Sodium Channels In Hippocampal Neurons Research Articles

RuO2 Supercapacitor Enables Flexible, Safe, and Efficient Optoelectronic Neural Interface

AbstractOptoelectronic biointerfaces offer a wireless and nongenetic neurostimulation pathway with high spatiotemporal resolution. Fabrication of low‐cost and flexible optoelectronic biointerfaces that have high photogenerated charge injection densities and clinically usable cell stimulation mechanism is critical for rendering this technology useful for ubiquitous biomedical applications. Here, supercapacitor technology is combined with flexible organic optoelectronics by integrating RuO2 into a donor–acceptor photovoltaic device architecture that facilitates efficient and safe photostimulation of neurons. Remarkably, high interfacial capacitance of RuO2 resulting from reversible redox reactions leads to more than an order‐of‐magnitude increase in the safe stimulation mechanism of capacitive charge transfer. The RuO2‐enhanced photoelectrical response activates voltage‐gated sodium channels of hippocampal neurons and elicits repetitive, low‐light intensity, and high‐success rate firing of action potentials. Double‐layer capacitance together with RuO2‐induced reversible faradaic reactions provide a safe stimulation pathway, which is verified via intracellular oxidative stress measurements. All‐solution‐processed RuO2‐based biointerfaces are flexible, biocompatible, and robust under harsh aging conditions, showing great promise for building safe and highly light‐sensitive next‐generation neural interfaces.

Transient Receptor Potential Vanilloid 4-Induced Modulation of Voltage-Gated Sodium Channels in Hippocampal Neurons.

Transient receptor potential vanilloid 4 (TRPV4) is reported to control the resting membrane potential and increase excitability in many types of cells. Voltage-gated sodium channels (VGSCs) play an important role in initiating action potentials in neurons. However, whether VGSCs can be modulated by the activation of TRPV4 in hippocampal pyramidal neurons remains unknown. In this study, we tested the effect of TRPV4 agonists (GSK1016790A and 4α-PDD) on voltage-gated sodium current (I Na) in hippocampal CA1 pyramidal neurons and the protein levels of α/β-subunit of VGSCs in the hippocampus of mice subjected to intracerebroventricular (icv.) injection of GSK1016790A (GSK-injected mice). Herein, we report that I Na was inhibited by acute application of GSK1016790A or 4α-PDD. In the presence of TRPV4 agonists, the voltage-dependent inactivation curve shifted to the hyperpolarization, whereas the voltage-dependent activation curve remained unchanged. The TRPV4 agonist-induced inhibition of I Na was blocked by the TRPV4 antagonist or tetrodotoxin. Moreover, blocking protein kinase A (PKA) markedly attenuated the GSK1016790A-induced inhibition of I Na, whereas antagonism of protein kinase C or p38 mitogen-activated protein kinase did not change GSK1016790A action. Finally, the protein levels of Nav1.1, Nav1.2, and Nav1.6 in the hippocampus increased in GSK-injected mice, whereas those of Nav1.3 and Navβ1 remained nearly unchanged. We conclude that I Na is inhibited by the acute activation of TRPV4 through PKA signaling pathway in hippocampal pyramidal neurons, but protein expression of α-subunit of VGSCs is increased by sustained TRPV4 activation, which may compensate for the acute inhibition of I Na and provide a possibility for hyper-excitability upon sustained TRPV4 activation.

Action potential changes associated with impairment of functional properties of sodium channels in hippocampal neurons induced by melamine

