Identification of Respiratory Neurons Regenerating Axons into Peripheral Nerve Grafts in Adult Rats

The present aimed to identify the origin of medullary and upper cervical respiratory neurons regenerating their axons into the peripheral nerve grafts in adult rats. We employed an antidromic activation technique and a retrograde horseradish peroxidase (HRP) tracing method for determining the origins of the regenerating axons. Autologous segments of the common peroneal nerve were successfully implanted ventrolaterally into the proximal cut end of the C2 spinal cord hemisection (n = 24). Two to 5 months after implantation, spontaneous multi-unit discharges were recorded in all grafts; respiratory-related discharges in 21 (87.5%) grafts; and non-respiratory discharges in 3 (12.5%) grafts. The respiratory discharge patterns were similar to those for normal respiratory efferent neurons in rats. After the recordings were completed, 5 of 182 respiratory units explored in the medullary ventral respiratory cell group (VRG) neurons were antidromically activated by electrical stimulation of the grafts (n = 15). The estimated axonal conduction velocities ranged from 5.6 to 7.4 (mean 6.7) m/s. Retrograde horseradish peroxidase (HRP) labelings applied to the distal cut end of the grafts (n = 13) revealed that HRP-labeled cells were located, predominantly ipsilaterally, in the brainstem up to 5-10 mm distant from the implanted site. Some HRP-labeled cells were observed in the region of the nucleus ambiguus where many respiratory neurons exist. These results demonstrate that peripheral nerve grafts implanted in the C2 segment can induce axonal regeneration of medullary VRG neurons conveying funcional efferent signals.