Transmitter-related retrograde labeling in the pigeon optic lobe: a high resolution autoradiographic study

High resolution light microscopic and electron microscopic autoradiography in a restricted area of the pigeon subtectal nucleus isthmi, pars parvocellularis (Ipc) 30 min after onset of a tectal [3H]glycine injection shows labeling mainly of the following elements: neuronal perikarya, initial axon segments and dendrites. Rapid labeling of such intrinsic Ipc elements and not of synaptic terminals possibly of tectal origin strongly suggests a fast retrograde migration of radioactivity within Ipc-tectal neurons. Fixation experiments indicate a soluble nature of this radioactive material. This type of retrograde labeling in the glycinergic Ipc-tectal pathway seems to be related to the transmitter specificity of the system.