Since the melamine-contamination event happened in September 2008, there have been lots of studies about melamine toxicity, but very limited studies focused on central nervous system (CNS). In the present study, we investigated the effects of melamine (5 × 10 −4, 5 × 10 −5 and 5 × 10 −6 g/ml) on voltage-gated sodium channels (VGSCs) in hippocampal CA1 neurons using whole-cell patch-clamp recordings technique. The results showed that only 5 × 10 −4 g/ml melamine reduced the amplitude of voltage-gated sodium current ( I Na). At the concentrations of 5 × 10 −5 and 5 × 10 −4 g/ml, melamine produced a hyperpolarizing shift in the steady-state activation curve of I Na and also enhanced the steady-state inactivate processing of I Na. Action potential properties and the pattern of repetitive firing were examined using current-clamp recording, which indicated that peak amplitude and overshoot of the evoked single action potential were decreased. The half-width and the firing rate of repetitive firing were increased in a concentration-dependent manner. The data suggest that melamine alters the action potential of hippocampal CA1 neurons by impairing the functional properties of VGSCs, which may be the underlie mechanisms of neurotoxicity induced by melamine.

A Possible Explanation for the Paradox of Hyperexcitability and Epilepsy in “Loss of Function” Voltage-Gated Sodium Channel Mutations

Reduced Sodium Current in GABAergic Interneurons in a Mouse Model of Severe Myoclonic Epilepsy in Infancy. Yu FH, Mantegazza M, Westenbroek RE, Robbins CA, Kalume F, Burton KA, Spain WJ, McKnight GS, Scheuer T, Catterall WA. Nat Neurosci 2006;9:1142–1149. Voltage-gated sodium channels (NaV) are critical for initiation of action potentials. Heterozygous loss-of-function mutations in NaV1.1 channels cause severe myoclonic epilepsy in infancy (SMEI). Homozygous null SCN1A–/– mice developed ataxia and died on postnatal day (P) 15 but could be sustained to P17.5 with manual feeding. Heterozygous SCN1A+/– mice had spontaneous seizures and sporadic deaths beginning after P21, with a notable dependence on genetic background. Loss of NaV1.1 did not change voltage-dependent activation or inactivation of sodium channels in hippocampal neurons. The sodium current density was, however, substantially reduced in inhibitory interneurons of SCN1A+/– and SCN1A–/– mice but not in their excitatory pyramidal neurons. An immunocytochemical survey also showed a specific upregulation of NaV1.3 channels in a subset of hippocampal interneurons. Our results indicate that reduced sodium currents in GABAergic inhibitory interneurons in SCN1A+/– heterozygotes may cause the hyperexcitability that leads to epilepsy in patients with SMEI.

Nicotinic acetylcholine receptor antagonistic property of the selective dopamine uptake inhibitor, GBR-12909 in rat hippocampal slices

Previously we found that inhibitors of noradrenaline (NA) and/or 5-HT reuptake are able to inhibit neuronal nicotinic acetylcholine receptors (nAChRs) in the CNS most probably by a channel blocker-type mechanism. The aim of our study was to clarify whether selective dopamine uptake inhibitors also possess this property, therefore we investigated the effect of GBR-12909 on the nicotine-evoked release of [3H]NA from rat hippocampal slices. GBR-12909, similar to selective NA and 5-HT uptake blockers, inhibited the nicotine-evoked release with an IC50 of 2.32 μM. The ability of monoamine uptake blockers to inhibit nicotine-evoked [3H]NA release (IC50) and NA reuptake (Ki) showed no correlation, indicating that the NA uptake system is not involved in the inhibition of the response to nicotine. Previously we have shown in whole cell patch clamp experiments, that GBR-12909, depending on the stimulation pattern, inhibits Na+-currents with an IC50 in the 6–35 μM concentration range [Mike A, Karoly R, Vizi ES, Kiss JP (2003) Inhibitory effect of the DA uptake blocker GBR-12909 on sodium channels of hippocampal neurons. Neuroreport 14:1945–1949]. To study whether the inhibition of Na+-channels is involved in the action of GBR-12909 on the nicotine-evoked [3H]NA release, we compared the effect of GBR-12909 and the Na+-channel blocker tetrodotoxin (TTX) on the electrical stimulation- and nicotine-evoked response. TTX prevented the release of [3H]NA induced by both types of stimulation, whereas GBR-12909 inhibited only the nicotine-induced response, indicating that under our experimental conditions the target of GBR-12909 is not the Na+-channel. These data indicate that the selective DA uptake inhibitor GBR-12909 is able to inhibit nAChRs, that is, the nAChR antagonistic property of monoamine uptake inhibitors is independent of their selectivity. The fact that monoamine uptake inhibitors with different chemical structure and selectivity are able to inhibit nAChRs may reveal some common properties of nicotinic receptors and monoamine uptake carriers.

