Fatty Acid and Related Potassium Kv2 Channel Blockers: Toxicity and Physiological Actions on Mosquitoes

Repellency, toxicity, and physiological actions of low molecular weight basic amines in mosquitoes

BACKGROUND

This study investigated the behavioral responses and toxicity of three basic amines: 1-methylpiperazine, 1-methylpyrrolidine, and triethylamine (TEA), compounds suggested previously to be anosmic in vapor exposures to caged mosquitoes.

RESULTS

These compounds showed repellency of Aedes aegypti mosquitoes, followed by flightlessness, knockdown, and paralysis, all increasing with exposure time and dosage. Electrophysiological experiments showed a blocking effect on nerve discharge of the Drosophila melanogaster larval central nervous system (CNS) with little evidence of hyperexcitation. Blockage of voltage-gated (Kv2) potassium channel currents under patch clamp occurred at similar concentrations. Involvement of K+ channels in the action of basic amines was supported by behavior and CNS recordings of a Shaker Kv1 mutant exposed to TEA, where instead of blockage, a hyperexcitation of nerve firing was observed. Experiments on cockroach leg mechanoreceptors demonstrated neuronal excitation and on mosquito antennae strong electroantennogram (EAG) signals with an augmentation of blank air responses after a single puff of basic amine.

CONCLUSIONS

The neurophysiological effects of basic amines are consistent with K+ channel block, whereas the antennal EAG response was not obviously associated with anosmia. The low-dose effects of basic amines appear to be repellency and bradykinesia. Overall, the findings provide key insights into the mechanisms underlying the biological activity of basic amines. © 2024 Society of Chemical Industry.

Identification and pharmacological characterization of the voltage-gated potassium channel Shab in diamondback moth, Plutella xylostella

BACKGROUND

Voltage-gated potassium channel Kv2 is the primarily delayed rectifier in insect nerves and muscles involved in several crucial biological processes, including action potential regulation, photoreceptor performance, and larval locomotor. It is a potential molecular target for developing a novel pesticide for mosquitos. However, there are few studies on the Kv2 channel in agricultural pests.

RESULTS

The only α-subunit gene of the Kv2 channel in Plutella xylostella (L.), PxShab, was cloned, and its expression profile was analyzed. The relative expression level of PxShab was highest in the pupal stage of both sexes and male adults but lowest in female adults. Meanwhile, PxShab had the highest expression in the head in both larvae and adults. Then, PxShab was stably expressed in the HEK-293 T cell line. Whole cell patch clamp recordings showed an outward current whose current-voltage relationship conformed to a typical delayed-rectifier potassium channel. 20 μM quinidine could effectively inhibit the potassium current, while the channel was insensitive to 4-AP even at 10 mM. Several potential compounds and botanical pesticides were assessed, and carvedilol (IC₅₀  = 0.53 μM) and veratrine (IC₅₀  = 2.22 μM) had a good inhibitory effect on the channel.

CONCLUSION

This study revealed the pharmacological properties of PxShab and screened out several high potency inhibitors, which laid the foundation for further functional research of PxShab and provides new insight into designing novel insecticides. © 2022 Society of Chemical Industry.

Recording central neurophysiological output from mosquito larvae for neuropharmacological and insecticide resistance studies

Resistance to currently utilized chemical insecticidal agents represents a significant threat to public health and food security worldwide. Better understanding the neurophysiological effects of available and candidate insecticidal molecules is valuable for characterizing the mechanisms of insecticide resistance, as well as the design and study of novel control chemistries. In this paper, we describe a method of recording nerve firing from the central nervous system of Aedes aegypti fourth instar larvae. In short, mosquito larvae were immobilized by placing small pins through the head and siphon of the larvae in a wax dish, ventral side down. A single, longitudinal, dorsal incision from the distal abdomen to the pronotum of the larva was made, the alimentary canal removed, and the ventral nerve cord severed between the second and third abdominal ganglia. A recording suction electrode was connected directly to axons within the severed end of the connective in a novel way to record nerve firing in the ventral nerve cord at a high signal-to-noise ratio with conventional electrophysiological equipment. Using this novel method, we report the effects of four neuroactive compounds using this method: octopamine, pilocarpine, nicotine, and γ-aminobutyric acid (GABA). The utility of this recording technique for elucidating target site mechanisms involved in insecticide resistance is demonstrated with p,p'-dichlorodiphenyltrichlorethane (DDT) and its difluoro analog (DFDT).

