The neuroleptic drug, fluphenazine, blocks neuronal voltage-gated sodium channels

Expanding the Toolbox: Novel Modulators of Endolysosomal Cation Channels

Functional characterization of endolysosomal ion channels is challenging due to their intracellular location. With recent advances in endolysosomal patch clamp technology, it has become possible to directly measure ion channel currents across endolysosomal membranes. Members of the transient receptor potential (TRP) cation channel family, namely the endolysosomal TRPML channels (TRPML1-3), also called mucolipins, as well as the distantly related two-pore channels (TPCs) have recently been characterized in more detail with endolysosomal patch clamp techniques. However, answers to many physiological questions require work in intact cells or animal models. One major obstacle thereby is that the known endogenous ligands of TRPMLs and TPCs are anionic in nature and thus impermeable for cell membranes. Microinjection, on the other hand, is technically demanding. There is also a risk of losing essential co-factors for channel activation or inhibition in isolated preparations. Therefore, lipophilic, membrane-permeable small-molecule activators and inhibitors for TRPMLs and TPCs are urgently needed. Here, we describe and discuss the currently available small-molecule modulators of TRPMLs and TPCs.

Characterization of Endogenous Sodium Channels in the ND7-23 Neuroblastoma Cell Line: Implications for Use as a Heterologous Ion Channel Expression System Suitable for Automated Patch Clamp Screening

The rodent neuroblastoma cell line, ND7-23, is used to express voltage-dependent sodium (Nav) and other neuronal ion channels resistant to heterologous expression in Chinese hamster ovary (CHO) or human embryonic kidney (HEK) cells. Their advantage is that they provide endogenous factors and signaling pathways to promote ion channel peptide folding, expression, and function at the cell surface and are also amenable to automated patch clamping. However, ND7-23 cells exhibit endogenous tetrodotoxin (TTX)-sensitive Nav currents, and molecular profiling has revealed the presence of Nav1.2, Nav1.3, Nav1.6, and Nav1.7 transcripts, but no study has determined which subtypes contribute to functional channels at the cell surface. We profiled the repertoire of functional Nav channels endogenously expressed in ND7-23 cells using the QPatch automated patch clamp platform and selective toxins and small molecules. The potency and subtype selectivity of the ligands (Icagen compound 68 from patent US-20060025415-A1-20060202, 4,9 anhydro TTX, and Protoxin-II) were established in human Nav1.3, Nav1.6, and Nav1.7 channel cell lines before application of selective concentrations to ND7-23 cells. Our data confirm previous studies that >97% of macroscopic Nav current in ND7-23 cells is carried by TTX-sensitive channels (300 nM TTX) and that Nav1.7 is the predominant channel contributing to this response (65% of peak inward current), followed by Nav1.6 (∼20%) and negligible Nav1.3 currents (∼2%). In addition, our data are the first to assess the Nav1.6 potency (50% inhibitory concentration [IC₅₀] of 33 nM) and selectivity (50-fold over Nav1.7) of 4,9 anhydro TTX in human Nav channels expressed in mammalian cells, confirming previous studies of rodent Nav channels expressed in oocytes and HEK cells.

Effects of phenothiazine-class antipsychotics on the function of α7-nicotinic acetylcholine receptors

The effects of phenothiazine-class antipsychotics (chlorpromazine, fluphenazine, phenothiazine, promazine, thioridazine, and triflupromazine) upon the function of the cloned α₇ subunit of the human nicotinic acetylcholine receptor expressed in Xenopus oocytes were tested using the two-electrode voltage-clamp technique. Fluphenazine, thioridazine, triflupromazine, chlorpromazine, and promazine reversibly inhibited acetylcholine (100 μM)-induced currents with IC₅₀ values of 3.8; 5.8; 6.1; 10.6 and 18.3 μM, respectively. Unsubstituted phenothiazine did not have a significant effect up to a concentration of 30 μM. Inhibition was further characterized using fluphenazine, the strongest inhibitor. The effect of fluphenazine was not dependent on the membrane potential. Fluphenazine (10 μM) did not affect the activity of endogenous Ca²⁺-dependent Cl⁻ channels, since the extent of inhibition by fluphenazine was unaltered by intracellular injection of the Ca²⁺ chelator BAPTA and perfusion with Ca²⁺-free bathing solution containing 2 mM Ba²⁺. Inhibition by fluphenazine, but not by chlorpromazine was reversed by increasing acetylcholine concentrations. Furthermore, specific binding of [¹²⁵I] α-bungarotoxin, a radioligand selective for α₇-nicotinic acetylcholine receptor, was inhibited by fluphenazine (10 μM), but not by chlorpromazine in oocyte membranes. In hippocampal slices, epibatidine-evoked [³H] norepinephrine release was also inhibited by fluphenazine (10 μM) and chlorpromazine (10 μM). Our results indicate that phenothiazine-class typical antipsychotics inhibit, with varying potencies, the function of α₇-nicotinic acetylcholine receptor.

