Histochemistry59, 81-95 (1978) Histochemistry 9 by Springer-Verlag1978 Acetylcholinesterase Activity in the Normal and Retino-deprived Optic Tectum of the Quail Light and Electron Microscopic Histochemistry and Biochemical Determination F. Ciani, A. Contestabile, and L. Villani Institute of Comparative Anatomy, Universityof Bologna, Via Belmeloro 8, 1-40126 Bologna, Italy Summary. Acetylcholinesterase localization has been studied by electron mic- roscopic histochemistry in the quail optic tectum. Ultrastructural analysis reveals that the different neuronal types in the tectum possess the metabolic pathways for AChE synthesis to different degrees. From the site of synthesis in cell bodies the enzyme spreads towards areas of neuropil. In the neuropil of AChE-rich areas a balance seems to exist between enzyme stored in dendrites (and sometimes axon terminals) and enzyme released into the extracellular spaces. Precise identification of cholinergic synapses by means of AChE localization is in most cases impossible, due to extensive spread of the enzyme through the extracellular compartments of the neuropil. Unilateral ocular ablation causes disappearance of the stratum opticum and decrease in thickness of the superficial tectal layers in the contralateral optic tectum, but only minor modifications in AChE localization. This find- ing is in agreement with biochemical results which show equivalence of the relative concentration of AChE in the right and left optic rectum 1 or 2 months after ablation of the right eye. The experimental evidence sug- gests that cholinergic mechanisms are not related to the discharge of retinal afferents on receptive tectal neurons, but more likely to intrinsic neural circuits which might be involved in the modulation of tectal activity. Introduction The optic tectum of non-mammalian vertebrates is usually a well differentiated nervous region showing elaborate cortical structure. Electron microscopic anal- ysis carried out on different vertebrates has revealed several interesting details of the ultrastructural organization of the tectal cortex (Ito, 1971 ; Szekely et al., 1973; Laufer and Vanegas, 1974; Ciani et al., 1975; Hayes and Webster, 1975; Angaut and Reperant, 1976; Davydova etal., 1976; Reperant and Angaut, 1977). In addition, histochemical researches (reviewed by Contestabile, 1976) and biochemical findings (Chakrabarti et al., 1974; Contestabile et al., 1978) 0301-5564/78/0059/0081/$03.00