Transneuronal passage of intraaxonally injected horseradish peroxidase (HRP) from group Ib and II fibers into the secondary neurons in the dorsal horn of the cat spinal cord

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Abstract
Intraaxonal injection of horseradish peroxidase (HRP) into physiologically identified group Ia, group Ib and muscle spindle group II fibers in the cat dorsal funiculus resulted in HRP labeling of the injected fibers and also spinal neurons in laminae V-VI. Staining of the neurons was dark enough to reveal most of the dendrites and a considerable length (up to 1 mm) of the axons. Serial reconstruction under light microscopy revealed that the vast majority of neurons were closely opposed to terminals of injected fibers, 47 neurons associated with 7 group Ib fibers, 17 other neurons with 3 spindle group II fibers, and none with group Ia fibers. The terminals of group Ib fibers contacted mainly the somata and proximal dendrites of the stained cells, whereas the majority of the group II fiber terminals were found on the distal dendrites. The neurons were probably stained by interneuronal transfer of HRP from the injected fibers. Possible mechanisms of the observed secondary staining are discussed.