Characteristics of the prolonged inhibition produced by a range of pyrethroids in the rat hippocampus

The effects of combined exposure to the pyrethroids deltamethrin and S-bioallethrin on hippocampal inhibition and skeletal muscle hyperexcitability in rats

The default assumption that different pyrethroid insecticides, sharing a common mode of action, will show additivity of toxicity has not always been supported by in vitro measures, some of which have indicated antagonism. Our intention was to see whether the antagonism between pyrethroids of different classes seen in vitro could be reproduced in vivo. We therefore investigated the effects of single and combined exposures to two commonly used pyrethroids, deltamethrin (type II) and S-bioallethrin (type I) given intravenously to anaesthetised rats. We used two quantitative measures that are responsive to pyrethroids: the duration of prolongation of hippocampal dentate granule cell inhibition and the amplitude of the abnormal electromyogram discharge. At equi-toxic doses, S-bioallethrin extended the inter-stimulus interval evoking 50% inhibition in the hippocampus by 30+/-2.2 ms, and deltamethrin extended it by 199+/-21 ms. Combined administration of the same doses of deltamethrin and S-bioallethrin extended hippocampal inhibition by 164+/-14 ms, which did not differ significantly from the effect of deltamethrin alone. S-bioallethrin was without any effect on the electromyogram, and produced no significant change in the amplitude of the abnormal muscle discharges evoked by deltamethrin. The increase in arterial blood pressure evoked by the combination was significantly less than that evoked by either pyrethroid alone (p<0.001). In summary, although our electrophysiological indices provide no support for functional antagonism between these two pyrethroids, they also fail to indicate any summation of effect.

A reassessment of the neurotoxicity of pyrethroid insecticides

The pyrethroids are a widely used class of insecticides to which there is significant human exposure. They are however generally regarded as safe to man, and there have been few reports of human fatalities. Their acute toxicity is dominated by pharmacological actions upon the central nervous system (CNS), predominantly mediated by prolongation of the kinetics of voltage-gated sodium channels, although other mechanisms operate. This review summarizes our present understanding of such actions and the pharmacological options to antagonize them. One significant problem is the very clear heterogeneity of pyrethroid sensitivity that is seen across sodium channel subtypes; however, the distribution and function of these across the central nervous system are poorly characterized. The review also provides an overview of recent studies that suggest additional effects of pyrethroids: developmental neurotoxicity, the production of neuronal death, and action mediated via pyrethroid metabolites. The evidence for these is at present equivocal, but all 3 carry important implications for human health.

5-HT loss in rat brain by type II pyrethroid insecticides

STUDY OBJECTIVE

Type II pyrethroids are a group of insecticides largely used in agriculture and public health. The nervous system is the main target for pyrethroids in insects and mammals. One notable form of toxicity associated with over exposure has been a facial cutaneous paraesthesia and irritation-related respiration symptoms including behavioural excitation mainly observed in workers spraying pyrethroids or in occupational settings. In acutely exposed rats, type II pyrethroids produce a severe syndrome characterized by salivation and choreoathetosis. Because many of the acute functional effects of type II pyrethoids can be associated with the neurotoxic effect on 5-hydroxytryptamine (5-HT) neurones, the objective of the present study was to examine whether deltamethrin, cyfluthrin and lambda-cyhalothrin administration results in changes of 5-HT content in rat brain. Characterizing this target will help us to better understand the toxicological effects of type II pyrethroids.

DESIGN

Rats were injected with either corn oil or pyrethroids (deltamethrin, 20 mg/kg per day, i.p., for 6 days; cyfluthrin, 14 mg/kg per day, i.p., for 6 days; lambda-cyhalothrin, 8 mg/kg per day, i.p., for 6 days). The frontal cortex, hippocampus, midbrain and striatum were removed at 24 hours post treatment and were analysed for content of 5-HT and 5-HIAA using a HPLC method with electrochemical detection.

RESULTS

A serotonin depleting effect was produced by these type II pyrethroids. The concentration of 5-HT and its metabolite 5-HIAA decreased in the brain regions from pyrethroid treated animals. Pyrethroids accelerated the turnover of 5-HT in midbrain and striatum areas. It is concluded that pyrethroids affect serotonin neurotransmission.

