Diuretics as Antiepileptic Drugs: Should We Go with the Flow?

Effects of the NKCC1 inhibitors bumetanide, azosemide, and torasemide alone or in combination with phenobarbital on seizure threshold in epileptic and nonepileptic mice

The sodium-potassium-chloride (Na-K-Cl) cotransporter NKCC1 is found in the plasma membrane of a wide variety of cell types, including neurons, glia and endothelial cells in the brain. Increased expression of neuronal NKCC1 has been implicated in several brain disorders, including neonatal seizures and epilepsy. The loop diuretic and NKCC inhibitor bumetanide has been evaluated as an antiseizure agent alone or together with approved antiseizure drugs such as phenobarbital (PB) in pre-clinical and clinical studies with varying results. The equivocal efficacy of bumetanide may be a result of its poor brain penetration. We recently reported that the loop diuretic azosemide is more potent to inhibit NKCC1 than bumetanide. In contrast to bumetanide, azosemide is not acidic, which should favor its brain penetration. Thus, azosemide may be a promising alternative to bumetanide for treatment of brain disorders such as epilepsy. In the present study, we determined the effect of azosemide and bumetanide on seizure threshold in adult epileptic mice. A structurally related non-acidic loop diuretic, torasemide, which also blocks NKCC1, was included in the experiments. The drug effects were assessed by determing the maximal electroshock seizure threshold (MEST) in epileptic vs. nonepileptic mice. Epilepsy was induced by pilocarpine, which was shown to produce long-lasting increases in NKCC1 in the hippocampus, whereas MEST did not alter NKCC1 mRNA in this region. None of the three loop diuretics increased MEST or the effect of PB on MEST in nonepileptic mice. In epileptic mice, all three diuretics significantly increased PB's seizure threshold increasing efficacy, but the effect was variable upon repeated MEST determinations and not correlated with the drugs' diuretic potency. These data may indicate that inhibition of NKCC1 by loop diuretics is not an effective means of increasing seizure threshold in adult epilepsy.

Role of NKCC1 and KCC2 in Epilepsy: From Expression to Function

As a main inhibitory neurotransmitter in the central nervous system, γ-aminobutyric acid (GABA) activates chloride-permeable GABAa receptors (GABAa Rs) and induces chloride ion (Cl⁻) flow, which relies on the intracellular chloride concentration ([Cl⁻]_i) of the postsynaptic neuron. The Na-K-2Cl cotransporter isoform 1 (NKCC1) and the K-Cl cotransporter isoform 2 (KCC2) are two main cation-chloride cotransporters (CCCs) that have been implicated in human epilepsy. NKCC1 and KCC2 reset [Cl⁻]_i by accumulating and extruding Cl⁻, respectively. Previous studies have shown that the profile of NKCC1 and KCC2 in neonatal neurons may reappear in mature neurons under some pathophysiological conditions, such as epilepsy. Although increasing studies focusing on the expression of NKCC1 and KCC2 have suggested that impaired chloride plasticity may be closely related to epilepsy, additional neuroelectrophysiological research aimed at studying the functions of NKCC1 and KCC2 are needed to understand the exact mechanism by which they induce epileptogenesis. In this review, we aim to briefly summarize the current researches surrounding the expression and function of NKCC1 and KCC2 in epileptogenesis and its implications on the treatment of epilepsy. We will also explore the potential for NKCC1 and KCC2 to be therapeutic targets for the development of novel antiepileptic drugs.

