Date Published: January 26, 2017
Publisher: Public Library of Science
Author(s): Yallappa Rajashekar, Thimmappa Shivanandappa, Frederic Marion-Poll.
Decalesides are a new class of natural insecticides which are toxic to insects by contact via the tarsal gustatory chemosensilla. The symptoms of their toxicity to insects and the rapid knockdown effect suggest neurotoxic action, but the precise mode of action and the molecular targets for decaleside action are not known. We have presented experimental evidence for the involvement of sodium pump inhibition in the insecticidal action of decaleside in the cockroach and housefly. The knockdown effect of decaleside is concomitant with the in vivo inhibition of Na+, K+ -ATPase in the head and thorax. The lack of insecticidal action by experimental ablation of tarsi or blocking the tarsal sites with paraffin correlated with lack of inhibition of Na+- K+ ATPase in vivo. Maltotriose, a trisaccharide, partially rescued the toxic action of decaleside as well as inhibition of the enzyme, suggesting the possible involvement of gustatory sugar receptors. In vitro studies with crude insect enzyme preparation and purified porcine Na+, K+ -ATPase showed that decaleside competitively inhibited the enzyme involving the ATP binding site. Our study shows that the insecticidal action of decaleside via the tarsal gustatory sites is causally linked to the inhibition of sodium pump which represents a unique mode of action. The precise target(s) for decaleside in the tarsal chemosensilla and the pathway linked to inhibition of sodium pump and the insecticidal action remain to be understood.
In view of the environmental and ecological concerns, human health hazards, and increasing insect resistance, many insecticides have been banned or replaced by newer chemicals . Mode of action of the major chemical classes of insecticides involves mainly three target sites in the nervous system: acetylcholinesterase, an enzyme of critical importance in the transmission of nerve impulse (organophosphorus and carbamates), voltage-gated sodium channels across the nerve membrane (pyrethoids and DDT), and the acetylcholine receptor (neonicotinoids) [1–5]. Selective insecticides such as juvenile hormone mimics (fenoxycarb and pyriproxyfen), ecdysone agonists and chitin synthesis inhibitors (Diflubenzuron) act on insect- specific targets that disrupt reproduction and development [6–8]. Among the insecticides derived from natural sources, azadirachtin, from the Indian neem tree, is a feeding deterrent and an insect growth regulator that suppresses fecundity, moulting, pupation and adult emergence [9–10]. Compounds that selectively act on the insect nicotinic acetylcholine receptor (neonicotinoids), such as imidacloprid, acetamiprid and thiomethaxam are among the modern insecticides used in pest management [11–13]. Avermectins, the insecticides of microbial origin, target GABA-gated chloride channels [14–15], whereas, the diamide insecticide acts on the ryanodine receptor [16–17]. Spinosyns, a new class of insecticides derived from actinomycetes, show high selectivity and low mammalian toxicity with eco-friendly behaviour [18–19].
Our study has demonstrated that Na+, K+ -ATPase is severely inhibited in insects (both house fly and cockroaches) exposed to decaleside I and II in the contact bioassay. The in vivo inhibition closely correlated (r = >0.9) with the knockdown effect of decaleside I and II in dose-response and time-course studies. The in vivo inhibition was seen in both head and thorax in the case of house fly and the nervous (ganglion) and muscle tissue in cockroaches exposed to decaleside. Further, experimental evidence shows that the inhibition of Na+, K+ -ATPase requires contact of the insect leg (tarsi) with insecticide treated surface, since no inhibition was seen in insects with tarsi ablated. The same was demonstrated with the application of wax on the tarsi, wherein toxic action of decaleside was abolished. In these experiments in which contact with tarsi for the insecticidal action of decaleside is required, also show concomitant inhibition of Na+, K+ -ATPase in vivo. The results lead us to conclude that tarsi-mediated insect toxicity of decaleside involves Na+, K+ -ATPase inhibition. Our results are the first report of a natural insecticide (novel trisaccharides) showing Na+, K+ -ATPase as the target in its mode of action. The molecular mechanisms involved in the insecticidal action of decaleside via the gustatory chemosensilla that lead to the knock-down effect, the toxic outcome finally leading to mortality, are not clear at present. Our results show that the insects exposed to decaleside, are initially hyperactive indicating neural excitation, which is followed by knockdown effect, symptoms somewhat similar to that of pyrethoids, suggesting a neurotoxic effect [43–44]. The basic difference, however, is that the decaleside action is mediated by contact with tarsi, unlike that of pyrethoids which act by contact at any point of the body surface of insects. It is known that pyrethroids act by interfering with the voltage gated sodium channels in the neurons which causes hyper excitation leading to knockdown [45–47]. In the case of decaleside action, inhibition of sodium pump is clearly demonstrated in our studies. The following hypothesis is proposed in order to explain the mode of action of decaleside via the gustatory receptors: on contact with the gustatory (sugar) receptors (step I) causes inhibition of sodium pump (step II) which is responsible for the hyperactivity due to increased neuronal excitation caused by excessive Na+ concentration (step III) finally leading to knockdown effect and mortality. Electrophysiological evidence is needed to support this hypothesis.