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Synaptic GABA release prevents GABA transporter type-1 reversal during excessive network activity.

Savtchenko L, Megalogeni M, Rusakov DA, Walker MC, Pavlov I - Nat Commun (2015)

Bottom Line: Here we combined a realistic kinetic model of GAT-1 with experimental measurements of tonic GABAA receptor currents in ex vivo hippocampal slices to examine GAT-1 operation under varying network conditions.Our simulations predict that synaptic GABA release during network activity robustly prevents GAT-1 reversal.We conclude that sustained efflux of GABA through GAT-1 is unlikely to occur during physiological or pathological network activity.

View Article: PubMed Central - PubMed

Affiliation: UCL Institute of Neurology, Queen Square, London WC1N3BG, UK.

ABSTRACT
GABA transporters control extracellular GABA, which regulates the key aspects of neuronal and network behaviour. A prevailing view is that modest neuronal depolarization results in GABA transporter type-1 (GAT-1) reversal causing non-vesicular GABA release into the extracellular space during intense network activity. This has important implications for GABA uptake-targeting therapies. Here we combined a realistic kinetic model of GAT-1 with experimental measurements of tonic GABAA receptor currents in ex vivo hippocampal slices to examine GAT-1 operation under varying network conditions. Our simulations predict that synaptic GABA release during network activity robustly prevents GAT-1 reversal. We test this in the 0 Mg(2+) model of epileptiform discharges using slices from healthy and chronically epileptic rats and find that epileptiform activity is associated with increased synaptic GABA release and is not accompanied by GAT-1 reversal. We conclude that sustained efflux of GABA through GAT-1 is unlikely to occur during physiological or pathological network activity.

No MeSH data available.


Related in: MedlinePlus

GAT-1 activity and [GABA]e simulations.(a) Current–voltage relationship of the steady-state GAT-1-mediated current (JGAT, molecules s−1; negative values—forward mode, positive values—reverse mode) at different concentrations of extracellular GABA (in μM). (b) Dependence of GAT-1 reversal potential (EGAT) on the concentration of extracellular GABA. (c) Membrane potential dependence of the steady-state cytosolic ([GABA]cyt; red symbols) and extracellular GABA concentrations ([GABA]e; black symbols) in the absence of synaptic release. Filled circles—for 100 synapses, open circles—for 10 synapses. (d) Membrane potential dependence of the steady-state [GABA]cyt and [GABA]e at various rates of synaptic release. (e) The effect of depolarization on the dynamics of GAT-1 operation and [GABA]e with (red) and without (black) synaptic GABA release. (f) The effect of presynaptic depolarization due to action potentials on the accumulation of extracellular GABA. Inset: simulated voltage changes. (g) Left: an increase in [GABA]e due to presynaptic spike-induced depolarization at different firing frequencies (% change compared with [GABA]e in the absence of simulated presynaptic action potentials). Right: GAT-1 current dynamics at 160 Hz presynaptic firing frequency.
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f1: GAT-1 activity and [GABA]e simulations.(a) Current–voltage relationship of the steady-state GAT-1-mediated current (JGAT, molecules s−1; negative values—forward mode, positive values—reverse mode) at different concentrations of extracellular GABA (in μM). (b) Dependence of GAT-1 reversal potential (EGAT) on the concentration of extracellular GABA. (c) Membrane potential dependence of the steady-state cytosolic ([GABA]cyt; red symbols) and extracellular GABA concentrations ([GABA]e; black symbols) in the absence of synaptic release. Filled circles—for 100 synapses, open circles—for 10 synapses. (d) Membrane potential dependence of the steady-state [GABA]cyt and [GABA]e at various rates of synaptic release. (e) The effect of depolarization on the dynamics of GAT-1 operation and [GABA]e with (red) and without (black) synaptic GABA release. (f) The effect of presynaptic depolarization due to action potentials on the accumulation of extracellular GABA. Inset: simulated voltage changes. (g) Left: an increase in [GABA]e due to presynaptic spike-induced depolarization at different firing frequencies (% change compared with [GABA]e in the absence of simulated presynaptic action potentials). Right: GAT-1 current dynamics at 160 Hz presynaptic firing frequency.

Mentions: To examine the impact of membrane depolarization and extracellular GABA concentrations ([GABA]e) on transporter operation in steady state, we adapted a previously established kinetic model of GAT-1 (ref. 21; Fig. 1a). Two important features emerged. First, at the low [GABA]e observed experimentally under baseline conditions both in vivo and in vitro22, the current–voltage relationship of the transporter displayed marked outward rectification. Thus, there is little transporter activity when the membrane potential (Vm) is below the spiking threshold. Second, EGAT is highly sensitive to changes in ambient GABA at low [GABA]e (Fig. 1b). Therefore, even a small increase in [GABA]e readily pushes EGAT to suprathreshold Vm, suggesting that GAT would be operating in the forward mode on detection of GABA release by even depolarized neurons. Furthermore, [GABA]e rises reduce the rectification, thus further facilitating GABA uptake (Fig. 1a).


