Neurotoxicity assessment of O-ethyl-O'-(2-diisopropylaminoethyl) methylphosphonite (QL) in hens

The pathogenesis of organophosphate polyneuropathy

This review discusses the facts regarding organophosphate-induced delayed polyneuropathy (OPIDP) as they are related to its pathogenesis rather than being a comprehensive review of all available data. Neuropathy target esterase (NTE) is considered to be the molecular target for OPIDP which is affected by several esterase inhibitors. Such inhibitors are ranked according to their toxicological effects as follows: 1. Phosphates, phosphoroamidates, and phosphonates cause OPIDP when high amounts of NTE are inhibited. In most cases 70 to 80% inhibition is enough, whereas in others much more is required. 2. Phosphinates, carbamates, and sulfonyl halides cause either protection from or promotion of OPIDP when given before or after a neuropathic OP, respectively. Both effects are related to doses that inhibit NTE. Neuropathy is also caused by the combined treatment with a carbamate and a sulfonyl fluoride. The potency of a given NTE inhibitor to cause OPIDP is related to the chemistry of the residue left attached to NTE, in addition to its affinity for the enzyme. The capability of inhibited NTE to undergo the aging process distinguishes inhibitors with high from those with negligible or very low potency to cause OPIDP. Therefore, protection from neuropathic doses of effective OPs is obtained when NTE is mostly inhibited with nonageable inhibitors. Promotion of OPIDP is likely to involve another site besides NTE because it might occur when almost all NTE is affected. Promotion affects either progression or expression of OPIDP after the initial biochemical lesion on NTE. Since only NTE inhibitors have been proven to be promoters, it is possible that this site is made available after the initiation of OPIDP and that it may have biochemical properties indistinguishable from those of NTE of naïve birds. Age-related resistance to OPIDP also seems to be related to either progression or expression of OPIDP and/or to the different physiology of NTE at a given age. Previously reported resistance of rats to clinical OPIDP seems also to be age-dependent. The physiological function(s) of NTE is unknown, but some practical gains have been obtained from its identification, including OPIDP risk assessment and biomonitoring.

Triphenyl phosphite neuropathy differs from organophosphorus-induced delayed neuropathy in rats

The topographies of neuropathic damage produced by triphenyl phosphite (TPP) and tri-ortho-cresyl phosphate (TOCP) are contrasted in the present study. Long-Evans, male rats were exposed (sc) to single (0.1 ml kg-1; 1.0 ml kg-1) or multiple (2 X 1.0 ml kg-1; 3 X 1.0 ml kg-1) doses of TPP, and sampled 1-3 weeks after exposure. Additional animals were dosed acutely with TOCP (1160 mg kg-1, p.o.) alone or in combination with multiple doses of TPP (2 X 1 ml kg-1). Functional changes, seen in multiple-dosed TPP-rats included tail-kinking, circling, and ataxia. Neuropathological damage seen in all but the 0.1 ml kg-1 treated animals consisted of degeneration of the ventrolateral and ventral columns of the spinal cord at all levels, and moderate peripheral nerve fibre damage. Medullary brainstem involvement consisted of axonal swellings and fragmented axons in the medial longitudinal fasciculus, the reticular formation, and the inferior cerebellar peduncles. Tissues examined from TOCP-treated rats displayed severe degeneration of the fasciculus gracilis at the cervical level, mild involvement of the dorsolateral columns at the lumbar levels and a sparing of all other cord and brainstem regions. Rats treated with neuropathic doses of both TPP and TOCP showed a composite pattern of degeneration that included damaged sites characteristic of the individual neurotoxicants. These data indicate that the neuropathic profile of TPP differs markedly from the delayed neuropathy (OPIDN) associated with exposure to model organophosphorus compounds such as TOCP.

Triphenyl phosphite: in vivo and in vitro inhibition of rat neurotoxic esterase

Organophosphorus compounds which, after acute administration, inhibit neurotoxic esterase (NTE) by greater than or equal to 65% and undergo a subsequent "aging" reaction, produce a delayed neuropathy characterized by degeneration of large and long nerve fibers (OPIDN). The present studies examine in detail the NTE-inhibiting properties of triphenyl phosphite (TPP), a plasticizer which produces ataxia and degeneration of the spinal cord in animals. A neurotoxic dosing regimen (1184 mg/kg/week, sc, for 2 weeks) inhibited both brain and spinal cord NTE (less than or equal to 40%) only marginally 4 and 48 hr postdosing. By contrast, TPP was shown in vitro to be a potent (150 = 0.98 microM) inhibitor of rat brain NTE relative to Mipafox or diisopropyl phosphorofluoridate. Compounds structurally related to TPP (i.e., triphenyl phosphate, triphenyl phosphine, trimethyl phosphite, and phenol) failed to inhibit NTE in vitro at less than 10 microM concentrations. Close examination of the TPP inhibition of NTE showed a nonlinear relationship between the duration of incubation time and loss of log(NTE activity). Preincubation of 10 microM TPP in buffer (37 degrees C) resulted in a time-dependent loss of TPP's ability to inhibit NTE. In summary, TPP is a powerful NTE inhibitor in vitro, but only a marginal NTE inhibitor after in vivo administration. These results raise questions as to the causal events mediating TPP-induced neuropathy in the rat.