snx-230 and decamethrin

Pyrethroid insecticides have potent actions on voltage-gated sodium channels (VGSC), inhibiting inactivation and increasing channel open times. These are thought to underlie, at least in part, the clinical symptoms of pyrethroid intoxication. However, disruption of neuronal activity at higher levels of organization is less well understood. In order to characterize pyrethroid effects on neurotransmitter release and neuronal excitability in glutamatergic networks, we examined the effects of deltamethrin (DM) and permethrin (PM) on neuronal activity in hippocampal neuronal cultures using patch-clamp and microelectrode array (MEA) recordings. In the presence of inhibitors of GABA receptors, spontaneous excitatory post-synaptic currents (sEPSCs) and spontaneous spike rates were reduced in a concentration-dependent manner by both DM and PM. IC(50) values were 0.037 and 0.70microM for inhibition of sEPSCs and 0.60 and 21.8microM for inhibition of spontaneous spike rate by DM and PM, respectively. Both compounds altered burst activity by decreasing the number of spikes during spontaneous bursting, the number of sEPSCs within a bursting release event and the duration of sEPSC bursts while increasing both the interspike interval and the time between sEPSCs. Exposure of neurons to the VGSC-specific modulator veratridine had effects similar to both DM and PM, while inhibition of voltage-gated calcium channels had no effect on spontaneous spike rates. In the absence of GABA receptor antagonists, both DM and PM increased spontaneous spike rates. Altogether, these data demonstrate that DM and PM disrupt network activity in vitro, largely via a VGSC-dependent mechanism.

Topics: Animals; Animals, Newborn; Calcium Channel Blockers; Calcium Channels; Cells, Cultured; Dose-Response Relationship, Drug; Excitatory Postsynaptic Potentials; Glutamic Acid; Hippocampus; Insecticides; Membrane Transport Modulators; Microarray Analysis; Microelectrodes; Nerve Net; Neurons; Nitriles; omega-Conotoxins; Patch-Clamp Techniques; Permethrin; Pyrethrins; Rats; Rats, Long-Evans; Sodium Channels; Synaptic Transmission; Time Factors; Veratridine

Pyrethroid insecticide modulation of the voltage-gated sodium channel (VGSC) is proposed to underlie their effects on neuronal excitability. However, some in vitro evidence indicates that target sites other than VGSCs could contribute to pyrethroid disruption of neuronal activity. VGSC-independent, pyrethroid-induced changes in neurotransmitter release were examined to investigate the possibility that target sites other than VGSCs contribute to pyrethroid effects. Using whole-cell patch clamp recordings, deltamethrin and permethrin effects on glutamate-mediated miniature excitatory postsynaptic currents (mEPSCs) from pyramidal neurons in mixed hippocampal cultures were examined. In the presence of the VGSC antagonist tetrodotoxin, the type I pyrethroid permethrin (10 microM) increased the average frequency of mEPSCs from a basal level of 1.0+/-0.4 to 3.5+/-0.6 Hz, with peak frequency of 9.9+/-1.5 Hz (n=6). Permethrin did not affect the distribution of current amplitudes, indicating that permethrin increased the probability of glutamate release at the presynaptic terminal without effects on postsynaptic responses. Removal of calcium from the extracellular solution following the induction of the permethrin-mediated effect decreased mEPSC frequency (6.8+/-1.8 Hz, n=3) to near control levels (1.9+/-0.8 Hz for control versus 2.5+/-0.6 Hz for permethrin minus Ca(2+), respectively). However, the N- and P/Q-type voltage-gated calcium channel antagonist omega-conotoxin MVIIC had no effect on the permethrin-dependent increase in mEPSC frequency. In contrast to permethrin, the type II pyrethroid deltamethrin (10 microM) failed to affect mEPSC frequency. These results indicate that permethrin causes a calcium-dependent increase in glutamate release from hippocampal neurons that is independent of effects on voltage-gated sodium or N- or P/Q-type voltage-gated calcium channels. The data indicate that permethrin increases mEPSC frequency via an alteration in intracellular calcium dynamics at the presynaptic terminal.

Topics: Action Potentials; Analysis of Variance; Animals; Animals, Newborn; Calcium; Calcium Channel Blockers; Cells, Cultured; Dose-Response Relationship, Drug; Drug Interactions; Excitatory Postsynaptic Potentials; Glutamic Acid; Hippocampus; Insecticides; Neurons; Nitriles; omega-Conotoxins; Patch-Clamp Techniques; Permethrin; Pyrethrins; Rats; Rats, Long-Evans; Tetrodotoxin