A potential role for astrocytes in mediating the antiepileptic actions of furosemide in vitro

GPR37L1 controls maturation and organization of cortical astrocytes during development

Astrocyte maturation is crucial to proper brain development and function. This maturation process includes the ramification of astrocytic morphology and the establishment of astrocytic domains. While this process has been well-studied, the mechanisms by which astrocyte maturation is initiated are not well understood. GPR37L1 is an astrocyte-specific G protein-coupled receptor (GPCR) that is predominantly expressed in mature astrocytes and has been linked to the modulation of seizure susceptibility in both humans and mice. To investigate the role of GPR37L1 in astrocyte biology, RNA-seq analyses were performed on astrocytes immunopanned from P7 Gpr37L1-/- knockout (L1KO) mouse cortex and compared to those from wild-type (WT) mouse cortex. These RNA-seq studies revealed that pathways involved in central nervous system development were altered and that L1KO cortical astrocytes express lower amounts of mature astrocytic genes compared to WT astrocytes. Immunohistochemical studies of astrocytes from L1KO mouse brain revealed that these astrocytes exhibit overall shorter total process length, and are also less complex and spaced further apart from each other in the mouse cortex. This work sheds light on how GPR37L1 regulates cellular processes involved in the control of astrocyte biology and maturation.

Furosemide prevents membrane KCC2 downregulation during convulsant stimulation in the hippocampus

In adults, γ-aminobutyric acid (GABA) type A receptor (GABA_AR)-mediated inhibition depends on the maintenance of low intracellular chloride anion concentration through neuron-specific potassium-chloride cotransporter-2 (KCC2). KCC2 has been widely reported to have a plasticity change during the course of epilepsy development, with an early downregulation and late recovery in neuronal cell membranes after epileptic stimulation, which facilitates epileptiform burst activity. Furosemide is a clinical loop diuretic that inhibits KCC2. Here, we first confirmed that furosemide pretreatment could effectively prevented convulsant stimulation-induced neuronal membrane KCC2 downregulation in the hippocampus in both in vivo and in vitro cyclothiazide-induced seizure model. Second, we verified that furosemide pretreatment rescued KCC2 function deficits, as indicated by E _GABA depolarizing shift and GABA_AR inhibitory function impairment induced via cyclothiazide treatment. Further, we demonstrated that furosemide also suppressed cyclothiazide-induced epileptiform burst activity in cultured hippocampal neurons and lowered the mortality rate during acute seizure induction. Overall, furosemide prevents membrane KCC2 downregulation during acute seizure induction, restores KCC2-mediated GABA inhibition, and interrupts the progression from acute seizure to epileptogenesis.

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.

Neuronal and astroglial correlates underlying spatiotemporal intrinsic optical signal in the rat hippocampal slice

Widely used for mapping afferent activated brain areas in vivo, the label-free intrinsic optical signal (IOS) is mainly ascribed to blood volume changes subsequent to glial glutamate uptake. By contrast, IOS imaged in vitro is generally attributed to neuronal and glial cell swelling, however the relative contribution of different cell types and molecular players remained largely unknown. We characterized IOS to Schaffer collateral stimulation in the rat hippocampal slice using a 464-element photodiode-array device that enables IOS monitoring at 0.6 ms time-resolution in combination with simultaneous field potential recordings. We used brief half-maximal stimuli by applying a medium intensity 50 Volt-stimulus train within 50 ms (20 Hz). IOS was primarily observed in the str. pyramidale and proximal region of the str. radiatum of the hippocampus. It was eliminated by tetrodotoxin blockade of voltage-gated Na(+) channels and was significantly enhanced by suppressing inhibitory signaling with gamma-aminobutyric acid(A) receptor antagonist picrotoxin. We found that IOS was predominantly initiated by postsynaptic Glu receptor activation and progressed by the activation of astroglial Glu transporters and Mg(2+)-independent astroglial N-methyl-D-aspartate receptors. Under control conditions, role for neuronal K(+)/Cl(-) cotransporter KCC2, but not for glial Na(+)/K(+)/Cl(-) cotransporter NKCC1 was observed. Slight enhancement and inhibition of IOS through non-specific Cl(-) and volume-regulated anion channels, respectively, were also depicted. High-frequency IOS imaging, evoked by brief afferent stimulation in brain slices provide a new paradigm for studying mechanisms underlying IOS genesis. Major players disclosed this way imply that spatiotemporal IOS reflects glutamatergic neuronal activation and astroglial response, as observed within the hippocampus. Our model may help to better interpret in vivo IOS and support diagnosis in the future.

