Peripheral specification of sensory neurons transplanted to novel locations along the neuraxis

Thoracic dorsal root ganglia in bullfrogs contain sensory neurons that innervate the skin of the trunk and have synaptic connections in the dorsal horn of the spinal cord. The ganglion that innervates the forelimb contains, in addition to cutaneous afferents, many muscle afferents that project more ventrally in the spinal cord and make monosynaptic connections with motoneurons. In the present study, we have transplanted thoracic sensory neurons to the brachial level in tadpoles to discover whether they can innervate forelimb muscles and, if so, whether they form central connections characteristic of forelimb muscle afferents. The ganglion that normally supplies the forelimb was removed from tadpoles and replaced with 2 thoracic ganglia. After the tadpoles completed metamorphosis, the peripheral and central connections of the transplanted thoracic sensory neurons were examined with anatomical and electrophysiological techniques. When the ganglia were transplanted at stage XIV or earlier, transplanted sensory neurons innervated the forelimb and projected into the brachial spinal cord. Electrical stimulation of forelimb muscle nerves evoked impulses in the dorsal root, indicating that some centrally projecting sensory neurons were muscle afferents. Furthermore, muscle afferents were also activated by stretching muscles which suggest that they terminated on spindles. HRP labeling of the central projections revealed that transplanted sensory neurons terminated at sites characteristic of both cutaneous and muscle afferents. The pattern of synaptic connections was assessed by recording intracellularly from motoneurons. Stimulation of muscle afferents produced monosynaptic EPSPs in motoneurons. As in normal frogs, triceps muscle afferents projected more strongly to triceps motoneurons than to subscapularis and pectoralis motoneurons, while subscapularis afferents projected to all 3 types of motoneurons. Thus, the transplanted sensory neurons formed central connections appropriate to their novel peripheral targets. These observations suggest that interactions between sensory neurons and their targets may be important in determining their central connections.