Brain function is based on communication between individual cells. This communication utilizes small neurotransmitter and neuromodulator molecules, which are transported through extracellular space (ECS). Substances released into the ECS spread predominantly by diffusion because the ECS lacks any active transport mechanism. Because its structure exerts a direct influence on the intercellular signaling and also on transport of nutrients, metabolites and therapeutic agents Diffusion can be exploited experimentally to quantify two major structural parameters of the ECS, volume fraction and diffusion permeability Activation of noradrenergic system is known to cause wakefulness, increase vigilance and facilitate the transition of cortical activity from the sleep state to the awake state [5]. We measured ECS parameters in a visual region of the rat neocortical slices under control conditions and after activation of noradrenergic system by the noradrenergic agonist isoproterenol. Using the Real-Time Iontophoretic (RTI) method [1], we found that α decreased from 0.22 to 0.18 when isoproterenol was applied, suggesting that activation of noradrenergic receptors mimics the awake state in a brain slice. Next, we utilized this experimental paradigm as an in vitro model of sleep and awake brain states to study diffusive spread of molecules with small and high molecular weights in these states. To this end, we quantified diffusion of a small cation tetramethylammonium (MW 74) with the RTI method and fluorescently-labeled macromolecule dextran (MW 3000) with Integrative Optical Imaging method [6] in the visual neocortex both under the control conditions and after application of isoproterenol. Our preliminary results show that θ TMA remained constant (0.376 vs. 0.381) while θ dextran decreased (0.346 vs. 0.289) after the application of isoproterenol. In conclusion, our pilot study suggests that the diffusive spread of macromolecules in brain ECS is more restricted during the awake-like state than during the sleep-like state. This result has important implications for transport of growth factors, proteins and macromolecular therapeutics in brain ECS. References [1] C. Nicholson, J. Phillips: Ion diffusion modified by tortuosity and volume fraction in the extracellular microenvironment of the rat cerebellum. J. Physiol. 321, 225-257 (1981)
