Influence of active synaptic pools on the single synaptic event

Abstract

The activity of the single synapse is the base of information processing and transmission in the brain as well as of important phenomena as the Long Term Potentiation which is the main mechanism for learning and memory. Although usually considered as independent events, the single quantum release gives variable postsynaptic responses which not only depend on the properties of the synapses but can be strongly influenced by the activity of other synapses. In the present paper we show the results of a series of computational experiments where pools of active synapses, in a compatible time window, influence the response of a single synapse of the considered pool. Moreover, our results show that the activity of the pool, by influencing the membrane potential, can be a significant factor in the NMDA unblocking from $M{g}^{2+}$ increasing the contribution of this receptor type to the Excitatory Post Synaptic Current. We consequently suggest that phenomena like the LTP, which depend on NMDA activation, can occur also in subthreshold conditions due to the integration of the dendritic synaptic activity.

Keywords: AMPA; LTP; NMDA; Synaptic modeling; Synaptic transmission.

Fig. 1

Schematic 3D representation of the simulated synaptic space. Each component of the synaptic space is shown in the legend and detailed explanation of the properties of the components are in the text. Height of the synaptic cleft is 20 nm, PSD diameter is 110 nm and total synaptic space has a diameter of 220 nm. AMPARs (yellow) and NMDARs (blue) protrude from the base of 7 nm. (Color figure online)

Fig. 2

Effect of the different firing frequency of the synaptic pools on the membrane potential. A Single run for each frequency; B average of $V_m$ over 100 runs; C peak levels of the EPSPs as function of the firing frequency of the pool (black single run, red averaged over 100 runs); D dependence of the peak level as a function of the firing frequency of the pool normalized to the peak level at $\varphi =0$ (black single run and red average on 100 runs). (Color figure online)

Fig. 3

Effect of the different firing frequency of the synaptic pool on EPSC amplitude produced by the synapse S. A single run for each frequency; B average over 100 runs; C amplitude of the EPSC as function of the firing frequency of the pool; D dependence of the EPSC peak amplitude, as a function of the firing frequency of the pool, normalized to the peak amplitude at $\varphi =0$

Fig. 4

Effect of the different firing frequency of the synaptic pool on AMPA-EPSC amplitude produced by the synapse S. A single run for each frequency; B average over 100 runs; C amplitude of the AMPA-EPSC as function of the firing frequency of the pool; D dependence of the AMPA-EPSC peak amplitude, as a function of the firing frequency of the pool, normalized to the peak amplitude at $\varphi =0$

Fig. 5

Effect of the different firing frequency of the synaptic pool on NMDA-EPSC amplitude produced by the synapse S. a single run for each frequency; b average over 100 runs; c amplitude of the NMDA-EPSC as function of the firing frequency of the pool; d dependence of the NMDA-EPSC peak amplitude, as a function of the firing frequency of the pool, normalized to the peak amplitude at $\varphi =0$

Fig. 6

Average number of NMDA receptors of the synapse $\mathit{S}$ activated at the different frequencies of the synaptic pool