Block of deltamethrin-modified sodium current in cultured mouse neuroblastoma cells: local anesthetics as potential antidotes

Effective Perturbations by Phenobarbital on INa, IK(erg), IK(M) and IK(DR) during Pulse Train Stimulation in Neuroblastoma Neuro-2a Cells

Phenobarbital (PHB, Luminal Sodium^®) is a medication of the barbiturate and has long been recognized to be an anticonvulsant and a hypnotic because it can facilitate synaptic inhibition in the central nervous system through acting on the γ-aminobutyric acid (GABA) type A (GABA_A) receptors. However, to what extent PHB could directly perturb the magnitude and gating of different plasmalemmal ionic currents is not thoroughly explored. In neuroblastoma Neuro-2a cells, we found that PHB effectively suppressed the magnitude of voltage-gated Na⁺ current (I_Na) in a concentration-dependent fashion, with an effective IC₅₀ value of 83 µM. The cumulative inhibition of I_Na, evoked by pulse train stimulation, was enhanced by PHB. However, tefluthrin, an activator of I_Na, could attenuate PHB-induced reduction in the decaying time constant of I_Na inhibition evoked by pulse train stimuli. In addition, the erg (ether-à-go-go-related gene)-mediated K⁺ current (I_K(erg)) was also blocked by PHB. The PHB-mediated inhibition on I_K(erg) could not be overcome by flumazenil (GABA antagonist) or chlorotoxin (chloride channel blocker). The PHB reduced the recovery of I_K(erg) by a two-step voltage protocol with a geometrics-based progression, but it increased the decaying rate of I_K(erg), evoked by the envelope-of-tail method. About the M-type K⁺ currents (I_K(M)), PHB caused a reduction of its amplitude, which could not be counteracted by flumazenil or chlorotoxin, and PHB could enhance its cumulative inhibition during pulse train stimulation. Moreover, the magnitude of delayed-rectifier K⁺ current (I_K(DR)) was inhibited by PHB, while the cumulative inhibition of I_K(DR) during 10 s of repetitive stimulation was enhanced. Multiple ionic currents during pulse train stimulation were subject to PHB, and neither GABA antagonist nor chloride channel blocker could counteract these PHB-induced reductions. It suggests that these actions might conceivably participate in different functional activities of excitable cells and be independent of GABA_A receptors.

An 80-Year-Old Woman with Alzheimer Disease and Accidental Poisoning with Pyrethroid Pesticide Successfully Treated with Intravenous Lipid Emulsion

Patient: Female, 80-year-old

Final Diagnosis: Poisoning

Symptoms: Seizure

Medication: —

Clinical Procedure: —

Specialty: Critical Care Medicine

Pyrethroid Poisoning

Introduction

Pyrethroid compounds are widely used as insecticides. These compounds not only have a versatile application, but also have favourable toxicological profiles with high selectivity and toxicity to insects and low toxicity to humans. Despite this, there have been several reports of toxicity to humans in both occupational exposure and deliberate ingestional poisoning.

Classical presentation and treatment

Two classical syndromic presentations are described. Type I syndrome is characterised predominantly by tremors and is seen with exposure to type I pyrethroids. Type II pyrethroids, which are structurally modified type I pyrethroids with the addition of a cyano group, can result in type II syndrome characterized by choreo-athetosis and salivation. Mega-dose poisoning and mixed poisoning, particularly with organophosphorus compounds, is associated with significant toxicity and death. Treatment is supportive and symptomatic. A favourable outcome can be expected in most patients.

How to cite this article

Ramchandra AM, Chacko B, Victor PJ. Pyrethroid Poisoning. Indian J Crit Care Med 2019;23(Suppl 4):S267–S271.