Infection of primary neurons mediated by nipah virus envelope proteins: role of host target cells in antiviral action

We have previously described heterotypic peptides from parainfluenza virus that potently inhibit Nipah virus in vitro but are not efficacious in vivo. In contrast, our second-generation inhibitors, featuring a cholesterol moiety, are also efficacious in vivo. The difference between in vitro and in vivo results led us to investigate the basis for this discrepancy. Here, we compare the activities of the compounds in standard laboratory cells and in cells relevant to the natural tropism of Nipah virus, i.e., primary neurons, and show that while our first-generation inhibitors are poorly active in primary neurons, the cholesterol-conjugated compounds are highly potent. These results highlight the advantage of evaluating antiviral potency in cells relevant to natural host target tissue.

Rapid effects of corticosterone in the mouse dentate gyrus via a nongenomic pathway

Corticosterone activates two types of intracellular receptors in the rodent brain: the high affinity mineralocorticoid receptor (MR) and lower affinity glucocorticoid receptor (GR). These receptors act as transcriptional regulators and mediate slow changes in neuronal activity in a region-dependent manner. For example, in CA1 pyramidal cells, corticosterone slowly changes Ca(2+) currents and glutamate transmission but dentate granule cells appear to be resistant. Recent studies have shown that corticosteroids also exert rapid MR-dependent, nongenomic effects on hippocampal CA1 cells [e.g. increasing the frequency of miniature excitatory postsynaptic currents (mEPSCs)]. In the present study, we investigated whether dentate granule cells are also resistant to the rapid effects of corticosterone. We found that, comparable to the CA1 area, corticosterone quickly and reversibly increases mEPSC frequency but not amplitude of dentate cells. This effect did not require protein synthesis and displayed the pharmacological profile of an MR- rather than GR-dependent event. These data support the hypothesis that, unlike the slow gene-mediated effects of corticosterone, rapid hormonal actions are quite similar for CA1 and dentate cells.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

Coregulation of genes in the mouse brain following treatment with clozapine, haloperidol, or olanzapine implicates altered potassium channel subunit expression in the mechanism of antipsychotic drug action

BACKGROUND

Antipsychotic drugs are the most effective treatment for the psychotic symptoms of schizophrenia, yet their mechanism of action remains largely unknown.

OBJECTIVES

Earlier studies have shown gene expression changes in rodent brains after treatment with antipsychotic drugs. We aimed to further characterize these changes using whole-genome transcript profiling to explore coregulation of genes after multiple antipsychotic drug treatment studies.

METHODS

This study involved transcript profile analysis after 7-day treatment of inbred C57BL/6 mice with conventional (haloperidol) or atypical (clozapine or olanzapine) antipsychotic drugs. Microarray analysis was undertaken using whole-brain mRNA on Affymetrix 430v2 arrays, with quantitative reverse transcriptase-PCR used to confirm gene expression changes. Western blotting was also used to explore translation of gene dysregulation to protein changes and to explore anatomical specificity of such changes.

MAIN RESULTS

Thirteen genes showed verified regulation by multiple antipsychotic drugs - three genes significantly upregulated and 10 genes significantly downregulated by treatment. These genes encode proteins that function in various biological processes including neurogenesis, cell adhesion, and four genes are involved in voltage-gated ion channels: neural precursor cell developmentally downregulated gene 4 (Nedd4), Kv channel interacting protein 3 (KChip3), potassium voltage-gated channel, shaker-related subfamily, alpha1 (Kcna1) encoding Kv1.1 protein and beta1 (Kcnab1) encoding Kvbeta1 protein. The translation of these gene expression changes to protein dysregulation for Kv1.1, KCHIP3, and NEDD4 was confirmed by western blot, with regional protein analyses undertaken for Kv1.1 and KCHIP3.

