THE DETERMINATION OF ACETYLCHOLINESTERASE ACTIVITY OF BRAIN SLICES AND ITS SIGNIFICANCE IN STUDIES OF EXTRACELLULAR ACETYLCHOLINESTERASE

The AChE activity of single slices obtained from the surface of the temporoparietal region of rat brains was measured colorimetrically under anaerobic conditions with acetylthiocholine as substrate. In intact slices from untreated rats AChE activity was only a small proportion of that of homogenates made from the slices, but this proportion increased with the surface area to weight ratio of slices and with an increase in substrate concentration, intact slices not showing substrate inhibition. The inhibition of AChE determined in slices from rats treated with DFP or paraoxon was less than that in homogenates obtained from the slices. When the access of substrate was limited to the cut surface of a slice, the rate of hydrolysis was four times greater than that observed when access was solely from the uncut surface. It is concluded that under anaerobic conditions the substrate diffuses into slices to a depth which is not constant but a function of both substrate and enzyme concentration. Thus the AChE activity of slices cannot be used as a measure of extracellular AChE.