The K+-Cl cotransporter KCC2 promotes GABAergic excitation in the mature rat hippocampus

GABAergic excitatory [K(+)](o) transients can be readily evoked in the mature rat hippocampus by intense activation of GABA(A) receptors (GABA(A)Rs). Here we show that these [K(+)](o) responses induced by high-frequency stimulation or GABA(A) agonist application are generated by the neuronal K(+)-Cl() cotransporter KCC2 and that the transporter-mediated KCl extrusion is critically dependent on the bicarbonate-driven accumulation of Cl() in pyramidal neurons. The mechanism underlying GABAergic [K(+)](o) transients was studied in CA1 stratum pyramidale using intracellular sharp microelectrodes and extracellular ion-sensitive microelectrodes. The evoked [K(+)](o) transients, as well as the associated afterdischarges, were strongly suppressed by 0.5-1 mm furosemide, a KCl cotransport inhibitor. Importantly, the GABA(A)R-mediated intrapyramidal accumulation of Cl(), as measured by monitoring the reversal potential of fused IPSPs, was unaffected by the drug. It was further confirmed that the reduction in the [K(+)](o) transients was not due to effects of furosemide on the Na(+)-dependent K(+)-Cl() cotransporter NKCC1 or on intraneuronal carbonic anhydrase activity. Blocking potassium channels by Ba(2+) enhanced [K(+)](o) transients whereas pyramidal cell depolarizations were attenuated in further agreement with a lack of contribution by channel-mediated K(+) efflux. The key role of the GABA(A)R channel-mediated anion fluxes in the generation of the [K(+)](o) transients was examined in experiments where bicarbonate was replaced with formate. This anion substitution had no significant effect on the rate of Cl() accumulation, [K(+)](o) response or afterdischarges. Our findings reveal a novel excitatory mode of action of KCC2 that can have substantial implications for the role of GABAergic transmission during ictal epileptiform activity.

Furosemide potentiates the anticonvulsant action of valproate in the mouse maximal electroshock seizure model

Accumulating evidence indicates that furosemide (FUR, a loop diuretic) exerts the anticonvulsant action in various in vitro and in vivo experiments. Therefore, the aim of this study was to assess the influence of FUR on the protective action of numerous conventional and newer antiepileptic drugs (carbamazepine [CBZ], lamotrigine [LTG], oxcarbazepine [OXC], phenobarbital [PB], topiramate [TPM] and valproate [VPA]) in the mouse maximal electroshock seizure (MES) model. Results indicate that FUR (up to 100mg/kg, i.p., 30 min before the test) neither altered the threshold for electroconvulsions nor protected the animals against MES-induced seizures in mice. FUR (100 mg/kg, i.p.) enhanced the anticonvulsant effects of VPA in the MES test by reducing its ED(50) value from 230.4 to 185.4 mg/kg (P<0.05). In contrast, FUR at 100 mg/kg had no significant effect on the antielectroshock action of the remaining drugs tested (CBZ, LTG, OXC, PB, and TPM) in mice. Estimation of free plasma and total brain VPA concentrations revealed that the observed interaction between FUR and VPA in the MES test was pharmacodynamic in nature because neither free plasma nor total brain VPA concentrations were altered after i.p. administration of FUR. In conclusion, one can ascertain that the selective potentiation of the antielectroshock action of VPA by FUR and lack of any pharmacokinetic interactions between drugs, make this combination of pivotal importance for epileptic patients treated with VPA and received FUR from other than epilepsy reasons.

K+ M-current regulates the transition to seizures in immature and adult hippocampus

PURPOSE

Loss-of-function mutations in Kv7.2 or Kv7.3 K(+) channel subunits underlies the neonatal epilepsy benign familial neonatal convulsions (BFNC). These two subunits interact to form a functional K(+) channel that underlies the M-current (I(M)), a voltage-dependent noninactivating K(+) current. In BFNC, seizures begin shortly after birth, and spontaneously remit in the first few months of life. The nature of this window of vulnerability is unclear. We address this issue using a hippocampal slice model, to study the effects of I(M) blockade or augmentation on epileptiform activity.

METHODS

We used the Mg(+)(+)-free seizure model in adult and immature (P8-P15) acute rat hippocampal slices. We recorded from both CA1 and CA3 regions using extracellular and intracellular methods.

RESULTS

When M-channels are blocked pharmacologically, the transition from interictal to ictal bursting becomes much more likely, especially in immature brain. We also show augmentation of I(M) is effective in stopping ictal events in immature brain, at the developmental age that approximates a human newborn in cortical development. I(M) appears to counter the sustained N-methyl-D-aspartate (NMDA) receptor-mediated depolarizations needed to trigger an ictal event. The increased likelihood of ictal bursting by I(M) blockade is not shared by other selective K(+) channel blockers that increase hippocampal excitability.

CONCLUSIONS

Voltage-dependent M-channels are activated during interictal bursts and contribute to burst termination. When these channels are compromised, interictal burst duration becomes sufficient to trigger the sustained depolarizations that underlie ictal bursts. This transition to ictal bursts upon I(M) blockade is especially likely to occur in immature hippocampus. This selective function of M-channels likely contributes to the transient window of vulnerability to seizures that occurs with BFNC.

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.