Biophysical Modeling Suggests Optimal Drug Combinations for Improving the Efficacy of GABA Agonists after Traumatic Brain Injuries

Abstract

Traumatic brain injuries (TBI) lead to dramatic changes in the surviving brain tissue. Altered ion concentrations, coupled with changes in the expression of membrane-spanning proteins, create a post-TBI brain state that can lead to further neuronal loss caused by secondary excitotoxicity. Several GABA receptor agonists have been tested in the search for neuroprotection immediately after an injury, with paradoxical results. These drugs not only fail to offer neuroprotection, but can also slow down functional recovery after TBI. Here, using computational modeling, we provide a biophysical hypothesis to explain these observations. We show that the accumulation of intracellular chloride ions caused by a transient upregulation of Na⁺-K⁺-2Cl^- (NKCC1) co-transporters as observed following TBI, causes GABA receptor agonists to lead to excitation and depolarization block, rather than the expected hyperpolarization. The likelihood of prolonged, excitotoxic depolarization block is further exacerbated by the extremely high levels of extracellular potassium seen after TBI. Our modeling results predict that the neuroprotective efficacy of GABA receptor agonists can be substantially enhanced when they are combined with NKCC1 co-transporter inhibitors. This suggests a rational, biophysically principled method for identifying drug combinations for neuroprotection after TBI.

Keywords: GABA; chloride; depolarization block; neuroprotection; potassium.

FIG. 1.

Depolarization block in the regular spiking (RS) neuron model. (A) Membrane potential traces plotted for three values of injected current. Note the lack of action potentials when the cell is injected with a large 2700 pA current. (B) Median membrane potential increases as a function of injected current, as expected. (C) Frequency–current relationship (F–I curve) of the RS neuron model. The minimum current injection that results in the cell entering depolarization block (I_DB) is apparent as a dramatic decrease in firing rate.

FIG. 2.

Ionic concentration ranges for extracellular potassium and intracellular chloride in normal, epileptic, and mild versus severe post-traumatic brain injury (TBI) brain states. This schematic is derived from values reported in the following references.^,,,,,,,,

FIG. 3.

The impact of potassium and chloride ion concentrations on regular spiking (RS) neuronal firing and depolariziaton block. (A) The membrane potential of the RS neuron model under normal physiological levels of extracellular potassium and intracellular chloride is shown in the lower panel. The cell maintains a low firing rate, with occasional spikes driven by the stochastic synaptic inputs. The precise values of extracellular potassium and intracellular chloride are shown in the upper panel. (B) Same as in A, but for high extracellular potassium conditions similar to those seen during epileptic seizures. Note the increase in firing rate as a function of increased extracellular potassium. (C) Same as in A, but for high extracellular potassium plus high intracellular chlroide conditions similar to those seen after a traumatic brain injury. The cell first increases its firing rate, but then enters depolariziation block under these conditions. (D) Median membrane potential of the RS neuron model as a function of both extracellular potassium and intracellular chloride. (E) Same as in D, but for firing rate of the RS neuron model. Note the lack of firing in the upper right quadrant, corresponding to depolariziaton block of the RS neuron under high extracellular potassium and high intracellular chloride conditions.

FIG. 4.

Effect of GABA_A receptor agonists on the response of regular spiking (RS) neuron model in pathological brain states, in the presence and absence of Na⁺-K⁺-2Cl^- (NKCC1) co-transporter blockers. (A) Response of the RS neuron model to GABA_A receptor agonists in an epileptic brain state. The values of extracellular potassium and intrcellular chloride are shown in the upper panel. Under these epileptic conditions, GABA_A receptor agonists can still help hyperpolarize the neuron, as would be expected from their common use as anti-epileptic drugs. (B) Same as in A, but for a post-traumatic brain injury (TBI) brain state. The GABA_A agonist is unable to rescue the neuron from depolarizaiton block. (C) Response of the RS neuron model to a combination of GABA_A receptor agonist and NKCC1 co-transporter blocker in a post-TBI brain state. Dotted line signifies the change in intracellular chloride caused by the effect of the NKCC1 co-transporter blocker. The combination of the two drugs is able to rescue the model neuron from depolariziation block.

FIG. 5.

GABA_A agonists, only in combination with Na⁺-K⁺-2Cl^- (NKCC1) co-transporter blockers, can rescue neurons from depolariziaton block across a range of pathological ion concentations. (A) Effect of GABA_A receptor agonists on the percent change in I_DB of the regular spiking (RS) neuron model when K_outside = 12.5 mM, reflecting mild traumatic brain injury (TBI). Note that values >0 reflect a therapeutic effect, whereas values <0 reflect a pathological worsening. Under these potassium conditions, GABA_A receptor agonists can reduce the propensity to depolarization block when intracellular chloride concentrations are close to the normal physiological range, but exert little or negative effect when chloride concentrations inside the cell exceed the physiological range. (B) Same as in A, but for the combination of a GABA_A receptor agonist and NKCC1 co-transporter blocker. By blocking NKCC1 co-transporter and hence reducing intracellular chloride concentration, this combination of drugs can rescue the model neuron from depolariziation block across a wide range of intracellular chloride concentrations. This suggests that this combination of drugs can have a therapeutic benefit in post-TBI brain states. (C, D) Same as A, B, but for K_outside = 18.5 mM, reflecting more severe TBI.