cAMP and Schwann cells promote axonal growth and functional recovery after spinal cord injury.

Central neurons regenerate axons if a permissive environment is provided; after spinal cord injury, however, inhibitory molecules are present that make the local environment nonpermissive. A promising new strategy for inducing neurons to overcome inhibitory signals is to activate cAMP signaling. Here we show that cAMP levels fall in the rostral spinal cord, sensorimotor cortex and brainstem after spinal cord contusion. Inhibition of cAMP hydrolysis by the phosphodiesterase IV inhibitor rolipram prevents this decrease and when combined with Schwann cell grafts promotes significant supraspinal and proprioceptive axon sparing and myelination. Furthermore, combining rolipram with an injection of db-cAMP near the graft not only prevents the drop in cAMP levels but increases them above those in uninjured controls. This further enhances axonal sparing and myelination, promotes growth of serotonergic fibers into and beyond grafts, and significantly improves locomotion. These findings show that cAMP levels are key for protection, growth and myelination of injured CNS axons in vivo and recovery of function.

The combination of human neuronal serotonergic cell implants and environmental enrichment after contusive SCI improves motor recovery over each individual strategy.

A human neuronal cell line, hNT2.19, which secretes serotonin (5-HT) after differentiation, was used as a transplant source to improve motor dysfunction following severe contusive spinal cord injury (SCI). Also, environmental enrichment (EE) was added to the interspinal transplant treatment paradigm. Motor testing was performed weekly before and following SCI, with and without EE and/or cell transplant conditions. Motor recovery was maximal when both cell transplant and EE were used. Individual treatment paradigms also significantly improved foot rotation and reduced footfall errors but not stride length or base of support dysfunction. This recovery of motor function after SCI suggests that the combinatory use of serotonergic hNT2.19 cell grafts plus EE is a meaningful strategy to modestly improve motor dysfunction that accompanies contusive SCI.

Loss of central inhibition: implications for behavioral hypersensitivity after contusive spinal cord injury in rats.

Behavioral hypersensitivity is common following spinal cord injury (SCI), producing significant discomfort and often developing into chronic pain syndromes. While the mechanisms underlying the development of behavioral hypersensitivity after SCI are poorly understood, previous studies of SCI contusion have shown an increase in amino acids, namely, aspartate and glutamate, along with a decrease in GABA and glycine, particularly below the injury. The current study sought to identify alterations in key enzymes and receptors involved in mediating central inhibition via GABA and glycine after a clinically-relevant contusion SCI model. Following thoracic (T8) 25.0 mm NYU contusion SCI in rodents, significant and persistent behavioral hypersensitivity developed as evidenced by cutaneous allodynia and thermal hyperalgesia. Biochemical analyses confirmed upregulation of glutamate receptor GluR3 with downregulation of the GABA synthesizing enzyme (GAD65/67) and the glycine receptor α3 (GLRA3), notably below the injury. Combined, these changes result in the disinhibition of excitatory impulses and contribute to behavioral hyperexcitability. This study demonstrates a loss of central inhibition and the development of behavioral hypersensitivity in a contusive SCI paradigm. Future use of this model will permit the evaluation of different antinociceptive strategies and help in the elucidation of new targets for the treatment of neuropathic pain.

Limitations of nerve repair of segmental defects using acellular conduits.

The authors present the case of a 20-year-old man who, 3 months after his initial injury, underwent repair of a 1.7-cm defect of the ulnar nerve at the wrist; repair was performed with an acellular nerve allograft. Given the absence of clinical or electrophysiological recovery at 8 months postrepair, the patient underwent reexploration, excision of the "regenerated cable," and rerepair of the ulnar nerve with sural nerve autografts. Histology of the cable demonstrated minimal axonal regeneration at the midpoint of the repair. At the 6- and 12-month follow-ups of the sural nerve graft repair, clinical and electrophysiological evidence of both sensory and motor reinnervation of the ulnar nerve and associated hand muscles was demonstrated. In this report, the authors describe a single case of failed acellular nerve allograft and correlate the results with basic science and human studies reporting length and diameter limitations in human nerve repair utilizing grafts or conduits devoid of viable Schwann cells.

Transplantation of Schwann cells in a collagen tube for the repair of large, segmental peripheral nerve defects in rats.

OBJECT: Segmental nerve defects pose a daunting clinical challenge, as peripheral nerve injury studies have established that there is a critical nerve gap length for which the distance cannot be successfully bridged with current techniques. Construction of a neural prosthesis filled with Schwann cells (SCs) could provide an alternative treatment to successfully repair these long segmental gaps in the peripheral nervous system. The object of this study was to evaluate the ability of autologous SCs to increase the length at which segmental nerve defects can be bridged using a collagen tube. METHODS: The authors studied the use of absorbable collagen conduits in combination with autologous SCs (200,000 cells/µl) to promote axonal growth across a critical size defect (13 mm) in the sciatic nerve of male Fischer rats. Control groups were treated with serum only-filled conduits of reversed sciatic nerve autografts. Animals were assessed for survival of the transplanted SCs as well as the quantity of myelinated axons in the proximal, middle, and distal portions of the channel. RESULTS: Schwann cell survival was confirmed at 4 and 16 weeks postsurgery by the presence of prelabeled green fluorescent protein-positive SCs within the regenerated cable. The addition of SCs to the nerve guide significantly enhanced the regeneration of myelinated axons from the nerve stump into the proximal (p < 0.001) and middle points (p < 0.01) of the tube at 4 weeks. The regeneration of myelinated axons at 16 weeks was significantly enhanced throughout the entire length of the nerve guide (p < 0.001) as compared with their number in a serum-only filled tube and was similar in number compared with the reversed autograft. Autotomy scores were significantly lower in the animals whose sciatic nerve was repaired with a collagen conduit either without (p < 0.01) or with SCs (p < 0.001) when compared with a reversed autograft. CONCLUSIONS: The technique of adding SCs to a guidance channel significantly enhanced the gap distance that can be repaired after peripheral nerve injury with long segmental defects and holds promise in humans. Most importantly, this study represents some of the first essential steps in bringing autologous SC-based therapies to the domain of peripheral nerve injuries with long segmental defects.