Selective retrograde labeling indicating the transmitter of neuronal pathways

A number of tritiated transmitter related compounds—amino acids and biogenic amines—were injected into the rat caudoputamen or substantia nigra in order to test (1) for the occurence of autoradiographic perikaryal labeling, (2) for a selectivity of perikaryal labeling relating certain compounds to certain pathways, and (3) for the relation of perikaryal labeling to known transmitter specificities of the systems involved. Perikaryal labeling was observed after injection of some but not all of the substances used and was best explained by retrograde labeling in pathways projecting to the injection sites. Six hours after injection of high concentrations of tritiated transmitter into the terminal area, perikaryal labeling was observed: (A) in substantial nigra compacta (A9), A10 (rostral) and A8 (all heavy), and in n. raphe dorsalis (light) after [ 3 H]‐dopamine and [ 3 H]‐norepinephrine injection into caudoputamen; (B) same pattern as in A, but heavy in n. raphe dorsalis after [ 3 H]‐serotonin injection into caudoputamen; perikaryal labeling absent in cortex and thalamus after injection of substances mentioned in A and B; (C) only in substantia nigra compacta (minimally) after [ 3 H]‐GABA injection into caudoputamen; (D) in cerebral cortex and thalamus but not in substantia nigra, A10, A8, nor in n. raphe dorsalis after injection of [ 3 H]‐D‐aspartate into caudoputamen; (E) in the rat caudoputamen but not in n. raphe dorsalis after [ 3 H]‐GABA injection into substantia nigra; (F) in n. raphe dorsalis but not in caudoputamen after [ 3 H]‐serotonin into substantia nigra. These results indicated, indeed, a certain selectivity—partly related to transmitter specificity—for perikaryal labeling patterns. As a method, transmitter specific retrograde tracing could be useful in pathways with dopamine‐, serotonin‐, and GABA‐mediated synaptic transmission.