Toxicity and mode of action of the aporphine plant alkaloid liriodenine on the insect GABA receptor

Liriodenine is a biologically active plant alkaloid with multiple effects on mammals, fungi, and bacteria, but has never been evaluated for insecticidal activity. Accordingly, liriodenine was applied topically in ethanolic solutions to adult female Anopheles gambiae, and found to be mildly toxic. Its lethality was synergized in mixtures with dimethyl sulfoxide and piperonyl butoxide. Recordings from the ventral nerve cord of larval Drosophila melanogaster showed that liriodenine was neuroexcitatory and reversed the inhibitory effect of 1 mM GABA at effective concentrations of 20-30 μM. GABA antagonism on the larval nervous system was equally expressed on both susceptible and cyclodiene-resistant rdl preparations. Acutely isolated neurons from Periplaneta americana were studied under patch clamp and inhibition of GABA-induced currents with an IC₅₀ value of about 1 μM were observed. In contrast, bicuculline did not reverse the effects of GABA on cockroach neurons, as expected. In silico molecular models suggested reasonable structural concordance of liriodenine and bicuculline and isosteric hydrogen bond acceptor sites. This study is the first assessing of the toxicology of liriodenine on insects and implicates the GABA receptor as one likely neuronal target, where liriodenine might be considered an active chemical analog of bicuculline.

Synergistic effects of potassium channel blockers and pyrethroids: mosquitocidal activity and neuronal mode of action†

BACKGROUND

The purpose of this research was to explore the possibility of co-applying pyrethroids (agonists of voltage-sensitive sodium channels) with potassium channel blockers in order to potentiate the neurological effects of pyrethroids on Anopheles gambiae. We hypothesized that the toxicity of pyrethroids caused by persistent sodium currents would be augmented by blockage of outward potassium current flow, which normally repolarizes the membrane potential during a nerve membrane action potential.

RESULTS

Topical treatments with LD₁₀ s (10% mortality doses) of synergists were given with pyrethroids. 2S-65465 (2S) showed the best synergism of permethrin (8.6-fold) and deltamethrin (7.2-fold), whereas piperonyl butoxide and 4-aminopyridine only showed 2.2- to 3.4-fold synergism with these pyrethroids. In electrophysiological recordings of Periplaneta americana giant axons, 2S (10 μm) and 4-AP (30 μm) caused multiple spikes after a single stimulation. Permethrin at 10 μm showed significant summating depolarization (4.5 ± 1.1 mV) after a train of ten stimuli were applied at 5 Hz, and deltamethrin at 0.03 μm showed significant membrane depolarization of 2.9 ± 0.4 mV without stimuli. 2S at 0.3 μm and 4-AP at 1-3 μm significantly synergized the effects of 3 μm permethrin and 0.01 μm deltamethrin.

CONCLUSIONS

Co-application of potassium channel blockers 2S and 4-AP with pyrethroids can synergize the mosquitocidal activities on An. gambiae, and these activities are correlated with synergistic effects at the level of the nerve membrane. If deployed in the field, this approach can potentially reduce the amount of chemicals needed for effective control of mosquitoes. © 2020 Society of Chemical Industry.

Toxicity and physiological actions of biflavones on potassium current in insect neuronal cells

Stellera chamaejasme L. is a Chinese traditional herb. It has a long history and many medicinal usages. Biflavones, one of the main active ingredients in S. chamaejasme's roots, possess excellent insecticidal activities both in vivo and in vitro. However, the mechanism of these compounds and its potential molecular targets on insect cell were still not clear. Here the whole cell patch clamp technique was used to investigate whether biflavones affects voltage-gated potassium channels (Kv) on insect neuronal cells (AW1 and WG2). The results confirmed that both the three biflavones: neochamaejasmin A (NCA), neochamaejasmin B (NCB) and isochamaejasmin A (ICM) can significantly inhibit the A-type potassium current (I_A) than delayed rectifier potassium current (I_K) expressed on insect cells. Moreover, ICM stood out as the strongest inhibition activity on I_A with IC₅₀ value of 106.75 μM. Multiple results suggest that the inhibition of potassium current was related to the gating modification of biflavones. ICM produced concentration dependent hyperpolarizing shifts in the voltage dependence of channel steady-state activation and inactivation. Maximal shifts of the ICM-induced V_0.5, were -15.1 mV for activation and -6.93 mV for inactivation. ICM also prolonged recovery from inactivation of current. Moreover, the biflavones could inhibited AW1 cell survival in both dose- and time-dependent manners with well correlation of K⁺ inhibitory activity. Our study showed that biflavones from S. chamaejasme exhibiting significant blocked effects on Kv of AW1 cells and inhibited cell proliferation. These findings may not only show the toxic mechanisms of biflavones on insect cells, but also suggest that Kv channel play an important role in biflavones' mode of action and may be the new targets for designing novel insecticides.

