Effects of acute insecticide exposure on neuronal activity in vitro in rat cortical cultures

Exposure to pesticides, such as carbamates, organophosphates, organochlorines and pyrethroids, has been linked to various health problems, including neurotoxicity. Although most in vivo studies use only male rodents, some studies have shown in vivo sex-specific effects after acute exposure. Since in vivo studies are costly and require a large number of animals, in vitro assays that take sex-specific effects into account are urgently needed. We therefore assessed the acute effects of exposure to different carbamates (methomyl, aldicarb and carbaryl), organophosphates (chlorpyrifos (CPF), chlorpyrifos-oxon (CPO) and 3,5,6-trichloropyridinol), organochlorines (endosulfan, dieldrin and lindane) and pyrethroids (permethrin, alpha-cypermethrin and 3-phenoxy-benzoic acid (3-PBA)) on neuronal network function in sex-separated rat primary cortical cultures using micro-electrode array (MEA) recordings. Our results indicate that exposure to the carbamate carbaryl and the organophosphates CPF and CPO decreased neuronal activity, with CPO being the most potent. Notably, (network) burst patterns differed between CPF and CPO, with CPO inducing fewer, but more intense (network) bursts. Exposure to low micromolar levels of endosulfan induced a hyperexcitation, most likely due to the antagonistic effects on GABA receptors. Interestingly, females were more sensitive to endosulfan than males. Exposure to dieldrin and lindane also increased neuronal activity, albeit less than endosulfan and without sex-specific effects. Exposure to type I pyrethroid permethrin increased neuronal activity, while exposure to type II pyrethroid alpha-cypermethrin strongly decreased neuronal activity. The increase seen after permethrin exposure was more pronounced in males than in females. Together, these results show that acute exposure to different classes of pesticides exerts differential effects on neuronal activity. Moreover, it shows that MEA recordings are suited to detect sex-specific neurotoxic effects in vitro.

Neuroactivity screening of botanical extracts using microelectrode array (MEA) recordings

Toxicity testing of botanicals is challenging because of their chemical complexity and variability. Since botanicals may affect many different modes of action involved in neuronal function, we used microelectrode array (MEA) recordings of primary rat cortical cultures to screen 16 different botanical extracts for their effects on cell viability and neuronal network function in vitro. Our results demonstrate that extract materials (50 μg/mL) derived from goldenseal, milk thistle, tripterygium, and yohimbe decrease mitochondrial activity following 7 days exposure, indicative of cytotoxicity. Importantly, most botanical extracts alter neuronal network function following acute exposure. Extract materials (50 μg/mL) derived from aristolochia, ephedra, green tea, milk thistle, tripterygium, and usnea inhibit neuronal activity. Extracts of kava, kratom and yohimbe are particularly potent and induce a profound inhibition of neuronal activity at the low dose of 5 μg/mL. Extracts of blue cohosh, goldenseal and oleander cause intensification of the bursts. Aconite extract (5 μg/mL) evokes a clear hyperexcitation with a marked increase in the number of spikes and (network) bursts. The distinct activity patterns suggest that botanical extracts have diverse modes of action. Our combined data also highlight the applicability of MEA recordings for hazard identification and potency ranking of botanicals.

In vitro neurotoxicity screening of engine oil- and hydraulic fluid-derived aircraft cabin bleed-air contamination

