Impact of synaptic localization and subunit composition of ionotropic glutamate receptors on synaptic function: modeling and simulation studies.

Ionotropic NMDA and AMPA glutamate receptors (iGluRs) play important roles in synaptic function under physiological and pathological conditions. iGluRs sub-synaptic localization and subunit composition are dynamically regulated by activity-dependent insertion and internalization. However, understanding the impact on synaptic transmission of changes in composition and localization of iGluRs is difficult to address experimentally. To address this question, we developed a detailed computational model of glutamatergic synapses, including spine and dendritic compartments, elementary models of subtypes of NMDA and AMPA receptors, glial glutamate transporters, intracellular calcium and a calcium-dependent signaling cascade underlying the development of long-term potentiation (LTP). These synapses were distributed on a neuron model and numerical simulations were performed to assess the impact of changes in composition and localization (synaptic vs extrasynaptic) of iGluRs on synaptic transmission and plasticity following various patterns of presynaptic stimulation. In addition, the effects of various pharmacological compounds targeting NMDARs or AMPARs were determined. Our results showed that changes in NMDAR localization have a greater impact on synaptic plasticity than changes in AMPARs. Moreover, the results suggest that modulators of AMPA and NMDA receptors have differential effects on restoring synaptic plasticity under different experimental situations mimicking various human diseases.

Impact of synaptic localization and subunit composition of ionotropic glutamate receptors on synaptic function: modeling and simulation studies.

Ionotropic NMDA and AMPA glutamate receptors (iGluRs) play important roles in synaptic function under physiological and pathological conditions. iGluRs sub-synaptic localization and subunit composition are dynamically regulated by activity-dependent insertion and internalization. However, understanding the impact on synaptic transmission of changes in composition and localization of iGluRs is difficult to address experimentally. To address this question, we developed a detailed computational model of glutamatergic synapses, including spine and dendritic compartments, elementary models of subtypes of NMDA and AMPA receptors, glial glutamate transporters, intracellular calcium and a calcium-dependent signaling cascade underlying the development of long-term potentiation (LTP). These synapses were distributed on a neuron model and numerical simulations were performed to assess the impact of changes in composition and localization (synaptic vs extrasynaptic) of iGluRs on synaptic transmission and plasticity following various patterns of presynaptic stimulation. In addition, the effects of various pharmacological compounds targeting NMDARs or AMPARs were determined. Our results showed that changes in NMDAR localization have a greater impact on synaptic plasticity than changes in AMPARs. Moreover, the results suggest that modulators of AMPA and NMDA receptors have differential effects on restoring synaptic plasticity under different experimental situations mimicking various human diseases.

Activation of GABAA receptors controls mesiotemporal lobe epilepsy despite changes in chloride transporters expression: In vivo and in silico approach.

Mesiotemporal lobe Epilepsy (MTLE), the most frequent form of focal epilepsy, is often drug-resistant. Enriching the epileptic focus with GABA-releasing engineered cells has been proposed as a strategy to prevent seizures. However, ex vivo data from animal models and MTLE patients suggest that, due to changes in chloride homeostasis, GABAA receptor activation is depolarizing and partly responsible for focal interictal discharges and seizure initiation. To understand how these two contradictory aspects of GABAergic neurotransmission coexist in MTLE, we used an established mouse model of MTLE presenting hippocampal sclerosis and recurrent hippocampal paroxysmal discharges (HPDs) 30-40days after a unilateral injection of kainate in the dorsal hippocampus. We first showed that injections of GABAA receptor agonists either systemically or directly into hippocampus suppressed HPDs. Western-blotting and immunostaining revealed that levels of α1, α3 and γ2 GABAA receptor subunits were increased in epileptic mice, compared to saline controls, while levels of R1 and R2 GABAB receptor subunits but also NR1, NR2A and NR2B NMDA receptor subunits and GluR1 and GluR2 AMPA receptor subunits were decreased. In addition, we showed that the expression of the transporter NKCC1, which load neurons with chloride, was increased, whereas KCC2, a chloride extruder, was decreased and that HPDs were suppressed by injection of blockers of NKCC1. These different changes were integrated in a numerical model, and in silico simulations supported the notion that chloride imbalance impair local inhibitory control of pyramidal neurons' activity in this model of MTLE. However, our numerical model also suggested that lasting activation of these receptors restore physiological intracellular chloride concentrations and suppress HPDs. Overall, our study suggests that activation of GABAA receptor remains an effective antiepileptic strategy to suppress focal seizures in MTLE, and demonstrates that modeling and simulation studies provide new insights about the cellular and synaptic mechanisms of this disease.

Modeling and simulation of organophosphate-induced neurotoxicity: Prediction and validation by experimental studies.

