Mechanisms of toxicity and tolerance to diisopropylphosphorofluoridate at the neuromuscular junction of the rat

Downregulation of nicotinic and muscarinic receptor function in rats after subchronic exposure to diazinon

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We studied the development of tolerance to subchronic p.o. administration of DZN in rats, under both in vivo and in vitro conditions

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As a consequence of AChE inhibition, ACh neurotransmitters are being accumulated in over stimulated nicotinic and muscarinic receptors.

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With isolated diaphragm and ileum, we examined the down regulation of nicotinic and muscarinic receptor function through EFS technique.

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The results of our research could be useful in forensic diagnostics of organophosphate poisoning.

Diazinon (DZN) is an organophosphate insecticide which exerts its effect through the inhibition of acetylcholinesterase enzyme (AChE). In this work, we studied the development of tolerance to subchronic p.o. administration of DZN in rats, under both in vivo and in vitro conditions. A group of 20 rats (2 groups, n = 10) was administered p.o. the 1/10 of established LD₅₀ DZN (namely 55.87 mg/kg bw) for 28 days. On the 14th and 28th day of study with isolated diaphragm and ileum, we examined the downregulation of nicotinic and muscarinic receptor function through Electrical Field Stimulation (EFS). Maximum contractility of the diaphragm was recorded on the 14th day of the study (25% higher compared to the non-treated rats), while on the 28th day the contractions almost did not differ from the values found in non-treated rats. EFS of isolated ileum on the 14th day of study caused significantly higher contractions compared to the non-treated rats, but after 28 days, ileum contractions decreased approximately to the level of contractions in non-treated rats. On the 14th study day, we also recorded increased amplitude of spontaneous ileum contractions, compared to non-treated rats. The application of increasing ACh concentrations caused dose-dependent ileum contractions, without statistically significant differences of median effective concentration (EC₅₀) values in non-treated and treated rats. Tolerance to subchronic DZN administration develops due to various adaptation mechanisms, including the most important one—downregulation of nicotinic and muscarinic receptor function.

Toxicity of Organophosphates and Carbamates

Organophosphate (OP) and carbamate (CM) compounds are commonly used as insecticides around the world. Some of them are extremely toxic to non-target species, including mammals. OP and CM insecticides are acetylcholinesterase (AChE) inhibitors and are commonly referred to as anticholinesterase agents. In addition to their cholinergic mechanisms, these insecticides exert toxicity through non-cholinergic mechanisms, thereby affecting several vital organs and body systems. The brain and skeletal muscles are the major target organs. Cardiovascular, respiratory and immune systems are also affected. There are similarities and differences between and among the toxicity profiles of OPs and CMs. This is due in part to variability in the interaction of each OP or CM with target and non-target receptors, enzymes and proteins. Treatment of CM poisoning rests with atropine, while the treatment of OP poisoning includes atropine in combination with an oxime.

Neurological evaluation of newborn infants of mothers working in citrus groves in Northern Thailand

The purpose of this pilot study was to assess neurological function in newborn infants born to mothers working in citrus orchards in Northern Thailand for a period in excess of one year where pesticide applications average 35 times a year. Forty-one infants from uncomplicated term births at the community hospital in Fang, Thailand, were given neurological evaluations during the first four days of life. This was a cross-sectional study in which nine mothers worked in citrus orchards and 32 mothers did not. Examiners were not given the exposure history of the mothers. Twelve infants--five of them born to mothers from citrus orchards--had examinations demonstrating abnormal muscle tone. Using logistic regression, the final model showed that maternal citrus grove exposure and anesthesia use were significant predictors (p < 0.05) of abnormal muscle tone with adjusted odds ratios of 9.82 (CI = 1.42, 68.07) and 5.99 (CI = 1.003, 35.85) for exposure and anesthesia respectively.

Adaptation to excess acetylcholine by downregulation of adrenoceptors and muscarinic receptors in lungs of acetylcholinesterase knockout mice

The acetylcholinesterase knockout mouse has elevated acetylcholine levels due to the complete absence of acetylcholinesterase. Our goal was to determine the adaptive changes in lung receptors that allow these animals to tolerate excess neurotransmitter. The hypothesis was tested that not only muscarinic receptors but also alpha(1)-adrenoceptors and beta-adrenoceptors are downregulated, thus maintaining a proper balance of receptors and accounting for lung function in these animals. The quantity of alpha(1A), alpha(1B), alpha(1D), beta(1), and beta(2)-adrenoceptors and muscarinic receptors was determined by binding of radioligands. G-protein coupling was assessed using pseudo-competition with agonists. Phospholipase C activity was measured by an enzymatic assay. Cyclic AMP (cAMP) content was measured by immunoassay. Muscarinic receptors were decreased to 50%, alpha(1)-adrenoceptors to 23%, and beta-adrenoceptors to about 50% of control. Changes were subtype specific, as alpha(1A), alpha(1B), and beta(2)-adrenoceptors, but not alpha(1D)-adrenoceptor, were decreased. In contrast, receptor signaling into the cell as measured by coupling to G proteins, cAMP content, and PI-phospholipase C activity was the same as in control. This shows that the nearly normal lung function of these animals was explained by maintenance of a correct balance of adrenoceptors and muscarinic receptors. In conclusion, knockout mice have adapted to high concentrations of acetylcholine by downregulating receptors that bind acetylcholine, as well as by downregulating receptors that oppose the action of muscarinic receptors. Tolerance to excess acetylcholine is achieved by reducing the levels of muscarinic receptors and adrenoceptors.