Chloride cotransporter KCC2 is essential for GABAergic inhibition in the SCN

One of the principal neurotransmitters of the central nervous system is GABA. In the adult brain, GABA is predominantly inhibitory, but there is growing evidence indicating that GABA can shift to excitatory action depending on environmental conditions. In the mammalian central circadian clock of the suprachiasmatic nucleus (SCN) GABAergic activity shifts from inhibition to excitation when animals are exposed to long day photoperiod. The polarity of the GABAergic response (inhibitory versus excitatory) depends on the GABA equilibrium potential determined by the intracellular Cl- concentration ([Cl-]i). Chloride homeostasis can be regulated by Cl- cotransporters like NKCC1 and KCC2 in the membrane, but the mechanisms for maintaining [Cl-]i are still under debate. This study investigates the role of KCC2 on GABA-induced Ca2+ transients in SCN neurons from mice exposed to different photoperiods. We show for the first time that blocking KCC2 with the newly developed blocker ML077 can cause a shift in the polarity of the GABAergic response. This will increase the amount of excitatory responses in SCN neurons and thus cause a shift in excitatory/inhibitory ratio. These results indicate that KCC2 is an essential component in regulating [Cl-]i and the equilibrium potential of Cl- and thereby determining the sign of the GABAergic response. Moreover, our data suggest a role for the Cl- cotransporters in the switch from inhibition to excitation observed under long day photoperiod.

Activity-dependent astrocyte swelling is mediated by pH-regulating mechanisms

During neuronal activity in the mammalian brain, the K⁺ released into the synaptic space is initially buffered by the astrocytic compartment. In parallel, the extracellular space (ECS) shrinks, presumably due to astrocytic cell swelling. With the Na⁺ /K⁺ /2Cl^- cotransporter and the Kir4.1/AQP4 complex not required for the astrocytic cell swelling in the hippocampus, the molecular mechanisms underlying the activity-dependent ECS shrinkage have remained unresolved. To identify these molecular mechanisms, we employed ion-sensitive microelectrodes to measure changes in ECS, [K⁺ ]_o and [H⁺ ]_o /pH_o during electrical stimulation of rat hippocampal slices. Transporters and receptors responding directly to the K⁺ and glutamate released into the extracellular space (the K⁺ /Cl^- cotransporter, KCC, glutamate transporters and G protein-coupled receptors) did not modulate the extracellular space dynamics. The HCO3--transporting mechanism, which in astrocytes mainly constitutes the electrogenic Na⁺ / HCO3- cotransporter 1 (NBCe1), is activated by the K⁺ -mediated depolarization of the astrocytic membrane. Inhibition of this transporter reduced the ECS shrinkage by ∼25% without affecting the K⁺ transients, pointing to NBCe1 as a key contributor to the stimulus-induced astrocytic cell swelling. Inhibition of the monocarboxylate cotransporters (MCT), like-wise, reduced the ECS shrinkage by ∼25% without compromising the K⁺ transients. Isosmotic reduction of extracellular Cl^- revealed a requirement for this ion in parts of the ECS shrinkage. Taken together, the stimulus-evoked astrocytic cell swelling does not appear to occur as a direct effect of the K⁺ clearance, as earlier proposed, but partly via the pH-regulating transport mechanisms activated by the K⁺ -induced astrocytic depolarization and the activity-dependent metabolism.

Outside the box: Medications worth considering when traditional antiepileptic drugs have failed

PURPOSE

Review and discuss medications efficacious for seizure control, despite primary indications for other diseases, as treatment options in patients who have failed therapy with traditional antiepileptic drugs (AEDs).

METHODS

Literature searches were conducted utilizing PubMed and MEDLINE databases employing combinations of search terms including, but not limited to, "epilepsy", "refractory", "seizure", and the following medications: acetazolamide, amantadine, bumetanide, imipramine, lidocaine, verapamil, and various stimulants.

RESULTS

Data from relevant case studies, retrospective reviews, and available clinical trials were gathered, analyzed, and reported. Experience with acetazolamide, amantadine, bumetanide, imipramine, lidocaine, verapamil, and various stimulants show promise for cases of refractory epilepsy in both adults and children. Many medications lack large scale, randomized clinical trials, but the available data is informative when choosing treatment for patients that have failed traditional epilepsy therapies.

CONCLUSIONS

All neurologists have encountered a patient that failed nearly every AED, diet, and surgical option. For these patients, we often seek fortuitous discoveries within small series and case reports, hoping to find a treatment that might help the patient. In the present review, we describe medications for which antiepileptic effect has been ascribed after they were introduced for other indications.

