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

Neurological aspects of organophosphate poisoning

Besides their well-known anticholinesterase action resulting in a typical acute cholinergic crisis, organophosphorus (OP) agents are capable of producing several subacute or chronic neurological syndromes. The acute over-stimulation at the neuromuscular junction results in muscle fiber necrosis. The significance of this OP-induced myopathy in human intoxication is unknown. Organophosphate-induced delayed neuropathy (OPIDN) arises 1-3 weeks after exposure to some OP compounds all capable of remarkably inhibiting a distinct esterase called neuropathy target esterase (NTE) during a critical time period. An experimental hen model has been designed to screen new OP compounds as to their delayed neurotoxic effects. The recently described intermediate syndrome emerges 1-4 days after an apparently well-treated cholinergic crisis. It main clinical features are sudden respiratory paralysis, cranial motor nerve palsies, and proximal limb muscle and neck flexor weakness. Whether or not this is a separate entity in OP agent toxicology remains to be seen. Further studies are required to further determine its clinical and paraclinical characteristics and the actual type of underlying neuromuscular dysfunction involved.

Prophylactic and therapeutic efficacy of memantine against seizures produced by soman in the rat

Male Sprague-Dawley rats injected sc with a single sublethal dose of the organophosphate nerve agent, soman (100 micrograms/kg), had motor limbic seizures within 5-15 min. Pretreatment with a single dose of memantine HCl (MEM, 18 mg/kg, sc), alone or in combination with atropine sulfate (ATS, 16 mg/kg, sc), before soman prevented seizures without sedation or ataxia. Rats appeared normal or demonstrated increased exploratory activity. Excessive salivation, a peripheral manifestation of soman intoxication, was decreased by ATS, but pretreatment with ATS alone did not prevent seizures. After seizure onset, MEM +/- ATS, but not ATS, abolished seizures. Acetylcholinesterase (AChE) activity in several brain regions (cortex, stem, striatum, and hippocampus) was markedly reduced by soman, but not by MEM, ATS, or MEM + ATS. Preadministration of MEM + ATS in vivo significantly protected AChE from inhibition by soman. Memantine reduced inhibition of AChE activity in crude brain homogenates by soman, but not by edrophonium (anionic site inhibitor) or decamethonium (peripheral site inhibitor). Thus, MEM may bind to a different modulatory site, not yet characterized, to protect AChE. When given after onset of soman-induced seizures, treatment with MEM +/- ATS did not reactivate AChE although seizures were controlled, suggesting additional anticonvulsant mechanisms of action. At concentrations (10(-4) to 5 x 10(-4) M) which did not significantly alter the spontaneous firing of action potentials (APs), MEM limited sustained high frequency repetitive firing (SRF) induced by depolarization of spinal cord (mouse and rat) and neocortical (mouse) neurons in monolayer-dissociated cell culture. In the same range of concentrations, ATS both limited SRF and suppressed spontaneous activity, suggesting toxicity. In addition, MEM and ATS reversibly produced use-dependent block of depolarizing responses to acetylcholine (ACh) applied by pressure ejection to spinal cord neurons. Thus, the anticonvulsant efficacy of MEM, with or without ATS, may have resulted from a combination of actions, including protection of AChE from inhibition by soman, limitation of high frequency firing of APs, and blockade of excitatory postsynaptic responses to ACh.

Prevention of diisopropylphosphorofluoridate-induced myopathy by botulinum toxin type A blockage of quantal release of acetylcholine

Botulinum toxin type A (BTx), which blocks quantal and partially reduces spontaneous nonquantal acetylcholine (ACh) release at neuromuscular junctions, was tested for its possible attenuating effect on diisopropylphosphorofluoridate (DFP)-induced muscle lesions. The extent of muscle lesion in extensor digitorum longus and soleus muscle of DFP injected rats with and without BTx pretreatment was evaluated using light and electron microscopic procedures. In parallel experiments, acetylcholinesterase (AChE) activity was measured and the functional state of muscles in experimental groups was determined by electrophysiological methods. The results show that pretreatment with BTx almost completely protects the muscles from DFP-induced spontaneous activity and lesions in spite of critically inhibited synaptic AChE. These results are consistent with the conclusion that the effect is not mediated by direct action of organophosphate on muscle, but by the accumulation of ACh resulting in muscle hyperactivity. Therefore, it is concluded that in conditions of acutely inhibited synaptic AChE, the quantal release of ACh is essential for lesion induction, whereas the spontaneous nonquantal ACh release, which is only partially affected in BTx-blocked nerve endings, seems not to be involved.

Effects of phenytoin, ketamine, and atropine methyl nitrate in preventing neuromuscular toxicity of acetylcholinesterase inhibitors soman and diisopropylphosphorofluoridate

Toxic manifestations of acetylcholinesterase inhibitors (AChE-I) include muscle twitching and muscle fiber necrosis, in addition to muscarinic manifestations of acetylcholine excess. The AChE-Is pinacolyl methylphosphonofluoridate (soman) or diisopropylphosphorofluoridate (DFP) were administered to rats to produce spontaneous muscle fiber discharges. Soman produced discharges that arose primarily from the central nervous system (CNS), while those due to DFP were generated from the peripheral nerves as well as the CNS. Three drugs were tested for their potential to reduce muscle fiber discharges: atropine methyl nitrate (AMN), ketamine, and phenytoin. Ketamine caused a significant decrease in discharges of CNS origin, while AMN and phenytoin had no effect. For muscle fiber discharges of peripheral origin, all three drugs produced a significant drop in muscle fiber discharges, but phenytoin showed slightly more efficacy than the others. AChE-I-induced muscle hyperactivity arises from actions on the CNS and on the peripheral nerve in varying proportions for different AChE-Is. Treatment for the toxicity of AChE-Is on muscle may be accomplished by administering drugs with distinctive pharmacological actions at target sites in the CNS and peripheral nervous system (PNS) where AChE-Is exert their effects. By attenuating the effects of AChE-Is at specific CNS or PNS sites, the neuromuscular toxicity can be reduced in a manner specific to the characteristic sites of toxicity of each AChE-I.

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.