Pioglitazone, a PPAR-γ Activator, Stimulates BKCa but Suppresses IK M in Hippocampal Neurons

Spontaneous Ca2+ events are linked to the development of neuronal firing during maturation in mice primary hippocampal culture cells

Calcium is one of the most vital intracellular secondary messengers that tightly regulates a variety of cell physiology processes, especially in the brain. Using a fluorescent Ca²⁺-sensitive Oregon Green probe, we revealed three different amplitude distributions of spontaneous Ca²⁺ events (SCEs) in neurons between 15 and 26 days in vitro (DIV) culture maturation. We detected a series of amplitude events: micro amplitude SCE (microSCE) 25% increase from the baseline, intermediate amplitude SCE (interSCE) as 25-75%, and macro amplitude SCE (macroSCE) - over 75%. The SCEs were fully dependent on extracellular Ca²⁺ and neuronal network activity and vanished in the Ca²⁺-free solution, 10 mM Mg²⁺-block, or in the presence of voltage-gated Na⁺-channel blocker, tetrodotoxin. Combined patch-clamp and Ca²⁺-imaging techniques revealed that microSCE match single action potential (AP), interSCE - burst of 3-12 APs, and macroSCE - 'superburst' of 10+ APs. MicroSCEs were blocked by a common α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainic acid (KA) receptor antagonist, CNQX. The γ-aminobutyric acid (GABA) A-type receptor (GABAAR) picrotoxin blockade and L-type voltage-dependent Ca²⁺-channel inhibitor diltiazem significantly reduced microSCE frequency. InterSCEs were inhibited by CNQX, but picrotoxin treatment significantly increased its amplitude. The N-methyl-d-aspartate (NMDA) receptor antagonist, D-APV, voltage-gated K⁺-channel blocker, tetraethylammonium, noticeably suppressed interSCE amplitude. We also demonstrate that macroSCEs were AMPA/KA receptor-independent.

Effective Activation by Kynurenic Acid and Its Aminoalkylated Derivatives on M-Type K+ Current

Kynurenic acid (KYNA, 4-oxoquinoline-2-carboxylic acid), an intermediate of the tryptophan metabolism, has been recognized to exert different neuroactive actions; however, the need of how it or its aminoalkylated amide derivative N-(2-(dimethylamino)ethyl)-3-(morpholinomethyl)-4-oxo-1,4-dihydroquinoline-2-carboxamide (KYNA-A4) exerts any effects on ion currents in excitable cells remains largely unmet. In this study, the investigations of how KYNA and other structurally similar KYNA derivatives have any adjustments on different ionic currents in pituitary GH₃ cells and hippocampal mHippoE-14 neurons were performed by patch-clamp technique. KYNA or KYNA-A4 increased the amplitude of M-type K⁺ current (I_K(M)) and concomitantly enhanced the activation time course of the current. The EC₅₀ value required for KYNA- or KYNA-A4 -stimulated I_K(M) was yielded to be 18.1 or 6.4 μM, respectively. The presence of KYNA or KYNA-A4 shifted the relationship of normalized I_K(M)-conductance versus membrane potential to more depolarized potential with no change in the gating charge of the current. The voltage-dependent hysteretic area of I_K(M) elicited by long-lasting triangular ramp pulse was observed in GH₃ cells and that was increased during exposure to KYNA or KYNA-A4. In cell-attached current recordings, addition of KYNA raised the open probability of M-type K⁺ channels, along with increased mean open time of the channel. Cell exposure to KYNA or KYNA-A4 mildly inhibited delayed-rectifying K⁺ current; however, neither erg-mediated K⁺ current, hyperpolarization-activated cation current, nor voltage-gated Na⁺ current in GH₃ cells was changed by KYNA or KYNA-A4. Under whole-cell, current-clamp recordings, exposure to KYNA or KYNA-A4 diminished the frequency of spontaneous action potentials; moreover, their reduction in firing frequency was attenuated by linopirdine, yet not by iberiotoxin or apamin. In hippocampal mHippoE-14 neurons, the addition of KYNA also increased the I_K(M) amplitude effectively. Taken together, the actions presented herein would be one of the noticeable mechanisms through which they modulate functional activities of excitable cells occurring in vivo.