Furosemide depresses the presynaptic fiber volley and modifies frequency-dependent axonal excitability in rat hippocampus

The loop diuretic furosemide is known to have anticonvulsant effects, believed to be exerted through blockade of glial Na⁺-K⁺-2Cl^- cotransport causing altered volume regulation in brain tissue. The possibility that direct effects of furosemide on neuronal properties could also be involved is supported by previous observations, but such effects have not been thoroughly investigated. In the present study we show that furosemide has two opposing effects on stimulus-induced postsynaptic excitation in the nonepileptic rat hippocampal slice: 1) an enhancement of e-s coupling, which depended on intact GABA_A transmission and was partially mimicked by selective blockade of K⁺-2Cl^- cotransport, and 2) a decrement of field excitatory postsynaptic potentials. The balance between these effects varied, depending on the amount of synaptic drive. In addition, the compound action potential (fiber volley) recorded from the stimulated Schaffer collateral axons in stratum radiatum showed a progressive decrease during perfusion of furosemide. This effect was activity-independent, was mimicked by the stilbene derivative DIDS, and could be reproduced on fiber volleys in the alveus. Furosemide also reduced the initial enhancement of the fiber volley observed during trains of high-frequency stimulation (HFS). Results of hyperosmotic expansion of the extracellular volume, with 30 mM sucrose, indicated that both the induction and antagonism of the HFS-induced enhancement were independent of signaling via the extracellular space. Furosemide caused an increased decay of paired-pulse-induced supranormal axonal excitability, which was antagonized by ZD7288. We conclude that furosemide decreases axonal excitability and prevents HFS-induced hyperexcitability via mechanisms downstream of blockage of anion transport, which could include hyperpolarization of axonal membranes.NEW & NOTEWORTHY This study shows that the anion transporter antagonists furosemide and DIDS cause a marked decrease of axonal excitability in rat hippocampal CA1 region and prevent the induction of activity-dependent hyperexcitability in Schaffer collateral axons. The data are consistent with direct effects on axonal membrane properties. We also find that activity-dependent enhancement and depression of axonal excitability can be modified independently, suggesting that these events are governed by different underlying processes.

A novel mechanism for the anticonvulsant effect of furosemide in rat hippocampus in vitro

Though both in vivo and in vitro studies have demonstrated an anticonvulsant effect of the loop diuretic furosemide, the precise mechanism behind this effect is still debated. The current study investigates the effect of furosemide on Cs-induced epileptiform activity (Cs-FP) evoked in area CA1 of rat hippocampal slices in the presence of Cs(+) (5mM) and ionotropic glutamatergic and GABAergic receptor antagonists. As this model diverges in several respects from other epilepsy models it can offer new insight into the mechanism behind the anticonvulsive effect of furosemide. The present study shows that furosemide suppresses the Cs-FP in a dose-dependent manner with a near complete block at concentrations ≥ 1.25 mM. Because furosemide targets several types of ion transporters we examined the effect of more selective antagonists. Bumetanide (20 μM), which selectively inhibits the Na-K-2Cl co-transporter (NKCC1), had no significant effect on the Cs-FP. VU0240551 (10 μM), a selective antagonist of the K-Cl co-transporter (KCC2), reduced the ictal-like phase by 51.73 ± 8.5% without affecting the interictal-like phase of the Cs-FP. DIDS (50 μM), a nonselective antagonist of Cl(-)/HCO3(-)-exchangers, Na(+)-HCO3(-)-cotransporters, chloride channels and KCC2, suppressed the ictal-like phase by 60.8 ± 8.1% without affecting the interictal-like phase. At 500 μM, DIDS completely suppressed the Cs-FP. Based on these results we propose that the anticonvulsant action of furosemide in the Cs(+)-model is exerted through blockade of the neuronal KCC2 and Na(+)-independent Cl(-)/HCO3(-)-exchanger (AE3) leading to stabilization of the activity-induced intracellular acidification in CA1 pyramidal neurons.

