Pseudocholinesterase Staining in the Primary Visual Pathway of the Macaque Monkey.

Abstract

Cholinesterases in brain tissue are divided into two main classes: ‘true’ cholinesterases (acetylcholinesterase, AChE, EC 3.1.1.7), which preferentially hydrolyse the neurotransmitter acetylcholine; and ‘pseudocholinesterases’ (for example, butyrylcholinesterase, BuChE, EC 3.1.1.8), which preferentially hydrolyse higher choline esters1. AChE is found at peripheral and central cholinergic synapses, is known to be the degradative enzyme of the cholinergic mechanism and may also have other functions in the central nervous system1–3. In contrast to AChE, the pseudocholinesterases have been assigned no certain function in neural transmission and initially were thought to occur mainly in neuroglia, Schwann cells and vascular endothelia1,4. Pseudocholinesterase activity has since been found in neurones and neuropil in several brain regions1,4,5, however, and in the superior cervical ganglion, BuChE has been localized to postsynaptic membranes6 and shown to exist in stable molecular forms corresponding to each of the known molecular forms of true cholinesterase7. These observations have led to the alternative interpretations that the pseudocholinesterases are either metabolic precursors of AChE8 or that they have functions closely related to those of AChE5,7 while being independent of true cholinesterases. We have directly compared the distributions of BuChE and AChE in the central visual pathway of the primate, a neural system in which anatomical and functional compartmentalization is well known9. We demonstrate here that the histochemical localization of Pseudocholinesterase rivals that of AChE in terms of specificity, and that BuChE is independent of AChE both in its normal distribution in the lateral geniculate body and striate cortex and in the response it shows to eye enucleation. We conclude that BuChE or its endogenous substrate may be a neuroactive substance in the primate brain.