In most airplanes, cabin air is extracted from the turbine compressors, so-called bleed air. Bleed air can become contaminated by leakage of engine oil or hydraulic fluid and possible neurotoxic constituents, like triphenyl phosphate (TPhP) and tributyl phosphate (TBP). The aim of this study was to characterize the neurotoxic hazard of TBP and TPhP, and to compare this with the possible hazard of fumes originating from engine oils and hydraulic fluids in vitro. Effects on spontaneous neuronal activity were recorded in rat primary cortical cultures grown on microelectrode arrays following exposure for 0.5h (acute), and 24h and 48h (prolonged) to TBP and TPhP (0.01 - 100µM) or fume extracts (1 - 100µg/mL) prepared from four selected engine oils and two hydraulic fluids by a laboratory bleed air simulator. TPhP and TBP concentration-dependently reduced neuronal activity with equal potency, particularly during acute exposure (TPhP IC₅₀: 10 - 12µM; TBP IC₅₀: 15 - 18µM). Engine oil-derived fume extracts persistently reduced neuronal activity. Hydraulic fluid-derived fume extracts showed a stronger inhibition during 0.5h exposure, but the degree of inhibition attenuates during 48h. Overall, fume extracts from hydraulic fluids were more potent than those from engine oils, in particular during 0.5h exposure, although the higher toxicity is unlikely to be due only to higher levels of TBP and TPhP in hydraulic fluids. Our combined data show that bleed air contaminants originating from selected engine oils or hydraulic fluids exhibit neurotoxic hazard in vitro, with fumes derived from the selected hydraulic fluids being most potent.

Mixture effects of tetrodotoxin (TTX) and drugs targeting voltage-gated sodium channels on spontaneous neuronal activity in vitro

Tetrodotoxin (TTX) potently inhibits TTX-sensitive voltage-gated sodium (Na_V) channels in nerve and muscle cells, potentially resulting in depressed neurotransmission, paralysis and death from respiratory failure. Since a wide range of pharmaceutical drugs is known to also act on Na_V channels, the use of medicines could predispose individuals to a higher susceptibility towards TTX toxicity. We therefore first assessed the inhibitory effect of selected medicines that act on TTX-sensitive (Riluzole, Chloroquine, Fluoxetine, Valproic acid, Lamotrigine, Lidocaine) and TTX-resistant (Carbamazepine, Mexiletine, Flecainide) Na_V channels on spontaneous neuronal activity of rat primary cortical cultures grown on microelectrode arrays (MEA). After establishing concentration-effect curves, binary mixtures of the medicines with TTX at calculated NOEC, IC₂₀ and IC₅₀ values were used to determine if pharmacodynamic interactions occur between TTX and these drugs on spontaneous neuronal activity. At IC₂₀ and IC₅₀ values, all medicines significantly increased the inhibitory effect of TTX on spontaneous neuronal activity of rat cortical cells in vitro. Subsequent experiments using human iPSC-derived neuronal co-cultures grown on MEAs confirmed the ability of selected medicines (Carbamazepine, Flecainide, Riluzole, Lidocaine) to inhibit spontaneous neuronal activity. Despite the need for additional experiments using human iPSC-derived neuronal co-cultures, our combined data already highlight the importance of identifying and including vulnerable risk groups in the risk assessment of TTX.

Culture of Rat Primary Cortical Cells for Microelectrode Array (MEA) Recordings to Screen for Acute and Developmental Neurotoxicity

Neurotoxicity testing of chemicals, drug candidates, and environmental pollutants still relies on extensive in vivo studies that are very costly, time-consuming, and ethically debated due to the large number of animals typically used. Currently, rat primary cortical cultures are widely used for in vitro neurotoxicity studies, as they closely resemble the in vitro brain with respect to the diversity of cell types, their physiological functions, and the pathological processes that they undergo. Common in vitro assays for neurotoxicity screening often focus on very target-specific endpoints such as morphological, biochemical, or electrophysiological changes, and such narrow focus can hamper translation and interpretation. Microelectrode array (MEA) recordings provide a non-invasive platform for extracellular recording of electrical activity of cultured neuronal cells, thereby enabling the evaluation of changes in neuronal (network) function as a sensitive and integrated endpoint for neurotoxicity screening. Here, we describe an in vitro approach for assessing changes in neuronal network function as a measure for neurotoxicity, using rat primary cortical cultures grown on MEAs. We provide a detailed protocol for the culture of rat primary cortical cells, and describe several experimental procedures to address acute, subchronic, and chronic exposure scenarios. We additionally describe the steps for processing and analyzing MEA and cell viability data. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Isolation and culture of rat primary cortical cells on 48-well MEA plates Support Protocol 1: Pretreatment and washing of 48-well MEA plates before first use or for re-use Support Protocol 2: Coating of 48-well MEA plates with 0.1% PEI solution Basic Protocol 2: MEA measurements during acute exposure Alternate Protocol 1: MEA measurements during subchronic exposure Alternate Protocol 2: MEA measurements during chronic exposure Support Protocol 3: Determination of cell viability after MEA experiments Basic Protocol 3: MEA data processing Basic Protocol 4: Analyzing MEA experiments after acute and subchronic exposure Alternate Protocol 3: Analyzing MEA experiments after chronic exposure.