Motor functions but not learning and memory are impaired upon repeated exposure to sub-lethal doses of methyl parathion

Our previous work showed that repeated exposure to methyl parathion (MP) caused a prolonged inhibition of acetylcholinesterase (AChE) activity (approximately 80%) and down-regulation of M(1) and M(2) muscarinic receptors (up to 38%) in rats at brain regions, including frontal cortex, striatum, hippocampus and thalamus. In the present neurobehavioral study, we found this repeated MP treatment had suppressant effects on rat's locomotor activity. However, we observed no evidence of long-term effects of MP on associative learning and memory. Our data demonstrated that repeated exposure to MP caused some functional deficits in CNS, but motor activity and associative learning/memory process might differ in the sensitivity to its toxic effect. The motor dysfunctions in MP-treated rats may be mediated via reciprocal balance between cholinergic and dopaminergic systems at striatum following cholinergic over-stimulation. Our findings also suggest that the CNS deficits induced by repeated exposure to MP or other organophosphate (OP) pesticides cannot be attributed entirely to the inhibition of AChE. To accurately assess the neuro-toxic risk by occupational exposure to sub-lethal doses of MP, novel biomarkers besides in vivo anticholinesterase potency are needed.

The electromyographic signal as a presymptomatic indicator of organophosphates in the body

Organophosphate (OP) compounds are present in household and agricultural pesticides as well as in nerve agents. The toxic effects of these chemicals result from their anticholinesterase activity, which disrupts nerve junctions and parasympathetic effector sites, leading to a variety of symptoms and possible death. When the anticholinesterase agents in OP compounds reach the neuromuscular junction, they cause a disruption in the firing of muscle fiber action potentials. This effect has the potential of altering the time course of the electromyographic (EMG) signal detected by surface electrodes. We investigated the association between OP compound dose, surface EMG changes, and overt signs of OP toxicity. Daily doses of 10-15 microg/kg of diisopropylfluorophosphate (DFP) were injected into the calf muscle of four rhesus monkeys while surface EMG signals were recorded from two thigh muscles bilaterally. With increasing number of doses, the EMG signal presented an increasing number of time gaps. The presence of the gaps was evident prior to any overt symptoms of cholinesterase toxicity. These findings can lead to the development of noninvasive technology for indicating the presence of OP compounds in muscle tissue prior to clinical abnormalities.

Adeno-associated viral glutamate decarboxylase expression in the lateral nucleus of the rat hypothalamus reduces feeding behavior

In vivo gene transfer of glutamate decarboxylase (GAD) has been explored as a means of inducing or increasing the production of the inhibitory amino-acid neurotransmitter, GABA. This strategy has been applied to neuroprotection, seizure prevention, and neuromodulation. In the present experiment, AAV2 was used to transfer the genes for green fluorescence protein (GFP) and GAD65 into the lateral nucleus of the rat hypothalamus. Microinjection of 500 nl of AAV2 resulted in transduction of a 0.25+/-0.04 mm(3) with targeting errors of X=0.48 mm, Y=0.18 mm, Z=0.37 mm using standard stereotactic technique. Pre- and postinjection food and water consumption, urine and feces production, and weight were recorded. In comparison with rAAVCAGGFP- and PBS-injected animals, rats treated with rAAVCAGGAD65 demonstrated reduced weight gain (P<0.014) and transiently reduced daily food consumption (P<0.007) during the postoperative period. No changes in water consumption or waste production were recorded. Effective GAD65 gene transfer was confirmed with in situ hybridization using a probe to the woodchuck post-transcriptional regulatory element sequence included in the vector. These findings suggest that increased GABA production in lateral nucleus of the hypothalamus induced by GAD65 gene transfer may reduce weight gain through reduced feeding.

