Dose- and time-dependent selective and non-selective effects of ricin (RCA 120) on rat primary sensory neurons

The distribution of degenerating fibers in the spinal cord was studied in Fink-Heimer-stained sections following treatment of the tibial nerve with ricinus communis agglutinin (RCA 120). The ricin was either injected into the nerve or applied in a capsule on the transected nerve. Short survival times and low doses of ricin resulted in degeneration in somatotopically appropriate parts of the medial dorsal horn. Longer survival times and higher doses resulted in degeneration which progressively expanded into inappropriate areas in the central and lateral parts of the dorsal horn and in deeper laminae regardless of the mode of application. Furthermore, the effect of a ricin injection into the tibial nerve on transganglionic transport of choleragenoid horseradish peroxidase (B-HRP) in the peroneal nerve was studied following a simultaneous or delayed B-HRP injection. A simultaneous ricin and a B-HRP injection resulted in primary afferent HRP labeling in the gray matter, regardless of the dose of ricin. Following a delayed B-HRP injection almost no primary afferent labeling was seen in the gray matter, unless a very low dose of ricin was injected. This study shows that treatment of a peripheral nerve with a high dose of ricin and a long survival time may result in a considerable non-selective degeneration of fibers in the spinal cord. A selective degeneration may, however, be obtained by using lower doses or shorter survival times.