Sciatic nerve transection triggers release and intercellular transfer of a genetically expressed macromolecular tracer in dorsal root ganglia.

We recently developed a genetic transneuronal tracing approach that allows for the study of circuits that are altered by nerve injury. We generated transgenic (ZW-X) mice in which expression of a transneuronal tracer, wheat germ agglutinin (WGA), is induced in primary sensory neurons, but only after transection of their peripheral axon. By following the transneuronal transport of the tracer into the central nervous system (CNS) we can label the circuits that are engaged by the WGA-expressing damaged neurons. Here we used the ZW-X mouse line to analyze dorsal root ganglia (DRG) for intraganglionic connections between injured sensory neurons and their neighboring "intact" neurons. Because neuropeptide Y (NPY) expression is strongly induced in DRG neurons after peripheral axotomy, we crossed the ZW-X mouse line with a mouse that expresses Cre recombinase under the influence of the NPY promoter. As expected, sciatic nerve transection triggered WGA expression in NPY-positive DRG neurons, most of which are of large diameter. As expected, double labeling for ATF-3, a marker of cell bodies with damaged axons, showed that the tracer predominated in injured (i.e., axotomized) neurons. However, we also found the WGA tracer in DRG cell bodies of uninjured sensory neurons. Importantly, in the absence of nerve injury there was no intraganglionic transfer of WGA. Our results demonstrate that intraganglionic, cell-to-cell communication, via transfer of large molecules, occurs between the cell bodies of injured and neighboring noninjured primary afferent neurons.