Experimental Study Of Muscle Pathological Changes In Myasthenia After Acute Omethoate Poisoning In Mice

The clinical neurotoxic effects of organophosphates (OPs)poisoning include acute cholinergic crisis, OPs induced delayedpolyneuropathy and intermediate myasthenia syndrome (IMS). IMSusually occurs after recovery from acute cholinergic crisis, and oftenseen from 1 to 4 days after the beginning of acute OPs poisoning. Theclinical characteristics of IMS have been described as weakness inmuscles innervated by cranial nerves, in neck flexors and proximallimb muscles as well as in respiratory muscles. Sudden respiratoryinsufficiency is the most life-threatening clinical symptom and is theprime cause of death in patients with IMS following acute OPspoisoning. The mechanism of IMS remains unclear. There are variousexplanations for the development of IMS following acute OPspoisoning, including persistent acetylcholinesterase (AChE) inhibition,necrotizing myopathy, oxidative damage, relating to the kinds of OPs,individual inherited characteristics and neuromuscular transmissionblock. Most authors concluded that the occurrence of myastheniafollowing acute OPs poisoning is closely concerned with thepostsynaptic block at the neuromuscular junctions. So in clinicalexperience, we can only administer some management to symptomsuch as mechanical ventilation, and we have not effctive treatments toit.Up to now, there have been no report on how to build the animalmodel of IMS. Omethoate is a high toxic organophosphorus pesticide,and there has been report that said incidence of respiratory failure inomethoate poisoning was higher than other OPs poisoning, and IMSoccurs in both rats and mice. In our experiment we choose KunMingmice as experimental animal.117 mice were randomly divided into three groups: normalcontrol group (6 mice);saline control group (6 mice);the rest 105mice were randomly divided into 7 groups, 15 mice in each group, 1dto 7d group. Normal control mice were given no injection, salinecontrol mice were intraperitoneally injected 0.9ml 0.9% saline, theother mice were administrated omethoate (omethoate cream wasdissolved in distilled water at the ratio of 1:200, prepared when usedeach time) at 50mg/kg intraperitoneally and at the same time theywere given atropine (atropine was dissolved in distilled water at theratio of 1:10) at 10mg/kg intraperitoneally in order to help the mice tosurvive the urgent period. At 1,2,3,4,5,6,7 day, the 1d to 7d groupmice and control mice were killed respectively by decapitation afterthey were asessed muscle strength according to the scoring criteriareported by Lennon et al, and diaphragm and musculus rectus femoriswere rapidly removed to prepare for HE staining and acetylcholineesterase staining. Then observe the pathological changes of muscleand change of motor end plate. Student's t-test was applied todetermine the statistical significances of differences among quantity ofmotor end-plate.In our experiment, acute toxic symptoms appeared 20 minutesafter administration of dimethoate in all mice, including decreasedactivity or no activity, titubation, decreased food appetite, acceleratedheart rate, shortness of breath. Fasciculation occured in most mice,myotonus all over the body happened on some mice, and a few diedduring acute time, and no mice died after acute time. Limb muscleweakness was evident, scoring 1~2 grade according to the criteriareported by Lennon, and neck flexors weakness and dyspnea were notevident, weakness in muscles innervated by cranial nerves could notbe observed. In this experiment, incidence of myasthenia was high,belonging to light category. There was not evident difference amongthe incidence of myasthenia every day. The result of HE staining ofmuscle showed inflammatory changes in both experiment groups andcontrol group, which manifested increasing nucleus underendomysium, nucleus ingression and inflammatory cell infiltration,and necrosis of muscle fiber was not evident. There was not obviousdifference in the inflammatory changes at different time. The result ofacetylcholine esterase staining showed that the distribution of positivespots was more discrete in myasthenia mice than in control mice, andthere was not obvious difference in staining among each group .Thestatistics results demonstrated that (1) the quantity of motor end-platewas less in each myasthenia group mice than in control group mice, (2)at most point the quantity of motor end-plate in non-myasthenia micewas similar to that in control mice, (3) most days the quantity of motorend-plate was less in myasthenia mice than in non-myasthenia mice,and some days there was not evident difference in the quantity ofmotor end-plate between them, (4) there was not evident difference inthe quantity of motor end-plate among each myasthenia group mice1~7 days after acute poisoning.Put together above all, we make out following conclusions: (1)the quantity of motor end-plate was less in myasthenia mice than innon-myasthenia mice, which may be related with the inhibition ofphosphate pesticide to acetylcholine esterase as well as the directdestruction of organophosphates to moter end-plate;(2) some micemanifested myasthenia after acute omethoate poisoning, and we canmake animal model of IMS by means of this, (3) inflammatorychanges of muscle were observed in both experiment groups andcontrol group, so inflammatory changes of muscle are not specificchange of IMS. (4) The density of motor end plate is higher indiaphragm than in musculus rectus femoris.