Cerebral metabolic effects of organophosphorus anticholinesterase compounds

Abstract

Rats treated intravenously with an organophosphoras anticholinesterase compound, paraoxon or soman, were sacrificed 2 to 131 min later, using 0.7sec of focused microwave irradiation (25 kW at 915 MHz). Brain regional rates of glucose utilization during 3-min intervals were determined with labeled glucose and fluorodeoxyglucase as tracers. Levels of glucose, lactate, ATP, and creatine phosphate were assayed in the same samples. The two compounds differed markedly in their effects on brain metabolism. Paraoxon (0.8LD₅₀) depressed rates of glucose use in all brain regions, without causing consistent changes in brain metabolite levels. This depressant effect was most pronounced during the first 30 min after toxin exposure and had largely disappeared by 2hr. Soman (0.8–0.95 LD₅₀) was variable in its effects. Animals that showed seizure-like behavior had marked increases in glucose use in diencephalon and cerebrum but no changes in cerebellum or brain stem. Rapid rates of glucose use were associated with high levels of lactic acid and lower levels of creatine phosphate. In cerebrum, but not diencephalon, levels of ATP fell by as much as 50% in strongly affected animals by 30–130 min after soman. All of these effects were reversible with atropine. Soman-treated animals that did not have seizure-like activity did not exhibit these brain metabolic changes. These results and those of others show that cholinergic compounds vary greatly in their effects on brain glucose and energy metabolism. Although noncholinergic mechanisms are a possibility, the most parsimonious explanation for these findings is that cholinesterase inhibitors vary in their affinity for different central nervous system (CNS) acetylcholine receptor populations.