Estimation of the risk of local and systemic effects in infants after ingestion of low-concentrated weak acids from descaling products

INTRODUCTION

The accidental ingestion of diluted household descaling products by infants is a phenomenon that poison control centers regularly encounter. Feeding infants with baby milk prepared with water from electric kettles still containing descaler is a common way of exposure. This study aimed to determine the risks related to ingestion of (diluted) descalers by infants.

METHODS

pH measurements were performed using acetic acid and three different commercially available electric kettle descalers. The pH of different dilutions was measured in the absence or presence of baby milk powder. In addition, an overview was made of pH values of different electric kettle descalers as given by the product information of the manufacturer. Finally, a simple pharmacokinetic (PK) model was used to predict changes in blood pH in infants after ingestion of acetic acid, which is the most commonly used descaler.

RESULTS

Several commercially available electric kettle descalers have a pH <2. Even after diluting such descalers up to 10 times the pH can remain low. The addition of milk powder increases the pH of descalers containing weaker acids, with a pH >1.5, while descalers with stronger acids and pH <1 show little pH increase after the addition of milk powder. Finally, a simple PBPK model for the ingestion of acetic acid predicted that the ingestion of larger amounts of acetic acid (>1000 mg) by an infant could result in relevant changes in blood pH.

CONCLUSIONS

Commercially available electric kettle descaling products may pose a health risk to infants in case of accidental ingestion since the pH of some of these products can be very low, even when diluted 10-times or in the presence of baby milk powder. Oral exposure of infants to the common descaler acetic acid, after accidental preparation of baby milk with cleaning vinegar, will probably not result in serious local effects, but changes in blood pH cannot be excluded when larger amounts of acetic acid are ingested.

Rational design of highly potent broad-spectrum enterovirus inhibitors targeting the nonstructural protein 2C

There is a great need for antiviral drugs to treat enterovirus (EV) and rhinovirus (RV) infections, which can be severe and occasionally life-threatening. The conserved nonstructural protein 2C, which is an AAA+ ATPase, is a promising target for drug development. Here, we present a structure-activity relationship study of a previously identified compound that targets the 2C protein of EV-A71 and several EV-B species members, but not poliovirus (PV) (EV-C species). This compound is structurally related to the Food and Drug Administration (FDA)-approved drug fluoxetine—which also targets 2C—but has favorable chemical properties. We identified several compounds with increased antiviral potency and broadened activity. Four compounds showed broad-spectrum EV and RV activity and inhibited contemporary strains of emerging EVs of public health concern, including EV-A71, coxsackievirus (CV)-A24v, and EV-D68. Importantly, unlike (S)-fluoxetine, these compounds are no longer neuroactive. By raising resistant EV-A71, CV-B3, and EV-D68 variants against one of these inhibitors, we identified novel 2C resistance mutations. Reverse engineering of these mutations revealed a conserved mechanism of resistance development. Resistant viruses first acquired a mutation in, or adjacent to, the α2 helix of 2C. This mutation disrupted compound binding and provided drug resistance, but this was at the cost of viral fitness. Additional mutations at distantly localized 2C residues were then acquired to increase resistance and/or to compensate for the loss of fitness. Using computational methods to identify solvent accessible tunnels near the α2 helix in the EV-A71 and PV 2C crystal structures, a conserved binding pocket of the inhibitors is proposed.

