Starting the simulation at time = 0 with no glutamate in the interior of the probe, the glutamate concentration rises with an exponential time constant ∼ 8.5 s to a steady state level (data not shown). At steady state, [Glu] inside the probe is elevated relative to the healthy region far from the probe (Fig. 4B1). With

sigma = 0 (i.e. no this website tissue damage), [Glu] in the probe is equal to the ambient [Glu] in the healthy tissue. With gradients of damage from sigma = 100 to 300 μm, steady-state glutamate levels in the probe range from ∼3 to 10 μM (Fig. 4B1). Decreasing the glutamate diffusion coefficient from its value in buffer, which is higher than in brain (Kullmann et al., 1999), increases the predicted steady state [Glu] measured in the probe (Fig. 4B2). Increasing or decreasing the leak rate L ( Fig. 4B3) also influences steady state [Glu] predicted in the probe volume. Glutamate transporters limit receptor activity on different time scales in the brain by restricting the spread of synaptically released glutamate as well as by maintaining low ambient glutamate concentrations (for reviews, see Danbolt, 2001, Tzingounis and Wadiche, 2007 and Vandenberg and Ryan, 2013). The steady-state ambient concentration of extracellular

glutamate at any click here point in brain reflects the balance of fluxes through sources and sinks in the neuropil. The data presented here indicate that transporters can establish steep concentration gradients when glutamate is supplied by passive Calpain diffusion from a pseudo-infinite source. Although we have used the neuronal transporter EAAT3 in these studies, its equilibrium thermodynamics are indistinguishable from the predominant astroglial transporter EAAT2 (Levy et al., 1998). With EAAT3 transporter

densities similar to those reported for EAAT2 in hippocampal astroglial membranes (∼104/μm2; Lehre and Danbolt, 1998) the concentration gradient between a 10 μM source concentration and the cell surface was found to exceed two orders of magnitude. The steepness of the gradient formed would be further increased if diffusion were reduced, as for example in tortuous neuropil (Kullmann et al., 1999). Conversely, reduction of transporter density or activity will reduce the steepness of the gradient and increase [Glu] at the cell surface. Reduced glutamate transport by loss or metabolic impairment is implicated in a broad range of neurodegenerative disorders (Sheldon and Robinson, 2007) including stroke (Rossi et al., 2000), traumatic brain injury (Goodrich et al., 2013), epilepsy (Coulter and Eid, 2012), Huntington’s disease (Faideau et al., 2010), ALS (Rothstein, 2009), and Alzheimer’s disease (Scimemi et al., 2013).