Comparative toxicity of fumigants and a phosphine synergist using a novel containment chamber for the safe generation of concentrated phosphine gas

Pesticidal Toxicity of Phosphine and Its Interaction with Other Pest Control Treatments

Phosphine is the most widely used fumigant for stored grains due to a lack of better alternatives, all of which have serious shortcomings that restrict their use. The extensive use of phosphine has led to the development of resistance among insect pests of grain, which threatens its status as a reliable fumigant. Understanding the mode of action of phosphine as well as its resistance mechanisms provides insight that may lead to improved phosphine efficacy and pest control strategies. The mechanisms of action in phosphine vary from disrupting metabolism and oxidative stress to neurotoxicity. Phosphine resistance is genetically inherited and is mediated by the mitochondrial dihydrolipoamide dehydrogenase complex. In this regard, laboratory studies have revealed treatments that synergistically enhance phosphine toxicity that may be used to suppress resistance development and enhance efficacy. Here, we discuss the reported phosphine modes of action, mechanisms of resistance and interactions with other treatments.

Suppression of a core metabolic enzyme dihydrolipoamide dehydrogenase (dld) protects against amyloid beta toxicity in C. elegans model of Alzheimer's disease

A decrease in energy metabolism is associated with Alzheimer's disease (AD), but it is not known whether the observed decrease exacerbates or protects against the disease. The importance of energy metabolism in AD is reinforced by the observation that variants of dihydrolipoamide dehydrogenase (DLD), is genetically linked to late-onset AD. To determine whether DLD is a suitable therapeutic target, we suppressed the dld-1 gene in Caenorhabditis elegans that express human Aβ peptide in either muscles or neurons. Suppression of the dld-1 gene resulted in significant restoration of vitality and function that had been degraded by Aβ pathology. This included protection of neurons and muscles cells. The observed decrease in proteotoxicity was associated with a decrease in the formation of toxic oligomers rather than a decrease in the abundance of the Aβ peptide. The mitochondrial uncoupler, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP), which like dld-1 gene expression inhibits ATP synthesis, had no significant effect on Aβ toxicity. Proteomics data analysis revealed that beneficial effects after dld-1 suppression could be due to change in energy metabolism and activation of the pathways associated with proteasomal degradation, improved cell signaling and longevity. Thus, some features unique to dld-1 gene suppression are responsible for the therapeutic benefit. By direct genetic intervention, we have shown that acute inhibition of dld-1 gene function may be therapeutically beneficial. This result supports the hypothesis that lowering energy metabolism protects against Aβ pathogenicity and that DLD warrants further investigation as a therapeutic target.

Development of detection methods for the diagnosis and analysis of highly toxic metal phosphides: A comprehensive and critical review

Metal phosphides, especially aluminum phosphide, and phosphine (PH₃ ) are widely used as insecticides and rodenticides for protection of grains during process of storage and transportation. The main reason of poisoning with this compound is related to the conscious ingestion of salts or accidental inhalation of PH₃ . So the early and accurate diagnosis of poisoning can significantly help to the effective clinical treatment or recognition of death cause. PH₃ is somewhat unstable due to reaction with oxygen or hemoglobin leading to formation of oxy-acids phosphorous. Here, we critically reviewed the literature introducing the quantitative and qualitative methods for the detection of metal phosphides, PH₃ , and its products. This study obviously demonstrates that during past years, different diagnosis methods have been remarkably progressed. Head-space gas chromatography and confirmatory colorimetric methods have been as the most popular techniques. Also, the gas sensors are a promising method that must be more progressed.