Comparative investigation of behavioral, neurotoxicological, and immunotoxicological indices in detection of subacute combined exposure with methyl parathion and propoxur in rats

The effects of 6 weeks of oral exposure to propoxur (PR; at doses of 0.851 and 8.51 mg/kg body wt.), methylparathion (MP; at doses of 0.218 and 0.872 mg/kg body wt.), and their combinations were investigated in male Wistar rats. Measurement endpoints of the investigation were certain general toxicological parameters (body weight gain, organ weights), plaque-forming cell (PFC) count from the spleen, open field (OF) behavior, auditory startle response (ASR), prepulse inhibition (PPI), rotarod performance, somatosensory and auditory cortical evoked potentials, and peripheral nerve conduction velocity. The treated rats did not show any sign of acute intoxication during the 6 weeks of exposure. The higher dose of PR, but not of MP, significantly decreased the relative liver weight. Both agents produced a significant dose-dependent increase of OF activity, with larger expression after 2 weeks than after 6 weeks. The number of ASR responses and the ASR amplitude increased. The amplitude after PPI was increased by MP but only minimally altered by PR and the combinations. There was a small, but with high-dose PR significant, increase in the latency of the somatosensory evoked potentials. Neither of the two substances alone had any effect on the PFC response. The effect of the combination of high-dose PR and low-dose MP was significantly different from that of high-dose PR alone on the liver weight, on the ASR amplitude, and on the PFC/10(6) cell and PFC/spleen counts. With high-dose MP and low-dose PR, no such interaction was observed. According to the results, the noneffective dose of MP can influence the toxicity of the effective dose of PR in a combined exposure situation.

Regulation of muscarinic acetylcholine receptor function in acetylcholinesterase knockout mice

Acetylcholinesterase (AChE) hydrolyzes acetylcholine to terminate cholinergic neurotransmission. Overstimulation of cholinergic receptors by excess acetylcholine is known to be lethal. However, AChE knockout mice live to adulthood, although they have weak muscles, do not eat solid food, and die early from seizures. We wanted to know what compensatory factors allowed these mice to survive. We had previously shown that their butyrylcholinesterase activity was normal and had not increased. In this report, we tested the hypothesis that AChE-/- mice adapted to the absence of AChE by downregulating cholinergic receptors. Receptor downregulation is expected to reduce sensitivity to agonists and to increase sensitivity to antagonists. Physiological response to the muscarinic agonists, oxotremorine (OXO) and pilocarpine, showed that AChE-/- mice were resistant to OXO-induced hypothermia, tremor, salivation, and analgesia, and to pilocarpine-induced seizures. AChE+/- mice had an intermediate response. The muscarinic receptor binding sites measured with [3H]quinuclinyl benzilate, as well as the protein levels of M1, M2, and M4 receptors measured with specific antibodies on Western blots, were reduced to be approximately 50% in AChE-/- brain. However, mRNA levels for muscarinic receptors were unchanged. These results indicate that one adaptation to the absence of AChE is downregulation of muscarinic receptors, thus reducing response to cholinergic stimulation.

The effects of topical ocular application of 0.25% demecarium bromide on serum acetylcholinesterase levels in normal dogs

OBJECTIVE

To assess the effects of topical ocular application of 0.25% demecarium bromide on serum acetylcholinesterase (AChE) levels in normal dogs.

ANIMALS

Nine adult mixed breed dogs weighing between 18 and 27 kg.

PROCEDURES

Fifty micro L of 0.25% demecarium bromide were applied to one eye of each dog every 8 h for 6 days. Blood was analyzed for AChE levels prior to commencement of eye drops, and at 45 min, 1 h 45 min, 4 h 45 min, 1 day, 3 days, and 7 days following commencement of eye drops using a 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) reaction.

RESULTS

Acetylcholinesterase levels declined over the first 24 h following commencement of demecarium administration in most dogs. This decline was highly variable and was statistically significant by 24 h. In some individuals AChE levels were suppressed to levels approaching clinical toxicity. By day 3 AChE levels had risen to levels above baseline in most dogs.

CONCLUSIONS

Topical ocular application of demecarium causes transient suppression of systemic acetylcholinesterase levels in most dogs. Acetylcholinesterase levels generally do not fall to toxic levels, but may do so in certain individuals. Demecarium bromide eye drops generally do not cause AChE toxicity, but dogs receiving such therapy should be monitored for signs of AChE toxicity, and concomitant use of other AChE inhibitors should be avoided.

Protective effect of pralidoxime on muscle fiber necrosis induced by organophosphate compounds

OBJECTIVE

To determine the protective effect of pralidoxime on muscle fiber necrosis induced by organophosphate acute intoxication in rats.

DESIGN

Adult male Wistar rats were given oral organophosphate compounds dissolved in glycerol formal: dichlorvos, isofenphos, metamidophos, and diazinon. Half of the animals also received pralidoxime mesylate (20 mg/kg, intraperitoneal). Control animals received only the solvent. Twenty-four hours after treatment, the diaphragm muscle was collected for histological counts of necrotic muscle fibers in transverse sections.

