Electrophysiologic changes associated with chronic administration of organophosphates

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.

Relative refractory period: a measure to detect early neuropathy in alcoholics

The absolute (ARP) and relative refractory period (RRP) of the median sensory nerve was determined in 26 control subjects and 24 alcoholics, nine of whom had symptoms of peripheral neuropathy. Recovery of latency to normal in response to the second stimulus was used to define RRP. A true RRP was calculated by subtracting ARP from measured RRP. Mean ARP for all subjects ranged from 0.75 to 0.80 msec; normal = 0.8 +/- 0.2 msec. The true RRP of control subjects was 2.1 +/- 0.5 msec, and for all alcoholic subjects it was 3.1 +/- 0.5 msec. True RRP for the nine symptomatic alcoholic subjects was 3.6 +/- 0.5 msec and 2.9 +/- 0.4 msec for those who were asymptomatic. Symptomatic and asymptomatic alcoholics differed significantly from one another, as well as from control subjects (P less than 0.001). Routine nerve conduction studies were normal in asymptomatic subjects. Three out of nine symptomatic alcoholics had increases in distal median motor or sensory latency, and three had slight slowing of median nerve conduction velocity. True RRP is more sensitive than routine measures of nerve conduction in the detection of axonal disorders influencing nerve conduction.

Electrophysiologic changes following treatment with organophosphorus-induced delayed neuropathy-producing agents in the adult hen

Although clinical, pathological, and biochemical effects of organophosphorus-induced delayed neuropathy (OPIDN) have been intensively investigated in the adult hen, detailed electrophysiological studies are lacking. Adult white leghorn hens were treated with a single oral dose of either 30 mg/kg tri-2-cresyl phosphate (TOCP), 750 mg/kg TOCP, 4 mg/kg di-n-butyl-2,2-dichlorovinyl phosphate (DBCV), or 30 mg/kg di-n-butyl-2,2-dichlorovinyl phosphinate (DBCV-P). The 750 mg/kg TOCP and DBCV, but not the 30 mg/kg TOCP and DBCV-P, treatments resulted in clinical signs of OPIDN and mild to marked damage of the tibial nerve 21 days after dose. Twenty-four hr lymphocyte neurotoxic esterase (NTE) inhibition was used as an index of brain NTE inhibition for the various organophosphorus compound (OP) treatment. Twenty-four hr lymphocyte NTE inhibition for 30 mg/kg TOCP, 750 mg/kg TOCP, DBCV, and DBCV-P was 54.1, 87.1, 84.8, and 68.3%, respectively. Twenty-one days after dose, the TOCP-treated hens exhibited some abnormalities in conduction velocity and action potential duration in the tibial or sciatic nerves. No abnormalities were observed in action potential parameters of either the DBCV or DBCV-P treatments. Neurotoxic OP (TOCP and DBCV) treatment resulted in decreased refractoriness in the tibial nerve, increased refractoriness in the sciatic nerve, and elevated strength duration threshold for both nerves. These changes were not present in nerves from DBCV-P (a non-neurotoxic NTE inhibitor)-treated hens. These results suggest that refractory period and strength duration abnormalities in peripheral nerve correlate well with the production of OPIDN and are evident without coincident clinical signs or histopathology.

Electrophysiologic changes in peripheral nerve following repeated exposure to organophosphorus agents

The purpose of this study was to examine the electrophysiologic changes which occur following repeated exposure to potent organophosphorus agents. The results show that DFP and soman each initially increase rat sciatic nerve conduction and reduce refractoriness. Continued exposure had a diminished effect with nerve excitability eventually returning to control. During recovery, the nerve membrane responsiveness to potassium-induced depolarization significantly changed in a manner which would indicate either decreased Na, K-ATPase activity or decreased potassium ion transmembrane flux. The data suggest that compensatory changes occur in rat nerve in response to organophosphorus exposure, and further, that these compensatory changes involve alteration in membrane ion fluxes.

Electrophysiologic deficits in peripheral nerve as a discriminator of early hexacarbon neurotoxicity

To determine the extent of neurotoxicity of parenterally administered hexacarbons, male Sprague-Dawley rats were given either n-hexane or 2,5-hexanedione for 35 consecutive days. Electrophysiologic measurements showed a lengthening of the sciatic and sural nerve action potentials (slower conduction velocities) and increased refractory periods. These effects correlated with a shift in the nerve membrane sensitivity to potassium-induced depolarization. A similar effect can be induced by ouabain, an Na+, K+-ATPase inhibitor. These effects were seen with both n-hexane and 2,5-hexane-dione. Although the treated animals gained weight more slowly than controls, they showed no loss of motor function when tested behaviorally, and there were no signs of histopathology in the peripheral nerves. These results show that hexacarbons produce a neurotoxicity that can be demonstrated by changes in nerve excitability, prior to overt behavioral neurotoxicity. Furthermore, these electrophysiologic changes may be related to a hexacarbon-induced disruption of nerve-membrane ATPase activity.