Hydroxyamide Analogs of Propofol Exhibit State-Dependent Block of Sodium Channels in Hippocampal Neurons: Implications for Anticonvulsant Activity

Although propofol is most commonly known for its general anesthetic properties, at subanesthetic doses, propofol has been effectively used to suppress seizures during refractory status epilepticus, a mechanism, in part, attributed to the inhibition of neuronal sodium channels. In this study, we have designed and synthesized two novel analogs of propofol, HS245 [2-(3-ethyl-4-hydroxy-5-isopropyl-phenyl)-3,3,3-trifluoro-2-hydroxy-propionamide] and HS357 [2-hydroxy-8-(4-hydroxy-3,5-diisopropyl-phenyl)-2-trifluoromethyl-octanoic acid amide], and determined their effects on sodium currents recorded from cultured hippocampal neurons. HS357 had greater affinity for the inactivated state of the sodium channel than propofol and HS245 (0.22 versus 0.74 and 1.2 microM, respectively) and exhibited the greatest ratio of affinity for the resting over the inactivated state. HS357 also demonstrated greater use-dependent block and delayed recovery from inactivation in comparison with propofol and HS245. Under current-clamp conditions, action potentials from hippocampal CA1 neurons in slices were evoked by current injection, or following perfusion with a zero Mg(2+)/7 mM K(+) artificial cerebrospinal fluid solution. Propofol and HS357 reduced the number of current-induced action potentials; however, HS357 caused a greater reduction in the number of spontaneous action potentials. Consistent with these electrophysiology studies, propofol and HS357 protected mice against acute seizures in the 6-Hz (22-mA) partial psychomotor model. Efficacious doses of propofol were associated with an impairment of motor coordination as assessed in the rotorod toxicity assay. In contrast, HS357 demonstrated a 2-fold greater protective index than propofol. Thus, propofol analogs represent an important structural class from which not only effective, but also safer, anti-convulsants may be developed.

The Mechanism of Activity-Dependent Sodium Channel Inhibition by the Antidepressants Fluoxetine and Desipramine

The effect of monoamine uptake inhibitor-type antidepressants on sodium channels of hippocampal neurons was investigated. Members of the tricyclic group of antidepressants are known to modify multiple targets, including sodium channels, whereas selective serotonin-reuptake inhibitors (SSRIs) are regarded as highly selective compounds, and their effect on sodium channels was not investigated in detail. In this study, a representative member of each group was chosen: the tricyclic antidepressant desipramine and the SSRI fluoxetine. The drugs were roughly equipotent use-dependent inhibitors of sodium channels, with IC(50) values approximately 100 microM at -150 mV holding potential, and approximately 1 microM at -60 mV. We suggest that therapeutic concentrations of antidepressants affect neuronal information processing partly by direct, activity-dependent inhibition of sodium channels. As for the mechanism of inhibition, use-dependent inhibition by antidepressants was believed to be due to a preferential affinity to the fast-inactivated state. Using a voltage and perfusion protocol by which relative affinities to fast-versus slow-inactivated states could be assessed, we challenged this view and found that the affinity of both drugs to slowinactivated state(s) was higher. We propose a different mechanism of action for these antidepressants, in which slow rather than fast inactivation plays the dominant role. This mechanism is similar but not equivalent with the novel mechanism of usedependent sodium channel inhibition previously described by our group (Neuroscience 125:1019-1028, 2004; Neuroreport 14:1945-1949, 2003). Our results suggest that different drugs can produce use-dependent sodium channel inhibition by different mechanisms.