Mosquitocidal activity of p,p'-difluoro-diphenyl-trichloroethane (DFDT)

New insecticides are urgently needed for the control of arthropod vectors of public health diseases. As resistance to many insecticides used for the control of public health pests is ubiquitous, all available chemistries should be evaluated for their potential to effectively control both insecticide-susceptible and insecticide-resistant strains of mosquitoes. This study aimed to evaluate p-p'-difluoro-diphenyl-trichloroethane (DFDT) as a mosquito control technology and relate its activity to that of DDT. We found that topical DFDT was significantly less toxic than DDT to both pyrethroid-susceptible and pyrethroid-resistant strains of Anopheles gambiae and Aedes aegypti. Direct nervous system recording from Drosophila melanogaster CNS demonstrated that DFDT is approximately 10-times less potent than DDT at blocking nerve firing, which may explain its relatively lower toxicity. DFDT was shown to be at least 4500 times more vapor-active than DDT, with an LC50 in a vapor toxicity screening assay of 2.2 μg/cm2. Resistance to DFDT was assessed in two mosquito strains that possess target-site mutations in the voltage-gated sodium channel and upregulated metabolic activity. Resistance ratios for Akdr (An. gambiae) and Puerto Rico (Ae. aegypti) strains were 9.2 and 12.2, respectively. Overall, this study demonstrates that DFDT is unlikely to be a viable public health vector control insecticide.

Venomics of the ectoparasitoid wasp Bracon nigricans

BACKGROUND

Venom is one of the most important sources of regulation factors used by parasitic Hymenoptera to redirect host physiology in favour of the developing offspring. This has stimulated a number of studies, both at functional and "omics" level, which, however, are still quite limited for ectophagous parasitoids that permanently paralyze and suppress their victims (i.e., idiobiont parasitoids).

RESULTS

Here we present a combined transcriptomic and proteomic study of the venom of the generalist idiobiont wasp Bracon nigricans, an ectophagous larval parasitoid of different lepidopteran species, for which we recently described the host regulation strategy and the functional role of the venom in the induction of physiological changes in parasitized hosts. The experimental approach used led to the identification of the main components of B. nigricans venom involved in host regulation. Enzymes degrading lipids, proteins and carbohydrates are likely involved in the mobilization of storage nutrients from the fat body and may concurrently be responsible for the release of neurotoxic fatty acids inducing paralysis, and for the modulation of host immune responses.

CONCLUSION

The present work contributes to fill the gap of knowledge on venom composition in ectoparasitoid wasps, and, along with our previous physiological study on this species, provides the foundation on which to develop a functional model of host regulation, based both on physiological and molecular data. This paves the way towards a better understanding of parasitism evolution in the basal lineages of Hymenoptera and to the possible exploitation of venom as source of bioinsecticidal molecules.

AW1 Neuronal Cell Cytotoxicity: The Mode of Action of Insecticidal Fatty Acids

To explore lead compounds for biological insecticides, nine fatty acids (FAs)' insecticidal activities against Helicoverpa zea (Lepidoptera, Noctuidae) and their cytotoxicity on H. zea neuronal cells (AW1 cells) were evaluated. The results showed that FAs at 1000 mg/L had a mortality rate of 10.0-83.33% and an inhibitory rate on AW1 cells with IC₅₀ values of 74.6-287.37 μM. Particularly, lauric acid exhibited the most excellent bioactivity both in vivo and in vitro among nine FAs. Further, its mode of action was investigated on the AW1 cells, and the results showed that lauric acid induced apoptosis on the AW1 cells, involving a decrease of mitochondrial membrane potential (ΔΨm) and an increase of caspase-9/3 activity and reactive oxygen species (ROS) levels. Furthermore, by detecting the expression of apoptosis protein, we found that the levels of Bcl-2 fell whereas the levels of cytochrome c and Bax rose remarkably. These results showed that FAs such as lauric acid could be potential lead compounds with a novel mode of action and highly insecticidal activity against H. zea.