Effects of pyrethroids on voltage-sensitive calcium channels: a critical evaluation of strengths, weaknesses, data needs, and relationship to assessment of cumulative neurotoxicity

The Food Quality Protection Act of 1996 requires that the U.S. Environmental Protection Agency conduct cumulative risk assessments for classes of pesticides that have a common mode or mechanism of action. For the pyrethroid insecticides, disruption of voltage-sensitive sodium channel function is generally accepted as the mechanism underlying acute neurotoxicity. However, data exist which suggest that voltage-sensitive calcium (Ca(2+)) channels (VSCC) may also be important targets of pyrethroid action. VSCC are important to neuronal function during development and for neurotransmitter release, gene expression, and electrical excitability in the nervous system. Disruption of these and other processes mediated by VSCC can result in neurotoxicity. If effects on VSCC are demonstrated to contribute to pyrethroid neurotoxicity, then such effects will have to be considered when making decisions regarding cumulative risk of exposure to this class of compounds. This document provides a critical review of the data related to the hypothesis that VSCC are important targets of pyrethroid effects. Data supporting effects of pyrethroids on VSCC have been generated by several different laboratories using different techniques and biological preparations. Thus, the many reports of effects on VSCC provide evidence that pyrethroids may interact with VSCC. However, evidence to support a role of VSCC in pyrethroid neurotoxicity is based entirely on in vitro observations, and numerous limitations exist in these data, including: (1) lack of defined concentration-response relationships, with some effects observed only at relatively high concentrations, (2) the use of indirect measures of VSCC function, (3) data from nonmammalian species, (4) data from studies that have not been peer-reviewed, (5) the need for replication of some effects, and (6) inconsistent or contradictory results from different laboratories/preparations. Thus, at the present time, it is premature to conclude that effects on VSCC play an important role in the acute neurotoxicity of pyrethroid insecticides in mammals. To demonstrate that VSCC are important targets of pyrethroid neurotoxicity in mammals, in vivo studies supporting a role for pyrethroid effects on VSCC are needed. Additional support could be provided by demonstration of direct effects of pyrethroid compounds on mammalian neuronal VSCC in vitro, including demonstration that concentration-response relationships are similar, or greater, in sensitivity to effects of pyrethroids on voltage-sensitive sodium channels. If such effects were to be demonstrated, the rationale for considering VSCC as targets of pyrethroid compounds when assessing cumulative risk would be strengthened. However, at the present time, the data available neither support nor refute conclusively the hypothesis that effects on VSCC are important to the acute neurotoxicity of 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.

Pyrethroid insecticides: poisoning syndromes, synergies, and therapy

BACKGROUND

Pyrethroid insecticides are widely used, but there have been relatively few reports of systemic poisoning. These reports have, however, shown that pharmacotherapy is difficult and that the duration of poisoning can be unexpectedly long. Pyrethroids are ion channel toxins prolonging neuronal excitation, but are not directly cytotoxic. Two basic poisoning syndromes are seen. Type I pyrethroids produce reflex hyperexcitability and fine tremor. Type II pyrethroids produce salivation, hyperexcitability, choreoathetosis, and seizures. Both produce potent sympathetic activation. Local effects are also seen: skin contamination producing paresthesia and ingestion producing gastrointestinal irritation. The slow absorption of pyrethroids across the skin usually prevents systemic poisoning, although a significant reservoir of pyrethroid may remain bound to the epidermis. Carboxyesterase inhibitors can enhance pyrethroid toxicity in high-dose experimental studies. Hence, the unauthorized pyrethroid/organophosphate mixtures marketed in some developing countries may precipitate human poisoning. Pyrethroid paresthesia can be treated by decontamination of the skin, but systemic poisoning is difficult to control with anticonvulsants. Pentobarbitone, however, is surprisingly effective as therapy against systemic type II pyrethroid poisoning in rats, probably due to its dual action as a chloride channel agonist and a membrane stabilizer.