RESULTS

Metamidophos- and isofenphos-treated animals showed the highest percentage of necrotic muscle fibers: 1.66 +/- 1.112 and 1.34 +/- 0.320, respectively. Diazinon-treated animals had a lower percentage of necrotic fibers: 0.40 +/- 0.032 (p < 0.05) compared to the first 2 products, and dichlorvos-treated animals showed the smallest: 0.05 +/- 0.021 (p < 0.05) when compared to the other 3 products. Pralidoxime reduced necrotic fibers about 20 times in metamidophos-treated animals, 10 times in isofenphos-treated animals and 6 times in diazinon-treated animals. Pralidoxime administration did not increase plasma cholinesterase activity in any group, although symptoms were reduced.

CONCLUSIONS

Oxime reduced diaphragmatic muscle necrosis in experimental organophosphate intoxication, despite little effect on plasma cholinesterase. Since respiratory insufficiency is an important cause of mortality and morbidity in organophosphate intoxications, early oxime administration may be particularly beneficial.

Neuromuscular transmission and its pharmacological blockade. Part 1: Neuromuscular transmission and general aspects of its blockade

Blockade of neuromuscular transmission is an important feature during anaesthesia and intensive care treatment of patients. The neuromuscular junction exists in a prejunctional part where acetylcholine is synthesized, stored and released in quanta via a complicated vesicular system. In this system a number of proteins is involved. Acetylcholine diffuses across the junctional cleft and binds to acetylcholinereceptors at the postjunctional part, and is thereafter metabolized by acetylcholinesterase in the junctional cleft. Binding of acetylcholine to its postjunctional receptor evokes muscle contraction. Normally a large margin of safety exists in the neuromuscular transmission. In various situations, apart from up-and-down regulation of acetylcholine receptors, adjustment of acetylcholine release can occur. Pharmacological interference can interrupt the neuromuscular transmission and causes muscle relaxation. For this reason both depolarizing and non-depolarizing muscle relaxants are clinically used. The characteristics of an ideal clinical muscle relaxant are defined. In the description of the pharmacology of the relaxants the importance of pharmacodynamic and pharmacokinetic parameters are defined. Stereoisomerism plays a role with the relaxants. Toxins and venoms also interfere with neuromuscular transmission, through both pre- and postjunctional mechanisms.

[Organophosphate-induced myotoxicity]

Organophosphates comprise a group of chemical compounds extensively used in farming as insecticides, which cause accidental poisoning in animals and men and are also used in suicide attempts. The toxicity of these compounds is due especially to the cardiac and respiratory impairment in consequence of autonomic nervous system disorders. However, it is known that some of these products induce a myopathy in experimental animals and humans. This myopathy is characterized by muscle cell degeneration, involving above all the respiratory muscles. Based on the fact that this involvement certainly enhances the respiratory impairment, this study offers an experimental method for routine evaluation of organophosphate myotoxicity, using a minimal and sufficient battery of stains and histochemical reactions, for muscle necrosis quantification. For this purpose, albino rats (Wistar) treated with the organophosphate paraoxon, were used both with and without antidotes (atropine or pralidoxime). Muscle fiber necrosis in the diaphragm of the rats treated with paraoxon or paraoxon and atropine, that affected about 15% of the fibers in some areas, was detected. In the group treated with paraoxon and pralidoxime, a minimal necrosis was seen, revealing a protective role of this later antidote during the development of myopathy.

Effects of myotoxins on skeletal muscle fibers

This review highlights various aspects of a number of experimental myological alterations, induced by different chemical toxicants, including anticholinesterase, colchicine, vincristine, chloroquine, tetanus toxin, botulinum toxin, reserpine and emetine. Despite their chemical diversity and mechanism(s) of action, it is evident from the data discussed here that remarkably different toxic agents exert quite similar effects and induce toxic myopathies. The latter include preferential involvement of slow-twitch red muscle, mitochondrial derangement, denervation-like alterations, formation of membranous whorls, tubular aggregates, autophagic vacuoles and axonal sprouts. The non-invasive experimental models discussed here are valuable in studying various aspects of myopathology in the absence of any mechanical damage to the innervating elements from neurons to axonal terminals.

Study of muscular effects of short-term pyridostigmine treatment in resting and exercising rats

1. Pyridostigmine (PYR) pretreatment is used by the military to obtain 20-30% whole blood acetylcholinesterase (AChE) inhibition in order to enhance the effectiveness of the standard therapeutic regimen for poisoning by an organophosphate anticholinesterase agent. The present study was undertaken to investigate in a rat model the potential muscle damage produced by this pretreatment when given alone or combined with physical exercise. 2. Grip strength and biochemical measurements, i.e. serum creatine phosphokinase (CPK) activity and creatine urinary excretion rate, together with histological studies, were performed in resting animals during a period of 14 days of PYR administration in a dose producing 20-30% whole blood acetylcholinesterase inhibition. No evidence was found of a deleterious effect of this treatment on the skeletal muscle. 3. In contrast, following physical exercise, the same treatment significantly exacerbated the biochemical changes reflecting a loss of integrity in skeletal muscles, namely, increased CPK and urinary creatine excretion rate. The significance of this observation remains to be clarified.

