Characterization of the pigeon isthmo-tectal pathway by selective uptake and retrograde movement of radioactive compounds and by golgi-like horseradish peroxidase labeling

Glycine application to the superficial sublayers of the optic tectum was a necessary condition for the appearance of label within Ipc neurons, and it is suggested that glycine and GABA, as putative neurotransmitters, are picked up by the Ipc-tectal terminals and that the label is transported retrogradely to the cell bodies.

Abstract

Autoradiographs of the pigeon optic lobe were prepared after injection of various tritiated amino acids or choline in the optic tectum or in the subtectal nucleus isthmi, pars parvocellularis (Ipc). Horseradish peroxidase (HRP) was also injected into Ipc. A topographically organized reciprocal connection was observed between the tectum and Ipc. The tectal-Ipc pathway had cell bodies lying in sublayer IIi. The acetylcholinesterase staining disappeared topographically in Ipc after tectal lesion. The Ipc-tectal fibers project mainly upon the superficial sublayers IId and IIb. These terminals, filled with HRP, were seen at the electron microscopic level to contain mainly round vesicles and primarily to contact horizontally running structures. In the lateral and caudal part of the tectum the application of [3H]glycine, [3H]serine, [3H]α-alanine or [3H]choline was followed by labelling of the cell bodies in Ipc, while the injection of all the other amino acids tested lead only to neuropil labeling. [3H]Glycine application to the superficial sublayers of the optic tectum was a necessary condition for the appearance of label within Ipc neurons. Rostral tectal injections of [3H]GABA, but not [3H]glycine, resulted in a labeling of rostral-Ipc somata. Glycine injected directly into Ipc was accumulated by neurons while little uptake of GABA or DABA was seen except for a small number of profiles within rostral regions of the nucleus. It is suggested that glycine and GABA, as putative neurotransmitters, are picked up by the Ipc-tectal terminals and that the label is transported retrogradely to the cell bodies. It is further proposed that glycine or a glycine related transmitter is active within portions of the Ipc-tectal pathway while rostrally located neurons of the subtectal nucleus are chemically differentiated and use GABA as their neurotransmitter.