Affinity of human brain acetylcholinesterase to some organophosphates and carbamates in vitro

Organophosphates/nerve agent poisoning: mechanism of action, diagnosis, prophylaxis, and treatment

OP/nerve agents are still considered as important chemicals acting on living organisms and are widely used. They are characterized according to their action as compounds influencing cholinergic nerve transmission via inhibition of AChE. Modeling of this action and extrapolation of experimental data from animals to humans is more possible for highly toxic agents than for the OP. The symptoms of intoxication comprise nicotinic, muscarinic, and central symptoms; for some OP/nerve agents, a delayed neurotoxicity is observed. Cholinesterases (AChE and BuChE) are characterized as the main enzymes involved in the toxic effect of these compounds, including molecular forms. The activity of both enzymes (and molecular forms) is influenced by inhibitors (reversible, irreversible, and allosteric) and other factors, such as pathological states. There are different methods for cholinesterase determination; however, the most frequent is the method based on the hydrolysis of thiocholine esters and subsequent detection of free SH-group of the released thiocholine. The diagnosis of OP/nerve agent poisoning is based on anamnesis, the clinical status of the intoxicated organism, and on cholinesterase determination in the blood. For nerve agent intoxication, AChE in the red blood cell is more diagnostically important than BuChE activity in the plasma. This enzyme is a good diagnostic marker for intoxication with OP pesticides. Some other biochemical examinations are recommended, especially arterial blood gas, blood pH, minerals, and some other specialized parameters usually not available in all clinical laboratories. These special examinations are important for prognosis of the intoxication, for effective treatment, and for retrospective analysis of the agent used for exposure. Some principles of prophylaxis against OP/nerve agent poisoning comprising the administration of reversible cholinesterase inhibitors such as pyridostigmine (alone or in combination with other drugs), scavengers such as preparations of cholinesterases, some therapeutic drugs, and possible combinations are given. Basic principles of the treatment of nerve agent OP poisoning are described. They are based on the administration of anticholinergics (mostly atropine but some other anticholinergics can be recommended) as a symptomatic treatment, cholinesterase reactivators as a causal treatment (different types but without a universal reactivator against all OP/nerve agents) as the first aid and medical treatment, and anticonvulsants, preferably diazepam though some other effective benzodiazepines are available. New drugs for the treatment are under experimental study based on new approaches to the mechanism of action. Future trends in the complex research of these compounds, which is important not only for the treatment of intoxication but also for the quantitative and qualitative increase of our knowledge of toxicology, neurochemistry, neuropharmacology, clinical biochemistry, and analytical chemistry in general, are characterized.

Anticholinesterase Activity of Aldicarb and Carbofuran in Neuron Cultures

The anticholinesterase activity of the carbamate insecticides, carbofuran and aldicarb, was examined in primary cultures of chick embryo forebrain neurons. Both compounds inhibited acetylcholinesterase, with carbofuran being more potent than aldicarb by more than two orders of magnitude. Preincubation of carbofuran with hepatic S9, derived from rat or chick, decreased its anticholinesterase activity, with chick S9 exerting the greater deactivating effect. Conversely, preincubation of aldicarb with rat control S9 increased its anticholinesterase activity, while chick S9 had only a slight effect. Preincubation of the test compounds with S9 obtained from animals induced with phenobarbitone or 3-methylcholanthrene generally had a slightly deactivating effect, with the exception of aldicarb, where preincubation with induced rat hepatic S9 fractions slightly increased its potency as an anticholinesterase agent when compared to the effects of control S9. Preincubation of aldicarb and carbofuran with hepatic S9 fractions from both rat and chick altered their anticholinesterase activities, so that they were more predictive of their relative potential species toxicities in vivo. The relative toxicities of aldicarb and carbofuran, within a given species, however, were not as well predicted. This model may be of use in assessing the potential toxicities of carbamate pesticides with respect to species differences.