| Abstract: | Data on distribution of biologically active substances in the turtle optic tectum are compared with results of similar experiments on other reptilian as well as on avian species. In two turtle species (Testudo horsfield and Emys orbicularis), immunoreactivity to monoamines (5-HT and TH), NPY, as well as NADPH-d activity were similarly distributed in neuropil of the SGFS retinorecipient part and in that of the SGP/SAP periventricular layers. Immunoreactivity to neuropeptides SP and m-Enk was maximal in neuropil of the SGFS non-retinorecipient part. The periventricular layers were characterized by the abundant radial SP- and mENK-ir as well as the NADPH-d-positive neurons. Diffusely dispersed ChAT-ir elements and many ir fibers perpenducilar to the tectal surface were observed in the SGFS retinorecipient part; the SGFS non-retinorecipient part contained a dense plexus of thick ir fibers and diffusely distributed ir terminals. The GABA ir cells were the most numerous in the tectum; they were spread in all tectal layers. Thus, various biologically active substance located in superficial retinorecipient tectal sublayers could affect processing and transmission of information via ascending dendrites of neurons in deeper layers. The cells containing SP, m-Enk, and NADPH-d had laminar organization in SGP; via the system of ascending and descending axons, they are able to affect other structures within and outside of the optic tectum. Putative sources of tectal modulatory innervation are discussed. In all studied reptilian and avian species, the principal similarity is revealed in the neurochemical organization. Some differences might be explained by the level of tectal differentiation due to factors of phylogenetic evolution and/or adaptive specialization. |
