Synaptic Efficacy as a Function of Ionotropic Receptor Distribution: A Computational Study

Abstract

Glutamatergic synapses are the most prevalent functional elements of information processing in the brain. Changes in pre-synaptic activity and in the function of various post-synaptic elements contribute to generate a large variety of synaptic responses. Previous studies have explored postsynaptic factors responsible for regulating synaptic strength variations, but have given far less importance to synaptic geometry, and more specifically to the subcellular distribution of ionotropic receptors. We analyzed the functional effects resulting from changing the subsynaptic localization of ionotropic receptors by using a hippocampal synaptic computational framework. The present study was performed using the EONS (Elementary Objects of the Nervous System) synaptic modeling platform, which was specifically developed to explore the roles of subsynaptic elements as well as their interactions, and that of synaptic geometry. More specifically, we determined the effects of changing the localization of ionotropic receptors relative to the presynaptic glutamate release site, on synaptic efficacy and its variations following single pulse and paired-pulse stimulation protocols. The results indicate that changes in synaptic geometry do have consequences on synaptic efficacy and its dynamics.

Fig 1. Probability of AMPAR open states as a function of receptor location.

(Top row) Probability of AMPAR open states as a function of the distance from release site when 2, 3 and 4 glutamate molecules are bound (shown by filled circles—bottom row). Middle row shows the normalized maximum values scaled between 0 and 1 to highlight the location where the probability of open state is maximum.

Fig 2. AMPAR-mediated EPSC as a function of the distance from the release site.

AMPAR-mediated EPSCs to a single pulse. A gradual decrease in EPSC amplitude was observed when the distance between the release site and the receptors increases. The peak value of EPSC was obtained at 0 nm and was 52% larger than the peak amplitude when AMPARs are located 200 nm away from the release site (A). The resulting EPSP values as a function of the distance are shown in B. As expected the EPSPs mediated by AMPARs closest to the glutamate release site had the highest amplitude, 50% larger than the peak amplitude of AMPAR located 200 nm away.

Fig 3. Time-to-peak of AMPAR-mediated EPSC as a function of the distance from the release site.

Simulated EPSCs as a function of AMPAR location. The time-to-peak of EPSCs show a 0.5 ms delay when AMPA receptors are 300 nm (marked by blue asterisk) away, as compared to receptors located at 0 nm (marked by red asterisk).

Fig 4. Desensitization of AMPAR as a function of the distance from the release site.

Individual plots (from top) represent the probability of desensitization when (A) 2 glutamate molecules are bound, (B) 3 glutamate molecules are bound and (C) 4 glutamate molecules are bound. (D) Overall desensitization, as a sum of all individual desensitized states including when 1 glutamate molecule is bound and no molecules are bound. Overall desensitization (sum of all desensitized D states) is relatively similar across all locations, as the receptor is likely to be in the desensitization state D0 (when no glutamate are bound) for a longer period of time.

Fig 5. Probability of glutamate occupancy (peak values) as a function of the distance from the release site.

The average AMPAR glutamate occupancy is 0.59.

Fig 6. Probability of the two NMDAR open states as a function of the distance from the release site.

There is a small variation in the individual open state probability as a function of distance. Open state 2 did show a slight variation as a function of receptor location, but the overall amplitude is very small (in the order of 10⁻³).

Fig 7. Lack of influence of NMDAR location on NMDAR-mediated EPSCs and EPSPs.

NMDAR-mediated EPSCs (A) and EPSPs (B) as a function of NMDAR location.

Fig 8. Summation of AMPAR- and NMDAR-mediated EPSCs at two magnesium concentrations.

EPSCs (shown in red) are a summation of AMPAR (blue dotted line)-mediated and NMDAR-mediated currents (green dotted line). When Mg²⁺ concentration varies from 1 mM to 0.5 mM, NMDAR-mediated current is larger, changing the overall EPSC (in red).

Fig 9. Schematic representation of the PSD and the receptors located on the postsynaptic membrane.

A: Postsynaptic responses were elicited by a paired pulse stimulus with an inter pulse interval of 10 ms. The EPSC waveforms (shown in red and blue) are a result of ionic current flow through both AMPAR (located right opposite to the glutamate release site and at 200 nm. respectively) and NMDAR channels. The paired pulse ratio (PPR) as explained in Box B is the ratio of the maximum amplitude of the second response divided by the maximum amplitude of the first response. When AMPARs are closer to the release site, PPR is ~0.85 and when they are farther away, PPR is ~0.96. Paired pulse ratios (collected from 16x11 simulations—AMPARs located from 0 nm to 300 nm (16 data points along the x-axis) and for inter pulse intervals varying from 10 to 2000 ms (11 data points along the y-axis) simulations). C: PPRs of synaptic EPSCs for [Mg ²⁺ ] = 1mM. Paired pulse ratios were plotted for AMPARs located from 0 nm to 300 nm (16 data points along the x-axis) and for inter pulse intervals varying from 10 to 2000 ms (11 data points along the y-axis) simulations). The paired pulse ratios vary between 0.8 and 1.18 (as indicated by the color bar). D: PPRs of synaptic EPSCs for [Mg ²⁺ ] = 0.5mM. [Mg²⁺] was maintained at 0.5 mM to simulate the effect of partial unblocking of NMDA receptors by Mg²⁺. PPRs significantly varied with AMPAR locations between 1 to 1.7, more specifically for relatively small inter pulse intervals (10–100 ms).

Fig 10. Schematic representation of the glutamatergic synapse model used in EONS. Some of the key elements are highlighted and details of the models of glutamate ionotropic receptors represented with kinetic rate constants.

Top Left: Schematic of 16-states AMPA receptor model, adapted from Robert and Howe (2003). Top Right: Kinetic schema of the 15 states NMDA receptor model with kinetic rate constants, adapted from Ambert et al., 2010.