Exposure to organophosphorus (OP) compounds, either pesticides or chemical warfare agents, represents a major health problem. As potent irreversible inhibitors of cholinesterase, OP may induce seizures, as in status epilepticus, and occasionally brain lesions. Although these compounds are extremely toxic agents, the search for novel antidotes remains extremely limited. In silico modeling constitutes a useful tool to identify pharmacological targets and to develop efficient therapeutic strategies. In the present work, we developed a new in silico simulator in order to predict the neurotoxicity of irreversible inhibitors of acetyl- and/or butyrylcholinesterase (ChE) as well as the potential neuroprotection provided by antagonists of cholinergic muscarinic and glutamate N-methyl-d-aspartate (NMDA) receptors. The simulator reproduced firing of CA1 hippocampal neurons triggered by exposure to paraoxon (POX), as found in patch-clamp recordings in in vitro mouse hippocampal slices. In the case of POX intoxication, it predicted a preventing action of the muscarinic receptor antagonist atropine sulfate, as well as a synergistic action with the non-competitive NMDA receptor antagonist memantine. These in silico predictions relative to beneficial effects of atropine sulfate combined with memantine were recapitulated experimentally in an in vivo model of POX in adult male Swiss mice using electroencephalic (EEG) recordings. Thus, our simulator is a new powerful tool to identify protective therapeutic strategies against OP central effects, by screening various combinations of muscarinic and NMDA receptor antagonists.

A computational model to investigate astrocytic glutamate uptake influence on synaptic transmission and neuronal spiking.

Over the past decades, our view of astrocytes has switched from passive support cells to active processing elements in the brain. The current view is that astrocytes shape neuronal communication and also play an important role in many neurodegenerative diseases. Despite the growing awareness of the importance of astrocytes, the exact mechanisms underlying neuron-astrocyte communication and the physiological consequences of astrocytic-neuronal interactions remain largely unclear. In this work, we define a modeling framework that will permit to address unanswered questions regarding the role of astrocytes. Our computational model of a detailed glutamatergic synapse facilitates the analysis of neural system responses to various stimuli and conditions that are otherwise difficult to obtain experimentally, in particular the readouts at the sub-cellular level. In this paper, we extend a detailed glutamatergic synaptic model, to include astrocytic glutamate transporters. We demonstrate how these glial transporters, responsible for the majority of glutamate uptake, modulate synaptic transmission mediated by ionotropic AMPA and NMDA receptors at glutamatergic synapses. Furthermore, we investigate how these local signaling effects at the synaptic level are translated into varying spatio-temporal patterns of neuron firing. Paired pulse stimulation results reveal that the effect of astrocytic glutamate uptake is more apparent when the input inter-spike interval is sufficiently long to allow the receptors to recover from desensitization. These results suggest an important functional role of astrocytes in spike timing dependent processes and demand further investigation of the molecular basis of certain neurological diseases specifically related to alterations in astrocytic glutamate uptake, such as epilepsy.

Modeling of the nervous system: from modulation of glutamatergic and gabaergic molecular dynamics to neuron spiking activity.

One of the fundamental characteristics of the brain is its hierarchical and temporal organization: scales in both space and time must be considered to fully grasp the system's underlying mechanisms and their impact on brain function. Complex interactions taking place at the molecular level regulate neuronal activity that further modifies the function of millions of neurons connected by trillions of synapses, ultimately giving rise to complex function and behavior at the system level. Likewise, the spatial complexity is accompanied by a complex temporal integration of events taking place at the microsecond scale leading to slower changes occurring at the second, minute and hour scales. These integrations across hierarchies of the nervous system are sufficiently complex to have impeded the development of routine multi-level modeling methodologies. The present study describes an example of our multiscale efforts to rise from the biomolecular level to the neuron level. We more specifically describe how we integrate biomolecular mechanisms taking place at glutamatergic and gabaergic synapses and integrate them to study the impact of these modifications on spiking activity of a CA1 pyramidal cell in the hippocampus.

Simulation of postsynaptic glutamate receptors reveals critical features of glutamatergic transmission.

Activation of several subtypes of glutamate receptors contributes to changes in postsynaptic calcium concentration at hippocampal synapses, resulting in various types of changes in synaptic strength. Thus, while activation of NMDA receptors has been shown to be critical for long-term potentiation (LTP) and long term depression (LTD) of synaptic transmission, activation of metabotropic glutamate receptors (mGluRs) has been linked to either LTP or LTD. While it is generally admitted that dynamic changes in postsynaptic calcium concentration represent the critical elements to determine the direction and amplitude of the changes in synaptic strength, it has been difficult to quantitatively estimate the relative contribution of the different types of glutamate receptors to these changes under different experimental conditions. Here we present a detailed model of a postsynaptic glutamatergic synapse that incorporates ionotropic and mGluR type I receptors, and we use this model to determine the role of the different receptors to the dynamics of postsynaptic calcium with different patterns of presynaptic activation. Our modeling framework includes glutamate vesicular release and diffusion in the cleft and a glutamate transporter that modulates extracellular glutamate concentration. Our results indicate that the contribution of mGluRs to changes in postsynaptic calcium concentration is minimal under basal stimulation conditions and becomes apparent only at high frequency of stimulation. Furthermore, the location of mGluRs in the postsynaptic membrane is also a critical factor, as activation of distant receptors contributes significantly less to calcium dynamics than more centrally located ones. These results confirm the important role of glutamate transporters and of the localization of mGluRs in postsynaptic sites in their signaling properties, and further strengthen the notion that mGluR activation significantly contributes to postsynaptic calcium dynamics only following high-frequency stimulation. They also provide a new tool to analyze the interactions between metabotropic and ionotropic glutamate receptors.