title: Review on Cowan WM, Gottlieb DI, Hendrickson AE, Price JL, Woolsey TA. 1972. The autoradiographic demonstration of axonal connections in the central nervous system. Brain Res 37: 21-51.
abstract: Axoplasmically transported proteins synthesized in neuronal somata labeled by radioactively labeled amino acids (tritium), following local targeted injections for tracing of pathways in the central nervous system using autoradiography. Results from a number of neuronal systems, including: the rat olfactory bulb; cortico-thalamic projections in the mouse; commissural connections of the rat hippocampus; and retinal projections in the monkey and chick are documented. Pathway origins are clear, as the number and distribution of the labeled cells and the normal structure of the injection site is preserved. Light and electron microscopic autoradiography shows that proteins are transported, at two rates: rapid transport (>100mm/day) of fewer proteins accumulating in axon terminals; and, slow transport (1-5mm/day) of the bulk of labeled proteins distributed along the length of axons. Different survival times can be selected to evaluate terminal projection field(s) or pathways from origin to termination. The clarity of autoradiographic labeling of pathways and their terminations is comparable to other techniques (such as the Nauta-Gygax and the Fink-Heimer methods and the electron microscopy of terminal degeneration). Labeled amino acids do not label molecules in fibers of passage and there is no retrograde transport of labeled material from the axon terminals. The functional polarity of fiber pathways can be easily established. We summarize the merits of this technique is based upon an established physiological properties of neurons that are summarized in contrast to currently used techniques dependent upon pathological changes in neurons, axons, or axonal terminals.
  
  
  
  This article considers a heavily cited Brain Research article that reported an extremely important turning point in the ability to demonstrate neuroanatomical pathways in the central nervous system. Using radioactive leucine microinjections into the brain, neurons synthesized proteins from this amino acid that were transported down their axons to the terminal synapses on the target neurons. Tracing the transport of the labeled protein by autoradiography permitted quantitative analysis of projections and pathways. As a result, pathway analysis was transformed from studying the degenerating processes of lesioned neurons to the study of intact pathways in non-manipulated brains. The classical protocol has since been widely applied and used to investigate countless brain circuits. This article is part of a Special Issue entitled SI:50th Anniversary Issue.
authors.affiliation: Washington University in St. Louis, St. Louis, MO, USA. Electronic address: tom63105@yahoo.com.
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Woolsey
mesh_terms: D000818:Animals; D001345:Autoradiography / Q000266:history*; D001370:Axonal Transport; D001369:Axons; D001921:Brain / Q000033:anatomy & histology*; D049673:History, 20th Century; D051379:Mice; D009434:Neural Pathways / Q000033:anatomy & histology; D056347:Neuroanatomical Tract-Tracing Techniques / Q000266:history*; D009445:Neuroanatomy / Q000266:history*; D009474:Neurons; D051381:Rats
keywords: Anterograde tract tracing; Autoradiography; Neuroanatomy; Quantitative analysis of brain pathways; Radioactive leucine