CONCLUSION

These results suggest that transcriptional regulation of ion channels, crucial for neurotransmission, may play a role in mediating antipsychotic drug effects.

The antipsychotic drug, fluphenazine, effectively reverses mechanical allodynia in rat models of neuropathic pain

RATIONALE

Fluphenazine is a potent antipsychotic drug used to treat schizophrenia and other psychotic symptoms. Its clinical benefit is mainly mediated by the antagonism of dopamine D2 receptors. We have recently discovered, however, that fluphenazine is also a potent sodium channel blocker, a property that may offer additional therapeutical indications, including analgesia.

OBJECTIVES

The present study sought to determine the analgesic effect of fluphenazine on neuropathic pain in animal models.

METHODS

The effect of fluphenazine on mechanical allodynia was assessed in three animal neuropathic pain models, including spinal nerve ligation, chronic constriction nerve injury (CCI), and sural-spared sciatic nerve injury models.

RESULTS

Systemic fluphenazine effectively attenuated mechanical allodynia in all three rat neuropathic pain models at doses (0.03-0.3 mg/kg) that approximate those used in rodent models of psychosis. In parallel with its in vivo antiallodynic effect, fluphenazine (3-30 microM) effectively suppressed the ectopic discharges in injured afferent fibers without affecting the propagation of action potentials evoked by electrical nerve stimulation in an ex vivo dorsal root ganglia (DRG)-nerve preparation excised from CCI rats. Furthermore, similar concentrations of fluphenazine significantly blocked sodium channels in DRG neurons.

CONCLUSIONS

The inhibitory action of fluphenazine on ectopic afferent discharges may be due to its ability to block voltage-gated sodium channels, and this may also provide a mechanistic basis for the drug's antiallodynic effect in animal models of neuropathic pain. In summary, our study demonstrates that the classic antipsychotic drug fluphenazine has antiallodynic properties in multiple rodent models of nerve injury-induced neuropathic pain.

Novel steroidal saponins, Sch 725737 and Sch 725739, from a marine starfish, Novodinia antillensis

Bioassay-guided fractionation of an active fraction from an extract of a marine starfish, Novodinia antillensis, led to the isolation and identification of two new saponins, Sch 725737 (1) and Sch 725739 (2). Compound 1 was identified as the NaV1.8 inhibitor with IC(50) of approximately 9 microM. The purification and the structure elucidation of these two saponins are described.

Small interfering RNA-mediated selective knockdown of NaV1.8 tetrodotoxin-resistant sodium channel reverses mechanical allodynia in neuropathic rats

The biophysical properties of a tetrodotoxin resistant (TTXr) sodium channel, Na(V)1.8, and its restricted expression to the peripheral sensory neurons suggest that blocking this channel might have therapeutic potential in various pain states and may offer improved tolerability compared with existing sodium channel blockers. However, the role of Na(V)1.8 in nociception cannot be tested using a traditional pharmacological approach with small molecules because currently available sodium channel blockers do not distinguish between sodium channel subtypes. We sought to determine whether small interfering RNAs (siRNAs) might be capable of achieving the desired selectivity. Using Northern blot analysis and membrane potential measurement, several siRNAs were identified that were capable of a highly-selective attenuation of Na(V)1.8 message as well as functional expression in clonal ND7/23 cells which were stably transfected with the rat Na(V)1.8 gene. Functional knockdown of the channel was confirmed using whole-cell voltage-clamp electrophysiology. One of the siRNA probes showing a robust knockdown of Na(V)1.8 current was evaluated for in vivo efficacy in reversing an established tactile allodynia in the rat chronic constriction nerve-injury (CCI) model. The siRNA, which was delivered to lumbar dorsal root ganglia (DRG) via an indwelling epidural cannula, caused a significant reduction of Na(V)1.8 mRNA expression in lumbar 4 and 5 (L4-L5) DRG neurons and consequently reversed mechanical allodynia in CCI rats. We conclude that silencing of Na(V)1.8 channel using a siRNA approach is capable of producing pain relief in the CCI model and further support a role for Na(V)1.8 in pathological sensory dysfunction.