Evidence for selective axon-terminal uptake and retrograde transport of label in cortico- and rubrospinal systems after injection of 3H-proline.

In tracing the macaque's ascending spinal connections by anterograde axonal flow, perikaryal labelling was also observed within the n. ruber pars magnocellularis as well as in the sensorimotor cortex. While neuropil labelling was demonstrated after single injections of leucine or proline, labelling of perikarya was only evident following proline injections. Perikaryal labelling is most likely to be due to uptake of label by axons and/or axons terminals within the injection field and subsequent retrograde axonal transport of radioactive compound to the parent cell bodies. Advantages and disadvantages of the uptake of label by axon terminals are discussed in relation to the use of 3H-proline for tracing neuronal connections. The selectively of this event and its relationship to the suggested inhibitory functional role of proline and its metabolites is not clear and needs further investigation.