Horseradish peroxidase retrograde labeling of primary sensory neurons: a comparison of four intraspinal application methods

Intraspinal grafting procedures: spinal cord effects induced in the adult rat: a clinical, histopathological, and immunohistochemical study

OBJECTIVE

Intraspinal grafting procedures using peripheral nerve grafts (PNG) or collagen guidance channels (CGC) have been recently used to treat brachial plexus injuries in humans and spinal cord injuries in animals. This study examined the effects of these procedures in the adult rat.

METHODS

In adult rats, we performed an avulsion of left C5, C6, and C7 nerve roots, followed by a myelotomy of the left ventrolateral aspect of the spinal cord between C5 and C6. The rats were subsequently assigned to one of three groups: group A (n = 10), no additional procedure; group B (n = 10), implantation of a PNG following myelotomy; group C (n = 10), implantation of a CGC. Clinical evaluation was postoperatively assessed. Rats were euthanized at day 6 or 21. Spinal cord lesions induced by surgery were assessed by measuring depth and rostrocaudal extent. Reactive astrogliosis, as a reaction to neuroglial damage, was assessed by revealing the glial fibrillary acidic protein with immunochemistry method.

RESULTS

No animal showed persistent neurological deficit at day 21. The depth and rostrocaudal extent of tissue damage was comparable in all groups at days 6 and 21. At day 6, the astrocytic reaction observed at the myelotomy/implantation site was statistically stronger in group C (CGC). At day 21, the astrocytic reaction became identical in all groups.

CONCLUSION

This study shows that grafting a PNG or a CGC into the spinal cord does not create significant additional iatrogenic effects and can be used in repair strategies to treat nerve root avulsions or spinal cord injuries.

Experimental bypass surgery between the spinal cord and caudal nerve roots for spinal cord injuries

Spinal cord injuries often cause permanent neurological deficits and are still considered as inaccessible to efficient therapy. Injured spinal cord axons are unable to spontaneously regenerate in adult mammalians. Re-establishing functional activity especially in the lower limbs by reinnervating the caudal infra-lesional territories could represent an attractive therapeutic strategy. For several years, we have studied and developed surgical bypasses using peripheral nerve grafts bridging the supra-lesional rostral spinal cord to the caudal infra-lesional lumbar roots. Main objectives were: 1- to overcome the spinal cord lesion and the consecutive glial barrier blocking the axonal regeneration; 2- to find and bring an alternative source of regenerating axons; 3- to guide those axons toward precisely definite targets (for example, lower limb muscles). We report here the results of our experimental research, which led us from animal experimental models (rodents, primates) to the first human experimentation. Limitations of the method (especially technical pitfalls) are numerous. However, we have obtained encouraging results in our attempts to "repair" the motor pathway. Functional recovery with strong evidence of centrifugal axonal regeneration from the spinal cord to the periphery has been observed. Regarding the sensory pathway, we have found evidence of centripetal axonal regeneration from the periphery toward the spinal cord. Further studies are obviously advocated, but our experimental model of spinal cord - nerve roots bypasses may be integrated in future "repair" strategies of both motor and sensory pathways following spinal cord injury.

Regeneration of primary sensory axons into the adult rat spinal cord via a peripheral nerve graft bridging the lumbar dorsal roots to the dorsal column

This study investigated the feasibility of using a peripheral nerve autograft (NAG) to promote and guide regeneration of sensory axons from the caudal lumbar dorsal roots to the rostral dorsal column following a lower thoracic cordotomy in adult rats. After a left hemicordotomy at the T13 vertebra level and ipsilateral L3 and L4 rhizotomies, a peripheral NAG (peroneal nerve) was connected to the distal roots stumps, then implanted into the left dorsal column 10 mm rostral to hemicordotomy site (n = 12). After surgery, all animals of the experimental group experienced complete anesthesia in their left hindlimb. Three months later, a slight response to nociceptive stimulation reappeared in L3 and/or L4 dermatomes in 6 of the 12 experimental animals. None of these animals exhibited self-mutilation. Nine months after surgery, we performed retrograde tracing studies by injecting horseradish peroxidase (HRP) into the left dorsal column 30 mm rostral to the NAG implantation site. In eight animals, we found HRP-stained neurons in the left L3 and/or L4 dorsal root ganglia (DRG). The mean number of HRP-stained neurons per DRG was 71 +/- 92 (range 2-259). In control groups, no HRP-stained neurons were found in L3 or L4 DRG. Histological analysis of the NAG showed evidence of axonal regeneration in all 8 animals with positive retrograde labeling of DRG neurons. However, we did not find a statistical correlation between the number of HRP-stained neurons and the degree of sensory recovery. This study demonstrates that an NAG joining dorsal roots to the dorsal column, thus shunting the original CNS-PNS junction, can support regeneration of central axons from DRG primary sensory neurons into the dorsal column over distances of at least 30 mm despite the inhibitory influence of the CNS white matter.