The role of voltage-gated chloride channels in type II pyrethroid insecticide poisoning

Pyrethroids act on mammalian sodium channels, but we have previously shown that low concentrations of the type II pyrethroid deltamethrin also decrease the open channel probability (P(o)) of voltage-gated chloride channels. This effect would be expected to amplify the sodium channel-mediated signs of poisoning produced by pyrethroids. In the present study we evaluated potential chloride channel agonists in vitro, and then tested the most effective of these on pyrethroid-poisoned rats to determine the practical significance of chloride channel effects in vivo. Patch clamp experiments showed that, for voltage-gated maxi chloride channels in excised, inside-out patches from mouse N1E 115 neuroblastoma cells, ivermectin (10(-7) M) and pentobarbitone (10(-6) M) significantly increased open channel probability (p </= 0.01 and p </= 0.02, respectively), whereas phenobarbitone, hexobarbitone, mephobarbitone, thiopentone, and barbituric acid did not. This suggested that, if chloride channels were important in vivo, ivermectin and pentobarbitone should antagonize type II pyrethroid poisoning and phenobarbitone should not. Male F344 rats were then pretreated with ivermectin (4 mg/kg iv), equisedative doses of either pentobarbitone (15 mg/kg ip) or phenobarbitone (45 mg/kg ip), or solvent controls. This was followed by deltamethrin (1.5 or 2 mg/kg iv) or the type I pyrethroid cismethrin (4 mg/kg iv). Ivermectin produced a marked fall in deltamethrin-induced salivation (p </= 0.05) and also (in anesthetized rats) in repetitive electromyogram discharge and muscle twitch (p </= 0.01 and p </= 0.05, respectively). Pentobarbitone significantly reduced the motor signs score due to deltamethrin (p </= 0.01). Ivermectin therefore protected against the peripheral signs of deltamethrin poisoning and pentobarbitone protected against the central signs. As expected phenobarbitone had no protective effects. The motor signs produced by the type I pyrethroid cismethrin (which does not act on chloride channels) were not diminished by either barbiturate. The peripheral benzodiazepine receptor blocker PK11195 did not diminish the protective action of ivermectin on the muscle twitch (p </= 0.05), although it partially reversed the block of salivation (p </= 0.05). These results support the hypothesis that the voltage-dependent chloride channel is a toxicologically significant additional site of action for deltamethrin and that the use of chloride channel agonists can provide a rationale for a novel and effective therapy against type II pyrethroid poisoning.

Inhibition of a neuronal voltage-dependent chloride channel by the type II pyrethroid, deltamethrin

Following the previous finding that the Type II pyrethroid, deltamethrin, increased membrane resistance in peripheral nerve and muscle in a chloride-dependent manner, the action of deltamethrin on neuronal voltage-dependent chloride channels was assessed using inside-out patches from NIE-115 neuroblastoma cells. These were bathed in symmetrical solutions, containing 149 mM chloride and the membrane potential stepped from 0 mV to voltages ranging from +/- 10 to 80 mV for 2 or 5 sec. Active patches contained large conductance channels (343 +/- 11 pS, n = 8), which inactivated relatively slowly during the voltage step and could be resolved into a number of substates. The channels were confirmed as being chloride specific on the basis of substitution experiments with isethionate and pharmacological blockade by 9-anthracene carboxylic acid (9-ACA). Within 20 min of adding deltamethrin (2 microM) to the bath solution, open channel probability (Po) fell from 0.50 +/- 0.06 to 0.24 +/- 0.04 (n = 11) a highly significant result. Glycerinformal solvent alone (0.1% v/v) caused a non-significant rise to 0.65 +/- 0.09 (n = 4). The decreased open channel probability after deltamethrin was due to an increased incidence of both the closed channel state and low conductance substates. In addition, deltamethrin frequently caused flickering between substrates similar to that seen after 9-ACA. Deltamethrin did not change single channel conductance, current-voltage relationship or time-dependent channel inactivation, but decreased open channel probability over the complete range of membrane voltage tested.(ABSTRACT TRUNCATED AT 250 WORDS)