Proconvulsive effect of hydrochlorothiazide in an in vitro rat seizure model

OBJECTIVES

Protective effects of diuretics, particularly of hydrochlorothiazide (HCT), for the development of epilepsy have been described in vivo. However, its mechanism of action is unknown.

MATERIALS AND METHODS

Extracellular field potentials were recorded from the CA1- and CA3-subfields of the hippocampus of rats. Epileptiform discharges were induced by omission of Mg2+ from the artificial cerebrospinal fluid (ACSF). HCT was added to the ACSF at a concentration of 2 mmol/l, 0.2 mmol/l or 0.02 mmol/l. Frequency, amplitude and duration of the epileptiform discharges were evaluated. Long-term potentiation (LTP) was induced with and without the presence of HCT (n=6; 2 mmol/l). In addition, rats were injected with HCT (n=4) or saline (n=2), and the brain tissue was analyzed using HPLC.

RESULTS

Application of 0.02, 0.2, and 2 mmol/l HCT accelerated the frequency of discharges by 50%, 91%, and 100%, respectively. The amplitude of burst discharges also increased by 9%, 54%, and 300%, and the duration of epileptiform discharges increased by 10%, 30% and 120%. All parameters returned close to the basal levels after 60min washout of the substance. HCT increased the electrical evoked potentials but did not affect the LTP in hippocampal tissues. There was no evidence of HCT in the rat brain after intraperitoneal injection.

CONCLUSION

Exposure of hippocampal slices to HCT enhanced epileptiform activity in a dose-dependent manner. In addition, HCT does not seem to cross the blood brain barrier in rats. Thus, the anticonvulsive effect of HCT most likely is not through direct neuronal effect.

Impact of acetazolamide and CPAP on cortical activity in obstructive sleep apnea patients

STUDY OBJECTIVES

1) To investigate the impact of acetazolamide, a drug commonly prescribed for altitude sickness, on cortical oscillations in patients with obstructive sleep apnea syndrome (OSAS). 2) To examine alterations in the sleep EEG after short-term discontinuation of continuous positive airway pressure (CPAP) therapy.

DESIGN

Data from two double-blind, placebo-controlled randomized cross-over design studies were analyzed.

SETTING

Polysomnographic recordings in sleep laboratory at 490 m and at moderate altitudes in the Swiss Alps: 1630 or 1860 m and 2590 m.

PATIENTS

Study 1: 39 OSAS patients. Study 2: 41 OSAS patients.

INTERVENTIONS

Study 1: OSAS patients withdrawn from treatment with CPAP. Study 2: OSAS patients treated with autoCPAP. Treatment with acetazolamide (500-750 mg) or placebo at moderate altitudes.

MEASUREMENTS AND RESULTS

An evening dose of 500 mg acetazolamide reduced slow-wave activity (SWA; approximately 10%) and increased spindle activity (approximately 10%) during non-REM sleep. In addition, alpha activity during wake after lights out was increased. An evening dose of 250 mg did not affect these cortical oscillations. Discontinuation of CPAP therapy revealed a reduction in SWA (5-10%) and increase in beta activity (approximately 25%).

CONCLUSIONS

The higher evening dose of 500 mg acetazolamide showed the "spectral fingerprint" of Benzodiazepines, while 250 mg acetazolamide had no impact on cortical oscillations. However, both doses had beneficial effects on oxygen saturation and sleep quality.

Rapamycin down-regulates KCC2 expression and increases seizure susceptibility to convulsants in immature rats