There is a great need for antiviral drugs to treat enterovirus and rhinovirus infections, which can be severe and occasionally life-threatening. This study describes novel small molecule inhibitors that target a broad spectrum of clinically relevant enterovirus species; a common mechanism of resistance development revealed the target to be a highly conserved binding pocket in the viral helicase 2C.

Snake Venom Gland Organoids

Wnt dependency and Lgr5 expression define multiple mammalian epithelial stem cell types. Under defined growth factor conditions, such adult stem cells (ASCs) grow as 3D organoids that recapitulate essential features of the pertinent epithelium. Here, we establish long-term expanding venom gland organoids from several snake species. The newly assembled transcriptome of the Cape coral snake reveals that organoids express high levels of toxin transcripts. Single-cell RNA sequencing of both organoids and primary tissue identifies distinct venom-expressing cell types as well as proliferative cells expressing homologs of known mammalian stem cell markers. A hard-wired regional heterogeneity in the expression of individual venom components is maintained in organoid cultures. Harvested venom peptides reflect crude venom composition and display biological activity. This study extends organoid technology to reptilian tissues and describes an experimentally tractable model system representing the snake venom gland.

Perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) acutely affect human α1β2γ2L GABAA receptor and spontaneous neuronal network function in vitro

Concerns about the neurotoxic potential of polyfluoroalkyl substances (PFAS) such as perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) increase, although their neurotoxic mechanisms of action remain debated. Considering the importance of the GABA_A receptor in neuronal function, we investigated acute effects of PFAS on this receptor and on spontaneous neuronal network activity. PFOS (Lowest Observed Effect Concentration (LOEC) 0.1 µM) and PFOA (LOEC 1 µM) inhibited the GABA-evoked current and acted as non-competitive human GABA_A receptor antagonists. Network activity of rat primary cortical cultures increased following exposure to PFOS (LOEC 100 µM). However, exposure of networks of human induced pluripotent stem cell (hiPSC)-derived neurons decreased neuronal activity. The higher sensitivity of the α₁β₂γ_2L GABA_A receptor for PFAS as compared to neuronal networks suggests that PFAS have additional mechanisms of action, or that compensatory mechanisms are at play. Differences between rodent and hiPSC-derived neuronal networks highlight the importance of proper model composition. LOECs for PFAS on GABA_A receptor and neuronal activity reported here are within or below the range found in blood levels of occupationally exposed humans. For PFOS, LOECs are even within the range found in human serum and plasma of the general population, suggesting a clear neurotoxic risk.

Modulation of human α4β2 nicotinic acetylcholine receptors by brominated and halogen-free flame retardants as a measure for in vitro neurotoxicity

Brominated flame retardants (BFRs) are abundant persistent organic pollutants with well-studied toxicity. The toxicological and ecological concern associated with BFRs argues for replacement by safer alternatives. However, the (neuro)toxic potential of alternative halogen-free flame retardants (HFFRs) is unknown. Previous research identified the nervous system as a sensitive target organ for BFRs, with modulation of excitatory nicotinic acetylcholine (nACh) receptors as one of the modes of action. Since it is essential to assess the (neuro)toxic potential of HFFRs before large scale use, we measured the effects of three BFRs and 13 HFFRs on the function of human α(4)β(2) nACh receptors, expressed in Xenopus oocytes, using the two-electrode voltage-clamp technique. The results demonstrate that some BFRs (TBBPA and to a lesser extent BDE-209) and HFFRs (TPP, Alpi, APP, MMT and to a lesser extent ATH, ATO, MHO, MPP, RDP and ZHS) act as nACh receptor antagonists. Contrary, BPS, BDP, DOPO and ZS were unable to modulate nACh receptors. Despite the lack of toxicological data on HFFRs and the need for additional studies to perform a full (neuro)toxic risk assessment, the current data on antagonistic effects on nACh receptors could be an important step in prioritizing viable HFFRs for substitution of BFRs.

