Co-injection of wheat germ agglutinin-HRP and choleragenoid-HRP into the sciatic nerve of the rat blocks transganglionic transport

We report on the surprising loss of transganglionic and retrograde labeling in the spinal cord of the rat after co-injection of the tracers wheat germ agglutinin-HRP (WGA-HRP) and choleragenoid toxin-HRP (CTB-HRP) into the sciatic nerve. Injection of WGA-HRP alone produced a pattern of transganglionic label consistent with transport by small-diameter primary afferent fibers. Small cell bodies were labeled in the ipsilateral dorsal root ganglion (DRG) and there was dense terminal labeling in the superficial dorsal horn of the lumbar spinal cord. Injection of CTB-HRP alone produced a pattern of transganglionic labeling consistent with transport by large- diameter primary afferent fibers. Large cell bodies were labeled in the DRG and there was dense terminal labeling in the nucleus proprius (Laminae III- V) in the spinal cord. CTB-HRP also produced extensive retrograde labeling of ventral horn motor neurons. When the two tracers were co-injected, we found few labeled cells in the ipsilateral DRG and there was almost complete loss of transganglionic terminal labeling in the lumbar spinal cord. Retrograde labeling of motor neurons was also significantly reduced. Even when one of the tracers (e.g., WGA-HRP) was injected 24 hr after and up to 10 mm proximal to the site of the first tracer (e.g., CTB-HRP), an inhibitory interaction was detected. The labeling pattern was always characteristic of the first tracer injected. Since the loss of labeling was observed in the DRG and in the spinal cord, we conclude that the mutually inhibitory interactions occur during uptake of the tracers by the sciatic nerve and/or in the mechanism of transport to the dorsal root ganglion and beyond. The fact that reduced labeling was observed even when the injections were separated temporally and spatially suggests that the underlying mechanism is more complicated than mere creation of a WGA-HRP/CTB-HRP complex that cannot be taken up and transported by the sciatic nerve.