The effects of combined exposure to the pyrethroids deltamethrin and S-bioallethrin on hippocampal inhibition and skeletal muscle hyperexcitability in rats

The default assumption that different pyrethroid insecticides, sharing a common mode of action, will show additivity of toxicity has not always been supported by in vitro measures, some of which have indicated antagonism. Our intention was to see whether the antagonism between pyrethroids of different classes seen in vitro could be reproduced in vivo. We therefore investigated the effects of single and combined exposures to two commonly used pyrethroids, deltamethrin (type II) and S-bioallethrin (type I) given intravenously to anaesthetised rats. We used two quantitative measures that are responsive to pyrethroids: the duration of prolongation of hippocampal dentate granule cell inhibition and the amplitude of the abnormal electromyogram discharge. At equi-toxic doses, S-bioallethrin extended the inter-stimulus interval evoking 50% inhibition in the hippocampus by 30+/-2.2 ms, and deltamethrin extended it by 199+/-21 ms. Combined administration of the same doses of deltamethrin and S-bioallethrin extended hippocampal inhibition by 164+/-14 ms, which did not differ significantly from the effect of deltamethrin alone. S-bioallethrin was without any effect on the electromyogram, and produced no significant change in the amplitude of the abnormal muscle discharges evoked by deltamethrin. The increase in arterial blood pressure evoked by the combination was significantly less than that evoked by either pyrethroid alone (p<0.001). In summary, although our electrophysiological indices provide no support for functional antagonism between these two pyrethroids, they also fail to indicate any summation of effect.

A reassessment of the neurotoxicity of pyrethroid insecticides

The pyrethroids are a widely used class of insecticides to which there is significant human exposure. They are however generally regarded as safe to man, and there have been few reports of human fatalities. Their acute toxicity is dominated by pharmacological actions upon the central nervous system (CNS), predominantly mediated by prolongation of the kinetics of voltage-gated sodium channels, although other mechanisms operate. This review summarizes our present understanding of such actions and the pharmacological options to antagonize them. One significant problem is the very clear heterogeneity of pyrethroid sensitivity that is seen across sodium channel subtypes; however, the distribution and function of these across the central nervous system are poorly characterized. The review also provides an overview of recent studies that suggest additional effects of pyrethroids: developmental neurotoxicity, the production of neuronal death, and action mediated via pyrethroid metabolites. The evidence for these is at present equivocal, but all 3 carry important implications for human health.

Identification of metabolic pathways involved in the biotransformation of tolperisone by human microsomal enzymes

The in vitro metabolism of tolperisone, 1-(4-methyl-phenyl)-2-methyl-3-(1-piperidino)-1-propanone-hydrochloride, a centrally acting muscle relaxant, was examined in human liver microsomes (HLM) and recombinant enzymes. Liquid chromatography-mass spectrometry measurements revealed methyl-hydroxylation (metabolite at m/z 261; M1) as the main metabolic route in HLM, however, metabolites of two mass units greater than the parent compound and the hydroxy-metabolite were also detected (m/z 247 and m/z 263, respectively). The latter was identified as carbonyl-reduced M1, the former was assumed to be the carbonyl-reduced parent compound. Isoform-specific cytochrome P450 (P450) inhibitors, inhibitory antibodies, and experiments with recombinant P450s pointed to CYP2D6 as the prominent enzyme in tolperisone metabolism. CYP2C19, CYP2B6, and CYP1A2 are also involved to a smaller extent. Hydroxymethyl-tolperisone formation was mediated by CYP2D6, CYP2C19, CYP1A2, but not by CYP2B6. Tolperisone competitively inhibited dextromethorphan O-demethylation and bufuralol hydroxylation (K(i) = 17 and 30 microM, respectively). Tolperisone inhibited methyl p-tolyl sulfide oxidation (K(i) = 1200 microM) in recombinant flavin-containing monooxygenase 3 (FMO3) and resulted in a 3-fold (p < 0.01) higher turnover number using rFMO3 than that of control microsomes. Experiments using nonspecific P450 inhibitors-SKF-525A, 1-aminobenzotriazole, 1-benzylimidazole, and anti-NADPH-P450-reductase antibodies-resulted in 61, 47, 49, and 43% inhibition of intrinsic clearance in HLM, respectively, whereas hydroxymethyl-metabolite formation was inhibited completely by nonspecific chemical inhibitors and by 80% with antibodies. Therefore, it was concluded that tolperisone undergoes P450-dependent and P450-independent microsomal biotransformations to the same extent. On the basis of metabolites formed and indirect evidences of inhibition studies, a considerable involvement of a microsomal reductase is assumed.

