Synaptic glutamate spillover increases NMDA receptor reliability at the cerebellar glomerulus

Abstract

Glutamate spillover in the mossy fiber to granule cell cerebellar glomeruli has been hypothesized to increase neurotransmission reliability. In this study, we evaluate this hypothesis using an experimentally based quantitative model of glutamate spillover on the N-methyl-d-aspartate receptors (NMDA-Rs) at the cerebellar glomerulus. The transient and steady-state responses of NMDA-Rs were examined over a physiological range of firing rates. Examined cases included direct glutamate release activation, glutamate spillover activation, and a combination of direct and spillover activation. Our results illustrate that the effects of spillover alone are equivalent to direct release and, notably, combined spillover and direct release effects on NMDA-Rs are not additive. Our results show that spillover does in fact provide a high degree of reliability given that the synaptic vesicle release rate must fall to approximately 15-25% of what is considered the normal baseline level in order to substantially alter neurotransmission across the examined range of frequencies. We suggest that the high reliability provided by activation due to glutamate spillover could be used to conserve energy by reducing the required overall glutamate load at higher frequencies.

Figure 1. Saftenku cerebellar glomerulus diffusion model (Saftenku 2005)

Mossy fiber terminal (MFT) is surrounded by a glial sheath. R_abs is the radius of the absorbing boundary representing glutamate uptake by the sheath, and R_dd is the thickness of a single dendritic digit.

Figure 2. Banke and Traynelis NMDA-R binding model (Banke and Traynelis 2003)

This model examines the binding of the two NR2 subunits, though co-agonist binding is necessary to open the ion channel. Glycine concentration is assumed to be high enough such that NR1 subunits are saturated. The binding of the first and second glutamate molecules is represented by RA and RA₂, respectively. The desensitized states are labeled RA₂d₁ and RA₂d₂. The fast and slow transition states are labeled RA_2f and RA_2s, respectively. The activated state is RA₂*. Adapted from (Banke and Traynelis 2003; Mitchell, Feng et al. 2007; Mitchell and Lee 2007).

Figure 3. Glutamate concentration and NMDA-R open probability profiles for direct release versus spillover activation at the cerebellar glomerulus

The figure illustrates a detailed example of model output at a firing rate of 1 Hz, where differences between direct release and spillover activation can best be detected. A.) Transient and steady-state open probability of NMDA-Rs over 10 seconds, due to either direct-release (blue) or spillover (red) activation. B.) Close-up illustration of the glutamate concentration of an individual spike (same units as A). C.) Close-up illustration of the open probability of NMDA-Rs of a steady-state spike. D.) Desensitization probability profile (probability of NMDA-Rs being in one of the two desensitized states shown in Figure 1) for direct release or spillover activation.

Figure 4. Effect of firing rate on NMDA-R open probability at the cerebellar glomerulus

The figure illustrates the impact of firing rate over a range of 1–32 Hz. A.) Transient and steady-state open probability of NMDA-Rs over 10 seconds. Given that the direct release and spillover profiles are essentially the same and thus indistinguishable, only the direct release profiles are shown. B.) Close up illustration of the open probability of NMDA-Rs of a steady-state spike at each simulated firing rate. C.) Plot of the maximum and minimum NMDA-R open probabilities across the entire range of firing rates.

Figure 5. Effect of combined activation on the NMDA-R open probability at the cerebellar glomerulus

A.) Transient and steady-state open probability of NMDA-Rs over 10 seconds. Notice that the combined activation (green) is essentially equivalent to direct release (blue) or spillover (red); that is, combined activation is does not result in additive effects on NMDA-R response. B.) Close-up illustration of the open probability of NMDA-Rs of an individual spike, highlighting the very small quantitative differences between combined, direct release, and spillover activation. C.) Plot of the maximum and minimum NMDA-R open probabilities for combined, direct release, and spillover activation across a range of firing rates. Notice that the different activation types become indistinguishable at higher frequencies. D.) Desensitization probability profile (probability of NMDA-Rs being in one of the two desensitized states shown in Figure 3) for direct release, spillover and combined activation.

Figure 6. Effect of missed glutamate vesicle release(s) on transmission reliability

A.) Transient and steady-state open probability of NMDA-Rs over 10 seconds at a firing rate of 1 Hz for steady-state (dotted) and missed spike (solid) cases. There is a missed release at 8 s, which results in a dropped spike. Notice that the next spike at 9 s (last spike in the trace) has increase in the maximum open probability. B.) Close-up illustration of the open probability of NMDA-Rs of for spike following missed release. C.) Plot of the maximum and minimum open probabilities for steady-state and missed spike properties across a range of firing rates.

Figure 7. Effect of constrained release of glutamate vesicles

The figure illustrates the effects of constrained vesicle release as a function of percent reliability (e.g. where 25% reliability equates to a 75% drop in vesicle release). A.) Transient and steady-state open probability of NMDA-Rs over 10 seconds at a firing rate of 4 Hz for 100%, 50%, 25% and 12.5% reliability. B.) Maximum steady-state open probability of NMDA-Rs at 100%, 50%, 25% and 12.5% reliability. C.) Minimum steady-state open probability of NMDA-Rs at 100%, 50%, 25% and 12.5% reliability.

Figure 8

Effect of enhanced glutamate uptake. A.) Transient and steady-sate open probability of NMDA-Rs over 4 seconds at a firing rate of 4 Hz. B.) B.) Maximum steady-state open probability of NMDA-Rs at 100%, 50%, 75% and 87.5.5% increased uptake. C.) Minimum steady-state open probability of NMDA-Rs at 100%, 50%, 75% and 87.5.5% increased uptake.