Effect of metabolic inhibition with piperonyl butoxide on rodent sensitivity to tri-ortho-cresyl phosphate

Lapatinib alleviates TOCP-induced axonal damage in the spinal cord of mouse

Neuregulin-1 (NRG1), a family of EGF-like factors that activates ErbB receptors, can regulate the proliferation, migration, and myelinating of Schwann cells. We previously reported that NRG1/ErbB signal is responsible for organophosphate (OP)-induced delayed neuropathy (OPIDN) in hens, a susceptive animal model to neuropathic organophosphorous compounds. Our previous study discovered that a neuropathic OP, tri-o-cresyl phosphate (TOCP) activated NRG1/ErbB signaling pathway in both spinal cord and sciatic nerves of hens during the formation of OPIDN and lapatinib, a non-selective antagonist of ErbB1 and ErbB2 receptors, alleviated the toxicity. In this study, we intended to further look into the potential role of NRG1 in the pathogenesis of TOCP-induced axon damage in spinal cord and sciatic nerves and whether lapatinib could also rescue this damage in mice, an OPIDN-resistant animal model. The results revealed that no obvious toxic signs were observed after single TOCP exposure. However, slight histopathological wreck in lumbar spinal cord and sciatic nerves was found following TOCP intoxication, and the damage in sciatic nerves was characterized by axon degeneration of myelin sheath but not the loss of neural skeleton. Only histopathological damage induced by TOCP in spinal cord could be prevented by lapatinib. The translational expression of NRG1/ErbB signaling molecules was analyzed by both in vivo and in vitro studies. In general, NRG1/ErbB pathway was activated by TOCP while combined treatment with lapatinib attenuated TOCP-induced NRG1/ErbB signaling cascade. The results implied that NRG1/ErbB system may predominately play functional role in spinal cord (central nervous system) but not in sciatic nerves (peripheral nervous system) of mouse subjected to neurotoxic OP, which was confirmed by the study in vitro that lapatinib was not able to attenuate TOCP-induced neurotoxicity in rodent Schwann cell line RSC 96 cells.

The homeostasis of phosphatidylcholine and lysophosphatidylcholine in nervous tissues of mice was not disrupted after administration of tri-o-cresyl phosphate

Neuropathy target esterase (NTE) is proven to act as a lysophospholipase (LysoPLA) in mice and phospholipase B (PLB) in cultured mammalian cells. In sensitive species, organophosphate (OP)-induced delayed neurotoxicity is initiated when NTE is inhibited by > 70% and then aged. It is hypothesized that homeostasis of phosphatidylcholine (PC) and/or lysophosphatidylcholine (LPC) in mice might be disrupted by the OPs since NTE and other phospholipases could be inhibited. To test this hypothesis, we treated mice using tri-o-cresyl phosphate (TOCP), which can inhibit and age NTE. Phenylmethylsulfonyl fluoride (PMSF), which inhibits NTE but cannot age, was used as a negative control. Effects on activity of NTE, LysoPLA, and PLB, the levels of PC, LPC, and glycerophosphocholine (GPC), and the aging of NTE in the brain, spinal cord, and sciatic nerve were examined. The results showed that the activities of NTE, NTE-LysoPLA, LysoPLA, NTE-PLB, and PLB were significantly inhibited in both TOCP- and PMSF-treated mice, and the inhibition of NTE and NTE-LysoPLA or NTE-PLB showed a high correlation coefficient. The NTE inhibited by TOCP was of the aged type, while nearly all NTE inhibited by PMSF was of the unaged type. Although the GPC level was remarkedly decreased, no significant change of PC and LPC levels was observed. However, the inhibition of these enzymes in mice by TOCP exhibited different characteristics from the TOCP-treated hens that we previously reported, which indicates that these enzymes were inhibited and then recovered more rapidly in mice than in hens. All results suggest that PC and LPC homeostasis was not disrupted in mice after exposure to TOCP. Differences in inhibition of NTE, LysoPLA, and PLB activities by TOCP between mice and hens may elucidate why these two species display different signs after exposure to the same neuropathic OPs.

Murine susceptibility to organophosphorus-induced delayed neuropathy (OPIDN)

This study reports that CD-1 strain mice are neuropathologically and biochemically responsive to acute doses of tri-ortho-cresyl phosphate (TOCP). Young (25-30 g) male and female animals were exposed (po) to a single dose of TOCP (580-3480 mg/kg) and sampled for neurotoxic esterase (NTE) activity at 24 and 44 hr postexposure and for neuropathic damage 14 days later. Biochemically, high intragroup variability existed at the lower doses, and at higher levels of TOCP exposure (i.e., greater than or equal to 1160 mg/kg), mean brain NTE inhibition never exceeded 68%. Hen and mouse brain NTE activity, assayed in vitro for sensitivity to inhibition by tolyl saligenin phosphate (TSP), the active neurotoxic metabolite of TOCP, showed similar IC50 values. Histologically, highly variable spinal cord damage was recorded throughout treatment groups and mean damage scores followed a dose-response pattern with no apparent correlation to threshold (i.e., greater than or equal to 65%) inhibition of brain NTE activity. Topographically, axonal degeneration in the mouse spinal cord predominated in the lateral and ventral columns of the upper cervical cord. Unlike the rat, which displays degeneration in the upper cervical cord's dorsal columns (i.e., gracilis fasciculus) in response to TOCP intoxication, treated mice showed minimal damage to this tract. To examine this discrepancy further, ultrastructural morphometric analysis of axon diameters in the cervical cord was performed in control mice and rats. These results indicated that in both species, the largest diameter (greater than or equal to 4 microns) axons are housed in the ventral columns of the cervical spinal cord, suggesting that axon length and diameter may not be the only criteria underlying fiber tract vulnerability in OPIDN.

The relationship between neurological damage and neurotoxic esterase inhibition in rats acutely exposed to tri-ortho-cresyl phosphate

A rodent model of organophosphorus-induced delayed neuropathy (OPIDN) has been developed using Long-Evans adult male rats exposed to tri-ortho-cresyl phosphate (TOCP). In the present study an attempt was made to relate neurochemical with neuropathological changes in rats exposed to single dosages of TOCP ranging from 145 to 3480 mg/kg. The degree of neurotoxic esterase (NTE) inhibition, measured at 20 and 44 hr and at 14 days postexposure was correlated with the appearance of spinal cord pathology 14 days postexposure in a separate group of similarly dosed rats. Those dosages that inhibited mean NTE activity in spinal cord greater than or equal to 72% and brain greater than or equal to 66% of control values within 44 hr postexposure produced marked spinal cord pathology 14 days postexposure in greater than or equal to 90% of similarly dosed animals. In contrast, dosages of TOCP which inhibited mean NTE activity in the spinal cord less than or equal to 65% and in the brain less than or equal to 57% produced spinal cord pathology in less than or equal to 15% of the animals. These data indicate that NTE inhibition may be used as a biochemical predictor for TOCP-induced neurological damage in rats.