Block of Two Subtypes of Sodium Channels in Cockroach Neurons by Indoxacarb Insecticides

doi:10.1016/j.neuro.2005.03.007

NeuroToxicology

Volume 26, Issue 3, June 2005, Pages 455-465

https://doi.org/10.1016/j.neuro.2005.03.007 Get rights and content

Abstract

Indoxacarb, a novel insecticide, and its decarbomethoxyllated metabolite, DCJW, are known to block voltage-gated Na⁺ channels in insects and mammals, but the mechanism of block is not yet well understood. The present study was undertaken to characterize the action of indoxacarb and DCJW on cockroach Na⁺ channels. Na⁺ currents were recorded using the whole-cell patch clamp technique from neurons acutely dissociated from thoracic ganglia of the American cockroach Periplaneta americana L. Two types of tetrodotoxin-sensitive Na⁺ currents were observed, with different voltage dependencies of channel inactivation. Type-I Na⁺ currents were inactivated at more negative potentials than type-II Na⁺ currents. As a result, these two types of Na⁺ channels responded to indoxacarb compounds differentially. At a holding potential of −100 mV, type-I Na⁺ currents were inhibited reversibly by 1 μM indoxacarb and irreversibly by 1 μM DCJW in a voltage-dependent manner, whereas type-II Na⁺ currents were not affected by either of the compound. However, type-II Na⁺ currents were inhibited by indoxacarb or DCJW at more depolarizing membrane potentials, ranging from −60 to −40 mV. The slow inactivation curves of type-I and type-II Na⁺ channels were significantly shifted in the hyperpolarizing direction by indoxacarb and DCJW, suggesting that these compounds have high affinities for the inactivated state of the Na⁺ channels. It was concluded that the differential blocking actions of indoxacarb insecticides on type-I and type-II Na⁺ currents resulted from their different voltage dependence of Na⁺ channel inactivation. The irreversible nature of DCJW block may be partially responsible for its potent action in insects.

Section snippets

INTRODUCTION

Indoxacarb is a new broad-spectrum insecticide effective against insects belonging to at least 10 orders and well over 30 families. It exhibits low mammalian toxicity and shows no cross-resistance to carbamates, pyrethroids, spinosad, cyclodienes, benzoylureas or organophosphates (Zhao et al., 2002, Wing et al., 2004). Indoxacarb was the first commercialized Na⁺ channel blocking insecticide of the oxadiazine class. Indoxacarb's selective toxicity against insects is due in part to the fact that

Isolation of Cockroach Neurons

The American cockroaches, Periplaneta americana L., were purchased from Carolina Biological Supply Company (Burlington, NC), and were reared in the dark at 29 °C on rat chow (Ralston Purina Company, St. Louis, MO) and water. Neurons from three thoracic ganglia of an adult male cockroach were prepared according to a modified version of the technique of Pinnock and Sattelle (1987). Briefly, three thoracic ganglia were isolated, desheathed and incubated for 20 min in cockroach saline solution

Two Types of Na⁺ Currents

The cockroach thoracic ganglia contain neurons of different sizes (10–100 μm) and different shapes. Most of them are with mainly round or pyriform somata with axon remnants. Na⁺ currents were rarely observed in small round-shaped neurons or large neurons without axon remnants, but were reliably recorded in large pyriform neurons. This is consistent with the fact that voltage-gated Na⁺ channels are mainly located on the axon or the initial segment of the soma, instead of the apical pole of

Two Kinds of Na⁺ Currents

Voltage-gated Na⁺ channels in dorsal unpaired median neurons of the cockroach are located mainly on the axon and the initial segment of the soma, as shown by immunocytochemical staining (French et al., 1993, Amat et al., 1998). The physiological and pharmacological properties of the fast voltage-gated Na⁺ currents in both giant axons and dorsal unpaired median neurons of the cockroach were studied using voltage clamp techniques (Sattelle et al., 1979, Pelhate et al., 1979, Lapied et al., 1990,

ACKNOWLEDGEMENTS

This study was supported by National Institutes of Health grant NS14143. Samples of indoxacarb (DPX-MP062) and its metabolite, DCJW, were provided by E.I. DuPont de Nemours and Company, Newark, Delaware. The authors wish to thank Dr. Ke Dong (Michigan State University, MI) for the helpful discussion on insect sodium channels. The authors thank Nayla Hasan and Sandra Guy for technical assistances and Julia Irizarry for secretarial assistance.

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While DCJW is a more potent blocker of sodium channels, it is well-documented in the literature that indoxacarb exhibits a modest level of blocking effect on most mammalian sodium channel isoforms expressed in Xenopus oocytes (von Stein et al., 2013) and in mammalian neurons (Zhao et al., 2003). Indoxacarb also inhibits cockroach sodium channels in primary neurons (Zhao et al., 2005) and cockroach sodium channels expressed in Xenopus oocytes (Silver et al., 2009). The effects of indoxacarb on wild-type cockroach sodium channels from our current study are consistent with those reported in our previous study (Silver et al., 2009).
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Block of Two Subtypes of Sodium Channels in Cockroach Neurons by Indoxacarb Insecticides

Abstract

Section snippets

INTRODUCTION

Isolation of Cockroach Neurons

Two Types of Na+ Currents

Two Kinds of Na+ Currents

ACKNOWLEDGEMENTS

J Neurosci Methods

J Biol Chem

Neurotoxicology

Neurotoxicology

Neuron

Na+-dependent neuritic spikes initiate Ca2+-dependnet somatic plateau action potentials in insect dorsal paired median neurons

J Neurophysiol

Immunocytochemical localization of sodium channels in an insect central nervous system using a site-directed antibody

J Neurobiol

Evolution of voltage-gated Na+ channels

J Exp Biol

Dorsal unpaired median neurones in the insect central nervous system: towards a better understanding of the ionic mechanisms underlying spontaneous electrical activity

J Exp Biol

Patch-clamp study of the properties of the sodium current in cockroach single isolated adult aminergic neurones

J Exp Biol

Indoxacarb, an oxadiazine insecticide, blocks insect neuronal sodium channels

Br J Pharmacol

Persistent tetradotoxin-sensitive sodium current resulting from U-to-C RNA-editing of an insect sodium channel

Proc Natl Acad Sci

The discovery of indoxacarb: oxadiazines as a new class of pyrozoline-type-Insecticides

Pest Management Sci

Suppression of voltage-gated sodium currents by the dihydropyrazole insecticide, DPX-JW062 in rat dorsal root ganglion neurons

J Pesticide Sci

Two Types of Na⁺ Currents

Two Kinds of Na⁺ Currents

Na⁺-dependent neuritic spikes initiate Ca²⁺-dependnet somatic plateau action potentials in insect dorsal paired median neurons

Evolution of voltage-gated Na⁺ channels