DARK Classics in Chemical Neuroscience: Krokodil

Krokodil is the street name for a homemade mixture that has been used as a cheap substitute for heroin. The main active substance in krokodil is desomorphine, an opioid that is 10 times more potent than morphine. Krokodil use began in Russia and Ukraine but has spread throughout several countries in Europe and North America. Krokodil is produced from codeine tablets in a bootleg reaction performed under clandestine and unsanitary conditions. The toxicity of krokodil is characterized by devastating symptoms that start as black ulcers at the injection site and evolve to gangrene and limb amputation. The dangerous effects of krokodil are associated with its homemade nature and lack of purification prior to use. In this review, we discuss the chemical and pharmacological properties and the metabolism of desomorphine, the preparation of krokodil, and how its homemade nature contributes to its toxicity. The synthesis of krokodil produces several other morphinans in addition to desomorphine that warrant further study as possible analgesic alternatives to morphine.

Changes in ferrous iron and glutathione promote ferroptosis and frailty in aging Caenorhabditis elegans

All eukaryotes require iron. Replication, detoxification, and a cancer-protective form of regulated cell death termed ferroptosis, all depend on iron metabolism. Ferrous iron accumulates over adult lifetime in Caenorhabditis elegans. Here, we show that glutathione depletion is coupled to ferrous iron elevation in these animals, and that both occur in late life to prime cells for ferroptosis. We demonstrate that blocking ferroptosis, either by inhibition of lipid peroxidation or by limiting iron retention, mitigates age-related cell death and markedly increases lifespan and healthspan. Temporal scaling of lifespan is not evident when ferroptosis is inhibited, consistent with this cell death process acting at specific life phases to induce organismal frailty, rather than contributing to a constant aging rate. Because excess age-related iron elevation in somatic tissue, particularly in brain, is thought to contribute to degenerative disease, post-developmental interventions to limit ferroptosis may promote healthy aging.

High-dose strategies for managing phosphine-resistant populations of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae)

BACKGROUND

Rhyzopertha dominica is a serious pest of stored grains and many populations have resistance to the fumigant phosphine. Some populations contain beetles with a 'strong resistance' phenotype. Recent work found the LC₅₀ values for two strong-resistant populations recently studied in North America, Belle Glade and Minneapolis were 100- and 595-fold higher, respectively, compared to LC₅₀ of a lab-susceptible strain. Populations with 'weak-resistant' phenotypes had LC₅₀ values 5- to 10-fold that of a susceptible strain. The work reported below aimed to determine the minimum phosphine concentrations and number of days of exposure needed to effectively control all life stages of representative weak- and strong-resistant strains, and then to recommend the treatment conditions needed to control strongly phosphine-resistant R. dominica in pest populations.

RESULTS

A dose-mortality assay estimated that phosphine fumigation over 48 h using 730-870 ppm at 25° C would control adults of both strongly resistant R. dominica populations. Fumigations with mixed life stage cultures found 200 ppm killed all susceptible and weak-resistant beetles in 2 days, but the strong-resistant Minneapolis and Belle Glade strains had substantial survivors at 200 ppm. Furthermore, the Belle Glade strain had beetles that survived 1000 ppm in 2-day fumigations. The strong-resistant Belle Glade strain needed nearly 10 days at over 400 ppm to have acceptable levels of control.

CONCLUSION

This study recommends protocols to manage strongly resistant R. dominica populations provided that a minimum phosphine concentration of 400 ppm be maintained at 25° C or higher for up to 10 days. © 2019 Society of Chemical Industry.