Cholinergic and noncholinergic changes in skeletal muscles by carbofuran and methyl parathion

The objective of this investigation was to determine the distribution of cholinergic (acetyl-cholinesterase, AChE) and noncholinergic markers in slow-, fast-, and mixed-fiber containing muscles (soleus, SOL; extensor digitorum longus, EDL; and diaphragm, DIA, respectively). Noncholinergic markers included high-energy phosphates (adenosine triphosphate, ATP; phosphocreatine, PCr; and their metabolites), and the activity of creatine kinase (CK) and lactate dehydrogenase (LDH) and their isoenzymes and subforms. All three types of muscles had only one CK isoenzyme, CK-MM, which totally consisted of MM3 subform. Levels of these determinants were highest in EDL followed by DIA and least in SOL. Another objective was to determine alterations of these markers under the influence of acute carbofuran (1.5 mg/kg) or methyl parathion (MPTH, 5 mg/kg) toxicity. Rats receiving either insecticide showed cholinergic signs with maximal severity including muscle fasciculations and convulsions within 15-30 min that lasted for about 2 h. At 1 h postinsecticide injection, when AChE was maximally inhibited (81-96%), significant depletion of ATP and PCr was evident in muscles (DIA > SOL > EDL), and activities of CK-MM and LDH were elevated in muscles and consequently in serum. Serum CK-MM3 activity was markedly reduced with sequential increase in MM2 and MM1 subforms, probably due to induced higher carboxypeptidase activity. These findings suggested that (1) the differences in levels of biochemical constituents in muscles depend upon the fiber type, (2) anticholinesterase insecticide-induced increased muscle activity produces characteristic changes in CK and LDH isoenzymes patterns, and (3) leakage of these enzymes/isoenzymes into serum is due to depletion of ATP and PCr, which are required to maintain the cell membrane permeability.

Transient opsoclonus in organophosphate poisoning

Opsoclonus transiently occurred in the acute stage of combined parathion and methyl-parathion poisoning. Despite persistent cholinesterase inhibition this symptom subsided spontaneously. This observation provides strong evidence for cholinergic pathways being reversibly involved in isolated opsoclonus.

Carbofuran-induced alterations (in vivo) in high-energy phosphates, creatine kinase (CK) and CK isoenzymes

Male Sprague-Dawley rats administered with an acute sublethal dose of carbofuran (1.5 mg/kg, s.c.) developed the signs of peak hypercholinergic activity during 30-60 min. At this time, in hemidiaphragm muscle, a significant decrease in ATP (28%) and phosphocreatine (PC) (29%) occurred without concurrent change in AMP and creatine (CR). A significant decrease in the levels of total adenine nucleotides (ATP + ADP + AMP) (20%) and total creatine compounds (PC + CR) (17%) was evident. The decline in the corresponding ratios of ATP/ADP (26%), ATP/AMP (39%), and PC/CR (20%) was therefore suggestive of greater utilization of ATP and PC in response to their increased demand for high-frequency muscle fasciculations. The energy charge = ATP + 1/2 ADP/(ATP + ADP + AMP), an index of high-energy phosphate adequacy in hemidiaphragm, remained unchanged. A significant (p less than 0.01) increase in serum magnesium with no concurrent change in calcium was also evident. The observed higher activity (152%) of total CK (EC 2.7.3.2) in the serum induced by carbofuran was possibly a reflection of more than a twofold increase in CK-BB isoenzyme (CK-1) and 141% increase in CK-MM isoenzyme (CK-3), which also strengthens our findings of enhanced synthesis of ATP and PC. Increased levels of CK-MM isoenzyme in the brain (253%) and hemidiaphragm (195%); and depletion of CK-BB isoenzyme in the hemidiaphragm (0%), heart (42%), and brain (77%), and of CK-MB isoenzyme (CK-2) in the brain (4%) and hemidiaphragm (14%), appeared to be the major contributory factors leading to enhanced serum CK activity.

