Transcellular transfer of HRP in the amphibian visual system

Unilateral intraocular injections of horseradish peroxidase (HRP) were made in the green tree frog, Hyla cinerea. Survival times ranged from 1 to 28 days. Control injections were placed in the orbit, peritoneum, or third ventricle. By 1 day after ocular injection anterogradely transported HRP was observed in the optic nerve and tract and in thalamic and midbrain retinal recipient zones. Retrograde filling of motor neurons was also observed by 1 day. At 3 days, HRP-positive magnocellular preoptic neurons became apparent. Finally, at 3-5 days post-injection, ependymal cells radially adjacent to HRP-positive neuropils, but not retrogradely filled cells, contained a small amount of reaction product. By 7 days these ependymal cells were densely filled and processes could be seen extending toward the neuropils. There was never evidence of HRP uptake by neurons in or adjacent to these HRP-positive neuropils. Neither retinal fibers nor ependymal cells were HRP-positive after any control injection or after processing uninjected material by the HRP histochemistry protocols. In contrast, motor cells were filled following orbital and peritoneal, but not ventricular, injections, suggesting blood-borne HRP reaching motor endplates could account for some of the motor neuron filling. Preoptic cells were filled after all control injections, demonstrating that they too could take up circulating HRP. The specificity of ependymal cell filling, however, suggests that anterogradely transported HRP can be released at the axon terminals and taken up specifically by ependymoglial cells.