Oxygen and Arsenite Synergize Phosphine Toxicity by Distinct Mechanisms

Phosphine is the only fumigant approved globally for general use to control insect pests in stored grain. Due to the emergence of resistance among insect pests and the lack of suitable alternative fumigants, we are investigating ways to synergistically enhance phosphine toxicity, by studying the mechanism of action of known synergists, such as oxygen, temperature, and arsenite. Under normoxia, exposure of the model organism Caenorhabditis elegans for 24 h at 20°C to 70 ppm phosphine resulted in 10% mortality, but nearly 100% mortality if the oxygen concentration was increased to 80%. In wild-type C. elegans, toxicity of phosphine was negatively affected by a decrease in temperature to 15°C and positively affected by an increase in temperature to 25°C. The dld-1(wr4) strain of C. elegans is resistant to phosphine due to a mutation in the dihydrolipoamide dehydrogenase gene. It also exhibits increased mortality that is dependent on hyperoxia, when exposed to 70 ppm phosphine at 20°C. As with the wild-type strain, mortality decreased when exposure was carried out at 15°C. At 25°C, however, the strain was completely resistant to the phosphine exposure at all oxygen concentrations. Arsenite is also a synergist of phosphine toxicity, but only in the dld-1(wr4)-mutant strain. Thus, exposure to 4 mM arsenite resulted in 50% mortality, which increased to 89% mortality when 70 ppm phosphine and 4 mM arsenite were combined. In stark contrast, 70 ppm phosphine rendered 4 mM arsenite nontoxic to wild-type C. elegans. These results reveal two synergists with distinct modes of action, one of which targets individuals that carry a phosphine resistance allele in the dihydrolipoamide dehydrogenase gene.

Response of Suidasia pontifica (Acaridida: Suidasiidae) to phosphine fumigation

The present study evaluated the level of phosphine toxicity of various strains of Suidasia pontifica, infesting stored agricultural products in the Philippines. The bioassays for the efficacy of phosphine fumigation followed the FAO standard method. A full assay comprised eight concentrations ranging from 0.005 to 1.00 mg/L with 20, 72 and 144-h exposure periods. A total of 15 strains were collected, representing 15 provinces in the country. Results revealed that the most susceptible strain (MR12gsn) was recorded from a private owned warehouse in General Santos City-LC₅₀ and LC₉₉ were 0.009 and 0.025 mg/L, respectively. On the other hand, the most resistant strain (LR3tar) was collected from a private owned warehouse in Tarlac City-LC₅₀ and LC₉₉ were 1.501 and 2.407 mg/L, based on discrimination dose tests. Eggs were more tolerant than the adults; complete mortality of eggs was achieved at 0.50 mg/L for 72 h and 0.35 mg/L for 144 h, whereas complete mortality of adults was obtained with 0.10 and 0.20 mg/L for 144 and 72 h exposure, respectively. The estimated minimum effective concentration (MEC) of phosphine to give complete kill of S. pontifica at various exposure periods must exceed 0.50 mg/L for 72 and 144 h. These results simply illustrate the increased efficiency (i.e., increased mortality) of phosphine fumigation of longer exposure of S. pontifica to lower concentrations.

Attenuation of radiation toxicity by the phosphine resistance factor dihydrolipoamide dehydrogenase (DLD)

Phosphine gas is an excellent fumigant for disinfesting stored grain of insect pests, but heavy reliance on phosphine has led to resistance in grain pests that threatens its efficacy. Phosphine-resistance was previously reported to be mediated by the enzyme DLD. Here we explore the relationship between phosphine toxicity and genotoxic treatments with the goal of understanding how phosphine works. Specifically, we utilized mutant lines either sensitive or resistant to phosphine, gamma irradiation or UV exposure. The phosphine-resistance mutation in the enzyme of energy metabolism, dihydrolipoamide dehydrogenase exhibited cross-resistance to UV and ionizing radiation. Two radiation-sensitive mutants that are defective in DNA repair as well as a mutant that is defective in the activation of the DAF-16 stress response transcription factor all exhibit sensitivity to phosphine that exceeds the sensitivity of the wild type control. A radiation resistance mutation in cep-1, the p53 orthologue, that is deficient in double strand break repair of DNA and is also deficient in apoptosis causes radiation-resistance results but sensitivity toward phosphine.

