Muscarinic drugs affect cholinesterase activity and development of eye structures during early chick development

During neurogenesis, markers of the cholinergic system are present in the eye and visual cortex of vertebrates. In adult vertebrates, a role for these molecules, including muscarinic acetylcholine receptors (mAChRs), in eye growth non-accommodative regulation is also known. In order to understand the biological mechanisms triggered by the cholinergic system in these events, we analysed the effects of a cholinergic agonist (10(-4) M carbachol) and an antagonist (10(-4) M atropine) of the muscarinic receptors, on early chick development. To establish if the cholinergic system also plays a role in the regulation of early neurogenetic signals, the drug treatments were made at stage 5-6 HH, during the formation of the cephalic process. Specific effects on forehead, and in particular on eye development were found; carbachol treated embryos presented huge and well pigmented eyes, significantly different from controls. The eyes of atropine-exposed embryos presented anomalies with different phenotypes ranging from strongly affected features to normal-like appearance. Generally, the eyes were smaller as compared to the controls, with a number of anomalies, also in the normal-like phenotype, including retina and lens defects. In these structures, distribution of cholinesterase activities was checked by histochemical methods, and the amount of cells undergoing nuclear disgregation was revealed by DAPI staining. We propose that the drugs affected the known nervous and pre-nervous functions of the cholinergic markers, such as cell signalling during primary induction, and regulation of cell death by ACh receptors.