High ability of zileuton ((±)-1-(1-benzo[b]thien-2-ylethyl)-1-hydroxyurea) to stimulate IK(Ca) but suppress IK(DR) and IK(M) independently of 5-lipoxygenase inhibition

Zileuton (Zyflo®) is regarded to be an inhibitor of 5-lipoxygenase. Although its effect on Ca²⁺-activated K⁺ currents has been reported, its overall ionic effects on neurons are uncertain. In whole-cell current recordings, zileuton increased the amplitude of Ca²⁺-activated K⁺ currents with an EC₅₀ of 3.2 μM in pituitary GH₃ lactotrophs. Furthermore, zileuton decreased the amplitudes of both delayed-rectifier K⁺ current (I_K(DR)) and M-type K⁺ current (I_K(M)). Conversely, no modification of hyperpolarization-activated cation current (I_h) was demonstrated in its presence of zileuton, although the subsequent addition of cilobradine effectively suppressed the current. In inside-out current recordings, the addition of zileuton to the bath increased the probability of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels; however, the subsequent addition of GAL-021 effectively reversed the stimulation of channel activity. The kinetic analyses showed an evident shortening in the slow component of mean closed time of BK_Ca channels in the presence of zileuton, with minimal change in mean open time or that in the fast component of mean closed time. The elevation of BK_Ca channels caused by zileuton was also observed in hippocampal mHippoE-14 neurons, without any modification of single-channel amplitude. In conclusion, except for its suppression of 5-lipoxygenase, our results indicate that zileuton does not exclusively act on BK_Ca channels, and its inhibitory effects on I_K(DR) and I_K(M) may combine to exert strong influence on the functional activities of electrically excitable cells in vivo.

The Specific Effects of OD-1, a Peptide Activator, on Voltage-Gated Sodium Current and Seizure Susceptibility

OD-1, a scorpion toxin, has been previously recognized as an activator of voltage-gated Na⁺ currents. To what extent this agent can alter hippocampal neuronal Na⁺ currents and network excitability and how it can be applied to neuronal hyperexcitability research remains unclear. With the aid of patch-clamp technology, it was revealed that, in mHippoE-14 hippocampal neurons, OD-1 produced a concentration-, time-, and state-dependent rise in the peak amplitude of I_Na. It shifted the I_Na inactivation curve to a less negative potential and increased the frequency of spontaneous action currents. Further characterization of neuronal excitability revealed higher excitability in the hippocampal slices treated with OD-1 as compared with the control slices. A stereotaxic intrahippocampal injection of OD-1 generated a significantly higher frequency of spontaneous seizures and epileptiform discharges compared with intraperitoneal injection of lithium-pilocarpine- or kainic acid-induced epilepsy, with comparable pathological changes. Carbamazepine significantly attenuated OD-1 induced seizures and epileptiform discharges. The OD-1-mediated modifications of I_Na altered the electrical activity of neurons in vivo and OD-1 could potentially serve as a novel seizure and excitotoxicity model.

Channels that Cooperate with TRPV4 in the Brain

Transient receptor potential vanilloid 4 (TRPV4) is a nonselective Ca²⁺-permeable cation channel that is a member of the TRP channel family. It is clear that TRPV4 channels are broadly expressed in the brain. As they are expressed on the plasma membrane, they interact with other channels and play a crucial role in nervous system activity. Under some pathological conditions, TRPV4 channels are upregulated and sensitized via cellular signaling pathways, and this can cause nervous system diseases. In this review, we focus on receptors that cooperate with TRPV4, including large-conductance Ca²⁺-activated K⁺(BKca) channels, N-methyl-D-aspartate receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors (AMPARs), inositol 1,4,5-trisphosphate receptors (IP3Rs), ryanodine receptors (RyRs), aquaporin 4 (AQP4), and other potential cooperative receptors in the brain. The data demonstrate how these channels work together to cause nervous system diseases under pathological conditions. The aim of this review was to discuss the receptors and signaling pathways related to TRPV4 based on recent data on the important physiological functions of TRPV4 channels to provide new clues for future studies and prospective therapeutic targets for related brain diseases.