Stress pre-conditioning with temperature, UV and gamma radiation induces tolerance against phosphine toxicity

Phosphine is the only general use fumigant for the protection of stored grain, though its long-term utility is threatened by the emergence of highly phosphine-resistant pests. Given this precarious situation, it is essential to identify factors, such as stress preconditioning, that interfere with the efficacy of phosphine fumigation. We used Caenorhabditis elegans as a model organism to test the effect of pre-exposure to heat and cold shock, UV and gamma irradiation on phosphine potency. Heat shock significantly increased tolerance to phosphine by 3-fold in wild-type nematodes, a process that was dependent on the master regulator of the heat shock response, HSF-1. Heat shock did not, however, increase the resistance of a strain carrying the phosphine resistance mutation, dld-1(wr4), and cold shock did not alter the response to phosphine of either strain. Pretreatment with the LD50 of UV (18 J cm-2) did not alter phosphine tolerance in wild-type nematodes, but the LD50 (33 J cm-2) of the phosphine resistant strain (dld-1(wr4)) doubled the level of resistance. In addition, exposure to a mild dose of gamma radiation (200 Gy) elevated the phosphine tolerance by ~2-fold in both strains.

Automated Wormscan

There has been a recent surge of interest in computer-aided rapid data acquisition to increase the potential throughput and reduce the labour costs of large scale Caenorhabditis elegans studies. We present Automated WormScan, a low-cost, high-throughput automated system using commercial photo scanners, which is extremely easy to implement and use, capable of scoring tens of thousands of organisms per hour with minimal operator input, and is scalable. The method does not rely on software training for image recognition, but uses the generation of difference images from sequential scans to identify moving objects. This approach results in robust identification of worms with little computational demand. We demonstrate the utility of the system by conducting toxicity, growth and fecundity assays, which demonstrate the consistency of our automated system, the quality of the data relative to manual scoring methods and congruity with previously published results.

Developing effective fumigation protocols to manage strongly phosphine-resistant Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae)

BACKGROUND

The emergence of high levels of resistance in Cryptolestes ferrugineus (Stephens) in recent years threatens the sustainability of phosphine, a key fumigant used worldwide to disinfest stored grain. We aimed at developing robust fumigation protocols that could be used in a range of practical situations to control this resistant pest.

RESULTS

Values of the lethal time to kill 99.9% (LT99 .9 , in days) of mixed-age populations, containing all life stages, of a susceptible and a strongly resistant C. ferrugineus population were established at three phosphine concentrations (1.0, 1.5 and 2.0 mg L(-1) ) and three temperatures (25, 30 and 35 °C). Multiple linear regression analysis revealed that phosphine concentration and temperature both contributed significantly to the LT99 .9 of a population (P < 0.003, R2 = 0.92), with concentration being the dominant variable, accounting for 75.9% of the variation. Across all concentrations, LT99.9 of the strongly resistant C. ferrugineus population was longest at the lowest temperature and shortest at the highest temperature. For example, 1.0 mg L(-1) of phosphine is required for 20, 15 and 15 days, 1.5 mg L(-1) for 12, 11 and 9 days and 2.0 mg L(-1) for 10, 7 and 6 days at 25, 30 and 35 °C, respectively, to achieve 99.9% mortality of the strongly resistant C. ferrugineus population. We also observed that phosphine concentration is inversely proportional to fumigation period in regard to the population extinction of this pest.

CONCLUSION

The fumigation protocols developed in this study will be used in recommending changes to the currently registered rates of phosphine in Australia towards management of strongly resistant C. ferrugineus populations, and can be repeated in any country where this type of resistance appears.

The rph2 gene is responsible for high level resistance to phosphine in independent field strains of Rhyzopertha dominica

The lesser grain borer Rhyzopertha dominica (F.) is one of the most destructive insect pests of stored grain. This pest has been controlled successfully by fumigation with phosphine for the last several decades, though strong resistance to phosphine in many countries has raised concern about the long term usefulness of this control method. Previous genetic analysis of strongly resistant (SR) R. dominica from three widely geographically dispersed regions of Australia, Queensland (SR_QLD), New South Wales (SR_NSW) and South Australia (SR_SA), revealed a resistance allele in the rph1 gene in all three strains. The present study confirms that the rph1 gene contributes to resistance in a fourth strongly resistant strain, SR2_QLD, also from Queensland. The previously described rph2 gene, which interacts synergistically with rph1 gene, confers strong resistance on SR_QLD and SR_NSW. We now provide strong circumstantial evidence that weak alleles of rph2, together with rph1, contribute to the strong resistance phenotypes of SR_SA and SR2_QLD. To test the notion that rph1 and rph2 are solely responsible for the strong resistance phenotype of all resistant R. dominica, we created a strain derived by hybridising the four strongly resistant lines. Following repeated selection for survival at extreme rates of phosphine exposure, we found only slightly enhanced resistance. This suggests that a single sequence of genetic changes was responsible for the development of resistance in these insects.

