Insects sodium channels

A main consideration in modeling the pyrethroid receptor on insect sodium channels in the open state was the determination that channel modification by cypermethrin and deltamethrin of cloned insect sodium channels expressed in Xenopus oocytes occurred only following repeated depolarizations [33, 55]. This use-dependency of some pyrethroids was the basis for the widely held opinion that these pyrethroids bind preferentially to open sodium channels and that state-dependent modification of sodium channels by pyrethroids was an important consideration for any receptor modeling. 

Tan J, Soderlund DM (2005) Identification of amino acids residues in the insect sodium channel critical for pyrethroids binding. Mol Pharmacol 67 513-522... 

Insecticides. Whereas fleas can be effectively dealt with by disinfection of clothes and living quarters, lice and mites require the topical application of insecticides to the infested subject. The following agents act mainly by interfering with the activation or inactivation of neural voltage-gated insect sodium channels. 

Pyrethroids are widely used to control many agriculturally and medically important insect pests. Due to intensive use of pyrethroids in pest control, many pest populations have developed resistance to these compounds. One major mechanism of pyrethroid resistance, conferred by the knock down resistance gene (Mr), is reduced target site (sodium channel) sensitivity to DDT and pyrethroids. Studies on the molecular basis of Mr and Mr-type resistance in various insects are enhancing our understanding of the structure and function of insect sodium channels and the molecular interaction between insect sodium channels and pyrethroids. In this chapter, I will review recent advances in... 

Cloning and Functional Expression of Insect Sodium Channels... 

Distinct Insect Sodium Channel Isoforms Generated by Alternative Splicing... 

Figure 2. Sodium channel protein topology indicating the naturally occurring mutations that are confirmed to reduce insect sodium channel sensitivity to pyrethroids. The sodium channel contains four repeated homologous domains (I-IV), each having six membrane-spanning segments (SI-6). See the text for the details.

Davies, T.G., Field, L.M., Usherwood, RN. and Williamson, M.S. (2007) DDT, pyrethrins, pyrethroids and insect sodium channels. lUBMB Life, 59,151-62. 

Several studies employing oocytes of the clawed frog, Xenopus laevis, for the in vitro translation of sodium channel encoding mRNAs (53-55) suggest that this experimental system may be particularly useful toward this end. The biophysical properties of sodium channels expressed in oocytes following injection of rat brain mRNA were similar to those of sodium channels in their native membrane environment, and were specifically inhibited by the sodium channel blockers tetrodotoxin and saxitoxin (i5.). Sodium channels encoded by mRNAs from rat skeletal muscle and eel electroplax have also been expressed in Xenopus oocytes (56-57). To date the expression of insect sodium channels in the Xenopus oocyte has not been reported, but the utility of this system for the translation and expression of insect acetylcholine receptor mRNA has recently been demonstrated (58). Successful application of this methodology to the expression of insect mRNAs encoding functional sodium channels offers a novel method to test some of the hypotheses for the molecular basis of the kdr mechanism. 

Finally, some differences have emerged between sodium channels of insects and vertebrates in terms of their intrinsic sensitivity to pyrethroids indicating that the selectivity of pyrethroids to insect is at least in part to their higher activity on insect sodium channels than vertebrate channels [15]. 

We concluded that paralysis in insects caused by injection of funnel web spider venom was caused by the joint actions of actions of the a-, p- and -agatoxins, each acting at a distinct ion channel target at the neuromuscular junction. The only toxins showing lethal actions when injected into insects were the p-agatoxins, which also appear to be specific for insect sodium channels. 

Modified toxins could be produced with site-specific mutations to produce toxins with amino acid residue substitutions, additions or deletions. These modified toxins could then be characterized by a variety of techniques including bioassays, binding studies. X-ray crystallography and NMR analyses. Additionally, these studies will lead to a better understanding of the insect sodium channel. In fact, results gathered from X-ray analysis, binding assays and NMR studies may... 

Arthropod-derived toxins are also among the list of classical examples of toxins used to probe biochemical processes. Scorpion toxins have been important in the experimental determination of the spacial arrangement of residues on the potassium channel (11), and also in the inference of properties of its quaternary structure (12). A scorpion toxin was used as a tool in the first isolation of a specific component of an insect sodium channel (13). Spider toxins have been similarly useful in characterization of calcium channels (14). 

Mode of Action. DDT and its analogues specifically affect the peripheral sense organs of insects and produce violent trains of afferent impulses that result in hyperactivity, convulsions, and paralysis. Death results from metaboHc exhaustion and the production of an endogenous neurotoxin. The very high lipophilic nature of these compounds faciUtates absorption through the insect cuticle and penetration to the nerve tissue. The specific site of action is thought to be the sodium channels of the axon, through inhibition of Ca " ATPase. 

The compounds featured in Table 1.1 are considered briefly here. Pyrethrins are lipophilic esters that occur in Chrysanthemum spp. Extracts of flower heads of Chrysanthemum spp. contain six different pyrethrins and have been used for insect control (Chapter 12). Pyrethrins act upon sodium channels in a manner similar to p,p -DDT. The highly successful synthetic pyrethroid insecticides were modeled on natural pyrethrins. 

Resistance to DDT has been developed in many insect species. Although there are some cases of metabolic resistance (e.g., strains high in DDT dehydrochlorinase activity), particular interest has been focused on kdr and super kdr mechanisms based upon aberrant forms of the sodium channel—the principal target for DDT. There are many examples of insects developing resistance to dieldrin. The best-known mechanism is the production of mutant forms of the target site (GABA receptor), which are insensitive to the insecticide. 

Pyrethroid insecticides are generally recognized as potent neurotoxicants that interfere with nerve membrane function by interaction with the sodium channel (Elliott and Janes 1978 Vijverberg et al. 1982 Gilbert et al. 1989 Haya 1989). Synthetic pyrethroids are more toxic against insect pests, up to 10 times more potent in some cases, than other insecticides now in general use (Bradbury... 

Soderlund DM (1995) Sodium channels. In Gilbert L (ed) Comprehensive molecular insect science, vol 5. Pergamon, Oxford, UK, pp 1-24... 

Lee SH, Soderlund DM (2001) The V410M mutation associated with pyrethroid resistance in Heliothis virescens reduces the pyrethroid sensitivity of house fly sodium channels expressed in Xenopus oocytes. Insect Biochem Mol Biol 31 19-29... 

Zlotkin E, The insect voltage-gated sodium channel as target of insecticides, Annu Rev EntomolAA-A23— 55, 1999. 

The target sites within the nervous system of insects known at present are very restricted. They consist of the sodium channel, the components of the nicotinic cholinergic synapse and the y-aminobutyric acid (GABA) and octopamine receptors. Benson4 (1991) lists potential target sites within the insect neuronal and muscular system. These are shown in Table 3.1. 

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