Effectiveness in the inhibition of dapagliflozin and canagliflozin on M-type K+ current and α-methylglucoside-induced current in pituitary tumor (GH3) and pheochromocytoma PC12 cells

Dapagliflozin (DAPA) or canagliflozin (CANA), Na⁺-dependent glucose co-transporter type 2 (SGLT2) inhibitors, were used for treatment of type II diabetes mellitus. Addition of DAPA or CANA suppressed M-type K⁺ current (I_K(M)) in pituitary tumor (GH₃) and pheochromocytoma PC12 cells. The IC₅₀ value for DAPA- or CANA-mediated inhibition of I_K(M) in GH₃ cells was 0.11 or 0.42 μM, respectively. The presence of DAPA (0.1 μM) shifted the steady-state activation of I_K(M) to less depolarized potential without changing the gating charge of the current. During high-frequency depolarizing pulses, I_K(M) magnitude was reduced by DAPA; however, DAPA-induced block of I_K(M) remained effective. The amplitude of neither erg-mediated K⁺ current nor hyperpolarization-activated cation current in GH₃ cells was modified in the presence of 1 μM DAPA. Alternatively, addition of DAPA, CANA, phlorizin or chlorotoxin effectively suppressed α-methylglucoside-(αMG-) induced current (I_αMG) in GH₃ cells, albeit inability of tefluthrin (activator of I_Na) to suppress this current. DAPA shifted the charge-voltage relation of presteady-state I_αMG in a rightward and downward direction with no change in the gating charge of the I_αMG. Under current-clamp recordings, subsequent additions of DAPA, but still in the continued presence of αMG, increased the firing rate of spontaneous action potentials stimulated by αMG. Our results suggested that activity of SGLT was expressed functionally in GH₃ and PC12 cells. Therefore, inhibitory actions of DAPA or CANA on the amplitude and gating of I_K(M) might provide a yet unidentified mechanism through which the SGLT1 or SGLT2 activity were attenuated in unclamped cells occurring in vivo.

Characterization of Effectiveness in Concerted Ih Inhibition and IK(Ca) Stimulation by Pterostilbene (Trans-3,5-dimethoxy-4'-hydroxystilbene), a Stilbenoid

Pterostilbene (PTER), a natural dimethylated analog of resveratrol, has been demonstrated to produce anti-neoplastic or neuroprotective actions. However, how and whether this compound can entail any perturbations on ionic currents in electrically excitable cells remains unknown. In whole-cell current recordings, addition of PTER decreased the amplitude of macroscopic I_h during long-lasting hyperpolarization in GH₃ cells in a concentration-dependent manner, with an effective IC₅₀ value of 0.84 μM. Its presence also shifted the activation curve of I_h along the voltage axis to a more hyperpolarized potential, by 11 mV. PTER at a concentration greater than 10 μM could also suppress l-type Ca²⁺ and transient outward K⁺ currents in GH₃ cells. With the addition of PTER, I_K(Ca) amplitude was increased, with an EC₅₀ value of 2.23 μM. This increase in I_K(Ca) amplitude was attenuated by further addition of verruculogen, but not by tolbutamide or TRAM-39. Neither atropine nor nicotine, in the continued presence of PTER, modified the PTER-stimulated I_K(Ca). PTER (10 μM) slightly suppressed the amplitude of l-type Ca²⁺ current and transient outward K⁺ current. The presence of PTER (3 μM) was also effective at increasing the open-state probability of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels identified in hippocampal mHippoE-14 neurons; however, its inability to alter single-channel conductance was detected. Our study highlights evidence to show that PTER has the propensity to perturb ionic currents (e.g., I_h and I_K(Ca)), thereby influencing the functional activities of neurons, and neuroendocrine or endocrine cells.

The Novel Direct Modulatory Effects of Perampanel, an Antagonist of AMPA Receptors, on Voltage-Gated Sodium and M-type Potassium Currents

Perampanel (PER) is a selective blocker of AMPA receptors showing efficacy in treating various epileptic disorders including brain tumor-related epilepsy and also potential in treating motor neuron disease. However, besides its inhibition of AMPA-induced currents, whether PER has any other direct ionic effects in different types of neurons remains largely unknown. We investigated the effects of PER and related compounds on ionic currents in different types of cells, including hippocampal mHippoE-14 neurons, motor neuron-like NSC-34 cells and U87 glioma cells. We found that PER differentially and effectively suppressed the amplitude of voltage-gated Na⁺ currents (I_Na) in mHippoE-14 cells. The IC₅₀ values required to inhibit peak and late I_Na were 4.12 and 0.78 μM, respectively. PER attenuated tefluthrin-induced increases in both amplitude and deactivating time constant of I_Na. Importantly, PER also inhibited the amplitude of M-type K⁺ currents (I_K(M)) with an IC₅₀ value of 0.92 μM. The suppression of I_K(M) was attenuated by the addition of flupirtine or ZnCl₂ but not by L-quisqualic acid or sorafenib. Meanwhile, in cell-attached configuration, PER (3 μM) decreased the activity of M-type K⁺ channels with no change in single-channel conductance but shifting the activation curve along the voltage axis in a rightward direction. Supportively, PER suppressed I_K(M) in NSC-34 cells and I_Na in U87 glioma cells. The inhibitory effects of PER on both I_Na and I_K(M), independent of its antagonistic effect on AMPA receptors, may be responsible for its wide-spectrum of effects observed in neurological clinical practice.