WormScan: a technique for high-throughput phenotypic analysis of Caenorhabditis elegans

Background

There are four main phenotypes that are assessed in whole organism studies of Caenorhabditis elegans; mortality, movement, fecundity and size. Procedures have been developed that focus on the digital analysis of some, but not all of these phenotypes and may be limited by expense and limited throughput. We have developed WormScan, an automated image acquisition system that allows quantitative analysis of each of these four phenotypes on standard NGM plates seeded with E. coli. This system is very easy to implement and has the capacity to be used in high-throughput analysis.

Methodology/Principal Findings

Our system employs a readily available consumer grade flatbed scanner. The method uses light stimulus from the scanner rather than physical stimulus to induce movement. With two sequential scans it is possible to quantify the induced phototactic response. To demonstrate the utility of the method, we measured the phenotypic response of C. elegans to phosphine gas exposure. We found that stimulation of movement by the light of the scanner was equivalent to physical stimulation for the determination of mortality. WormScan also provided a quantitative assessment of health for the survivors. Habituation from light stimulation of continuous scans was similar to habituation caused by physical stimulus.

Conclusions/Significance

There are existing systems for the automated phenotypic data collection of C. elegans. The specific advantages of our method over existing systems are high-throughput assessment of a greater range of phenotypic endpoints including determination of mortality and quantification of the mobility of survivors. Our system is also inexpensive and very easy to implement. Even though we have focused on demonstrating the usefulness of WormScan in toxicology, it can be used in a wide range of additional C. elegans studies including lifespan determination, development, pathology and behavior. Moreover, we have even adapted the method to study other species of similar dimensions.

The rph1 gene is a common contributor to the evolution of phosphine resistance in independent field isolates of Rhyzopertha dominica

Phosphine is the only economically viable fumigant for routine control of insect pests of stored food products, but its continued use is now threatened by the world-wide emergence of high-level resistance in key pest species. Phosphine has a unique mode of action relative to well-characterised contact pesticides. Similarly, the selective pressures that lead to resistance against field sprays differ dramatically from those encountered during fumigation. The consequences of these differences have not been investigated adequately. We determine the genetic basis of phosphine resistance in Rhyzopertha dominica strains collected from New South Wales and South Australia and compare this with resistance in a previously characterised strain from Queensland. The resistance levels range from 225 and 100 times the baseline response of a sensitive reference strain. Moreover, molecular and phenotypic data indicate that high-level resistance was derived independently in each of the three widely separated geographical regions. Despite the independent origins, resistance was due to two interacting genes in each instance. Furthermore, complementation analysis reveals that all three strains contain an incompletely recessive resistance allele of the autosomal rph1 resistance gene. This is particularly noteworthy as a resistance allele at rph1 was previously proposed to be a necessary first step in the evolution of high-level resistance. Despite the capacity of phosphine to disrupt a wide range of enzymes and biological processes, it is remarkable that the initial step in the selection of resistance is so similar in isolated outbreaks.

