Retrograde axonal transport of horseradish peroxidase. Uptake at mouse neuromuscular junctions following systemic injection.

Eye muscles were examined histochemically for the presence of horseradish peroxidase (HRP) activity after intravenous injections of large doses of this tracer protein. HRP had penetrated through the blood vessels and diffused into the areas of motor end plates, where it outlined axon terminals at the synaptic clefts. HRP was incorporated into pinocytotic vesicles in the axons from where an intraaxonal transport in the retrograde direction to the nerve cell bodies in the brainstem followed. Accumulation of HRP in perikarya of motor neurons can therefore be the result of a physiological process of pinocytosis at the axon terminals. In this way exogenous macromolecules in the blood can by-pass the blood-brain barrier and reach the lower motor neurons in the CNS.