Tracing motor neurons and primary sensory afferents of the monkey spinal cord with cholera toxin subunit B

Abstract Nonhuman primates are increasingly being used as animal models in neuroscience research. However, efficient neuronal tracing techniques for labeling motor neurons and primary sensory afferents in the monkey spinal cord are lacking. Here, by injecting the cholera toxin B subunit into the sciatic nerve of a rhesus monkey, we successfully labeled the motor neurons and primary sensory afferents in the lumbar and sacralspinal cord. Labeled alpha motor neurons were located in lamina IX of the L6–S1 segments, which innervate both flexors and extensors. The labeled primary sensory afferents were mainly myelinated Aβ fibers that terminated mostly in laminae I and II of the L4–L7 segments. Together with the labeled proprioceptive afferents, the primary sensory afferents formed excitatory synapses with multiple types of spinal neurons. In summary, our methods successfully traced neuronal connections in the monkey spinal cord and can be used in spinal cord studies when nonhuman primates are used.