Managing aluminum phosphide poisonings

Aluminum phosphide (AlP) is a cheap, effective and commonly used pesticide. However, unfortunately, it is now one of the most common causes of poisoning among agricultural pesticides. It liberates lethal phosphine gas when it comes in contact either with atmospheric moisture or with hydrochloric acid in the stomach. The mechanism of toxicity includes cellular hypoxia due to the effect on mitochondria, inhibition of cytochrome C oxidase and formation of highly reactive hydroxyl radicals. The signs and symptoms are nonspecific and instantaneous. The toxicity of AlP particularly affects the cardiac and vascular tissues, which manifest as profound and refractory hypotension, congestive heart failure and electrocardiographic abnormalities. The diagnosis of AlP usually depends on clinical suspicion or history, but can be made easily by the simple silver nitrate test on gastric content or on breath. Due to no known specific antidote, management remains primarily supportive care. Early arrival, resuscitation, diagnosis, decrease the exposure of poison (by gastric lavage with KMnO(4), coconut oil), intensive monitoring and supportive therapy may result in good outcome. Prompt and adequate cardiovascular support is important and core in the management to attain adequate tissue perfusion, oxygenation and physiologic metabolic milieu compatible with life until the tissue poison levels are reduced and spontaneous circulation is restored. In most of the studies, poor prognostic factors were presence of acidosis and shock. The overall outcome improved in the last decade due to better and advanced intensive care management.

Caenorhabditis elegans generates biologically relevant levels of genotoxic metabolites from aflatoxin B1 but not benzo[a]pyrene in vivo

There is relatively little information regarding the critical xenobiotic-metabolizing cytochrome P450 (CYP) enzymes in Caenorhabditis elegans, despite this organism's increasing use as a model in toxicology and pharmacology. We carried out experiments to elucidate the capacity of C. elegans to metabolically activate important promutagens via CYPs. Phylogenetic comparisons confirmed an earlier report indicating a lack of CYP1 family enzymes in C. elegans. Exposure to aflatoxin B(1) (AFB(1)), which is metabolized in mammals by CYP1, CYP2, and CYP3 family enzymes, resulted in significant DNA damage in C. elegans. However, exposure to benzo[a]pyrene (BaP), which is metabolized in mammals by CYP1 family enzymes only, produced no detectable damage. To further test whether BaP exposure caused DNA damage, the toxicities of AFB(1) and BaP were compared in nucleotide excision repair (NER)-deficient (xpa-1) and NER-proficient (N2) strains of C. elegans. Exposure to AFB(1) inhibited growth more in xpa-1 than N2 nematodes, but the growth-inhibitory effects of BaP were indistinguishable in the two strains. Finally, a CYP-nicotinamide adenine dinucleotide phosphate reductase-deficient strain (emb-8) of C. elegans was found to be more resistant to the growth-inhibitory effect of AFB(1) exposure than N2, confirming that the AFB(1)-mediated growth inhibition resulted from CYP-mediated metabolism. Together, these results indicate that C. elegans lacks biologically significant CYP1 family-mediated enzymatic metabolism of xenobiotics. Interestingly, we also found that xpa-1 nematodes were slightly more sensitive to chlorpyrifos than were wild type. Our results highlight the importance of considering differences between xenobiotic metabolism in C. elegans and mammals when using this alternative model in pharmaceutical and toxicological research.

Mitochondrial uncouplers act synergistically with the fumigant phosphine to disrupt mitochondrial membrane potential and cause cell death

Phosphine is the most widely used fumigant for the protection of stored commodities against insect pests, especially food products such as grain. However, pest insects are developing resistance to phosphine and thereby threatening its future use. As phosphine inhibits cytochrome c oxidase (complex IV) of the mitochondrial respiratory chain and reduces the strength of the mitochondrial membrane potential (DeltaPsi(m)), we reasoned that mitochondrial uncouplers should act synergistically with phosphine. The mitochondrial uncouplers FCCP and PCP caused complete mortality in populations of both wild-type and phosphine-resistant lines of Caenorhabditis elegans simultaneously exposed to uncoupler and phosphine at concentrations that were individually nonlethal. Strong synergism was also observed with a third uncoupler DNP. We have also tested an alternative complex IV inhibitor, azide, with FCCP and found that this also caused a synergistic enhancement of toxicity in C. elegans. To investigate potential causes of the synergism, we measured DeltaPsi(m), ATP content, and oxidative damage (lipid hydroperoxides) in nematodes subjected to phosphine-FCCP treatment and found that neither an observed 50% depletion in ATP nor oxidative stress accounted for the synergistic effect. Instead, a synergistic reduction in DeltaPsi(m) was observed upon phosphine-FCCP co-treatment suggesting that this is directly responsible for the subsequent mortality. These results support the hypothesis that phosphine-induced mortality results from the in vivo disruption of normal mitochondrial activity. Furthermore, we have identified a novel pathway that can be targeted to overcome genetic resistance to phosphine.

