Rapid volume pulsations of the extracellular space accompany epileptiform activity in trauma-injured neocortex and depend on the sodium-bicarbonate cotransporter NBCe1

Post traumatic epilepsy (PTE) is a treatment-resistant consequence of traumatic brain injury (TBI). Recently, it has been revealed that epileptiform activity in acute chemoconvulsant seizure models is accompanied by transient shrinkages of extracellular space (ECS) called rapid volume pulsations (RVPs). Shrinkage of the ECS surrounding neurons and glia may contribute to ictogenic hyperexcitability and hypersynchrony during the chronic phase of TBI. Here, we identify the phenomenon of RVPs occurring spontaneously in rat neocortex at ≥ 3 weeks after injury in the controlled cortical impact (CCI) model for PTE. We further report that blocking the electrogenic action of the astrocytic cotransporter NBCe1 with 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) eliminates both RVPs and epileptiform activity in ex-vivo CCI neocortical brain slices. We conclude that NBCe1-mediated extracellular volume shrinkage may represent a new target for therapeutic intervention in PTE.

Rapid volume pulsation of the extracellular space coincides with epileptiform activity in mice and depends on the NBCe1 transporter

KEY POINTS

Extracellular space (ECS) rapid volume pulsation (RVP) accompanying epileptiform activity is described for the first time. Such RVP occurs robustly in several in vitro and in vivo mouse models of epileptiform activity. In the in vitro 4-aminopyridine model of epileptiform activity, RVP depends on the activity of the electrogenic Na⁺ /HCO₃ ^- cotransporter (NBCe1). NBCe1 pharmacological inhibition suppresses RVP and epileptiform activity. Inhibition of changes in ECS volume may be a useful target in epilepsy patients who are resistant to current treatments. ​ ABSTRACT: The extracellular space (ECS) of the brain shrinks persistently by approximately 35% during epileptic seizures. Here we report the discovery of rapid volume pulsation (RVP), further transient drops in ECS volume which accompany events of epileptiform activity. These transient ECS contractions were observed in multiple mouse models of epileptiform activity both in vivo (bicuculline methiodide model) and in vitro (hyaluronan synthase 3 knock-out, picrotoxin, bicuculline and 4-aminopyridine models). By using the probe transients quantification (PTQ) method we show that individual pulses of RVP shrank the ECS by almost 15% in vivo. In the 4-aminopyridine in vitro model, the individual pulses of RVP shrank the ECS by more than 4%, and these transient changes were superimposed on a persistent ECS shrinkage of 36% measured with the real-time iontophoretic method. In this in vitro model, we investigated several channels and transporters that may be required for the generation of RVP and epileptiform activity. Pharmacological blockages of Na⁺ /K⁺ /2Cl^- cotransporter type 1 (NKCC1), K⁺ /Cl^- cotransporter (KCC2), the water channel aquaporin-4 (AQP4) and inwardly rectifying potassium channel 4.1 (Kir4.1) were ineffective in halting the RVP and the epileptiform activity. In contrast, pharmacological blockade of the electrogenic Na⁺ /HCO₃ ^- cotransporter (NBCe1) by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS) eliminated both the RVP and the persistent ECS shrinkage. Importantly, this blocker also stopped the epileptiform activity. These results demonstrate that RVP is closely associated with epileptiform activity across several models of epileptiform activity and therefore the underlying mechanism could potentially represent a novel target for epilepsy management and treatment.

Real-time Iontophoresis with Tetramethylammonium to Quantify Volume Fraction and Tortuosity of Brain Extracellular Space

This review describes the basic concepts and protocol to perform the real-time iontophoresis (RTI) method, the gold-standard to explore and quantify the extracellular space (ECS) of the living brain. The ECS surrounds all brain cells and contains both interstitial fluid and extracellular matrix. The transport of many substances required for brain activity, including neurotransmitters, hormones, and nutrients, occurs by diffusion through the ECS. Changes in the volume and geometry of this space occur during normal brain processes, like sleep, and pathological conditions, like ischemia. However, the structure and regulation of brain ECS, particularly in diseased states, remains largely unexplored. The RTI method measures two physical parameters of living brain: volume fraction and tortuosity. Volume fraction is the proportion of tissue volume occupied by ECS. Tortuosity is a measure of the relative hindrance a substance encounters when diffusing through a brain region as compared to a medium with no obstructions. In RTI, an inert molecule is pulsed from a source microelectrode into the brain ECS. As molecules diffuse away from this source, the changing concentration of the ion is measured over time using an ion-selective microelectrode positioned roughly 100 µm away. From the resulting diffusion curve, both volume fraction and tortuosity can be calculated. This technique has been used in brain slices from multiple species (including humans) and in vivo to study acute and chronic changes to ECS. Unlike other methods, RTI can be used to examine both reversible and irreversible changes to the brain ECS in real time.

T-type calcium channels contribute to calcium disturbances in brain during hyponatremia

Disturbance of calcium homeostasis is implicated in the normal process of aging and brain pathology prevalent in the elderly such as Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis. Previous studies demonstrated that applying a hyponatremic iso-osmotic (low-NaCl) artificial cerebrospinal fluid (ACSF) to rodent hippocampus causes extracellular calcium to rapidly decrease. Restoring normonatremia after low-NaCl treatment causes a rapid increase in extracellular calcium that overshoots baseline. This study examined the amplitude, timing, and mechanism of these surprising calcium changes. We also tested whether hyponatremia increased calcium entry into brain cells or calcium binding to chondroitin sulfate (CS), a negatively charged constituent of the extracellular matrix (ECM) that may be occupied by sodium during normonatremia. We report three major findings. First we show that CS does not contribute to extracellular calcium changes during low-NaCl treatments. Second, we show that the time to minimum extracellular calcium during low-NaCl treatment is significantly shorter than the time to maximum extracellular calcium in recovery from low-NaCl treatment. Third, we show that the decrease in extracellular calcium observed during hyponatremia is attenuated by ML 218, a highly selective T-type calcium channel blocker. Together these data suggest that calcium rapidly enters cells at the onset of low-NaCl treatment and is extruded from cells when normonatremia is restored. Calcium binding to CS does not significantly contribute to calcium changes in brain during hyponatremia. Differences in timing suggest that extracellular calcium changes during and in recovery from hyponatremia occur by distinct mechanisms or by a multistep process. Finally, partial block of extracellular calcium influx by ML 218 suggests that T-type channels are involved in calcium entering cells during hyponatremia. Given the high prevalence of hyponatremia among elderly patients and the growing understanding of calcium's role in multiple neurologic pathologies, this study promotes a novel approach for studying and potentially preventing the effects of hyponatremia on calcium dysregulation in brain tissue.