Receptor-mediated presynaptic facilitation of quantal release of acetylcholine induced by pralidoxime in Aplysia

1. Possible interactions of contrathion (pralidoxime sulfomethylate), a reactivator of phosphorylated acetylcholinesterase (AChE), with the regulation of cholinergic transmission were investigated on an identified synapse in the buccal ganglion of Aplysia californica. 2. Transmitter release was evoked either by a presynaptic action potential or, under voltage clamp, by a long depolarization of the presynaptic cell. At concentrations higher than 10(-5) M, bath-applied contrathion decreased the amplitude of miniature postsynaptic currents and increased their decay time. At the same time, the quantal release of ACh was transiently facilitated. The facilitatory effect of contrathion was prevented by tubocurarine but not by atropine. Because in this preparation, these drugs block, respectively, the presynaptic nicotinic-like and muscarinic-like receptors involved in positive and negative feedback of ACh release, we proposed that contrathion activates presynaptic nicotinic-like receptors. 3. Differential desensitization of the presynaptic receptors is proposed to explain the transience of the facilitatory action of contrathion on ACh release. 4. The complexity of the synaptic action of contrathion raises the possibility that its therapeutic effects in AChE poisonings are not limited to AChE reactivation.

On the development of behavioral tolerance to organophosphates. II: Neurophysiological aspects

In a recent study (9) it was found in rats that chronic treatment with the irreversible cholinesterase inhibitors DFP or soman led to behavioral tolerance in the case of DFP, but not in the case of soman. Biochemically, no explanation was found for this difference between these two inhibitors. Notably, chronic administration of each of these inhibitors did not affect the availability of the nicotinic receptors at the motor endplate, in spite of very low cholinesterase activity. In an attempt to explain the different effects of these inhibitors a neurophysiological approach seemed appropriate. The spontaneous quantal release of acetylcholine from diaphragm muscles in vitro from animals chronically treated with each inhibitor showed a similar trend; compared with controls the MEPP frequency was decreased, which was significant for DFP, and the MEPP amplitude was increased, which was significant for soman. Neuromuscular function of muscle strips obtained from both DFP- or soman-treated animals appeared significantly more sensitive to additional inhibitor added in vitro. This could simply be explained by the high preexisting level of cholinesterase inhibition, but seems in contrast with the phenomenon of tolerance.

Prevention and antagonism of acute carbofuran intoxication by memantine and atropine

Male Sprague-Dawley rats administered with a sublethal acute dose of carbofuran (1.5 mg/kg, sc) developed the observable toxic signs of anticholinesterase nature within 5-7 min. The toxic signs with increasing propensity to maximal severity including tremors, generalized muscle fasciculations, and convulsions were evident during 15 min to 1 h and lasted for 2 h. Thereafter, signs were seen up to 3 h with reduced intensity. By the end of 3.5 h toxic signs were completely subsided. Maximal acetylcholinesterase (AChE) inactivation occurred at 1 h in discrete brain regions (cortex, stem, striatum, and hippocampus) and hemidiaphragm muscle when most severe signs of toxicity were also evident. A single sc dose of memantine HCl (MEM, 18 mg/kg) and atropine sulfate (ATS, 16 mg/kg) 60 and 15 min, respectively, prior to carbofuran administration completely prevented the expected gross toxic signs and significantly (p less than .01) attenuated the carbofuran-induced inhibition of AChE activity. When given therapeutically, this combined treatment completely reversed the clinical evidence of carbofuran toxicity within 15 min and also markedly reduced AChE inactivation. Memantine or atropine when given alone was less effective compared to their combined administration. The results of this study suggested that, in addition to cholinolytic effects of atropine, memantine may prevent and antagonize the acute toxicity of carbofuran by (a) protection of AChE activity and its rapid reactivation from inhibition and (b) rapid elimination of carbofuran.

Prevention of diisopropylphosphorofluoridate (DFP)-induced skeletal muscle fiber lesions in rat

The objective of the present investigation was to assess the comparative efficacy of prophylactic treatment with d-tubocurarine (d-TC) (0.075 mg/kg), atropine sulfate (16 mg/kg), and atropine methylnitrate (16 mg/kg), employed singly or in combination against the diisopropylphosphorofluoridate (DFP)-induced myopathy in rat. DFP (1.5 mg/kg, s.c.) produced signs of cholinergic toxicity with predominantly peripheral involvement manifest as severe muscle fasciculations beginning within 5-7 min and persisting in excess of 4-6 h. Maximal muscle fiber necrosis was observed within 24 h. Rats were protected against the apparent behavioural and morphological changes as well as electrophysiological signs of neuromuscular toxicity by all pretreatment agents. Combined pretreatment with d-TC (0.075 mg/kg, s.c.) and atropine methylnitrate (16 mg/kg, s.c.) was found to be most effective in attenuating DFP-induced muscle fiber necrosis as evidenced by complete absence of lesions and the prevention of DFP-induced hyperactivity in nerve and muscle. Significant protection was afforded by all pretreatment agents when given alone. It is suggested that the pretreatment agents act presynaptically by preventing drug-induced backfiring and muscle fasciculations possibly by reducing the release of acetylcholine (ACh). The protective drugs in the concentrations used had no significant effect on the normal characteristics of conduction and transmission.

