Fig. 12.
Comparison with reported glutamate concentration time courses. A, Glutamate time courses determined using low-affinity glutamate receptor antagonists are slower than predicted here. The difference is difficult to account for only by slowing diffusion within the synaptic cleft. Time courses reported by: Clements et al. (1992) (CLTJW 1; dashed line); Clements (1996)(CLTJW 2; dashed curve); Diamond and Jahr (1997)(DJ1,2; dotted). The predictions of the present model (solid lines, average concentration over the PSD) are for instantaneous release of one vesicle in the absence of transporters (ctrl), and the same but with an extreme obstruction of the synaptic cleft (obstructed; αcleft = 0.1, λcleft = 1.9). The peak concentrations for ctrl and obstructed time courses are 10.9 and 109 mm, respectively. B, There is a discrepancy between the glutamate time courses reported by Rusakov and Kullmann (1998b) and Rusakov et al. (1999) and the present simulations, which agree with an approximate theoretical prediction. The glutamate concentration at 300 nm reported by Rusakov and Kullmann (RK; dot-dashed line) is approximately twice that produced using very similar parameters in the present simulation (dashed line). The accuracy of the present simulation is confirmed by its close agreement with the analytical solution for diffusion from a point source in a porous medium (dotted line). Some of the (slight) difference between the present simulation and the analytical solution is attributable to the presence of transporters, because a similar simulation without transporters (solid line) agrees even more closely with the analytical solution.