Modification by pyrethroids and DDT of phosphorylation activities of rat brain sodium channel

DDT and Other Chlorinated Insecticides

The use of organochlorine insecticides such as DDT, lindane and cyclodieneshas declined markedly worldwide over the last decades. Most are now banned or not used. At an acute toxicity level they have been relatively safe in use for humans. However, the greatest concerns are their persistence in people, wildlife and the environment due to their slow metabolism. Although their carcinogenicity for humans has not been supported by strong epidemiological evidence, their potential to be modulators of endocrine and immune function at levels remaining in the environment or associated with residual spraying of DDT continue to be of concern. At present, DDT is still allowed by the United Nations for combating malaria, with continual monitoring and assessment where possible. The toxicological consequences of exposure of animals and people to DDT is discussed as well as some analogues and other insecticides such as lindane, dieldrin and chlordecone that, although little used, continue to persist in surroundings and people. Because of circumstances of world health brought about by climate change or human activities that have yet to develop, there may come a time when the importance of some may re-emerge.

Neurotoxic implications of the agonistic action of CS-syndrome pyrethroids on the N-type Ca(v)2.2 calcium channel

BACKGROUND

Cismethrin (T-syndrome) and deltamethrin (CS-syndrome) pyrethroids have been previously shown to increase membrane depolarization and calcium influx, but only deltamethrin increased Ca(2+)-dependent neurotransmitter release from rat brain synaptosomes. Deltamethrin's action was blocked by omega-conotoxin GVIA, delineating a separate action at N-type Ca(v)2.2 channels that is consistent with the in vivo release of neurotransmitter. It is hypothesized that other CS-syndrome pyrethroids will elicit similar actions at presynaptic nerve terminals.

RESULTS

Nine additional pyrethroids were similarly examined, and these data were used in a cluster analysis. CS-syndrome pyrethroids that possessed alpha-cyano groups, cypermethrin, deltamethrin and esfenvalerate, all caused Ca(2+) influx and neurotransmitter release and clustered with two other alpha-cyano pyrethroids, cyfluthrin and cyhalothrin, that shared these same actions. T-syndrome pyrethroids, bioallethrin, cismethrin and fenpropathrin, did not share these actions and clustered with two non-alpha-cyano pyrethroids, tefluthin and bifenthrin, which likewise did not elicit these actions. Deltamethrin reduced peak current of heterologously expressed wild-type Ca(v)2.2, increased peak current of T422E Ca(v)2.2 and was 20-fold more potent on T422E Ca(v)2.2 than on wild-type channels, indicating that the permanently phosphorylated form of Ca(v)2.2 is the preferred target.

CONCLUSIONS

Ca(v)2.2 is directly modified by deltamethrin, but the resulting perturbation is dependent upon its phosphorylation state. The present findings may provide a partial explanation for the different toxic syndromes produced by these structurally distinct pyrethroids.

Pyrethroid action on calcium channels: neurotoxicological implications

Actions of cismethrin versus deltamethrin were compared using two functional attributes of rat brain synaptosomes. Both pyrethroids increased calcium influx but only deltamethrin increased Ca(2+)-dependent neurotransmitter release following K(+)-stimulated depolarization. The action of deltamethrin was stereospecific, concentration-dependent, and blocked by omega-conotoxin GVIA. These findings delineate a separate action for deltamethrin and implicate N-type rat brain Ca(v)2.2 voltage-sensitive calcium channels (VSCC) as target sites that are consistent with the in vivo release of neurotransmitter caused by deltamethrin. Deltamethrin (10(-7) M) reduced the peak current (approx. -47%) of heterologously expressed wild type Ca(v)2.2 in a stereospecific manner. Mutation of threonine 422 to glutamic acid (T422E) in the alpha(1)-subunit results in a channel that functions as if it were permanently phosphorylated. Deltamethrin now increased peak current (approx. +49%) of T422E Ca(v)2.2 in a stereospecific manner. Collectively, these results substantiate that Ca(v)2.2 is directly modified by deltamethrin but the resulting perturbation is dependent upon the phosphorylation state of Ca(v)2.2. Our findings may provide a partial explanation for the different toxic syndromes produced by these structurally-distinct pyrethroids.

