Abstract
Abstract: :
Purpose: To estimate the relative spatial location of NMDA receptors postsynaptic to ribbon synapses of bipolar cells and test the hypothesis that extrasynaptic NMDA receptor location contributes to their differential contribution to miniature versus light-evoked EPSCs. Methods: Simulations of the diffusion of glutamate from a single synaptic vesicle into a synaptic cleft were run using the program, MCell, to track the position of molecules in 3D space. Glutamate concentration was measured in these simulations for a volume 4.7 x 105 nm3 directly below the mouth of the pore and an equivalent volume displaced by various distances. Simulations were run for diffusion rates from the free diffusion of glutamate in aqueous solution (7.6 x 10-6 cm2 per sec) to slower rates more likely to reflect the tortuosity and viscosity of the extracellular space (a minimum of 0.5 x 10-6 cm2 per sec). The timecourse of glutamate from these simulations was applied as a stimulus to a simulated patch of membrane containing NMDA receptors in the program NEURON to determine if the simulated single vesicle release event would be sufficient to activate NMDA receptors at a distance, which it should not do based on experimental data (Taylor et al, J. Physiol. 1995). Nine different kinetic models for NMDA receptors (Chen and Murphy, Molec. Pharmacol. 2001) were tested in these simulations. Results: For a 25 nm diameter vesicle filled with 4000 molecules of glutamate, the peak concentration at the postsynaptic membrane below the mouth of the pore reached 2.6 mM. The peak concentration reached 0.11, 0.06 and 0.02 mM at 500, 700 and 900 nm away from the release site, respectively. Slowing the rate of diffusion delays the time to the peak glutamate concentration but not the peak value itself. Only when NMDA receptors (all kinetic models) were stimulated with a concentration waveform consistent with their being 900 nm from the release site, was there no NMDA response as physiological data predict. Conclusions: If the lack of an NMDA component to mini-EPSCs in ganglion cells results from a spatial localization of receptors, NMDA receptors must be 900 nm equidistant from all ribbon synapses. Reduced vesicle concentration of glutamate, or possible modulation of co-agonist D-serine could allow for more proximal locations.
Keywords: computational modeling • ganglion cells • excitatory amino acid receptors