Acute tabun toxicity; biochemical and histochemical consequences in brain and skeletal muscles of rat

Male Sprague-Dawley rats injected s.c. with an acute non-lethal dose (200 micrograms/kg) of ethyl N,N-dimethylphosphoramidocyanidate (tabun) showed onset of hypercholinergic activity within 10-15 min. The maximal severity of toxicity signs was evident within 0.5-1 h and persisted for 6 h. Except for mild tremors no overt toxicity signs were evident after 24 h. Within 1 h a dramatic decline of acetylcholinesterase (AChE) activity occurred in all the brain structures (less than 3%) and skeletal muscles (less than 10% in soleus and hemi-diaphragm; and 32% in extensor digitorum longus (EDL)). No significant recovery was seen up to 48-72 h. Within 7 days rats became free of toxicity signs and AChE activity had recovered to about 40% in brain structures (except cortex, 14%) and 65-70% in skeletal muscles. Within 1 h the 16 S molecular form of AChE located at the neuromuscular junction was most severely inhibited in soleus, followed by hemi-diaphragm and least in the EDL, and had fully recovered in all the muscles when examined after day 7. Muscle fiber necrosis developed within 1-3 h in soleus and hemi-diaphragm and after a delay of 24 h in EDL. The highest number of necrotic lesions in all muscles was seen at 72 h with the hemi-diaphragm maximally affected and EDL the least. To determine detoxification of tabun by non-specific binding, the activity of butyrylcholinesterase (BuChE) and carboxylesterase (CarbE) was measured. The inhibition and recovery pattern of BuChE activity was quite similar to that of AChE, except that the rate of recovery was more rapid. Within 1 h the remaining activity of CarbE was 10% in plasma, about 30% in brain structures, and 79% in liver; recovery was complete within 7 days. The inhibition of BuChE and CarbE can serve as a protective mechanism against tabun toxicity by reducing the amount available for AChE inhibition. The prolonged AChE inhibition in muscle and brain may indicate storage of tabun and delayed release from non-enzymic sites. Since tabun is a cyanophosphorus compound, the toxic effects from the released cyanide (CN) could be another reason for the delayed recovery after tabun.

Interaction of cycloheximide and diisopropylphosphorofluoridate (DFP) during subchronic administration in rat

Male Sprague-Dawley rats daily treated with DFP (0.5 mg/kg/day, sc) exhibited signs of cholinergic toxicity such as tremors and muscle fasciculations between Days 3 and 5 comparable to those observed 15 min after a single acute signs-producing dose (1.5 mg/kg, sc). Further administration of DFP (0.5 mg/kg/day, sc) for 6-14 days led to tolerance development as evidenced by disappearance of the described toxicity signs. The protein synthesis inhibitor cycloheximide, when given in a nontoxic dose (0.5 mg/kg/day, sc) 1 hr before DFP (0.5 mg/kg/day, sc) administration, potentiated the DFP toxicity and rats died after the fifth injection. DFP-tolerant rats developed toxicity signs when subsequently treated with cycloheximide (0.5 mg/kg/day, sc) and DFP (0.5 mg/kg/day, sc). Each drug when given alone for 4 days caused 30-50% reduction of [14C]valine uptake in vivo into the free amino acids pool as well as its incorporation into proteins of brain and skeletal muscles. A combination of these drugs caused a significantly greater inhibitory effect on [14C]valine incorporation into proteins. Cycloheximide (0.5 mg/kg/day, sc) administered for 4 days did not significantly alter the levels of acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), or carboxylesterase (CarbE) activities but potentiated the DFP-induced inhibition of the activities of these enzymes. It is concluded that the cycloheximide pretreatment potentiates DFP toxicity by a mechanism that is related to inhibition of the synthesis of proteins such as AChE, BuChE, and CarbE.

Prevention of phospholine-induced myopathy with d-tubocurarine, atropine sulfate, diazepam, and creatine phosphate

Acute administration of phospholine [diethyl-S-(2-dimethyl aminoethyl)phosphorothioate] at 0.2 mg/kg sc produces a myopathy characterized by initial focal changes in the subsynaptic area of the skeletal muscle. The onset of the myopathy is associated with fasciculations of high frequency. Agents that either prevent or reduce the fasciculations, such as d-tubocurarine, atropine sulfate, and diazepam, were effective in reducing the number of muscle lesions. These agents may reduce spontaneous muscle activity by blocking the postsynaptic receptor, by modifying the ionic-channel characteristics, by reducing presynaptic acetylcholine (ACh) release, or by a combination of any of these mechanisms. Creatine phosphate (CP) does not reduce fasciculations, but it is effective in reducing the number of necrotic fibers, probably by stimulating and sustaining the mechanism of Ca2+ uptake into the sarcoplasmic reticulum. It is postulated that an increase in the sarcoplasmic Ca2+ concentration triggers the events that lead to muscle necrosis.

