Intraneuronal injection of horseradish peroxidase labels glial cells associated with the axons of the giant metacerebral neuron of Aplysia.

After injecting the cell body of the giant serotonergic neuron (GCN) in the cerebral ganglion of Aplysia with horseradish peroxidase (HRP), we found that some glial cells were labeled along the cerebrobuccal connective and posterior lip nerve, the two nerves that contain the major axon branches of the injected neuron. Starting at a distance of about 3 mm from the site of the injection, only glial cells in the nerve that are close to an axon branch of the GCN contained HRP. Labeling appears to be selective because, at most, only one out of three glial cells was labeled within an area of 20 micrometer of the axon. Moreover, no HRP was detected in any of the many glial cells within the cerebral ganglion. Reaction product diffusely labeled the cytoplasm of glial cell bodies and processes after intraneuronal injection of HRP. This distribution was markedly different from that observed in glial cells after the uptake of HRP from the extracellular space; HRP, presumed to be taken up by endocytosis, was found to be localized to vesicles, tubules, and multivesicular bodies.