diffusion in the molecular layer was compared with the spherical di stribution in the granular layer and agarose gel by computing isoconce ntration ellipsoids. 6. One functional consequence of these results wa s demonstrated by measuring local changes in [K+]+ and [Ca2+]o after m icroiontophoresis of a cerebellar transmitter, glutamate. The ratios o f ion shifts in the x- and y-axes in the granular layer were close to unity, with a ratio of 1.04 +/- 0.08 for the rise in [K+]o and 1.03 +/ - 0.17 for the decrease in [Ca2+]o. In contrast, ion shifts in the mol ecular layer had an x:y ratio of 1.44 +/- 0.14 for the rise in [K+]o a nd 2.10 +/- 0.42 for the decrease in [Ca2+]o. 7. These data demonstrat e that the structure of cellular aggregates can channel the migration of substances in the extracellular microenvironment, and this could be a mechanism for volume transmission of chemical signals. For example, the preferred diffusion direction of glutamate along the parallel fib ers would help constrain an incoming excitatory stimulus to stay ''on- beam.''