Multiple novel modes of action involved in the in vitro neurotoxic effects of tetrabromobisphenol-A

Neurotoxicological data on the widely used brominated flame retardant tetrabromobisphenol-A (TBBPA) is limited. Since recent studies indicated that inhibitory GABA(A) and excitatory α(4)β(2) nicotinic acetylcholine (nACh) receptors are sensitive targets for persistent organic pollutants, we investigated the effects of TBBPA on these receptors, expressed in Xenopus oocytes, using the two-electrode voltage-clamp technique. Our results demonstrate that TBBPA acts as full (≥ 10 μM) and partial (≥ 0.1 μM) agonist on human GABA(A) receptors, whereas it acts as antagonist (≥ 10 μM) on human α(4)β(2) nACh receptors. Next, neuronal B35 cells were used to further study the effects of TBBPA on calcium-permeable nACh receptors using single-cell fluorescent calcium imaging. These results demonstrate that TBBPA (≥ 1 μM) inhibits acetylcholine (ACh) receptors as evidenced by a reduction in the ACh-evoked increases in the intracellular calcium concentration ([Ca(2+)](i)). Additionally, TBBPA (> 1 μM) induced a strong and concentration-dependent increase in basal [Ca(2+)](i) in B35 cells. Similarly, TBBPA (> 1 μM) increases basal [Ca(2+)](i) in dopaminergic PC12 cells. This increase is also evident under calcium-free conditions, indicating it originates from intracellular calcium stores. Moreover, depolarization-evoked increases in [Ca(2+)](i) are strongly reduced by TBBPA (≥ 1 μM), indicating TBBPA-induced inhibition of voltage-gated calcium channels. Our in vitro studies thus demonstrate that TBBPA exerts several adverse effects on functional neurotransmission endpoints with effect concentrations that are only two orders of magnitude below the highest cord serum concentrations. Although epidemiological proof for adverse TBBPA effects is lacking, our data justify the quest for flame retardants with reduced neurotoxic potential.

Potentiation of the human GABA(A) receptor as a novel mode of action of lower-chlorinated non-dioxin-like PCBs

PCBs are still ubiquitous pollutants despite the ban on their industrial and commercial use. To date, risk characterization and assessment of non-dioxin-like PCBs (NDL-PCBs), especially with respect to neurotoxicity, is hampered by a lack of data. Therefore, the effects of six common NDL congeners (PCB28, 52, 101, 138, 153 and 180) on human GABA(A) receptors, expressed in Xenopus oocytes, were investigated using the two-electrode voltage-clamp technique. When coapplied with GABA (at EC(20)), PCB28 and PCB52 concentration-dependently potentiate the GABA(A) receptor-mediated ion current. Though the LOEC for both PCB28 and PCB52 is 0.3 microM, PCB28 is more potent than PCB52 (maximum potentiation at 10 muM amounting to 98.3 +/- 12.5% and 25.5 +/- 1.4%, respectively). Importantly, coapplication of PCB28 (0.3 microM) and PCB52 (10 microM) resulted in an apparently additive potentiation of the GABA(A) response, whereas coapplication of PCB28 (0.3 microM) and PCB153 (10 microM) attenuated the PCB28-induced potentiation. The present results suggest that the potentiation of human GABA(A) receptor function is specific for lower-chlorinated NDL-PCBs and that higher molecular weight PCBs may attenuate this potentiation as a result of competitive binding to human GABA(A) receptors. Nonetheless, this novel mode of action could (partly) underlie the previously recognized NDL-PCB-induced neurobehavioral alterations.

