Cytochemical study of cholinesterases in the normal and retino-derived optic tectum of reptiles.

An electron microscopic histochemical study has been carried out on the localization of acetylcholinesterase (AChE) in the optic tectum of a lizard and a turtle. Several types of AChE-synthetizing elements have been described. In both species these neurons constitute only a fraction of the total neuronal popultio- of the optic tectum. The area of maximum accumulation of AChE activity is in both cases at level of the neuron pil of the stratum fibrosum et griseum superficiale, where most retinal afferents are known to terminate. There is however a peculiar difference in the ultrastructural localization of the enzyme activity in the stratum et fibrosum et griseum superficiale of the two species. While in the turtle most activity is localized inside dendritic structures, a definite prevalence of extracellular localization, including synaptic spaces, has been noticed of the lizard. The main effect of eye deprivation on AChE localization in the optic tectum has been noticed for the lizard, in which a remarkable decrease of extracellular AChE occurs 2 months after contralateral enucleation. In the lizard a peculiar localization of pseudocholinesterase AChE occurs 2 months after contralateral enucleation. In the lizard a peculiar localization of pseudocholinesterase (BuChE) has been in addition demonstrated in the spaces between axon bundles in the stratum opticum and stratum album centrale. The relevance of these findings is briefly discussed also in relation to the existence of cholinergic mechanisms of neurotransmission in the optic tectum.