Reduced sodium current in GABAergic interneurons in a mouse model of severe myoclonic epilepsy in infancy

Voltage-gated sodium channels (Na(V)) are critical for initiation of action potentials. Heterozygous loss-of-function mutations in Na(V)1.1 channels cause severe myoclonic epilepsy in infancy (SMEI). Homozygous null Scn1a-/- mice developed ataxia and died on postnatal day (P) 15 but could be sustained to P17.5 with manual feeding. Heterozygous Scn1a+/- mice had spontaneous seizures and sporadic deaths beginning after P21, with a notable dependence on genetic background. Loss of Na(V)1.1 did not change voltage-dependent activation or inactivation of sodium channels in hippocampal neurons. The sodium current density was, however, substantially reduced in inhibitory interneurons of Scn1a+/- and Scn1a-/- mice but not in their excitatory pyramidal neurons. An immunocytochemical survey also showed a specific upregulation of Na(V)1.3 channels in a subset of hippocampal interneurons. Our results indicate that reduced sodium currents in GABAergic inhibitory interneurons in Scn1a+/- heterozygotes may cause the hyperexcitability that leads to epilepsy in patients with SMEI.

Inhibitory effect of the DA uptake blocker GBR 12909 on sodium channels of hippocampal neurons.

The effect of the selective dopamine uptake inhibitor GBR 12909 on TTX-sensitive sodium channels of cultured hippocampal neurons was investigated using whole cell patch-clamp technique. GBR 12909 dose-dependently inhibited sodium currents evoked by trains of depolarizing pulses with an IC50 of 6.3 microM. A weaker inhibition (IC50 = 17-35 microM) could be observed when currents were evoked by either single pulse depolarization or from hyperpolarized holding membrane potential. These data indicate that the extent of inhibition caused by GBR 12909 depends on the physiological activity pattern of neurons. Our results suggest that caution is needed for the interpretation of data when GBR 12909 is used for the inhibition of dopamine uptake at concentrations above the submicromolar range.

Effect of phenytoin on sodium and calcium currents in hippocampal CA1 neurons of phenytoin-resistant kindled rats

About 20–30% of patients with epilepsy continue to have seizures despite carefully monitored treatment with antiepileptic drugs. The mechanisms explaining why some patients' respond and others prove resistant to antiepileptic drugs are poorly understood. It has been proposed that pharmacoresistance is related to reduced sensitivity of sodium channels in hippocampal neurons to antiepileptic drugs such as carbamazepine or phenytoin. In line with this proposal, a reduced effect of carbamazepine on sodium currents in hippocampal CA1 neurons was found in the rat kindling model of temporal lobe epilepsy (TLE), i.e. a form of epilepsy with the poorest prognosis of all epilepsy types in adult patients. To address directly the possibility that neuronal sodium currents in the hippocampus play a crucial role in the pharmacoresistance of TLE, we selected amygdala-kindled rats with respect to their in vivo anticonvulsant response to phenytoin into responders and nonresponders and then compared phenytoin's effect on voltage-activated sodium currents in CA1 neurons. Furthermore, in view of the potential role of calcium current modulation in the anticonvulsant action of phenytoin, the effect of phenytoin on high-voltage-activated calcium currents was studied in CA1 neurons. Electrode-implanted but not kindled rats were used as sham controls for comparison with the kindled rats. In all experiments, the interval between last kindled seizure and ion channel measurements was at least 5 weeks. In kindled rats with in vivo resistance to the anticonvulsant effect of phenytoin (phenytoin nonresponders), in vitro modulation of sodium and calcium currents by phenytoin in hippocampal CA1 neurons did not significantly differ from respective data obtained in phenytoin responders, i.e. phenytoin resistance was not associated with a changed modulation of the sodium or calcium currents by this drug. Compared to sham controls, phenytoin's inhibitory effect on sodium currents was significantly reduced by kindling without difference between the responder and nonresponder subgroups. Further studies in phenytoin-resistant kindled rats may help to elucidate the mechanisms that can explain therapy resistance.