Chicken retinospheroids as developmental and pharmacological in vitro models: acetylcholinesterase is regulated by its own and by butyrylcholinesterase activity.

The phylo- and ontogenetically related enzymes butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) are expressed consecutively at the onset of avian neuronal differentiation. In order to investigate their possible co-regulation, we have studied the effect of highly selective inhibitors on each of the cholinesterases with respect to their expression in rotary cultures of the retina (retinospheroids) and stationary cultures of the embryonic chick tectum. Adding the irreversible BChE inhibitor iso-OMPA to reaggregating retinal cells has only slight morphological effects and fully inhibits BChE expression. Unexpectedly, iso-OMPA also suppresses the expression of AChE to 35%-60% of its control activity. Histochemically, this inhibition is most pronounced in fibrous regions. The release of AChE into the media of both types of cultures is inhibited by iso-OMPA by more than 85%. Control experiments show that AChE suppression by the BChE inhibitor is only partially explainable by direct cross-inhibition of iso-OMPA on AChE. In contrast, the treatment of retinospheroids with the reversible AChE inhibitor BW284C51 first accelerates the expression of AChE and then leads to a rapid decay of the spheroids. After injection of BW284C51 into living embryos, we find that AChE is expressed prematurely in cells that normally express BChE. We conclude that the cellular expression of AChE is regulated by the amount of both active BChE and active AChE within neuronal tissues. Thus, direct interaction with classical cholinergic systems is indicated for the seemingly redundant BChE.