Synaptic GABA release prevents GABA transporter type-1 reversal during excessive network activity.

Savtchenko L, Megalogeni M, Rusakov DA, Walker MC, Pavlov I - Nat Commun (2015)

GAT-1 activity and [GABA]e simulations.(a) Current–voltage relationship of the steady-state GAT-1-mediated current (JGAT, molecules s−1; negative values—forward mode, positive values—reverse mode) at different concentrations of extracellular GABA (in μM). (b) Dependence of GAT-1 reversal potential (EGAT) on the concentration of extracellular GABA. (c) Membrane potential dependence of the steady-state cytosolic ([GABA]cyt; red symbols) and extracellular GABA concentrations ([GABA]e; black symbols) in the absence of synaptic release. Filled circles—for 100 synapses, open circles—for 10 synapses. (d) Membrane potential dependence of the steady-state [GABA]cyt and [GABA]e at various rates of synaptic release. (e) The effect of depolarization on the dynamics of GAT-1 operation and [GABA]e with (red) and without (black) synaptic GABA release. (f) The effect of presynaptic depolarization due to action potentials on the accumulation of extracellular GABA. Inset: simulated voltage changes. (g) Left: an increase in [GABA]e due to presynaptic spike-induced depolarization at different firing frequencies (% change compared with [GABA]e in the absence of simulated presynaptic action potentials). Right: GAT-1 current dynamics at 160 Hz presynaptic firing frequency.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4374149&req=5

f1: GAT-1 activity and [GABA]e simulations.(a) Current–voltage relationship of the steady-state GAT-1-mediated current (JGAT, molecules s−1; negative values—forward mode, positive values—reverse mode) at different concentrations of extracellular GABA (in μM). (b) Dependence of GAT-1 reversal potential (EGAT) on the concentration of extracellular GABA. (c) Membrane potential dependence of the steady-state cytosolic ([GABA]cyt; red symbols) and extracellular GABA concentrations ([GABA]e; black symbols) in the absence of synaptic release. Filled circles—for 100 synapses, open circles—for 10 synapses. (d) Membrane potential dependence of the steady-state [GABA]cyt and [GABA]e at various rates of synaptic release. (e) The effect of depolarization on the dynamics of GAT-1 operation and [GABA]e with (red) and without (black) synaptic GABA release. (f) The effect of presynaptic depolarization due to action potentials on the accumulation of extracellular GABA. Inset: simulated voltage changes. (g) Left: an increase in [GABA]e due to presynaptic spike-induced depolarization at different firing frequencies (% change compared with [GABA]e in the absence of simulated presynaptic action potentials). Right: GAT-1 current dynamics at 160 Hz presynaptic firing frequency.
Mentions: To examine the impact of membrane depolarization and extracellular GABA concentrations ([GABA]e) on transporter operation in steady state, we adapted a previously established kinetic model of GAT-1 (ref. 21; Fig. 1a). Two important features emerged. First, at the low [GABA]e observed experimentally under baseline conditions both in vivo and in vitro22, the current–voltage relationship of the transporter displayed marked outward rectification. Thus, there is little transporter activity when the membrane potential (Vm) is below the spiking threshold. Second, EGAT is highly sensitive to changes in ambient GABA at low [GABA]e (Fig. 1b). Therefore, even a small increase in [GABA]e readily pushes EGAT to suprathreshold Vm, suggesting that GAT would be operating in the forward mode on detection of GABA release by even depolarized neurons. Furthermore, [GABA]e rises reduce the rectification, thus further facilitating GABA uptake (Fig. 1a).

Bottom Line: Here we combined a realistic kinetic model of GAT-1 with experimental measurements of tonic GABAA receptor currents in ex vivo hippocampal slices to examine GAT-1 operation under varying network conditions.Our simulations predict that synaptic GABA release during network activity robustly prevents GAT-1 reversal.We conclude that sustained efflux of GABA through GAT-1 is unlikely to occur during physiological or pathological network activity.

View Article: PubMed Central - PubMed

Affiliation: UCL Institute of Neurology, Queen Square, London WC1N3BG, UK.

ABSTRACT
GABA transporters control extracellular GABA, which regulates the key aspects of neuronal and network behaviour. A prevailing view is that modest neuronal depolarization results in GABA transporter type-1 (GAT-1) reversal causing non-vesicular GABA release into the extracellular space during intense network activity. This has important implications for GABA uptake-targeting therapies. Here we combined a realistic kinetic model of GAT-1 with experimental measurements of tonic GABAA receptor currents in ex vivo hippocampal slices to examine GAT-1 operation under varying network conditions. Our simulations predict that synaptic GABA release during network activity robustly prevents GAT-1 reversal. We test this in the 0 Mg(2+) model of epileptiform discharges using slices from healthy and chronically epileptic rats and find that epileptiform activity is associated with increased synaptic GABA release and is not accompanied by GAT-1 reversal. We conclude that sustained efflux of GABA through GAT-1 is unlikely to occur during physiological or pathological network activity.

No MeSH data available.


Related in: MedlinePlus