Seizure susceptibility to neurological insults, including chemical convulsants, is age-dependent and most likely reflective of overall differences in brain excitability. The molecular and cellular mechanisms underlying development-dependent seizure susceptibility remain to be fully understood. Because the mammalian target of rapamycin (mTOR) pathway regulates neurite outgrowth, synaptic plasticity and cell survival, thereby influencing brain development, we tested if exposure of the immature brain to the mTOR inhibitor rapamycin changes seizure susceptibility to neurological insults. We found that inhibition of mTOR by rapamycin in immature rats (3-4 weeks old) increases the severity of seizures induced by pilocarpine, including lengthening the total seizure duration and reducing the latency to the onset of seizures. Rapamycin also reduces the minimal dose of pentylenetetrazol (PTZ) necessary to induce clonic seizures. However, in mature rats, rapamycin does not significantly change the seizure sensitivity to pilocarpine and PTZ. Likewise, kainate sensitivity was not significantly affected by rapamycin treatment in either mature or immature rats. Additionally, rapamycin treatment down-regulates the expression of potassium-chloride cotransporter 2 (KCC2) in the thalamus and to a lesser degree in the hippocampus. Pharmacological inhibition of thalamic mTOR or KCC2 increases susceptibility to pilocarpine-induced seizure in immature rats. Thus, our study suggests a role for the mTOR pathway in age-dependent seizure susceptibility.

Diuretics and epilepsy: will the past and present meet?

Clinical studies from over half a century ago suggested efficacy of a variety of diuretics in focal and generalized epilepsies as well as in status epilepticus, but these findings have not been translated into modern epilepsy training or practice. Recent advances in our understanding of neuronal maturation and the pathophysiology of neonatal seizures provide fresh insight into the mechanisms by which diuretics might reduce susceptibility to seizures. In vitro and in vivo rodent studies and human epilepsy surgical cases have shown that specific diuretic agents targeting the cation-chloride cotransporters decrease neuronal synchrony and neuronal hyperexcitability. These agents are thought to convey their antiepileptic activity by either expanding the extracellular space or promoting a cellular chloride transport balance that reflects a more developmentally "mature," less excitable state. It may be time to reexamine whether diuretics could serve as adjunctive therapies in the treatment of refractory epilepsies.

Cation-chloride cotransporters and neuronal function

Recent years have witnessed a steep increase in studies on the diverse roles of neuronal cation-chloride cotransporters (CCCs). The versatility of CCC gene transcription, posttranslational modification, and trafficking are on par with what is known about ion channels. The cell-specific and subcellular expression patterns of different CCC isoforms have a key role in modifying a neuron's electrophysiological phenotype during development, synaptic plasticity, and disease. While having a major role in controlling responses mediated by GABA(A) and glycine receptors, CCCs also show close interactions with glutamatergic signaling. A cross-talk among CCCs and trophic factors is important in short-term and long-term modification of neuronal properties. CCCs appear to be multifunctional proteins that are also involved in shaping neuronal structure at various stages of development, from stem cells to synaptogenesis. The rapidly expanding work on CCCs promotes our understanding of fundamental mechanisms that control brain development and functions under normal and pathophysiological conditions.

Tonic activation of group I mGluRs modulates inhibitory synaptic strength by regulating KCC2 activity

The furosemide-sensitive potassium-chloride cotransporter (KCC2) plays an important role in establishing the intracellular chloride concentration in many neurons within the central nervous system. Consequently, modulation of KCC2 function will regulate the reversal potential for synaptic GABAergic inputs, thus setting the strength of inhibitory transmission. We show that tonic activation of group I metabotropic glutamate receptors (mGluR1s) regulates inhibitory synaptic strength via modulation of KCC2 function in pyramidal neurons of the hippocampal CA3 area. Specifically, group I mGluRs signal via activation of a protein kinase C-dependent pathway to alter KCC2 activity, thereby altering the intracellular chloride concentration, and thus inhibitory synaptic input. This interaction between the glutamatergic and chloride transport systems highlights a novel homeostatic mechanism whereby ambient glutamate levels directly regulate the inhibitory synaptic tone by setting the activity level of KCC2. Thus, mGluRs are poised to play a pivotal role in providing a direct interplay between the excitatory and inhibitory systems in the hippocampus.