Mitochondrial modulation of phosphine toxicity and resistance in Caenorhabditis elegans

Phosphine is a fumigant used to protect stored commodities from infestation by pest insects, though high-level phosphine resistance in many insect species threatens the continued use of the fumigant. The mechanisms of toxicity and resistance are not clearly understood. In this study, the model organism, Caenorhabditis elegans, was employed to investigate the effects of phosphine on its proposed in vivo target, the mitochondrion. We found that phosphine rapidly perturbs mitochondrial morphology, inhibits oxidative respiration by 70%, and causes a severe drop in mitochondrial membrane potential (DeltaPsim) within 5 h of exposure. We then examined the phosphine-resistant strain of nematode, pre-33, to determine whether resistance was associated with any changes to mitochondrial physiology. Oxygen consumption was reduced by 70% in these mutant animals, which also had more mitochondrial genome copies than wild-type animals, a common response to reduced metabolic capacity. The mutant also had an unexpected increase in the basal DeltaPsim, which protected individuals from collapse of the membrane potential following phosphine treatment. We tested whether directly manipulating mitochondrial function could influence sensitivity toward phosphine and found that suppression of mitochondrial respiratory chain genes caused up to 10-fold increase in phosphine resistance. The current study confirms that phosphine targets the mitochondria and also indicates that direct alteration of mitochondrial function may be related to phosphine resistance.

Calcium-activated potassium channels in insect pacemaker neurons as unexpected target site for the novel fumigant dimethyl disulfide

Dimethyl disulfide (DMDS), a plant-derived insecticide, is a promising fumigant as a substitute for methyl bromide. To further understand the mode of action of DMDS, we examined its effect on cockroach octopaminergic neurosecretory cells, called dorsal unpaired median (DUM) neurons, using whole-cell patch-clamp technique, calcium imaging and antisense oligonucleotide strategy. At low concentration (1 microM), DMDS modified spontaneous regular spike discharge into clear bursting activity associated with a decrease of the amplitude of the afterhyperpolarization. This effect led us to suspect alterations of calcium-activated potassium currents (IKCa) and [Ca(2+)](i) changes. We showed that DMDS reduced amplitudes of both peak transient and sustained components of the total potassium current. IKCa was confirmed as a target of DMDS by using iberiotoxin, cadmium chloride, and pSlo antisense oligonucleotide. In addition, we showed that DMDS induced [Ca(2+)](i) rise in Fura-2-loaded DUM neurons. Using calcium-free solution, and (R,S)-(3,4-dihydro-6,7-dimethoxy-isoquinoline-1-yl)-2-phenyl-N,N-di-[2-(2,3,4-trimethoxy-phenyl)ethyl]-acetamide (LOE 908) [an inhibitor of transient receptor potential (TRP)gamma], we demonstrated that TRPgamma initiated calcium influx. By contrast, omega-conotoxin GVIA (an inhibitor of N-type high-voltage-activated calcium channels), did not affect the DMDS-induced [Ca(2+)](i) rise. Finally, the participation of the calcium-induced calcium release mechanism was investigated using thapsigargin, caffeine, and ryanodine. Our study revealed that DMDS-induced elevation in [Ca(2+)](i) modulated IKCa in an unexpected bell-shaped manner via intracellular calcium. In conclusion, DMDS affects multiple targets, which could be an effective way to improve pest control efficacy of fumigation.