Differences in central and peripheral neural actions between soman and diisopropyl fluorophosphate, organophosphorus inhibitors of acetylcholinesterase

Toxic doses of acetylcholinesterase (AChE) inhibitors produce prominent motor symptoms (fasciculations, fibrillations, and body tremors) and muscle fiber necrosis. The severity and quality of motor symptoms and fiber necrosis depend upon the specific AChE inhibitor. To examine the importance of nerve and muscle activity in producing muscle necrosis, we recorded electromyographic activity from normal and acutely denervated rat gastrocnemius muscle following administration of the organophosphorus AChE inhibitors soman and diisopropyl fluorophosphate (DFP). The motor symptoms induced on the denervated side represented activity originating at the nerve terminal while those of the contralateral nondenervated muscle represented the sum of peripheral plus central descending activity. The results indicate that soman and DFP produce different responses. At nonlethal toxic doses, the majority of motor symptoms induced by soman is due to impulses descending from the central nervous system, and a proportion of these symptoms are epileptiform activity. This activity is not generated at spinal levels. In contrast, DFP produces motor symptoms mainly by peripheral action that is dependent on a functioning nerve terminal. At lethal doses, both agents have central and peripheral effects. Different patterns of electrical activity are associated with each of the motor symptoms. We found that muscle fiber necrosis correlates best with peripherally generated high-frequency repetitive discharges.

Biochemical and histochemical alterations following acute soman intoxication in the rat

Rats injected with a nonlethal acute dose (100 micrograms/kg, sc) of soman (pinacolyl methylphosphonofluoridate) exhibited signs of anticholinesterase toxicity beginning at 5-15 min with increasing severity and lasting for 4-6 hr. Generalized tremors and seizure activity indicated comparatively greater involvement of the central cholinergic system than peripheral neuromuscular effects. During peak toxicity, all the brain regions tested showed more than 95% inhibition of acetylcholinesterase (AChE) activity. The cortex area was maximally affected (99% inhibition). Among skeletal muscles, soleus AChE was most severely affected (94%) and extensor digitorum longus (EDL) the least (72%). Inhibition of EDL AChE occurred at a much slower rate than in brain and other muscles. Significant recovery of AChE activity was seen by 48-72 hr after soman treatment in both brain and skeletal muscles. By Day 7, recovery was virtually complete in skeletal muscles but not in brain, although significant recovery had occurred by this time. Muscle fiber necrosis developed within 6 hr in the soleus and diaphragm, while no necrotic fibers were found in the EDL. The 16 S AChE molecular form showed the fastest recovery of the AChE isozymes in all three muscles. Full recovery was seen after 7 days in soleus and was increased to greater than control activity in diaphragm and EDL. The inhibition pattern of butyrylcholinesterase (BuChE) activity was similar to that described for AChE activity, but the recovery was comparatively faster. Carboxylesterase activity in plasma was decreased to less than 10% of control within 1 hr and recovered to 53% of control within 24 hr. No significant inhibition was seen in hepatic carboxylesterase activity. It can be concluded that soman-induced acute toxicity is directly related to the rate and degree of AChE inhibition. A significant amount of soman binds to non-AChE enzymes with serine sites such as BuChE and carboxylesterases.

Role of uptake of [14C]valine into protein in the development of tolerance to diisopropylphosphorofluoridate (DFP) toxicity

In a subchronic toxicity study male Sprague-Dawley rats were daily treated with diisopropylphosphorofluoridate (DFP) (0.5 mg/kg, sc) for 14 days. Maximum signs of anticholinesterase toxicity were observed during Days 4 and 5 comparable to those seen 10-15 min following a single sublethal dosage (1.5 mg DFP/kg, sc). Signs disappeared after Days 6-7 of exposure and rats became apparently normal during the remainder of the treatment period. Significant hypothermia was seen following the second to fifth doses with maximum effect after the fifth injection. Subsequent injections of DFP did not cause any reduction in temperature. Incorporation of [14C]valine was measured 24 hr after the 5th and 14th injections of DFP, at a time when body temperature had recovered to control values. The rate of in vivo incorporation of [14C]valine was measured 0.5, 1.0, and 2.0 hr after a subcutaneous injection of L-[1-14C]valine at a dose of 5 microCi/mmol/100 g body wt. After five injections the rate of L-[1-14C]valine uptake into the free amino acid pool and the incorporation into the protein bound pool was significantly (p less than 0.01) reduced in discrete brain regions, liver, kidney, and skeletal muscles. At the end of the 14-day treatment, protein synthesis in all the skeletal muscles tested had recovered completely (p greater than 0.01) to the values of nontreated control animals. In brain, liver, and kidney, however, no recovery was seen during this period. The recovery of protein synthesis in skeletal muscle may be one of the mechanisms that lead to tolerance development during prolonged administration of subacute concentrations of DFP.