Mechanisms of pyrethroid neurotoxicity: implications for cumulative risk assessment

The Food Quality Protection Act (FQPA) of 1996 requires the United States Environmental Protection Agency to consider the cumulative effects of exposure to pesticides having a 'common mechanism of toxicity.' This paper reviews the information available on the acute neurotoxicity and mechanisms of toxic action of pyrethroid insecticides in mammals from the perspective of the 'common mechanism' statute of the FQPA. The principal effects of pyrethroids as a class are various signs of excitatory neurotoxicity. Historically, pyrethroids were grouped into two subclasses (Types I and II) based on chemical structure and the production of either the T (tremor) or CS (choreoathetosis with salivation) intoxication syndrome following intravenous or intracerebral administration to rodents. Although this classification system is widely employed, it has several shortcomings for the identification of common toxic effects. In particular, it does not reflect the diversity of intoxication signs found following oral administration of various pyrethroids. Pyrethroids act in vitro on a variety of putative biochemical and physiological target sites, four of which merit consideration as sites of toxic action. Voltage-sensitive sodium channels, the sites of insecticidal action, are also important target sites in mammals. Unlike insects, mammals have multiple sodium channel isoforms that vary in their biophysical and pharmacological properties, including their differential sensitivity to pyrethroids. Pyrethroids also act on some isoforms of voltage-sensitive calcium and chloride channels, and these effects may contribute to the toxicity of some compounds. Effects on peripheral-type benzodiazepine receptors are unlikely to be a principal cause of pyrethroid intoxication but may contribute to or enhance convulsions caused by actions at other target sites. In contrast, other putative target sites that have been identified in vitro do not appear to play a major role in pyrethroid intoxication. The diverse toxic actions and pharmacological effects of pyrethroids suggest that simple additivity models based on combined actions at a single target are not appropriate to assess the risks of cumulative exposure to multiple pyrethroids.

kdr-Type resistance in insects with special reference to the German cockroach, Blattella germanica

The phenomenon of knockdown resistance (kdr) was first noted in the housefly (Musca domestica), and has subsequently be found (i.e. kdr-type resistance) in several other insect pests including the German cockroach (Blattella germanica). This type of resistance causes insensitivity of the nervous system to pyrethroids, DDT and a limited number of sodium channel neurotoxins. In the German cockroach, kdr-type resistance is incompletely recessive, monogenic and not sex linked or due to cytoplasmic factors. Additionally, kdr-type resistance is not associated with a change in sodium channel density. kdr or kdr-type loci are tightly linked or identical to the para-homologous sodium channel locus in German cockroach, housefly and tobacco budworm (Heliothis virescens), suggesting that kdr and kdr-type resistance are due to mutations in the para-homologous sodium channel gene. kdr-Type resistance in the German cockroach appears similar, although not necessarily identical, to kdr in houseflies.

Differential sensitivity of tetrodotoxin-sensitive and tetrodotoxin-resistant sodium channels to the insecticide allethrin in rat dorsal root ganglion neurons

The pyrethroid insecticides are known to modify neuronal sodium channels to cause a prolongation of whole cell current. The sodium channels expressed in the dorsal root ganglion neurons of the rat are of two types, one highly sensitive to tetrodotoxin and the other highly resistant to tetrodotoxin. The pyrethroid allethrin exerted profound effects on tetrodotoxin-resistant sodium channels while causing minimal effects on tetrodotoxin-sensitive sodium channels. Currents derived from tetrodotoxin-resistant sodium channels were greatly prolonged during a step depolarization; the tail currents upon repolarization were also augmented and prolonged. In the tetrodotoxin-sensitive sodium channel currents, these changes caused by allethrin were much smaller or negligible. The activation and inactivation voltages of tetrodotoxin-resistant peak sodium currents were not significantly altered by allethrin. The differential action of allethrin on the two types of sodium channels would be important not only in identifying the target molecular structure but also in interpreting the symptoms of poisoning in mammals.

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.