The cannabinoid receptor antagonist AM251 increases paraoxon and chlorpyrifos oxon toxicity in rats

Indole-3-propionic acid mitigates chlorpyrifos-mediated neurotoxicity by modulating cholinergic and redox-regulatory systems, inflammatory stress, apoptotic responses and DNA damage in rats

This study probed the neuroprotective influence of indole-3-propionic acid (IPA) in rats exposed to chlorpyrifos (CPF) alone at 5 mg/kg body weight or co-administered with IPA at 12.5 and 25 mg/kg for 14 days. Behavioral data indicated that IPA significantly (p < 0.05) abated CPF-mediated anxiogenic-like behaviors with concomitant improvement in the locomotor and exploratory behaviors as substantiated by track plots and heat maps data. Also, IPA mitigated CPF-mediated diminution in cholinergic and antioxidant defense systems whereas it markedly improved thioredoxin level and thioredoxin reductase activity in cerebral and cerebellar tissues of the animals. Co-administration of IPA significantly enhanced anti-inflammatory cytokine, interleukin-10 but suppressed oxidative and inflammatory stress, caspase-9 and caspase-3 activation with concomitant reduction in 8-hydroxy-2'-deoxyguanosine (8-OHdG) level and histological damage. Collectively, IPA-mediated neuroprotection involves modulation of cholinergic and redox-regulatory systems, inflammatory stress, apoptotic responses and DNA damage in cerebrum and cerebellum of rats.

Retrograde activation of CB1R by muscarinic receptors protects against central organophosphorus toxicity

The acute toxicity of organophosphorus-based compounds is primarily a result of acetylcholinesterase inhibition in the central and peripheral nervous systems. The resulting cholinergic crisis manifests as seizure, paralysis, respiratory failure and neurotoxicity. Though overstimulation of muscarinic receptors is the mechanistic basis of central organophosphorus (OP) toxicities, short-term changes in synapse physiology that precede OP-induced seizures have not been investigated in detail. To study acute effects of OP exposure on synaptic function, field excitatory postsynaptic potentials (fEPSPs) were recorded from Schaffer collateral synapses in the mouse hippocampus CA1 stratum radiatum during perfusion with various OP compounds. Administration of the OPs paraoxon, soman or VX rapidly and stably depressed fEPSPs via a presynaptic mechanism, while the non-OP proconvulsant tetramethylenedisulfotetramine had no effect on fEPSP amplitudes. OP-induced presynaptic long-term depression manifested prior to interictal spiking, occurred independent of recurrent firing, and did not require NMDA receptor currents, suggesting that it was not mediated by activity-dependent calcium uptake. Pharmacological dissection revealed that the presynaptic endocannabinoid type 1 receptor (CB1R) as well as postsynaptic M₁ and M₃ muscarinic acetylcholine receptors were necessary for OP-LTD. Administration of CB1R antagonists significantly reduced survival in mice after a soman challenge, revealing an acute protective role for endogenous CB1R signaling during OP exposure. Collectively these data demonstrate that the endocannabinoid system alters glutamatergic synaptic function during the acute response to OP acetylcholinesterase inhibitors.

Comparative in vitro and in vivo effects of chlorpyrifos oxon in the outbred CD-1 mouse (Mus musculus) and great plains toad (Anaxyrus cognatus)

We compared biochemical, functional, and behavioral responses to the organophosphorus anticholinesterase chlorpyrifos oxon (CPO) in mice (Mus musculus, CD-1) and toads (Anaxyrus cognatus, Great Plains toad). Toads were substantially less sensitive to acute lethality of CPO based on the maximum tolerated (nonlethal) dose (toads, 77 mg/kg; mice, 5.9 mg/kg). Sublethal exposures led to classical signs of toxicity (increased involuntary movements, autonomic secretions) in mice but hypoactivity in toads. Motor performance in an inclined plane test was not affected by CPO in mice but was altered at the highest dosage in toads. Acetylcholinesterase (AChE), butyrylcholinesterase, monoacylglycerol lipase, and fatty acid amide hydrolase activities in brain were inhibited in mice but not in toads, and fatty acid amide hydrolase activity in the liver was inhibited in both species. Toad brain AChE was less sensitive to in vitro inhibition by CPO (50% inhibitory concentration [IC50; 20 min, 37 °C], 101 vs 7.8 nM; IC50 [20 min, 26 °C], 149 vs 6.2 nM), and studies of inhibitor kinetics indicated substantially lower anticholinesterase potency of CPO against the toad brain enzyme. Using an in vitro indirect inhibition assay, preincubation of CPO with toad brain homogenate was more effective than an equivalent mouse brain homogenate at reducing CPO potency. These data suggest that the relatively low sensitivity of toads to cholinergic toxicity is based on the low sensitivity of brain AChE, which in turn may be attributable to more effective target-site detoxification. Environ Toxicol Chem 2018;37:1898-1906. © 2018 SETAC.

Comparative effects of parathion and chlorpyrifos on endocannabinoid and endocannabinoid-like lipid metabolites in rat striatum

Parathion and chlorpyrifos are organophosphorus insecticides (OPs) that elicit acute toxicity by inhibiting acetylcholinesterase (AChE). The endocannabinoids (eCBs, N-arachidonoylethanolamine, AEA; 2-arachidonoylglycerol, 2AG) are endogenous neuromodulators that regulate presynaptic neurotransmitter release in neurons throughout the central and peripheral nervous systems. While substantial information is known about the eCBs, less is known about a number of endocannabinoid-like metabolites (eCBLs, e.g., N-palmitoylethanolamine, PEA; N-oleoylethanolamine, OEA). We report the comparative effects of parathion and chlorpyrifos on AChE and enzymes responsible for inactivation of the eCBs, fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL), and changes in the eCBs AEA and 2AG and eCBLs PEA and OEA, in rat striatum. Adult, male rats were treated with vehicle (peanut oil, 2 ml/kg, sc), parathion (27 mg/kg) or chlorpyrifos (280 mg/kg) 6-7 days after surgical implantation of microdialysis cannulae into the right striatum, followed by microdialysis two or four days later. Additional rats were similarly treated and sacrificed for evaluation of tissue levels of eCBs and eCBLs. Dialysates and tissue extracts were analyzed by LC-MS/MS. AChE and FAAH were extensively inhibited at both time-points (85-96%), while MAGL activity was significantly but lesser affected (37-62% inhibition) by parathion and chlorpyrifos. Signs of toxicity were noted only in parathion-treated rats. In general, chlorpyrifos increased eCB levels while parathion had no or lesser effects. Early changes in extracellular AEA, 2AG and PEA levels were significantly different between parathion and chlorpyrifos exposures. Differential changes in extracellular and/or tissue levels of eCBs and eCBLs could potentially influence a number of signaling pathways and contribute to selective neurological changes following acute OP intoxications.