Total cholinesterase activity in discrete brain regions and retina of normal horses.

The organophosphorus and carbamate insecticides inhibit cholinesterase, causing an increase of acetylcholine at neuromuscular junctions. Toxicosis due to anticholinesterase compounds can be diagnosed by finding depressed cholinesterase activity in the blood, brain, and/or retina. Cholinesterase activity < 50% of normal has been associated with significant exposure to an anticholinesterase agent. Cholinesterase activity <25% of normal is considered indicative of severe poisoning. Previous studies have demonstrated that cholinesterase activity differs among regions of the central nervous system. Miniature swine have twice the cholinesterase activity in the cerebellum as in the cerebral cortex. The acetylcholinesterase activity in the cerebral cortex is about two-thirds lower than that in the cerebellum of guinea pigs but is about the same in both regions for the rat and monkey. In rabbits, the retina has lower acetylcholinesterase activity than do regions of the brain. The cerebellum, occipital lobe, and parietal lobe have similar activities, which are between those for the retina and those for the temporal lobe. The frontal lobe has acetylcholinesterase activity up to twice that of other regions of the brain. Previous work measuring cholinesterase activity was performed using homogenates of entire brains. However, when submitting samples from large animals such as horses and cows, it is common practice for the referring veterinarian to only submit part of the brain. Because cerebral tissue appears similar regardless of the area sampled, the diagnostician often does not know which part of the brain is being tested. Therefore, the cholinesterase activity of the sample could possibly differ from the normals established using whole brain even if the animal has not been exposed to an anticholinesterase agent. The purpose of this study was to measure the total cho-