Total flavones of Rhododendron simsii Planch flower protect rat hippocampal neuron from hypoxia-reoxygenation injury via activation of BKCa channel

OBJECTIVES

To study the effects of total flavones of Rhododendra simsii Planch flower (TFR) on hypoxia/reoxygenation (H/R) injury in rat hippocampal neurons and its underlying mechanism.

METHOD

Model of H/R was established in newborn rat primary cultured hippocampal neuron. Lactate dehydrogenase (LDH) and neuron-specific enolase (NSE) activity as well as malondialdehyde (MDA) content in cultured supernatants of the neurons were examined. Methyl thiazolyl tetrazolium assay and Hoechst33258 staining were, respectively, used to detect cell viability and apoptosis of neurons. Protein expression and current of BK_Ca channel were assessed by using Western blotting and whole-cell patch-clamp methods, respectively.

KEY FINDINGS

In the ranges of 3.7-300 mg/l, TFR significantly inhibited H/R-induced decrease of neuronal viability and increases of LDH, NSE and MDA in the supernatants as well as apoptosis; TFR 33.3, 100 and 300 mg/l markedly increased current of BK_Ca channel rather than the BK_Ca channel protein expression in the neurons.

CONCLUSIONS

Total flavones of R. simsii Planch flower had a protective effect against H/R injury in rat hippocampal neuron, and activation of BK_Ca channel may contribute to the neuroprotection.

Characterization of Perturbing Actions by Verteporfin, a Benzoporphyrin Photosensitizer, on Membrane Ionic Currents

Verteporfin (VP), a benzoporphyrin derivative, has been clinically tailored as a photosensitizer and recently known to suppress YAP-TEAD complex accompanied by suppression of the growth in an array of neoplastic cells. However, the detailed information is little available regarding possible modifications of it and its related compounds on transmembrane ionic currents, despite its growing use in clinical settings. In this study, from whole cell recordings, VP (0.3-100 μM) increased the amplitude of Ca²⁺-activated K⁺ currents (I _K(Ca)) in pituitary tumor (GH₃) cells in a concentration-dependent manner with an EC₅₀ value of 2.4 μM. VP-stimulated I _K(Ca) in these cells was suppressed by further addition of either paxilline, iberiotoxin, or dithiothreitol, but not by that of tobultamide or TRAM-39. VP at a concentration of 10 μM mildly suppressed the amplitude of delayed-rectifier K⁺ current; however, it had minimal effects on M-type K⁺ current. In cell-attached current recordings, addition of VP to the recording medium enhanced the activity of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels. In the presence of VP, additional illumination with light intensity of 5.5 mW/cm² raised the probability of BK_Ca-channel openings further. Addition of VP decreased the peak amplitude of L-type Ca²⁺ current together with slowed inactivation time course of the current; however, it failed to modify voltage-gated Na⁺ current. Illumination of GH₃ cells in continued presence of VP also induced a non-selective cation current. Additionally, VP increased the activity of BK_Ca channels in human 13-06-MG glioma cells with an EC₅₀ value of 1.9 μM. Therefore, the effects of VP on ionic currents described herein tend to be upstream of its inhibition of YAP-TEAD complex and they are conceivably likely to contribute to the underlying mechanisms through which it and its structurally similar compounds effect the modifications in functional activities of pituitary or glial neoplastic cells, if the in vivo findings occur.

Concerted suppression of Ih and activation of IK(M) by ivabradine, an HCN-channel inhibitor, in pituitary cells and hippocampal neurons