Selective inhibition of human heteromeric alpha9alpha10 nicotinic acetylcholine receptors at a low agonist concentration by low concentrations of ototoxic organic solvents

Ethylbenzene and para-xylene (p-xylene), but not the chemically closely related organic solvents ortho-xylene (o-xylene) and meta-xylene (m-xylene), are known to cause ototoxicity and irreversible hearing loss, though the underlying mechanisms are still unknown. In this study, effects of ethylbenzene and of p-, o-, and m-xylene on human heteromeric alpha9alpha10 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes were investigated using the two-electrode voltage clamp technique. ACh dose-dependently evoked an alpha9alpha10 nAChR-mediated ion current with an EC(50) of 137 microM. When ACh is applied at a low concentration (10 microM), the nAChR-mediated ion current is inhibited by a low concentration (10 microM) of ethylbenzene and p-xylene, but not by the same concentration of the non-ototoxic solvents. At a high solvent concentration (300 microM), all solvents cause inhibition of the ion currents evoked by 10 microM ACh. Ion currents evoked by a near maximum-effective concentration ACh (1mM) are inhibited by the selected organic solvents only at 300 microM. These results demonstrate that low concentrations of the known ototoxic solvents ethylbenzene and p-xylene inhibit alpha9alpha10 nAChR-mediated ion currents, whereas the structurally related, non-ototoxic solvents m-xylene and o-xylene do not, indicating that the alpha9alpha10 nAChR is a potential target for solvent-induced ototoxicity.

Hydroxylation increases the neurotoxic potential of BDE-47 to affect exocytosis and calcium homeostasis in PC12 cells

BACKGROUND

Oxidative metabolism, resulting in the formation of hydroxylated polybrominated diphenyl ether (PBDE) metabolites, may enhance the neurotoxic potential of brominated flame retardants.

OBJECTIVE

Our objective was to investigate the effects of a hydroxylated metabolite of 2,2',4,4'-tetra-bromodiphenyl ether (BDE-47; 6-OH-BDE-47) on changes in the intracellular Ca2+ concentration ([Ca2+]i) and vesicular catecholamine release in PC12 cells.

METHODS

We measured vesicular catecholamine release and [Ca2+]i using amperometry and imaging of the fluorescent Ca2+-sensitive dye Fura-2, respectively.

RESULTS

Acute exposure of PC12 cells to 6-OH-BDE-47 (5 microM) induced vesicular catecholamine release. Catecholamine release coincided with a transient increase in [Ca2+]i, which was observed shortly after the onset of exposure to 6-OH-BDE-47 (120 microM). An additional late increase in [Ca2+]i was often observed at > or =1 microM 6-OH-BDE-47. The initial transient increase was absent in cells exposed to the parent compound BDE-47, whereas the late increase was observed only at 20 microM. Using the mitochondrial uncoupler carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) and thapsigargin to empty intracellular Ca2+ stores, we found that the initial increase originates from emptying of the endoplasmic reticulum and consequent influx of extracellular Ca2+, whereas the late increase originates primarily from mitochondria.

CONCLUSION

The hydroxylated metabolite 6-OH-BDE-47 is more potent in disturbing Ca2+ homeostasis and neurotransmitter release than the parent compound BDE-47. The present findings indicate that bioactivation by oxidative metabolism adds considerably to the neurotoxic potential of PBDEs. Additionally, based on the observed mechanism of action, a cumulative neurotoxic effect of PBDEs and ortho-substituted polychlorinated biphenyls on [Ca2+]i cannot be ruled out.

Neonatal exposure to brominated flame retardant BDE-47 reduces long-term potentiation and postsynaptic protein levels in mouse hippocampus

BACKGROUND

Increasing environmental levels of brominated flame retardants raise concern about possible adverse effects, particularly through early developmental exposure.

OBJECTIVE

The objective of this research was to investigate neurodevelopmental mechanisms underlying previously observed behavioral impairments observed after neonatal exposure to polybrominated diphenyl ethers (PBDEs).