GABAergic input onto CA3 hippocampal interneurons remains shunting throughout development

In hippocampus, the net flow of excitability is controlled by inhibitory input provided by the many populations of local circuit inhibitory interneurons. In principal cells, GABA(A) receptor-mediated synaptic input undergoes a highly coordinated shift from depolarizing early in life to a more conventional hyperpolarizing inhibition on maturation. This switch in inhibitory input polarity is controlled by the developmental regulation of two chloride cotransporters (NKCC1 and KCC2) that results in a net shift from high to low intracellular Cl(-). Whether inhibitory input onto inhibitory interneurons demonstrates a similar developmental shift in intracellular Cl(-) is unexplored. Using the gramicidin perforated-patch configuration, we recorded from CA3 hippocampal stratum lucidum interneurons and pyramidal cells to monitor inhibitory input across a broad developmental range. GABA(A) receptor-mediated synaptic input onto stratum lucidum inhibitory interneurons was shunting in nature across the entire developmental age range tested, as resting membrane potential and the IPSC reversal potential remained within a few millivolts (1-4 mV) between postnatal day 5 (P5) and P31. Furthermore, sensitivity to block of the two chloride cotransporters KCC2 and NKCC1 did not differ across the same age range, suggesting that their relative expression is fixed across development. In contrast, pyramidal cell synaptic inhibition demonstrated the well described switch from depolarizing to hyperpolarizing over the same age range. Thus, in contrast to principal cells, inhibitory synaptic input onto CA3 interneurons remains shunting throughout development.

Differential effects of cation-chloride co-transport-blocking diuretics in a rat hippocampal slice model of epilepsy

The cation-Cl- co-transport-blocking loop diuretics have clinically known anticonvulsant activity, though they can also induce seizures. We explored the effects of ethacrynic acid (ETA), furosemide (FUR) and bumetanide (BUM), prototypical blockers of cation-Cl- co-transport, on the epileptiform field potentials induced in CA3 area of hippocampal slices from 5-weeks-old rats by a high K+-low Ca2+ perfusion fluid. That milieu induces frequent spontaneous field bursts, making single fimbrial stimuli to evoke several repetitive population spikes, of increased amplitude. ETA (0.25-1 mM) concentration-dependently reduced spontaneous field bursting, up to terminating it. FUR, 5 mM also inhibited spontaneous field bursting, while BUM (12.5-100 microM) only presented an inconsistent tendency. Both ETA and FUR showed a less marked ability to depress the evoked responses, but approximately mM concentrations significantly reduced the number of repetitive population spikes and their amplitude. BUM only modestly reduced population spike amplitude, without concentration-dependence. This study shows that K+-Cl- co-transport-blocking diuretics ETA and FUR inhibit high K+-induced epileptiform activity in hippocampal slices from (nearly) adult rats, while the Na+-K+-2Cl- co-transport-preferring diuretic BUM had only negligible activity. These results support that neuronal K+-Cl- co-transport-blockade provides antiepileptic effects.

Reduced anesthetization during the intracarotid amobarbital (Wada) test in patients taking carbonic anhydrase-inhibiting medications

PURPOSE

Failure to show adequate anesthetization during the intracarotid amobarbital procedure (IAP or "Wada test") is a rare complication. After an unusually high rate of recent anesthetization failures, we sought to determine the frequency of reduced anesthetization and any common factors underlying these failures.

METHODS

We reviewed the records of all patients who underwent IAP tests through the UCLA Seizure Disorder Center between September 1999 and May 2002. Age, date, epileptogenic focus, radiologist, and current medications were all considered.

RESULTS

Of a total of 56 patients who underwent our intracarotid amobarbital examination, 11 (19.6%) showed either very rapid recovery (<or=1 min) or anesthetization failure. Of these, 10 (91%) of 11 were taking a medication with some carbonic anhydrase-inhibiting (CAI) properties (topiramate (TPM), n=7; zonisamide (ZNS), n=2; hydrochlorothiazide and furosemide, n=1 each). The only patient of 40 (2.5%) who was not taking any CAI drugs and showed an early IAP recovery (55 s) had recently discontinued TPM. IAPs performed on patients after weaning from TPM showed a correlation between length of drug discontinuation and duration of anesthesia effect.

CONCLUSIONS

Our data strongly suggest that recent failures of anesthetization during the IAP are associated with a possible interaction between amobarbital and medications possessing CAI properties. We suggest that the IAP be performed only after >/=8 weeks after discontinuation of such medications.