COINJECTION 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 lab eling in the spinal cord of the rat after co-injection of the tracers wheat germ agglutinin-HRP (WGA-HRP) and choleragenoid toxin-HRP (CTB-H RP) into the sciatic nerve. Injection of WGA-HRP alone produced a patt ern of transganglionic label consistent with transport by small-diamet er primary afferent fibers. Small cell bodies were labeled in the ipsi lateral dorsal toot ganglion (DRG) and there was dense terminal labeli ng in the superficial dorsal horn of the lumbar spinal cord. Injection of CTB-HRP alone produced a pattern of transganglionic labeling consi stent with transport by large-diameter primary afferent fibers. Large cell bodies were labeled in the DRG and there was dense terminal label ing in the nucleus proprius (Laminae III-V) in the spinal cord. CTB-HR P also produced extensive retrograde labeling of ventral horn motor ne urons. When the two tracers were co-injected, we found few labeled cel ls in the ipsilateral DRG and there was almost complete loss of transg anglionic terminal labeling in the lumbar spinal cord. Retrograde labe ling 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 inhibitor y interaction was detected. The labeling pattern was always characteri stic of the first tracer injected. Since the loss of labeling was obse rved in the DRG and in the spinal cord, we conclude that the mutually inhibitory interactions occur during uptake of the tracers by the scia tic nerve and/or in the mechanism of transport to the dorsal root gang lion 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 W GA-HRP/CTB-HRP complex that cannot be taken up and transported by the sciatic nerve.