Pyrethroid insecticides: poisoning syndromes, synergies, and therapy

BACKGROUND

Pyrethroid insecticides are widely used, but there have been relatively few reports of systemic poisoning. These reports have, however, shown that pharmacotherapy is difficult and that the duration of poisoning can be unexpectedly long. Pyrethroids are ion channel toxins prolonging neuronal excitation, but are not directly cytotoxic. Two basic poisoning syndromes are seen. Type I pyrethroids produce reflex hyperexcitability and fine tremor. Type II pyrethroids produce salivation, hyperexcitability, choreoathetosis, and seizures. Both produce potent sympathetic activation. Local effects are also seen: skin contamination producing paresthesia and ingestion producing gastrointestinal irritation. The slow absorption of pyrethroids across the skin usually prevents systemic poisoning, although a significant reservoir of pyrethroid may remain bound to the epidermis. Carboxyesterase inhibitors can enhance pyrethroid toxicity in high-dose experimental studies. Hence, the unauthorized pyrethroid/organophosphate mixtures marketed in some developing countries may precipitate human poisoning. Pyrethroid paresthesia can be treated by decontamination of the skin, but systemic poisoning is difficult to control with anticonvulsants. Pentobarbitone, however, is surprisingly effective as therapy against systemic type II pyrethroid poisoning in rats, probably due to its dual action as a chloride channel agonist and a membrane stabilizer.

Neuronal ion channels as the target sites of insecticides

Certain types of neuronal ions channels have been demonstrated to be the major target sites of insecticides. The insecticide-channel interactions that have been studied most extensively are pyrethroid actions on the voltage-gated sodium channel and cyclodiene/lindane actions on the GABAA receptor chloride channel complex. With the exception of organophosphate and carbamate insecticides which inhibit acetylcholinesterases, most insecticide commercially developed act on the sodium channel and the GABA system. Pyrethroids show the kinetics of both activation and inactivation gates of sodium channels resulting in prolonged openings of individual channels. This causes membrane depolarization, repetitive discharges and synaptic disturbances leading to hyperexcitatory symptoms of poisoning in animals. Only a very small fraction (approximately 1%) of sodium channel population is required to be modified by pyrethroids to produce severe hyperexcitatory symptoms. This toxicity amplification theory applies to pharmacological and toxicological action of other drugs that go through a threshold phenomenon. Selective toxicity of pyrethroids between invertebrates and mammals can be explained based largely on the responses of sodium channels and partly on metabolic degradation. The pyrethroid-sodium channel interaction is also supported by Na+ uptake and batrachotoxin binding experiments. Cyclodienes and lindane exert a dual action on the GABAA system, the initial transient stimulation being followed by a suppression. The stimulation requires the presence of the gamma 2 subunit. The suppression of the GABA system is also documented by Cl- flux and ligand binding experiments. It appears that the sodium channel and the GABA system merit continuing efforts for development of newer and better insecticides. Nitromethylene heterocycles including imidacloprid act on nicotinic acetylcholine receptors. Insect receptors are more sensitive to these compounds than mammalian receptors. Single-channel analyses of the nicotinic acetylcholine receptor of PC12 cells have shown that imidacloprid increases the activity of subconductance state currents and decreases that of main conductance state currents. This may explain the imidacloprid suppression of acetylcholine responses.

Neurotoxicological effects and the mode of action of pyrethroid insecticides

Neuroexcitatory symptoms of acute poisoning of vertebrates by pyrethroids are related to the ability of these insecticides to modify electrical activity in various parts of the nervous system. Repetitive nerve activity, particularly in the sensory nervous system, membrane depolarization, and enhanced neurotransmitter release, eventually followed by block of excitation, result from a prolongation of the sodium current during membrane excitation. This effect is caused by a stereoselective and structure-related interaction with voltage-dependent sodium channels, the primary target site of the pyrethroids. Near-lethal doses of pyrethroids cause sparse axonal damage that is reversed in surviving animals. After prolonged exposure to lower doses of pyrethroids axonal damage is not observed. Occupational exposure to pyrethroids frequently leads to paresthesia and respiratory irritation, which are probably due to repetitive firing of sensory nerve endings. Massive exposure may lead to severe human poisoning symptoms, which are generally treated well by symptomatic and supportive measures.