METHODS

C57Bl/6 mice received a single oral dose of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) on postnatal day (PND) 10 (i.e., during the brain growth spurt). On PND17-19, effects on synaptic plasticity, levels of postsynaptic proteins involved in long-term potentiation (LTP), and vesicular release mechanisms were studied ex vivo. We investigated possible acute in vitro effects of BDE-47 on vesicular catecholamine release and intracellular Ca(2+) in rat pheochromocytoma (PC12) cells.

RESULTS

Field-excitatory postsynaptic potential (f-EPSP) recordings in the hippocampal CA1 area demonstrated reduced LTP after exposure to 6.8 mg (14 micromol)/kg body weight (bw) BDE-47, whereas paired-pulse facilitation was not affected. Western blotting of proteins in the postsynaptic, triton-insoluble fraction of hippocampal tissue revealed a reduction of glutamate receptor subunits NR2B and GluR1 and autophosphorylated-active Ca(2+)/calmodulin-dependent protein kinase II (alphaCaMKII), whereas other proteins tested appeared unaffected. Amperometric recordings in chromaffin cells from mice exposed to 68 mg (140 micromol)/kg bw BDE-47 did not reveal changes in catecholamine release parameters. Modest effects on vesicular release and intracellular Ca(2+) in PC12 cells were seen following acute exposure to 20 microM BDE-47. The combined results suggest a post-synaptic mechanism in vivo.

CONCLUSION

Early neonatal exposure to a single high dose of BDE-47 causes a reduction of LTP together with changes in postsynaptic proteins involved in synaptic plasticity in the mouse hippocampus.

Cholinergic drugs potentiate human nicotinic α4β2 acetylcholine receptors by a competitive mechanism

Effects of cholinergic drugs on human alpha4beta2 nicotinic acetylcholine receptors expressed in Xenopus oocytes have been investigated in electrophysiological and ligand binding experiments. Atropine, scopolamine, physostigmine, and tacrine combine potentiation of ion current induced by low concentrations of acetylcholine with inhibition of ion current evoked by high concentrations of acetylcholine. Rivastigmine, galanthamine, and dichlorvos cause only inhibition of ion current evoked by low concentrations of acetylcholine. Binding experiments show that the potentiating cholinergic drugs atropine, scopolamine, and physostigmine are competitive ligands of human alpha4beta2 nicotinic acetylcholine receptors. Conversely, the inhibitory cholinergic drugs galanthamine and rivastigmine are non-competitive. The non-competitive drugs are not allosteric, since they do not affect the saturation curve of the radioligand [3H]cytisine. Effects of potentiating cholinergic drugs on nicotinic acetylcholine receptors are consistent with and predicted by a model comprising competitive drug effects at two equivalent agonist recognition sites on the nicotinic acetylcholine receptor combined with non-competitive ion channel block.

Block of neuronal nicotinic acetylcholine receptors by organophosphate insecticides

Chronic and acute exposure to organophosphate (OP) pesticides may lead to persistent neurological and neurobehavioral effects, which cannot be explained by acetylcholinesterase (AChE) inhibition alone. It is suggested that other brain proteins are involved. Effects of commonly used organophosphate pesticides on rat neuronal alpha4beta2 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus laevis oocytes have been investigated using the two-electrode voltage clamp technique. Several OP pesticides, e.g., parathion-ethyl, chlorpyrifos and disulfoton, inhibited the ACh-induced ion current with potencies in the micromolar range. The potency of inhibition increased with increasing concentrations of the agonist ACh. Comparison of the potency of nAChR inhibition with the potency of AChE inhibition demonstrated that some OPs inhibit nAChRs more potently than AChE. Binding experiments on alpha4beta2 nAChRs showed that the OPs noncompetitively interact with nAChRs. The inhibitory effects on nAChRs are adequately described and explained by a sequential two-step mechanism, in which rapidly reversible OP binding to a separate binding site leads to inhibition followed by a stabilization of the blocked state or receptor desensitization. It is concluded that OPs interact directly with neuronal alpha4beta2 nAChRs to inhibit the agonist-induced response. This implicates that neuronal alpha4beta2 nAChRs are additional targets for some OP pesticides.