Ivabradine (IVA), a heart-rate reducing agent, is recognized as an inhibitor of hyperpolarization-activated cation current (I_h) and also reported to ameliorate inflammatory or neuropathic pain. However, to what extent this agent can perturb another types of membrane ion currents in neurons or endocrine cells remains to be largely unknown. Therefore, the I_h or other types of ionic currents in pituitary tumor (GH₃) cells and in hippocampal mHippoE-14 neurons was studied with or without the presence of IVA or other related compounds. The IVA addition caused a time- and concentration-dependent reduction in the amplitude of I_h with an IC₅₀ value of 0.64 μM and a K_D value of 0.68 μM. IVA (0.3 μM) shifted the I_h activation curve to a more negative potential by approximately 8 mV, despite no concomitant change in the gating charge. Additionally, IVA was found to increase M-type K⁺ current (I_K(M)) together with a rightward shift in the activation curve. In cell-attached current recordings, IVA (3 μM) applied to the bath increased the open probability of M-type K⁺ channels; however, it did not modify single-channel conductance of the channel. In current-clamp voltage recordings, IVA suppressed the firing of spontaneous action potentials in GH₃ cells; and, further addition of linopirdine attenuated its suppression of firing. In hippocampal mHippoE-14 neurons, IVA also effectively increased I_K(M) amplitude. In summary, both inhibition of I_h and activation of I_K(M) caused by IVA can synergistically combine to influence electrical behaviors in different types of electrically excitable cells occurring in vivo.

Evidence for Effective Multiple K+-Current Inhibitions by Tolvaptan, a Non-peptide Antagonist of Vasopressin V2 Receptor

Tolvaptan (TLV), an oral non-peptide antagonist of vasopressin V₂ receptor, has been increasingly used for managements in patients with hyponatremia and/or syndrome of inappropriate antidiuretic hormone secretion. However, none of the studies have thus far been investigated with regard to its possible perturbations on membrane ion currents in endocrine or neuroendocrine cells. In our electrophysiological study, the whole-cell current recordings showed that the presence of TLV effectively and differentially suppressed the amplitude of delayed rectifier K⁺ (I_K(DR)) and M-type K⁺ current (I_K(M)) in pituitary GH₃ cells with an IC₅₀ value of 6.42 and 1.91 μM, respectively. This compound was also capable of shifting the steady-state activation curve of I_K(M) to less depolarized potential without any appreciable change in the gating charge of this current. TLV at a concentration greater than 10 μM also suppressed the amplitude of erg-mediated K⁺ current or the activity of large-conductance Ca²⁺-activated K⁺ channels; however, this compound failed to alter the amplitude of hyperpolarization-activated cation current in GH₃ cells. In vasopressin-preincubated GH₃ cells, TLV-mediated suppression of I_K(M) remained little altered. Under current-clamp condition, we also observed that addition of TLV increased the firing of spontaneous action potentials in GH₃ cells and further addition of flupirtine could reverse TLV-mediated elevation of the firing. In Madin-Darby canine kidney (MDCK) cells, the K⁺ current elicited by long ramp pulse was also effectively subject to inhibition by this compound. Findings from the present study were thus stated as saying that the suppression by TLV of multiple type K⁺ currents could be direct and independent of its antagonism of vasopressin V₂ receptors. Our study also reveals an important aspect that should be considered when assessing aquaretic effect of TLV or its structurally similar compounds.

Bisoprolol, Known to Be a Selective β₁-Receptor Antagonist, Differentially but Directly Suppresses IK(M) and IK(erg) in Pituitary Cells and Hippocampal Neurons

Bisoprolol (BIS) is a selective antagonist of β₁ adrenergic receptors. We examined the effects of BIS on M-type K⁺ currents (IK(M)) or erg-mediated K⁺ currents (IK(erg)) in pituitary GH3, R1220 cells, and hippocampal mHippoE-14 cells. As GH₃ cells were exposed to BIS, amplitude of IK(M) was suppressed with an IC50 value of 1.21 μM. The BIS-induced suppression of IK(M) amplitude was not affected by addition of isoproterenol or ractopamine, but attenuated by flupirtine or ivabradine. In cell-attached current, BIS decreased the open probability of M-type K⁺ (KM) channels, along with decreased mean opening time of the channel. BIS decreased IK(erg) amplitude with an IC50 value of 6.42 μM. Further addition of PD-118057 attenuated BIS-mediated inhibition of IK(erg). Under current-clamp conditions, BIS depolarization increased the firing of spontaneous action potentials in GH₃ cells; addition of flupirtine, but not ractopamine, reversed BIS-induced firing rate. In R1220 cells, BIS suppressed IK(M); subsequent application of ML-213(Kv7.2 channel activator) reversed BIS-induced suppression of the current. In hippocampal mHippoE-14 neurons, BIS inhibited IK(M) to a greater extent compared to its depressant effect on IK(erg). This demonstrated that in pituitary cells and hippocampal neurons the presence of BIS is capable of directly and differentially suppressing IK(M) and IK(erg), despite its antagonism of β₁-adrenergic receptors.